RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-4. Hadron Physics

Study of direct photon polarization to shed on strong magnetic fieldin heavy ion collisions at PHENIX†
T. Hoshino∗1,∗2A strong magnetic ﬁeld is expected to be createdin non-central heavy-ion collisions. This ﬁeld is created by the eﬀect of both of the collision participants
and spectators. The ﬁeld direction is perpendicular
to the collision reaction plane. The magnitude of theﬁeld created in the collisions at BNL-RHIC top energy is estimated to reach as high as 1014 teslas. Ifthe magnitude of the magnetic ﬁeld in heavy-ion collisions is compared with the QED critical magnetic ﬁeld
(Bc = m2e /e = 4×109 teslas) and the surface of magnetars (1011 teslas), the ﬁeld magnitude is much higher inheavy ion collisions. The ﬁeld magnitude is decreased
rapidly but keeps the magnitude above the critical ﬁeldof an electron for about 3 fm/c.
Creation of a strong magnetic ﬁeld in heavy ion collisions was ﬁrst expected in 19761) . The strong magnetic ﬁeld in heavy ion collisions has recently attracted
the interest of researchers in theories and experiments
in recent years for the following reasons. First, themagnetic ﬁeld can reach beyond the critical ﬁeld andinteresting phenomena such as Chiral magnetic eﬀects
and non-linear QED eﬀects can appear in such a strong
ﬁeld. Second, quark-gluon plasma (QGP) is expected
to be created in a strong magnetic ﬁeld thus the evolution of the plasma may have an eﬀect by the strong
magnetic ﬁeld. The state of the initial stage is another topic of recent interest. Measuring the eﬀect ofthe ﬁeld should be the key to understanding the initialstage.
Chiral magnetic eﬀect is measured by experiments
using charged particle asymmetry with respect to thereaction plane which is reported by STAR and ALICE
experiments2)3) . However the ﬁeld has not yet been
directly detected by experiments. If the eﬀects of themagnetic ﬁeld can be detected, we can directly accessto the initial stage of collisions. Electro-magnetic particles are a clean probe for accessing the initial stage
because they are not aﬀected by the strong interaction.
Dilepton production rate in an external magnetic
ﬁeld is calculated using a photon vacuum polarization
tensor, and the production rate depends on the direction of the ﬁeld. The production rate is higher inperpendicular direction to the ﬁeld than that parallelto the ﬁeld.
Dilepton polarization is also measured to understandthe production mechanism of quarkonia. The PHENIX
experiment measured J/ψ polarization with dielectron4). While the physics motivation and mechanism are
diﬀerent between the measurement of quarkonia and∗1∗2

RIKEN Nishina CenterDepartment of Physical Science, Hiroshima University

Fig. 1. Definitions of polarization axis

virtual photon polarization, we are now using the same
parametrization with that analysis.
dN∝ 1 + λ cos(θ)
d cos(θ)(1)

where θ is the angle between the positive lepton ina dilepton rest frame and a certain polarization axis,and λ is the polarization parameter; if λ is positive(negative), polarization is transverse (longitudinal).
Three deﬁnitions of polarization frames are usuallyused. In the helicity (HX) frame, the axis is the dilepton momentum in the laboratory frame. In the CollinsSoper (CS) frame, the axis is the bisector between two
beams in the dilepton rest frame. In the GottfriedJackson (GJ) frame, the axis is the direction of thebeam in the dilepton rest frame.
We use the data set of Au+Au and p+p at 200
GeV/nucleon pair collected in 2004 and 2006, respectively. The PHENIX experiment has excellent electron
identiﬁcation capabilities and has succeeded in measurement of direct virtual photons.
The direct virtual photon is the probe and π 0 and ηDalitz decay electrons are candidates of control probes
because they are from a later stage of the collisions.
Polarization measurement in p + p collisions can also
act as a control probe since a ﬁeld is not created in thecollisions.
We are measuring polarization with the following
mass regions, 0.12 < Mee < 0.3 (containing virtualphoton) and Mee < 0.12 GeV/c2 (π 0 Dalitz decay
dominant). The acceptance eﬀect is included in thecos(θ) distribution. Thus we need to correct the eﬀect
with simulation. Since the simulation does not containthe eﬀect of the magnetic ﬁeld, we can estimate the effect. We are now estimating the acceptance eﬀect withsingle particle simulations.
References
1) B.Muller et al.: Phys. Rev. Lett. 36, 517 (1976)
2) B. I. Abelev et al., Phy. Rev. Lett. 103, 251601 (2009)3) B. Avelev et at., Phys. Rev. Lett. 110, 012301 (2013)
4) A. Adare et. al., Phys. Rev. D 82, 012001 (2010)- 116 -完全版2014_本文.indd 116

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