Study of di‐jet proper/es in p+p
collisions at √s = 7 TeVby the LHC‐ALICE experiment

Tatsuya Chujo
for the ALICE collabora1on(University of Tsukuba)Outline• Physics mo/va/on• ALICE experiment and data set• First look at (di‐) jet in p+p √s = 7 TeV in ALICE• Dijet Calorimeter project• Summary and outlook

JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

2Why jets?
•••••

Jet is a well deﬁned object, and produced by the hard
scaJng of partons at the ini@al stage of the collision.
Studied in many high energy experiments for many years.
Jet provides a powerful prove to study the hot and dense
QCD maNer created in high energy heavy ion collisions.
At LHC energy, jet produc/on is dominant, compared tothat in RHIC.
Jet measurements in p+p at LHC provide an important
baseline to heavy ion data, as well as the further
understa@ng of QCD.

UA2 (1982)

JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

Delphi (1992)

CDFSTAR3

Jet quenching at RHIC
Phys. Rev. Lett. 91, 072304 (2003).

Pedestal&flow subtracted• Disappearance of away‐side jet in Au+Au
central at RHIC (√sNN = 200 GeV).
• Jet quenching by hot and dense medium.
• indica@ng energy density: ε > 100 ε0• First measurement of full jet
Fall mee@ng (Sep.13, 2010), T. Chujo
reconstruc@on at RHICJPS2010(STAR).

?
4

Jet ID using TPC & ITS in ALICE•ITS (Inner Tracking System), TPC– Charged par@cles reconstructed Δη = 1.8.
– ITS: σrφ ~ few 10 um, σz~few 100 um (SPD,SDD)••••

SPD: Silicon Pixel
SSD: Silicon Strip
SDD: Silicon DribMaterial budget: ~1% X/X0 per layer.

– TPC: Excellent momentum resolu@on.
•~7% at 10 GeV.

Momentum resolu1on (ΔpT/pT)
JPS 2010, Sep. 13, T. Chujo

5Analyzed data sample• p+p at √s = 7.0 TeV, reconstructed data from
LHC10c and LHC10d periods.
• Minimum bias trigger: 128 M events.
– elimina@ng non‐physics events, and requiring z‐vertex< +‐ 10 cm.
– MB trigger: “SPD or V0‐A or V0‐C”• at least one charged par@cle in 8 pseudorapidity units• η cut:– single charged tracks within |η| < 0.9.
– jets |η| < 0.5.
JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

6Jet ﬁnding algorithms
R=2(Δη) + (Δφ )

2

1.

Jet

Cone algorithm:––€•

Simple geometric mo@va@on.
Split/merging procedure foroverlapping cone.
UA1
SIS cone
Seedless Infrared Safe Conealgorithm
insensi@ve to sob radia@on.

•2.

outgoing parton
Hard scatter

Sequen@al recombina@on
algorithm:––KT jetCone jetAn@‐kT jet••

Cluster pairs of objects close inrela@ve pT.
Deﬁne “distance” between pairs.
kT algorithm
Star@ng from low pT par@cle.

an@‐kT algorithm

JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

Star@ng from high pT par@cle.

7Dijet event in p+p 7 TeV inALICEη-φ grid

JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

88

(Raw) single jet spectrum in p+p 7 TeV

Jet can be measured pT ~ 70 GeV/c with current sta/s/cs.
JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

9Di‐jet invariant mass plot

di‐jet mass can be measured pT ~ 50 GeV with current sta/s/cs.
JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

10

Leading and second jet pT raw
spectrum and correla/on (forALICE
performance
Uncorrected
30/08/2010

Leading jet pJPS2010
is harder
thanthat for 2nd jet.
Fall mee@ng(Sep.13, 2010),T. Chujo
T spectrum

ALICE
performance
Uncorrected
30/08/201011

Acoplanarity of jets (Raw)• Azimuthal angle
diﬀerence between 1st(leading) jet and 2nd jet.
• leading jet pT cut: > 10GeV/c• Acoplanarity:
ALICE performanceUncorrected
30/08/2010– φJet1 – φJet2 + π• Next step:– comparison withmodel.
– Comparison to that inheavy ion, which will
start data taking inNov. 2010,Pb+Pb 2.76
TeV.

JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

12

ALICE Dijet Calorimeter (DCal) Project
DCal:• Extend the acceptance ofEMCal (Pb‐Scin1. sampling).

– EMCal: Δφ = 110°
– DCal: Δφ = 60° (on opposite
side of EMCal)– Δη = 0.7 for both EMCal andDCal + PHOS
– ~10%/√E• Allow back‐to‐back hadron‐jet,di‐jet measurements in ALICE,
with R = 0.4, up to pT ~ 150
GeV/c.
• Enhance jet, γ trigger
capability.
–Catania, CERN, Fransca1, Grenoble, INFN,Jyväskylä, Nantes, Stranbourg, Tsukuba,ORNL, LBNL, Yale, Huston,LANL, Wuhan• To be installed in 2012.
JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

13

Summary and outlook• First look at jet and dijet in p+p 7 TeV in ALICE.
• Using the 128 M MB sta@s@cs, ALICE can measure single jet upto pT ~ 70 GeV/c, and dijet mass < 50 GeV/c2.
• Outlook:– Analyze full sta@s@cs data sample (so far, 700 M MB data in p+p 7
TeV as of Sep. 2010).
– Correc@ons to the raw (di‐) jet spectra.
– Acoplanarity:• Model comparison.
• > 2 jets study.

– Prepare for the ﬁrst Pb+Pb run (√sNN = 2.76 TeV, Nov. 2010)!

• Any diﬀerence in φ balance, pT balance in Pb+Pb compared to p+p?

– Use EMCal & PHOS info, to enhance di‐jet measurement.
JPS2010 Fall mee@ng (Sep.13, 2010), T. Chujo

14