1. Relativistic Heavy Ions: the UK perspective
STAR
Peter G. Jones
University of Birmingham, UK
NuPECC Meeting, University of Glasgow, 3-4 October 2008

2. The nuclear phase diagram
early universe
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)
R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005)
GSI-SIS
chemical freeze-out curve
BNL-AGS
CERN-SPS
T0 ≤ 2Tc (RHIC)
T0 ≈ 4-5 Tc (LHC)
200
250
150
100 50
Chemical Temperature Tch [MeV]
critical point?
quark-gluon plasma
Lattice QCD
deconfinement
chiral restoration
hadron gas
neutron stars
atomic nuclei
200
400
600
800
1000
1200
Baryonic Potential B [MeV]

3. UK participation
Involved since the inception of the CERN Heavy Ion programme
16O, 32S
208Pb
208Pb, 197Au
208Pb
WA85
WA94
WA97
NA57
ALICE
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)
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)
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
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
NA36
NA49
STAR
J.M. Nelson
R. Zybert
P.G. Jones (1992)
E.G. Judd (1993)
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
1987
1994
1999
2008

4. 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

5. Statistical/thermal models
Hadrons are produced statistically – enhancement explained?
CERN-pp
CERN-AA
RHIC–AA
strangeness s
STAR
Chemical freezeout temperature Tch
net-baryon density B
Strangeness saturation factor
net-strangeness density S = 0

6. Soft versus Hard QCD The advantage of high energy colliders
, K, N, …
, K, N, …  f
Hadron gas
s = 1?
(H)
QGP
Light-cone trajectory
(Q)
s = 0.4
Parton formation and thermalisation
0 = q z
Soft process
e.g. strangeness
Hard process
e.g. jets, charm A A
Soft processes occur over the lifetime of the system.
Hard processes occur at early times and serve as a “standard candle”.

7. High pT particle production
High pT jets are well described by perturbative QCD
heavy nucleus
radiated
gluons
key prediction: jets are quenched
X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480
Jet of high pT hadrons
Fragmentation
Leading hadron pT pL
pTOT
Parton distribution functions
– initial state
Hard scattering cross-section
– pQCD calculable
Fragmentation function
– final state

8. High-pT hadrons in A+A collisions
Central
STAR: Phys. Rev. Lett. 89 (2002)
STAR
Central
Peripheral
Peripheral
binary collisions
scale factor
p+p reference

9. Measuring jets by two-particle correlations
STAR
8 < pT(trigger) < 15 GeV/c
STAR: Phys. Rev. Lett. 97 (2006) Df
Trigger particle
Associated (near-side)
Associated (away-side)

10. Away side broadening or quenching?
Measure “jet” yields as a function of zT = pT(assoc)/pT(trig)
STAR: Phys. Rev. Lett. 97 (2006)
STAR
Near-side
Away-side
|| < 0.63
|| < 0.63
Suppression by factor 4-5 in central Au+Au.
No suppression

11. 2-d ( correlations
|h| ~ 1
h ~ 0
Trigger particle
Trigger particle Df Dh

12. 2-d ( correlations
In vacuo (pp)
fragmentation
static medium
broadening
flowing medium
anisotropic shape
(Armesto et al, PRL 93, (2004); Eur. Phys. J. C )
d+Au
Au+Au Dh Dh Df
Away-side Df
Away-side
Near-side
Near-side
Disappearance of away-side correlation = jet quenching.
Modification of near-side correlation = coupling of jet to the medium?

13. Extracting near-side “jet” yields
Au+Au 20-30%
3 < pT,trig. < 4 GeV/c and pT,assoc. > 2 GeV/c
yield,) 
STAR  1
Ridge yield  -1
Jet yield -2 2
 ()
Npart
Birmingham analysis: particle-type composition of the jet/ridge.
Strange particles now being used as a diagnostic tool.

14. Access to a wide range of observables in one experiment!
ALICE at the LHC
Access to a wide range of observables in one experiment!
ITS
Low pt tracking
Vertexing
TPC
Tracking, dEdx
TRD
Electron ID
TOF
PID
HMPID
PID high pt
PHOS
g,p0
MUON
m-pairs
PMD
g multiplicity

15. UK–ALICE Birmingham’s role in ALICE ALICE trigger
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)
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

16. 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. l+
jets p l– c c g* p g b b K p p

17. 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)

18. 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 ALICE
Low transverse momentum coverage.
Particle tracking.
Particle identification.
More speculative …
Multiplicity: pp (LHC) = CuCu (RHIC)
QGP in pp collisions?
p + p  0 + X
STAR

19. UK–ALICE Physics First physics Correction for trigger biases
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).

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 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.

21. The nuclear phase diagram
early universe
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)
R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005)
T0 ≈ 4-5 Tc (LHC)
200
250
150
100 50
T0 ≤ 2Tc (RHIC)
Chemical Temperature Tch [MeV]
critical point?
quark-gluon plasma
SPS
Lattice QCD
AGS
deconfinement
chiral restoration
hadron gas
SIS
chemical freeze-out curve
neutron stars
atomic nuclei
200
400
600
800
1000
1200
Baryonic Potential B [MeV]

22. Expectations from lattice QCD
F Karsch: Quark Gluon Plasma 3 (World Scientific)
Central Au+Au √sNN = 200 GeV
RHIC
LHC ?
Energy density at RHIC
RHIC: T0/Tc = 1.5–2.0
LHC: 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

23. Origin of surviving jets
Surface bias
RAA sets a lower bound on the density
Wicks, Horowitz, Djordjevic and Gyulassy, nucl-th/
Origin of surviving jets
(pT = 15 GeV/c)
More penetrating probes needed to explore the medium.

24. 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
Opacity (twist) expansion
Static medium
Transport coefficient
For 1-d longitudinal expansion:
Guylassy, Levai, Vitev,
Wang, Wang, …
Baier, Dokshitzer, Mueller, Peigne, Schiff,
Salgado, Wiedemann, …

25. 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.

26. ALICE – Observables ALICE is a general purpose detector
Access to a wide range of observables in one experiment!