Abstract

STAR has recently reported observations consistent with the presence of Electromagnetic field effects in Au+Au collisions at center-of-mass energies of 200 and 27 GeV. Such effects are predicted to result in a negative ∆v1, defined by the difference in rapidity-odd directed flow (v1) between positively and negatively charged particles, in peripheral collisions.We present the STAR BES-II results of ∆v1 for charged π+-, K+-, p(pbar) and Λ(Λ bar) as a function of rapidity, transverse momentum (pT ), and centrality in the rapidity window of −1 < y < 1 in Au+Au collisions at √sNN = 19.6-7.7 GeV. Our measurements are consistent with the dominance of Faraday+Coulomb effect in peripheral collisions and a stronger electromagnetic effect at lower collision energies, expected from the  longer lifetime of the electromagnetic field and shorter lifetime of the fireball at lower collision energies. Moreover, the non-zero ∆v1 of  Λ(Λ bar) is consistent with the action of electromagnetic forces on constituent charged quarks, expected from the presence of deconfined quark gluon plasma phase.

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Conclusion

We presented ∆dv1/dy as a function of centrality in Au+Au collisions at BES-II energies at RHIC. ∆dv1/dy is negative in peripheral collisions and becomes more negative at lower collision energies, consistent with expectations from the dominance of Faraday+Coulomb effect and a stronger electromagnetic effect at lower collision energies. ∆v1 shows hints of larger negative values at higher pT in peripheral collisions expected from theoretical predictions. ∆dv1/dy (Λ,Λ bar) is consistent with ∆dv1/dy (p,pbar)-∆dv1/dy (K+,K-), as expected from the action of electromagnetic forces on charged quarks followed by quark coalescence. These measurements can help constrain the initial strength of electromagnetic fields in heavy ion collisions, the conductivity of Quark Gluon Plasma and its evolution in the presence of strong electromagnetic fields.