Heavy quarkonia and Quark-Gluon Plasma: a saga with (at least) three-episodes

Heavy quarkonia and Quark-Gluon Plasma:a saga with (at least) three-episodes

E. Scomparin (INFN Torino)

Nikhef, Amsterdam, June 15, 2012

SPS: the discovery of theanomalous suppression

“A new hope”

RHIC: puzzling observationsfrom America

“The Empire strikes back”

LHC: towards a new era ?

“Return of the Jedi”

Heavy quarkonia states

Almost 40years of physics!

Spectroscopy Decay Production In media

See 182 pages review On arXiv:1010.5827

Which medium ? We want to study the phase diagram of strongly interacting matter Is it possible to deconfine quarks/gluons and create a Quark-Gluon Plasma (QGP) ?

Only way to do that in the lab ultrarelativistic HI collisions Problems !

Quark-Gluon Plasma is short-lived ! Only final state hadrons are observed in our detectors (indirect observation)

Probing the QGP One of the best way to study QGP is via probes which are sensitive to the short-lived QGP phase

Ideal properties of a QGP probe

Production in elementary NN collisions under control

Not (or slightly) sensitive to the final-state hadronic phase High sensitivity to the properties of the QGP phase

None of the probes proposed up to now (including heavy quarkonia!) actually satisfies all of the aforementioned criteria

So what makes heavy quarkonia so attractive ?

Interaction with cold nuclear matter under control

VACUUM

HADRONICMATTER

QGP

Everything in one slide.....

Perturbative Vacuum

cc

Color Screening

ccScreening of

strong interactionsin a QGP

• Different states, different sizes• Screening stronger at high T

• D maximum size of a bound state, decreases when T increases

Resonance melting

QGP thermometer

How the whole story began…First paper on the topic

1986, Matsui and Satz

The most famous paper inour field (1570 citations!)

Keywords

1)Hot quark-gluon plasma

2)Colour screening

3)Screening radius

4)Dilepton mass spectrum

Unambiguous signature ofQGP formation

…and how first measurements looked like• NA38: first measurement of J/ suppression at the SPS, O-U collisions at 200 GeV/nucleon(1986)

periferiche centrali NJ/

cont. (2.7-3.5)

• J/ is suppressed (factor 2!) moving from peripheral towards central events

Peripheralevents

Centralevents

Do we see a QGP effect in O-U collisions at SPS?Is this the end of the story ?

...but the story was not so simple

• Are there any other effects, not related to colour screening, that may induce a suppression of quarkonium states ?

... so let’s start from the beginning !

• Is it possible to define a “reference” (i.e. unsuppressed) process in order to properly define quarkonium suppression ?

• Which elements should be taken into account in the design of an experiment looking for qurkonium suppression?

None of these questions has a trivial answer....

• Can the melting temperature(s) be uniquely determined ?

• Do experimental observations fit in a coherent picture ?

• In particular:

Sequential suppression

9

Sequential suppression of the resonances

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

More bound states smaller size

Debye screening condition r0 > D will occur at different T

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

Do we need to measure all the states?

10

Quarkonium 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 production

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

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

Pro

mp

tN

on

-pro

mp

t

B-decay component “easier” to separate displaced production

Direct60%

B decay10%

Feed Down30%

Low pT

Suppression pattern

J/

(3S) b(2P)(2S)

b(1P)

(1S)

(2S)c(1P)

J/

Digal et al., Phys.Rev. D64(2001)

094015

• Since each resonance should have a typical dissociation temperature, one should observe «steps» in the suppression pattern of the measured J/ when increasing T

• Ideally, one could vary T• by studying the same system (e.g. Pb-Pb) at various s• by studying the same system for various centrality classes

How can we get Tdiss ? Lattice QCD calculations are our main source of information on the dissociation temperatures Early studies showed that the complete disappearance of the J/ peak occurred at very high temperatures (~2Tc) However spectral functions expected to change rather smoothly

How to pin down Tdiss ?

Recent results on Tdiss

• Binding energies for the various states can be obtained from potential models too• Assume a state “melts” when Ebind < T Tdiss ~1.2 Tc

Ebin Tweak binding

Ebin Tstrong binding

•Latest results from lattice:•No clear sign of bound state beyond T=1.46 Tc

•Region close to Tc now under studyNew!

O.Kaczmarek@HP12

Measurement via decays

14

beam

MuonOther

hadron absorber

and tracking

target

muon trigger

magnetic fi

eld

Iron w

all

Place a huge hadron absorber to reject hadronic background Implement a trigger system, based on fast detectors, to select

Reconstruct tracks in a spectrometer

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

Correct for multiple scattering and energy loss

Approach adopted by NA50, PHENIX and ALICE

Measurement via decays

15

Approach adopted by NA60, CMS and foreseen in PHENIX and ALICE upgrades

Dipole magnet

target

vertex tracker

or

!

hadron absorberMuonOther

and trackingmuon triggerm

agnetic fi

eld

Iron w

all

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 resolution

Determine origin of the muons

ExperimentsFrom fixed targetat the SPS

(muons only)…

NA60

to RHIC collider(muons+electrons)…

STAR

PHENIX

Experiments…to the LHC(electrons+muons)

CMS (high pT)

ALICE (low pT )

ATLAS (high pT)

ALICE dedicated HI experimentCMS+ATLAS mainly pp, but verygood capability for charmonia andbottomonia in HI

Quantifying the suppression (1) High temperature should indeed induce a suppression of the charmonia and bottomonia states

How can we quantify the suppression ?

Low energy (SPS) Normalize the charmonia yield to the Drell-Yan dileptons

g

g

c

c

J/

q

q

*

+

-

+

-

Advantages Same final state, DY is insensitive to QGP Cancellation of syst. uncertainties

Drawbacks Different initial state (quark vs gluons)

Quantifying the suppression (2)

At RHIC, LHC Drell-Yan is no more “visible” in the dilepton mass spectrum overwhelmed by semi-leptonic decays of charm/beauty pairs

Solution: directly normalize to elementary collisions (pp), via nuclear modification factor RAA

= RAA<1 suppressionRAA>1 enhancement

Advantagessame process in nuclear environment and in vacuum DrawbacksSystematics more difficult to handle (no cancellations)

An ideal normalization would be the open heavy quark yieldHowever, this poses several practical problems (see later)

Mechanisms for suppression We have seen earlier that a suppression of the J/ was observed already in low-energy O-U collisions at the SPS Was this a sign of J/ dissociation by QGP-related effects ? Not really. Look at what happens in p-A collisions, where no QGP formation would be expected !

ApppA

NA50, pA 450 GeV

a = 1 no nuclear effectsa <1 nuclear effects

A significant reduction of the yield per NN collision is observed, usually parametrized by effective quantities (, abs)

N.B.: J/pA/(A J/

pp )is equivalent to RpA

Nuclear absorption

21

The most direct interpretation of the previous observations isnuclear absorption once the J/ is produced, it must cross a thickness L of nuclear matter, where it may interact and dissociate

If the cross section for nuclear absorption is abs

J/, one expects

absLpppA Ae ~

L

… a behaviour indeed seen in the data… which may affect J/ production in AA collisions too, and mask genuine QGP-related effects

J/ vs (2S)

Less bound quarkonium states should be easier to break…. and indeed this is the case

• It is important to note that the charmonium production process happens on a rather long timescale

p

c

cg

J/• The nucleus “sees” the cc in a (mainly) color octet state• Hadronization can take place outside the nucleus

At SPS energies

mb 0.98.3σψ'abs

mb 0.54.5σ J/ψabs

Is nuclear absorption the whole story ?

23

Collection of results from many fixed target pA experiments

Nuclear effects show a strong variation vs the kinematic variables

Very likely we observe a combination of several nuclear effects

lower s

higher s

J/

J/ vs (2S)

Fast (2S) as absorbed as J/!

Nuclear shadowing

valence quarks sea quarks gluons

24

PDF in nuclei are strongly modified with respect to those in a free nucleon

i(x,Q2)=Ri(A,x,Q2) x i(x,Q2)

free proton PDFnPDF: PDF of proton in a nucleus

Various parameterizationsavailable

Significant uncertainties for the gluon modifications, the more relevant for quarkonia production

From enhancement to suppression, moving towards higher energy

SPS

RHICLHC

Consequences

SPSTevatron (FT) RHIC

Increasing √s From anti-shadowing to shadowing

At SPS, the “true” nuclear absorption cross section is larger than the “effective” one

Shadowing can be factorized: is nuclear absorption what remains ?

Results on d-Au from RHIC

PHENIX, J/ , J/ ee

Data favour rather small absorption cross sections, ~2-3 mb (depending on pdfparameterization), much lower than at fixed target

s-dependence of nuclear absorption

• Global interpretation of cold nuclear matter effects not easy (other ingredients such as initial state energy loss can play a role)

• Tendency towards vanishing J/abs when s increases

• Collect pA data in the same kinematic domain of AA data

PbPb results at sNN =17.2 GeV (SPS)

NA50 and the discovery of the anomalous J/ suppression

N.B.: the cold nuclear matter effects were extrapolated from pA results obtained at higher s (27.4 GeV)

Cold nuclear matter effects at 17.2 vs 27.4 GeV

When finally pA collisions were studied at the very same energy of the nuclear collisions (s=17.2 GeV), it was found that cold nuclear matter effects are stronger at that energy Need to re-normalize Pb-Pb suppression to the new reference

s = 17.2 GeV

s = 27.4 GeV

SPS “summary” plot

After correction for EKS98 shadowing

In-In 158 GeV (NA60)Pb-Pb 158 GeV (NA50)

Let’s compare NA50 (Pb-Pb) and NA60 (In-In) results:

Anomalous suppression for central PbPb collisions(up to ~30%, compatiblewith (2S) and c melting)

Agreement between PbPb and InIn in the common Npart region

PbPb data not precise enough to clarify the details of the pattern!

B. Alessandro et al., EPJC39 (2005) 335R. Arnaldi et al., Nucl. Phys. A (2009) 345

Size of anomalous suppression smaller wrt first estimates(due to stronger than expected cold nuclear matter effects)

Is (2S) suppressed too ?

Yes, but already for light-nuclei projectiles (S-U collisions)

Makes sense, the less bound (2S) state may need lower temperatures to melt

Up to now, the most accurate set of results on (2S) production in nuclear collisions

New results from LHC in a few minutes….

Moving to RHIC: expectations Two main lines of thought

1) We gain one order of magnitude in s. In the “color screening” scenario we have then two possibilitiesa) We reach T>Tdiss

J/ suppression becomes stronger than at SPSb) We do not reach T>Tdiss

J/ suppression remains the same

2) Moving to higher energy, the cc pair multiplicity increases

A (re)combination of cc pairsto produce quarkonia may take place at the hadronization J/ enhancement ?!

J/ RAA: SPS vs RHIC Let’s simply compare RAA (i.e. no cold nuclear effects taken into account)

Qualitatively, very similar behaviour at SPS and RHIC !

PHENIX experiment measured RAA at both central and forward rapidity: what can we learn ?

Do we see (as at SPS) suppression of (2S) and c ?

Or does (re)generation counterbalance a larger suppression at RHIC ?

RHIC: forward vs central y

34

Comparison of results obtained at different rapidities

Stronger suppression at forward rapidities

Mid-rapidity

Forward-rapidity

Not expected if suppression increases with energy density (which should be larger at central rapidity) Are we seeing a hint of (re)generation, since there are more pairs at y=0?

Suppression vs recombinationDo we have other hints telling us that recombination can play a role at RHIC ?

Recombination could be measured in an indirect way

J/ y distribution should be narrower wrt pp

J/ pT distribution should be softer (<pT

2>) wrt pp

J/ elliptic flow J/ should inherit the heavy quark flow

charmOpen

Closed

Difficult to conclude

<pT2> vs system size

No clear decrease of <pT

2> wrt pp at RHIC, as expected in case of recombination

…still, at the SPS, there was a very clear increase from elementaryto nucleus-nucleus collisions

Difficult to conclude

Comparisons with models

In the end, models can catch the main features of J/ suppression at RHIC, but no quantitative understanding

In particular, no clear conclusion on

(2S) and c onlysuppression

vs

All charmonia suppressed+ (re)generation

An interesting comparison Up to now we concentrated on RAA at RHIC What happens if we try taking into account cold nuclear matter effects and compare with the same quantity at the SPS

Nice “universal” behavior

Note that 1) charged multiplicity is proportional to the energy density in the collision2) Maximum suppression ~40-50% (still compatible with only (2S) and c

melting)

Go to the LHC and getmore data!

First results at the LHC

Great expectation (as for many other observables) from the first LHC heavy-ion runs (Pb-Pb @ 2.76 TeV)

Advent of upsilon family (seen also at RHIC with small statistics) (Re)generation negligible Observe suppression of less bound (2S), (3S) wrt (1S)

Solve/clarify issue with J/ suppression/(re)generation

Complementary acceptance ALICE low pT J/ CMS/ATLAS high pT J/, excellent resolution

Massr0

Very first result A couple of weeks after the end of the 2010 data taking, ATLAS published the first LHC result on J/ suppression!

Interesting, but a bit deceiving! ~ Same suppression as at RHIC,SPS However this comparison is not sound. Different pT explored!

ALICE results Results from both 2010 and 2011 (high luminosity ~ 100 b-1) now available. 2011 results public since just a couple of weeks (Hard Probes conference, Cagliari)

e+e-

+-

New!

RAA vs pT (0<pT<8 GeV/c)

RAA at forward rapidity flattens for Npart >100 Similar behaviour at central rapidity (increse for central ?)

New!

ALICE vs CMS, low vs high pT

Less suppression at ALICE than at PHENIX (low pT dominated) Larger suppression at high pT

Are we observing an effect of (re)generation ? Indeed (re)generated J/ should sit predominantly at low pT

(where the bulk of the charm yield is)

Let’s then look at the suppression vs pT

J/ suppression vs pT

Less suppression at low pT (where regeneration is expected) Effect not present at RHIC energy Models reproduce this low-pT enhancement

New!

Do J/ observed at LHC inherit the charm quark flow?

Open charm and J/ flow

3 indication for D-meson flow from ALICE Hint of non-zero flow for J/ at LHC energy (2.2 significance)

New!

A word of caution At both SPS and RHIC energies studies of cold nuclear matter effects were very important to establish a “full picture”

A priori, nuclear absorption should be negligible (crossing time of the two nuclei extremely fast at LHC energies) Shadowing plays a role Quantitative estimate from 2012 pPb LHC run

New!

From charmonia to bottomonia: CMS

Evident suppression of the less bound (2S), (3S) states !

New!

Relative suppression and RAA

New!

(2S) almost disappears for central Pb-Pb events Also (1S) is suppressed, compatible with feed-down from 2S+3S

Comparison with RHIC

STAR measures an inclusive RAA (all the states together) and sees a suppression, compatible with the one observed by CMS

STARCMS

ALICE can complement CMS measurement by studying forward rapidity s (results expected soon)

Not everything is clear (1) Thanks to the very good resolution CMS can accurately measure (2S) in Pb-Pb

Double ratio (2S)/J/ in Pb-Pb vs pp Striking difference between “low pT, large y” and “large pT, low y” To be understood

New!

Not everything is clear (2)

Open charm is the natural reference for J/ suppression Is the striking similarity of open and close charm suppression telling us something ?

Conclusions Heavy quarkonia and QGP: after >25 years still a very lively field of investigation, with surprises still possible

Very strong sensitivity of quarkonium states to the medium created in heavy-ion collisions: interpretation not always easy

Two main mechanisms at play1) Suppression by color screening2) Re-generation (for charmonium only!) at high s

can qualitatively explain the main features of the results

This is just one of the multi-faceted aspects of QGP-related studies: many more observables are being looked at!

Future of quarkonia multi-differential suppression studies cold nuclear matter at LHC full LHC energy complete characterization of excited states in the QGP

Size: 16 x 26 metersWeight: 10,000 tonsDetectors: 18

Focus on Pb-Pb

•Centrality estimate: standard approach

PRL106 (2011) 032301

•Glauber model fits•Define classes corresponding to fractions of the inelastic Pb-Pb cross section

Charged multiplicity – Energy density

• dNch/d = 1584 76

• (dNch/d)/(Npart/2) = 8.3 0.4

• ≈ 2.1 x central AuAu at √sNN=0.2 TeV

• ≈ 1.9 x pp (NSD) at √s=2.36 TeV• Stronger rise with √s in AA w.r.t. pp • Stronger rise with √s in AA w.r.t. log

extrapolation from lower energies55

PRL105 (2010) 252301

• Very similar centrality dependence at LHC & RHIC, after scaling RHIC results (x 2.1) to the multiplicity of central collisions at the LHC

PRL106 (2011) 032301

c)GeV/(fm15 2 Bj

System size

56

• Spatial extent of the particle emitting source extracted from interferometry of identical bosons

• Two-particle momentum correlations in 3 orthogonal directions -> HBT radii (Rlong, Rside, Rout)

• Size: twice w.r.t. RHIC

• Lifetime: 40% higher w.r.t. RHIC

ALICE: PLB696 (2011) 328 ALICE: PLB696 (2011) 328