Global polarization of $\Lambda$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV, with no significant difference between $\Lambda$ and $\bar{\Lambda}$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity $\eta^H$ within $|\eta^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2\sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.

/pdf/global-polarization-of-l-hyperons-in-au-au-collisions-at-snn-29v02am1z6.pdf

Global polarization of Λ hyperons in Au + Au collisions at sNN =200 GeV

The chiral magnetic effect (CME) is predicted to occur as a consequence of a local violation of P and CP symmetries of the strong interaction amidst a strong electromagnetic field generated in relativistic heavy-ion collisions. Experimental manifestation of the CME involves a separation of positively and negatively charged hadrons along the direction of the magnetic field. Previous measurements of the CME-sensitive charge-separation observables remain inconclusive because of large background contributions. To better control the influence of signal and backgrounds, the STAR Collaboration performed a blind analysis of a large data sample of approximately 3.8 billion isobar collisions of Ru4496+Ru4496 and Zr4096+Zr4096 at sNN=200 GeV. Prior to the blind analysis, the CME signatures are predefined as a significant excess of the CME-sensitive observables in Ru+Ru collisions over those in Zr+Zr collisions, owing to a larger magnetic field in the former. A precision down to 0.4% is achieved, as anticipated, in the relative magnitudes of the pertinent observables between the two isobar systems. Observed differences in the multiplicity and flow harmonics at the matching centrality indicate that the magnitude of the CME background is different between the two species. No CME signature that satisfies the predefined criteria has been observed in isobar collisions in this blind analysis.20 MoreReceived 31 August 2021Accepted 7 December 2021DOI:https://doi.org/10.1103/PhysRevC.105.014901©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle correlations & fluctuationsQuark-gluon plasmaRelativistic heavy-ion collisionsNuclear Physics 

Search for the chiral magnetic effect with isobar collisions at 
sNN=200
 GeV by the STAR Collaboration at the BNL Relativistic Heavy Ion Collider

The Λ (Λ[over ¯]) hyperon polarization along the beam direction has been measured in Au+Au collisions at sqrt[s_{NN}]=200  GeV, for the first time in heavy-ion collisions. The polarization dependence on the hyperons' emission angle relative to the elliptic flow plane exhibits a second harmonic sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction, expected due to elliptic flow. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum (p_{T}) dependence at p_{T} greater than 1  GeV/c. The magnitude of the signal is about 5 times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, the kinematic vorticity calculations in the blast-wave model tuned to reproduce particle spectra, elliptic flow, and the azimuthal dependence of the Gaussian source radii measured with the Hanbury Brown-Twiss intensity interferometry technique reproduce well the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the polarization signal.

/pdf/polarization-of-l-l-over-hyperons-along-the-beam-direction-p413jn9mbn.pdf

Polarization of Λ (Λ[over ¯]) Hyperons along the Beam Direction in Au+Au Collisions at sqrt[s_{NN}]=200 GeV.

Author(s): Adam, J; Adamczyk, L; Adams, JR; Adkins, JK; Agakishiev, G; Aggarwal, MM; Ahammed, Z; Ajitanand, NN; Alekseev, I; Anderson, DM; Aoyama, R; Aparin, A; Arkhipkin, D; Aschenauer, EC; Ashraf, MU; Atetalla, F; Attri, A; Averichev, GS; Bai, X; Bairathi, V; Barish, K; Bassill, AJ; Behera, A; Bellwied, R; Bhasin, A; Bhati, AK; Bielcik, J; Bielcikova, J; Bland, LC; Bordyuzhin, IG; Brandenburg, JD; Brandin, AV; Brown, D; Bryslawskyj, J; Bunzarov, I; Butterworth, J; Caines, H; Calderon de la Barca Sanchez, M; Campbell, JM; Cebra, D; Chakaberia, I; Chaloupka, P; Chang, F-H; Chang, Z; Chankova-Bunzarova, N; Chatterjee, A; Chattopadhyay, S; Chen, JH; Chen, X; Chen, X; Cheng, J; Cherney, M; Christie, W; Contin, G; Crawford, HJ; Das, S; Dedovich, TG; Deppner, IM; Derevschikov, AA; Didenko, L; Dilks, C; Dong, X; Drachenberg, JL; Dunlop, JC; Efimov, LG; Elsey, N; Engelage, J; Eppley, G; Esha, R; Esumi, S; Evdokimov, O; Ewigleben, J; Eyser, O; Fatemi, R; Fazio, S; Federic, P; Federicova, P; Fedorisin, J; Filip, P; Finch, E; Fisyak, Y; Flores, CE; Fulek, L; Gagliardi, CA; Galatyuk, T | Abstract: We report first measurements of e^{+}e^{-} pair production in the mass region 0.4

/pdf/low-pt-e-e-pair-production-in-au-au-collisions-at-snn-200-42eq8l0sro.pdf

Low- pT e+e- Pair Production in Au +Au Collisions at sNN =200 GeV and U + U Collisions at sNN =193 GeV at STAR

Author(s): Adamczyk, L; Adkins, JK; Agakishiev, G; Aggarwal, MM; Ahammed, Z; Alekseev, I; Aparin, A; Arkhipkin, D; Aschenauer, EC; Attri, A; Averichev, GS; Bai, X; Bairathi, V; Bellwied, R; Bhasin, A; Bhati, AK; Bhattarai, P; Bielcik, J; Bielcikova, J; Bland, LC; Bordyuzhin, IG; Bouchet, J; Brandenburg, JD; Brandin, AV; Bunzarov, I; Butterworth, J; Caines, H; Calderon De La Barca Sanchez, M; Campbell, JM; Cebra, D; Chakaberia, I; Chaloupka, P; Chang, Z; Chatterjee, A; Chattopadhyay, S; Chen, JH; Chen, X; Cheng, J; Cherney, M; Christie, W; Contin, G; Crawford, HJ; Das, S; De Silva, LC; Debbe, RR; Dedovich, TG; Deng, J; Derevschikov, AA; Di Ruzza, B; Didenko, L; Dilks, C; Dong, X; Drachenberg, JL; Draper, JE; Du, CM; Dunkelberger, LE; Dunlop, JC; Efimov, LG; Engelage, J; Eppley, G; Esha, R; Evdokimov, O; Eyser, O; Fatemi, R; Fazio, S; Federic, P; Fedorisin, J; Feng, Z; Filip, P; Fisyak, Y; Flores, CE; Fulek, L; Gagliardi, CA; Garand, D; Geurts, F; Gibson, A; Girard, M; Greiner, L; Grosnick, D; Gunarathne, DS; Guo, Y; Gupta, S; Gupta, A; Guryn, W; Hamad, AI | Abstract: We present measurements of second-order azimuthal anisotropy (v2) at midrapidity (|y|l1.0) for light nuclei d,t,He3 (for sNN=200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV) and antinuclei d; (sNN=200, 62.4, 39, 27, and 19.6 GeV) and Hē3 (sNN=200 GeV) in the STAR (Solenoidal Tracker at RHIC) experiment. The v2 for these light nuclei produced in heavy-ion collisions is compared with those for p and p. We observe mass ordering in nuclei v2(pT) at low transverse momenta (pTl2.0 GeV/c). We also find a centrality dependence of v2 for d and d. The magnitude of v2 for t and He3 agree within statistical errors. Light-nuclei v2 are compared with predictions from a blast-wave model. Atomic mass number (A) scaling of light-nuclei v2(pT) seems to hold for pT/Al1.5GeV/c. Results on light-nuclei v2 from a transport-plus-coalescence model are consistent with the experimental measurements.

/pdf/measurement-of-elliptic-flow-of-light-nuclei-at-snn-200-62-4-1nvk5341p4.pdf

Measurement of elliptic flow of light nuclei at sNN =200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV at the BNL Relativistic Heavy Ion Collider

The 2×3 channel pseudo Vertex Position Detector (pVPD) in the STAR experiment at RHIC has been upgraded to a 2×19 channel detector in the same acceptance, called the Vertex Position Detector (VPD). This detector is fully integrated into the STAR trigger system and provides the primary input to the minimum-bias trigger in Au+Au collisions. The information from the detector is used both in the STAR Level-0 trigger and offline to measure the location of the primary collision vertex along the beam pipe and the event “start time” needed by other fast-timing detectors in STAR. The offline timing resolution of single detector channels in full-energy Au+Au collisions is ~100 ps, resulting in a start time resolution of a few tens of picoseconds and a resolution on the primary vertex location of ~1 cm.

The STAR Vertex Position Detector

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The STAR Vertex Position Detector