Hadronic resonance production in pp and Pb Pb collisions at LHC with the ALICE experiment

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1 Journal of Physics: Conference Series OPEN ACCESS Haronic resonance prouction in pp an Pb Pb collisions at LHC with the ALICE experiment o cite this article: A Baalá et al 3 J. Phys.: Conf. Ser Relate content - Haronic resonances at ALICE A G Knospe an the Alice Collaboration - Haronic resonance prouction in Pb Pb collisions at the ALICE experiment A G Knospe an the Alice Collaboration - Strangeness prouction in ALICE Domenico Elia an the ALICE Collaboration View the article online for upates an enhancements. his content was ownloae from IP aress on 6//8 at 3:6

2 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 Haronic resonance prouction in pp an Pb Pb collisions at LHC with the ALICE experiment A. Baalá an H. Oeschler for the ALICE collaboration INFN-Sezione i Catania, V. S. Sofia 64, Catania, I955, Italy Institut für Kernphysik, echnische Universität Darmstat, Darmstat, Germany Angela.Baala@ct.infn.it Abstract. Resonances, with their lifetimes comparable to that of the fireball, can be use to estimate the time span an haronic interaction cross section in the phase between chemical an kinetic freeze-out. In pp collisions, measurements of resonances provie an important baseline for heavy-ion ata an allow for the tuning of QCD-inspire particle prouction moels. he ALICE collaboration measure K (89) an φ() prouction, both in pp collisions at s=7 ev an in Pb Pb collisions at s NN=.76 ev. he inelastic yiel of Σ(385) ± in pp collision at s=7 ev was also measure. ransverse momentum spectra, ratios to stable particles an a comparison between central an peripheral prouction are shown.. Introuction he stuy of resonance prouction plays an important role both in elementary an in heavy ion collisions. In pp an e + e collisions, it contributes to the unerstaning of haron prouction [, ] as the ecay proucts of resonances represent a large fraction of the final state particles. In aition, it provies a reference for tuning event generators inspire by Quantum Chromoynamics (QCD) such as PHOJE [3] an PYHIA [4]. Haronic resonances are a sensitive probe of the ynamical evolution of the fireball. Due to their short lifetime (a few fm/c) a significant fraction ecays uring the evolution from chemical to kinetic freeze-out an their haronic aughters interact with the meium uring the fireball expansion [5, 6, 7, 8]. In particular, proucts of their haronic ecay may rescatter reucing the measure resonance signal. Resonances may also be regenerate through collisions of harons. he competition between resonance-generating processes an rescattering, an therefore the ratio of resonance yiels to non-resonance yiel is governe by the lifetime an the temperature of the haronic meium. hermal moels [7, 9,, ] preict particle ratios as function of the chemical freezeout temperature ( ch ) an the time between chemical an thermal freeze-out. Particularly interesting is the comparison of φ an K prouction, consiering the ifferent lifetimes (about a factor ) of the two resonances. Due to the large lifetime (44 fm/c) the φ is expecte to ecay outsie the hot an ense interacting meium.. Experimental setup an ata analysis he results reporte in this paper refer to analyses carrie out using a sample of minimum-bias pp ata at s=7 ev (6 to 5 millions events, for the ifferent resonances analyze) an Content from this work may be use uner the terms of the Creative Commons Attribution 3. licence. Any further istribution of this work must maintain attribution to the author(s) an the title of the work, journal citation an DOI. Publishe uner licence by Lt

3 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 of minimum-bias Pb Pb ata at s NN =.76 ev (about millions events), collecte uring. he events are selecte with a primary vertex within cm of the etector center... Experimental setup he ALICE etector [, 3] provies extensive particle tracking an ientification in the central pseuorapiity range ( η.9) as well as muon tracking an ientification at forwar angles (.5 > η > 4). For the analysis escribe in this paper, only the central barrel has been use. he central tracking an particle ientification etectors inclue, from the innermost outwars, the Inner racking System (IS), the ime Projection Chamber (PC) an the ime of Flight array (OF). he central etectors are embee in a.5 solenoial fiel. he moerate fiel, together with a low material buget permits the reconstruction of low p tracks. Furthermore, two forwar scintillator hooscopes (VZERO) place along the beam irection at -.9 m an 3.3 m on either sie of the interaction point, which cover the pseuorapiity regions 3.7 < η <.7 an.8 < η < 5., were use for minimum-bias triggering an for rejecting beam-gas interactions. he Inner racking System (IS) is a silicon etector that surrouns the interaction point, with six layers between raii 3.9 cm to 43 cm from the beam axis. he two innermost layers, base on silicon pixels (SPD), are also use as an on-line trigger an to reconstruct the collision vertex with a resolution better than µm. he PC [4] provies track reconstruction with up to 59 three-imensional space points per track in a cylinrical active volume of about 9 m 3. he stanar tracking use in this analysis combines the information from the IS an PC. he momentum resolution of the PC is in the range -7% for pions with <p < GeV/c. Furthermore it provies very goo resolution in the istance of closest approach to the vertex (impact parameter resolution in the transverse irection is < µm for p > GeV/c) an hence an excellent separation of primary an seconary particles. he PC ientifies particles via the specific energy loss E/x etermine with a truncatemean proceure. It achieves a resolution ranging from 5-6.5% (tracks with 59 clusters - mean over all the reconstructe tracks) in pp an Pb Pb collisions. Ientification is achieve by calculating the ifference between the measure energy loss an the one expecte for ifferent mass hypotheses. A selection on this ifference, normalize to the resolution σ PC, is optimize for each analysis an epens in general on the signal to backgroun ratio an on the transverse momentum. he PC E/x measurements allow pions to be separate from kaons for momenta up to p.7 GeV/c, while the proton/antiproton ban starts to overlap with the pion/kaon ban at p GeV/c. he electron/positron E/x crosses the other bans at various momenta. In orer to ensure high efficiency an goo E/x resolution an to minimize the contamination from seconaries an fakes, tracks were require to have at least 7 reconstructe clusters in the PC. o improve the resolution (<% at p GeV/c), tracks were accepte only in the range η <.8 (i.e. well within the PC acceptance) an with p.5 GeV/c. he ime-of-flight Detector (OF) is an array of multi-gap resistive-plate chambers place at a raius of 37 to 399 cm. Particles are ientifie by the ifference between the measure time-of-flight an the one expecte from a given particle (π, K, p). he selection is expresse in units of the estimate resolution σ OF for each track, which has a mean value of 6 ps an 85 ps in pp an Pb Pb collisions, respectively. he OF allows pions an kaons to be unambiguously ientifie up to p.5-. GeV/c. he two mesons can be istinguishe from (anti)protons up to p.5 GeV/c. For the analyses escribe in this paper the start time of the collision (event time zero) is measure by the etector, an array of Cherenkov counters locate at +35 cm an -7 cm along the beam line. For events in which the signal is not present, it is estimate using the particle arrival times at the OF or the average collision time observe in the fill.

4 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 Counts/( MeV/c ) 4 3 /8/ Data Quaratic Backgroun Fit BreitWigner Peak Fit mass=.97±.8 GeV/c with=5.4±. MeV/c centrality %.5 GeV/c < p < GeV/c statistical uncertainties ALI PERF φ K K in PbPb s NN =.76 ev Invariant Mass (GeV/c ) ALI-PERF-96 Figure. he K + K invariant mass istribution in Pb Pb collisions at s NN =.76 ev. he fitting function is the sum of a Breit-Wigner function an a polynomial. Figure. he Kπ invariant mass istribution in Pb Pb collisions at s NN =.76 ev. he fitting function is the sum of a Breit- Wigner function an a polynomial. ALI-PERF-6695 Figure 3. he Λπ + invariant mass istribution in pp collisions at s=7 ev after backgroun subtraction. he soli line is the result of the combine fit: a Breit-Wigner function plus a polynomial. he ashe line escribes the resiual backgroun... Raw yiel extraction an p spectrum Resonances are ientifie by their main haronic ecay (K π ± +K, φ K + +K, Σ ± Λπ ± ). Due to their very short lifetimes, ecay proucts cannot be istinguishe from particles coming from the primary vertex. heir yiel is obtaine by computing the invariant mass spectrum of all primary caniates (tracks or hyperons) an then subtracting a combinatorial backgroun. his was performe by the event-mixing or the like-sign technique. he signal, after subtracting the combinatorial backgroun, was then fitte with a Breit-Wigner plus a polynomial for the resiual backgroun. A Voigtian function (convolution of Breit-Wigner function an Gaussian) was use in pp collisions for the extraction of the φ raw yiel. For the Σ, the resiual backgroun originating from correlate Λπ pairs coming from Λ(5) ecay, has been estimate by Monte-Carlo simulations an subtracte before fitting the invariant mass spectrum. Some examples of invariant mass spectra are presente in Figs., an 3. he mass an with of analyze resonances are close to the PDG values. In particular, in Pb-Pb collisions no mass shift nor broaening has been observe for the φ(), nor for K (89), similar to what is observe in pp collisions. In orer to extract the total prouction yiel, the raw counts were correcte for the ecay branching ratio an for the losses ue to geometrical acceptance an etector efficiency, which was etermine by Monte-Carlo simulation using PYHIA or HIJING generators, for pp an 3

5 (GeV/c) yp N inel N pp INEL, ALICE (K* PHOJE s= 7 ev +K* ) / PYHIA D6 (9) PYHIA ALASCSC (36) PYHIA Perugia (3) PYHIA Perugia (35) International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 (GeV/c) yp N inel N pp INEL, s = 7 ev ALICE φ() PHOJE PYHIA D6 (9) PYHIA ALASCSC (36) PYHIA Perugia (3) PYHIA Perugia (35) MODEL / DAA 3 ALI PUB Figure 4. Comparison of the K p spectrum in inelastic pp collisions at s=7 ev with PHOJE an PYHIA tunes [5]. 4 MODEL / DAA ALI PUB Figure 5. Comparison of the φ p spectrum in inelastic pp collisions at s=7 ev with PHOJE an PYHIA tunes [5]. (GeV/c) y <.8 N y p evts N MODEL / DAA ALI PREL (Σ* + Σ* ) / ALICE Preliminary uncert. = stat + syst pp INEL, s = 7 ev + ALICE (Σ* + Σ* ) / PHOJE PYHIA D6 (9) PYHIA ALASCSC (36) PYHIA Perugia (35) Figure 6. Comparison of the (Σ + + Σ )/ p spectrum in inelastic pp collisions at s=7 ev with PHOJE an PYHIA tunes. (GeV/c) p ALI PREL 747 π uncert. = stat + syst y ~ K Ks ρ * K p φ Λ ALICE pp 7 ev ALICE pp.9 ev SAR AuAu. ev ISR parameterization Ξ + Ξ Particle mass (GeV/c ) Figure 7. Mean transverse momentum p as a function of the particle mass. Au-Au ata at s NN =. ev []; ALICE ata for pp collisions at s=.9 ev [3, 4]; ALICE ata at s=7 ev [5]. *+ Σ Ω Pb Pb ata, respectively. Final yiels were obtaine normalizing to the number of inelastic collisions (pp collisions) or to the number of analyze events in a given centrality range (Pb Pb collisions). he trigger efficiency was also taken into account. he analyses of K (89) an φ() mesons in pp collisions at s=7 ev are escribe in etail in [5]. 4

6 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 ) (GeV/c) yp evt N / (N 3 ALI PREL 7598 uncertainties: stat. (bars), syst. (boxes) * * (K + K )/ % 4 % 46 % 68 % PbPb at s NN =.76 ev y < Figure 8. K p spectra in Pb Pb collisions at s NN =.76 ev at ifferent centralities. (GeV/c) y N/p + φ K K, y <.5, PbPb s =.76 ev NN uncertainties: stat. (bars), sys. (boxes) 3 ALI PREL Centrality 5% 4 5% % % 3% 34% 45% 56% 67% 78% Figure 9. φ() p spectra in Pb Pb collisions at s NN =.76 ev at ifferent centralities. (GeV/c) p * K (89) PbPb.76 ev (ALICE) pp 7 ev (ALICE) y <.5 uncertainties: stat. (bars), syst. (boxes) AuAu 6.4 GeV (SAR) AuAu GeV (SAR) CuCu 6.4 GeV (SAR) CuCu GeV (SAR) pp GeV (SAR) (GeV/c) p.4. φ Mesons N part ALI PREL 7645 Figure. Mean transverse momentum of K meson as a function of the mean number of participants for SAR [3] an ALICE ata ALI PREL 73 ALICE pp s=7 ev ALICE Preliminary PbPb SAR pp s= GeV SAR AuAu s NN = GeV uncertainties: stat. (bars), sys. (boxes) s NN =.76 ev N part Figure. Mean transverse momentum of φ() meson as a function of the mean number of participants for SAR [, 8] an ALICE ata [5]. 3. Results 3.. pp collisions at s=7 ev Figures 4, 5 an 6 show the K, φ() an (Σ + + Σ + )/ spectra with a comparison to a number of PYHIA [6, 7, 8] tunes an PHOJE [3]. he best agreement is foun for the recent PYHIA Perugia tune. In fact there is a goo agreement of the moel to the ata for the K. It reprouces only the high p part (p >3 GeV/c) of the φ spectrum. Very poor agreement is foun for the baryonic resonance. he spectra have been fitte by a sallis function [9] an the extracte n values (6.±.7±.8 an 6.7±.±.4 for K an φ, respectively) are similar to those quote by the SAR experiment at RHIC for the φ measure in pp collisions at GeV (n = 8.3 ±.) []. We enote by K the average of K an K. 5

7 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 In contrast, the slope parameters (54±±8 an 7±4± MeV for K an φ, respectively) are significantly higher than the values obtaine at RHIC, = ±4 ± MeV for φ []. For the φ the inelastic yiel increases proportionally to the charge particle multiplicity from.9 to 7 ev. he mean transverse momenta p for K, φ() an Σ(385) ± are presente in Fig. 7 as a function of the particle mass. hey follow the tren observe for stable particles inicating a common prouction mechanism. In this figure the p for ifferent system an various collision energies (ALICE pp ata at.9 an 7 ev, SAR Au-Au ata at s NN =. ev) are also shown. Particle Ratio.8.6 PbPb at =.76 ev s NN φ()/k 4 K*(89)/K / K K* PbPb.76 ev (ALICE) pp 7 ev (ALICE) AuAu GeV (SAR) pp GeV (SAR) y <.5 uncertainties: syst + stat uncertainties: stat (bar), syst (box) 3 4 N part ALI DER 3557 Figure. K /K, φ/k ratios as a function of mean number of participants in Pb Pb collisions at s NN =.76 ev.. ALI PREL 7658 s NN (GeV) Figure 3. K /K ratio as a function of the collision energy. Open symbols are for pp ata, close symbols for ion-ion ata [3, 5]. 3.. Pb Pb collisions at s NN =.76 ev he transverse momentum spectra for K an φ in several event centrality bins are shown in Figs. 8 an 9. he mean transverse momentum as a function of the mean number of participant N part at LHC an at RHIC energies for K an φ() is shown in Figs. an, respectively. We note that the p measure in pp collisions at s=7 ev is equal to the value measure in Pb Pb peripheral collisions at s NN =.76 ev. For the φ, the p at LHC energies is larger than the one at RHIC energies. his is consistent with a stronger raial flow at LHC than RHIC. In fact, a global blast-wave fit of π, K, p shows a % increase in β with respect to RHIC [9], for central collisions. In heavy ion collisions, the yiels for stable an long-live harons reflect the thermoynamic conitions (temperature, chemical potentials) at freeze-out, whereas the yiel for short-live resonances can be moifie by final-state interactions insie the hot an ense reaction zone [5, 3]. Particularly interesting is the comparison of φ() an K prouction, consiering the ifferent lifetimes (about a factor of ) of the two resonances. While the φ/k ratio is inepenent of the collision centrality, the K /K ratio ecreases with increasing centrality (Fig.). Figure 3 shows that the K /K ratio in pp collisions is the same at RHIC an at the LHC. In contrast, this ratio ecreases in heavy ion collisions an this effect seems to be larger at the LHC energies. he ifference in K* an φ prouction coul be relate to the interaction of K with haronic meium, which oes not affect the φ yiel ue to its long lifetime. Figure 4 shows the ecrease of the K /K ratio with respect to the system size represente by (N ch /η) /3 for the ifferent ata sets of ifferent collision systems an energies. he fact that they all fall on a common line coul inicate that the observe ecrease is relate to the raial extent of the fireball. he ecrease coul be cause by the pion 6

8 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 rescattering mechanism (σ(π, π)), which estroyes the pion-kaon correlation of the K ecay proucts. However, there is also the possibility that the ratio K*/K changes from chemical to kinetic freeze-out just by the variation of the temperature. /K K* PbPb.76 ev (ALICE) AuAu 6.4 GeV (SAR) AuAu GeV (SAR) CuCu 6.4 GeV (SAR) CuCu GeV (SAR) pp 7 ev (ALICE).3.. uncertainties: bars(stat.), boxes(syst.) ALI DER /3 (N ch /η) Figure 4. K /K ratio as a function of (N ch /η) /3. SAR ata are from [3]. 4. Conclusions he haronic resonance K (89), φ() an Σ(385) ± have been measure in pp collisions at s=7 ev an in Pb Pb collisions at s NN =.76 ev for ifferent event centrality bins by the ALICE experiment at the LHC. ransverse momentum spectra of K (89), φ() an Σ(385) ± measure in pp collisions have been compare to PHOJE an ifferent PYHIA tunes. Only the K (89) is reprouce by Perugia PYHIA tune. None of tunes gives a satisfactory escription of the φ() an Σ(385) ± ata. In proton-proton collisions the mean transverse momenta increase with the collision energy an they follow the tren of the stable particles. While the φ/k is rather flat versus centrality, the K /K ratio ecreases with centrality. It is shown that this ecrease is relate to the raial extension of the fireball. References [] Aguilar-Benitez M et al. (LEBC-EHS Collaboration) 99 Inclusive particle prouction in 4 GeV/c ppinteractions Z. Phys. C 5 45 [] Albrecht H et al. 994 Inclusive prouction of K (89), ρ (77), an ω(78) mesons in the Υ energy region Z. Phys. C 6 [3] Engel R 995 Photoprouction within the two component ual parton moel. Amplitues an cross-sections Z. Phys. C 66 3; Engel R an Ranft J 996 Haronic photon-photon interactions at high-energies Phys. Rev. D 54, 444. [4] Sjöstran, Mrenna S an Skans P 6 PYHIA 6.4 physics an manual J. High Energy Phys. 5 6 [5] Bleicher M an Aichelin J Strange resonance prouction: Probing chemical an thermal freezeout in relativistic heavy ion collisions Phys. Lett. B 53 8 [6] Bleicher M an Stöcker H 4 Dynamics an freeze-out of haron resonances at RHIC J. Phys. Nucl. Part. Phys. G. 3 S [7] Markert C et al. Strange haron resonances: freeze-out probes in heavy-ion collisions Proceeings of PASI an hep-ph/66 [8] Vogel S an Bleicher M 5 Resonance absorption an regeneration in relativistic heavy ion collisions Proceeings of Nucl. Phys. Winter Meeting 5 in Bormio an nucl-th/557v 7

9 International Workshop on Discovery Physics at the LHC (Kruger) Journal of Physics: Conference Series 455 (3) 3 oi:.88/ /455//3 [9] Anronic A et al. 9 hermal haron prouction in relativistic nuclear collisions: the haron mass spectrum, the horn, an the QCD phase transition Phys. Lett. B [] Anronic A et al. he statistical moel in Pb-Pb collisions at the LHC arxiv.774v [] orrieri G an Rafelski J Strange haron resonances as a signature of freeze-out ynamics Phys. Lett. B [] Aamot K et al. (ALICE Collaboration) 8 he ALICE experiment at the CERN LHC J. Instrum. 3 S8 [3] Aamot K et al. (ALICE Collaboration) Alignment of the ALICE Inner racking System with cosmicray tracks J. Instrum. 5 P33 [4] Alme J et al. he ALICE PC, a large 3-imensional tracking evice with fast reaout for ultra-high multiplicity events Nucl. Instrum. Meth. in Phys. Res. A 6 36 [5] Abelev B et al. (ALICE Collaboration) Prouction of K (89) an φ() in pp collisions at s=7 ev Eur. Phys. J. C 7 83 [6] Fiel R 8 Physics at the evatron Acta Phys. Pol. B 39 6; Fiel R 8 Stuying the unerlying events at CDF an the LHC Proceeings of the First International Workshop on Multiple Partonic Interactions at the LHC, (MPI8), Perugia(Italy) 7-3 October(8),pag. arxiv:3.4 [7] Buttar C et al. 4 Simulations of minimum bias events an the unerlying event, MC tuning an preictions for the LHC Acta Phys. Pol. B [8] Skans P Z uning Monte Carlo Generators: he Perugia unes Phys. Rev. D [9] sallis C 988 Possible generalization of Boltzmann-Gibbs statistics J. Stat. Phys [] Abelev B I et al. (SAR Collaboration) 9 Measurements of φ meson prouction in relativistic heavy-ion collisions in the BNL Relativistic Heavy Ion Collier (RHIC) Phys. Rev. C [] Aams J et al. (SAR Collaboration) 5 K(89) resonance prouction in Au+Au an p+p collisions at snn = GeV Phys. Rev. C [] Abelev B I et al. (SAR Collaboration) 7 Strange particle prouction in p+p collisions at s= -GeV Phys. Rev. C [3] Aamot K et al. (ALICE Collaboration) Strange particle prouction in proton-proton collisions at s=.9 ev with ALICE at the LHC Eur. Phys. J. C [4] Aamot K et al. (ALICE Collaboration) Prouction of pions, kaons an protons in pp collisions at s=9 GeV with ALICE at the LHC Eur. Phys. J. C [5] Abelev B et al. (SAR Collaboration) Multi-strange baryon prouction in pp collisions at s=7 ev with ALICE Phys. Lett. B 7 39 [6] Abelev B et al. (SAR Collaboration) 6 Strange baryon resonance prouction in s NN= GeV p+p an Au+Au collisions Phys. Rev. Lett [7] Abelev B et al. (SAR Collaboration) 9 Systematic measurements of ientifie particle spectra in pp, +Au, an Au+Au collisions at the SAR etector Phys. Rev. C [8] Aams J et al. (SAR Collaboration) 5 φ meson prouction in Au+Au an p+p collisions at snn= GeV Phys. Lett. B 6 8 [9] Abelev B et al. (ALICE Collaboration) Pion, kaon, an proton prouction in central Pb-Pb collisions at s NN=.76 ev Phys. Rev. Lett [3] Markert C 5 Resonance prouction in heavy ion collisions J. Phys. Nucl. Part. Phys. G 3 S897 [3] Aggarwal M M et al. (SAR Collaboration) K prouction in Cu+Cu an Au+Au collisions at snn=6.4 GeV an GeV Phys. Rev. C