Transcript Heavy-Ion Physics from the SPS to the LHC David Evans The University of Birmingham McCubbinFest – RAL 8th November 2011

Heavy-Ion Physics
from the SPS to the LHC
David Evans
The University of Birmingham
McCubbinFest – RAL 8th November 2011
Aims of Heavy Ion Physics
 Study strongly interacting matter at extreme energy
densities over large volumes and long time-scales (study of
QCD at its natural scale, QCD ~ 200 MeV).
 Study the QCD phase transition from hadronic matter to a
deconfined state of quarks and gluons - The Quark-Gluon
Plasma.
Study the physics of the Quark-Gluon Plasma (QCD under
extreme conditions).
Phases of Strongly Interacting Matter
Both statistical and lattice QCD predict that
hadronic matter will undergo a phase
transition, in to a deconfined state of
quarks and gluons – a quark-gluon plasma,
at a temperature of,
T ~ 170 MeV and energy density,
 ~ 1 GeV/fm3.
Lattice QCD, mB = 0
ALICE will far exceed these
temperatures & densities at the
LHC.
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Heavy Ion Collisions
Create QGP by colliding ultra-relativistic heavy ions
pre-equilibration  QGP 
hadronisation  freeze out
Colliders: AGS, SPS, RHIC, LHC
SNN (GeV) =
5.4
19
200
2760 (5500)
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Signatures / Observables
Need to study signatures in quantities that will not change
during hadronisation.
 Quantum numbers which remain unchanged by strong
interaction.
 Particles that do not interact strongly
• Strangeness Enhancement
• J/ Suppression
• Thermal dilepton production
• Direct Photons
Etc.
Study yields, particle
distributions (pT, y etc),
correlations, …..
SPS Experiments
Early Experiments using 16O (1986-1987) &
32S (1988-1993) Ions:
Collaboration
Main detectors
Physics signatures
NA34
(Helios)
Muon spectrometer &
calorimeter
Dimuons, J/, K/, s
NA35
Streamer chamber
K0, , 
NA36
TPC
K0, , 
NA38
Muon spectrometer
Dimuons, J/, 
WA80
Plastic ball, EM calorimeter
0 and 
Target fragmentation
WA85/WA94
 Spectrometer with
MWPCs
K 0 , ,  ,   ,  
HELIOS
An energetic collaboration
Aim of Early Experiment
• The aim of these early experiments was to see
if we could really extract physics from this
difficult, high multiplicity, environment and find
a smoking gun for QGP formation.
A quick aside!
My First Encounter with the
McCubbin
A young Evans
McCubbin running the show
RAL Summer School 1989
Results from Sulphur Beams
at the SPS
Di-electron Enhancement
Di-Muon Enhancement
Both Helios and NA38
saw an enhancement of
di-muons but no centrality
dependence
1.5 < mmm < 2.5 GeV/c2
J/ & ’ Suppression
NA38 – dN/dM vs Mmm
Strangeness Enhancement
Enhancement of strange particles
seen by WA85/94 with multistrange particles enhanced more.
Enhance of Lambda hyperons and
kaons also seen by NA35 and NA36.
K+ enhancement seen by Helios and 
enhancement seen by NA38
Summary of Early Results
• Enhancement of intermediate mass di-leptons seen.
• Suppression of J/ reported with ’ suppressed even
more.
• Strangeness enhancement seen with multi-strange
hyperons having largest enhancement.
• No excess of s seen – no direct photon production
observed (can’t win them all!)
• A promising start!
Lead Ions @ SPS
(1994-2001)
• 1994 saw the first lead collisions and a new generation of
improved HI experiments and analysis techniques.
• The aim was to find convincing evidence for the QGP.
NA57
NA49
Si micro-strips (WA97) and pixel detectors
(NA57) was used for the first time.
Pb-Pb Results
 The lead data from the SPS produced a host of
impressive results, generally of much better quality than
previously produced.
 Probably the most striking were the results on J/
suppression and strangeness enhancement.
J/ Suppression
NA50
Ratio: measured to
expected as a function of
the energy density of the
fireball.
Strangeness Enhancement
Enhancement increases with strangeness content
 enhanced by up to a factor of 20.
NA57
A New State of Matter
• Taking all the results, from lead collisions from
the SPS, in to account ….
• The case for a deconfined state of matter was
compelling and
• In February 2001, CERN annouced that a new
state of matter had indeed be created in Pb
collisions at CERN.
HI Colliders
• While the SPS experiments were using lead beams, a
new HI collider was being built – RHIC.
• At CERN, we were working on a HI experiment for
the LHC (later to be called ALICE).
• In 1999, Birmingham was given approval to join the
ALICE experiment and build the central trigger
processor.
– Norman McCubbin was one of the referees and made a
major contribution in getting us approved.
RHIC Collider
• RHIC – results from SPS exps
confirmed, plus
• Eliptic flow: axial anisotropy in
collective flow of final state
hadrons wrt to the reaction plane
– ie. medium behaves like an
ideal liquid rather than a gas as
expected.
• Jet quenching: suppression of
high pT hadrons wrt yield
expected from superposition of
nucleon-nucleon collisions.
Statistical Hadronisation
at RHIC
23/51
• Chemical freeze-out;
• Two parameters:
• Tch and mB;
 Tch = (160±2) MeV
 mB ;= (20±4) MeV
A. Andronic et al., arXiv:nucl-th/0511071v3
Phases of Strongly Interacting
Matter
colour
Evidence of collective
behaviour
Equal energy
density contours
P
pT
v2

d 3 N 1 d 2 N  

1  2vn cosn  r 
E 3 

d p 2 pT dpT dy  n1
Fourier coefficient
V1 = directed flow.
Angle of reaction plane
V2 = elliptic flow.

v 2  cos2
Measuring elliptic flow
Data reaches hydro limit
Data fits ideal hydro-dynamical models i.e. zero viscosity
Jets in heavy ion collisions
• Studying deconfinement with jets
key QCD prediction: jets are quenched
Fragmentation
X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480
radiated
gluons
soft beam jet
pTOT
pT
quark
pL
di-quark
jet
heavy nucleus
Interaction at the quark (parton) level
• Models of jet suppression
Various approaches; main points:
The same interaction at the hadron level
Multiple soft scattering: Weidemann et al.
Opacity expansion: Gyulassy et al.
Twist expansion: Wang et al.
DEmed is independent of parton energy.
DEmed depends on length of medium, L.
kT2
DEmed gives access to gluon density dNg/dy or transport coefficient qˆ 

Leads to a deficit of high pT hadrons compared to p+p collisions (no medium).
Suppression of high pT
hadrons
Central collisions
Nuclear modification factor
d 2 N AA /dpT d
RAA ( pT ) 
TAAd 2 NN /dpT d
Scale factor
p+p reference
no. binary collisions

pp
TAA  Nbinary  inelastic

Peripheral collisions
Divide PT Spectra of AA
by pp (with scaling
factor)
First Pb-Pb Collisions Sunday
7th November 2010
• The ALICE age begins
• Almost all observables in a
single, general purpose, HI
detector.
• Aims to study the QGP in
detail.
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ALICE Detector
Technologies:18
Tracking: 7
PID:
6
Calo. :
5
Trigger,
30 Nch:11
Detector:
Size: 16 x 26 metres
Weight: 10,000 tons
Collaboration:
> 1000 Members
> 100 Institutes 30
> 30 countries
Event Characterisation
 Particle Production and Energy density :
 Produced Particles: dNch/d ~ 1600 ± 76 (syst)
Matter
underinextreme
conditions:
 ~ 30,000
particles
total, ~ 400
times pp !
(at moment
condenses
to
 somewhat
on highQGP
side of
expectations
‘normal’
matter)
 growth
with energy
faster
in AA than pp
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 > 15 GeV/fm
~ 50 times core of a neutron star
(40 billion tons/cm3)
50 protons packed into the volume of
one !
Temperature > 4 x 1012 K
> 200,000 times centre of Sun !
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Charged Jets in Pb-Pb
Large suppression of
jets seen in central
collisions
10-20% peripheral
168 GeV
Another way to study this effect is to look at high pT192 GeV
suppression.
0-10% central
102 GeV
D
47 GeV
D
bin size: 0.1x0.1
Back-to-back highenergy jets seen in
peripheral collisions
D
D
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Nuclear Modification Factor
Divide pT spectra in Pb-Pb by p-p
(with suitable normalisation factor)
D
d AA
/ dpT
R ( pT ) 
D
 Ncoll  d pp
/ dpT
D
AA
• particle interaction with
medium
• stronger suppression at LHC
• ~factor 7 at 7 GeV/c
quark-gluon plasma
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RAA vs centrality
• Suppression
increases with
centrality.
• Minimum remains
around 6-7 GeV
RAA for different particles
Peripheral collisions
Central collisions
RAA Open Charm:
Nuclear Modification Factor
D
d

D
AA / dpT
RAA
( pT ) 
D
 Ncoll  d pp
/ dpT
• mass ordering predicted:
DE() > DE(D) > DE(B)
• charm RAA is approaching 
at pT> 6 GeV/c
• Possible indication of
smaller charm suppression
at low pT but overall
comparable with s.
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RAA Heavy Quarks to Leptons:
Nuclear Modification Factor
• semi-leptonic decays:
• b, c → e+X
• b, c → m+X
• heavy quark suppressed
by factor ~ 3
• B feed-down
uncorrected
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RAA J/:
Nuclear Modification Factor
D
d AA
/ dpT
R ( pT ) 
D
 Ncoll  d pp
/ dpT
D
AA
• J/ suppressed by factor ~ 2
• inclusive J/: ~10% B feed-down
• cold nuclear effect not
measured: need pPb collisions
• small centrality dependence
• a test of statistical hardonization
will need data in mid rapidity:
PbPb runs in this year
Inclusive J/ RAA = 0.49  0.03 (stat.)
 0.08 (syst.)
Rather small suppression and less than at RHIC
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 & ’ Suppression from
CMS
 Primes suppression even more
(as expected from colour
screening in QGP).
 Suppression seen at CMS – colour
screening in QGP strong enough to
break-up bb bound states?
Elliptic Flow
 v2 shape is identical to
RHIC
 v 2 ~30% larger than
RHIC: larger <pT>
 ,K reproduced by
hydro calculation
 p ok for semiperipheral but is a
ALICE, Phys. Rev. Lett. 105 (2010) 252302.
remaining challenge for
Hydro-calcs.: Shen, Heinz, Huovinen and Song, arXiv:1105.3226.
central collisions
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Does QGP Flow?
 Result: The QGP is a (almost) perfect liquid
 unexpected result
 QGP initially though to behave like a gas
(weakly interacting)
 closest Theory prediction /S > 1/4≈ 0.08
 AdS/CFT:
SUSY string theory in 5 dimensions !
Viscosity/Entropy /s
 perfect liquid  (‘response to pressure gradients’)
(strong interactions in the liquid)
 QGP almost ideal fluid, /S < 0.2 - 0.5
 usually use Viscosity/Entropy
(/S dimensionless number)
QGP
1012 0C
Most perfect liquid ever produced!
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Temperature
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Azimuthal Flow:
What next ?
 Precision measurement of /s:
 current RHIC limit: /S < (2-5) x 1/4
 /S < 1/4 => conjectured AdS/CFT limit is wrong
 /S > 1/4 => measure 
 /S ≈ 1/4 => quantum corrections
which are O(10-30%) in AdS/CFT!
 20% in v2 ~ 1/4 need few % precision

2m kT

Precision: How ?
 fix initial conditions (geometrical shape is model dependent, eg Glauber, CGC)
 quantify flow fluctuations  (influence measured v2, depending on method)
 measure non-flow correlations d (eg jets)
2
2
 improve theory precision (3D hydro, 'hadronic afterburner', ...) vn {2}  vn   n  d
 .........
2
2
vn {4}  vn   n
QM2011 J. Schukraft
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Higher Harmonics: v3, v4,
v5, ...
• fluctuations in the
initial distribution
• non zero v3
• particle correlation is
described by first five
harmonics
Summary
•
•
•
•
Heavy-ion physics has come a long way in the last 25 years
and given us a number of surprises along the way.
The more we learn about the QPG, the more exotic it appears.
We are studying the highest temperatures, highest densities,
and (oddly enough) the most ideal liquid every created in an
experiment.
• ALICE is at the very beginning of a 10+ year programme and
we have just scratched the surface (with ~1% of required
data).
• lots more work and exciting physics to look forward to.
• Probably a few more surprises as well.
Thanks Norman
• Thank you Norman for your help, support, and
encouragement over the years.
• Have a great retirement.
• Many thanks to every else for listening.