Production of Strange Particles in Jets in Heavy-ion Collisions in ALICE

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1 WD'3 Proceedings of Contributed Paers, Part III, 5 57, 23. IBN MAFYZPRE Production of trange Particles in Jets in Heavy-ion Collisions in ALICE V. učera Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Reublic. Nuclear Physics Institute, he Academy of ciences of the Czech Reublic, Řež, Czech Reublic. Institut Pluridiscilinaire Hubert Curien, Ecole Doctorale de Physique et Chimie-Physique, Université de trasbourg, trasbourg, France. Abstract. he current status of an analysis of roduction of strange articles in s in heavy-ion collisions measured with the ALICE exeriment at the LHC is resented. he main goal of the analysis is to investigate roerties of hot and dense strongly interacting matter created in ultrarelativistic collisions of nuclei by studying hadronisation rocesses occuring in this medium. For this urose, strange V articles (baryon, meson ) are chosen to be studied. V candidates are reconstructed using selection criteria alied on the toological arameters of their decay. Jets are reconstructed from charged tracks using the anti-k t algorithm. he uncorrected fragmentation functions are extracted for the strange articles found in the selected s. Introduction Motivation he theory of strong interaction, Quantum Chromodynamics (QCD), redicts a hase transition of hadronic matter at high temeratures and high energy densities. Under these conditions, quarks and gluons become deconfined and form a state called quark gluon lasma (QGP). It is believed that hadronic matter existed in this state during an early stage of evolution of the Universe. According to the results of the latest exeriments, it seems that ultrarelativistic collisions of heavy ions (HIC) enable to create a state of strongly interacting matter resembling QGP and to robe different regions of the hase diagram of hadronic matter. tudying roerties of this matter at high temeratures and high energy densities is exected to imrove our understanding of QCD. Matter created in HIC at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) seems to have roerties close to those of erfect liquid, manifests resence of artonic degrees of freedom and shows indications of suression of article roduction at high transverse momenta ( ) which is observed for the constituents as quenching. Fig. shows the nuclear modification factor R AA which exresses how sectra of hadrons roduced in HIC differ from those in roton collisions. Jet as a robe of strongly interacting medium In a rocess involving hard scattering and strong interaction, there are artons roduced with high. uch arton created in hard scattering radiates softer artons, resulting in a cascade of artons. his set of artons is called a. Partons cannot be observed so at the exerimental level, a is detected as a collimated sray of hadrons with high. Production of s in roton collisions is a rocess well described by erturbative QCD and s can therefore be used as a robe to study collisions of nuclei. As a roagates through the matter created in HIC, the roerties are exected to be modified by strong interaction of constituents with the medium. his mechanism can modify the energy, its shae or article comosition. 5

2 UČERA: RANGE PARICLE IN JE 2.5 P P 7.3 GeV (PbPb) π WA98 (-7%) RHIC 2 GeV (AuAu) π PHENIX (-%) ± h AR (-5%) LHC 2.76 ev (PbPb) ± h CM (-5%) GLV: g /dy = 4 GLV: g /dy = 4 GLV: g /dy = 2 YaJEM-D elastic, small P esc elastic, large P esc YaJEM AW R AA ± h ALICE (-5%) 2 PQM: <q> = 3-8 GeV /fm CM reliminary.5 RHIC Figure. Measurements of modification of hadron roduction in HIC erformed with different exeriments []. he rocess where a hadron h is roduced from a arton q is described in QCD by the fragmentation functions D h q (z, j ), z = E h /E q, where E h is the hadron energy and j is the transverse momentum of the hadron relative to the momentum of the arton with energy E q. tudying fragmentation functions of articles in s allows to investigate the effect of the medium on hadronisation mechanisms in s, as function of quark flavour. ince the momentum of the initial arton cannot be measured, the arton energy is commonly relaced by the energy of the measured. Modification of fragmentation functions by the medium is exected to be ronounced mostly for softer constituents, i.e. for low z. In order to focus on this region, fragmentation in s in HIC is described using distribution of a variable ( ) ξ = ln /h. Hadron roduction ectra of hadrons measured at the LHC manifest a strong increase of baryon/meson ratio in HIC relative to collisions at intermediate. his behaviour has been observed at lower collision energies at RHIC and is also resent at the LHC energies [2]. Maximum of the ratio increases with increasing centrality (quantity directly related to the imact arameter of colliding nuclei). he henomenon is observed for hadrons containing the lightest quarks (u, d) and also for strange hadrons (containing the quark s). Fig. 2 shows the ratios of sectra of baryon and meson measured with ALICE. Hadron roduction by fragmentation cannot exlain this anomaly of baryon/meson ratio. A scenario, roosed to describe it, assumes that hadrons are roduced by coalescence and arton recombination in the region of low. In the region of intermediate these mechanisms are suosed to comete with fragmentation which is a rocess dominating at high. Particles suitable for investigating these rocesses are the V articles. hese are strange neutral articles decaying into two charged articles (daughters). Mother article is reconstructed using the toology of its V-shaed decay. his rocedure is using various arameters describing the satial configuration of the decay as it is dislayed in Fig. 3. Presented analysis is focused on the reconstruction of baryon () and meson using 52

3 UČERA: RANGE PARICLE IN JE Figure 2. Baryon/meson enhancement observed with ALICE for baryon and meson [3]. ointing angle decay length ositive-daughter track V momentum DCA between daughters negative-daughter track rimary vertex DCA of daughter to rimary vertex B Figure 3. oological arameters used in the reconstruction of V articles. the following decay channels: π + +π (b. r. 69 %), +π (b. r. 64 %), +π + (b. r. 64 %). Analysis Exeriment A Large Ion Collider Exeriment (ALICE) is one of the four main exeriments at the LHC at CERN and the one dedicated to study extreme states of strongly interacting matter. It rovides a unique article identification erformance given by using various secific detectors as the ime Projection Chamber (PC), the Inner racking ystem (I), the ime Of Flight detector (OF) etc. Another imortant feature of the aaratus is detection of events with high multilicity of tracks ( 4 ). he central tracking detectors, oerating in a moderate magnetic field of induction of.5, enable measurement of momenta of charged articles down to MeV/c. Jets Jets need to be reconstructed using a algorithm. he most oular class of algorithms, used in the current analyses, are sequential recombination algorithms. Examles of them are k t [4], anti-k t [5], Cambridge/Aachen. hey all use the same formalism and differ in 53

4 UČERA: RANGE PARICLE IN JE value of the arameter : ( ) ij d ij = min i, j R 2, 2 ij = (y i y j ) 2 + (φ i φ j ) 2 = k t Cambridge/Aachen anti-k t where i, y i, φ i are resectively transverse momentum, raidity and azimuth of a article i and R is the resolution arameter of the algorithm. he rocedure of clustering articles into s starts with high- articles for the anti-k t algorithm whereas the k t algorithm clusters low- articles first which makes the former suitable for reconstruction of signal s and the latter for background estimation. Presented results have been obtained using data from the Pb + Pb collisions at = 2.76 ev measured with ALICE in 2. Inut for the algorithm are charged tracks measured in the I and the PC. ignal s are reconstructed by the anti-k t algorithm with R =.4 and a threshold for minimum of charged tracks min = 5 MeV/c [6]. In a collision of nuclei there is an imortant contribution of background consisting of soft articles coming from underlying event. he average density of corresonding to this background is estimated using the k t s with the same setting as for the signal s, where 2 hardest s are excluded from each event [7]. he density ρ is estimated as ρ = median { } A where A is area of. Raw of a reconstructed signal is corrected by subtracting corresonding to the background in the area, using a 4-vector formalism: P bg = ρa, P corr = P P bg. he signal s are selected from the leading s in a range of seudoraidity η <.35 and are required to have the hardest charged track with > 5 GeV/c to reduce contribution of fake s. Fig. 4 shows the background-subtracted sectra of selected leading charged s in different centrality bins. hese sectra still have to undergo corrections for background fluctuations and detector effects, using an unfolding rocedure. trange articles election criteria used for the reconstructed V candidates are tuned to reduce contribution of fake candidates from the combinatorial background. he rocedure enables to obtain a very clean eak in the invariant-mass sectrum. Candidates within a defined range of invariant mass are selected as signal in further analysis. Fig. 5 shows the uncorrected inclusive sectra of strange articles for different centrality ranges. Next ste is to search for the V articles within the cone. his requirement has a significant imact on the statistics of candidates. he ratio of number of Vs found in s to number of inclusive Vs is : 3. he uncorrected sectra of strange articles in s in the most central collisions are shown in Fig. 6 for different ranges of momentum. tatistical uncertainties will be reduced when using data measured in 2, where a much higher statistics is available. ( ) Another ste in the analysis is to calculate the value of ξ = ln /h for each V candidate and obtain the distribution /dξ. he uncorrected fragmentation functions for different momenta are shown in Fig

5 UČERA: RANGE PARICLE IN JE Leading-charged- sectrum,ch,ch d N ev - % % 2 % 4-6 % 6-8 % -5-6 Pb + Pb, 2, = 2.76 ev ,ch Figure 4. ectrum of leading charged s in different centrality bins, corrected for average background density. d inclusive sectrum (mass eak) Pb + Pb, 2, = 2.76 ev - % % 2 % d inclusive sectrum (mass eak) Pb + Pb, 2, = 2.76 ev - % % 2 % N ev % 6-8 % N ev % 6-8 % Figure 5. Uncorrected inclusive sectra of strange articles in different centrality bins. Hard constituents are lotted in the low ξ region. hese articles should be roduced mainly by fragmentation. oft articles oulating the high ξ region might be roduced with articiation of other hadronisation mechanisms like coalescence and arton recombination. When the measured fragmentation functions are fully corrected, the results will be comared with theoretical redictions (see Fig. 8) which exect the modification of fragmentation functions by medium in HIC to be sensitive to the tye of article. Fig. 9 shows uncorrected baryon/meson ratio for inclusive candidates and for candidates found in s in range 2 GeV/c. he ratio enhancement and its centrality deendence are well visible for inclusive articles. Higher statistics will enable comarison of ratio of inclusive articles with that of articles in s. Conclusion An analysis of strange articles in s in heavy-ion collisions in ALICE has been resented together with its first results. Charged s were reconstructed and corrected for average contribution of background. trange articles and were reconstructed and found in s. 55

6 UČERA: RANGE PARICLE IN JE sectrum of in s, cent - % Pb + Pb, 2, = 2.76 ev : GeV/c sectrum of in s, cent - % Pb + Pb, 2, = 2.76 ev : GeV/c d N - : 2 GeV/c : 3 GeV/c : 4- GeV/c d N - : 2 GeV/c : 3 GeV/c : 4- GeV/c Figure 6. Uncorrected sectra of strange articles in s in the most central collisions. dξ N - ξ distribution of in s, cent - % : GeV/c : 2 GeV/c : 3 GeV/c : 4- GeV/c dξ N - ξ distribution of in s, cent - % : GeV/c : 2 GeV/c : 3 GeV/c : 4- GeV/c Pb + Pb, 2, = 2.76 ev Pb + Pb, 2, = 2.76 ev ξ = ln( / ) ξ = ln( / ) Figure 7. Uncorrected fragmentation functions of strange articles in the most central collisions. h /dξ (b) E =4.5GeV Θ c = π/2 π ± (vacuum) ± (vacuum) ( _ 2.5 ) (vacuum) 2.5 π ± (medium) ± (medium) ( _ 2.5 ) (medium) ξ = ln[e / h ] Figure 8. heoretical rediction for fragmentation functions of different tyes of articles in vaccum and in medium [3]. 56

7 UČERA: RANGE PARICLE IN JE /.6.4 Ratio of sectra (inclusive, mass eak) - % % /.6.4 Ratio of sectra (in s GeV/c, mass eak) - % %.2 2 % 4-6 %.2 2 % 4-6 % 6-8 % 6-8 % Pb + Pb, 2, = 2.76 ev h.2 Pb + Pb, 2, 2 = 2.76 ev h Figure 9. Uncorrected baryon/meson ratio of strange articles for inclusive candidates (left) and candidates in s (right), in different centrality bins. Uncorrected inclusive sectra, sectra of articles in s and fragmentation functions were obtained. Future analysis stes include otimisation of V reconstruction criteria and imrovement of signal extraction. Further corrections need to be alied to the V reconstruction rocedure and to the sectra. Another ste in the analysis is using information from electromagnetic calorimeter. his would enable erforming of reconstruction of full s including their neutral comonent. When the results are fully corrected, conclusions can be drawn from comarison of measured strange-article sectra with redictions of fragmentation models. References [] he CM Collaboration: Centrality deendence of the nuclear modification factor for charged article transverse momentum sectra in PbPb collisions at = 2.76 ev, CM-PA-HIN--5 (2), htt://cdsweb.cern.ch/record/ [2] B. B. Abelev et al. [ALICE Collaboration]: and roduction in Pb-Pb collisions at = 2.76 ev, CERN-PH-EP32 (23), arxiv: [nucl-ex]. [3] J. Belikov: Lambda/s ratios as a function of t in centrality bins in PbPb events at 2.76 ev, ALI-PREL-884 (2), htts://aliceinfo.cern.ch/figure/node/286. [4] M. Cacciari and G. P. alam: Diselling the N 3 myth for the k t -finder, Phys. Lett. B 64 (26) 57, arxiv:he-h/522. [5] M. Cacciari, G. P. alam and G. oyez: he anti-k t clustering algorithm, JHEP 84 (28) 63, arxiv:82.89 [he-h]. [6] M. Verweij [ALICE Collaboration]: Measurement of sectra with charged articles in Pb-Pb collisions at =2.76 ev with the ALICE detector, Nucl. Phys. A (23) 5c, arxiv: [nucl-ex]. [7] B. Abelev et al. [ALICE Collaboration]: Measurement of Event Background Fluctuations for Charged Particle Jet Reconstruction in Pb-Pb collisions at = 2.76 ev, JHEP 23 (22) 53, arxiv: [he-ex]. [8]. aeta; U. Wiedemann: Jet hadrochemistry as a characteristics of quenching, Eur. Phys. J. C 55 (28) 293, arxiv: [he-h]. 57