Jet probes of nuclear collisions: From RHIC to LHC
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Jet probes of nuclear collisions: From RHIC to LHC. Dan Magestro, The Ohio State University. Midwest Critical Mass October 21-22, 2005. Where’s the LHC?. CERN. CERN. BNL. Where’s the LHC?. CERN. Site of ALICE Experiment. CERN. Jets in hadronic collisions. DM, Hard Probes 2004.

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Jet probes of nuclear collisions: From RHIC to LHC

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Jet probes of nuclear collisions: From RHIC to LHC

Dan Magestro, The Ohio State University

Midwest Critical Mass

October 21-22, 2005


Where’s the LHC?

CERN

CERN

BNL

Dan Magestro, Ohio State University


Where’s the LHC?

CERN

Site of ALICEExperiment

CERN

Dan Magestro, Ohio State University


Jets in hadronic collisions

DM, Hard Probes 2004

  • High-energy hadronic collisions: collisions of constituent partons

    • Each parton carries fraction (x) of hadron’s momentum

    • “Hard-scattered” outgoing partons back-to-back in azimuth (),

    • not back-to-back in pseudorapidity () due to different x

Side view of collision

Transverse plane

p

p

Dan Magestro, Ohio State University


Jets as probes of nuclear collisions - 1



  • Partons in vacuum fragment characteristically →calibrated probe

    Can it be calibrated at LHC?

Particle Data Group, PLB 592 (2004)

parton

= pT(hadron) / pT(parton)

Dan Magestro, Ohio State University


Jets as probes of nuclear collisions - 2

QCD hard scattering, jets

QGP and

hydrodynamic expansion

hadronic phase

initial state

pre-equilibrium

hadronization

1 fm/c

5 fm/c

50 fm/c

10 fm/c

  • Hard parton scatterings early in collision  direct probe

    • ALICE is the only LHC detector that will measure all HI observables

Momentum-space anisotropy

QCD hard scattering, jets

Heavy particles (charm)

Strange baryon spectra

Direct photons, leptops

Resonance production

Nuclear coalescence

Nuclear modification

HBT interferometry

, K, p spectra

Particle ratios

Dan Magestro, Ohio State University


Observable #1: Inclusive spectra

baryons

Particle production in AA relative to expectations from p+p

RHIC measurements

  • RHIC: Initial + final state effects  RAB ~ constant

STAR, Phys. Rev. Lett. 91 (2003) 072304

B. Cole, QM2005

Dan Magestro, Ohio State University


Observable #1: Inclusive spectra

LHC expectation

  • Significant hardening of spectra

  • Reduced sensitivity to initial-state kinematic effects

  • Larger variation of energy-loss with pT

Dan Magestro, Ohio State University


Observable #1: Inclusive spectra

LHC expectation

  • RAA exhibits strong pT dependence

I. Vitev

5x change!

Note: collisional energy loss not considered in this model  even MORE suppression?

Dan Magestro, Ohio State University


Jet and dijet measurements at LHC

Trigger



Near side

Away side

0

/2

0

 (radians)

ET < 20 GeV

ET >(>) 20 GeV

Calorimetry, jet-cone algorithms

  • Try to measure full jet energy

  • Difficult for lower energy jets dueto background

Tracking, 2-particle correlations

  • Correlate high-pTtrigger particles with associated particles

  • Sample fragmentation function, neutral energy (~1/3) lost

Background contains jets at LHC!

Dan Magestro, Ohio State University


Jet correlations and the underlying event

PRL 90 (2003) 082302

  • Spatial anisotropy in collision induces 2nd-order harmonic

p+p

Au+Au, 20-40%

Dan Magestro, Ohio State University


Observable #2: Back-to-back dijets

Pedestal&flow subtracted

RHIC results (3)

Higher pT→ Away-side suppression

Lower pT→ Away-side enhancement

pT(assoc) > 2 GeV/c

pT(assoc) > 0.15 GeV/c

STAR, PRL 95 (2005) 152301

STAR, PRL 91 (2003) 072304

4 < pT(trig) < 6 GeV/c

Dan Magestro, Ohio State University


Observable #2: Back-to-back dijets

RHIC results (3)

Higher pT→ Away-side peak emerges: Dijets

8 < pT(trig) < 15 GeV/c

DM (STAR), nucl-ex/0510002

Dan Magestro, Ohio State University


Aside: At RHIC we’ve now reached pQCD regime

O. Barannikova (STAR), QM2005

  • At RHIC, pQCD fragmentation dominates hadron production above ~6 GeV/c

R. Fries

  • At LHC, pQCD dominance predicted to set in a bit higher

Dan Magestro, Ohio State University


Observable #2: Back-to-back dijets

LHC expectation

  • Low-ET jets: Two-particle correlations

  • High-ET jets: Jet reconstuction above background

Strong pT dependence of energy loss at LHC requires dijet analyses over full pT range!!

Dan Magestro, Ohio State University


Observable #2: Back-to-back dijets

LHC expectation – Low-ET

  • Two-particle correlations necessary

  • Difficult: large suppression + poor acceptance for away-side jet at low-ET

Dan Magestro, Ohio State University


Observable #2: Back-to-back dijets

Taken from B. Wyslouch, Hard Probe 2004

LHC expectation – High-ET

  • Multiple jets + background reduces purity at “intermediate” ET

  • Trigger capabilities NEEDED to access high ET

Dan Magestro, Ohio State University


LHC operations plan

  • Main objectives:

    • Terminate installation in Febr. 2007

    • First pp collisions in summer 2007, √s = 14 TeV

    • First long heavy ion (Pb+Pb) run end of 2008, √s = 5.5 TeV

  • Higher jet rates: first jet-quenching questions can be answered with ~104-105 events, e.g. prediction for strong pT dependence of energy loss

  • Parallel efforts statistical & reconstructed jets needed to explore full pT range

Dan Magestro, Ohio State University


An opportunity for early Pb beam?

  • 2005 LHC Project Workshop (Chamonix XIV)

  • Pb 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 late 2007

    • Depends on ability to commission ion cycle in SPS in 2006

    • Luminosity = (few) x 1024 cm-2s-1 ~105 events in < 1 day

This could be a great opportunity to answer first (zeroth) energy loss questions with a one-day pilot run

Dan Magestro, Ohio State University


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