ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Jet physics in heavy ion collisions with
Compact Muon Solenoid at the LHC
I.Lokhtin
for the CMS Collaboration
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Motivation to study jets in heavy ion collisions at LHC
CMS detector
Survey of physics channels for CMS jet analysis
Jet and high momentum hadron reconstruction in CMS
Monte-Carlo tools to simulate jet quenching
Examples of jet observables
Summary and outlook
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Motivation to study jets in heavy ion
collisions at the LHC
Detection and studies of properties of nuclear matter under
extreme conditions (energy density, temperature, pressure)
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CERN-SPS – first indication of QGP formation (J/ψ suppression, ...)
BNL-RHIC – observation of new phenomena strongly supporting idea of
QGP formation (high-pT hadron suppression, azimuthal anisotropy)
CERN-LHC – new regime of HI physics where hard and semi-hard QCD
production dominates over underlying soft events and probes hot and long
lived QGP: complementary measurements from ALICE & CMS
ALICE: most of “soft probes”+ selected aspects of “hard probes”
(low-pT particle tracking & ID, forward muons (ψ, ), photon multiplicity,...)
CMS/ATLAS: most of “hard probes”+ selected aspects of “soft probes”
(high-pT particle tracking, central  (ψ, , Z), jets with calorimetry & tracker,...)
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Phenomena expected to probe dense
matter in HIC at the LHC
Jet quenching (medium-induced partonic energy loss)
quenching of total rates of hard jets and high-pT particles
● medium-modified jet fragmentation with leading particles
● p -imbalance in +jet and */Z+jet production processes
T
● medium-modified spectra of high-mass dimuons and B→J/ψ
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Quarkonium dissociation in dense matter
medium-modified spectra of J/ψ and ψ '
● melting of , ' and ''
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Collective behaviour of nuclear matter
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radial and elliptic flow
Exotic events
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Strangelets, Centauros, DCCs
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Jet quenching in heavy ion collisions
∆E ∝T03 (temperature), g (number degrees of freedom) ⇒ ∆E|QGP >> ∆E|HG
LHC, central Pb+Pb:
T0, QGP ~ 1 GeV >> T0, HGmax ~ 0.2 GeV,
gQGP > gHG
⇓
∆EQGP / ∆EHG ≥ (1 GeV / 0.2 GeV)3 ~ 102
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
CMS detector
Si tracker with pixels ||<2.4
good efficiency and low fake
rates for pt > 1GeV,
excellent momentum
resolution, p/p~1%
Muon chambers ||<2.4
Fine grained high resolution
calorimetry (HCAL, ECAL,
HF) with hermetic coverage
up to ||<5
CASTOR (5.3<||<6.9)
ZDC(z=∓140 m)
B=4 T
Fully functional at highest
multiplicities; high rate
capability for (pp, pA, AA),
HLT capable of selecting
HI events in real time
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Physics channels for jet studies in
CMS
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high-pt QCD jets (HCAL+ECAL, TK)
● jets tagged by leading h±, 0 (HCAL+ECAL, TK)
● B-jets tagged by leading  (HCAL+ECAL, TK, MU)
● +jet, */Z(→+-)+jet (HCAL+ECAL, TK, MU)
● BB, DD → +- (TK, MU)
● particle spectra (TK)
● energy flow (HCAL+ECAL, HF, CASTOR)
Requires adequate reconstruction:
jets
● /0
● charged particle tracking in jets
● muons
● event centrality
● event plane
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I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Jet cross section & expected event rate
Jet cross section
Expected statistics for CMS acceptance
A.Accardi, N. Armesto, I. Lokhtin,
hep-ph/02111314
(no trigger and reconstruction efficiency)
|ηjet,γ| < 3, |ηh,µ| < 2.4
t = 1.2 × 106 s,
Channel
σAA = A2σpp, A = Pb
(Pythia6.2, CTEQ5M)
jet+jet, ETjet > 100 GeV
jet tagged by h±,π0,
ETjet > 100 GeV, zh±,π0 > 0.5
4 × 106
2 × 105
B-jet tagged by µ,
ETjet > 100 GeV, zµ > 0.3
700
ETjet > 50 GeV, zµ > 0.3
2 × 104
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Reconstruction: jets with calorimetry
The algorithm is based on event-by-event η-dependent background subtraction
PYTHIA (100 GeV jet) + HIJING (Pb+Pb, dN± /dη = 5000) + full GEANT
I. Vardanyan
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Reconstruction: jets with calorimetry
measured energy
efficiency, purity
energy resolution
ϕ-resolution
I. Vardanyan
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
η-resolution
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Charged particle tracking
The track finder is based on Kalman Filtering and includes seed generation,
track propagation trajectory updating and smoothing +
primary vertex finding and restriction of the vertex region for HI collisions
Efficiency
∆pT/pT < 1%
|η| < 0.7
Fakes
High efficiency and low fake rate even at high track density and good resolution;
further improvement is using tracker information for jet energy correction
C.Roland
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Jet fragmentation with tracker
Longitudinal momentum fraction z
along the thrust jet axis
C.Roland
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
Transverse momentum relative
to thrust jet axis
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Reconstruction: event centrality
Based on correlation between energy deposition and impact parameter
HIJING (generator level, acceptance of HF and CASTOR)
C.Teplov
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Reconstruction of event plane
The algorithm is based on event-by-event calculation of anisotropic energy flow
HYDRO (Pb+Pb, b=6 fm) + full GEANT (dominant role of endcaps)
I.Lokhtin, S.Petrushanko, L.Sarycheva, A.Snigirev, CMS Note 2003/019
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Monte-Carlo HI simulatons at LHC
There are two kind of MC HI simulations in HIC at LHC:
1. “Hard probes” signal event (jets, quarkonia, heavy quarks, Z) is
generated with standard pp generator (PYTHIA,...) and
superimposed on background of full heavy ion event
2. Global observables (particle spectra) and multiplicity background
for “hard probes” are generated with a heavy ion event generator
(HIJING,...)
Problem: In most existing HI event generators such important effects as jet
quenching and elliptic flow are not included or implemented poorly
Solution: Develop MC tools for adequate, fast simulation of physics
phenomena
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Fast MC tools to simulate jet
quenching and flow effects
PYQUEN - fast code to simulate jet quenching (modify
PYTHIA6.2 jet event), http://cern.ch/lokhtin/pyquen
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HYDRO - fast code to simulate transverse and elliptic
flow in central and semi-central AA collisions at LHC,
http://cern.ch/lokhtin/hydro
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HYDJET - merging HYDRO (flow effects), PYTHIA
(hard jet production) and PYQUEN (jet quenching)
http://cern.ch/lokhtin/hydro/hydjet.html
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I. Lokhtin, A. Snigirev
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
PYQUEN (PYthia QUENched)
Initial parton configuration
PYTHIA6.2 w/o hadronization: mstj(1)=0
↓
Hard parton rescattering and energy loss + emitted gluons
PYQUEN (input: beam atomic number and centrality)
↓
Parton hadronization and final particle formation
PYTHIA6.2 with hadronization: mstj(1)=1, call PYEXEC
More details on internal PYQUEN physics sets can be found in:
I.Lokhtin, A.Snigirev, “Nuclear geometry of jet quenching”, EPJ C16 (2000) 527;
“Fast simulation of jet quenching...”, hep-ph/0406038; and references therein.
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
PYQUEN: medium-modified jet
fragmentation function
Longitudinal jet profile
Transverse jet profile
Pb+Pb (b=0), √s=5.5A TeV, ETjet > 100 GeV
Significant low-z enhancement and high-z suppression + slight kT softening
PYQUEN (PYQUENm) - vacuum shower before (after) in-medium radiation
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
HYDJET (HYDrodynamics + JETs)
generates njet NN subcollisions and formation of
jet-induced state by calling (PYTHIA+PYQUEN) njet times
start filling JETSET arrays with npyt lines and HYDJET
arrays with nl (corresponding to nl partons) lines
calculation of multiplicity of HYDRO-induced particles,
nhyd=n-npyt, and adding new particles in JETSET arrays
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
HYDJET: quenching of particle
spectra
30,000 minimum bias Pb+Pb events, √s=5.5A TeV (pT, min=10 GeV, ntot=30000)
pT-distribution
-distribution
PT-spectra: strong hardening due to jets and softening due to jet quenching
-spectra: broadening due to jets and small influence due to jet quenching
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
HYDJET: elliptic flow
v2(pT)
v2()
v2(pT > 2 GeV): sharp drop due to jets and additional v2 due to jet quenching
v2(): ~20%-reduction due to jets and small influence due to jet quenching
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Summary and outlook
With its large acceptance, nearly hermetic fine granularity hadronic
and electromagnetic calorimetry, and good muon and tracking
systems, CMS is an excellent device for the study of mediuminduced energy loss by light and heavy quarks (''jet quenching'') in
various physics channels at LHC.
Adequate jet reconstruction, high-pT particle tracking (for isolated
tracks like muons and tracks in jets) and the determination of
global event characteristics with CMS in the high multiplicity
environment are possible.
Significant progress in development of Monte-Carlo tools for
simulation of jet quenching in heavy ion collisions is achieved
(PYQUEN, HYDJET).
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''
ICPAQGP'2005, Kolkata, India, February 8­12, 2005
Acknowledgments
I would like to thanks many colleagues for support, fruitful
discussions and providing the material for this report:
Daniel Denegri, Olga Kodolova, David Lopez, Sergey
Petrushanko, Ludmila Sarycheva, Alexander Snigirev,
Sergey Shmatov, Christof Roland, Constantin Teplov, Irina
Vardanyan, Ramona Vogt, Bolek Wislouch, Pablo Yepes,...
I appreciate to the organizers of the Conference for the
invitation and warm welcome
I.Lokhtin, ''Jet physics in heavy ion collisions with CMS detector at LHC''