An experimental perspective on An experimental perspective on first jet measurements at LHC: first jet measurements at LHC: Lessons from RHICLessons from RHICDan Magestro, The Ohio State University
ALICE-USA Collaboration Meeting
October 15-16, 2005
ALICE-USA 10/2005 2 Dan Magestro, Ohio State University
Jets in hadronic collisionsJets in hadronic collisions
• High-energy hadronic collisions: collisions of constituent partonsHigh-energy hadronic collisions: collisions of constituent partons
• Each parton carries fraction (x) of hadron’s momentumEach parton carries fraction (x) of hadron’s momentum
• ““Hard-scattered” outgoing partons back-to-back in azimuth (Hard-scattered” outgoing partons back-to-back in azimuth (), ),
• not back-to-back in pseudorapidity (not back-to-back in pseudorapidity () due to different x) due to different x
p
p
Transverse planeSide view of collision
DM, Hard Probes 2004
ALICE-USA 10/2005 3 Dan Magestro, Ohio State University
Jets as probes of nuclear collisions - 1Jets as probes of nuclear collisions - 1
1.1. Partons in vacuum fragment characteristically Partons in vacuum fragment characteristically →→ calibratedcalibrated probe probe
Can it be calibrated at LHC? Can it be calibrated at LHC?
part
on gq,
Particle Data Group, PLB 592 (2004)
= pT(hadron) / pT(parton)
ALICE-USA 10/2005 4 Dan Magestro, Ohio State University
1 fm/c 5 fm/c 10 fm/c 50 fm/chadronization
initial state
pre-equilibrium
QGP andhydrodynamic expansion
hadronic phase
Jets as probes of nuclear collisions - 2Jets as probes of nuclear collisions - 2
, K, p
spec
tra
Mom
entu
m-s
pace
anis
otro
py
Partic
le ra
tios
Reson
ance
pro
ducti
on
HBT inte
rfero
met
ry
Nuclea
r coa
lesce
nce
Strang
e ba
ryon
spec
tra
QCD har
d sc
atte
ring,
jets
Heavy
par
ticles
(cha
rm)
Nuclea
r mod
ificat
ion
Direct
phot
ons,
lepto
ps
QCD har
d sc
atte
ring,
jets
2.2. Hard parton scatterings early in collision Hard parton scatterings early in collision directdirect probe probe
• ALICE is the only LHC detector that will measure all HI observablesALICE is the only LHC detector that will measure all HI observables
ALICE-USA 10/2005 5 Dan Magestro, Ohio State University
Observable #1: Inclusive spectraObservable #1: Inclusive spectra
• RHIC: Initial + final state effects RHIC: Initial + final state effects R RABAB ~ constant ~ constant
STAR, Phys. Rev. Lett. 91 (2003) 072304B. Cole, QM2005
baryons
RHIC measurements
ALICE-USA 10/2005 6 Dan Magestro, Ohio State University
Observable #1: Inclusive spectraObservable #1: Inclusive spectra
• Significant hardening of Significant hardening of spectraspectra
LHC expectation
1. Reduced sensitivity to initial-state kinematic effects
2. Larger variation of energy-loss with pT
ALICE-USA 10/2005 7 Dan Magestro, Ohio State University
Observable #1: Inclusive spectraObservable #1: Inclusive spectra
• RRAAAA exhibits strong p exhibits strong pTT dependence dependence I. Vitev
LHC expectation
5x change!
Note: collisional energy loss not considered in this model even MORE suppression?
ALICE-USA 10/2005 8 Dan Magestro, Ohio State University
Jet and dijet measurements at LHCJet and dijet measurements at LHC
Calorimetry, jet-cone algorithmsCalorimetry, jet-cone algorithms
• Try to measure full jet energyTry to measure full jet energy
• Difficult for lower energy jets dueDifficult for lower energy jets dueto backgroundto background
Tracking, 2-particle correlationsTracking, 2-particle correlations
• Correlate high-pCorrelate high-pTT triggertrigger particles particles
with with associatedassociated particles particles
• Sample fragmentation function, Sample fragmentation function, neutral energy (~1/3) lostneutral energy (~1/3) lost
Trigger
0 /2
(radians)
0
Near side
Away side
ET < 20 GeV ET >(>) 20 GeV
Background contains jets at LHC!
ALICE-USA 10/2005 9 Dan Magestro, Ohio State University
Jet correlations and the underlying eventJet correlations and the underlying event
• Spatial anisotropy in collision induces 2Spatial anisotropy in collision induces 2ndnd-order harmonic-order harmonic
PRL 90 (2003) 082302
p+pAu+Au, 20-40%
ALICE-USA 10/2005 10 Dan Magestro, Ohio State University
Observable #2: Back-to-back dijetsObservable #2: Back-to-back dijets
RHIC results (3)
Higher pHigher pTT →→ Away-side suppression Away-side suppression
Pedestal&flow subtractedSTAR, PRL 91 (2003) 072304
Lower pLower pTT →→ Away-side enhancement Away-side enhancement
pT(assoc) > 2 GeV/c pT(assoc) > 0.15 GeV/c
4 < pT(trig) < 6 GeV/c
STAR, PRL 95 (2005) 152301
ALICE-USA 10/2005 11 Dan Magestro, Ohio State University
Observable #2: Back-to-back dijetsObservable #2: Back-to-back dijets
RHIC results (3)
Higher pHigher pTT →→ Away-side peak emerges: Away-side peak emerges: DijetsDijets8 < pT(trig) < 15 GeV/c
DM (STAR), nucl-ex/0510002
ALICE-USA 10/2005 12 Dan Magestro, Ohio State University
Aside: At RHIC we’ve now reached pQCD regimeAside: At RHIC we’ve now reached pQCD regime
O. Barannikova (STAR), QM2005
• At RHIC, pQCD fragmentation At RHIC, pQCD fragmentation dominates hadron production dominates hadron production above ~6 GeV/cabove ~6 GeV/c
R. Fries
• At LHC, pQCD dominance At LHC, pQCD dominance predicted to set in a bit higher predicted to set in a bit higher
ALICE-USA 10/2005 13 Dan Magestro, Ohio State University
Observable #2: Back-to-back dijetsObservable #2: Back-to-back dijets
LHC expectation
Strong pT dependence of energy loss at LHC requires dijet analyses over full pT range!!
• Low-ELow-ETT jets: Two-particle correlations jets: Two-particle correlations
• High-EHigh-ETT jets: Jet reconstuction above background jets: Jet reconstuction above background
ALICE-USA 10/2005 14 Dan Magestro, Ohio State University
Observable #2: Back-to-back dijetsObservable #2: Back-to-back dijets
LHC expectation – Low-ET
• Two-particle correlations necessaryTwo-particle correlations necessary
• Difficult: large suppression + poor Difficult: large suppression + poor acceptance for away-side jet at low-Eacceptance for away-side jet at low-ETT
ALICE-USA 10/2005 15 Dan Magestro, Ohio State University
Observable #2: Back-to-back dijetsObservable #2: Back-to-back dijets
LHC expectation – High-ET
• Multiple jets + background reduces Multiple jets + background reduces purity at “intermediate” Epurity at “intermediate” ETT
• Trigger capabilities NEEDED to Trigger capabilities NEEDED to access high Eaccess high ETT
Taken from B. Wyslouch, Hard Probe 2004
ALICE-USA 10/2005 16 Dan Magestro, Ohio State University
LHC operations planLHC operations plan
• Main objectives:Main objectives:
• Terminate installation in Febr. 2007Terminate installation in Febr. 2007
• First pp collisions in summer 2007, First pp collisions in summer 2007, √s = 14 TeV√s = 14 TeV
• First long heavy ion (Pb+Pb) run end of 2008, First long heavy ion (Pb+Pb) run end of 2008, √s = 5.5 TeV√s = 5.5 TeV
• Higher jet rates: first jet-quenching questions can be answered Higher jet rates: first jet-quenching questions can be answered with ~10with ~1044-10-1055 events, e.g. prediction for events, e.g. prediction for strong pstrong pTT dependence of dependence of
energy lossenergy loss
• Parallel efforts statistical & reconstructed jets needed to explore Parallel efforts statistical & reconstructed jets needed to explore full pfull pTT range range
ALICE-USA 10/2005 17 Dan Magestro, Ohio State University
An opportunity for early Pb beam? An opportunity for early Pb beam?
• 2005 LHC Project Workshop (Chamonix XIV)2005 LHC Project Workshop (Chamonix XIV)
• Pb ions may be available at very early stage of LHC operationPb ions may be available at very early stage of LHC operation
• Jowett (CERN): Likely short Pb “Pilot Run” end of p+p Pilot Run in Jowett (CERN): Likely short Pb “Pilot Run” end of p+p Pilot Run in late 2007late 2007
• Depends on ability to commission ion cycle in SPS in 2006Depends on ability to commission ion cycle in SPS in 2006
• Luminosity = (few) x 10Luminosity = (few) x 102424 cm cm-2-2ss-1-1 ~10~1055 events in < 1 day events in < 1 day
This could be a great opportunity to answer first (zeroth) energy loss
questions with a one-day pilot run
