QGP (II)

E. Scapparone 24 Maggio, 2010 Gli adroni ad alto P T sono prodotti da partoni con Hard-scattering iniziale. I partoni hanno bisogno di un tempo finito per uscire dalla zone della collisione, durante il quale si forma il “dense medium”  durante questo tempo subiscono Interazione forte col mezzo e quindi costituiscono una “sonda” per studiare il mezzo stesso. Ci sono due variabile che si possono studiare: 1) Rapporto degli adroni emessi in collisioni tra nuclei rispetto a p-p ( corretto per effetti geometrici e scalando col numero di collisioni) ; 2) Disappearance del jet “back to back” nell’angolo azimuthale ( “jet quenching”);

PHENIX Nessun effetto in d-Au  effetto associato a interazione tra ioni pesanti.

The results are consistent with the effects of parton energy loss in traversing dense medium, predicted before the data were available

Per ripdodurre l’intensita’ della soppressione, questi MC devono assumere Una densita’ di gluoni 30 volte maggiore di quella della materia fredda e Confinata ( densita’ di energia 100 volte maggiore). centralità 90% 50% 0%

Altra possible normalizzazione: I fotoni

As one increases the collision energy in nucleus nucleus collisions, the produced plasma reaches higher energy and particle densities, the system stays longer in the QGP phase, and correspondingly the traversing partons are more quenched.

Il meccanismo dominante di perdita di energia dei partoni nel QGP e’ la radiazione di gluoni (“gluonstrahlung”).

Near side: dove seleziono la particella “trigger” Away side: lato opposto  Maggiori effetti del mezzo

Select a High Pt particle as trigger and measure the Δ F = F

−

F

trig angle

Evoluzione da low to high pt PHENIX(2008) Medium Fragmentation Shape simile a p-p + soppressione

• Experimentally, the centrality is evaluated by measuring one or more of these variables: –

N ch

: number of charged particles produced in a given rapidity interval (near mid-rapidity) – • increases (~ linearly) with

N part E T

: transverse energy = S

E i

sin q

i

• increases (~ linearly) with

N part

– E ZDC : energy collected in a “zero degree” calorimeter • increases (~ linearly) with

N spectators

Bjorken’s formula

• To have an estimate of the energy density reached in the initial stages of the collisions, we can project back in time the energy carried by the collision products (“Bjorken’s estimate”) Consider a thin cylindrical slab of transverse dimension

S

of expanding matter contained within a thickness

dz

at time t .

dz dE dv = c d dz = 2

t b

= (c/2) dy dv = c dV = S dz = S c dE =

e

dV

t

dy

t (non rel.:

dy y =

b ) In the center-of-mass frame

v

=0 at the center of the slab -

dv v

=0

dv

e  1

Sc

t

dE dy y

 0

Bjorken’s formula

Initial energy density

Estimate for central (head-on) Pb-Pb collisions at the SPS e 

Sc

1 t 0

dE dy Bjorken’s formula y

 0 Initial time t 0 : usually taken to be ~ 1 fm/c i.e.: equal to the “formation time”: the time it takes for the energy initially stored in the field to materialize into hadrons

dE dy



dE T dy



m T dN dy y

 0

y

 0

y

 0  400 GeV Transverse dimension

S

:

S

  

R

2

Pb R Pb S

 1 .

2 fm  160 fm 2

R A

 1 .

2 fm   ( 208 ) 1 / 3  7 .

A

1 / 3 1 fm

RHIC

e

~ 5 GeV/fm3

e ~ (400/160) GeV/fm 3 ~ 2.5 GeV/fm 3 Enough for deconfinement!

Published estimate from NA49:

e

= 3.2



0.3 GeV/fm 3 [Phys. Rev. Lett. 75 (1995), 3814]

Strangeness enhancement

• restoration of c symmetry -> increased production of s – mass of strange quark in QGP expected to go back to current value • m S ~ 150 MeV ~ Tc  copious production of ss pairs, mostly by gg fusion [Rafelski: Phys. Rep. 88 (1982) 331] [Rafelski Müller: P. R. Lett. 48 (1982) 1066] K + • deconfinement  stronger effect for multi-strange – can be built recombining uncorrelated s quarks produced in independent  + X microscopic reactions  strangeness enhancement increasing W + with strangeness content [Koch, Müller & Rafelski: Phys. Rep. 142 (1986) 167] d u d d u s s u d u d u u u d d u d d u s d s s s u d u s u d u d d u u s d s s u u u u s u s s d d s u d d u d s u d s d u L p  -

•

Charmonium as a Probe of QGP

Matsui and Satz predicted J/

y

production suppression in Quark Gluon Plasma because of color screening

Charmonium suppression

• QGP signature proposed by Matsui and Satz, 1986 • In the plasma phase the interaction potential is expected to be screened beyond the Debye length l

D

(analogous to e.m. Debye screening): • Charmonium (cc) and bottonium (bb)

states with r >

l

D

will not bind

; their production will be suppressed l

D

, and therefore which onium states will be suppressed, depends on the temperature

Debye screening

In an electromagnetic plasma, the potential of a charge is screened by the field of the electrons that surround it [see e.g.: Jackson p. 494] F (

r

) 

Ze

exp( 

r r

/ l

D

) with l

D

 4 

kT n

0

e

2

n

0 : = density of electrons in the plasma In a QGP, the colour field is likewise going to be screened.

In order to have a back-of-envelope estimate the screening length, one can take the above formula, and substitute:

e

2 (Gauss system)  a QCD ~ 1 getting:

n

= 28.8 10 6 MeV 3

n

0 

n

= 3.6

T 3

(Stefan-Boltzmann law for QGP) l

D

 4  200 ( MeV)  28.8

 10 6 ( MeV 3 )  7 .

43  10  4 MeV 1

kT

~ 200 MeV and, using: 1 MeV -1 = 197.3 fm: l

D

 0.15 fm

NO QGP c c V= kr -

a

/ R V = -

a

e – r/

l

d R

c

QGP

Come si identifica una risonanza (esempio J/ Y ) ?

- Identificazione dei leptoni ( canale a 3  - misura del loro momento; molto difficile); - Calcolo della massa invariante e selezione.

m inv = sqrt( (E 1 +E 2 ) 2 – (p 1 +p 2 ) 2 )

NA38

Dimuon Spectrum

• The measured dimuon spectrum is fitted to a source cocktail in order to extract the J/ y , y ’ and Drell-Yan contributions NA50 dimuon spectrum (Pb-Pb, 158 A GeV/c) Quarkonium production is usually normalised to Drell-Yan production (which is not influenced by strong interactions) m + m 

Why do we keep using Drell-Yan ?

Drell-Yan (muon pairs) is a well known computable process, proportional to the # of elementary nucleon-nucleon collisions, with the following priceless advantages : • • • • identical identical identical identical experimental biases inefficiencies selection criteria } cuts as

J/

y Therefore the corrections cancel out 

(

J/

y

)

 in the ratio

(

DY

)

which is insensitive to normalization factors/uncertainties PUNTI NEGATIVI : 1) statistica DY << statistica J/ y 2) normalizzazione isospin

Branching to muons

Nuclear absorption

• There is a “normal” suppression of the production of J/ y , observed already in pA and lighter ion collisions and attributed to nuclear absorpion • The Pb-Pb point falls below the nuclear absorption curve (“anomalous” suppression)

• From a fit of experimental p-A data (NA38,NA50):

 abs

=4.18 +- 0.35 mb (

hep-ex/0412036

) • In S-U collisions the same suppression is observed • The normal suppression is interpreted as the absorption, in the nuclear environment, of the c-cbar pair before the J/

y

(or

y

’ or

c

) formation : preresonance absorption.

The CERN Pb ion programme

• Started in 1994 • Pb nuclei accelerated to 158 A GeV/c (40 A GeV/c in 1999) collide on fixed targets (typically 4-6 weeks/year) • 7 experiments: – NA44 (single arm spectrometer: particle spectra, interferometry, particle correlations) – NA45 (e+e- spectrometer: low mass lepton pairs) – NA49 (large acceptance TPC: particle spectra, strangeness production, interferometry, …) – NA50 (dimuon spectrometer: high mass lepton pairs, J/ y production) – NA52 (focussing spectrometer: strangelet search, particle production) – WA97/NA57 (silicon pixel telescope spectrometer: production of strange and multiply strange particles) – WA98 (photon and hadron spectrometer: photon and hadron production)

A Pb-Pb collision at the SPS

• “Busy” events! (thousands of produced particles) • High granularity detectors are employed (TPC, Si Pixels,...)

NA38 first results

O+U at 200 GeV/c:

S



J continuum

/ y ( 2 .

7  3 .

5 )

Factor 2 suppression… but… including: • normal nuclear absorption !!!

• IMR charm-like excess !!!

(fit starts from 1.7 GeV/c

2

!!)

Experiment NA50

• Aim: study the production of J/ y in Pb-Pb collisions • Experimental technique: – absorb all charged particles produced in the collision except muons – detect J/ y by reconstructing the decays J/ y  m + m  (B.R.  5.9 %)

Anomalous J/

y

suppression

• J/ y normalized to Drell-Yan as a function of the transverse energy (i.e. centrality) • The data points deviate from the solid curve, which indicates the prediction for nuclear absorption • The deviation increases with increasing collision centrality

• Attempts at describing the NA50 data within purely hadronic models without deconfinement – dissociation of the J/ y final state hadronic in interactions with comovers • try harder...

Projectile Target J / y L

NA60: stesso rivelatore di NA50 con aggiunta di rivelatori al Silicio (tracking).

Alcune delle motivazioni dell’esperimento: If the J/psi suppression pattern in Pb-Pb collisions indicates that central Pb-Pb collisions produce a state of matter where colour is no longer confined, we should move on to the detailed understanding of how deconfinement sets in, and what physics variable governs the threshold behaviour of charmonia ( c c ) suppression: (local) energy density, density of wounded nucleons, density of percolation clusters, etc. This requires collecting data with smaller nuclear systems like In-In. The J/psi data collected in central Pb-Pb collisions indicate that we are already beyond the point where the phase transition takes place, but do not provide any information on the value of the critical temperature. Finite temperature lattice QCD tells us that the strongly bound J/psi ccbar state should be screened when the medium reaches temperatures 30-40 % higher than T_c, while the large and more loosely bound psi' state should melt near T_c. The NA38 experiment has shown that the psi' is significantly suppressed when going from p-U to peripheral S-U collisions. We need to see if this suppression follows a smooth pattern or a sudden transition, within a single collision system rather than comparing p-U to S-U data.

If the Y ' suppression is due to Debye screening, its suppression pattern could provide a clear measurement of T_c. However, the hadronic "comovers" produced in S-U collisions may "absorb" the psi' mesons, since its binding energy is only around 40 MeV. What mechanism is responsible for the Y ' suppression? The presently existing results are not clear in what concerns the onset and pattern of the psi' suppression. A new measurement is needed, with improved mass resolution to have a cleaner separation of the psi‘ peak with respect to the J/ Y shoulder, and which scans the energy density region from the p-U to the S-U data. In-In collisions are also well placed for this study.

NA60’s detector concept Idea: place a

high granularity

and radiation-hard silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering and energy loss in the absorber

~ 1m Muon Spectrometer Hadron absorber

MWPC’s

Toroidal Magnet

Iron wall

Target area

beam m m

ZDC

Trigger Hodoscopes

Dipole field 2.5 T BEAM TARGET BOX BEAM TRACKER VERTEX TELESCOPE

not to scale

IC MUON FILTER

Matching in coordinate and in momentum space  • • Origin of muons can be accurately determined Improved dimuon mass resolution

ZDC

allows studies vs. collision centrality

Specific questions that remain open Is the anomalous suppression also present in lighter nuclear systems?

Study collisions between other systems, such as Indium-Indium Which is the variable driving the suppression?

Study the J/ y suppression pattern as a function of different centrality variables, including data from different collision systems S-U In-In Pb-Pb What is the normal nuclear absorption cross-section at the energy of the heavy ion data?

Study J/ y production in p-A collisions at 158 GeV pure Glauber calculation N part What is the impact of the c c feed-down on the observed J/ Study the nuclear dependence of c c y suppression pattern?

production in p-A collisions New and accurate measurements are needed to answer these questions

Comparison with previous results An “ anomalous suppression ” is present already in Indium-Indium The normal absorption curve is based on the NA50 results. Its uncertainty (~ 8%) at 158 GeV is dominated by the (model dependent) extrapolation from the 400 and 450 GeV data

Spiegazioni alternativa al QGP 1) Comovers -Comovers model. La J/ Y puo’ interagire con gli adroni “comovers”. La sezione d’urto e’ molto difficile da stimare.

Na50 J/

y

suppression can be reproduced by DPM with absorption by comovers. The number of comovers in Capella model is proportional to number of participants and also to number of collisions.

A. Capella, D. Sousa, nucl-th/0303055

Suppression by produced hadrons (“comovers”) The model takes into account nuclear absorption and comovers interaction with  co = 0.65 mb (Capella-Ferreiro) In-In @ 158 GeV nuclear absorption comover + nuclear absorption (E. Ferreiro, private communication) NA60 In-In 158 GeV Pb-Pb @ 158 GeV The smeared form (dashed line) is obtained taking into account the resolution on N Part due to our experimental resolution

2) Percolation

[First works: Baym , Physica (Amsterdam) 96A, 131 (1979) Celik et al., Phys. Lett. 97B (1980) 128] Forma di deconfinamento geometrica, di pre-equilibrio. Pre-requisito al deconfinamento Vero e proprio, applicabile ai sistemi finiti. Se il condensato di partoni contiene partoni abbastanza “hard”, puo’ dissociare la J/ Y Superficie di area  R 2 R N dischi di raggio r<< R Densita’ n= N/  R 2 Aumentanto la densita’ si trovano cluster di area sempre maggiore Quando N,R  ∞ e n finito , la cluster size diverge a n= 1.13/  R 2 . Per N,R finiti si ha Percolazione quando il cluster piu’ largo Riempie tutta la superficie.

1 0.5

1 1.5

r/R=1/100 n =n(r/R) 2

A causa dell’overlap, alla soglia di percolation, solo 2/3 dell’area e’ riempita.

 Local Percolation: Hard probe, come gli stati c-cbar risentono del mezzo localmente.

Si ottiene a 1.72/  r2

3) Regeneration

Regeneration models[2,3,4] predict an enhanced production of hidden charm states for sufficiently high charm densities

This would imply thermalization of charm quarks, and by extension, the light quarks that comprise the QGP

Osservazione sperimentale (Gazdzicki and Gorenstein) Il rapporto di J/ Y /  - = cost

“a dominant fraction of the J

c

mesons produced in hadronic and nuclear collisions at CERN SPS energies is created at hadronization according to the available hadronic phase space”

L. Grandchamp and R. Rapp, Phys. Lett. B523 60 (2001) regeneration suppression

“Two component” model: suppression in hadronic and QGP phase + statistical production at hadronization

J/

y

at SPS

• J/

y

in NA60 poorly reproduced by models which fit NA50 data

Satz, Digal, Fortunato Rapp, Grandchamp, Brown Capella, Ferreiro

A RHIC tutto piu’ complicato - Lattice Gauge calculations now indicate that the J/ψ remains bound up to 1.5 to 2 times the deconfinement temperature (the J/ψ is very small in radius, and would reasonably require a higher density to become unbound due to screening). The maximum temperature reached in central Au+Au collisions at RHIC is thought to be ~ 2 times the transition temperature, so it is now not clear if the J/psi is expected to melt at RHIC. - some higher states that feed down to the J/ψ are expected to melt just above the deconfinement temperature; - the large charm quark production cross section at RHIC leads to predictions that J/ψ will be formed by random coalescence of unrelated charm pairs in central Au+Au collisions, even if the initial group of forming J/ψ is destroyed in deconfined matter; - modifications of gluon densities at low momentum-fraction in heavy nuclei are expected to start to be significant at RHIC and perhaps modify the initial charm anti-charm production cross sections.

R

AuAu

(y~0) ~ R

AuAu

(SPS)

• Lower rapidity R

AA

look surprisingly similar, while there are obvious differences:

– Cold nuclear matter effects (x Bjorken ,…) – Energy density – … ±12% global syst ±7% global syst ±11% global syst

R

AuAu

(y~0) > R

AuAu

(y~1.7)

• More suppression at forward rapidity !

±12% global syst ±7% global syst 60% R AA (y~1.7) R AA (y~0)

Quick comparison to SPS

J/ψ survival beyond CNM

• At mid-rapidity , the amount of surviving J/ ψ @ RHIC is compatible with SPS (~60%) but depends a lot on CNM (and pp references)… • At forward rapidity, RHIC anomalous suppression is much stronger !

±11% global systematics ±35% global systematics ±30% global systematics 44 ± 23% 25 ±12%

J/ ψ R

AA

over CNM in Cu+Cu and Au+Au

Calculations by M.J. Leitch using break-up cross-section and errors estimated from 2008 data Differences between mid and forward rapidity measurement is washed out.

Suppression beyond cold nuclear matter effects is observed, consistent with de confinement 48

Conclusions

• Two qualitative possible scenarios 1. Large melting + some regeneration 2. Initial effects (CGC) + melting (of ψ

’

, χ

c

?)

• Need better handle of CNM • • • Need better open charm measurements !

Smoking gun would have been a J/ψ rise … v 2 could become the smoking gun – (maybe run7 with 4 x run4 and reaction plane detector)

• Data is young, new ideas may arise

…

Johanna Stachel

Model prediction