Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 48 (2015)

Measurement of high-pT neutral mesons with a high-energy photon
trigger in ALICE
S. Yano∗1,∗2 and K. Shigaki∗10E0d3 σ pp→π X
11 Nπ
1 1 σMB=(1)3dp
2π pT Nevt ε × Acc BR ∆y∆pT

Here σM B is the cross-section of minimum-bias (MB),0N π is the number of reconstructed neutral pions, andAcc is the PHOS acceptance correction. Nevt is thenumber of analyzed events in the minimum-bias trigger data analysis. In case of the PHOS trigger dataanalysis, it is the number of analyzed PHOS trigger
events times the rejection factor. In the minimum-bias
trigger data analysis, ε indicates the reconstruction ef∗1
∗2

Department of Physics Science, Hiroshima University
RIKEN Nishina Center

ﬁciency, whereas in the PHOS trigger data analysis, it
indicates the reconstruction eﬃciency times the trigger
eﬃciency for a neutral-pion meson.
The invariant cross-section of a neutral pion in pp
collisions at LHC energies are shown in Fig. 1. As
shown in Fig. 1, the 0.9, 2.76 and 7 TeV analyses
used only minimum-bias (MB) trigger data, but the 8TeV analysis (this analysis) used not only minimumbias but also PHOS trigger data. The very-low pT(∼ 1GeV/c) region cross-section was measured by thePhoton Conversion Method (PCM)2) with the tracking detectors of the central burrel1) . For the 8 TeV
result, the minimum-bias and PHOS trigger data werecombined to measure the very wide pT range neutralpion meson. The dashed line denotes the result ﬁtted
to Tsalis functions.
Ed3σ/dp3 (pb GeV-2/c3)

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-pTparticle production is a powerful tool for characterizing
the QGP because the interaction between its fast partons depends on the QGP transport properties. Thehadron yields in heavy-ion collisions can be quantiﬁed by the nuclear modiﬁcation factor (RAA ), whichis the ratio of the particle yield in heavy-ion collisions
normalized by the number of inelastic nucleon–nucleon
collisions to the yield in pp collisions. Previous experiments have shown that RAA at high pT is signiﬁcantly
smaller than 1, which can be explained by the energy
loss of fast partons traversing in QGP.
The ALICE experiment includes a high-resolution
and high-granularity electromagnetic calorimeter calledPHOS1) . One of the main physics goals achievable
by PHOS is the study of 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) by requiring the measured energy to be above a threshold.
The threshold
was set to be 2 and 4 GeV in pp colli√
sions at s = 8 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 inpp collisions.√ Further, neutral-pion production in pp
collisions at s = 8 TeV are compared with results forother LHC energies (0.9, 2.76 and 7 TeV)2)3) . In thisanalysis, 0.3nb−1 and 70nb−1 MB-trigger and√ PHOStrigger data respectively, in pp collisions at s = 8TeV
are used.
The invariant cross-section can be calculated as

pp s = 8 TeV
pp s = 7 TeV × 10-1
pp s = 2.76 TeV × 10-2-3

pp s = 900 GeV × 10110pT (GeV/c)

Fig. 1. Neutral-pion cross section in pp collisions at LHCenergies.

By comparing the results for diﬀerent LHC energies, pertubative Quantum Chromodynamics (pQCD)
can be tested at LHC energies and the fragmentation
function (FF) of gluon to the neutral-pion model canbe restricted. Further, this comparison can help predict a neutral-pion production cross-section in pp collisions at other energies. This has not been measured
yet and more precise references to study phenomena
induced by AA collisions can be estimated for QGP
study.
References
1) The ALICE Collaboration, JINST 3, S08002 (2008).
2) The ALICE colaboration, Phys.Lett.B 717 (2012), pp.
162 - 172.
3) The ALICE colaboration, Eur. Phys. J. C (2014) 743108.

- 13 -

完全版2014_本文.indd 13

15/10/16 17:39