Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Azimuthal distributions of jets with respect to high-pT neutral pion
triggers in pp collisions from ALICE
D. Watanabe∗1,∗2 and T.Chujo∗2
Jet measurements play a critical role in probing thehot and dense QCD medium created in heavy-ion collisions. Detailed transport properties of the medium
can be extracted through measurements of parton energy loss and medium response with respect to the lostenergy.

and Inner-Tracking System (ITS).
This analysis used the shower shape and cluster
splitting method4) to identify high pT π 0 . With thismethod, high pT π 0 around 40 GeV/c can be identiﬁed
with the signal-to-noise ratio of 90%.

In general, the energy loss of recoil jets and leading
jets depends on the path length in the medium (i.e,creation point and moving direction in the medium).
For example, jet pairs with a large energy asymmetry in the ﬁnal states can be from the surface of themedium,as shown in Fig.1. While leading jets escapethe medium from the surface recoil jets traverse in themedium with loss to its energy. We use this surface
bias to acquire deeper insight into the medium properties: The stronger the surface bias, the greater is thepath length in the dense medium of the recoiling jet atthe opposite azimuth. By measuring the full jets in therecoil side rather than measuring high-pT hadrons, we
can perform a more comprehensive and direct study of
jet interactions in medium.1)

Fig.2 shows the azimuthal angular correlation between π 0 and jet in pp collisions, where π 0 pT is from
8 to 12 GeV/c and the associated jet pT is higher than10 GeV/c. From this distribution, two main observables will be discussed as the functions of trigger andassociated pT . One is the away-side yield per trigger yield, and the other is the width of the near-side
and away-side correlation. These quantities will be discussed in the future, and similar analysis will be done
for p-Pb and Pb-Pb collisions in order to understandthe ordinal nuclear matter eﬀects and the propertiesof the hot and dense medium.

Fig. 1. Simple geometry of hadron-jet correlation with theleading particle in a recoil jet momentum thresholdpT > 0.5 (GeV/c)2)

Fig. 2. π 0 -jet azimuthal correlation with trigger pT regionof 8-12 GeV/c and associated jet threshold pT,jet > 10GeV/c

In this paper, we report the jet azimuthal distribu√
tion with neutral pion triggers in pp collisions at s
= 7 TeV from LHC-ALICE, which is very important
as a baseline study for heavy-ion collisions. The ALICE detector was built as a general-purpose detector
for measurements of ultra relativistic heavy ion collision at the LHC.3) For neutral pion identiﬁcation, anelectromagnetic calorimeter (EMCAL) is used. Jets
are measured by a Time-Projection Chamber (TPC)∗1 RIKEN Nishina Center∗2 University of Tsukuba

References
1) ALICE collaboration, An Addendum to the EMCal
Technical Design Report (DCAL), CERN-LHCC-2010011
2) STAR collaboration, M. Ploskon et al., Nucl. Phys.
A830, 255c
3) ALICE collaboration, Performance of the ALICE Experiment at the CERN LHC, arXiv:1402.4476v2
4) ALICE collaboration, X. ZHU, Analysis of the π 0 charged hadron correlations using ALICE EMCAL,
https://indico.cern.ch/event/181055/session/37/contribution/451- 100 -