Measurement Of Higher-order Cumulants Of Net-（k+Λ） Multiplicity Distributions In (S_NN)^1/2=27GeV Au+Au Collisions With STAR

The main goal of the RHIC beam energy scan program is to explore the QCD phase diagram and search for the location of the QCD Critical Point.The QCD critical point is the end point of a first-order phase transition line,and at the critical point the length diverges.We search for the critical point by studying the non-monotonic behavior of the ratio of susceptibilities with respect to collision energy.This non-monotonic behavior suggests that as the collision energy varies,the hot dense matter produced in the collision passes through the critical region and is influenced by critical fluctuations.Susceptibilities are related to the cumulants of the conserved charge multiplicity distribution.When relating the susceptibilities to the cumulants,a volume term appears,making it difficult to compare different collision systems and centralities.Moreover,in relativistic heavy-ion collisions,it is often difficult to determine the system’s volume.Therefore,we use the ratio of susceptibilities to eliminate the volume term,which is equal to the ratio of cumulants of conserved charges in experiments.We need to choose the appropriate conserved charges.In previous studies,the net charge,net baryon and net strangeness are the three conserved charges,and the net charge,net proton number and net kaon number are considered to the proxies respectively.However,in relativistic heavy-ion collisions,the cumulants of net(Lambda+Kaon)multiplicity distribution is more sensitive to the net strangeness susceptibilities due to the presence of Lambda in the calculation.Therefore,it can be used to search for the QCD critical point and study the flavor dependence of the chemical freeze-out parameters in the QCD phase diagram.In this paper,for the first time,using(Lambda+Kaon)to represent net strangeness and measuring the cumulants of its multiplicity distribution.This paper measures the higher-order cumulants(1 to 4th order)of net(Lambda+Kaon)multiplicity distribution and their ratios with the data(?)=27 GeV Au+Au collisions with the STAR experiment taken in the year 2018.The Lambda,Kaon,and their anti-particles are measured with transverse momentum between 0.4 and 1.6 GeV/c,and rapidity |y|<0.5.Before calculating the higher-order cumulants of net(Lambda+Kaon),it is necessary to select a good sample of collision events by applying some analysis condition.Then,the identification of K mesons and reconstruction of Lambda particles in the events are performed.After identifying the particles,one can calculate the cumulants of net(Lambda+Kaon).However,due to the efficiency of the detectors,which refers to the number of particles detected by the detectors not being equal to the actual number of particles produced in each collision event,a correction for detector efficiency is needed.The widely used method for efficiency correction recently is the track-by-track method,where the efficiency of each track is individually corrected.The advantage of this method is that the efficiency of the detector depends on variables such as centrality and transverse momentum.Therefore,using the corresponding efficiency for each track allows for a more accurate efficiency correction.The idea behind this method is that if an event actually generates N particles,then the number of particles detected by the detector,n,follows a Binomial distribution,denoted as BP,N(n),where p is the detector efficiency.It is found that the factorial cumulants between the detected particle number and the generated particle number are related as κm(n)=pmκm(N).Thus,we use this relationship to correct for efficiency.For each specific centrality,the collision parameter is not a fixed value but rather a range.In other words,each centrality corresponds to a width of collision parameters.This means that even events with the same centrality can have different initial collision geometries,referred to as centrality bin width effect.To suppress the artificial effects introduced when dividing the centrality,we first calculate the value of the cumulants for unit bin in the multiplicity distribution that is used for centrality class.Then,using the event number as weights,we calculate the weighted average of the cumulants in each centrality interval to obtain the value of the cumulants within each centrality.This method is called centrality bin width correction.Afterwards,we estimate the statistical and systematical uncertainties.In estimating the statistical uncertainty,we employ the Bootstrap method which derives conclusions about population characteristics from the existing samples.As a result,we obtain the cumulants up to the fourth order for net(Kaon+Lambda).C1 increases linearly with increasing average number of participant overall,and its value is positive.This indicates that the yield of net Kaon is higher compared to net Lambda,suggesting the dominance of strange mesons.This is clearly inconsistent with the results from the UrQMD model,and this discrepancy may be caused by the disagreement between the decay yields of high-mass resonance particles in the UrQMD model and experimental data.The decay channels of high-mass resonances are included in the UrQMD model events.The cumulants are extensive quantities proportional to the system volume,and the volume effect dominates the value of the cumulants.In order to eliminate the volume effect,we calculate the ratio of the cumulants,C2/C1,which have a weak dependence on the collision centrality.This is because the ratio of the cumulants is not a function of volume.At the same time,we also compared the experimental data with the calculated results from UrQMD and found that the experimental results of C1/C2 do not agree well with the calculated results from UrQMD.Similarly,the values of C1/C2 and C4/C2 also show a weak dependence on the collision centrality.This paper presents the net conserved charge for the first time using a combination of two particles,and uses the existing track by track method for efficiency correction.The cumulants of the net(Lambda+Kaon)number with the data(?)=27 GeV Au+Au collisions with STAR taken in the year 2018 is calculated,and the centrality bin width correction is also performed,and statistical and systematical uncertainties are estimated.The UrQMD model was also used to calculate the cumulants under different collision energies as the baseline.A complete analysis framework has been constructed,which can be used to calculate the cumulants of any combination of multiple particles and can be directly used to analyze the high-order cumulants of other collision systems.Using this method to analyze the high-order cumulants of other particle combinations and other collision energies is of certain significance for finding the critical point of QCD phase transition.