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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

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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?