Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Measurement of high-pT neutral mesons with a high-energy photon
trigger at ALICE
S. Yano∗1 and K. Shigaki∗1with√ the PHOS trigger (closed circles) in pp collisions
at s = 8 TeV. Only the statistical errors are shownin Fig. 2. Up to 40 GeV/c, neutral pions can be measured with PHOS trigger. No other experiments havesuccessfully measured neutral pions up to 40 GeV/c.
At the low-pT region, MB- and PHOS-trigger results
are consistent with each other within statiscical errors.

∗1Efficiency

ALICE, one of the experiments at the Large Hadron
Collider (LHC) at CERN, is aimed at studying heavyion collisions and the properties of a deconﬁned stateof matter, the quark-gluon plasma (QGP)1) . High-pT
particle production is a powerful tool for characterizing
the QGP because the interaction of its fast partonsdepends on the QGP transport properties. The hadronyields in heavy-ion collisions can be quantiﬁed by thenuclear modiﬁcation factor (RAA ), which is the ratioof the particle yield in heavy-ion collisions normalizedby the number of inelastic nucleon–nucleon collisions
to the yield in pp collisions. Previous experiments haveshown that RAA at high pT is signiﬁcantly smaller than1, which can be explained by the energy loss of fast
partons traversing in QGP.
The ALICE experiment has a high-resolution andhigh-granularity electromagnetic calorimeter calledPHOS1) . One of the main achievable physics goals
by PHOS is the study of the energy loss through themeasurement of high-pT neutral mesons (π 0 and η).
Three PHOS modules are installed in the ALICE experiment, which covers azimuthal angles in the range
260◦ < ϕ < 320◦ and pseudorapidity |η| < 0.125.
PHOS provides a photon trigger (PHOS trigger) byrequiring the measured energy to be above a threshold. The threshold
was set to be 2 and 4 GeV in pp√collisions at s = 8 TeV and 7 GeV in p-Pb collisions√at sNN = 5.02 TeV. By using the PHOS trigger, highpT neutral mesons can be eﬃciently measured in theALICE experiment. This paper discribes the analysis
status of neutral-pion production measured with thePHOS trigger and minimum-bias (MB) trigger data in
pp collisions.
In this analysis, 0.3nb−1 MB-trigger data
√ and70nb−1 PHOS-triggered data in pp collisions at s = 8TeV are used. The PHOS-trigger eﬃciency as a function of measured photon energy is evaluated with realMB-trigger data. By using this eﬃciency, the eﬃciency
for a parent particle is estimated through a simulation.
For instance,√ neutral-pion trigger eﬃciency in pp collisions at s = 8 TeV is shown in Fig. 1.
The rejection factor of the PHOS trigger for the MBtrigger data, R, was determined with real data in thisanalysis. R corresponds to the number of MB-trigger
events inspected while one PHOS trigger is issued. Itis 150 for 2-GeV threshold in pp at 8 TeV, 4800 for
4-GeV threshold in pp at 8 TeV, and 6500 for 7-GeV
threshold in p-Pb at 5.02 TeV.
Fig. 2 shows the invariant raw yield of neutral pions
measured with the MB trigger (open circles) and that

1.2

π0 trigger efficiency (Threshold: 4 GeV)Work in progress

1Single π0 simulation

0.8

DATA (AnyINT)

0.6

0.40.200510152025303540p (GeV/c)
T

1/2πp 1/Nevtd2N/dp dηFig. 1. Neutral-pion trigger eﬃciency.

TWork in progress

TDATA INT7 s=8 TeVDATA PHI7 s=8 TeV
pp Tsalis s=8 TeV

0510152025303540pT (GeV/c)

Fig. 2. Invariant raw yield of neutral pions in pp collisions
at 8 TeV.

In summary, we began investigating PHOS-triggereddata and attempting to extract the invariant yield of
neutral mesons for the PHOS-triggered data. The ﬁnal result of the invariant yield up to 40 GeV/c in pp
collisions is expected to be obtained shortly. Furthermore, 1.6nb−1 PHOS-triggered data are recorded in√
p-Pb collisions at sN N = 5.02 TeV. Neutral pions
above 40 GeV/c can be measured with this data set.
The analysis of PHOS-trigger data will extend our understanding of high-pT particle production beyond thepreviounsly published result2) of MB-tigger data. InPb-Pb collisions, neutral pions near 40 GeV/c can bemeasured with data taken in 2011. By analyzing thisdata set, the extraction of RAA for single particles up
to 40 GeV/c will be possible, which is one of the futureplans of our data analysis.
References
1) The ALICE Collaboration, JINST 3, S08002 (2008).
2) The ALICE colaboration, Phys.Lett.B 717 (2012), pp.
162 - 172.

Department of Physics Science, Hiroshima University- 99 -