Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Study of v2 depending on multiplicity with ZDC energy event
categorization
R. Takahama∗1,∗2 for the PHENIX Collaboration
The PHENIX experiment was operated at BNLRHIC, which collides heavy ions at relativistic energy
to create a hot state in a second. In this state, quarks
and gluons are deconﬁned, and this state is called
quark-gluon plasma (QGP) the collective motion ofQGP is known. In hydrodynamic models, the detailed
azimuthal correlation structure of emitted particles resulting from the collective motion is typically characterized by its Fourier components, known as the elliptic
ﬂow (v2 ).1)
In the past decade, some results have suggested
the possibility of QGP-like matter creation during extremely high multiplicity events in a lighter collisionsystem than that of heavy ions.3–7) However, the mechanism of that possibility has not been revealed. One
helpful fact is that the ridge structure and collectivemotion are measured only in extremely high multiplicity events, where multiparton interactions becomemore relevant.8)
The eﬀect of multiparton interactions in heavy ion
collision is of interest to us. Selecting events with thesame number of participating nucleons in the collision
is crucial to study the multiparton interaction eﬀect
because much more nucleons interact in heavy ion collisions than in proton-proton collisions. We can not directory measure the number of participating nucleons
because they change to new particles after interactions.
However, the number of spectator (not participated incollision) neutrons can be directory detected by thezero degree calorimeter (ZDC). In high-energy heavyion collisions, most of the spectator nucleons outside of
the overlap region are causally disconnected from thematter produced by the participating nucleons, so wemeasure the energy of spectator neutrons captured byZDC. We classify the events with the number of participating nucleons in the collision as in p + p collisionby selecting the events with the same energy measured
by ZDC, thus creating a simple situation to study multiparton collision. Furthermore, the multiplicity has awidth at some ZDC energy sum events. We assumedthat the eﬀect of the multiparton interaction makes
this width. If the multiparton interaction eﬀect is more
relevant, a larger multiplicity should be obtained fromevents with the same number of participating nucleons. Because v2 increases as a function of the numberof tracks in p + p collision,7) v2 will be larger in events
where the multiparton interaction eﬀect is more relevant. Analyzing v2 as a function of multiplicity at
the same energy in the ZDC detector can tell us if the∗1∗2

multiparton interaction eﬀect is worth consideration inheavy ion collision.
Our goal is to analyze v2 as a function of multiplicity using the ZDC energy sum event categorization in√
Au + Au collision data taken at sN N = 200 GeV in
the PHENIX experiment. Such an event categorization has never been used to analyze v2 , so we workedon the reaction plane angle calibration to make its distribution ﬂat. Figure 1 shows the reaction plane angle
distribution after calibration of the one of the eventclasses. A ﬁtting to these distributions with a straightline gave a χ2/ndf of approximately 1.0. We checked
the ﬂatness for all event classes for all runs and ﬁnallyconﬁrmed that the calibration had been done successfully. We are now at the stage of analyzing v2 as afunction of multiplicity using calibrated events. Theresults will be published in the near future.
×χχ−−−−−−

Fig. 1. Reaction plane angle distribution after calibrationin events whose the number of normalized FVTX tracks
is larger than 0.50 and less than 0.55. Left: with noZDCe event selection. Right: events in 900 < ZDCe <910.

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