Transcript Strong interactions and multiparticle production in heavy

High energy heavy ion interactions
and the search for the Quark-Gluon
Plasma
Alberta Marzari-Chiesa / Univ. TORINO
Luciano Ramello / Univ. Piemonte Orientale
NURT 2003 - La Habana, Cuba, October 27-31, 2003
Plan of the presentation






Introduction to Quark Gluon Plasma physics and
heavy ion collisions
Experimental observables to determine the centrality
of each collision
Present and future experimental facilities at CERN
and at Brookhaven National Lab.
Some results on the global features of the collisions
Specific QGP signatures: enhancement of strange
particles, charmonium suppression
Why go to higher energy (experiment ALICE at the
LHC) ?
A. Marzari-Chiesa and L. Ramello - NURT 2003
2
Nuclear matter and QGP



“Ordinary” Nuclear Matter is made of nucleons and electrons.
Nucleons (and other hadrons) are made of quarks: a nucleon is
made of 3 quarks, a meson (, K, ,…) is made of a quark and an
antiquark. In ordinary matter the quarks are never free: they
are confined inside the hadrons, and their mass is mumd300
MeV, ms 500 MeV.
The energy density of ordinary nuclear matter, for a nucleus of
mass number A and radius R = r0A1/3, ro=1.2 fm is 0.14 GeV/fm3
Quark Gluon Plasma (QGP) is a state of matter in which quarks
and gluons are free, and their mass, being the “bare” mass, is
smaller: mumd5 MeV, ms 150 MeV. QCD lattice calculations
predict that this state occurs when the density is   10 o: when
density is increased enough “interpenetration can occur and eventually
each quark will find very many others in its immediate neighbourhood. [...] it
has no way to remember which of these were the partners in the low-density
nucleonic state” (H. Satz - Nature 324 (1986) 116)
A. Marzari-Chiesa and L. Ramello - NURT 2003
3
Phase transition to QGP
Such a high density can be obtained:
• by compressing baryons
• by heating a mesonic medium,
increasing its density by particle
production in collisions
QCD lattice calculations predict that
the phase transition from ordinary
nuclear matter to QGP can occur for
temperatures T~150-200 MeV and/or
for energy densities ε > 2.5-3 GeV/fm3
A. Marzari-Chiesa and L. Ramello - NURT 2003
4
QGP formation: where (and when) ?

In early Universe
(<1ms after Big
Bang)

possibly in the core
of neutron stars

a transient state in
Heavy Ion collisions
A. Marzari-Chiesa and L. Ramello - NURT 2003
5
Why HEAVY ION interactions ?


The system must be “large” (d>>1 fm) and of “long lifetime”
( > 1 fm/c = formation time)
Moreover it must be near equilibrium, and this can be
realized only if the number of collisions per particle is > 1
The first condition is satisfied by heavy nuclei:
RPb  1.2(208)1/3  7 fm.
Also the second one is satisfied, since the mean free path for a
hadron is much lower than the dimensions of a heavy nucleus (with 
= 0.14 fm-3 and  = 40 mb = 4 fm2  lh=1/ = 1.6 fm << d ). For
quarks and gluons lq 0.5 fm , lg 0.2 fm : the situation is therefore
even better.
Other conditions (temperature, energy density) cannot be “a
priori” estimated: they must be determined experimentally.
A. Marzari-Chiesa and L. Ramello - NURT 2003
6
Heavy nuclei are extended objects

the collision can be quite different, depending on the
way in which the nuclei interact. The parameter that
describes the collision is the impact parameter b,
defined as the minimum distance between the
centers of the two nuclei:
b
Central
b0
Peripheral
b0
Very peripheral
b = r 1+ r2
A. Marzari-Chiesa and L. Ramello - NURT 2003
7
Geometrical spectator/participant picture



The nucleons inside the interaction volume are called
participants, and the other spectators.
Spectators proceed almost unperturbed with momentum
close to the one of the beam
Participants interact, and many n-n collisions occur in the
interaction volume, producing secondary particles
A. Marzari-Chiesa and L. Ramello - NURT 2003
8
Experimental observables for centrality
NA50 experiment at CERN SPS
Transverse Energy ET
Multiplicity Nch
Forward energy EF
A. Marzari-Chiesa and L. Ramello - NURT 2003
9
Experimental observables (cont’d)
Forward Energy is measured with a ZDC (Zero Degree Calorimeter).
Spectators proceed in beam direction at very forward angles (at SPS: θ <0.5
mrad) and their energy per nucleon is the same of the beam. A calorimeter
covering angles < 0.5 mrad measures the total energy of spectators
(contribution of secondaries coming from the interactions is negligible at
these angles). The number of spectators (Nspec) can be obtained dividing
EZDC by the energy per nucleon of the beam, Ebeam (158 GeV for Pb at
SPS). The number of participants Npart will be: Npart = A – Nspec = A EZDC/Ebeam
 Transverse energy (defined as ET=Eisinθi) depends only on energy
deposited in the interaction volume. It is therefore proportional to the number
of participants. ET is invariant under the boost from the C.M. system to the
lab system. Some experiments measure the electromagnetic transverse
energy ET0, i.e. the e.m. showers from the gamma rays which arise mainly
from the neutral pion decays. ET0 is proportional to total transverse energy.
 Charged Multiplicity is measured by (e.g.) a silicon detector, and it scales
with the number of participants as well, similarly to transverse energy. The
number of charged particles is proportional to the total number of particles
(e.g.: +0- are produced in equal amounts).

A. Marzari-Chiesa and L. Ramello - NURT 2003
10
Transverse energy distributions
The agreement between the
model (Venus 4.12) and the
data is clearly visible.
It is therefore possible,
with a rather simple
calculation, convert ET or
Nch into the number of
participants.
A. Marzari-Chiesa and L. Ramello - NURT 2003
11
Multiplicity distributions (1)
NA57 experiment at SPS: charged multiplicity
Nch in the pseudorapidity range 2<η<4
measured with silicon microstrip detectors
Events have been classified in five
centrality classes, corresponding to
given fractions of the total inelastic
Pb-Pb cross-section at 158 A GeV
A. Marzari-Chiesa and L. Ramello - NURT 2003
12
Multiplicity distributions (2)
PHENIX, one of the four RHIC experiments, measures charged multiplicity
with two different detectors
A. Marzari-Chiesa and L. Ramello - NURT 2003
13
Heavy ion facilities at CERN and BNL
Accelerators for fixed target experiments:
• AGS (1986) Si, Au beams
E(max,lab) = 14.5-11.5 AGeV
Colliders:
sNN  5.5 - 5GeV
• SPS (1986) O, S, In, Pb beams • RHIC (2000) Au beams
100+100 AGeV
E(max,lab) = 200-160 AGeV
• LHC (2007) Pb beams
sNN  19.5 -17.5GeV
2.7 + 2.7 ATeV
@SPS : experiments extremely specialized
in studying particular phenomena
@ RHIC and LHC: multipurpose
experiments
A. Marzari-Chiesa and L. Ramello - NURT 2003
14
CERN accelerators
A. Marzari-Chiesa and L. Ramello - NURT 2003
15
The SPS heavy ion physics program
dimuons
2003
hadrons
•
•
multistrange
1986 - 1987 :
Oxygen @ 60 & 200 2000
GeV/nucleon
1987 - 1992 :
Sulphur @ 200
GeV/nucleon
P
b
photons
hadrons
NA57
hadrons
•
1994 - 2000 : Lead
@ 40, 80 & 158
GeV/nucleon
2002 - 2003 :
Indium and Lead @
158 GeV/nucleon
NA49
NA45
Ceres
1994
NA52
dimuons
NA44
hadrons
1992
WA94
S
O
strangelets
NA50
WA98 WA97
•
NA60
dielectrons
NA34/3
Helios-3
WA80
WA85
NA34/2
Helios-2
NA35 NA36 NA38
1986
A. Marzari-Chiesa and L. Ramello - NURT 2003
16
Brookhaven National Lab
A. Marzari-Chiesa and L. Ramello - NURT 2003
17
Complex events



The events are very
complex, having a very
high multiplicity ( 500
charged tracks a @ SPS,
> 1000 tracks @ RHIC).
Nevertheless many
measurements have been
made and understood
 Formidable
experimental challenge
expecially for tracking
NA49 at CERN SPS uses magnets to
separate out charged particles and
Time Projection Chambers to measure
their trajectories
A. Marzari-Chiesa and L. Ramello - NURT 2003
18
June 12, 2000 at 9pm
RHIC event
This is one of the first
Au-Au collisions
recorded by the Time
Projection Chamber
(TPC) of STAR, one
of the four RHIC
experiments
A. Marzari-Chiesa and L. Ramello - NURT 2003
19
Global Measurements (to check whether the phase
transition is possible): (1) temperature
If a system of particles is in thermal equilibrium at temperature T,
the transverse mass distribution is:
1 dN
 e - mT / T
mT dmT
mT 
2
pT + m 2
 measuring the inverse slope of the mT distribution, we can obtain T
From the mT spectra (see next slide), it is evident that they are consistent
with an exponential low. BUT it is also evident that the slope parameter
increases with the particle mass.
This was explained with a collective flow (expansion of the interaction
volume):
Ekin  Ekin + m   flow 2
Th .
which introduces a term that depends on the particle’s mass.
A. Marzari-Chiesa and L. Ramello - NURT 2003
20
Temperature and flow from mTspectra
NA49: Blast wave fit indicates
temperature of 122-127 MeV
and average flow velocity of
0.48
Pions were not included
in the blast wave fit due
to significant resonance
contribution at low mT
pions and deuterons not included in fit
V. Friese, NA49, Strange Quark Matter 2003
A. Marzari-Chiesa and L. Ramello - NURT 2003
21
mT spectra at RHIC

First mT spectra from
STAR (for negatively
charged pions) show
higher temperature with
respect to SPS
experiments
T~ 190 MeV
A. Marzari-Chiesa and L. Ramello - NURT 2003
22
Global Measurements: (2) energy density
It is estimated from transverse energy
using Bjorken’s model:
o= formation time = 1 fm/c
R=1.12 A1/3 fm;  = rapidity
dET
 Bj ( 0 )  2
 R  0 d
1
It is evident that in central Pb-Pb interactions the energy
density (3.2 GeV/fm3) is well above the value expected for
the phase transition (crit  1 GeV/fm3)
A. Marzari-Chiesa and L. Ramello - NURT 2003
23
Energy density at RHIC
ET per Participant & per Charged Particle is even higher at RHIC:
PHENIX
PRELIMINARY
NA49-WA98 @ SPS
PHENIX
PRELIMINARY
Number of Clusters in PC1
dET/d per participant @ percentile

Au+Au 130A
GeV
Energy in EMCal
(GeV)
ET/participant is 50% larger than for SPS; dET/d is 40% higher than SPS,
As a consequence, Energy density is higher: approximately, more than 40%
larger at RHIC than at SPS since the parameters of the Bjorken formula
were calculated for SPS.
A. Marzari-Chiesa and L. Ramello - NURT 2003
24
Quark Gluon Plasma signatures

A probe for deconfined matter (QGP) must:




Several signatures were proposed, and most of them were
searched for. The results must be carefully studied, taking into
account that:



(obviously) distinguish between confined and deconfined matter
be present in the initial stages of the interaction (the QGP phase)
preserve a memory of the initial state during the evolution of the
system
Signals compete with backgrounds emitted from “normal” nuclear
sources
Signals are modified by final-state interactions: after the QGP
phase, as soon as the temperature becomes lower, a
hadronisation phase occurs, in which the quarks become bound
Here we will present only two signatures:


strangeness enhancement
J/Ψ suppression
A. Marzari-Chiesa and L. Ramello - NURT 2003
25
How to validate QGP signatures

The way of analyzing the results is common to all
the signatures:



The effect is measured in light systems as p-p or pnucleus, where no QGP can be present, and then it is
“extrapolated” to heavier systems.
The extrapolation is made assuming that a nucleusnucleus interaction is the superposition of many
nucleon-nucleon interactions.
If the experimental results are different from this
extrapolation, one concludes that something different
happened.
A. Marzari-Chiesa and L. Ramello - NURT 2003
26
Strangeness enhancement


In hadron interactions, strange particle production occurs via
associated production.
The reaction that requires the minimum energy is:
pp  pK
Q  M  + M K - M p  670MeV
for which:

Strange anti-baryon production requires more energy:
pp  pp
Q  2M   2.2GeV
pp  ppKK
Q  2M K  986MeV
In a QGP the energy threshold is lower,
being the energy to produce an ss couple:
Q  2ms  300MeV
A. Marzari-Chiesa and L. Ramello - NURT 2003
27
Multi-strange hadrons

The strangeness enhancement is not conclusive if limited to K/ ratio:
the K production enhancement can in fact be explained through rescattering:
+K+K
BUT

++
for multistrange baryons or multistrange antibaryons the situation is different:
 - ( s s s ) ( M   1670MeV ) can be produced via:
with a very high threshold
 +   
or via a long series of interactions:
 + N   + K ......+ N production
N + K   +
 +   +
 + K   +

which take a long time (~100 fm/c,
to be compared to 5-10 fm/c of a
single N-N collision)
in a QGP with strangeness enhancement factor Es the hadrons containing N
strange quarks are produced with a rate EsN times higher than in an
environment with no strangeness enhancement. So in QGP: E>E>E
A. Marzari-Chiesa and L. Ramello - NURT 2003
28
Experimental measurement of strangeness
Strangeness production was
measured by experiments
WA97 and NA57 at CERN
SPS, with Pb beam at 158
GeV/nucleon
A. Marzari-Chiesa and L. Ramello - NURT 2003
29
Multi-strange hyperon enhancement
WA97 has seen a clear enhancement:
for  and anti- there is a factor 17 with
respect to extrapolation of p-Be and p-Pb
results. NA57 later confirmed the result.
A. Marzari-Chiesa and L. Ramello - NURT 2003
30
K/ ratio vs. center of mass energy
   +2( K + + K - )
ES 
 
where  not published :
 (  +  -  K - ) / 0.8
Data at 30 AGeV support
phase transition scenario
(Statistical Model of the Early Stage)
Volker Friese (NA49), Strange Quark Matter 2003, Atlantic City, March 2003
A. Marzari-Chiesa and L. Ramello - NURT 2003
31
K/ ratio vs. energy
BRAHMS results at y=0 seem to indicate saturation of K+/π+ reached at top SPS energy
A. Marzari-Chiesa and L. Ramello - NURT 2003
32
Charmonium suppression




Quark binding can be dissolved in quark matter. The mechanism is
similar to the Debye screening observed in atomic physics:
“The force between the charged partners of a bound state is
considerably modified, if this bound state is placed in an environment of
many other such objects. The Coulomb potential between two electric
charges e, separated by a distance r, in vacuum is proportional to e2/r.
In the presence of many other charges it becomes subject to Debye
screening:
e2
e2

 e - r / rD
r
r
where the screening radius rD is inversely proportional to the overall
charge density of the system.
If in atomic matter the Debye radius becomes less than the atomic
radius rA , then the binding force between electron and nucleus is
effectively screened, and the electron becomes “free”. For atomic
systems, an increase in density thus results in an insulator-conductor
transition” (H. Satz, Nature, 1986).
Something similar can happen in a deconfined medium for the colour
charge between a quark and an antiquark
A. Marzari-Chiesa and L. Ramello - NURT 2003
33
Charmonium suppression (cont’d)
For the colour charge, in a “normal” nuclear medium:
V ( r )  r -

r
where r is the term responsible of the quark confinement and /r is the
Coulomb-like term
In a QGP where quarks are deconfined and many colour charges are present:
 -r / r
V (r ) 
r
e
C
If rC is smaller than the distance at which a quark and an antiquark become
bound to form a particle, the bound state cannot be formed.
rC is inversely proportional to the charge density. Since the quark density
is proportional to the temperature, we expect that rC is decreasing with
temperature.
A. Marzari-Chiesa and L. Ramello - NURT 2003
34
Charmonium suppression (cont’d)
J/Ψ, Ψ’ and χc are different
bound states of the charmanticharm system
(charmonium)
Each of them has a different
bound state radius ri: when
temperature T is high enough
so that rD(T) < ri, then the i-th
charmonium state is
dissolved by the QGP.
This means that as soon as 1.1 TC (TC is the phase transition
critical temperature) is reached, c (and ’) cannot be formed,
while 1.3 TC is needed to dissolve also J/.
A. Marzari-Chiesa and L. Ramello - NURT 2003
35
Experimental study of charmonium

EXPERIMENTS NA38 & NA50 at CERN
studied the charmonium suppression measuring the J/ production as a function
of the number of participants.
NA50 is the upgrade of NA38, having three centrality detectors instead of one,
and a higher rate capability.
J/’s were detected through their decay in m+m-: the experimental
apparata consisted therefore essentially of a dimuon spectrometer +
centrality detector(s). Characteristic of these experiments is the high
beam intensity ( 107 Pb/s), due to the low J/ production cross section
and to the low branching ratio in two muons:
J /   mm  6 10-2
 '  mm  7 10-3
A. Marzari-Chiesa and L. Ramello - NURT 2003
36
The NA50 experiment
J/Ψ
Drell-Yan
The absorber stops all the hadrons, and
only muons can reach the last chambers.
Measuring the emission angle and the
curvature of both muons, it is possible to
reconstruct the m+m- invariant mass
A. Marzari-Chiesa and L. Ramello - NURT 2003
37
The Drell-Yan reference process
Drell-Yan is a rare, “hard” collision process and its cross-section
scales with the number of nucleon-nucleon collisions.
 DY
AB
  DY
NN
 A B
A nucleons
NN
 DY

B nucleons
This is in effect what is observed: had an
absorption been present, it would change
the scaling to (AB) , with  < 1.
Drell Yan reactions are therefore taken as a reference and many of the
J/ results were presented as a ratio J//DY.
A. Marzari-Chiesa and L. Ramello - NURT 2003
38
Nuclear absorption
J/ absorption with respect to DrellYan was already observed by the
NA38 experiment.
Unfortunately, it was not possible to
conclude that the QGP had been
observed since the suppression,
observed in Oxygen and Sulphur
interactions, is already present in pnucleus interactions.
The plot Bmm vs AB, that for Drell
Yan events is flat, here is consistent
with a continous decreasing
pattern from p-p to S-U
interactions:
Bmm(J/)  (AB)0.920.015
This behaviour can be accounted for
by nuclear absorption.
A. Marzari-Chiesa and L. Ramello - NURT 2003
39
Anomalous charmonium suppression
The observations in p-A, A-B collisions can be fitted by the empirical law:
 AB  AP BT NN J /  exp(-o L N )
abs
where 0 = nuclear density, L =
length of nuclear matter crossed
by the charm quark-antiquark
pair after its formation
L can be calculated using a simple
geometrical model (hard spheres) or
with more refined models of the
nuclei.
Going to heavier systems the situation
changes: for Pb-Pb the “normal” nuclear
absorption does not justify the results
and an “anomalous” additional
suppression is clearly present.
A. Marzari-Chiesa and L. Ramello - NURT 2003
40
Anomalous suppression (cont’d)

In this figure the ratio
J//D.Y. is divided by the
same ratio expected under
the hypothesis of “normal”
nuclear absorption.
The number of participants is
obtained from the measured
transverse energy.
A. Marzari-Chiesa and L. Ramello - NURT 2003
41
Anomalous suppression (cont’d)
The same analysis is possible with
EZDC and Nch as centrality
variables. Here the ZDC analysis
is reported. It is clear that the
suppression pattern is compatible
with a double step in EZDC. The
first could be due to the 
absorption, the second to the J/
one. All the models, based on
“normal” nuclear effects, are ruled
out.
A. Marzari-Chiesa and L. Ramello - NURT 2003
42
Why go to higher energies ?
Significant quantitative improvements
in the experimental conditions are
expected when going from SPS
energy to RHIC (already running
since June 2000) and later to LHC
(startup foreseen in 2007)
Energy density, volume and
lifetime of the plasma are very
much improved by going to
RHIC, and even more by going
to LHC
A. Marzari-Chiesa and L. Ramello - NURT 2003
43
More extended baryon free region
A net-baryon free region (no excess of protons over antiprotons) allows
easier comparison with theory
Net protons distributions indicate high degree of stopping at AGS energies,
less stopping at top SPS energy and almost full transparency at RHIC
A. Marzari-Chiesa and L. Ramello - NURT 2003
44
“Onium” suppression revisited
The main advantage of LHC for
“onium” physics will be the
access to Y (beauty-antibeauty)
states: this will allow
unambiguous confirmation of
the results already obtained from
charmonium studies at lower
energies.
RHIC is presently accumulating
data on charmonium, which
should allow access to a higher
transverse momentum region
than the one previously
explored.
A. Marzari-Chiesa and L. Ramello - NURT 2003
45
The ALICE experiment
A. Marzari-Chiesa and L. Ramello - NURT 2003
46
The ALICE Internal Tracking System
6 cylindrical layers of
silicon detectors:
 pixel detectors
 drift detectors
 double sided
microstrip detectors
Layer 3
Layer 4
14.9
23.8
Ladders
14
22
SDDs per ladder
6
8
Radius (mm)
A. Marzari-Chiesa and L. Ramello - NURT 2003
47
Drift
segmented
Drift
2 x 256 anodes
MOS charge injectors for
drift velocity monitoring
Wafer: 5”, NTD, 3 k.cm, 300 mm
Active area: 7.02  7.53 cm2
guard region
implanted HV voltage dividers
256 anodes (294 mm pitch)
A. Marzari-Chiesa and L. Ramello - NURT 2003
48
The Internal Tracking
System mechanical
support
A. Marzari-Chiesa and L. Ramello - NURT 2003
49
Challenges: rates and data volumes
ALICE will push the previous limits
of high energy physics experiments
in the direction of very large data
volumes, while other LHC experiments
will be demanding very high trigger
rates and Data Acquisition bandwidth
To insure that ALICE data will be
analyzed in a timely manner, the offline
software is being prepared well in
advance of the beginning of data taking
in 2007, and is being tested with M.C.
events.
The GRID software technology is being
developed in order to be able to process
data stored in several regional centers in
an efficient way, moving around
programs rather than data.
A. Marzari-Chiesa and L. Ramello - NURT 2003
50
High Energy Heavy Ion bibliography



C.Y. Wong - Introduction to high energy heavy-ion collisions, ed.
World Scientific (1994)
J. Schukraft & H. Schmidt - The Physics of ultrarelativistic heavyion collisions, Journal of Physics G 19 (1993)
Proceedings of the QM (Quark Matter) Conferences:
1991 - Gatlinburg (USA) - Nucl. Phys. A 544
1993 - Borlange (Sweden) - Nucl. Phys. A 566
1995 - Monterey (USA) - Nucl. Phys. A 590
1996 - Heidelberg (Germany) - Nucl. Phys. A 610
1997 - Tsukuba (Japan) - Nucl. Phys. A 638
1999 - Torino (Italy) - Nucl. Phys. A 661
2001 – Stony Brook (USA) – Nucl. Phys. A 698
2002 - Nantes (France) – Nucl. Phys. A715
A. Marzari-Chiesa and L. Ramello - NURT 2003
51