Description
----------
Talk title: Observation of the Breit-Wheeler Process in Heavy-Ion Collisions
----------
Speaker : Dr. Daniel Brandenburg (Brookhaven National Lab)
----------
Abstract:
Ultra-relativistic heavy ion collisions are expected to produce some of the
strongest magnetic fields ($10^{13}-10^{16}$ Tesla) in the Universe[1].
Recently, there has been increased interest in the magnetic fields produced
by heavy ion collisions and their possible observational impacts through
emergent magnetohydrodynamical phenomena in Quantum Chromodynamics, like
the Chiral Magnetic Effect[2]. The initial strong electromagnetic fields
produced in heavy ion collisions have been proposed as a source of
linearly-polarized, quasi-real photons[3] that can interact via the
Breit-Wheeler process to produce $e^+ e^-$ pairs[4].
In this talk I present STAR measurements of $e^+ e^{-}$ pair production in
ultra-peripheral and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200
GeV. A comprehensive study of the pair kinematics is presented to
distinguish the $\gamma\gamma \rightarrow e^+ e^-$ process from other
possible production mechanisms.
Furthermore, the measured distribution of $e^+e^-$ pairs reveals a
striking fourth-order angular modulation which is related to vacuum
birefringence[5], a phenomenon predicted in 1936 in which empty space can
split light according to its polarization components when subjected to a
strong magnetic field. These measurements provide the first direct
experimental evidence that ultra-relativistic heavy ion collisions are
capable of producing the strongest magnetic fields in the known Universe
over an extended spatial distribution.
[1] V. Skokov, A. Illarionov, and V. Toneev. International Journal of
Modern Physics A 24 (2009): 5925–32.
[2] Kharzeev, D. E., et al. Prog. Part. Nucl. Phys., 88 (2016)1–28
[3] C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25.
[4] G. Breit and J. A. Wheeler. Physical Review 46 (1934): 1087
[5] Heisenberg, W., and H. Euler. Zeitschrift für Physik, (1936) arXiv:
physics/0605038
----------
Self-introduction: My name is Daniel Brandenburg. I graduated from Rice University in 2018. Asa PhD student I worked on measurements of jet quenching signatures as partof the Beam Energy Scan phase I and performed the first STAR measurement ofdimuon production. I am currently a Goldhaber Fellow at BrookhavenNational Laboratory (BNL). Before that I was in a joint Postdoc positionbetween BNL and SDU (2018 - 2020). While at BNL I have taken a leadershipposition as the software coordinator in STAR's ongoing forward rapidityupgrade program. My physics interests are focussed on ultra-peripheralcollisions and the ongoing Beam Energy Scan phase 2 and the opportunitiesto study the possible QCD critical point.
----------
