Relativistic Heavy Ions:
the UK perspective
STAR
Peter G. Jones
University of Birmingham, UK
NuPECC Meeting, University of Glasgow, 3-4 October 2008
The nuclear phase diagram
Location of critical point uncertain:
F. Karsch, BNL Workshop, 9-10 March 2006.
Z. Fodor, S. Katz, JHEP 0203 (2002) 014, 0404 (2004) 050
C. R. Alton et al., Phys. Rev. D71 (2005) 054508
R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005) 114014
T0 ≈ 4-5 Tc (LHC)
250
T0 ≤ 2Tc (RHIC)
200
critical point?
100
quark-gluon plasma
CERN-SPS
150
Lattice QCD
Chemical Temperature Tch [MeV]
early universe
BNL-AGS
hadron gas
deconfinement
chiral restoration
GSI-SIS
50
neutron stars
atomic nuclei
0
0
200
400
600
800
1000
1200
Baryonic Potential B [MeV]
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
2/20
UK participation
• Involved since the inception of the CERN Heavy Ion programme
16O, 32S
208Pb
WA85
WA94
WA97
NA57
J. Kinson
J.N. Carney
O. Villalobos-Baillie
M.F. Votruba
R. Lietava
A. Kirk
D. Evans (1992)
J.P. Davies (1995)
A.C. Bayes (1995)
M. Venables (1997)
NA36
J.M. Nelson
R. Zybert
P.G. Jones (1992)
E.G. Judd (1993)
1987
J. Kinson
D. Evans
G.T. Jones
O. Villalobos-Baillie
I. Bloodworth
P. Jovanovic
A. Jusko
R. Lietava
P. Norman (1999)
M. Thompson (1999)
R. Clarke (2004)
P. Bacon (2005)
S. Bull (2005)
208Pb, 197Au
ALICE
J. Kinson
D. Evans
G.T. Jones
O. Villalobos-Baillie
A. Bhasin
P. Jovanovic
A. Jusko
R. Lietava
R. Platt (2007)
D. Tapia Takaki (2008)
H. Scott
NA49
STAR
J.M. Nelson
R. Zybert
P.G. Jones
H. Caines (1996)
L. Hill (1997)
T. Yates (1998)
L. Barnby (1999)
R. Barton (2001)
J.M. Nelson
P.G. Jones
L. Barnby
M. Lamont (2002)
J. Adams (2005)
L. Gaillard (2008)
A. Timmins (2008)
T. Burton
E. Elhahuli
1994
208Pb
1999
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
ALICE
D. Evans
P.G. Jones
C. Lazzeroni
G.T. Jones
O. Villalobos-Baillie
L. Barnby
R. Lietava
M. Bombara
A. Jusko
M. Krivda
Z. Matthews
S. Navin
R. Kour
P. Petrov
A. Palaha
2008
3/20
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
WA97
NA57
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
4/20
Statistical/thermal models
• Hadrons are produced statistically – enhancement explained?
STAR
s
strangeness
Strangeness saturation factor
net-baryon density B
  E      1
Ni T , B , S 
g
S
2
B
S 1
  s i  p exp i
  dp
2
V
T
 
2 0  
ch
Chemical freezeout temperature Tch
net-strangeness density S = 0
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
5/20
Soft versus Hard QCD
• The advantage of high energy colliders
, K, N, …
, K, N, …
f
Hadron gas
(H)
s = 1?
(Q)
s = 0.4
QGP
Light-cone trajectory
0 = q
Parton formation and
thermalisation
z
A
Soft process
e.g. strangeness
Hard process
e.g. jets, charm
A
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
6/20
High pT particle production
• High pT jets are well described by perturbative QCD
X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480
Jet of high pT hadrons
key prediction: jets are quenched
Leading hadron
Fragmentation
radiated
gluons
pTOT
pT
pL
heavy nucleus
d hpp
Dh0 /c
 K   dx a dxb f a (x a ,Q ) fb (xb ,Q )
(ab  cd)
2
ˆ
dt
 zc
dyd pT
abcd
2
Parton distribution functions
– initial state
Hard scattering cross-section
– pQCD calculable
Fragmentation function
– final state

2 d
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
7/20
High-pT hadrons in A+A collisions
Central
STAR: Phys. Rev. Lett. 89 (2002) 202301
STAR
Central
Peripheral
Peripheral
d 2 N AA /dpT d
RAA ( pT ) 
TAA d 2 NN /dpT d
scale factor
p+p reference
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
binary collisions
8/20
Measuring jets by two-particle correlations
STAR
Trigger particle
8 < pT(trigger) < 15 GeV/c
Associated (near-side)
Df
Associated (away-side)
STAR: Phys. Rev. Lett. 97 (2006) 162301
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
9/20
Away side broadening or quenching?
• Measure “jet” yields as a function of zT = pT(assoc)/pT(trig)
STAR: Phys. Rev. Lett. 97 (2006) 162301
STAR
Near-side
Away-side
|Df| < 0.63
|Df| < 0.63
Suppression by
factor 4-5 in central
Au+Au.
No suppression
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
10/20
2-d (DDf correlations
|| ~ 1
Trigger particle
~0
Df
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
Trigger particle
D
11/20
2-d (DDf correlations
d+Au
In vacuo (pp)
fragmentation
D
Au+Au
static medium
broadening
Df
Away-side
D
flowing medium
anisotropic shapeNear-side
Df
Away-side
Near-side
(Armesto et al, PRL 93, (2004); Eur. Phys. J. C 38 461)
Disappearance of away-side correlation = jet quenching.
Modification of near-side correlation = coupling of jet to the medium?
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
12/20
Extracting near-side “jet” yields
Au+Au 20-30%
yieldDf,D)

D
3 < pT,trig. < 4 GeV/c and pT,assoc. > 2 GeV/c
1
STAR

0
-1
Jet yield
-2
0
2
Df ()
Npart
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
13/20
ALICE at the LHC
Access to a wide range of observables in one experiment!
HMPID
PID (RICH) @ high pt
TOF
PID
TRD
Electron ID
PMD
 multiplicity
TPC
Tracking, dEdx
ITS
Low pt tracking
Vertexing
MUON
-pairs
PHOS
,0
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
14/20
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 trigger
Up to 60 inputs (every 25 ns)
David Evans / ALICE trigger
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
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
15/20
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.
l+
jets
• Some new theoretical developments:

l–
AdS/CFT correspondance
Connection between string theory and ...
… strongly-coupled gauge theories.
*
cc


bb
Provides an alternative to (lattice) QCD.
Some (limited) success so far.
K

Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008

16/20
New ideas in Hadronization
David 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
17/20
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.
p + p  0 + X
• Advantages of ALICE
Low transverse momentum coverage.
STAR
Particle tracking.
Particle identification.
• More speculative …
Multiplicity: pp (LHC) = CuCu (RHIC)
QGP in pp collisions?
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
18/20
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 biases
Important for all papers reporting cross-sections (All).
• Longer term proton-proton physics – Pb-Pb physics
Resonances – 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
19/20
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
20/20
The nuclear phase diagram
Location of critical point uncertain:
F. Karsch, BNL Workshop, 9-10 March 2006.
Z. Fodor, S. Katz, JHEP 0203 (2002) 014, 0404 (2004) 050
C. R. Alton et al., Phys. Rev. D71 (2005) 054508
R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005) 114014
T0 ≈ 4-5 Tc (LHC)
250
T0 ≤ 2Tc (RHIC)
200
quark-gluon plasma
critical point?
150
100
SPS
Lattice QCD
Chemical Temperature Tch [MeV]
early universe
AGS
deconfinement
chiral restoration
SIS
hadron gas
50
neutron stars
atomic nuclei
0
0
200
400
600
800
1000
1200
Baryonic Potential B [MeV]
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
21/20
Expectations from lattice QCD
Central Au+Au √sNN = 200 GeV
F Karsch: Quark Gluon Plasma 3 (World Scientific)
RHIC
LHC ?
Energy density at RHIC
0 
dET
R2 0 dy
1
 5 15 GeV / fm3
y0
RHIC:
LHC:
T0/Tc = 1.5–2.0
T0/Tc = 3.0–4.0
J D Bjorken: Phys. Rev. D 27 (1983) 40
RHIC and LHC permit a detailed study of the high T phase of QCD
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
22/20
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
23/20
Models of energy loss
Initial ideas based on collisional energy loss.
J D Bjorken, FERMILAB-Pub-82/59-THY
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
GLV formalism
BDMPS formalism
DE 
Opacity (twist) expansion

L

  glueL
Transport coefficient
qˆ 


For 1-d longitudinal expansion:
Guylassy, Levai, Vitev,
Wang, Wang, …

CR S 2
qˆL
4
kT2
medium

DE  L
Static medium
  glue
Baier, Dokshitzer, Mueller, Peigne, Schiff,
Salgado, Wiedemann, …
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
24/20
Use RAA to determine the medium density
• Nuclear modification factor, RAA, for pions
Eskola, Honkanen, Salgado, Wiedemann (2004)
The medium is dense (30-50 x normal matter), but
RAA provides limited sensitivity.
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
25/20
ALICE – Observables
• ALICE is a general purpose detector
Access to a wide range of observables in one experiment!
Peter G Jones, NuPECC Meeting, Glasgow, 3-4 October 2008
26/20