Relativistic Heavy Ions:the UK perspective

Peter G. Jones

University of Birmingham, UK

NuPECC Meeting, University of Glasgow, 3-4 October 2008

STAR

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 2/20

The nuclear phase diagram

Baryonic Potential B [MeV]

Chemical Temperature T

ch [MeV]

0

200

250

150

100

50

0 200 400 600 800 1000 1200

quark-gluon plasma

hadron gas

neutron stars

early universe

deconfinementchiral restoration

Lattice

QCD

atomic nuclei

Location of critical point uncertain:F. Karsch, BNL Workshop, 9-10 March 2006.Z. Fodor, S. Katz, JHEP 0203 (2002) 014, 0404 (2004) 050C. R. Alton et al., Phys. Rev. D71 (2005) 054508R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005) 114014

critical point?

GSI-SIS

chemical freeze-out curve

BNL-AGS

CERN-SPS

T0 ≤ 2Tc (RHIC)

T0 ≈ 4-5 Tc (LHC)

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 3/20

UK participation•Involved since the inception of the CERN Heavy Ion programme

WA85WA94

NA36 NA49 STAR

ALICE

WA97NA57

J.M. NelsonR. ZybertP.G. Jones (1992)E.G. Judd (1993)

J.M. NelsonR. ZybertP.G. JonesH. Caines (1996)L. Hill (1997)T. Yates (1998)L. Barnby (1999)R. Barton (2001)

J.M. NelsonP.G. JonesL. BarnbyM. Lamont (2002)J. Adams (2005)L. Gaillard

(2008)A. Timmins (2008)T. BurtonE. Elhahuli

D. EvansP.G. JonesC. LazzeroniG.T. JonesO. Villalobos-

BaillieL. BarnbyR. LietavaM. BombaraA. JuskoM. KrivdaZ. MatthewsS. NavinR. KourP. PetrovA. Palaha

J. KinsonJ.N. CarneyO. Villalobos-BaillieM.F. VotrubaR. LietavaA. KirkD. Evans (1992)J.P. Davies (1995)A.C. Bayes (1995)M. Venables (1997)

J. KinsonD. Evans G.T. Jones O. Villalobos-BaillieI. BloodworthP. JovanovicA. JuskoR. LietavaP. Norman (1999)M. Thompson (1999) R. Clarke (2004)P. Bacon (2005)S. Bull (2005)

ALICE

J. KinsonD. Evans G.T. Jones O. Villalobos-BaillieA. BhasinP. JovanovicA. JuskoR. LietavaR. Platt (2007)D. Tapia Takaki (2008)H. Scott

1987 1994 1999 2008

16O, 32S 208Pb 208Pb, 197Au 208Pb

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 4/20

Strangeness at the CERN-SPS•Strangeness enhancement as a signature of QGP formation

If T > TC ≈ ms, expect copious thermal s-quark production.

Gluon fusion shown to dominate over light quark annihilation.

Enhancement is measured relative to proton-proton collisions.

NA35/NA49

WA97NA57

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 5/20

Statistical/thermal models•Hadrons are produced statistically – enhancement explained?

€

Ni T ,μ B ,μ S( )V

= γ sS gi

2π 2 p2 exp− Ei − μ B − μS( )

Tch

⎡ ⎣ ⎢

⎤ ⎦ ⎥±1

⎡

⎣ ⎢

⎤

⎦ ⎥

0

∞∫

−1dp

Chemical freezeout temperature Tch

net-baryon density BStrangeness saturation factor

net-strangeness density S = 0

strangeness

s

STAR

CERN-pp

CERN-AA

RHIC–AA

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 6/20

Soft versus Hard QCD•The advantage of high energy colliders

Hadron gas

Parton formation and thermalisation

z

, K, N, …

f

(H)

(Q)

QGP

A A

0 = q

Light-cone trajectory

s = 0.4

s = 1?

Soft processe.g. strangeness

Hard processe.g. jets, charm

, K, N, …

Soft processes occur over the lifetime of the system.Hard processes occur at early times and serve as a “standard candle”.

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 7/20

•High pT jets are well described by perturbative QCD

High pT particle production

pT

pL

pTOT

€

dσ pph

dyd2 pT= K dxa∫ dxb

abcd∑ fa (xa ,Q2) fb(xb,Q2) dσ

dˆ t (ab → cd)

Dh /c0

π zc

Parton distribution functionsHard scattering cross-sectionFragmentation function

– initial state– pQCD calculable– final state

Fragmentation Leading hadron

heavy nucleus

radiatedgluons

key prediction: jets are quenchedX.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480

Jet of high pT hadrons

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 8/20

High-pT hadrons in A+A collisions

Central

Peripheral

STAR: Phys. Rev. Lett. 89 (2002) 202301

p+p reference

€

RAA (pT ) = d 2N AA /dpTdηTAAd 2σ NN /dpTdη

scale factor

Central

Peripheral

binary collisions

STAR

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 9/20

Measuring jets by two-particle correlations

Trigger particle Associated (near-side)

Associated (away-side)

8 < pT(trigger) < 15 GeV/c

STAR: Phys. Rev. Lett. 97 (2006) 162301

STAR

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 10/20

Away side broadening or quenching?•Measure “jet” yields as a function of zT = pT(assoc)/pT(trig)

Near-side Away-side

No suppression

|| < 0.63 | | < 0.63

Suppression by factor 4-5 in central

Au+Au.

STAR: Phys. Rev. Lett. 97 (2006) 162301

STAR

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 11/20

2-d ( correlations

~ 1 ~ 0 Trigger particle

Trigger particle

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 12/20

2-d ( correlations

d+Au Au+Au

Near-side

Away-side

Disappearance of away-side correlation = jet quenching.Modification of near-side correlation = coupling of jet to the medium?

In vacuo (pp)fragmentation static medium

broadening

flowing mediumanisotropic shape

(Armesto et al, PRL 93, (2004); Eur. Phys. J. C Eur. Phys. J. C 38 38 461461)

Near-side

Away-side

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 13/20

0

Extracting near-side “jet” yields

Au+Au 20-30%

yield,

)

3 < pT,trig. < 4 GeV/c and pT,assoc. > 2 GeV/c

()2-2

-1

1

0

Npart

Jet yield

Ridge yield

STAR

Birmingham analysis: particle-type composition of the jet/ridge.Strange particles now being used as a diagnostic tool.

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 14/20

ALICE at the LHCAccess to a wide range of observables in one

experiment!

ITSLow pt trackingVertexing

TPCTracking, dEdx

TRDElectron ID

TOFPID

HMPIDPID (RICH) @ high pt

PHOS,0

MUON -pairs

PMD multiplicity

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 15/20

UK–ALICE•Birmingham’s role in ALICE

The ALICE central trigger system.

Only major subsystem which is the responsibility of a single university group.

Strong involvement in the science (Physics Performance Reports).

Now one of the largest university groups in ALICE.

•ALICE triggerUp to 60 inputs (every 25 ns)

24 L0 – 1.6 s (100 ns decision time)

24 L1 – 6 s

12 L2 – 90 s

50 trigger classes / 6 detector clusters

Pb-Pb collisions: 8 kHz interaction rate

p-p collisions: 200 kHz interaction rate

David Evans / ALICE trigger

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 16/20

ALICE - Key Physics•Study QCD on its natural (energy) scale T > TC ≈ QCD.•Explore quark and gluon dynamics in a hot medium.•Hot topics:

Collective behaviour – sQGP.

Opacity to jets – gluon density.

Heavy flavour production – Debye screening.

•Some new theoretical developments:AdS/CFT correspondance

Connection between string theory and ...

… strongly-coupled gauge theories.

Provides an alternative to (lattice) QCD.

Some (limited) success so far.

K

l+

l–

jets

b

cc

b

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 17/20

New ideas in HadronizationDavid d'Enterria (CERN)David Evans (Birmingham)Nick Evans (Southampton)Nigel Glover (IPPP)Peter Jones (Birmingham)Frank Krauss (IPPP)Kasper Peeters (MPI)Marija Zamaklar (Durham)

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 18/20

ALICE – pp physics•ALICE has a competitive programme of pp physics

Precision measurements of inelastic cross-sections.

Particle production as a function of pT.

Test of QCD calculations.

Study of diffractive events.

Probes nucleon structure.

•Advantages of ALICELow transverse momentum coverage.

Particle tracking.

Particle identification.

•More speculative …Multiplicity: pp (LHC) = CuCu (RHIC)

QGP in pp collisions?

p + p 0 + X

STAR

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 19/20

UK–ALICE Physics•First physics

Multiplicity and transverse momentum distributions.

Initial tests of QCD; input to fragmentation functions.

Are parton distributions sufficiently well understood?

•Correction for trigger biasesImportant for all papers reporting cross-sections (All).

•Longer term proton-proton physics – Pb-Pb physicsResonances – sensitive to hadronic phase (Villalobos-Baillie).

Charmonium ( J/) production – Debye screening (Lazzeroni).

High-pT and jet physics – energy loss (Barnby, Bombara, Evans, Lietava).

Anomalous high multiplicity pp events? – (Jones).

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 20/20

Outlook and Summary

Unclear whether there will be a Pb-run in 2009.

From 2010, expect 1 month of Pb per year.

First few years, Pb-Pb collisions @ 5.5 TeV per nucleon.

Option of changing beam species/energy in subsequent years.

e.g. p-Pb, symmetric light ions, lower energy(ies).

LHC will achieve first collisions in March 2009.

ALICE has a full physics programme.

UK is helping to shape that programme.

First physics proton-proton collisions Pb-Pb collisions.

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 21/20

The nuclear phase diagram

Baryonic Potential B [MeV]

Chemical Temperature T

ch [MeV]

0

200

250

150

100

50

0 200 400 600 800 1000 1200

AGS

SIS

quark-gluon plasma

hadron gas

neutron stars

early universe

deconfinementchiral restoration

Lattice

QCD

chemical freeze-out curve

SPS

atomic nuclei

Location of critical point uncertain:F. Karsch, BNL Workshop, 9-10 March 2006.Z. Fodor, S. Katz, JHEP 0203 (2002) 014, 0404 (2004) 050C. R. Alton et al., Phys. Rev. D71 (2005) 054508R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005) 114014

critical point?

T0 ≤ 2Tc (RHIC)

T0 ≈ 4-5 Tc (LHC)

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 22/20

Expectations from lattice QCD

€

0 = 1πR2τ 0

dETdy

y=0

= 5 −15 GeV /fm3

Energy density at RHIC

Central Au+Au √sNN = 200 GeV

RHIC: T0/Tc = 1.5–2.0LHC: T0/Tc = 3.0–4.0

RHIC and LHC permit a detailed study of the high T phase of QCDJ D Bjorken: Phys. Rev. D 27 (1983) 40

F Karsch: Quark Gluon Plasma 3 (World Scientific)

RHIC

LHC ?

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 23/20

Surface bias•RAA sets a lower bound on the density

Wicks, Horowitz, Djordjevic and Gyulassy, nucl-th/0512076

Origin of surviving jets(pT = 15 GeV/c)

More penetrating probes needed to explore the medium.

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 24/20

Models of energy loss

Initial ideas based on collisional energy loss.

Radiative energy loss was found to be dominant for light quarks.Soft gluon emission suppressed (Landau, Pomeranchuk, Midgal effect).

Energy loss is independent of parton energy, E, and becomes a function of the path length L in the medium.

Two example approaches (others exist)

Few hard(er) interactions Multiple soft interactions

€

E ≈ CRα S4

ˆ q L2

€

ˆ q =kT

2

mediumλ

∝ ρ glue

For 1-d longitudinal expansion:

Static medium

€

χ =Lλ

= σρ glueL

Opacity (twist) expansion

€

E ∝ L

Transport coefficient

GLV formalism BDMPS formalism

J D Bjorken, FERMILAB-Pub-82/59-THY

Guylassy, Levai, Vitev, Wang, Wang, …

Baier, Dokshitzer, Mueller, Peigne, Schiff,Salgado, Wiedemann, …

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 25/20

Use RAA to determine the medium density

•Nuclear modification factor, RAA, for pions

The medium is dense (30-50 x normal matter), but RAA provides limited

sensitivity.

Eskola, Honkanen, Salgado, Wiedemann (2004)

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008 26/20

ALICE – Observables•ALICE is a general purpose detector

Access to a wide range of observables in one experiment!