Highlights from the STAR experiment

We present an overview of the recent results obtained by the STAR Collaboration at RHIC. We report the beam energy dependence of directed flow of strange particles. These results support the assumption, that directed flow is formed before hadronization and observed particles are formed via coalescence of constituent quarks. First evidence of a non-zero directed flow of D0 meson is reported. We present the precision measurement of elliptic flow of D0 meson and the first measurement of Λc baryon in Au+Au collisions at √ sNN = 200 GeV, which suggests thermalization of charm quarks. We report results of global Λ polarization measurements in Au+Au collisions, together with the investigation of polarization dependence of centrality and transverse momentum pT . Results from the STAR fixed-target program show good agreement with previously obtained results. A precise measurement of the 3 Λ H lifetime is presented. Mesonic decay modes are used to reconstruct the 3 Λ H from Au+Au collisions. The measured lifetime is about 50% shorter than the one of a free Λ, indicating a strong hyperon-nucleon interaction in the hypernucleus system. Finally, we give an outlook to detector upgrades for the Beam Energy Scan phase II. 1 Azimuthal anisotropy measurements One of the most important observables in heavy-ion experiments is the azimuthal anisotropic flow [1] that is usually quantified by the Fourier coefficients of the azimuthal distribution of the final state particles relative to the collision symmetry planes Ed3N/dp3 = 1/2π · dN/ptdptdη · (1 + Σ2vncos[n(φ − Ψn)]). The first-order coefficient, called the directed flow, is argued to be sensitive to the equation of state of the matter, and could serve as a possible signature of the QGP phase transition. STAR has recently performed directed flow measurements at mid-rapidity for Λ, Λ̄,K±,K0 s , and φ at √ sNN = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions. Results show good sensitivity for testing a picture where the flow is assumed to be formed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. Fig. 1 represents v1(y) for indicated particles at two centralities. One can see that within errors, the plotted species have a near linear v1(y) over the acceptance of the STAR detector. Directed flow slope dv1/dy versus beam energy for Λ, Λ̄,K±,K0 s , φ was also studied [2] and compared with the data for π±, p and p̄ [3]. Combined data for the ten particle species available allow one to carry out a detailed investigation of constituent quark v1. Obtained results support the assumption called the coalescence sum rule [4]. In this scenario the directed ∗e-mail: aparin@jinr.ru , 0 (201 E Web of Conferences https://doi.org/10.1051/e onf /201920401018 PJ pjc 9) 204 10 Baldin ISHEPP XXIV 18 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). flow v1 is formed at the pre-hadronic stage, specific types of quarks have the same directed flow and the detected hadrons are formed via coalescence.


Azimuthal anisotropy measurements
One of the most important observables in heavy-ion experiments is the azimuthal anisotropic flow [1] that is usually quantified by the Fourier coefficients of the azimuthal distribution of the final state particles relative to the collision symmetry planes Ed 3 N/d p 3 = 1/2π · d 2 N/p t d p t dη · (1 + Σ2v n cos[n(φ − Ψ n )]). The first-order coefficient, called the directed flow, is argued to be sensitive to the equation of state of the matter, and could serve as a possible signature of the QGP phase transition. STAR has recently performed directed flow measurements at mid-rapidity for Λ,Λ, K ± , K 0 s , and φ at √ s NN = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions. Results show good sensitivity for testing a picture where the flow is assumed to be formed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. Fig. 1 represents v 1 (y) for indicated particles at two centralities. One can see that within errors, the plotted species have a near linear v 1 (y) over the acceptance of the STAR detector. Directed flow slope dv 1 /dy versus beam energy for Λ,Λ, K ± , K 0 s , φ was also studied [2] and compared with the data for π ± , p andp [3]. Combined data for the ten particle species available allow one to carry out a detailed investigation of constituent quark v 1 . Obtained results support the assumption called the coalescence sum rule [4]. In this scenario the directed * e-mail: aparin@jinr.ru   Direct measurements of charmed hadrons require a very good vertex detector due to their very short lifetimes. STAR had installed the Heavy Flavor Tracker (HFT) [5], the MAPSbased vertex detector for run years 2014-2016. This allowed one to perform precise measurements of the flow for heavy flavor particles [6]. Fig. 2 shows the first measurement of the directed flow for charmed meson D 0 produced in Au-Au collisions for 10-80% centrality. It is clear that within the error value of v 1 (y) is nonzero and follows the linear trend obtained previously for the light particle sector. The second harmonic of the charmed meson D 0 has been recently studied using data from run years 2014 and 2016. The measured D 0 v 2 in 0-80% centrality Au+Au collisions can be described by a viscous hydrodynamic calculation for transverse momentum p T less than 4GeV/c. The D 0 v 2 as a function of transverse kinetic energy (m Tm 0 , where m T = p 2 T + m 2 ) is consistent with that of light mesons in 10-40% centrality Au+Au collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium produced in these collisions.
The energy dependent measurements reported here will be enhanced after STAR acquires greatly increased statistics using the upgraded detectors in Phase-II of the RHIC Beam Energy Scan (BES-II).

Heavy flavor production
The excellent HFT resolution enabled a study of charmed hadrons with even smaller decay lengths than D 0 , in particular Λ c . Fig. 3 shows the reconstructed invariant mass of pKπ in 10-60% Au+Au collisions at √ s NN = 200 GeV, with topological cuts optimized for Λ c . A peak at the Λ c mass is observed clearly. STAR has performed the first measurements of centrality dependence of Λ c production in heavy ion-collisions. It has been found that Λ c /D 0 ratio increases from peripheral to central, that indicates hot medium effects. The measured STAR data point was compared to model calculations with various levels of charm quark thermalization in the medium and different coalescence implementations of the coalescence mechanism. The energy loss of charm and bottom quarks is of great interest since quarks are expected to exhibit different radiative energy losses depending on their mass. The nuclear modification

Global polarization measurements
STAR has recently reported measurements of global Λ polarization in Au+Au collisions [8,9]. The existence of alignment between the angular momentum of a non-central collision and the spin of emitted particles was demonstrated for the first time, revealing that the fluid produced in heavy-ion collisions is by far the most vortical system ever observed. The measured effect for Λ andΛ hyperons has shown positive polarization of the order of a few percent for energies of BES and of tens of percent for the highest RHIC energy. The signal increases with decreasing collision energy, and systematic splitting between Λ andΛ is observed which might indicate an additional magnetic component. Spin-orbit coupling can generate a spin alignment, or polarization, along the direction of the vorticity which is on the average parallel to the overall angular momentum of the system. Fig. 4 shows the measured global polarization of Λ andΛ as a function of the collision energy for the 20-50% centrality bin in Au+Au collisions. A set of different theoretical models is argued to describe the measured polarization [10]- [12].
Calculations for primary Λ and all Λ taking into account the effect of feed-down, from a (3+1)D viscous hydrodynamic model vHLLE with the UrQMD initial state [10] are shown for comparison. The model calculations agree with the data over a wide range of collision energy, including √ s NN = 200 GeV within the current accuracy of experimental measurements.
Calculations from a Multi-Phase Transport (AMPT) model [11] predict slightly higher polarization than the hydrodynamic model, but are also in good agreement with the data within uncertainties. Fig. 5 shows differential measurements of the polarization, versus the collision centrality, and hyperon's transverse momentum. With the given large uncertainties, it is not clear if the polarization saturates or even starts to drop off in the most peripheral collisions (Fig. 5,  left). The polarization dependence on p T is weak or absent (Fig. 5, right), considering the large uncertainties, which is consistent with the expectation that the polarization is generated by a rotation of the system. No significant dependence on pseudorapidity or transverse momentum was observed. The statistical uncertainties need to be improved to reach a definitive conclusion on the event-by-event charge asymmetry, which is consistent with a possible contribution to the global polarization from the axial current induced by the initial magnetic field.  pipe has been done recently. During 3 minutes run 1M events of 0-30% centrality Au+Au were recorded. Fig. 6 shows energy dependence of the measured directed flow slope parameter dv 1 /dy| y=0 , comparing the data from the fixed-target mode to BES STAR [13] and AGS results [14]. Within the uncertainties the data are consistent with those measured in a collider experiments. Fig. 6, left, shows the slope parameter for the lightest mesons (π ± , K ± and K 0 S ) and Fig. 6, right, the same for lightest baryons (p, Λ andp). Fig. 7 demonstrates STAR capabilities for particle identification in the fixed-target regime for strange particles. Spectra are plotted in dependence on transverse kinetic energy for K 0 S , left, and Λ, right.

Hypertriton lifetime measurements
STAR has performed a precise measurement of the 3 Λ H and 3 ΛH lifetime. Mesonic decay modes were used to reconstruct 3 Λ H from Au+Au collisions data. All 3 Λ H measurements, regardless of beam energy, were combined to increase the statistics. The hypertriton candidates were reconstructed from the invariant mass distributions of the daughters: 3 He + π − for the 2-body, and d + p + π − for the 3-body decay channel of 3 Λ H and 3H e + π + for the 2-body, and d +p + π + for the 3-body decay channel for 3 Λ H and 3 ΛH , respectively. A minimum χ 2 estimation is used to determine the lifetime of τ = 142 + 24 − 21(stat.) ± 31(syst.) ps. This lifetime is about 50% shorter than the lifetime τ = 263 ± 2 ps of a free Λ, indicating strong hyperon-nucleon interaction in the hypernucleus system. Fig. 8 shows the experimentally measured hypertriton lifetime.  The STAR experiment will collect large datasets for Au+Au collisions during BES phase II in years 2019-2020, which will further reduce experimental uncertainties on the 3 Λ H lifetime and will likely provide a new insight into its structure.   search for the QCD critical point and signatures of a phase transition between hadronic gas and QGP phases. In order to increase detector acceptance and particle identification capabilities, three major subsystem upgrades were proposed must be ready till the end of 2018: Event Plane Detector (EPD), the inner TPC upgrade (iTPC) and the endcap Time-Of-Flight detector (eTOF). The EPD was fully installed and became operational for the run year 2018, replacing the Beam-Beam Counter (BBC) as a minimum-bias trigger detector. EPD allows one to carry out forward measurements of both centrality and event plane determination reducing systematic uncertainty due to autocorrelations for midrapidity analysis. This new detector consists of two disks that are placed on both sides of the STAR detector. EPD has pseudorapidity acceptance of 2.1 < |η| < 5.1 with 16 radial segments and 24 azimuthal segments.

Detector upgrades for BES-II
The iTPC will increase the acceptance of the TPC up to |η| = 1.5 increasing number of readout pad rows from 13 to 40 and maximum number of hits per track from 45 to 72. It will improve the dE/dx resolution, and allow tracks to be reconstructed down to p T of 60 MeV/c. A single inner sector was installed for tests for the run year 2018 and the full complement of 24 will be installed for the run year 2019.
The eTOF will be installed on one side of STAR, which will extend PID capabilities at forward rapidity. Three modules of eTOF were installed behind one of the TPC sectors for the run year 2018 and the full complement of 36 will be installed for the run year 2019. Combining all three detector systems will reduce significantly the systematic uncertainties of many observables and improve the statistical precision of STAR BES measurements.

Summary
STAR has presented a variety of experimental results in a wide energy √ s NN range from 4.5 GeV to 200 GeV obtained in the fixed-target and collider modes. A systematic study of collision energy dependence of strange particles indicates the constituent quark coalescence. Results on heavy flavor production measured with HFT include the directed and elliptic flows of D 0 mesons and reconstruction of Λ c baryons, which supports the idea of charm thermalization at RHIC energies. Global Λ polarization measurements in Au+Au collisions previously made for the energies of the BES now has been presented revealing a non-zero effect for the highest RHIC energy 200 GeV. These observations can provide a new insight on the vorticity in heavy-ion collisions. Three detector upgrades are expected to be ready prior to the run year 2019 for the upcoming Beam Energy Scan phase II. The iTPC, eTOF and EPD detectors will improve the acceptance, particle identification capabilities, and reduce systematic uncertainties for many observables.