Measurement of charged jet spectrain pp and PbPb collisions at 𝑠"" = 5.02 TeV with ALICE
Ritsuya Hosokawa for the ALICE collaboration

LPSC CNRS/IN2P3, Université Grenoble-Alpes
University of Tsukuba, Center for Integrated Research in Fundamental Science and Engineering (CiRfSE)

Abstract

Jets originate from hard scattered partons at the initial stage of collisions. Jets are sensitive probes of the hot and dense medium created in head-on
heavy ion collisions. The jet nuclear modification factor RAA helps quantifying partonic energy loss in central Pb-Pb collisions. For this a precise pp baseline measurement at thesame centre-of-mass energy is crucial. In this contribution, we present charged jet spectra in Pb-Pb collisions at 𝑠"" = 5.02 TeV and charged jet cross section in pp collisions at𝑠 = 5.02 TeV measured with the ALICE detector at the LHC. The pp measurements are compared to pQCD calculations at LO and NLO accuracy. The cone radius dependence ofthe pp cross sections and the dependence on the leading constituent bias of the Pb-Pb spectra are shown. We compare the nuclear modification factor 𝑅** to similarmeasurements at 𝑠"" = 2.76 TeV[1],[2].

Analysis details

Physics motivation• Jets in pp collisions• Good tests of pQCD calculations and MC generators for high energy physics• Provide a references for heavy ion collisions at same collision energy

Event selection• Jets are well established probe for Quark-Gluon Plasma (QGP) properties• Jets are modified while traversing the QGP → Jet quenching effect• QGP properties can be probed by evaluating the quenching effect• QGP lifetime in heavy ion collisions is very short (~ 10-23 s)• Self produced probes, like jets, allow to access QGP properties• Jets are produced at very early stage of collisions• Entire QGP evolution can be probed

Track selection• Jets in heavy ion collisions

Common• pp collisions|ηtrack| < 0.9, pT,track > 0.15 GeV/c•• Anti-kt clustering algorithm[3],[4](Signal jets for pp,Pb-Pb)

Jet cone radii R = 0.2 (pp, Pb-Pb), 0.4 (pp)|ηjet| < 0.9 - R • pT,leading > 5 GeV/c (Pb-Pb)••

Underlying Event(UE)subtraction

Ø Central barrel tracking system (ITS + TPC) for charged particle track reconstruction.
Ø The V0 detector for event trigger and centrality determination. In this analysis,
Minimum bias trigger is determined as V0 AND trigger (V0A AND V0C).

V0

•

ITS

• TPC (Time projection chamber)••••

Filled with Ar-CO2 (9:1)
Max drift time: 92 μs
Acceptance (Full tracking): |η|<0.9•

Consists of 3 type silicondetectors

Results
pp collisions•𝛿𝑝6 = Q 𝑝62=>;R − 𝐴 A 𝜌UDetector effects
correction

Detector & UEeffects correction• Corrected by the SVDunfolding method[5],[6]Jet Spectrum

Detector responses are
extracted fromPYTHIA8[7] + GEANT3[8]MC simulations

Pb-Pb collisions

𝑑𝑁4/211
𝑇-- 𝑁/0/12 𝑑𝑝6 𝑑𝜂
𝑅** =𝑑𝜎99
𝑑𝑝6 𝑑𝜂
H.Yokoyama
H.Yokoyama••

The charged jet cross section
shows good agreement with thePOWHEG NLO-pQCD prediction[9].

The cross section ratio R=0.2 / R = 0.4 is sensitive to the jet
structure and rises as function of jet pT, indicating stronger
collimation at higher pT. The ratio is well described byPYTHIA6[10], PYTHIA8 and POWHEG.

• Strong jet suppression
is observed• Jet suppression strength
increases from peripheral
to central collisions• Jet RAA is comparablewith previous results

H.Yokoyama• At higher collision energies...

Ø Harder collision → jet spectrum becomes flatter → larger RAA
Ø Hotter/denser medium → strong suppression → smaller RAA

Spectrum flattening is compensated by stronger jetsuppression

Summary• First measurements of charged jet spectra and RAA have been performed for LHC Run2 data at ALICE• pp collisions• Inclusive charged jet differential cross section is well described by NLO-pQCD calculation• Jet cross section ratio is well described by POWHEG and PYTHIA• Reference charged jet spectra for Pb-Pb collisions are established (~60 GeV/c)• Pb-Pb collisions

𝐴4/2 : Jet area

*NOMST•

Full tracking w/ TPC: |η|<0.9

𝜌 = 𝑚𝑒𝑑𝑖𝑎𝑛

H.Yokoyama

SPD (Silicon Pixel Detector)
SDD (Silicon Drift Detector)
SSD (Silicon Strip Detector)• Acceptance•LM
9H,IJK

H.Yokoyama

32+32 scintillators
Acceptance:
2.8<η<5.1 (V0A),−3.7<η<−1.7 (V0C)

ITS (Inner Tracking System);<===>?
• 𝑝6,4/2= 𝑝6,4/2- 𝜌 A 𝐴4/2

UE fluctuations are estimatedwith random cone method•

the LHC.
• Key detectors for charged jet measurement•• Pb-Pb collisions• kt clustering algorithm (Background for Pb-Pb)

The ALICE detector is a multipurpose detector dedicated to heavy ion physics at••

Pb-Pb collisions• 25.5 M minimum• 3.36 M minimum
bias events
bias events• Charged tracks by ITS + TPC

Jet reconstruction

ALICE Detector•pp collisions• Charged jet nuclear modification factors (RAA) are estimated•Strong suppression is observed at central collisions•
Comparable with the results in 𝑠"" = 2.76 TeV collisions•
Effect of spectrum flattening is compensated by the stronger jet suppression at higher collision energy

References[1] ATLAS collaboration, Phys.Rev.Lett. 114 (2015) no.7, 072302[2] ALICE collaboration, Phys.Lett. B746 (2015) 1-14
[3] Matteo Cacciari et al. JHEP 0804:063, (2008)[4] Matteo Cacciari et al. Eur. Phys.J. C72 (2012) 1896
[5] Andreas Hoecker, Vakhtang Kartvelishvili, Nucl. Instrum. Meth.
A 372 (1996) 469[6] Tim Adye, arXiv:1105.1160 [physics.data-an], (2011)[7] Torbjorn Sjostrand et al., Comput.Phys.Commun. 191 (2015)
159-177[8] Rene Brun et al. CERN-W5013 (1994)[9] Simone Alioli et al., JHEP 1104 (2011) 081[10] Torbjorn Sjostrand et al., JHEP 0605 (2006) 026

r.hosokawa@cern.ch