High energy heavy ion interactions and the search for the Quark-Gluon Plasma

High energy heavy ion interactions and the search for the Quark-Gluon Plasma

Alberta Marzari-Chiesa / Univ. TORINOLuciano Ramello / Univ. Piemonte Orientale

NURT 2003 - La Habana, Cuba, October 27-31, 2003

A. Marzari-Chiesa and L. Ramello - NURT 2003

2

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) ?


3

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 mumd300 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: mumd5 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)


4

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


5

QGP formation: where (and when) ? In early Universe

(<1s after Big Bang)

possibly in the core of neutron stars

a transient state in Heavy Ion collisions


6

Why HEAVY ION interactions ?

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.

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


7

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:

Central b 0

Peripheralb 0

Very peripheralb = r1+ r2

b


8

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


9

Experimental observables for centrality

Multiplicity Nch

Transverse Energy ET

Forward energy EF

NA50 experiment at CERN SPS


10

Experimental observables (cont’d)

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.: are produced in equal amounts).

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.


11

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.


12

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


13

Multiplicity distributions (2)PHENIX, one of the four RHIC experiments, measures charged multiplicitywith two different detectors


14

Heavy ion facilities at CERN and BNLAccelerators for fixed target experiments:• AGS (1986) Si, Au beams E(max,lab) = 14.5-11.5 AGeV

• SPS (1986) O, S, In, Pb beams E(max,lab) = 200-160 AGeV

GeVsNN 55.5

GeVsNN 5.175.19

Colliders:• RHIC (2000) Au beams 100+100 AGeV• LHC (2007) Pb beams 2.7 + 2.7 ATeV

@SPS : experiments extremely specialized in studying particular phenomena

@ RHIC and LHC: multipurpose experiments


15

CERN accelerators


16

The SPS heavy ion physics program

• 1986 - 1987 : Oxygen @ 60 & 200 GeV/nucleon

• 1987 - 1992 : Sulphur @ 200 GeV/nucleon

• 1994 - 2000 : Lead @ 40, 80 & 158 GeV/nucleon

• 2002 - 2003 : Indium and Lead @ 158 GeV/nucleon NA35 NA36

NA49

NA34/2Helios-2

NA34/3Helios-3

NA44

NA45Ceres

NA38

NA50

NA60

WA80

WA98

WA85

WA97

NA57

NA52

WA94SO

Pb

multistrange

photonshadrons

dimuons

dielectrons

1986

1994

2000

hadrons

strangeletshadrons

hadronsdimuons

1992

2003


17

Brookhaven National Lab


18

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 toseparate out charged particles andTime Projection Chambers to measuretheir trajectories


19

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

June 12, 2000 at 9pm


20

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:

Tm

TT

Tedm

dN

m/1 22

mpm TT

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):

which introduces a term that depends on the particle’s mass.

2. flowTh

kinkin mEE


21

Temperature and flow from mTspectra

pions and deuterons not included in fit

158 AGeV

NA49: Blast wave fit indicates temperature of 122-127 MeVand average flow velocity of 0.48

Pions were not included in the blast wave fit due to significant resonancecontribution at low mT

V. Friese, NA49, Strange Quark Matter 2003


22

mT spectra at RHIC

First mT spectra from STAR (for negatively charged pions) show higher temperature with respect to SPS experiments

T~ 190 MeV


23

Global Measurements: (2) energy densityIt is estimated from transverse energy

using Bjorken’s model:o= formation time = 1 fm/cR=1.12 A1/3 fm; = rapidity

dTdE

RBj0

201

)(

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)


24

Energy density at RHIC ET per Participant & per Charged Particle is even higher at RHIC:

dE

T/d

per

par

tici

pan

t @

per

cen

tile

NA49-WA98 @ SPS Num

ber

of C

lust

ers

in P

C1

Energy in EMCal (GeV)

PHENIX PRELIMINARY

Au+Au 130A GeV

PHENIX PRELIMINARY

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.


25

Quark Gluon Plasma signatures A probe for deconfined matter (QGP) must:

(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 Several signatures were proposed, and most of them were

searched for. The results must be carefully studied, taking into account that: 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


26

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 p-

nucleus, where no QGP can be present, and then it is “extrapolated” to heavier systems.

The extrapolation is made assuming that a nucleus-nucleus interaction is the superposition of many nucleon-nucleon interactions.

If the experimental results are different from this extrapolation, one concludes that something different happened.


27

Strangeness enhancement In hadron interactions, strange particle production occurs via

associated production. The reaction that requires the minimum energy is:

MeVMMMQ

Kppp

pK 670

for which:

Strange anti-baryon production requires more energy:

MeVMQKppKpp

GeVMQpppp

K 9862

2.22

In a QGP the energy threshold is lower, being the energy to produce an couple:ss

MeVmQ s 3002


28

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:

)1670()( MeVMsss

with a very high threshold

K

KN

productionNKN ......

which take a long time (~100 fm/c, to be compared to 5-10 fm/c of a single N-N collision)

or via a long series of interactions:

can be produced via:

in a QGP with strangeness enhancement factor Es the hadrons containing N strange quarks are produced with a rate Es

N times higher than in an environment with no strangeness enhancement. So in QGP: E>E>E


29

Experimental measurement of strangenessStrangeness production was measured by experiments WA97 and NA57 at CERN SPS, with Pb beam at 158 GeV/nucleon


30

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.


31

)(2 KK

ES

8.0/)( Kwhere not published :

K/ ratio vs. center of mass energy

Volker Friese (NA49), Strange Quark Matter 2003, Atlantic City, March 2003

Data at 30 AGeV supportphase transition scenario

(Statistical Model of the Early Stage)


32

K/ ratio vs. energy

BRAHMS results at y=0 seem to indicate saturation of K+/π+ reached at top SPS energy


33

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:

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

Drrer

e

r

e /22


34

Charmonium suppression (cont’d)For the colour charge, in a “normal” nuclear medium:

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:

rrrV )(

Crrer

rV /)(

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.


35

Charmonium suppression (cont’d)

J/Ψ, Ψ’ and χc are different bound states of the charm-anticharm system (charmonium)

Each of them has a different bound state radius ri: whentemperature 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/.


36

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 +-: 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:

3

2

107'

106/

J


37

The NA50 experiment

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 +- invariant mass

J/Ψ

Drell-Yan


38

The Drell-Yan reference processDrell-Yan is a rare, “hard” collision process and its cross-section scales with the number of nucleon-nucleon collisions.

A nucleons

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.


39

Nuclear absorptionJ/ absorption with respect to Drell-Yan 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 p-nucleus interactions.The plot B vs AB, that for Drell Yan events is flat, here is consistent with a continous decreasing pattern from p-p to S-U interactions:B(J/) (AB)0.920.015

This behaviour can be accounted for by nuclear absorption.


40

Anomalous charmonium suppression

)exp(/abs

NoJNNTPAB LBA 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.

The observations in p-A, A-B collisions can be fitted by the empirical law:

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.


41

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.


42

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.


43

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


44

More extended baryon free region

Net protons distributions indicate high degree of stopping at AGS energies, less stopping at top SPS energy and almost full transparency at RHIC

A net-baryon free region (no excess of protons over antiprotons) allowseasier comparison with theory


45

“Onium” suppression revisitedThe main advantage of LHC for “onium” physics will be the access to Y (beauty-antibeauty) states: this will allow unambiguous confirmation ofthe 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.


46

The ALICE experiment


47

The ALICE Internal Tracking System 6 cylindrical layers of

silicon detectors: pixel detectors drift detectors double sided

microstrip detectorsLayer 3 Layer 4

Radius (mm) 14.9 23.8

Ladders 14 22

SDDs per ladder 6 8


48

guard region

implanted HV voltage dividers

256 anodes (294 m pitch)

MOS charge injectors for drift velocity monitoring

Drif

tD

rift

segmented

2 x 256 anodes

Wafer: 5”, NTD, 3 k.cm, 300 m Active area: 7.02 7.53 cm2


49

The Internal TrackingSystem mechanicalsupport


50

Challenges: rates and data volumesALICE will push the previous limitsof high energy physics experimentsin the direction of very large data volumes, while other LHC experimentswill be demanding very high trigger rates and Data Acquisition bandwidth

To insure that ALICE data will beanalyzed in a timely manner, the offlinesoftware 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 processdata stored in several regional centers in an efficient way, moving around programs rather than data.


51

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 heavy-

ion 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

Documents

High energy heavy ion interactions and the search for the Quark-Gluon Plasma