II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Analysis of Quark-Gluon plasma properties based on jets with ALICE
experimental data
T. Kumaoka,∗1,∗2 for the ALICE Collabolation
Quark-Gluon plasma (QGP) is deﬁned as a matter
phase at high temperatures and high densities. It is predicted using quantum chromodynamics. A few QGP evidences have been discovered through the LHC and RHIC
experiments. However, QGP features are not revealed,and various measurements are performed to clarify QGP
properties. Among these properties, the stopping power
and jet quenching mechanism can be clariﬁed by measuring the nuclear modiﬁcation factor (RAA ) in a wide
jet transverse momentum (pT ) range. RAA is the ratioof the jet yield in nuclear-nuclear collisions to that in
p-p collisions (Fig. 1). When RAA is less than 1, it implies that jets are suppressed by the QGP created owing
to nuclear collisions. Measured distributions can restrict
theoretical models and clarify the jet quenching mechanism.
In this study, we measure RAA in Pb-Pb collisions at√sNN = 5.02 TeV by using the ALICE experimental dataof 2018. This data set possesses an integrated luminosity of 0.4 nb−1 , which is larger compared to the previousstudy.1) The data are expected to include rare high-pT
jets than the previous measurements, extending the RAA
measurements to the higher pT region.a) Moreover, thelarge data statistics enables the study on RAA at various centralities (Fig. 2), which were limited owing to thelarge number of background particles along the low pT
direction in the case of the previous study. As peripheral
collisions provide a smaller background than central collisions, studies in the lower pT region become possible.
This study clariﬁes the correlation between the partonenergy suppression and centrality.
Data quality requirements were applied to the data,
and those marked as good demonstrate three times larger
statistics than the previous study.1) Jets were recon-Fig. 1. RAA deﬁnition. The numerator and denominatordemonstrate the jet yields in heavy ion collisions and p-p.
∗1∗2a)

Department of Physics, University of Tsukuba
RIKEN Nishina CenterIn this measurement,
we will use the jet pT spectrum in p-p√
collisions at s = 5.02 TeV, which derived from the ALICE
experiment conducted in 2018, as the denominator of RAA .

Fig. 2. Collision centrality. The overlap of heavy ions is
known as centrality. In other words, centrality is deﬁned
as the percentile of the hadronic cross section corresponding to particle multiplicity, considering the count from themost central collisions.

structed using the anti-kT algorithm2) of the fast-jetpackage3) without any modiﬁcation. The expected contribution to the jet pT based on background particles wasestimated for each jet on an event-by-event basis. By
subtracting the background, we obtained jet pT spectra
for various centralities. Figure 3 shows the normalized
jet pT spectrum for various centralities. Moreover, it
shows that jets are more suppressed in the central collisions compared with that in the peripheral collisions.

Fig. 3. Jet pT spectrum for each centrality. The horizontal
axis denotes jet pT and the vertical axis denotes jet yields
normalized by the pseudorapidity range and number of binary collisions. In the previous study, RAA was measured
in the area between the two dashed lines.

References
1) S. Acharya et al., Phys. Rev. C 101, 034911 (2020).
2) M. Cacciari et al., J. High Energy Phys. 4, 063 (2008).
3) M. Cacciari et al., Eur. Phys. J. C 72, 1896 (2012).

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