Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Search for the strong magnetic field via di-electron measurement
in heavy-ion collisions at RHIC-PHENIX
T. Hoshino∗1,∗2 for the PHENIX collaboration
A strong magnetic ﬁeld is expected to be createdin high-energy heavy-ion collisions. The intensity ofthe magnetic ﬁeld created in the collisions at BNLRHIC is estimated to reach about 1014 teslas. Theﬁeld creation can be considered to be due to the eﬀectof both of collision participants and collision spectatorsand the ﬁeld direction is perpendicular to the reaction
plane (Fig. 1).
The possibility of ﬁeld creation was presented ﬁrst
about 35 years ago1) . In recent years, this began toattract attention because achieving an increase in theﬁeld intensity with increasing energy of the collider iswell beyond the critical ﬁeld of an electron (eBc = m2e ).
The time evolution of this ﬁeld is also calculated basedon theories. According to the theoretical calculation,
the ﬁeld intensity decreases rapidly, but maintains fora few fm/c above the critical magnetic ﬁeld of an electron2) . Chiral magnetic eﬀects and other interesting
eﬀects, such as non-linear QED eﬀects, are discussed
based on the theories to be caused by the strong ﬁeld.
From experimental studies, charged particle asymmetry with respect to the reaction plane3) and directphoton azimuthal anisotropy4) suggest the presence ofthe strong ﬁeld. However, the ﬁeld itself is yet to be
directly detected experimentally.
Direct detection of the ﬁeld in high-energy nuclear
collisions is a very important issue. Detection of thestrongest ﬁeld in the universe has a major impact initself. Further, the observation of the ﬁeld leads to theconﬁrmation of the chiral magnetic eﬀect. Moreover,there is a possibility to verify non-linear quantum electrodynamics eﬀects such as vacuum-birefringence andthe decay of real photons.
Direct photons/virtual photons are good candidates
for probing the ﬁeld detection, because they are not affected by the strong interaction and they maintain theinitial information. Pi0 decay photons/Dalitz-decay
electron pairs are candidates for control probes because
they are from the later time. Combinatorial pairs frommixed events could also be used as a control probe.
According to the calculation of photon vacuum polarization in a strong magnetic ﬁeld, the production
rate of di-electrons from virtual photon decay depends
on the ﬁeld direction5) . Since PHENIX has a goodelectron-identiﬁcation capability, we focus on virtualphoton decays with a dependence on the magnetic ﬁeld
direction. By using electron pairs from virtual-photon
decays as the probe, polarization measurement is possible without the using of a polarimeter.
∗1∗2Fig. 1. Schematic image of the magnetic field creation inheavy-ion collision√We are using the sN N = 200 GeV Au+Au collisions data set collected in 2004 at the RHIC-PHENIX.
Electrons and positrons are identiﬁed by RICH andEMCal, and momentum is decided by Drift Chamberand Pad Chamber. Global variables, z-vertex, centrality and reaction plane, are decided by Beam-BeamCounter.
We select two di-electron invariant-mass region,
0.12 < mee < 0.3 GeV/c2 and 0 < mee < 0.1 GeV/c2 .
The ﬁrst region contains virtual photon components,and thus, this region is expected to contain the signal. The second is dominant Dalitz-decay di-electron,
which has no-physics eﬀect to polarization. The polarization is measured using the angular distributionsof di-electron with a correlation to the reaction plane,
as a function of centrality, because it depends on thestrength of the created ﬁeld. Now, we focus on polarizations of Dalitz-dacay pairs and combinatorial pairs,
which is important for background subtraction. We
discuss what kind of background is included, how tosubtract backgrounds, and how to extract the signal.
References
1) B.Muller et al.: Phys. Rev. Lett. 36, 517 (1976)
2) D.E.Kharzeev et al.: Nucl. Phys. A 803, 227 (2008)3) B.I.Abelev et al. (STAR): Phys. Rev. Lett. 103, 251601(2009)
4) A.Adare et al. (PHENIX): Phys. Rev. Lett. 109, 122302(2012)
5) K.Ishikawa et al.: Int. J. Mod. Phys. A28, 1350100(2013)RIKEN Nishina Center
Department of Physical Science, Hiroshima University- 80 -