jet experiments at lhc - mcgill physicssome key results on jets in lhc hi collisions some comments...
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Jet Experiments at LHC
Gunther Roland
JET Collaboration Symposium Montreal June 2015
(as opposed to “Jet Physics at RHIC”)
Gunther Roland JET SymposiumJet Experiments at LHC 2
Heavy-ion jet results at LHCDijet asymmetries Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at sqrt(s) = 2.76 TeV with the ATLAS Detector at the LHC (ATLAS; 1011.6182) Observation and studies of jet quenching in PbPb collisions at sqrt(s) =2.76TeV (CMS; 1102.1957) Jet momentum dependence of jet quenching in PbPb collisions at sNN =2.76TeV (CMS; 1202.5022v2) Photon-jet asymmetries Studies of jet quenching using isolated- photon + jet correlations in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV (CMS; 1205.0206) Single-jet suppression Measurement of the jet radius and transverse momentum dependence of inclusive jet suppression in lead-lead collisions at sNN = 2.76 TeV with the ATLAS detector (ATLAS; 1208.1967v2) Measurement of charged jet suppression in Pb-Pb collisions at sNN = 2.76 TeV (ALICE; 1311.0633) Measurement of jet suppression in central Pb–Pb collisions at √sNN= 2.76 TeV (ALICE; 1502.01689v1.pdf) Jet structure Measurement of jet fragmentation into charged particles in pp and PbPb collisions at sqrt(s[NN]) = 2.76 TeV (CMS; 1205.5872) Modification of jet shapes in PbPb collisions at sqrt(s) = 2.76TeV (CMS; 1310.0878) Measurement of inclusive jet charged-particle fragmentation functions in Pb+Pb collisions at√sNN =2.76TeV with the ATLAS detector (ATLAS; 1406.2979v2) Measurement of jet fragmentation in PbPb and pp collisions at√sNN =2.76TeV (CMS;1406.0932v2) B-tagged jets Evidence of b-jet quenching in PbPb collisions at √sNN =2.76TeV (CMS;1312.4918) Jet-hadron correlations Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb–Pb collisions at √sNN = 2.76 TeV (ALICE; 1506.03984v1) Jets in pPb Studies of dijet transverse momentum balance and pseudorapidity distributions in pPb collisions at sNN =5.02TeV (CMS; 1401.4433v2.pdf) Measurements of the Nuclear Modification Factor for Jets in Pb+Pb Collisions at √sNN = 2.76 TeV with the ATLAS Detector (ATLAS; 1411.2357v2) Centrality and rapidity dependence of inclusive jet production in √sNN = 5.02 TeV proton–lead collisions with the ATLAS detector (ATLAS; 1412.4092v1) Measurement of charged jet production cross sections and nuclear modification in p–Pb collisions at sNN = 5.02 TeV (ALICE; 1503.00681v1) Measurement of dijet kT in p–Pb collisions at √sNN = 5.02 TeV (ALICE; 1503.03050v1)
Some key results on jets in LHC HI collisions
Some comments on what to expect from Run II
Some comments on experiment/theory interactions
Jet Experiments at LHC
Gunther Roland JET SymposiumJet Experiments at LHC 4
Jets as tools to characterize QGP
Medium effects on jets allow extraction of QGP transport coefficients:
• q: transverse momentum diffusion (radiative energy loss)
• e: longitudinal drag (collisional energy loss)^
^
Jets as tools to understand QGP
How does the strongly coupled liquid emerge from QCD? • Jets probe QGP at different (controllable) length scales • Scattering sensitive to quasi-particle nature of the medium
“pQCD plasma”
“AdS/CFT goo”
Jets as tools to manipulate QGP
How does QGP respond to local energy deposition by jets?
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LHC jet experiments (in heavy ion collisions)
Achieved good experimental control • Reconstruction efficiency (close to 100%@50+ GeV) • Jet energy scale (2-4% above 30 GeV) • Resolution and UE fluctuations (~15%@100GeV)
Jet measurement approach: • Clustering with anti-kT algorithm • Subtraction of event-by-event background energy
estimate • Jet energy corrections using pp calibration and MC
correction factors
This was not obvious a-priori; success enabled by nature of observed jet modifications}
Complementary approaches to reconstruction biases • At low pT: efficiency vs fake jets • Restrict measurement to high pT (CMS/ATLAS) • Ensemble-based measurement with combinatorial
correction (ALICE/STAR)
For sufficiently high trigger pT, all away-side jets are found
Gunther Roland JET SymposiumJet Experiments at LHC 6
+ RHIC
Jets in LHC Run-2 (and beyond)
PbPb √s = 5TeV
Collision rate ≈ 20kHz Lint ≈ 3/nb
√s = 5.5TeV Collision rate ≈ 50kHz Lint ≈ 10/nb (run 2+3)
LHC schedule
Compared to LHC Run1: x60 due to higher luminosity; x3 due to higher √s
HI
HI
HI
HI HI HI
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“RA
A”
“x+J
et”
JetsHadrons
b Jets
RHIC today LHC today RHIC tomorrow LHC tomorrow
Dijets (pT,1)
γ+jets (pTγ)
Z0+jets (pTZ)
Double b-tag (pT,1)
Kinematic reach: Now and tomorrow(*)(*)Artist’s impression
B mesonsD mesons
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Kinematic reach: Now and tomorrow(*)“R
AA”
“x+J
et”
D mesons
Hadrons
B mesonsb Jets
RHIC today LHC today RHIC tomorrow LHC tomorrow
Dijets (pT,1)
γ+jets (pTγ)
Z0+jets (pTZ)
Double b-tag (pT,1)
New observables; smaller uncertainties; broader pT reach RHIC+LHC overlap
Jets
(*)Artist’s impression
Ensemble-based measurements and x+hadron correlations
add low pT reach
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The importance of photon-jet correlations
40 < pTγ < 50GeV 50 < pTγ < 60GeV 60 < pTγ < 80GeV pTγ > 80GeV
Using isolated photons to tag away-side jets • determines initial parton energy to ≈ 15% • determines initial direction of the parton • tags parton to be a light quark
photon pT <50GeV: Spectrum of PbPb jets is
suppressed vs pp Biased jet selection
photon pT >80GeV: Spectrum of PbPb jets is
shifted vs pp Unbiased jet selection
jet pT > 30 GeV
Sufficiently high γ pT or sufficiently low jet pT yield unbiased selection of jets
Gunther Roland JET SymposiumJet Experiments at LHC 10
q determined with about 35% uncertainty Combined RHIC and LHC data: • Test model consistency • First hint of temperature dependence Quantitative extraction of e awaits more precise heavy flavor data
^
^
Similar information from jet RAA, dijet asymmetries, γ-jet
momentum shift
Extracting QGP transport coefficients
Gunther Roland JET SymposiumJet Experiments at LHC 11
RAA now and in Run-2
Essential to not let systematic, quantitative exp/theo comparison go stale • Add new Run-1 data • Predict/prepare for Run-2 data
(GeV/c)T
Charged Particle p20 40 60 80 100 120 140 160
AA R
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1| < 0.8ηALICE, || < 1.0ηATLAS, |
| < 1.0ηCMS, |
| < 0.8ηALICE, || < 1.0ηATLAS, |
| < 1.0ηCMS, |PbPb 0 - 5%, 2.76 TeVpp, 2.76 TeV
300
LHC Run-2
200 250
?
?ATLAS; 1504.04337
Gunther Roland JET SymposiumJet Experiments at LHC 12
~ Agreement betweenCMS and ATLASat high jet pT
Agree with charged particle RAA at high pT
It would be nice to have low pT CMS data / ATLAS RAA with R=0.2 / ALICE high pT data
RAA for charged hadrons and jets
ATLAS; 1504.04337
Gunther Roland JET SymposiumJet Experiments at LHC 13
Collisional energy loss
Radiative energy loss
The B’s are coming…
This is what google thinks of dead cones…
Gunther Roland JET SymposiumJet Experiments at LHC 14
Flavor Dependence of Jet Quenching
π
D
b-jet
bàJ/ψ
Pb+Pb
Flavor/mass dependence: Yes, no, maybe?
Qualitatively not too dissimilar wrt. expectations Quantitatively there are many subtleties
Gunther Roland JET SymposiumJet Experiments at LHC 15
b jets D mesons
▪ Non-negligible for both jets and hadrons! ▪ Even more important for charm than for bottom at LHC energy!
Gluon Splitting Contribution
~40-50% ~20%
▪ HF studies: matched partons are not necessary heavy quarks!Non-Prompt J/ψ
~30-40%
Estimated from PYTHIA 6
Not all heavy quarks are created equalfrom Yen-Jie Lee
RHIC users meeting
Gunther Roland JET SymposiumJet Experiments at LHC 16 25
2.0<|η|<2.5
0<|η|<0.5
Flavor/mass dependence: Yes, no, maybe?
Forward spectra are steeper (RAA smaller) But it’s 85% quark jets!
It might be good to look at a global analysis of all data relevant to q/g flavor dependence and mass dependence to verify some of
the basic beliefs
from Yen-Jie Lee RHIC users
meeting
Gunther Roland JET SymposiumJet Experiments at LHC 17
Energy flow and medium response
Do we have a medium, if there’s no medium response?
Experimental and theoretical challenges: • Strength and angular structure of medium response
unknown • Jets are correlated with the complex e-by-e flow fields
through quenching • How to distinguish medium-response from modified jet
branching?
Neufeld, Vitev PRC 2012Ma, Wang PRL 2011
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Jet-track correlations in CMSEnhancement of low-pT correlated yields at large angles to jet axis
Is this “trivial” jet-medium coupling or signature of modified QCD branching (“turbulent flow”)?AJ > 0.22
PbPb –pp
PbPb 0-30%
CMS dijet “missing pT”
Jet-track correlations for leading/subleading jet Common interpretation with MPT analysis is highly non-trivial Need jet MC to translate one to the other
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Jet measurements as QGP microscope
Gunther Roland JET SymposiumJet Experiments at LHC 20
Jet measurements as QGP microscope
“pQCD plasma”
“AdS/CFT goo”
Kurkela, Wiedemann, arXiv:1407.0293
Angular and momentum structure of intra-jet parton cascadeLarge angle inter-jet scattering
D’Eramo, Rajagopal, ‘12 updated calculation underway
Differential (event-by-event) measurements may allow glimpse at “QCD Rutherford scattering” off QGP constituents
Need ≈ x10 better precision
CMS γ+jet data
STAR jet-hadron correlations (1506.03984v1)
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We have observed medium modifications of jet momentum and angular structure
Broadening/excess at large r, low pT
(~2% of jet energy)
Narrowing/depletion at intermediate r, pT
Little change at small r, high pT
Radius r
Jet shapes Fragmentation functions RAA radius dependence
ΔRecoil radius dependence
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Qualitative consistent results between CMS and ATLAS
But: Experiment-comparison suggests that we do NOT YET have sufficient control over systematics necessary for our decadal programHigh pT particles Low pT particles
CMS: PRC 90 (2014) 024908ATLAS: PLB 739 (2014) 320-342
Z = p||Trk / pJet
But there is a long road ahead…
Gunther Roland JET SymposiumJet Experiments at LHC 23
Jet structure in pp and PbPbLiliana Apolinario
Lisbon HI jet workshop
Q-Pythia
Jewel
Jet structure variables sensitive to (in-medium) shower evolution
Critical effort in pp highest sensitivity searches (q/g and boosted object discrimination)
Stability vs pp pileup is major issue
Convergence of AA and pp needs
Jesse Thaler, Boost 2014
Dani Pablos Lisbon HI jet workshop
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• Rich (overwhelming?) harvest of jet results from LHC Run-1• Theory/experiment dialogue is essential
• JET collaboration qhat extraction from RAA is paradigmatic• Needs to be continuous process, not one and done (e.g. new RAA
measurements)• Complex questions will need global analyses (e.g. flavor dependence)• Full MC essential for interpretation of complex measurements (e.g. jet-medium
correlations) • Step forward with LHC Run-2 may prove as big as LHC Run-1• Will experiment or theory lead?
Summary