hard probes at lhc theoretical perspective
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Hard Probes at LHC
Theoretical PerspectiveBerndt Müller
Hard Probes 2006Monterrey June 9-16, 2006
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The LH-I-C
Heavy ion physics at the LHC is only ~2 years away
The LHC program will be the last step of RHI physics into uncharted territory for a long time (forever?)
Are we (the theory community) ready for it?
What is our grand strategy for the RHIC + LHC era?
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Physics goals for the LHC
If RHIC is a “discovery facility”, then the LHC should be the “confirmation / consolidation facility”: Does the theoretical framework developed at RHIC hold up?
LHC will provide quantitative tests of the models developed to describe the RHIC data: Saturation of the initial gluon density (Almost) ideal hydrodynamic evolution of matter (v2)
Scaling of parton energy loss with ∫d Color screening, quark recombination Major new probes: contained jets and b-quarks, permitting much
improved control of theoretical predictions.
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QCD phase diagram
LHC
LHCinitial
state
RHIC
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QCD equation of state
RHIC
2
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LHC ?
170 340 510 MeV
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sQGP or tQGP ?
Low viscosity and large parton energy loss indivate that QGP probed at RHIC is strongly coupled (sQGP).
Recent studies of effects of plasma instabilities suggest that “strong coupling” effects may be produced by turbulent behavior of an expanding weakly coupled QGP (tQGP).
Consequences for LHC are very different: sQGP picture suggests that physics of weakly coupled QGP at
LHC (at early times) could be totally different tQGP effects persist under LHC conditions picture suggests that
medium produced at LHC will behave very similar to that produced at RHIC.
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LHC kinematic range
Bulk physics probes 10-4 < x <
10-3
100 GeV jets similar to 2 GeV hadrons at
RHIC
Forward/backward regions provide
access to very small x 10-5
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The “soft” environment
What kind of state do we expect to be
produced in Pb+Pb collisions at the
LHC ?
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Initial state: Saturated glue
~ 1/Q2
2 1/3
2 2 2
(parton density
, )= r 1 a ea sA
A
xG x Q A x
R Q Q
2sat ( , )Q x A
Glassy gluons are liberated – quark pairs are produced rapidly.
“Glasma” turns into a quark-gluon plasma..
Nonlinear interactions among classical fields lead to the saturation of gluon density in the transverse plane.
“Color Glass Condensate”
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ECM dependence: dN/dy, dE/dy
NLO pQCD with geometric parton saturation (Eskola et al. - EKRT)
RHIC
LHC
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RHIC
LHC
4
expansion
3in in200 GeV/fm at 0.2 fm/c
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Limiting fragmentation
RHIC data (PHOBOS) show universal dN/d in fragmentation region with central plateau, which grows with ECM.Gelis, Staśto, Venugopalan - hep-ph/0605087
beam
LHC 2.8GeVs
0
1500 2250y
dN
dy
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ECM dependence of dN/dy
3
2
Geometric scaling à la Golec-Biernat & Wüsthoff
1/
2 2 0sat 0 2
with 0.288, 0.79
( , )A
x AQ x A Q
x R
(Armesto et al. hep-ph/0407018)
From fit to HERA e-p and NMC nuclear photoabsorption data.
2 2sat,/ A AdN dy Q R
LHC RHICin in3 500 MeVT T
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LHC conditions: synopsis
Higher energy density 0 at earlier time 0.
Jet physics can be probed to pT > 100 GeV. b, c quarks are plentiful, good probes. Increased lifetime of QGP phase (10-15 fm/c) makes
preequilibrium effects less important. Even more rapid expansion reduces life-time of final-state hadron
gas.
LHC kinematic range is much larger than RHIC’s:
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Hard Probes…
Hard Probes are Standard Model observables that can be predicted perturbatively, with the exception of some infrared sensitive parts that either: Can be determined from other measurements or by means of reliable lattice
simulations, Or, are the quantities to be probed.
It is important to know, in which range of kinematic parameters a probe can be considered as “hard” in this sense.
It is important to understand how strongly the extraction of physics from a hard probe depends on a detailed understanding of (and sufficiently realistic modeling of) bulk matter properties.
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List of (possible) hard probes
High-pT hadrons
High-pT di-hadrons (or + hadron) Single jets -jet correlations Heavy quarkonia High invariant mass lepton pairs High pT photons W and Z bosons
accessible at RHIC and LHC
accessible at LHC (but not at
RHIC ?)
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Hard yields at LHC
Charm (bottom) production predicted to increase from RHIC to LHC by factor 10 (100)
~100 c pairs and ~5 b pairs per central Pb-Pb collision
20 jets with Etot > 50 GeV per
second 1 W-boson per second 1 Z-boson every 3 seconds
But note that dN/dy increases by factor ~3, dET/dy by factor ~5.
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Are we ready…
…to make predictions of hard probes for the
LHC ?The ground work has been laid:
Hard probes in heavy ion collisions at the LHC
• PDF’s, shadowing, and pA collisions – hep-ph/0308248• Jet physics – hep-ph/0310274• Heavy flavour physics – hep-ph/0311048• Photon physics – hep-ph/0311131
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State of theory
Do we have a coherent theoretical framework ?
High-pT hadrons: YES
High-pT di-hadrons (or + hadron): MAYBE
Single jets: YES -jet correlations: YES Heavy quarkonia: NO High invariant mass lepton pairs: YES
High pT photons: MAYBE
W and Z bosons: NO
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Ubi sumus?
An illustrative example:
High pT hadrons
(aka. Jets)
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PQCD framework: Jets
(1) (' ' )'2
2'
, '
AA hh X
XA p A p
p p
Fd
dQF
F(1) F(2)
D(1
)
D(2
)
Q2
' '
2'
pp pp
pp
d
dQ
(1) (2)' 'p h p hD D
Factorization:(1) (2)
' 'p h p hD D
Medium modified fragmentation functions:
( )1 /p h p h
zD z D
E E
q qg
L
Measured medium property: ˆ ~ ( ) (0)i
iq dx F x F
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The devil is in the details
A general theoretical framework is not enough: Extraction of q^ requires detailed modeling of reaction geometry
(scattering vertices, density distribution, path lengths, expansion, etc.). Value of q^ extracted from data is correlated with assumptions about path
length L and expansion pattern. Extracted values range from q^ = 0.5 GeV2/fm 15 GeV2/fm.
Prediction of RAA for heavy quarks has failed: Observed suppression of D mesons can be (barely) reconciled with help
of additional elastic collisional energy loss mechanism,… …if down-feeding from b-quarks is ignored.
Contribution from quark recombination at intermediate pT was not expected: Domain of dominance depends sensitively on transverse flow.
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Flat or rising RAA ?
Vitev et al (GLV)
LHC
Armesto et al (ASW)
Extrapolations to LHC energy vary widely due to modeling differences:
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Why RAA is flat
Most partons produced in the center are totally absorbed
Long path lengths are exploited only by high momentum partons RAA does not increase at high pT
What holds for singles, does not hold for coincidences…
Prod. points in (x,y) for partons giving hadrons with pT > 5 GeV:
increasing s
Loizides, nucl-ex/0501017
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Di-hadrons probe interior
Detector
Deeper penetration of higher-pT probe leades to increased “punch-
through” of away side jet.
T. Renk
Vertex distribution for trigger hadrons of pT – 25 GeV/c in Pb+Pb @ LHC
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B/h ratio at LHC
• For 10 < pT < 20 GeV, dominant effect due to b mass.
• RB/h enhancement at “low” pT probes mass dependence of (radiative) energy loss
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Hadron production at LHC
r = 0.75
r = 0.85
r = 0,65
R.J. Fries, BM, nucl-th/0307043
includes parton energy loss
Study of RAA can be extended to full measurement of modified Dph(z).
Full energy loss distribution becomes relevant.
Suggestion to theorists: Publish w(E) integrated over emission points, angles, for comparison among models.
Recombination:
Increased flow at LHC pushes domain of reco dominance to higher pT.
Thermal-shower reco may even dominate for baryon production to very large pT (Hwa et al.)
reco
frag
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“Jet quenching”
Jet energy is not lost, but just redistributed inside the jet cone to larger kt than in vacuum fragmentation (LPM effect).
Separation of fully evolved jet from background will become possible at LHC for large jet energies.
kT
Medium modifications of jet shape can tell us about the mechanism of energy loss of the initiating parton.
Are jets induced by b-quarks modified differently than those induced by light quarks/gluons?
400 GeV2
TdE
dy
Distributed over ~300 particles.
Soft background:
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Jet-medium interactions
Medium not only leads to modification of jet due to induced radiation of gluons (due to elastic interactions with the radiated gluons), but also interacts inelastically with the radiation:
Radiated gluons can deposit energy and momentum into the medium.
q q
g
Differentiation of collisional and radiative energy loss must be subsumed into an analysis of the elastic and inelastic interactions of the developing jet with the medium. The theoretical approach needs to be developed to match onto experimentally accessible kinematic observables and/or cuts.
But if the gluon gets absorbed, how can we distinguish the process from collisional energy loss, which involves absorption of a virtual gluon by the medium?
abs. rad.
coll.
Is there a Mach cone?
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Other hard probes
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Quarkonium production
pQCD framework: Either NRQCD factorization including color octet components of the (Q-Qbar) state and feed-down from excited states or color evaporation model.
J/
CEM
NRQCDNRQCD
CEM
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Quarkonium emission
(a) Spectral function:
LQCD simulations, with analytic continuation to real time, suggest Td ~ 1.5 - 2Tc for J/ and .
But width (T) unknown.
Datta et al.
No comparable theoretical framework exists, in which to treat medium modifications to (Q-Qbar) spectral function, (in-)elastic dissociation by the medium, recombination in a unified manner.
(b) Inelastic dissociation:
pQCD calculations of gluon absorption give substantial rates for LHC conditions.
J/
20 0
0
( ) ln 100 fmfgd
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Quarkonium emission II
Ionization by thermal gluons:
J/
LHCRHIC
Recombination is unavoidable, if c-quarks are “dense” and thermalized.
(Semi-)realistic calculation with full geometry and flow are needed, but the contribution is potentially large, and growing with c.m. energy.
If gluons are responsible for parton energy loss, they must also dissociate J/.
Are the predictions really compatible with the J/ data at RHIC energy?
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Electromagnetic probes
Well developed theoretical framework Importance of sophisticated collision modeling recognized Direct probes of thermal medium emission
Low mass dileptons enhanced by long lifetime and pushed to large pT by flow
Credible predictions possible after NA60 data Thermal photons probe initial T
EM probes will increasingly be useful in conjunction with other hard probes: J/ and Photon tagging of jets Jet-to-photon conversion as part of the modified Dp(z)
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Weak(er) probes ?
W and Z have widths O(2 GeV) decay on time scale of QGP formation and thermalization at LHC.
Does the medium influence the hadronic decay channels? [Probably not significantly, because M counts, not .]
But the medium will affect the final-state propagation of the (hadronic) decay products, e.g. Z b-bbar.
Can this be used as a probe of the medium?
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Summary
Theory of different hard probes for LHC energies in different stages of development: EM probes in rather good shape Hard parton / jet probes require improved (realistic) treatment of
evolving fireball geometry/flow; inelastic interaction between jet and medium requires conceptual clarification
Unified QCD based framework for quarkonium probes missing Do we anticipate a transition from sQGP (at RHIC) to pQGP
(at LHC – at early times)? Is the RHIC medium a strongly coupled plasma or just a turbulent
plasma with anomalous transport properties? How can hard probes help to decide between these pictures?
Are there new “surprises” which we can anticipate?
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