Light Nuclei ( d , t ) Production in Au + Au Collisions at s NN − − − − √ = 7.7 - 200 GeV
NNuclear Physics A 00 (2020) 1–4
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
Light Nuclei ( d , t ) Production in Au + Au Collisions at √ s N N = Dingwei Z hang (for the STAR Collaboration) Key Laboratory of Quark & Lepton Physics (MOE) and Institute of Particle Physics, Central China Normal University, Wuhan, 430079, China
Abstract
In high-energy nuclear collisions, light nuclei can be regarded as a cluster of baryons and their yields are sensitive tothe baryon density fluctuations. Thus, the production of light nuclei can be used to study the QCD phase transition,at which the baryon density fluctuation will be enhanced. A yield ratio of light nuclei, defined as N ( t ) × N ( p ) / N ( d ), ispredicted to be a sensitive observable to search for the 1st-order phase transition and / or QCD critical point in heavy-ioncollisions. In this paper, we present the energy and centrality dependence of (anti)deuteron and triton production inAu + Au collisions at √ s NN = B ( d ) and B ( t ), particle ratios, d / p , t / p ,and t / d , and the yield ratio of N ( t ) × N ( p ) / N ( d ). More importantly, non-monotonic energy dependence is observed forthe yield ratio, N ( t ) × N ( p ) / N ( d ), in 0-10% central Au + Au collisions with a peak around 20-30 GeV. Their physicsimplications on QCD critical point search and change of the equation of state will be discussed.
Keywords:
Triton, Coalescence parameters, Neutron density fluctuation
1. Introduction
In relativistic heavy-ion collisions, the study of the phase transition between the Quark-Gluon Plasma(QGP) and the hadronic matter attracts great interest [1]. Production of light nuclei with small bindingenergy is sensitive to the local nucleon density, which can provide a unique tool to probe the essentialfeature of the QCD phase diagram [2]. In the coalescence picture, the density of the cluster is proportionalto the proton density times the probability of finding a neutron within a small sphere around the protonmomentum [3]. Nucleon coalescence mechanism can be described as: E A d N A d p A = B A E p d N p d p p Z (cid:32) E n d N n d p n (cid:33) A − Z ≈ B A E p d N p d p p A , (1)where A and Z are the mass and charge number of the light nucleus under study. p p , p n , and p A are momentaof proton, neutron, and nucleus respectively, with p A = Ap p , assuming p p ≈ p n . a r X i v : . [ nu c l - e x ] M a y / Nuclear Physics A 00 (2020) 1–4
In the vicinity of the critical point or the first order phase transition, density fluctuations become larger.Based on the coalescence picture, the yield ratio, N ( t ) × N ( p ) / N ( d ), is sensitive to the neutron density fluc-tuation, ∆ n = (cid:104) ( δ n ) (cid:105) / (cid:104) n (cid:105) , where (cid:104) n (cid:105) denotes the average value over space and δ n denotes the fluctuationof neutron density from its average value (cid:104) n (cid:105) [4]. The yield ratio of light nuclei, which connects neutrondensity fluctuation, can be approximated as: N ( t ) × N ( p ) / N ( d ) = g (1 + ∆ n ) , (2)with g =
2. Results and discussions
The results presented in this paper are obtained from the data taken with the STAR experiment in Au + Aucollisions at √ s NN = | y | < - - - - - - - - - - - - - - -
27 GeV - - -
39 GeV
STAR Preliminary - - - - - -
200 GeV - - - · · · · Blast-wave
Triton from Au+Au Collision (GeV/c) T Transverse Momentum p ] - ) [( G e V / c ) T dydp T p p N / ( d (GeV/c) T p - - - - - ] - )[( G e V / c ) T dydp T p p N / ( d Au + Au 54.4 GeVTriton
STAR Preliminary ´ ´ ´ ´ (GeV/c) T p - - - -
10 110 ] - )[( G e V / c ) T dydp T p p N / ( d Au + Au 54.4 GeVProton
STAR Preliminary (GeV/c) T p - - - - - - - ] - )[( G e V / c ) T dydp T p p N / ( d Au + Au 54.4 GeVAnti-Deuteron
STAR Preliminary ´ ´ ´ ´ ´ (GeV/c) T p - - - - ] - )[( G e V / c ) T dydp T p p N / ( d Au + Au 54.4 GeVDeuteron
STAR Preliminary ´ ´ ´ ´ ´ Fig. 1. Transverse momentum spectra of t (left) from BES-I and high-statistical data set at 54.4 GeV (right) measured at midrapidityin Au + Au collisions for di ff erent collision centralities. The dashed lines are the individual fits to the data with blast-wave functions.The vertical lines and boxes show the statistical and systematic errors, respectively. momentum spectra for tritons in Au + Au collisions at √ s NN = | y | < | y | < | y | < + Au collisionsat √ s NN = In the left panel of Fig. 2, the p T / A dependence of B is shown at √ s NN = ff erentcollision centralities. It is found that the value of B increases from central to peripheral collisions andincreases with increasing p T / A , which can be explained by the decreasing of source volume. In the rightpanel of Fig. 2 we compare the results of B and √ B in the 0-10% collision centrality at p T / A = / c (This value corresponds to p T = / c for d , p T = / c for t ). At energies below √ s NN =
20 GeV, the coalescence parameters B and √ B decrease with increasing collision energy, whichimplied that the size of the particle-emitting source increases. When √ s NN >
20 GeV, the decreasing trendseems to change, which might imply a change of the equation of states of the medium in those collisions [8].The B and √ B are consistent within uncertainties except for √ s NN =
200 GeV, which might be that theproduction mechanisms for d and t are di ff erent at STAR top energy. Nuclear Physics A 00 (2020) 1–4 /A (GeV/c) T p ) / c ( G e V t B - - - STAR Preliminary 40~80% 20~40% 10~20% 0~10%
Au+Au 7.7 GeV /A (GeV/c) T p ) / c ( G e V t B - - - STAR Preliminary40~80% 20~40% 10~20% 0~10%
Au+Au 200 GeV (GeV) NN s ) / c ( G e V A B A - - / A = 0.65 GeV/c T p t: A = 3d: A = 2 Au+Au (0-10%)
STAR Preliminary (GeV) NN s d 2 / B t B Fig. 2. (Left panel) Coalescence parameter B as a function of p T / A for triton measured from 7.7 and 200 GeV collisions in 0-10%,10-20%, 20-40% and 40-80% central Au + Au collisions. (Right panel) Coalescence parameter B (open star) and √ B (red dot) as afunction of collision energy in 0-10% central Au + Au collisions. The vertical lines and square brackets show statistical and systematicerrors, respectively.
The yields of triton and deuteron are obtained by extrapolating the measured p T range to the unmeasured p T regions with various parameterizations. The extrapolation is done by individual blast-wave fit for eachparticle. The deuteron integral yields are from the new data sets 54.4 GeV, which confirm the previous > part 10 1 . G e V D eu t e r on STAR Preliminary (GeV) NN s P a r ti c l e R a ti o s - - - - - ThermalSTAR d/p t/p 0.2 ´ t/d STAR Preliminary Au + Au, 0-10% > part Fig. 3. Centrality dependence of dN / dy of d (left panel) and t (middle panel) in Au + Au collisions. Energy dependence of d / p , t / p ,and t / d (right panel) ratios for 0-10% central Au + Au collisions at BES-I energies. The dotted lines are thermal model predictions. Theerrors represent combined systematic and statistical uncertainties. measurements at other energies by STAR [8]. We also show the particle ratios of d / p , t / p , and t / d as afunction of collision energy in 0-10% central Au + Au collisions in the right panel of Fig. 3. The dashedlines are the thermal model calculations [9]. They describe the ratios of d / p very well but overestimates the t / p and t / d particle ratios. In Fig. 4, we show the energy dependence of yield ratio of light nuclei ( N ( t ) × N ( p ) / N ( d )), which isdirectly related to the neutron density fluctuation, in 0-10% central Au + Au collisions. Non-montonic energydependence is observed with a peak around 20-30 GeV. The light nuclei yield ratios measured by STARexperiment below 20 GeV are consistent with the results calculated from NA49 experiment [4]. The resultsfrom the high statistic Au + Au collisions at 54.4 GeV follow the trend very well. The maximum of thisnon-monotonic behavior might indicate that the density fluctuations become strongest at this energy region.Since there is no critical physics implemented in the JAM model, the results of central ( b < + Aucollisions from JAM model is also plotted as blue band in Fig. 4 for comparison. We found that the JAMmodel results show a flat energy dependence of yield ratio and cannot describe the data [10]. To providea definite physics conclusion on this non-monotonic structure, we still need further studies, especially theprecise experimental measurements and theoretical understanding. / Nuclear Physics A 00 (2020) 1–4 NN s0.20.40.6 d2 / N p N · t N = 0.3 GeV/c d,t P D JAM (b<3 fm), = 3.4 fm t R D = 4.0 fm, d R D STAR Preliminary Fig. 4. Energy dependence of the light nuclei yield ratio N ( t ) × N ( p ) / N ( d ) in central (0 − − + Au collisions at BES energies by the STAR experiment and the open squares arethe results calculated from the Pb + Pb data of NA49 experiment [4]. The blue band represents the results of central Au + Au collisions( b < 3. Conclusions We presented STAR results of d and t production in heavy-ion collisions at √ s NN = B and √ B from central collision show the same trends except for 200 GeV and they seem to reacha minimum around 20 GeV, indicating a change in the equation of state. The thermal model cannot describethe triton production, which is still an open question. Non-monotonic energy dependence of the light nucleiyield ratio is observed for 0-10% central Au + Au collisions with a peak around 20-30 GeV. The ratio below20 GeV are consistent with the results calculated from NA49 experiment. The results from JAM model,without physics of phase transition and critical point, show flat energy dependence and cannot describe theexperimental data. To make definite conclusion, we still need dynamical modelling of heavy-ion collisionswith a more realistic equation of state. Acknowledgements This work is supported by the National Key Research and Development Program of China (2018YFE0205201),the National Natural Science Foundation of China (No.11828501, 11575069, 11890711 and 11861131009). References [1] K. Fukushima and C. Sasaki, “The phase diagram of nuclear and quark matter at high baryon density,” Prog. Part. Nucl. Phys. , 99 (2013)[2] H. H. Gutbrod, A. Sandoval et al. “Final State Interactions in the Production of Hydrogen and Helium Isotopes by RelativisticHeavy Ions on Uranium,” Phys. Rev. Lett. , 667 (1976).[3] L. P. Csernai and J. I. Kapusta, “Entropy and Cluster Production in Nuclear Collisions,” Phys. Rept. , 223 (1986).[4] K. J. Sun, L. W. Chen, C. M. Ko, J. Pu and Z. 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