Observation of Centrality-Dependent Acoplanarity for Muon Pairs Produced via Two-Photon Scattering in Pb+Pb Collisions at sNN

Abstract

This Letter presents a measurement of γγ → μ þ μ − production in Pb þ Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at ffiffiffiffiffiffiffi ffi s NN p ¼ 5.02 TeV with an integrated luminosity of 0.49 nb −1. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from γγ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.

Dielectron production at low transverse momentum in Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}}=5.02 $ TeV with ALICE

“…The STAR collaboration found an excess with respect to the hadronic expectation of low-p T,ee dielectrons in Au-Au and U-U collisions, which is compatible with calculations for coherent photo-production [1][2][3]6] and which cannot be linked to hadronic sources [7]. The ATLAS collaboration observes photoproduced dimuons in Pb-Pb collisions, which exhibit a centrality dependent broadening effect of their angular pair correlations that seems not to be present in UPC [8,9]. In the following, the first dedicated measurement of low-mass dielectron production at low p T,ee in Pb-Pb collisions at √ s NN = 5.02 TeV is reported and compared to various models of dielectron photo-production.…”

Experimental searches for the chiral magnetic effect in heavy-ion collisions

“…Other physical reasons are nevertheless also possible. For example, ATLAS has measured an angular broadening of the back-to-back muon pairs and attributed their observation to Coulomb scatterings in the QGP medium [87]. Calculations confirm that the acoplanarity of dilepton pairs can be attributed to QED radiation in the QGP [88].…”

Probing the linear polarization of photons in ultraperipheral heavy ion collisions

“…It is not surprising to find that they are identical to each other in this approximation, just like the relation established between the dipole amplitude and the polarized gluon distributions [3,[28][29][30]. In the present paper, we propose to test this theoretical predication by measuring cos 2φ and cos 4φ asymmetries in di-lepton production induced by the linearly polarized photon distribution.Recently, the STAR collaboration at RHIC [27] and the ATLAS collaboration [26] at LHC have carried out the measurements of transverse momentum spectra of lepton pairs for various invariant mass regions with high precision. The significant q ⊥ (total transverse momentum of lepton pair) broadening effect found in hadronic heavy-ion collisions in comparison to those in UPCs has stimulated a lot of theoretical progress [20][21][22][23][24], as the transverse momentum broadening effect plays a crucial role in understanding the properties of the hot medium created in heavy-ion collisions.…”