Baryon production from small to large collision systems at ALICE
BBaryon production from small to large collisionsystems at ALICE
Omar Vázquez Rueda ∗ on behalf of the ALICE Collaboration Lund UniversityE-mail: [email protected]
Studies of the production of light- and heavy-flavor baryons are of prominent importance tocharacterize the partonic phase created in ultrarelativistic heavy-ion collisions and to investigatehadronization mechanisms at the LHC. Studies performed in p–Pb and pp collisions haverevealed unexpected features, qualitatively similar to what is observed in larger collision systemsand, in the charm sector, not in line with the expectations from e + e − and e − p interactions. TheALICE experiment has exploited its excellent tracking and particle identification capabilitiesdown to low transverse momentum to perform an extensive study of protons, hyperons andcharmed baryons. In this paper, a discussion of the most recent results on light (protons andhyperons) and heavy-flavor ( Λ c ) baryon production is presented, together with a comparison tophenomenological models. European Physical Society Conference on High Energy Physics - EPS-HEP2019 -10-17 July, 2019Ghent, Belgium ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ h e p - e x ] J a n aryon production from small to large collision systems at ALICE Omar Vázquez Rueda
1. Introduction
Measurements of ultrarelativistic heavy-ion collisions at the top LHC energies have corroboratedthe formation of a strongly coupled partonic state of matter, the so-called Quark–Gluon Plasma(QGP). Some of the features of this system are strong collective radial and anisotropic flow andopacity to jets [1]. Collective radial flow in central collisions is observed as hardening of the trans-verse momentum ( p T ) spectra of heavy particles such as protons while parton energy loss comesout as the suppression of the production of high- p T particles [2]. Recent measurements in high-multiplicity pp and p–Pb collisions have revealed flow-like patterns even in these small collisionsystems [3, 4]. Furthermore, it has been observed that the yields of strange hadrons normalizedto the one of pions ( π + + π − ) increase significantly with the charged-particle multiplicity of theevent [5]. This increase scales with the strangeness content of baryons. In high-multiplicity events,strangeness production reaches values similar to those observed in heavy-ion collisions, where aQGP is formed.In this paper, a brief review of selected results of baryon production is reported. The results includeboth the multiplicity and p T -dependent baryon-to-meson ratios in pp, p–Pb and Pb–Pb collisions inthe light and heavy-flavor sectors, as well as comparisons with model predictions. Measurementsof the yields of strange hadrons relative to the one of pions across energy and system size are alsodiscussed. The paper is organized as follows. In section 2, the ALICE apparatus and some ofthe techniques used to measure the p T spectra of identified particles are described. Results anddiscussions are presented in section 3. Final remarks are summarized in section 4.
2. ALICE apparatus
ALICE (A Large Ion Collider Experiment) is the dedicated heavy-ion experiment at the LHC withunique capabilities for tracking and particle identification (PID) over a wide range of p T . Thetrigger selection and event classification into multiplicity classes are accomplished using the V0detector. It is composed of a pair of forward scintillator hodoscopes, which cover the pseudorapid-ity ranges 2 . < η < . − . < η < − . | η | ≤ . ( d E / d x ) with a resolution ofabout 6%. The Time Projection Chamber (TPC) [7] consists of a hollow cylinder with a sym-metry axis parallel to the beam axis. The active volume is about 90 m filled with a mixture ofgas consisting of Ne-CO -N at atmospheric pressure. An electrode at the center of the cylinderprovides, together with a voltage dividing network at the surface of the outer and inner cylinder,a drift field of 400 V / cm. Its acceptance covers the pseudorapidity interval | η | < . E / d x in the gas volume.1 aryon production from small to large collision systems at ALICE Omar Vázquez Rueda
The Time-Of-Flight (TOF) [7] detector is a large area array of Multigap Resistive Plate Chambers(MRPC), positioned at 370 −
399 cm from the beam axis and covering the full azimuthal angle andthe pseudorapidity range | η | < .
9. The TOF detector can provide information about the produc-tion of identified particles in the intermediate p T region, from ∼ / c up to 3 GeV / c for pions,kaons and (anti-)protons. Strange hadrons such as K and Λ ( Λ ) are reconstructed at mid-rapidity ( | y | < . ) via their characteristic weak decay topologies: K → π + + π − , Λ ( Λ ) → p ( ¯p ) + π − ( π + ) .The Λ + c and Λ − c are measured by reconstructing the hadronic decay modes: Λ + c → pK − π + and Λ + c → pK (and charge conjugates).
3. Results and discussions
The study of particle production as a function of multiplicity can be performed by calculating theratios of particle p T spectra to that of pions. Pions are commonly taken as the reference speciessince they are the most abundant particles and least affected by collective flow. Figure 1 showsthe p T -differential proton-to-pion and Λ / K ratio in pp, p–Pb and Pb–Pb collisions. The resultsbetween the lowest and highest event multiplicity classes in each of the systems are contrasted. Forthe three different colliding systems the same pattern is observed: At low- p T the high-multiplicityratio is lower than the low-multplicity ratio, but this changes at high p T . This suggests that particlesheavier than pions are shifted from low to high p T as the charged particle multiplicity increases.While the maximum in the proton-to-pion ratio reaches an approximate value of 0 . .
8. This is interpreted in terms of collective radial flow, whose effects are morerelevant for heavier particles. Moreover, the largest value of the maximum seen in central Pb–Pbcollisions reveals the presence of the strongest radial flow as discussed in terms of a Blast-Waveanalysis in [2].The ALICE Collaboration has also measured the Λ + c / D baryon-to-meson ratio in pp, p–Pb [8]and Pb–Pb collisions at different energies. Due to the large mass of the charm quark ( m c ∼ = . / c ) , it is produced in hard-partonic scattering processes during the initial stages of thecollision. In Pb–Pb collisions they are even produced before the formation of the QGP, allowingthem to interact with and experience the whole evolution of the medium. Figure 2 shows the p T -differential Λ + c / D ratio in pp, p–Pb, central and peripheral Pb–Pb collisions at √ s NN = .
02 TeV.It is observed that in central Pb–Pb collisions the ratio is higher than in peripheral ones at in-termediate p T ( (cid:46) p T (cid:46)
10 GeV / c ) . The result from central collisions shows a maximum at p T ≈ / c , which is consistent with the one measured in the proton-to-pion ratio of centralPb–Pb collisions although at a smaller value of p T . At low- p T ( p T (cid:46) / c ) a hint of suppres-sion is observed for central collisions, however, this has to be treated with care as the systematicuncertainties are too large to draw firm conclusions. Also, the analogous measurements from ppand p–Pb collisions at intermediate p T are found to be smaller than in Pb–Pb collisions. The en-hancement of the baryon-to-meson ratios in the light-flavor and heavy-flavor sector at intermediate p T suggest that the baryon production in an environment with a high density of partons occurs by2 aryon production from small to large collision systems at ALICE Omar Vázquez Rueda ) - π + + π ) / ( p ( p + = 7 TeVsALICE Preliminary pp = 21.3 〉 η /d ch dN 〈 V0M Class I, = 2.3 〉 η /d ch dN 〈 V0M Class X, (V0M Multiplicity Classes) ) c (GeV/ T p = 5.02 TeV NN sALICE p-Pb = 45.1 〉 η /d ch dN 〈 〉 η /d ch dN 〈 = 2.76 TeV NN sALICE Pb-Pb = 1601.0 〉 η /d ch dN 〈 〉 η /d ch dN 〈 ALI-PREL-110279
ALI-PREL-110561
Figure 1: p T -differential proton-to-pion (top) and Λ / K (bottom) ratios. From the left to the right subpanelin each of the figures, the results from pp, p–Pb and Pb–Pb collisions are shown. Two event multiplicityclasses are shown: red (blue) markers represent the results from the highest (lowest) multiplicity eventsin the respective colliding systems. The error bars show the statistical uncertainty, while the empty boxesshows the total systematic uncertainty. recombination as discussed in [9].Figure 3 shows the p T -differential Λ + c / D ratio in pp collisions at √ s = .
02 TeV and at √ s = + e − collisions. The mea-sured values at the two different energies are consistent within uncertainties. The PYTHIA8 predic-tions include the Monash tune and modes where color reconnection can lead to baryon junctions.The baryon junction formation in PYTHIA brings the predictions closer to data. What remainsto be seen is if one can also find a tune in which the junction mechanism also can describe cor-rectly the light-flavor baryon production - in particular for protons, which are not enhanced inhigh-multiplicity events. The model parameters corresponding to Mode0 used in these compar-isons are described in [6]. All the MC generators including the expectations from e + e − were foundto significantly underestimate the data at low p T while at high p T data seem to tend to the valuespredicted by these models. The color reconnection mechanism in PYTHIA enhances flow-like ef-fects, bringing the predictions closer to data.The yields of K , Λ , Ξ and Ω normalized to the one of pions are shown in Fig. 4 as a function of the3 aryon production from small to large collision systems at ALICE Omar Vázquez Rueda
10 20 ) c (GeV/ T p / D + c Λ ALICE Preliminary | < 0.5 y = 5.02 TeV, | NN s -extrapolated pp reference T p calc. with prompt f Open marker: Pb −
10% Pb − −
50% Pb − y Pb, -0.96 < − p ALI−PREL−321706
Figure 2: p T -differential Λ + c / D ratio in pp, p–Pb, central and peripheral Pb–Pb collisions at √ s NN = .
02 TeV measured at mid-rapidity ( | y | < . ) . The error bars show the statistical uncertainty, while theempty boxes show the total systematic uncertainty. ) c (GeV/ T p / D + c Λ = 7 TeV s pp, | < 0.5 y | data (JHEP 04 (2018) 108)PYTHIA8 (Monash) , Mode0c c → qPYTHIA8 gg,qPYTHIA8 SoftQCD, Mode0DIPSY (ropes)HERWIG7 ALICE Preliminary = 5.02 TeV s pp, | < 0.5 y | data ALI−DER−314630
Figure 3: p T -differential Λ + c / D ratio in pp collisions at √ s = .
02 TeV and at √ s = mean charged-particle density ( (cid:104) d N ch / d η (cid:105) ) in pp at √ s = √ s NN = .
02 TeV andPb–Pb collisions at √ s NN = .
76 TeV. Significant enhancement of strange hadrons with respectto pions ( π + + π − ) with increasing multiplicity is observed. Moreover, the strangeness productionincreases with increasing strange-quark content. The origin of strangeness production in hadronic4 aryon production from small to large collision systems at ALICE Omar Vázquez Rueda collisions is apparently driven by the characteristics of the final state rather than by the collisionsystem or energy [5]. Predictions from MC generators were compared and it was found that whilePYTHIA8 [10] and EPOS LHC [11] underestimate the yield ratios, DIPSY [12] describes the databest. DIPSY is a model where interactions among strings allow the formation of ‘color ropes’which are expected to produce more strange hadrons than strings, since the ‘ropes’ have strongercolor fields (higher string tension).
ALI-PREL-134498
Figure 4: p T -integrated yield ratios to pions ( π + + π − ) as a function of (cid:104) d N ch / d η (cid:105) . Results from pp at √ s = √ s NN = .
02 TeV and Pb–Pb collisions at √ s NN = .
76 TeV are shown. The errorbars show the statistical uncertainty, whereas the empty and dark-shaded boxes show the total systematicuncertainty and the contribution uncorrelated across multiplicity bins, respectively.
4. Conclusions
The ALICE experiment has made precise measurements of spectra of identified particles down tolow p T in pp, p–Pb and Pb–Pb collisions over a broad range of collision energies allowing theexploration of the non-perturbative QCD regime. It has been observed that the light-flavor baryon-to-meson ratios in high multiplicity pp and p–Pb collisions showed qualitative similarities to the5 aryon production from small to large collision systems at ALICE Omar Vázquez Rueda ones in heavy-ion collisions and that similar effects are also present in the heavy-flavor sector. Fur-thermore, precise measurements of the production of identified-charged particles as a function ofmultiplicity have shown that multiplicity is a key variable for studying the relative particle abun-dances.
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