Nuclear Physics at Tera Electron Volt Energies

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Nuclear Physics at Tera Electron Volt Energies
Y.P. Viyogi, VECC
Aim Study of Quark Gluon Plasma (a deconfined
phase of hadronic matter)
? Predictions from Quantum Chromodynamics (the
theory of strong interactions) Collisions of
heavy nuclei at TeV energies may produce large
energy density and compression such that hadronic
matter will momentarily dissolve into a plasma of
quarks and gluons moving freely. This will be a
transient state.
? Connections with Cosmology A few microseconds
after Big Bang, matter in the early Universe was
in the form of quark gluon plasma.
? Connection with Astrophysics Matter in the
dense core of neutron stars may be in the form of
quark gluon plasma.
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Phase Diagram
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Time evolution of nuclear collisions
?s
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Pre-equilibrium
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What to look for ?
Energy density Temperature initial, critical,
freeze-out (kinetic, chemical) Particle
production Thermalisation / Equilibration Collect
ive flow Jet quenching dE/dx of quarks and
gluons Black body radiation thermal direct
photons low mass dileptons Deconfinement
colour screening Phase transition fluctuation
at phase boundary Chiral symmetry
restoration Melting of resonances Disoriented
chiral condensates Energy dependence of
parameters what to expect in future ?
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Facilities
Alternating Gradient Synchrotron (AGS)
accelerator at BNL (New York) Au ions upto 11A
GeV (fixed target experiments), CM energy 5.A
GeV Super Proton Synchrotron (SPS) accelerator at
CERN (Geneva) Lead ions at 158.A GeV (fixed
target experiments), CM energy 17.A GeV for lead
target Relativistic Heavy Ion Collider (RHIC) at
BNL (New York) Au ions at 100A GeV 100A
GeV Large Hadron Collider (LHC) at CERN (Geneva)
Lead ions at 2.75A TeV 2.75A TeV (to be
operational in 2007).
Experimental Equipments Several types of
detectors for tracking, calorimetry, multiplicity
measurements, time of flight measurements etc.
supported by large international collaborations
(100-150 at SPS, 300-500 at RHIC, 1000-2000 at
LHC). A small component, preshower Photon
Multiplicity Detector (PMD) from India for SPS,
RHIC and also LHC. A part of tracking chambers of
muon spectrometer for LHC also from India.
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CERN (Geneva, Switzerland)
LHC
SPS
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Brookhaven National Laboratory, New York
RHIC
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Indian participation in experimental QGP program
Emulsion experiment EMU01 Collaboration
Chandigarh, Jaipur, Jammu since 1986 EMU14
Jadavpur Online experiments (Inspirational
leadership of Bikash Sinha) WA93/WA98
Collaboration at CERN SPS VECC, IOP,
Chandigarh, Jaipur, Jammu since 1990, with an
Indian detector (PMD) STAR Collaboration at RHIC
VECC, IOP, Chandigarh, Jaipur, Jammu, IITB with
PMD PHENIX Collaboration at RHIC BARC,
BHU ALICE Collaboration at CERN LHC SINP, VECC,
IOP, Aligarh, Chandigarh, Jaipur, Jammu (PMD and
Tracking chambers)
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Preshower Photon Multiplicity Detector (PMD)

• At SPS (WA93/WA98 Experiments)
• Scintillator pads with wavelength shifting fibres
  using image intensifier CCD camera systems
  readout. 3 X0 thick Lead converter
• Scintillator pads of size 10, 15, 20, 25 mm2
• WA93 (1990-92) 8000 pads covering 3m2
• WA98 (1993-96) 53000 pads covering 21m2

At RHIC and LHC (STAR and ALICE
experiments) Honeycomb gas proportional counter
with copper honeycomb cathode, gold plated
tungsten wire anode, anode signal processing
using GASSIPLEX and MANAS, 3X0 thick lead
converter. STAR 83,000 cells, 1 sq.cm
cross-section, 8mm gas depth ALICE 220,000
cells, 0.22 sq.cm cross-section, 5mm gas depth
PMD probes thermalisation (flow), phase
transition (multiplicity fluctuation), Chiral
symmetry restoration (charged-neutral fluctuation)
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WA98 Experiment at CERN SPS
Common coverage of PMD and SPMD
Charged-neutral correlation
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WA98 PMD
53000 pads, 21 sq.m. area
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Centrality Selection
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The tracking complexity
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Particle production
WA98 photons _at_ SPS
PHOBOS Charged particles _at_ RHIC
130 GeV
200 GeV
20 GeV
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Transverse Energy
Increasing role of hard processes, scaling with
Nbinary
Transverse energy per particle almost same at SPS
and RHIC Extra energy goes into particle
production, not in increasing Pt
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Energy Density
Bjorken estimate
t0 formation time ( 1fm/c)
NA50
PHENIX _at_ RHIC estimate 4.6 GeV/fm3 for top
0-5 centrality
Energy density 30 times nuclear matter density
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Energy evolution of B/B ratio
Pair- production is now much larger than baryon
transport!
(ISR)
Baryon free region At LHC
STAR preliminary
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Thermalisation Flow
Initial space anisotropy ?
Carried to final state momentum anisotropy
Pressure gradient in the overlap zone
collective flow in the reaction plane
n1 directed N2 elliptic
1 2 S vn cos nf
dN --- df
v1 shift of centroid, v2 measure of
ellipticity ?1 , ?2 specify orientation
v1,v2 should decrease with increasing centrality
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Azimuthal Anisotropy WA93 PMD
Second order anisotropy coeff. (elliptic flow) of
photons
First observation of Collective Flow at SPS
Phys. Lett. B403 (1997) 390
Contribution from p decay
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Anisotropy in WA98 Experiment
Charged particles
photons
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Anisotropy in Neutral Pions
Simulation of a large number of data set for
various combinations of flow and multiplicity
Phys. Lett. B489 (2000) 9
Parameter ?m can be determined from experimental
data
Scaling relation
V( ? ) a -------- ---------- c,
Vin(p) (? b)²
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Azimuthal Anisotropy WA98 PMD
Photon flow assuming Vn (p) V n(p)
directed
elliptic
Advantage of p flow No coulomb effect
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Flow at RHIC
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Thermalisation Jet quenching
Binary collision limit (hard scattering)
Effect on Pt distribution Cronin effect Nuclear
shadowing Jet quenching
Pt (GeV/c)
At low Pt enhancement due to Cronin effect At
high Pt jet quenching dominates
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Jets in STAR
?? ? 0.5 units
First observation of jet-like structure in
nuclear collisions
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Jet Quenching Theory
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Chiral symmetry restoration DCC
At T gt Tc Chiral symmetry restored ? Vacuum
expectation value of chiral field is zero. At T lt
Tc Chiral symmetry broken ? Vacuum may be
oriented in one of the pion directions
(disoriented wrt normal vacuum directions) Disorie
nted chiral condensates (DCC) formed in domains
of (?,f) emission of low pt pions Distribution
of neutral pion fraction () very different for
DCC and generic events p ? 2? shows up in
photon detectors p? shows up in charged
particle detectors
Look at Ng vs. Nch fluctuations
Apart from correlated fluctuation, individual
fluctuations in Ng and Nch important for the
study of phase transition
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A normal event in WA98 PMD and SPMD
SPMD hits projected onto PMD plane
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Charged particle depleted events
An event of WA98 PMD -- SPMD
Anti-CENTAURO event of JACEE 36 photons, 1
charged particle
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Ng vs. Nch Fluctuation

• Top 5 central events ONLY
• Bins in f 1,2, 4, 8, 16
• Discrete Wavelet Analysis
• Correlation Analysis

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Sensitivity to DCC
Simulation with a simple DCC model p/ p
introduced at freezout
Mixed events for PMD/SPMD Breaks different
correlations (detector effects) M1 both
individually mixed M2 N? and Nch from different
events M3? PMD no, SPMD mixed M3ch SPMD no,
PMD mixed
nDCC event sample with some fraction of DCC
events
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RMS widths of Data and Mixed events
Comparison with 4 types of mixed events
Data width gt M1
First observation of non-statistical fluctuation
in particle multiplicity
Phys. Rev. C64 (2001) 011901
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Formation of DCC upper limits
0-5 central
Global DCC
Localized DCC domain
? fraction of pions as DCC pions
Phys. Lett. B420 (1998) 169
Upper limit for DCC-like localized
fluctuations 3x10-3 for central collisions.
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Thermal radiation direct photons
Photons (real or virtual) primordial origin,
very large mean free path ? very good signal of
QGP formation, direct measure of black body
radiation But, weak signal, direct photons only
2.5 of decay photons at SPS, expected 10 at
RHIC Overwhelming background of decay photons (
???2?), difficult to measure Extraction of
direct photon spectra vary careful estimate of
hadronic cocktail (contribution from all known
resonances) Systematic errors large (subtraction
of ?? and ?? contributions)
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First Observation of Direct photons at SPS

WA98 _at_ SPS
Phys. Rev. Lett. 85 (2000)3595
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Direct photons theory
Huovinen/Ruuskanen
Srivastava/Sinha
S/S plasma, hydrodynamics, Ti 335 MeV,
suggest at least half of the total photon
spectrum should be thermal H/R fix Ti
210-250 MeV, attempt to explain data with (EOS A)
and without (EOS H) phase transition. But for
EOS-H, energy density of hadron gas 8 GeV/fm3
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Thermal radiation Low mass dilepton production
Virtual photons ?lepton pairs, large background
in muon channel, ee- cleaner
NA45 _at_ SPS
Hadronic cocktail
Excess over hadronic cocktail could be from
thermal radiation No peak for ?, f resonances
melting may signal chiral transition Explained by
both QGP and medium-modified hadronic models
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Deconfinement transition colour screening
J/y Suppression
J/y production direct, feed-down from ?c,
y? J/y suppression colour Debye
screening Strong colour field of the QGP prevents
formation of attractive potential for c-cbar
binding to J/y But J/y also have nuclear
absorption (pA data)
D-
Experiment measure ? ?- pair invariant mass
spectrum, wrt to Drell-Yan background
D
J/y ? ee- also important, being tried at RHIC,
LHC
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J/y Suppression
NA50 _at_ SPS
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J/? suppression Theory
Plasma model
Comover absorption model
Models based on QGP explain all the features of
data. Models based on comover absorption also
explain good part of the data. Bulk of comover
absorption takes place around energy density 1
GeV/fm³ or more. The question can comovers
exist in high energy density medium ?
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How dense a hadron gas can be ?
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Future what to expect at LHC ?
Very high initial temperature 900 MeV Energy
density 3-5 times _at_RHIC Plasma lifetime 5-7
times _at_RHIC Quarkonium charm to bottom (?
region) Plenty of jets going to 100 GeV and
beyond
ALICE experiment at LHC designed to handle 8000
particles at y0
What are our preparations for future ?
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STAR experiment at RHIC, BNL
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STAR PMD in Wide Angle Hall
Veto plane
Preshower plane
Engineering run of STAR PMD during 2002-2003 RHIC
running period
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Charged-neutral correlation in STAR
PMD behind Forward TPC, which measures charged
particle momenta
Cut on pt (ch) greatly enhances the strength
?
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TOF
TRD
HMPID
TPC
PMD
ITS
Muon Arm
PHOS
ALICE Set-up
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J/? anisotropy in ALICE using PMD event plane
A simulation study
Absorption of J/? by comovers in the forward
rapidity region predicted to be azimuthally
anisotropic. Anisotropy can be studeid using
event plane from PMD.
Results Input v1 v2 0.05 Event plane
v2 PMD ?like 0.0520.002 p 0.0530.003
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How to study jets in forward direction in ALICE?
Low-cost EM-calorimeter behind PMD (pp physics,
jets, E_t Flow.)
Five planes of 1cm lead 5mm Scintillator
PMD
A new direction in both instrumentation and
physics incentive to younger generation
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Summary
A new form of matter seems to have been created
with very high energy density and temperature,
which undergoes rapid expansion and is at least
partly thermalised.
Thermal direct photons have been measured. Colour
screening of charmonium resonances observed.
Chiral symmetry seems to be restored in the new
matter.
However some of the results seem to be explained
by a dense (probably almost unphysical) hadron
gas.
Indian team has made significant contribution to
the experimental program of ultra-relativistic
nuclear collisions.Contributions growing in
coming years.
The future at RHIC and LHC is going to be much
more exciting. Crucial for resolving ambiguities.
Keep in touch.
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An artists impression of QGP
THANK YOU
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Event-by-event fluctuation
Thermodynamic quantities ? fluctuation in global
observables
phase transition large fluctuation at
phase boundary
At tricritical point singularities in
thermodynamical quantities large e-by-e
fluctuations in experimental observables
First order phase transition supercooling ?
density fluctuation ? droplet formation ?
rapidity fluctuation (spikes, gaps)
In high energy nuclear collisions, large particle
multiplicity (400-4000) statistical fluctuation
only a few percent. Convenient to study
event-by-event fluctuation.
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Gaussians
STATISTICAL DYNAMICAL
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Multiplicity Fluctuation
Multiplicity distributions are GAUSSIANS
for narrow bins in centrality. The physics
(statistical dynamical) is in the width of the
distribution. The amount of fluctuation
w s2/ lt N gt
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Multiplicity Fluctuations for various
centralities
Charged Particles
Central
Peripheral
Data agree fairly well with participant model
calculations