Dilepton production and the onset of deconfinement

International Workshop “Critical Point and Onset of Deconfinement”

Firenze, July 3-6 2006

E. ScomparinINFN-Torino (Italy)

• Introduction• J/ suppression studies at SPS energies

• NA38/NA50: p-A, S-U, Pb-Pb• NA60: p-A, In-In• Other inputs: E866, HERA-B

• Other J/ related topics: v2, pT, polarization• Conclusions

The SPS energy range

SPS low energy~ 20 GeV/nucleon

SPS high energy~ 200 GeV/nucleon

• In the dilepton sector, SPS is well positioned to study • Onset of deconfinement J/ suppression (this talk)• Approach to chiral symmetry restoration in-medium modifications of

vector mesons

SPS probably sitting in the region close to

Deconfinementthreshold

Critical point

Charmonium production at SPS

• Study carried out by NA38/NA50/NA60 at the SPS from 1986 until today• Essentially the same experiment, although with very significant upgrades

• Large set of results with very good statistics• (Lots of) systems studied, including:

• p-p, p-d, p-Be, p-C, p-Al, p-Cu, p-Ag, p-W, p-Pb, p-U, O-Cu, O-U, S-U, In-In, Pb-Pb

• Similar (but not identical) energy/kinematical domain between various data sets

• Very significant contributions (in a slightly higher energy range) by E866 and HERA-B

• Many relevant questions to be answered by studying charmonium production in heavy-ion collisions at the SPS

• Is (at least part of the) suppression of charmonia that we observe in the data NOT due to usual hadronic processes ?• Do we have evidence for a “threshold behaviour” of the suppression, that might be connected with the onset of deconfinement ?• Can we observe the predicted “suppression hierarchy” for the various charmonia states ?

The NA38/NA50/NA60 experimentsBased on the same muon spectrometer (inherited by NA10)

no apparatus-dependent systematics

Many updates in the target region, in parallel with the availability of radiation hard detectors

NA50

MUON FILTER

BEAMTRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

BEAM

IC

not on scale

NA60

pA collisions: the reference

• Glauber fit to BµµJ/ at 400-450 GeV• J/

abs= 4.48 0.42 mb

Main problem:extrapolation to 158 GeV/c

• S-U data (200 GeV) should not be used (absorption sources different wrt pA might be present)

• Obtain normalization (J/pp)

at 200 GeV • using only pA data • assuming J/

abs does not depend on s

• High statistics 400/450 data: J//DY ratios• Obtain J/

abs= 4.18 0.35 mb

Expected (J/)/DY at 158 GeV • NA50 uses Drell-Yan as a reference process to study J/ suppression• Is (J/)/DY equivalent to J/ cross section per N-N collision ? Yes, Drell-Yan A-dependence measured DY = 0.995 0.016 (stat.) 0.019 (syst.)

• Start from J/ pp/DY

pp @ 450 GeV (1.4% error)

• Rescale to 200 GeV• J/ see previous page (7.8% error, SU not used)• DY LO calculation (2.5 % error)

• Rescale to 158 GeV• J/ fit a la Schuler to measured J/ cross sections (1.5% error)• DY LO calculation (negligible error)

• Use Glauber (with neutron halo) to calculate centrality dependence of expected J/ /DY • Include experimental smearing on centrality determination (ET, EZDC, Nch)

Direct measurement of J/ /DY at 158 GeV would significantly decrease such errors (NA60)

Open questions on J/ production in p-A

• Solid theoretical understanding is still missing• There are indications that J/ may depend on s inside the SPS energy range (becomes much smaller at RHIC!)• May have consequences for expected nuclear absorption at 158 GeV • There is feed-down from ’ and c is an effective quantity (eff)

• Use of eff (or equivalently, eff) could introduce a bias (fraction of

measured J/ coming from higher resonances can vary between p-A and A-A, due to different suppression mechanisms in the two systems)

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.0

0.9

0.8

0.7

B&KB&KVogt: final state Vogt: final state absorptionabsorption

- R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539- K.G. Boreskov & A.B. Kaidalov, K.G. Boreskov & A.B. Kaidalov, JETPL JETPL 77 77 (2003) 599(2003) 599

E866 38.8 GeV Be/Fe/WE789 38.8 GeV Be/C/Cu/WE772 38.8 GeV H2/C/Ca/Fe/WNA50 29.1 GeV Be/Al/Cu/Ag/WNA3 22.9 GeV H2/Pt

xF

J/ /DY in Pb-Pb collisions at 158 GeV• Final NA50 set of data

Old reference(include S-U in the determination)

Small error, but assumes S-U is normal

New reference (only p-A collisions are used)

Larger error, but no assumption on S-U

Comparison between centrality estimators (ET, EZDC, Nch)

Fair agreement between various centrality estimators

Suppression pattern (S-U vs Pb-Pb) • Is J/ suppressed beyond nuclear absorption ? Yes, in central and semi-central PbPb collisions

• Does the suppression exhibit a threshold behavior vs centrality? not easy to answer

• Is there any sign of a “second drop” in the suppression pattern ? not evident, but no saturation of the suppression

• Are we observing the suppression of c in nuclear collisions, expected to occur at T~Tc ?

Not obvious, recent HERA-B result =0.210.05) (J/ψσ

)J/ψΣσ(χR

INCL

c(i)χ c

Recent news from SPS: NA60• Having observed an anomalous suppression in Pb-Pb collisions it is important to have a systematic study also with lighter ions

Compare suppression patternas a function of variouscentrality variables

Try to single out a scalingvariable for the anomalous suppression

Study J/ suppression in Indium-Indium collisions

hadron absorberMuonOther

and trackingMuon trigger

magnetic field

Iron wall

NA50 spectrometer2.5 T dipole magnet

Matching in coordinate and momentum space

targets

beam tracker

vertex tracker

or!

NA60: detector concept

• Improved dimuon mass resolution• Origin of muons can be accurately determined

~ 200 m in the longitudinal coordinate~ 20 m in the transverse coordinateExcellent vertex resolution

J/ / DY analysisSet A (lower ACM current)

• Combinatorial background (, K decays) from event mixing method (negligible)• Multi-step fit: a) DY (M>4.2 GeV), b) IMR (2.2<M<2.5 GeV), c) charmonia (2.9<M<4.2 GeV)• Mass shape of signal processes from MC (PYTHIA+GRV94LO pdf)

• Results from set A and B statistically compatible use their average in the following

• Stability of the J/ / DY ratio:• change of input distributions in MC calculation 0.3% (cos), 1% (rapidity) • level of muon spectrometer target cut < 3%

Set B (higher ACM current)

• Data points have been normalized to the expected J/ normal nuclear absorption, calculated with

as measured with p-A NA50 data

J/ / DY vs. centrality

J/abs = 4.18 0.35 mb

• Qualitative agreement with NA50 results plotted as a function of Npart

bin1 Npart = 63bin2 Npart = 123bin3 Npart = 175

B. Alessandro et al., Eur. Phys. J. C39(2005) 335

3 centrality bins,defined through

EZDC

Anomalous suppression present in Indium-Indium

A different analysis techniqueMeasured J/ events are compared to the expected J/ centrality distribution, calculated assuming nuclear absorption as the only suppression source

Nuclearabsorption

Normalization of the nuclear absorption curve

we require the ratio measured/expected, integrated over centrality, to be equal to the same quantity for the J//DY analysis (0.87 ± 0.05)

Very small statistical errorsMany centrality binsMore sensitive to systematics

Measured / Expected vs. Npart

• Departure from the expected normal nuclear absorption in peripheral events• Saturation in more central events ?

Comparison with NA38/NA50

The J/ suppression patterns are in fair agreement when plotted against Npart

Comparison with the extreme case of a step-like function

Npart

Mea

s/E

xp

1

Step position

A1A2

Step position: Npart = 82 ± 9A1= 0.98 ± 0.03A2= 0.85 ± 0.012/dof = 2.0

• Resolution on Npart estimate (due to the measured EZDC resolution) taken into account• A certain amount of physics smearing can be accommodated by the data

Comparison with a recent model

Maximum hadronic absorption

(Hagedorn gas)not enough

to reproduce In-In and Pb-Pb

Becattini, Maiani et al., Phys. Lett. B632(2006) 233

Mechanisms atthe parton level

must beinvoked

Nucl. abs. onlyNucl. abs. + hadron gas

Summary on systematic errors

Various sources of systematic errors have been investigated and their effect on the measured suppression pattern is the following:

The most central bin is affected by a sizeable systematic error relatively to the others. There is also a ~10% systematic error, independent on centralityThe shape of the suppression pattern can be accurately evaluated, but its absolute normalization is more uncertain

• Event selection 1-2%• Input to Glauber model (In density distributions)

• Link EZDC – Npart

• Error on J/pp(450 GeV) 8% centrality independent

• Error on abs 3-4 % (almost) centrality independent

• Error due to the J//DY normalization ~ 6% centrality independent

>10% for EZDC < 3 TeVnegligible elsewhere

5 -10 % for EZDC < 3 TeVnegligible elsewhere

J/ suppression studies: where are we ?• Results for various p-A and A-A systems indicate that

• J/ is suppressed beyond normal nuclear absorption• in Pb-Pb collisions (NA50)• in In-In collisions (NA60)

• J/ is not suppressed beyond normal nuclear absorption• in S-U collisions (NA38)

• Is there a threshold effect ?

• Results are not conclusive• Anomalous suppressions sets in at Npart~100 at SPS energy

• Coherent interpretation of RHIC and SPS results is challenging

• J/ regeneration at RHIC ?• Sequential suppression, with only c melting observed at SPS/RHIC ?

Azimuthal distribution of J/• Possible sources of J/ v2

• Charm elliptic flow For J/ formed by cc recombination, if c quarks thermalize early

Not likely to occur atSPS energies

Greco, Ko, Rapp, PLB595(2004) 202

• cc break-up on co-moving hadrons More pions in-plane than out-of-plane (pion elliptic flow) J/ exiting in-plane more absorbed

Gives negative valuesof v2 with smooth centrality dependence

• cc break-up by QGP hard gluons Parton density azimuthally anisotropic J/ exiting out-of-plane more absorbed

Give positive valuesof v2 with sudden onset when critical conditions for QGP are reached

Heiselberg, Mattiello, PRC60(1999)44902

Wang, Yuan, PLB540(2002) 62Zhu, Zhuang, Xu, PLB607 (2005) 107

Preliminary NA50 results (Pb-Pb)

Correction for event plane resolution still under investigation

• Calculate v’n = cos[n( - n)] • v’2 always smaller than v2

• Reaction plane estimated using e.m. calorimeter (6 azimuthal sectors) determine event plane 2

• Small positive J/ v2 on average More J/ exiting in plane

• Negative J/ v2 more unlikely no major role for breakup by comovers

Preliminary NA60 results (In-In)

• More peripheral data hint for a non isotropic emission pattern with positive v2 ?• Only 50% of the statistics analyzed

central peripheral

• Event plane method has been used• Correction for reaction plane resolution applied

J/ transverse momentum distributions

pT2 = pT

2pp + agN L

pT2

J/ (G

eV/c

)2

L (fm)

preliminary

Study, for A-A collisions, the dependence of pT2 on L

Both Pb-Pb and In-In pointsare well reproduced assumingthat pT distributions arebroadened by initial-stateparton multiple scattering

pT2pp

[(GeV/c)2] agN

[(GeV/c)2/ fm] Pb-Pb + In-In 1.10 0.02 0.081 0.003

J/ central to peripheral ratio “RCP”

DY,1

Tψ,1

iDY,

Tiψ,T

iCP N

)(pNN

)(pN)(pR

Define

i=1 most peripheral bini=5 most central bin

• J/ is suppressed mainly at low transverse momentum

• For pT > 3.5 GeV/c, the centrality dependence of J/ suppression is weak

Polarization of the J/The polarization of the J/ provides a detailed test of quarkonium production models

Quarkonium polarization:•CSM: predicts transverse polarization•CEM: predicts no polarization•NRQCD: predicts transverse polarization at large pT

In nucleus-nucleus collisionsB.L. Ioffe and D.E. Kharzeev, PRC68 (2003) 061902 “Quarkonium Polarization in Heavy-ion collisions as a possible signature of the QGP”

Results up to now (E866, CDF…) do not show an increase of the polarization for high pT

“…polarization exhibits strong non-perturbative effects. The QGP is expected to screen away the non perturbative physics: the J/ which escape from the plasma should possess polarization as predicted by perturbative QCD…”

0 < pT < 5 GeV/c3.2 < yLAB < 3.8- 0.7 < cosH < 0.7

• cosH distribution is fitted with

Polarization of the J/Polarization of the J/ in Indium-Indium (NA60) in Indium-Indium (NA60)

cosH

bin corresponding to 1 < pT < 2 GeV/c

• Study performed in the kinematical region

H2

H

θλcos1dcosθ

dσ

• Acceptance correction performed using a 3-D method

• Polarization angle is computed in the helicity frame (z-axis coincident with the J/ direction in the center of mass frame)

According to theory, in case of QGP formation the expected value for the polarization is = 0.6 (for pT ~ 0),and even taking into account the initial transverse momentum of gluons, remains significantly higher than zero

Preliminary NA60 results

xF

pT (GeV/c) = 0.35 – 0.4

… values closer to zero

Conclusions• Study of quarkonium production very important for HI collisions

• Onset of deconfinement• Thermometer of the medium (sequential suppression)

• SPS results indicate anomalous suppression (signal is there !)

• Quantitative and detailed comparison of J/ suppression between various systems is still not conclusive

• Pb-Pb vs In-In qualitative agreement when plotted against Npart• S-U (asymmetric system) seems to be in disagreement Role of different energy density profile ?

• Understanding the SPS+RHIC set of results is crucial

• Next steps• NA50 analysis without Drell-Yan essential to compare Pb-Pb and In-In• NA60 pA results @ 158 GeV, study of A-dependence of c production