jets quenching: what’s next?

Jet quenching: what's next? 1ETD-HICJuly 16-19, 2007

Jets quenching: what’s next?Peter Jacobs

Lawrence Berkeley National Laboratory


LRP Phases of QCD: Recommendation #11. Our central goal is a dramatic advance in our understanding of QCD Matter, through quantitative comparison of theory and experiment to determine the properties of the strongly coupled Quark-Gluon Plasma discovered in the initial phase of RHIC operations, and through further exploration of the QCD phase diagram at non-zero baryon density where a critical point has been predicted. The essential requirements for the success of this scientific program are therefore our highest priority:

• Effective utilization of the RHIC facility and completion of the ongoing detector upgrade program;• The RHIC II luminosity upgrade, which will enable quantitative study of rare processes;• Strong support for the ongoing theoretical studies of QCD matter, including finite temperature and finite baryon density lattice QCD studies and phenomenological modeling, and an increase of funding to support new initiatives enabled by experimental and theoretical breakthroughs.

What does

this mean

?


GLV

ASW

χ2 fit to RAA

1-pv

alue

Towards precision: measuring

PHENIX QM06 B. Sahlmueller et al, nucl-ex/0701060

q̂

RAA constrains theory parameters to ~factor 2


Quantitative jet quenching: dihadron correlations at higher pT…

Recoil jet clearly seen above background but at suppressed ratedifferential measurement of`E upper bound on qhat

trigger

recoil

?

pTtrigger>8 GeV/c

Yie

ld p

er tr

igge

r

STAR, nucl-ex/0604018


Zhang, Owens, Wang and Wang nucl-th/0701045

from inclusive and di-hadon suppression

fmq 2GeV3.08.2~ˆ

Consistent minima for two independent measurements

q̂


RHIC Performance: Run 7

slopeRun7 ~ 2 X slopeRun4


Transport Coefficients

RHIC I AuAu 2nb-1

(Recorded?)

Systematics dominated.

Run-4AuAu 0.2 nb-1

10%)ty (Probabili

/fmGeV 24ˆ6 2

q

RHIC I ++


Jet in LHC Heavy Ions

Pb+Pb at 5.5 TeV: huge kinematic reach

P. Jacobs and M. van LeeuwenNucl. Phys A774, 237 (2006)


Jets at RHIC in the fb era (from Jamie Dunlop’s talk on Monday)

Jet reco under optimization

Data in hand: p+p, Cu+Cu, Au+Au

From LHC studies should work for Et>~20-30 GeV

Nbin projection from p+p# Jets in bin at 40 GeV

Run 6 Cu+Cu: ~50Run 7 Au+Au: ~1000

600 ub-1

, 23 pb-1

p+p equiv. RHIC II: ~50, 000 Precise

D(z)

2 pb/GeV

40% precision with 0.3 pb-1, half barrel

STAR PRL 97 (2006) 252001


• Fully reconstructed single jets

– much reduced geometric bias

– Jet shape and fragmentation modified by the medium

• Observables

– “RAA” of jets

– Fragmentation function

– Acoplanarity of dijets

– Jet-– Jet-Z0

– Multi jets

– …

c

d

a b

ATLAS

Jet observables


Jet reconstruction: generic issues

Two broad classes of algorithms:“kT/Durham”: merge all tracks/energy clusters that

are nearby in phase space“cone”: fixed shape; stable energy-weighted maxima around seeds; special rules for merging/splitting

colinear safety: finite calorimeter threshold misses jet on left?

infrared safety: one or two jets?


Jet reconstruction in heavy ion collisions100 GeV jet in central Pb+Pb

En

erg

y (

Ge

V)

Large backgrounds optimal resolution using small jet cones R~0.3?

Complex underlying event fluctuations in heavy ion events:• full jet reconstruction is difficult• jet trigger is tricky (large background fluctuations)


Soft Background in Jet Cones

ET = 100 GeV, R = 0.4

(TPC+EMCal)

(TPC)

(TPC – like RHIC)

Cone radius R=√(Δη2+ΔΦ2)

Large cone radius large background

Radius cut of .4 + pT cut lowers background > 80% of Ejet

R

Energy in cone R: background and jets

Central Pb+Pb

Preserve “most” of jet while strongly suppressing bkgd:

pT>2 GeV/cR~0.4


Small cones and jet splittingET = 100 GeV, R = 0.4

(TPC+EMCal)

(TPC)

(TPC – like RHIC)

But why this huge tail for a mono-energetic jet sample with Ejet

T=100 GeV?


Input Jet Energy [GeV]

fraction of events with Nreconstructed>1

Jet splitting/Sub-jet summing

Jet Energy [GeV]#

Je

ts

all particles, R=0.3, pt>2GeV

- input - Njets,rec=1- Njets,rec>=1 highest jet- Njets,rec>=1 mid-cone- Njets,rec>=1 sum

• Small cone radius splits jets - need a correction pass to sum sub-jets and recover jet energy accurately• Internal structure of jet and its modification may also be of great interest…

Work in

progress


kT at a hadron collider: CDF inclusive jets

…same performance as modern cone algorithms


2

222

,min ijTjTiij

jijiij

Rkkd

R

• Infrared safe measure area using zero-energy ghost particles• Potential advantage over cone: smaller effective area, lower integrated background

PhysLett B641, 57 (2006)

Fast kT


Fast kT for LHC Heavy Ions: ATLAS


Benchmark measurement: modified fragmentation function

• MLLA: good description of vacuum fragmentation (basis of PYTHIA)• introduce medium effects at parton splitting Borghini and Wiedemann, hep-ph/0506218

Jet quenching fragmentation strongly modified at pT

hadron~1-5 GeV

=ln(EJet/phadron)

pThadron~2 GeV

Jet quenching


Measuremement of modified fragmentation

Ratio of purple/red

Kinematic reach beyond ~200 GeV

175 GeV jets in ALICE acceptance

Jet quenching

Dashed line = no jet quenching


So what?

What is learned by probing it with ~200 GeV jets?

is a transport property of a medium at T=200 MeV q̂

22

2~ˆ TT

trT q

dq

ddqq


Evolution of qhat

2 2 22

2 2ˆ( , ) ( ) ( , )

1 (2 ) 2R T T

Tc

g C d q qq E dx x x q

N Ep

High energy jet small x

Large momentum transfer large scale

22

4 ( )1

s AT T

c

Cx G x

N

2 22

2

( , ) 1( , )

ln(1/ ) ln 2

xG xxG x

x

(DLA)

k=E

p

q

Casalderrey-Solana and Wang, arXiv 0705.1352


Evolution of qhat cont’d

Casalderrey-Solana and WangarXiv:0705.1352

Consider jet quenching similar to DIS jet energy variation probes QGP structure at small and varying x…


Frag Fn modification via elastic scattering

H. Pirner et al (LHC Final Predictions workshop): • Frag Fn modified by scattering in a screened gluon gas (ng~T3)• “evolution” is kinematic in origin

ln(1/x)

dN/d

ln(1

/x)

Virtuality Q2Je

t mul

tipl

icit

y


Direct measurement of qhat: dijet acoplanarity

X.-N. Wang

2 ˆ( , )T q Edy yq jet,

jetT Eq ~


q,g

+ jet : photon ET = parton ET at LO

Detailed measurement of modified fragmentation

needs RHIC II luminosity

Phys.Rev.C74:034906,2006.Phys.Rev.Lett.77:231-234,1996.

Run-4 AuAu 0.2 nb-1RHIC I AuAu 2 nb-1RHIC II AuAu 20 nb-1

Direct Photons at RHIC II


p+p

Pb+Pb/

hep-ph/0311131

Direct Photons in LHC Heavy Ions

104/year

This is a difficult and limited measurementcannot be the flagship of the LHC heavy ion program


Final remarks

Jet quenching is well-established• multiple high-pT signatures with large experimental effects enables detailed quantitative study• theory: qualitative successes but quantitative gaps

New provocations and speculations: AdS/CFT, Mach cones/whatever,...

Qualitatively new opportunities at RHIC II and LHC

But the Gee-Whiz Era of RHIC Physics is overnow vital to turn our qualitative successes into solid quantitative measurements of hot QCD matter with credible error bars

ongoing, intensive collaboration of experiment and theory

jets quenching: what’s next?

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