Probing the effects of strong electromagnetic fields with charge-dependent directed flow in Pb-Pb collisions at the LHC

Abstract

The first measurement at the LHC of charge-dependent directed flow (v1) relative to the spectator plane is presented for Pb–Pb collisions at √ sNN = 5.02 TeV. Results are reported for charged hadrons and D0 mesons for the transverse momentum intervals pT > 0.2 GeV/c and 3 < pT < 6 GeV/c in the 5–40% and 10–40% centrality classes, respectively. The difference between the positively and negatively charged hadron v1 is found to have a positive slope as a function of pseudorapidity η , d∆v1/dη =[1.68 ± 0.49 (stat.) ± 0.41 (syst.)] ×10−4, with a 2.6σ significance. The same measurement for D0 and D0 mesons yields a positive value d∆v1/dη= [4.9 ± 1.7 (stat.) ± 0.6 (syst.)]×10−1, which is about three orders of magnitude larger than the one of the charged hadrons, and is larger than zero with significance of 2.7σ . These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in non-central heavy-ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed ∆v1 of charged hadrons and D0 mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy-ion collision. These observations challenge some of the recent theoretical calculations incorporating effects of the strong electromagnetic field, which predicted a negative and an order of magnitude smaller value of d∆v1/dη for both light-flavour and charmed hadrons. ∗See Appendix A for the list of collaboration members ar X iv :1 91 0. 14 40 6v 1 [ nu cl -e x] 3 1 O ct 2 01 9 Charge-dependent directed flow of hadrons and D mesons ALICE Collaboration Quantum Chromo-dynamic (QCD) calculations on the lattice [1–6] predict at very high temperatures and energy densities the existence of a deconfined state of quarks and gluons, known as the quark–gluon plasma (QGP). Characterizing the QGP properties is among the main goals of the experimental program with ultra-relativistic heavy-ion collisions at the Large Hadron Collider (LHC). Measurements of the anisotropic transverse flow [7–11] at the LHC, quantified by the second (elliptic flow) and higher order (n > 2) harmonic coefficients vn, allowed one to characterize the different phases of a heavy-ion collision and to constrain the properties of the QGP [12–16]. The directed flow, v1, has a special role due to its sensitivity to the three-dimensional spatial profile of the initial conditions and the pre-equilibrium early time dynamics in the evolution of the heavy-ion collision. The space-momentum correlations in particle production from a longitudinally tilted source results in a finite v1. The tilt arises from the asymmetries in the number of forward and backward moving nucleons at different positions in the transverse plane [17–19]. The directed flow of charged hadrons at the LHC [20] has significantly smaller magnitude compared to that at lower RHIC energies [21], which can be interpreted as a smaller initial tilt at the LHC [22–24]. Charm quarks are produced early in the collision via hard scattering processes. Their emission region does not have a tilt in the longitudinal direction [19] unlike the one of light quarks, which are predominantly produced in soft processes at later stages of the collision [18, 25]. Consequently, the region of the charm quark production in the transverse plane is shifted with respect to that of the light quarks and gluons. This results in an enhanced dipole asymmetry in the charm quark distribution [19]. During the system expansion, charm quarks would be dragged by the flow of the light quarks in the transverse direction of the shift, which is predicted to result in a larger directed flow of hadrons containing charm quarks as compared to light-flavour hadrons [19, 26]. Consequently, the measurements of the charge-integrated directed flow of hadrons containing light (u, d, and s) and heavy (c) quarks together with their difference in magnitude are of great interest and allow one to probe the three-dimensional space–time evolution of the matter produced in a heavy-ion collision. Ultra-relativistic heavy-ion collisions are also characterized by extremely strong electromagnetic fields primarily induced by the spectator protons, which do not undergo inelastic collisions. There is a strong interest in measuring and understanding the time evolution of these fields, which are estimated to reach 1018 – 1019 Gauss in the very early stages (< 0.5 fm) of Pb–Pb collisions at LHC energies [27, 28]. Several phenomena are predicted to occur in the presence of this strong electromagnetic field, such as the chiral magnetic effect (CME), which is driven by the generation of an electric current along the magnetic field in a medium with chiral imbalance [29–32]. While the experimental results for charge-dependent correlations are in qualitative agreement with theoretical expectations for the CME [33–35], the possible background contributions, such as effects of local charge conservation coupled with the anisotropic flow, prevent their unambiguous interpretation [36] and have led to upper limits on the CME a LHC energies. Thus it is fundamental to use other observables with direct sensitivity to the electromagnetic fields in order to constrain their magnitudes and time evolution in heavy-ion collisions. The charge dependence of the directed flow of the produced particles relative to the collision-spectator plane is directly sensitive to the presence of the electromagnetic fields. The spectator plane is defined by the deflection direction of the collision spectators. On average its orientation is perpendicular to the direction of the magnetic field generated by the positively charged spectators. In the center-of-mass system of two colliding nuclei the charge dependence of the directed flow comes from two competing effects. The first one is the Lorentz force experienced by a charged particle propagating in the magnetic field. The second one, opposite to the first, is generated by the electric field induced by the rapidly decreasing magnetic field. In an electrically conducting plasma, the induced electric field creates charged currents that might greatly slow down the decay of the magnetic field [27]. The measurement of chargedependent directed flow may also provide constraints to the QGP electric conductivity.