1

Hard ProbesRoberta Arnaldi

INFN Torino

5th International School on QGP and Heavy Ions Collisions: past, present and future

Torino, 5-12 March 2011

2

Outlook:1) Hard probes: definitions

2) High pT hadrons

3) Heavy Flavours

4) Quarkonia1) Theoretical expectations2) SPS results3) RHIC results4) LHC perspectives and first results

3

Quarkonium: introductionQuarkonium is considered since a long time as one of the most striking signatures for the QGP formation and its study in AA collisions is already a 25 years long story

SPS RHIC LHC

17 GeV/c 200 GeV/c 2.76 TeV/c√s

years 1990 ~2000 20101986

…but, as for the other hard probes, in order to understand quarkonium behaviour in the hot matter (AA collisions), its interactions with the cold nuclear matter should be under control (pA/dAu collisions)

4

What is Quarkonium?Quarkonium is a bound state of and

qq

with

Charmonium () family Bottomonium () family

According to the quantum numbers, several quarkonium states exists

5

At T=0, the binding of the and quarks can be expressed using the Cornell potential:

krr

rV )(

Coulombian contribution, induced by a g exchange between and

Confinement term

Quarkonium

The QGP consists of deconfined colour charges the binding of a pair is subject to the effects of colour screening

What happens to a pair placed in the QGP?

krr

rV )( Dre

rrV /)(

• The “confinement” contribution disappears• The high color density induces a screening

of the coulombian term of the potential

qq

qq

6

c c

vacuum

r

J/

r

c c

Temperature T<Td

r

c c

Temperature T>Td

J/D

D

The screening radius D(T) (i.e. the maximum distance which allows the formation of a bound qq pair) decreases with the temperature T

if resonance radius > D(T) no resonance can be formed

At a given T:if resonance radius < D(T) resonance can be formed

Debye screening

7Phys.Lett. B178 (1986) 416

This is the idea behind the suggestion (by Matsui and Satz) of the J/ as a signature of QGP formation (25 years ago!)

Charmonium suppression

Very famous paper, cited ~ 1400 times!

8

Sequential suppression of the resonances

The quarkonium states can be characterized by • the binding energy• radius

More bound states have smaller size

Debye screening condition r0 > D will occur at different T

Sequential screening

state J/ c (2S)Mass(GeV) 3.10 3.53 3.69

E (GeV) 0.64 0.20 0.05

ro(fm) 0.25 0.36 0.45

state (1S) (2S) (3S)

Mass(GeV) 9.46 10.0 10.36

E (GeV) 1.10 0.54 0.20

ro(fm) 0.28 0.56 0.78 (2S) J/c

T<Tc

Tc

thermometer for the temperature reached in the HI collisions

(2S) J/c

T~Tc

Tc

(2S) J/c

T~1.1Tc

Tc

(2S) J/c

T>>Tc

Tc

J/ (quarkonium) can be studied through its decays:

J/ +-

J/ e+e-

Quarkonium decay

10

Quarkonium productionQuarkonium production can proceed:

• directly in the interaction of the initial partons• via the decay of heavier hadrons (feed-down)

For J/ (at CDF/LHC energies) the contributing mechanisms are:

Direct productionFeed-down from higher charmonium states:~ 8% from (2S), ~25% from c

B decaycontribution is pT dependent~10% at pT~1.5GeV/c

Prom

ptDi

spla

ced

Feed down and J/ from B, if not properly taken into account, may affect physics conclusions

Direct60%

B decay10%

Feed Down30%

11

beam

MuonOther

hadron absorber

and tracking

target

muon trigger

magnetic field

Iron wall

Place a huge hadron absorber to reject hadronic background Implement a trigger system, based on fast detectors, to select muons Reconstruct muon tracks in a spectrometer (magnetic field + tracking detectors)

Extrapolate muon tracks back to the target Vertex reconstruction is usually rather poor (z~10 cm)

Correct for multiple scattering and energy loss

Standard way of measuring pairsApproach adopted by NA50, PHENIX and ALICE (forward region)

12

Upgraded way of measuring pairsApproach adopted by NA60, LHC exp. and foreseen in future PHENIX and ALICE upgrades (in the forward muon)

2.5 T dipole magnet

targets

vertex tracker

or

!

hadron absorberMuonOther

and trackingmuon triggerm

agnetic field

Iron wall

Use a silicon tracker in the vertex region to track muons before they suffer multiple scattering and energy loss in the hadron absorber.

Improve mass resolutionDetermine origin of the muons

13

J/ is produced in two steps that can be factorized:

Quarkonium production in pp

Production of the pair perturbativeEvolution of pair into a bound quarkonium state non perturbative

Different descriptions of this evolution are at the basis of the various theoretical models

1) Color singlet model

2) Color evaporation model

3) NRQCD

14

Models for quarkonium production in ppColor Singlet Model Color Evaporation M. NRQCD

Proposed soon after the J/ discovery

pair is produced in a color singlet state, with the same quantum numbers of the final quarkonium

Unable to describe Tevatron data. However, recently NLO and NNLO corrections have been included to improve the agreement

pair evolves in quarkonium if <mD independently of its color and spin

Probability to evolve into a certain quarkonium state depends by a constant F which is energy and process independent

Works rather well, but no detail on the hadronization of the qq pair towards the bound state

Inclusive quarkonium production cross section is a sum of short distance coeff. and long distance matrix elements:

This approach includes CSM and CEM as special cases

Charmonium can be produced also through the creation of a color octet state

/)/(ˆ J

nn

ijnQQ OCJij

15

Production models and CDF resultsThe first CDF results on J/ direct production revealed a striking discrepancy wrt LO CSM

factor 50!

The agreement improves in NRQCD approach

…but situation still puzzling, because polarization is not described!

Open questions, to be investigated at LHC!

Recently many step forwards (i.e. NLO and NNLO corrections…)

Quarkonium production in pAAs the other hard probes, quarkonium may be affected by initial and final state effects

16

allow the understanding the J/ behaviour in the cold nuclear medium complicate issue, because of many competing mechanisms:

provide a reference for the study of charmonia dissociation in a hot medium approach followed at SPS and similarly at RHIC (with dAu data)

pA collisions

Final state: cc dissociation in the medium, final energy loss

p

μμ

J/ Initial state: shadowing, parton energy loss, intrinsic charm

Useful to investigate initial state effects

17

ApppA

These effects can be quantified, in pA collisions, in two ways:

absLpppA Ae ~

Cold Nuclear Matter effects

Effective quantities which include al initial and final state effects

In pA collisions, no QGP formation is expected

NA50, pA 450 GeV

= 1 no nuclear effects <1 nuclear effects

The larger abs, the more important are the nuclear effects

in principle, no J/ suppression. however a reduction of the yield per nucleon-nucleon collisions is

observed

18

Nuclear absorptionOnce the J/ has been produced, it must cross a thickness L of nuclear matter, where it may interact and disappear

If the cross section for nuclear absorption is abs

J/, one expects

absLpppA Ae ~

L

19

Nuclear effects vs. xFCollection of results from many fixed target pA experiments

Nuclear effects show a strong variation vs the kinematic variables

Because of the dependence on xF and energy the reference for the AA suppression must be obtained under the same kinematic/energy domain as the AA data

I. Abt et al., arXiv:0812.0734

lower s

higher s

20

Nuclear effects

• anti-shadowing (with large uncertainties on gluon densities!)• final state absorption…

Interpretation of results not easy many competing effects affect J/ production/propagation in nuclei

need to disentangle the different contributionsSize of shadowing effects may be large and has to be taken into account when comparing results at different energies

C. Lourenco, R. Vogt and H.Woehri, JHEP 0902:014,2009F. Arleo and Vi-Nham Tram Eur.Phys.J.C55:449-461,2008, arXiv:0907.0043

Clear tendency towardsstronger absorption at low √s

21

Why CNM are important? The cold nuclear matter effects present in pA collisions are of course present also in AA and can mask genuine QGP effects

It is very important to measure cold nuclear matter effects before any claim of an “anomalous” suppression in AA collisions

L

J//N

coll

L J

//N

coll/n

ucl.

Abs.

1

Anomalous suppression!

pA

AA

CNM, evaluated in pA, are extrapolated to AA, in order to build a reference for the J/ behaviour in hadronic matter

Measured/Expected

22

CNM, evaluated in pA, are extrapolated to AA, in order to build a reference for the J/ behaviour in hadronic matter

J/ in AA collisions @ SPS

After correction for EKS98 shadowing

B. Alessandro et al., EPJC39 (2005) 335R. Arnaldi et al., Nucl. Phys. A (2009) 345R.A., P. Cortese, E. Scomparin Phys. Rev. C 81, 014903

In-In 158 GeV (NA60)Pb-Pb 158 GeV (NA50) Using the previously

defined reference:Central Pb-Pb: Anomalous suppression ~ 30% effect

In-In: almost no anomalous suppression?

23

J/ @ RHICPHENIX J/e+e- |y|<0.35 & J/+- |y| [1.2,2.2] STAR J/e+e- |y|<1

pp, dA collisionspp 200 GeV/nucleondAu 200 GeV/nucleon

All data have been collected with the same collision energy (√s = 200 GeV) and kinematics

AA collisionsAu-Au 200 GeV/nucleon Cu-Cu 200 GeV/nucleon

24

pp results should help to• understand the J/ production mechanism• provide a reference for AA collisions (RAA)

J/ @ RHIC – pp, dAu collisions

In a similar way as at SPS, CNM effects are obtained from dAu data

RHIC data exploit different x2 regions corresponding to shadowing (forward and midrapidity) anti-shadowing (backward rapidity)

BackwardMid Forward

dAu collisions

pp collisions

25

J/ @ RHIC – AuAu collisionsComparison at different rapidities

Stronger (unexpected) suppression at forward rapidities

Coalescence of charm pairs in the medium?

Different CNM effects?

Mid-rapidity

Forward-rapidity

26

Comparison with SPS resultsResults are shown as a function of the multiplicity of charged particles (~energy density, assuming SPS~RHIC)

Comparison with SPS results

Both Pb-Pb and Au-Au seem to depart from the reference

curve at NPart~200

For central collisions more important suppression in Au-Au with respect to Pb-Pb

27

Interpretation of the results

Several theoretical models have been proposed in the past, starting from the following observations

• RAA at forward y is smaller than at midrapidity• similar suppression at SPS and RHIC

Different approaches proposed:

SPS RHIC LHCs (GeV) 17.2 200 5500

Ncc ≈ 0.2 ≈10 ≈100-200

1) Only J/ from ’ and c decays are suppressed at SPS and RHIC

The 2 effects may balance: suppression similar to SPS

2) Also direct J/ are suppressed at RHIC but cc multiplicity high

J/ regeneration ( Ncc2) contributes to the J/ yield

same suppression is expected at SPS and RHIC reasonable if Tdiss (J/) ~ 2Tc

Some interpretations

28

RecombinationModels including J/ regeneration qualitatively describe the RAA data(X. Zhao, R. Rapp arXiv:0810.4566, Z.Qu et al. Nucl. Phys. A 830 (2009) 335)

J/ elliptic flow J/ should inherit the positive heavy quark flowJ/ y distribution should be narrower wrt pp

J/ pT distribution should be softer (<pT

2>) wrt pp

Results are not precise enough to assess the amount of regeneration

Indirect way some distributions should be affected by regeneration

Direct way for quantitative estimate accurate measurement of charm

Recombination

29

Quarkonium @ LHCMany questions still to be answered at LHC energy

• Role of the large charm quark multiplicityWill J/ regeneration dominate the picture for charmonium ? (RHIC results still not conclusive, at this stage)

• Bottomonium physicsStill (almost) unexplored in HI collisions

Regeneration?

Further suppression?

30

Quarkonium @ LHCInvestigated by the 4 LHC experiments: ATLAS mid-rapidity ||<2.5 (+-)CMS mid-rapidity ||<2.5 (+-)ALICE mid rapidity ||<0.9 (e+e- channel) forward rapidity 2.5< <4 (+-)LHCb forward rapidity 2.5< <4 (+-)

LHCb

CMS

ATLAS

ALICE

31

Quarkonium LHC results in pp

New results presented by the 4 experiments

Differential distributions (y, pT)

Fraction of J/ from B

Preliminary theory comparison

32

results @ LHC in pp hardly seen at RHIC, while now at LHC the family is fully accessible

Extremely important measurement:

More robust theory calculation (due to heavy bottom quark and absence of b-hadron feed-down)

arXiv:1012.5545

33

First J/ in Pb-Pb collisions!

We expect few thousands J/ from 2010 statistics

no correction for feed-downs, J/ from B

ATLAS: arXiv:10125419

J/ with pT>3GeV/c and ||<2.5

A centrality dependent suppression is observed

=

34

Backup

35

Statistical hadronizationJ/ production by statistical hadronization of charm quarks (Andronic, BraunMunzinger, Redlich and Stachel, PLB 659 (2008) 149)

• charm quarks produced in primary hard collisions• survive and thermalize in QGP • charmed hadrons formed at chemical freeze-out (statistical laws)• no J/ survival in QGP

yA. Andronic et al. arXiv:0805.4781

Good agreement between data and model

Recombination should be tested on LHC data!

Statistical hadronization

36

yT esmx /2

x2 scaling

• Shadowing effects (in the 21 approach) and final state absorption

• • scale with x2

2

21~xxms JNJ

if parton shadowing and final state absorption were the only relevant mechanisms should not depend on √s at constant x2

37

J/ @ RHIC – AuAu collisionsComparison at different rapidities

Comparison between different systems

CuCu explores a smaller Npart range

Stronger (unexpected) suppression at forward rapidities

Coalescence of charm pairs in the medium?Different CNM effects?

38

High pT J/ in Cu-Cu

RCuCu up to pT = 9 GeV/c suppression looks roughly constant up to high pT

PHENIX (minimum bias) STAR (centrality 0-20% & 0-60%)

RCuCu =1.4±0.4±0.2 (pT>5GeV/c) RAA increases from low to high pT

Difference between high pT results, but strong conclusions limited by poor statisticsBoth results in contradiction with AdS/CFT+Hydro

Increase at high pT already seen at SPS

NA50: Pb-Pb

pT dependence