Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Towards measurment of direct photons via external conversions inhigh multiplicity pp collisions at 13 TeV
H.Murakami∗1,∗2γdir (pT ) = γinc (pT ) − γdec (pT )γdec (pT )) · γinc (pT )= (1 −γinc (pT )(1)= (1 − Rγ−1 (pT )) · γinc (pT ),

(3)

(2)

where the fraction of inclusive photons above decay
photons Rγ (pT ) is called the direct photon fraction.
If Rγ (pT ) is larger than unity, direct photons are produced in the collision. The fraction can be calculated
from the decay photon spectrum. The major sourceof decay photons are π 0 and η , and other sources are∗1∗2

Department of Physics, University of Tokyo
RIKEN Nishina Center- 108 -γγ

calculated by using mT scaling based on these neutralmeson’s pT spectra. Therefore, the determination ofthe pT spectra of neutral meson is very important.
×γγ

A Large Ion Collider Experiment (ALICE) at theCERN, LHC is dedicated to the study of Quark GluonPlasma (QGP), a strongly interacting QCD medium.
The study of the QGP was mainly performed by using relativistic heavy ion collisions. The collective behavior of hadronic particles has been observed in highmultiplicity pp and p-Pb collisions at the LHC.1) Theseobservations could be indicating the formation of thermalized systems such as the QGP, even in small systems.2) A direct photon is an ideal probe to study suchthermalized systems due to its penetrating nature. In
photons produced
this study, we aim to measure direct √in high multiplicity pp collisions at s =13 TeV.
In the ALICE experiment, we measure direct photons in two ways, via a calorimeter or via the measurement of photon conversions (external conversions).
The latter allows us to measure very low transverse momentum pT photons. The main tracking system3) ofthe ALICE detector consists of the Inner Tracking System (ITS), the Time Projection Chamber (TPC), andthe Time of Flight (TOF). Direct photons will convert
into dielectrons in the material of the ITS and TPC,
and leave two tracks. For multiplicity estimation, V0detectors, which are made of two arrays of scintillationcounters are used and they are set on both sides of theALICE interaction point.
Photons are produced in several stages during thetime-space evolution of the QGP and there are mainly
two groups: direct photons and decay photons. The
former originates from quark-anti-quark annihilationand the QCD Compton scattering, and the lattercomes from particle decay such as π0 → γγ. It is diﬃcult to measure direct photons separately, since there
is huge background from decay photons. Therefore,
we measure all the emitted photons(inclusive photons)
and decay photons. The invariant yield of direct photons γdir (pT ) can be expressed in terms of the inclusive
photons γinc (pT ) and that of decay photons γdec (pT ) :π → γ γγγγ

Fig. 1. Invariant mass distribution of reconstructed photon
pairs Mγγ

In this paper, we report the status of neutral meson
analysis. In 2015, ALICE successfully collected over√
400M events which correspond to 0.007 pb−1 in s=13 TeV pp collisions with minimum bias (MB) trigger and we used this data sample. The ﬁrst step of theanalysis is to ﬁnd photons with the V 0 reconstruction
method. The V 0 is the secondary vertex from unknown
particle decays such as Λ0 , Λ0 , Ks0 , γ. In order to select pure photon samples from V 0 particles, several
cuts such as particle identiﬁcation cuts, a 2D cut for
dielectrons and a cut using the Armenteros-Podolanskiplot4) were applied. Figure 1 shows the invariant mass
spectrum of reconstructed photon candidates which
are from external conversion pairs. The blue line is thecombinatorial background(BG), which is calculated using the event mixing technique, in which photons aretaken from diﬀerent events and combined to form pairs.
On top of the BG, a clear peak can be seen for π 0 mass
0.135 GeV/c2 . Subtraction of BG and evaluation of
the invariant raw yield of π 0 are work in progress. Not
only the raw yield of π 0 , but also that of η will be calculated and eventually, we will evaluate the invariant
cross section. Moreover, we will carry out an analysis
focused on high multiplicity events.
References1) The CMS collaboration, J. High Energy Phys. 09 (2010)091.
2) C.Shen et al, arXiv:nucl-th/1504.07989v1(2015).
3) The ALICE Collaboration, JINST 3, S08002 (2008).
4) J. Podolansky and R. Armenteros. Phil. Mag. 45(13),1954.