Jean-Noël Capdevielle

Reda Attallah
email: reda.attallah@yahoo.com

Mohamed Cherif Talai
email: mctalai@wissal.dz

Coplanar emission near the LHC energy range (observed with XREC in the stratosphere)

The alignment of very high energy secondary cosmic rays was observed at both stratospheric and mountain altitudes by several X-ray emulsion chamber experiments. Extensive simulation with CORSIKA demonstrates that such phenomena can be explained by fluctuations with standard physics. However, in the case of two events observed in the stratosphere, specific features contradicts such explanation. According to the properties of those events with a minimal cascading, we explore the hints of new physics which could explain the alignment in terms of relativistic strings and diquark breaking. One description of the consequent coplanar emission expected in colliders is proposed.

I. INTRODUCTION

Two decades ago the alignment of secondary particles has been observed in X ray emulsion chambers (XREC) exposed at mountains such as Pamir [START_REF] Baradzei | Proc. 3rd Int. Symp. on Very High Energy Cosmic Ray Interactions[END_REF], [START_REF] Borisov | Proc. 3rd Int. Symp. on Very High Energy Cosmic Ray Interactions[END_REF], [START_REF] Borisov | Proc. 28th Int. Cosmic Ray Conf[END_REF] and Kanbala [START_REF] Xue | Proc. 26th Int. Cosmic Ray Conf[END_REF], or boarded in a plane like Concorde [START_REF] Capdevielle | Proc. 25th Int. Cosmic Ray Conf[END_REF], [START_REF] Capdevielle | [END_REF], or else during Siberian Balloon Flights (SBF) such as RUNJOB [START_REF] Galkin | Proc. 27th Int. Cosmic Ray Conf[END_REF]; γ-ray families exhibiting clear geometrical alignments have also been registrated. The coplanar emission appears significantly above a threshold energy of about 10 PeV at mountain altitude [START_REF] Borisov | [END_REF] and a similar threshold is ascertained also in the stratosphere [START_REF] Capdevielle | Proc. 13th Int. Symp. on Very High Energy Cosmic Ray Interactions (Wei-Hai)[END_REF]. We have pointed out from extensive simulations with COR-SIKA [START_REF] Attallah | [END_REF] that the aligments can result from fluctuations in a context of standard physics. In contrast, several common features between the two events collected in the stratosphere (JF2AF2 in Concorde, STRANA on a balloon flown by FIAN) belies this last conclusion and the present work is concentrated again on new elements suggesting some hints of new physics in terms of valence diquark breaking and stretching of relativistic strings at very high tension. Our paper is outlined as follows: the geometric characterization of the alignment is reviewed in section 2 and the properties of JF2AF2 and STRANA are considered in section 3, after a detailed presentation of the experimental data and the simulation results. Section 4 is devoted to the methodological approaches of the origin of the interaction, the effect of the experimental energy threshold in XREC's. The last sections concern the new theoretical propositions explaining the coplanar emission and the description of the coplanar emission which might be expected in colliders at energies exceeding (s) = 3000 GeV.

II. GEOMETRICAL AND STATISTICAL CHARACTERIZATION OF THE COPLANAR EMISSION

The asymmetry of the geometrical structure of detected particles (hadrons or electrons and photons) can be characterized by the linear correlation coefficient r determined by fitting to a straight line by the least-squares method the coordinates derived from the tracks (or dark spots) left on the sensitive plates [START_REF] Capdevielle | Proc. 25th Int. Cosmic Ray Conf[END_REF]:

r = n i (x i -x)(y i -ȳ) n i (x i -x) 2 n i (y i -ȳ) 2 (1)
where n is the number of particles under consideration, (x i , y i ) the coordinates of the i th particle and (x, ȳ) the average values. The degree of alignment can also be described by the parameter λ n introduced by Pamir Collaboration [START_REF] Borisov | Proc. 3rd Int. Symp. on Very High Energy Cosmic Ray Interactions[END_REF] and defined as:

λ n = n i =j =k cos 2ϕ k ij n(n -1)(n -2) (2)
where ϕ k ij is the angle between the straight lines joining the i th and j th particles to the k th one (0 ≤ ϕ k ij ≤ π). If all the n points are perfectly aligned along a straight line, r = ±1 and λ n = 1 ; both tend respectively to 0 and -1 n-1 in case of an isotropic distribution. When λ n ≥ 0.8, the event is considered as aligned or elongated. This corresponds for more than 90% of the events to |r| ≥ 0.94.

III. A PAIR OF COPLANAR EVENTS IN THE STRATOSPHERE, JF2AF2(CONCORDE) AND STRANA (BALLOON)

Among stratospheric experiments, only ECHOS-Concorde [START_REF] Capdevielle | Proc. 25th Int. Cosmic Ray Conf[END_REF], [START_REF] Capdevielle | [END_REF] and the earliest FIAN SBF [START_REF] Apanasenko | Proc. 15th Int. Cosmic Ray Conf[END_REF] experiments reported the observation of coplanar emission. The events registrated in Concorde above 1 PeV are non contained events, under the form of γ-ray families, resulting mainly from the primary hadronic interaction. Strictly speaking, that are "near direct events" that we have sorted in the category of "direct observations". It must be emphasized that in the experimental emulsion chamber terminology, the symbol γ's denotes all particles generating electromagnetic cascades in the chamber, i.e. photons, electrons and positrons.

A. ECHOS-Concorde experiment

During more than two decades, 8 X-ray emulsion chambers were flown with the supersonic aircraft Concorde connecting Paris to New York to achieve different measurements: very high energy cosmic ray jets, stratospheric γ-ray families, γray flux [START_REF] Iwai | [END_REF], [START_REF] Capdevielle | Proc. 16th Int. Cosmic Ray Conf[END_REF], [START_REF] Capdevielle | Proc. 20th Int. Cosmic Ray Conf[END_REF], [START_REF] Capdevielle | [END_REF], hyperstrange baryonic matter [START_REF] Capdevielle | Proc. 24th Int. Cosmic Ray Conf[END_REF] and dosimetry. Regular Atlantic flights provide indeed a plateau exposure of more than 2 hours. Hence, the same X-ray emulsion chamber, boarded a hundred times, can be exposed during at least 200 hour (the supersonic flights at an average altitude of 17 km, corresponds to an atmospheric depth of about 100 g cm -2 ). The thickness of all the detectors, except for the last one, was large enough to measure the energy of secondary γ-rays (γ+e ± ) at least up to 1000 TeV, allowing the collection of valuable information on the hadronic multiproduction in the energy range lying between the limits of the present colliders and the future LHC. However, because of the limited weight resulting from the compromise between area, aperture and time of exposure, the chamber was not thick enough to measure the energy of secondary hadrons. Three events were observed above 1 PeV; two of them, near 10 6 GeV, exhibit respectively a multicluster structure and hints of nuclear fragments. The most energetic event, arrived under a zenith angle of 52 • with a deposited energy of 1600 TeV by the e.m. component, approaching a primary energy of 10 PeV. This exceptional event shows a perfect alignment of the most energetic γ-rays (γ+e ± ) on the X-ray film [START_REF] Capdevielle | Proc. 25th Int. Cosmic Ray Conf[END_REF]. The photography of this event, named JF2af2, visible to the naked eyes on the X-ray film, is reproduced on figure 1. The 211 γ-rays (above 200 GeV) composing this event are identified by their respective coordinates and energies. Their analysis, focused on multiclustering and planarity, following the information of the emulsion sheets scanned after superposition of the X-ray film, indicating the passage of the cascades, suggests a multijet structure [START_REF] Capdevielle | [END_REF]. When sorted in order of decreasing energy, it comes out that the most energetic γ-rays stand along a perfect geometrical straight line. The topology of JF2af2 is displayed as a lego-plot on figure 2 for the 4 most energetic γ-rays (above 50 TeV). The linear correlation factor r determined by fitting to a straight line the coordinates of the 4 aligned γ-rays is equal to 0.9993 (λ 4 = 0.9972). Moreover, r remains close to unity (0.992) for the 38 most energetic γ-rays containing 808 TeV, i.e. 51% of all the visible energy.

In order to examine the relevance of this remarkable event, we have carried out a special set of simulations (10 4 events) under the same experimental conditions. Among all individual events simulated, we have selected for a complete analysis those with a linear correlation factor |r| larger than 0.94. An event virtually identical with JF2af2 in this sample shows a nice alignment of 23 γ-rays (figure 3a). The similarity of clusters (γ-rays above 10 TeV) to those of figure 1 is especially interesting. Tracing back the genetics of this event, it comes out that it arises from a normal non-single diffractive (NSD) primary interaction with a high multiplicity. The altitude of the first collision is about 40 km and the linear correlation factor for the 23 γ-rays (above 10 TeV) is 0.991; the total energy deposited is about 964 TeV. Figure 3b shows another aligned event with similar characteristics obtained with a second set of simulations. In this case, the first interaction takes place at an altitude of 28 km and the linear correlation factor for the 18 aligned γ-rays above 10 TeV forming this event is equal to 0.999.

The main features producing the alignment, here, are the high multiplicity and the large geometrical distance between the chamber and the first collision. Both circumstances combine as follows: the probability to get a large transverse momentum p t is enhanced in high multiplicity events and this large p t can be assigned to a high energy γ-ray. The rest of the cluster is displayed in the opposite direction (p t conservation) and the maximal separation between γ-rays appears in the horizontal plane (the emulsion or X sheets) with a characteristic gap.

Table I shows the calculated fraction (%) of events with

B. SBF and RUNJOB experiments

The RUssian-Nippon JOint Balloon (RUNJOB) experiment consists in a balloon-borne X-ray emulsion chamber flown at an average altitude of about 30 km (∼ 10 g cm -2 ) since 1995 [START_REF] Apanasenko | Proc. 25th Int. Cosmic Ray Conf[END_REF]. We emphasize here that the event STRANA was registrated earlier in similar conditions during a preliminary flight and that the alignement was ascertained during a recent analysis. The design of this detector gives the opportunity to study the geometry of secondary particles after the interaction of primary cosmic rays in a target module. The energy range covered by RUNJOB lies between the limits of the present accelerators and the very high energy cosmic rays. The data of four balloon flights performed during 1995 and 1996 were analyzed in order to search for alignments in the electromagnetic component [START_REF] Galkin | Proc. 27th Int. Cosmic Ray Conf[END_REF].

In order to estimate the situation of balloon flights measurements we have simulated 100 sets of stratospheric γ-ray families, each set containing exactly the same number of events (170) as observed, under the same RUNJOB experimental conditions. That is a carbon nucleus as target (Lucite target), a primary energy ranging from 10 13 to 10 15 eV and a detection threshold energy of 100 GeV. The primary particle is assumed to be in turn a proton, an α-particle ( 4 2 He), a carbon ( 12 6 C) nucleus, a magnesium ( 2412 Mg) nucleus and in the end an iron ( 5626 Fe) nucleus. Besides, the simulated events satisfy the requirement of falling within the fiducial area of the detector (0.4 m 2 ). The zenith angle is chosen at random in the range 0-70 • and the first interaction of the primary cosmic particle is fixed at an altitude of 30 km (∼ 10 g cm -2 ). The observation level lies 20 cm below. The asymmetry parameter λ n is calculated for the 3 and 4 most energetic secondary γ-rays (γ+e ± ). Figure 4 shows the average fractions of the aligned events with λ 3 ≥ 0.8 (a) and λ 4 ≥ 0.8 (b) for the different hadronic interaction models as a function of the mass number A of the primary particle. The error bars are purely statistical and represent one standard deviation. It is clear that all the models give values very close to each other and, within the error bars, independently of the type of primary particle . These fractions equal on average (22 ± 2) for the events with λ 3 ≥ 0.8 and (7 ± 1) for λ 4 ≥ 0.8. Our simulation at RUNJOB level provided also accidental alignments [START_REF] Attallah | [END_REF] and a situation similar to table I (as shown in figure 4).

In the case of the earliest SBF, the calorimeter was deep enough to collect the energy deposited also by the secondary hadrons with one general energy threshold for γ's and hadrons of 2 TeV. In the case of the XREC of Concorde, it was not possible to follow the hadronic contribution, but the energy threshold for γ's of 0.2 TeV provided a better resolution.

C. JF2AF2 and STRANA

Taking into account the different energy thresholds of the XREC's used for both events, respectively 200 GeV for JF2af2 and 2 TeV for STRANA [START_REF] Managadze | Proc. 29th Int. Cosmic Ray Conf[END_REF] , we observe first that the visible energy deposited in γ rays is very similar, as shown in table III.

In the case of STRANA, the hadronic accompaniement is observed for the central jet with 30 hadrons depositing a total energy of 2500 TeV.

IV. APPROACH OF THE ORIGIN OF THE INTERACTION

For non contained events, the tracks appears parallel as far as the distance to the vertex exceeds more than 50 m and the geometrical convergence measurements are no more valid. The height of the primary collision can be obtained by the invariant mass method, assuming that the γ's originates mainly from neutral pion decay, by the < ER > factor assuming an average transverse momentum of 0.2 GeV/c for γ's and by the best fit of pseudo-rapidity distribution for a given height.

A. The invariant mass method

The invariant pseudo-rapidity distributions have been determined for JF2af2 [START_REF] Capdevielle | [END_REF] for the total event as well as for the individual jets A, Ap, B and we have repeated those calculations, obtaining from the maxima of the histograms respective heights around 80 m (for total), 65 m, 75 m. Those values are however submitted to large uncertainties (η meson production, an error of 10-20% on coordinates and energy measurements, proportion of γ's of 1st generation, but they suggest that the γ's of the 3 major jets are generated at less than 300 m above the chamber.

B. The < ER > factor

We observe on table II that the < ER > factor indicates an origin of 325 m for Jet B, instead of an origin of 175 m for Jet A and 65 m for Jet Ap. In the case of STRANA, the invariant mass method and the other approaches suggested also different values from 100 m up tp 1000 m in the case of pseudo rapidity distribution fitted for an α primary. We have not the coordinates of the particles of STRANA, but we note that the central Jet Ap in JF2af2 is very concentrated, suggesting an interaction close from the chamber and could be followed by a hadronic cascade developed under the XREC.

C. Effect of the energy threshold of the XREC

Our simulation to check the limits of validity of the vertex determination are in progress and we started to appreciate the effect of the XREC threshold; some discrepancies with the so-called Bristol convention (relation of transverse momenta from γ's to neutral pions) appears as other consequences on N γ and inelasticity K γ shown on table IV.

We observed also that the KNO variable z has to be taken into account in any interpretation by fitting the pseudo rapidity.

V. THEORETICAL HYPOTHESIS FOR THE COPLANAR EMISSION

A. Earliest conjectures

This intriguing phenomenon motivated several theoretical investigations [START_REF] Borisov | [END_REF], [20]. Specific interaction features such as the breakdown of linear strings, due to the production of gluon jets in the main interaction [21], or the double inelastic diffraction [22] were discussed as a possible origin of the alignment. Other works proposed new physical processes such as the coplanar diffractive production of strange baryons not far from the chamber [START_REF] White | Proc. 8th Int. Symp. on Very High Energy Cosmic Ray Interactions[END_REF].

B. Strings between valence quark and valence diquark breaking

Following the proposition of one of us [START_REF] Capdevielle | Proc. 13th Int. Symp. on Very High Energy Cosmic Ray Interactions (Wei-Hai)[END_REF], we introduce here the combination of the valence diquark and the breakdown of relativistic strings as a possible explanation of the coplanar emission 5.

According to the simplified presentation of Wong [START_REF] Wong | Introduction to High Energy Heavy-Ion Collisions[END_REF], one pair q -q is created when the distance L separating both valence quarks exceeds a threshold value. The string fragmentation corresponds to a tension κ = 1/2πα ′ of about 1 GeV/fm, α ′ being the Regge slope. The transverse momentum of the quarks emitted is related to the tension by the relation (3):

< p t > 2 = κ π (3)
Such relation provides the classical values of p t = 0.2 GeV/c for quarks and 0.4 GeV/c for the pions where the pairs q -q are recombined.

Above an energy threshold of about 200 GeV (in CMS) per valence quarks (corresponding to a proton projectile of 10 PeV in the Laboratory system, a new string appears between the partners of the valence diquark [START_REF] Capdevielle | [END_REF]. The tension increases with the distance and the breakdown happens s soon as the minimal energy of excitation required is available; the maximal distance between the valence quarks associated to this minimal energy is obtained when the 3 quarks are aligned.

Such circumstance excludes the classic recombination of the leading cluster (one valence diquark with one quark of the sea giving a pilot proton, a neutron or a ∆ resonance); this could explain why the penetrating power of the cosmic air showers appears to level off in the "knee" energy range. Following this assumption, at energies more close to the LHC, the fragmentation of 3 separated valence quarks will still be observed, but the alignment will be smeared out, the energy required for the rupture being available for all geometrical configurations. The most simple recombination for the 3 valence quarks of the projectile will happen with antiquarks of the sea giving the emission of 3 energetic hadrons. Around the threshold energy, the coplanar emission will result in the emission of one characteristic collimated trident, of 3 charged pions, for example.

VI. COPLANAR EMISSION AND COLLIDERS

Coming back to JF2af2 and STRANA, the assumption of the emission of 3 hadrons could have several advantages, explaining in both cases the electromagnetic and hadronic components, as well as the original transverse momenta. In the case of JF2af2, the external jets (< ER >) factor and some secondary peaks at the largest values of the invariant mass indicate a maximal distance of the primary collision at 2.2 km above the chamber. We notice that 3 coplanar charged pions carrying the e.m. energy deposited in A, Ap and B interacting respectively at about 175, 60 and 320 m above the chamber are able to reproduce the structure observed and require a primary energy of about 5 PeV (reduced by 50% when compared to normal hadronic collision); this energy is very close of the "knee" of the primary energy spectrum and in better agreement with the number of high energy events expected with the modest exposures on Concorde and on SBF. The transverse momenta required for those pions at the emission would be 10.0, 3.3 and 6.5 GeV/c, comparable to the values required for STRANA. Such circumstance indicate a maximal tension of the string concerning the diquark of 20-30 GeV/fm instead of 1 GeV/fm. The transformation in Center of mass system could give a crude description of a possible coplanar emission in colliders.

-characteristics coplanar tridents collimated (emitted inside 0.3-1.5 • ); -pseudo-rapidities between 4.3 and 5.8, energies between 0.4 and 0.7 TeV; -p t 's around 3-10 GeV/c; -energy threshold √ s = 3000 GeV (an energy reachable by Fermi collider working in pulse mode) corresponding to valence quarks with individual energies above 200 GeV.

VII. DISCUSSION

We have investigated in this work the physical relevancy of some experimental data concerning the phenomenon of alignment observed for secondary particles in high energy cosmic ray interactions. The standard collider physics can explain the alignment via fluctuations and accidental large transverse momenta of very energetic secondaries. Some contradiction however remains (height of the primary collision, multicluster structure, very high p t , energy distribution).

The frequency of occurrence of the coplanar emission at high energy is another point worth discussing. Let us take, for instance, the event JF2af2 observed by ECHOS-Concorde experiment assuming that it is the outcome of a coplanar interaction at an altitude of 17 km of a 10 7 GeV primary nucleon with an atmospheric nitrogen. Taking into consideration the total primary integral energy spectrum [START_REF] Berezinsky | Astrophysics of Cosmic Rays[END_REF], the area of the exposed detector, and an aperture of 60 • , the total exposure would have to be multiplied by 20 to collect one event of 10 7 GeV. The energy of the primary particle is usually estimated from the so-called visible energy, i.e. the energy of the electromagnetic component of the considered interaction assuming that an average of one third of the available energy is allocated to this channel with a total inelasticity near 0.5. However this assumption which results from standard physics at low energy is in actual fact extrapolated to very high energy. Some theoretical work have recently proposed new unconventional processes that contradict this hypothesis. For instance the Concorde events with γ-inelasticity factor of 0.53 would correspond to the primary energy of 3 10 6 GeV requiring just to increase the expected exposure time by 50%.

Among the new ideas to explain the coplanar emission, the diquark breaking mechanism combined with relativistic strings fragmentation leads to a reduction by about a factor 2 of the primary energy when converted from the visible energy, which results in an enhancement of the frequency of occurrence of coplanar emission; it may also explain the reduction of the penetrating power of the cosmic ray cascades near the energies of the cosmic ray knee [START_REF] Capdevielle | Proc. 28th Int. Cosmic Ray Conf[END_REF]. It may also be connected with the decrease of high energy secondaries in the fragmentation region suggested by hybrid EAS-γ families experiment. The collimated coplanar tridents could be a simple signature to recognize or eliminate with collider experiments and more generally , it could be useful to present the remarkable cosmic ray events in Center of Mass for systematic comparisons. An experiment at low luminosity in the LHC, measuring the secondaries in the very forward region [START_REF] Haguenauer | Proc. Int. Conf. on Cosmic Rays and Dark Matter[END_REF], could probably clarify the question of coplanar emission, as well as the extension of the present emulsion chamber experiments, as it is now planned in Pamir. Another possibility could be to use the test flights of the Airbus A380 with an arrangement of emulsion bricks (like in the Opera neutrino experiment) on both decks of the airliner.

Fig. 1 .Fig. 2 .

 12 Fig.1. JF2af2 event as it appears to the naked eye on the X-ray film.

Fig. 3 .

 3 Fig. 3. Examples of 2 simulated events similar to JF2af2 with 23 (a) and 18 (b) aligned γ-rays (γ+e ± ) above 10 TeV.

Fig. 4 .

 4 Fig. 4. Calculated fractions (%) of the aligned events with λ 3 ≥ 0.8 (a) and λ 4 ≥ 0.8 (b) for RUNJOB experimental conditions obtained by different hadronic interaction models: DPMJET (open circle), HDPM (open triangle), QGSJET (full square), SYBILL (open square) and VENUS (open diamond). The dashed line represents in both cases the average value.

Fig. 5 .

 5 Fig. 5. Valence quarks and diquarks, string tension.

Fig. 6 .

 6 Fig. 6. Maximal tension for a threshold excitation energy, stretching of a high tension string between valence quarks of the diquark, diquark breaking.

TABLE I CALCULATED

 I FRACTIONS (%) OF THE ALIGNED EVENTS WITH AT LEAST 4 γ-RAYS (γ+E ± ) ABOVE 10 TEV FOR WITH |r| ≥ 0.94 (FIRST ROW) AND λ 4 ≥ 0.8 (SECOND ROW) FOR DIFFERENT HIGH ENERGY HADRONIC

		INTERACTION MODELS.
		DPMJET	HDPM	QGSJET	SIBYLL	VENUS
	|r| ≥ 0.94	0.7	1.5	0.6	0.5	0.9
	λ 4 ≥ 0.8	7.4	8.0	7.4	7.4	7.1

at least 4 γ-rays (γ+e ± ), i.e. four e.m. cascades in a family (each above 10 TeV) for which |r| ≥ 0.94 (first row) and λ 4 ≥ 0.8 (second row) for different hadronic interaction models. We reproduce hereafter in table II the typical structure of JF2af2 with 3 dominant clusters, Jet A, Jet Ap and Jet B with respective characteristics, (E γ ) (visible energy), X, Y (coordinates of individual energy weighted centers), N γ , average radius R and average < ER > factor ; distances X, Y , R are in millimeters, E energy of individual γ's are in TeV.

TABLE II 3

 II MAIN CLUSTERS IN JF2AF2.

		(Eγ )	X	Y	Nγ	R	< ER >
	Jet A	331.	81.3	7.1	60	8.62	35.8
	Jet Ap	455.4	100.5 11.2	10	0.49	13.1
	Jet B	610.6	114.	22.	77	10.26	75.6

TABLE III TOTAL

 III ENERGY DEPOSITED FOR JF2AF2 AND STRANA (E.M.

		COMPONENT
		(Eγ )	Nγ	E th
	JF2AF2	1586.	211	0.2
	STRANA	1400.	76	2.

TABLE IV EFFECT

 IV OF THE ENERGY THRESHOLD OF THE XREC ON TRANSVERSE MOMENTUM, Nγ , Kγ AVERAGED ON 1000 PROTON COLLISIONS OF 10 PEV. Nγ , Kγ ARE RESPECTIVELY 81 AND 0.26 FOR E th = 0 TEV. z = 2 IS THE KNO FACTOR AND THE LAST LINE ILLUSTRATES THE CASE OF FLUCTUATIONS IN CASE OF HIGH MULTIPLICITY.

		ptγ	p tπ 0	Nγ	Kγ
	E th = 0.2	0.233 0.534	43	0.259
	E th = 2.0	0.249 0.541	29	0.257
	E th = 2.0 (z = 2)	0.294 0.654	66	0.324


Tight binding modeling of halid perovskites

Soline Boyer-Richard 1 , Reinhard Scholz 2 , Jean-Marc Jancu 1 , Claudine Katan 3 , Jacky Even The free-electron model is built upon the assumption that electrons propagate almost freely and that their wave functions can be approximated by plane waves. As an "opposite" starting point, one can use localized atomic orbitals as a basis set to perform band-structure calculations; this is the tight binding (TB) or linear combination of atomic orbitals (LCAO) approach. This method has been introduced for periodic crystals by Slater and Koster 1 and was extensively used in the past to rationalize the physical properties of a wide range of compounds, from transition metals to semiconductor crystals 2 .

In this paper we use this approach to investigate the properties of HOP, considering the reference cubic structures of the 3D ABX 3 family. Our tight-binding model is based on a 16 functions basis without SOC or a 32 functions basis with SOC. For cubic ABX 3 halide perovskites, we consider one "s" and three "p" orbitals for the B atom, and the same for each of the three X atoms in the simple cubic unit cell, as already presented for a qualitative analysis of the band structure 3 . No basis function is taken into account for the A organic molecule, which's position is not fixed because it has rotational degrees of freedom/disorder in the cubic phase. We only consider first neighbor interactions, only between B and X atoms. We have chosen the minimal sp 3 basis as developed by Slater and Koster and we have tried to reproduce the already known experimental data like energy band-gap and effective masses around the bandgap with our semi-empirical TB model. Nine parameters are considered without SOC: four different diagonal matrix elements related to the atomic energies of the chemical constituents B and X: E sB , E pB , E sX and E pX . Five transfer matrix elements refer to the overlap integral of atomic functions: V ss , V sBpX , V sXpB , V ppσ , V ppπ . SOC only appears for p orbitals with two SOC values Δ soB and Δ soX . MAPbI 3 dispersion relation has been calculated using the sp 3 tight binding method including SOC. Figure 1 presents the band diagram obtained, fitting existing experimental results recalled in ref. 2. The bandgap is obtained at R point with E G = 1.620 eV. Effective masses around the bandgap are the following: ! " * = 0.235 ! + and ! , * = 0.229 ! + . The dispersion relation shape is in very good agreement with the one presented by Brivio using DFT calculation 4 and with previous symmetry analyses 5,6 .

Figure 1 .

 1 Figure 1. MAPbI 3 band diagram obtained within the TB scheme with SOC. The energy of valence band maximum is set to zero. Bands are colored according to their main orbital character: red depicts I 5p, pink depicts Pb 6p, black depicts Pb 6s and blue depicts I 5s. Points denoted M, X and R are zone-boundary points close to (1/2,1/2,0), (1/2,0,0) and (1/2,1/2,1/2) respectively. The Γ point is the BZ center.

1 1

 1 Université Européenne de Bretagne, INSA, FOTON UMR 6082, 35708 Rennes, France 2 Institut für Angewandte Photophysik, TU Dresden, Deutschland 3 Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS -Université de Rennes 1, France


Nicola Ferro
email: ferro@dei.unipd.it

Fabio Moens

Josiane Silvestri
email: silvestr@yahoo-inc.com

Fabio Crestani
email: fabio.crestani@usi.ch

Marie-Francine Moens

Josiane Mothe
email: josiane.mothe@irit.fr

Fabrizio Silvestri

Jaana Kekäläinen
email: jaana.kekalainen@uta.fi

Paolo Rosso
email: prosso@dsic.upv.es

Paul Clough
email: p.d.clough@sheffield.ac.uk

Gabriella Pasi
email: pasi@disco.unimib.it

Christina Lioma
email: c.lioma@di.ku.dk

Stefano Mizzaro
email: mizzaro@uniud.it

Giorgio Maria Di

Claudia Hauff
email: c.hauff@tudelft.nl

Omar Alonso
email: omar.alonso@microsoft.com

Pavel Serdyukov

Gianmaria Silvello
email: silvello@dei.unipd.it

Report on ECIR 2016: 38th European Conference on Information Retrieval

keynotes, scientific and social programme, industry day, tutorials, workshops and student support.

Introduction

The European Conference on Information Retrieval (ECIR) is the main European forum for the presentation of new research results in the field of information retrieval and cognate areas. Initiated as a BCS Information Retrieval Specialist Group (IRSG) Colloquium for Information Retrieval in 1979, the event developed in a renowned international scientific meeting. To reflect its European scale, since 1998 the conference was held alternately in the UK and continental Europe. From 2012, this rule has been removed. The most recent ECIR editions were held in Glasgow, UK (2008); Toulouse, France (2009); Milton Keynes, UK (2010); Dublin, Ireland (2011); Barcelona, Spain (2012), Moscow, Russia (2013), Amsterdam, The Netherlands (2014); and, Vienna, Austria (2015).

The 2016 edition of ECIR1 [START_REF] Ferro | Advances in Information Retrieval[END_REF] was hosted by the Information Management Systems

Venues

ECIR 2016 has been organized in three separate venues, shown in Figure 1: workshops and tutorials have been hosted at the Department of Geosciences; the first day of the conference has been hosted at the University of Padua Botanical Garden, which is the world's oldest academic botanical garden that is still in its original location; the other two days of the conference and the industry day have been hosted at the Bo Palace, the main historical building of the University of Padua.

Social Programme

ECIR 2016 featured a very rich social program. The Welcome Reception on Sunday evening offered the merry environment of a little country festival, with typical local dishes, such as "porchetta" (Italian spit-roasted pork), hand made pizza, and draft beer. The poster session and the reception on Monday evening were hold at Caffè Pedrocchi, one of the most important historical cafes in Italy, inaugurated in 1831, keeping a mixture of neoclassic and neo-Gothic styles. Since its opening, the Caffè Pedrocchi welcomes eminent guests such as the writer Stendhal, who named its desserts as some of the best he ever tried, as well as students who always spent hours in the reading room discussing all sorts of matters, sometimes producing innovative ideas, sometimes the revolution, as they did in 1848 during Asburgic domination. Here, in the Rossini room, dedicated to the composer Rossini, after the Poster session, ECIR 2016 participants were greeted with the typical north-eastern Italian wine "Prosecco" and a dinner with Italian cheeses, cured meats, shellfish, and other Italian culinary specialities. Finally, the social dinner was hosted at the "San Gaetano" Cultural Centre, built in 2008 renovating the old city courthouse. The Cultural Centre is a modern five-storeyed structure, which preserves ancient signs of old city: the excavation made in order to create the basement places revealed late medieval pavements and a considerable amount of archaeological findings stratified from the Renaissance back to the Roman age, partially visible in the foyer. The social dinner was prepared by one of the best caterers in town and gave participants the opportunity to taste fine Italian ingredients, dishes, wines and dozens of different types of "grappa".

Keynotes

ECIR 2016 included three keynotes: two keynotes were the Karen Spärck Jones (KSJ) Award ones -Jordan Boyd-Graber (University of Colorado, USA) and Emine Yilmaz (University College London, UK) -while the third keynote, Domonkos Tikk (Gravity R&D, Hungary), was part of the industry day.

Machine Learning Shouldn't be a Black Box

Jordan Boyd-Graber gave the first KSJ keynote, entitled "Machine Learning Shouldn't be a Black Box". Machine learning is ubiquitous: detecting spam e-mails, flagging fraudulent purchases, and providing the next movie in a Netflix binge. But few users at the mercy of machine learning outputs know what's happening behind the curtain. Jordan Boyd-Graber tried to demystify the black box for non-experts by creating algorithms that can inform, collaborate with, compete with, and understand users in real-world settings, providing several examples ranging from topic models, simultaneous machine interpretation in translation, and language-based games in diplomacy, politics, and trivia.

A Task-Based Perspective to Information Retrieval

Emine Yilmaz gave the second KSJ keynote, entitled "A Task-Based Perspective to Information Retrieval". Ideally, an information retrieval system should be able to understand the reason that caused the user to submit a query and it should help the user achieve the actual task by guiding her through the steps (or subtasks) that need to be completed. Devising task based information retrieval systems have several challenges that have to be tackled and Emine Yilmaz described them, in comparison and contrast with the traditional way in building information retrieval systems.

The main challenges to be faced are: (1) devising methodologies for accurately extracting and representing tasks, (2) building and designing new interfaces for task based information retrieval systems, (3) devising methodologies for evaluating the quality of task based information retrieval systems, and (4) task based personalization of information retrieval systems.

Lessons Learned at Building Recommendation Services in Industry Scale

Domonkos Tikk gave the industry keynote, entitled "Lessons learnt at building recommendation services in industry scale". He has shown how Gravity's recommendation technology, top contender in Netflix, has evolved from the big pile of task specific program codes to scalable services that serve billions of recommendation requests monthly. Having academic origin with strong research focus, the recommendation quality has always been the primary differentiating factor at Gravity but, over times, they also learnt that machine learning competitions are different from scalable and robust services. Therefore, Domonkos Tikk discussed and presented some lessons learnt on this road to create a solution from a offspring of the Netflix competition.

Programme

ECIR 2016 received a total of 284 submissions in three categories: 201 full papers out of which 7 papers in the Reproducibility track, 66 short papers, and 17 demonstrations.

The geographical distribution of the submissions was as follows: 51% were from Europe, 21% from Asia, 19% from North and South America, 7% from North Africa and the Middle East, and 2% from Australasia.

All submissions were reviewed by at least three members of an international two-tier Program Committee. Of the full papers submitted to the conference, 42 were accepted for oral presentation (22% of the submitted ones) and 8 as posters (4% of the submitted ones). Of the short papers submitted to the conference, 20 were accepted for poster presentation (30% of the submitted ones). In addition, 6 demonstrations (35% of the submitted ones) were accepted. The accepted contributions represent the state of the art in information retrieval, cover a diverse range of topics, propose novel applications, and indicate promising directions for future research. Figure 3 provides a break-down of the submitted and accepted papers and demos by country and highlights the global reach of ECIR as a premier international conference.

ECIR 2016 continued the Reproducibility track introduced in ECIR 2015 which specifically invited the submission of papers reproducing a single paper or a group of papers from a third party, where the authors were not directly involved in the original paper. Authors were requested to emphasize the motivation for selecting the papers to be reproduced, the process of how results were attempted to be reproduced (successfully or not), the communication that was necessary to gather all information, the potential difficulties encountered, and the result of the process. Of the 7 papers submitted to this track, 4 were accepted (57% of the submitted ones).

The Programme Committee consisted of 183 members, 21 of which were Meta-Reviewers and 162 of which were Reviewers. 59 Additional Reviewers entered some reviews on behalf of a Reviewer. The full paper decisions were made based on a two-tier review process involving both Reviewers and Meta-Reviewers, while the decisions on the short papers and demos were made based only on reviews. A panel on "Data-driven Information Retrieval" was organized at ECIR by Maristella Agosti (University of Padua, Italy) with Omar Alonso (Microsoft Bing, USA), Raffaele Perego, (ISTI CNR, Italy), and Maarten de Rijke (University of Amsterdam, The Netherlands) as panelists. The panel stems from the fact that IR has been always concerned with finding the "needle in a haystack" to retrieve the most relevant information from huge amounts of data, able to best address user information needs. Nevertheless, nowadays we are facing a radical paradigm shift, common also to many other research fields, and IR is becoming a more and more data-driven science due, for example, to recent developments in machine learning, crowd-sourcing, user interaction analysis, and so on. The goal of the panel is to discuss the emergent trends in this area, their advantages, their pitfalls, and their implications for the future of the field.

Finally, two moments have been dedicated to the "Women in IR" meeting and discussionsone during the coffe break in the afternoon of the first day of the conference and the other during the lunch of the second day of the conference. In order to give visibility to these events and solicit awareness in the community, the participants met around an ECIR 2016 totem dedicated to Elena Lucrezia Cornaro Piscopia, the first woman in the world to receive an academic degree at the University of Padua in 1678.

Awards

Test of Time Award

Considering the long history of ECIR, which is now at it 38th edition, ECIR 2016 introduced a new award, the Test of Time (ToT) Award, to recognize research that has had long-lasting influence, including impact on a subarea of information retrieval research, across subareas of information retrieval research, and outside of the information retrieval research community (e.g. non-information retrieval research or industry).

The ECIR ToT Award went to Cyril Goutte and Eric Gaussier for the paper entitled "A Probabilistic Interpretation of Precision, Recall and F-Score, with Implication for Evaluation" presented at ECIR 2005 [START_REF] Goutte | A Probabilistic Interpretation of Precision, Recall and F-Score, with Implication for Evaluation[END_REF].

There were also two honorable mentions: Giambattista Amati, Claudio Carpineto, Giovanni Romano for the paper entitled "Query Difficulty, Robustness, and Selective Application of Query Expansion" presented at ECIR 2004 [START_REF] Amati | Query Difficulty, Robustness, and Selective Application of Query Expansion[END_REF]; and, Iadh Ounis, Gianni Amati, Vassilis Plachouras, Ben He, Craig Macdonald, Douglas Johnson for the poster entitled "Terrier Information Retrieval Platform" presented at ECIR 2005 [START_REF] Ounis | Terrier Information Retrieval Platform[END_REF].

The ToT Award Committe was chaired by Norbert Fuhr (University of Duisburg-Essen, Germany) and its members were: Maristella Agosti (University of Padua, Italy), Pia Borlund (University of Copenhagen, Denmark), Djoerd Hiemstra (University of Twente, The Netherlands), Kalervo Järvelin (University of Tampere, Finland), Gabriella Kazai (Lumi.do, UK), Iadh Ounis (University of Glasgow, UK), and Jacques Savoy (University of Neuchatel, Switzerland). The ToT Award has been supported by Springer.

Best Paper Award

The Best Paper Award went to Mengdie Zhuang, Elaine G. Toms, and Gianluca Demartini for the paper entitled "The Relationship between User Perception and User Behaviour in Interactive Information Retrieval Evaluation" [START_REF] Zhuang | The Relationship between User Perception and User Behaviour in Interactive Information Retrieval Evaluation[END_REF].

There were also three honorable mentions: Liora Braunstain, Oren Kurland, David Carmel, Idan Szpektor, and Anna Shtok for the paper entitled "Supporting Human Answers for Advice-Seeking Questions in CQA Sites" [START_REF] Braunstain | Shtok. Supporting Human Answers for Advice-Seeking Questions in CQA Sites[END_REF]; Anjie Fang, Craig Macdonald, Iadh Ounis, and Philip Habel for the paper entitled "Topics in Tweets: A User Study of Topic Coherence Metrics for Twitter Data" [START_REF] Fang | Topics in Tweets: A User Study of Topic Coherence Metrics for Twitter Data[END_REF]; and, Johannes Jurgovsky, Michael Granitzer, and Christin Seifert for the paper entitled "Evaluating Memory Efficiency and Robustness of Word Embeddings" [START_REF] Jurgovsky | Evaluating Memory Efficiency and Robustness of Word Embeddings[END_REF].

The Best Paper Award Committee was chaired by Jaap Kamps (University of Amsterdam, The Netherlands) and its members were: Charles L.A. Clarke (University of Waterloo, Canada) and Jussi Karlgren (Gavagai, Sweden). The Best Paper Award has been supported by Yahoo!.

During the poster and demo session, participants have been asked to vote for the best poster and the best demo. The best poster was awarded to Martin Potthast, Sebastian Kpsel, Benno Stein and Matthias Hagen for the poster entitled "Clickbait Detection" [START_REF] Potthast | Clickbait Detection[END_REF]; the best demo was awarded to Pedro Saleiro, Jorge Teixeira, Carlos Soares and Eugénio Oliveira for the demo entitled "TimeMachine: Entity-centric Search and Visualization of News Archives" [START_REF] Saleiro | TimeMachine: Entity-centric Search and Visualization of News Archives[END_REF].

Tutorials

Collaborative Information Retrieval: Concepts, Models and Evaluation

Recent work have shown the potential of collaboration for solving complex or exploratory search tasks allowing to achieve synergistic effects with respect to individual search, which is the prevalent information retrieval (IR) setting this last decade. An important challenge is related to the design of Collaborative Information Retrieval (CIR) models and their effectiveness evaluation since individual IR frameworks and measures do not totally fit with the collaboration paradigms.

The CIR tutorial presented first a general overview of collaborative search introducing the main underlying notions. Then, it focused on related work dealing with collaborative ranking models and their effectiveness evaluation.

Group Recommender Systems: State of the Art, Emerging Aspects and Techniques, and Research Challenges

A recommender system aims at suggesting to users items that might interest them and that they have not considered yet. A class of systems, known as group recommendation, provides suggestions in contexts in which more than one person is involved in the recommendation process. The tutorial provided the audience with an overview on group recommendation, illustrating the recommender system principles. It then formally introduced the problem of producing recommendations to groups, and presented a survey based on the tasks performed by these systems. It also analyzed challenging topics like their evaluation and present emerging aspects and techniques in this area.

Living Labs for Online Evaluation: From Theory to Practice (LiLa2016)

Experimental evaluation has always been central to Information Retrieval research. The field is increasingly moving towards online evaluation, which involves experimenting with real, unsuspecting users in their natural task environments, a so-called living lab. Specifically, with the recent introduction of the Living Labs for IR Evaluation initiative at CLEF and the OpenSearch track at TREC, researchers can now have direct access to such labs. With these benchmarking platforms in place, online evaluation will be an exciting area to work on in the future. This half-day tutorial aimed to provide a comprehensive overview of the underlying theory and complement it with practical guidance.

Real-Time Bidding based Display Advertising: Mechanisms and Algorithms (RTBMA 2016)

Real-Time Bidding (RTB), which allows selling and buying in real-time one ad impression at a time. The ability of making impression level bid decision and targeting to an individual user in real-time has fundamentally changed the landscape of the digital media. The further demand for automation, integration and optimisation in RTB brings new research opportunities in the IR fields, including information matching with economic constraints, CTR prediction, user behaviour targeting and profiling, personalised advertising, and attribution and evaluation methodologies.

The tutorial teamed up presenters from both the industry and academia and it brought the insightful knowledge from the real-world systems, by providing an overview of the fundamental mechanism and algorithms with the focus on the IR context.

Workshops

3rd International Workshop on Bibliometric-enhanced Information Retrieval (BIR2016)

The BIR workshop brought together experts in Bibliometrics and Information Retrieval. First, both communities only partly overlap, albeit sharing various interests. Second, it is profitable for both sides to tackle some of the emerging problems that scholars face today when they have to identify relevant and high quality literature in the fast growing number of electronic publications available worldwide. Bibliometric techniques are not yet used widely to enhance retrieval processes in digital libraries, although they offer value-added effects for users. Information professionals working in libraries and archives, however, are increasingly confronted with applying bibliometric techniques in their services. The third BIR workshop aimed to foster a common ground for the incorporation of bibliometric-enhanced services into scholarly search engine interfaces. In particular it addressed specific communities and studies on large, cross-domain collections like Mendeley and ResearchGate.

1st International Workshop on Modeling, Learning and Mining for Cross/Multilinguality (MultiLingMine 2016)

The increasing availability of text information coded in many different languages poses new challenges to modern information retrieval and mining systems in order to discover and exchange knowledge at a larger world-wide scale. The 1st International Workshop on Modeling, Learning and Mining for Cross/Multilinguality (dubbed MultiLingMine 2016) provided a venue to discuss research advances in cross-/multilingual related topics, focusing on new multidisciplinary research questions that have not been deeply investigated so far (e.g., in CLEF and related events relevant to CLIR). This included theoretical and experimental on-going works about novel representation models, learning algorithms, and knowledge-based methodologies for emerging trends and applications, such as, e.g., cross-view cross-/multilingual information retrieval and document mining, (knowledge-based) translation-independent cross-/multilingual corpora, applications in social network contexts, and more.

ProActive Information Retrieval: Anticipating Users' Information Needs (ProAct IR)

Given the increasing popularity of smartphones and Internet enabled wearable devices, how can the information retrieval systems use the additional data, and better interact with the user so as to better understand, and even anticipate her precise information needs? Building such zero query or minimum user effort systems require research efforts from multiple disciplines covering algorithmic aspects of retrieval models, user modeling and profiling, evaluation, context modeling, novel user interfaces design, etc. The ProAct IR workshop gathered together the researchers from academia and industry practitioners with these diverse backgrounds to share their experiences and opinions on challenges and possibilities of developing such proactive information retrieval systems.

1st International Workshop on Recent Trends in News Information Retrieval (NewsIR'16)

The news industry has gone through seismic shifts in the past decade with digital content and social media completely redefining how people consume news. Readers check for accurate fresh news from multiple sources throughout the day using dedicated apps or social media on their smartphones and tablets. At the same time, news publishers rely more and more on social networks and citizen journalism as a frontline to breaking news. In this new era of fast-growing instant news delivery and consumption, publishers and aggregators have to overcome a great number of challenges. These include the verification or assessment of a source's reliability; the integration of news with other sources of information; real-time processing of both news content and social streams in multiple languages, in different formats and in high volumes; deduplication; entity detection and disambiguation; automatic summarization; and news recommendation. The NewsIR stimulated discussion around new and powerful uses of IR applied to news sources and the intersection of multiple IR tasks to solve real user problems. To promote research efforts in this area, NewsIR released a new dataset consisting of one million news articles to the research community and introduced a data challenge track as part of the workshop.

Industry Day

The goal of the Industry Day track was to bring an exciting programme that contained a mix of invited talks by industry leaders with presentations of novel and innovative ideas from the search industry. The final program consisted of invited talks and accepted proposal talks.

The invited invited talks were:

• Domonkos Tikk (Gravity), see Section 3.3 above;

• Etienne Sanson (Criteo), who presented an overview of the challenges R&D had to face over the last 10 years at Criteo;

• Debora Donato (StumbleUpon), who presented how to improve user engagement through genre diversification;

• Nicola Montecchio (Spotify), who presented music search, personalization and discovery at Spotify.

Four proposal talks were accepted but just three have been actually presented:

• "Worst Practices for Designing Production Information Access Systems" by Fernando Diaz (Microsoft Research NYC) who presented on the gap between studying and implementing information access systems. Fernando's presentation covered open problems in production systems that academia is better suited for addressing than industry;

• "Get on with it! Recommender system industry challenges move towards real-world, online evaluation" by Roberto Turrin (Moviri/ContentWise), Martha Larson (Delft University of Technology), and Daniel Kohlsdorf (XING);

• "Multilingual query categorization" by Michal Laclavik (Magnetic), Marek Ciglan (Magnetic), Sam Steingold (Magnetic), and Alex Dorman (Magnetic).

Student Support

ECIR has traditionally a strong focus on student participation. This year student support included a mentoring program and student grants.

The student mentorship was aimed to assist young authors in preparing papers for submission and improve quality of submissions. We invited about 30 researchers to participate as mentors and 21 students applied for the programme. Students submitted six full papers and four short papers to the conference. Both mentors and mentees regarded the program as satisfactory. One student paper was accepted to ECIR 2016 conference.

Student grant program included travel and accommodation grants supported by BCS and ELIAS. We received 34 grant applications in total and 27 students were granted with travel and accommodation support. Criteria for selection were: relevance of the research topic to ECIR; PhD year and track record (preference was given to students at an earlier stage of their career and with evidence of an exceptionally strong track record); presentation of a full/short/demo paper; strong letter of support. Preference was given to full-time students studying/working in developing countries and presenting their work and to those who had no alternative travel support.

The Student Grant Committee was chaired by John Tait (JohnTait.net, UK) and its members were: Jaime Arguello (UNC Chapel Hill, USA), Pavel Braslavski (Ural Federal University / Kontur Labs, Russian Federation), Jaap Kamps (University of Amsterdam, The Netherlands), Diane Kelly (University of North Carolina, USA), Mihai Lupu (Vienna University of Technology, Austria), and Nils Pharo (Oslo and Akershus University College of Applied Sciences, Norway).

Communication and Media

The ECIR 2016 Web site has been launched at ECIR 2015 and provided all the necessary information about the conference, its program and its logistics.

Figure 4 summarizes about one year of activity of the ECIR 2016 Web site, from its launch at ECIR 2015 in April 2015 to April 2016. The site has been visited by about 17,000 users, amounting to around 33,000 session and 80,000 page views. The geographic distribution of the accesses shows how ECIR has attracted interest from most of the countries in the world, confirming its global reach.

ECIR 2016 had a presence on Twitter with the account @ecir2016 and the hashtag #ecir2016, followed by 468 followers.

The main plenary sessions (opening, keynote talks, panel, and closing) were live streamed and are now available online at:

• http://ecir2016.dei.unipd.it/video/opening

• http://ecir2016.dei.unipd.it/video/ksj-keynote-jordan-boyd-graber

• http://ecir2016.dei.unipd.it/video/ksj-keynote-emine-yilmaz

• http://ecir2016.dei.unipd.it/video/industry-keynote-domonkos-tikk

• http://ecir2016.dei.unipd.it/video/panel-data-driven-ir

• http://ecir2016.dei.unipd.it/video/closing Overall, in less than one month, since late March 2016 to mid April 2016, they have been watched around 2,000 times.

The photo stream of the conference is available at the following address: http://ecir2016. dei.unipd.it/photo-stream.

Final Remarks

At the end of the conference, and in the subsequent days, we conducted a survey among the participants to ECIR 2016 to gather their feedback and opinion on different aspects of the conference and its organization. A total of 115 participants answered the survey and the results are summarized in Figure 5. The overall impression on ECIR 2016 was positive or more than positive for 96% of the participants and 90% or more of the participants were satisfied or very satisfied with all the different aspects of the conference, detailed in Figure 5.

ECIR 2016 introduced, for the first time, the Test of Time Award in order assess the maturity and impact of the scientific production of its community. Moreover, ECIR 2016 continued an innovation introduced at ECIR 2015, namely the Reproducibility Track, in order to stress the importance of the methodologies we adopt to conduct our experiment as well as to share and analyze their results.

ECIR 20175 will be held in Aberdeen, UK, from 8th to 13th April 2017, hosted by Robert Gordon University (RGU) and AECC (Aberdeen Exhibition and Conference Centre).

  2 (IMS) research group at the Department of Information Engineering 3 of the University of Padua 4 , Italy, from 20th to 23rd of March 2016. A total of 220 people attended ECIR 2016. 2 Organizing Committee, Venues and Social Program 2.1 Organizing Committee ECIR 2016 featured a large and multidisciplinary organizing committee, almost perfectly gender balanced: General Chair, Nicola Ferro (University of Padua, Italy); Program Chairs, Fabio Crestani Università della Svizzera Italiana (USI), Switzerland) and Marie-Francine Moens (KU Leuven, Belgium); Short Paper Chairs, Josiane Mothe (ESPE, IRIT, Université de Toulouse, France) and Fabrizio Silvestri (Yahoo! Labs, London); Student Mentorship Chairs, Jaana Kekäläinen (University of Tampere, Finland) and Paolo Rosso (Universitat Politècnica de València, Spain); Workshop Chairs, Paul Clough (University of Sheffield, UK) and Gabriella Pasi (University of Milano Bicocca, Italy); Tutorial Chairs, Christina Lioma (University of Copenhagen, Denmark) and Stefano Mizzaro (University of Udine, Italy); Demo Chairs Giorgio Maria Di Nunzio (University of Padua, Italy) and Claudia Hauff (TU Delft, The Netherlands); Industry Day Chairs, Omar Alonso (Microsoft Bing, USA) and Pavel Serdyukov (Yandex, Russia); Local Organization Chair, Gianmaria Silvello (University of Padua, Italy).

  (a) Botanical garden: plenary lecture room. (b) Botanical garden: parallel lecture room. (c) Bo Palace: plenary lecture room. (d) Bo Palace: parallel lecture room.

Figure 1 :

 1 Figure 1: ECIR 2016 main venues.

Figure 2 :

 2 Figure 2: ECIR 2016 social events.

  (a) Full papers. (b) Short papers. (c) Demos.

Figure 3 :

 3 Figure 3: Breakdown of submitted and accepted papers by country.

  (a) Web site access statistics. (b) Geographic distribution of Web site accesses.

Figure 4 :

 4 Figure 4: Participants survey results.

Figure 5 :

 5 Figure 5: Participants survey results.

http://ecir2016.dei.unipd.it/

http://ims.dei.unipd.it/

http://www.dei.unipd.it/en/

http://www.unipd.it/en/

http://ecir2017.org/

Acknowledgements

We wish to express our gratitude to all the committee members, mentors, authors, and participants who determined the real success of ECIR 2016.

We wish to greatly thanks all the people in the local organizing team who contributed to the organization and running of ECIR 2016. Research staff: Maristella Agosti, Linda Cappellato, Sergio Canazza, Emanuele Di Buccio, Marco Ferrante, Maria Maistro, and Ivano Masiero; administrative staff: Silvia Bedin, Antonio Camporese, Alessandro Cardinale, Debora Leoncini, and Sabrina Michelotto; technical services staff: Alessandra Angarano, Federico Beccaro, Carlo Calore, Marco Filippi, Vittorio Gallo, Andrea Graziani, Lorenzo Franceschin, Giorgio Paolucci, Agostino Pinto, Elisabetta Piva, Dario Da Re, Lorenzo Sartoratti, Maria Daniela Toffan, and Maurizio Vedaldi; student volunteers: Eugenio Angriman, Giulio Busato, Andrea Carraro, Jessica Mangano, Stefano Marchesin, Jennypher Mottola, Agata Rotondi, Daniel Zilio, Virginia Zorzi. ECIR 2016 has been held under the patronage of: Regione del Veneto (Veneto Region), Comune di Padova (Municipality of Padua), University of Padua, Department of Informantion Engineering, and Department of Mathematics. Finally, ECIR 2016 would have not been possible without the generous financial support from our sponsors: Google (gold level); Elsevier, Spotify, and Yahoo! Labs (palladium level); Springer (silver level); and, Yandex (bronze level). The conference was supported by the ELIAS Research Network Program of the European Science Foundation, British Computer Society, University of Padua, Department of Information Engineering, and Department of Mathematics.