SPS charm(onium) and bottom(onium)measurements

E. ScomparinINFN Torino (Italy)

• Introduction• Past heavy quark and quarkonium measurements: NA38/NA50 (Helios-3)• Present heavy quark and quarkonium measurements: NA60• What remains to be learned ?• Conclusions

Heavy quarkonia• Matsui and Satz prediction (1986) at the origin of the whole field

• No experiment was explicitly intended for charmonia detection• Even NA38 (proposed in March 1985) was aiming at the study of thermal dimuon production• Experimental facts

• Relatively small cross section (@ s=20 GeV, BµµJ/~10 nb)• J/µµ channel relatively clean• Need large luminosities and a very selective trigger

• NA38 happened to be in a very good situation to study charmonium (its ancestor, NA10, studied high mass Drell-Yan and production)

Charmonium production:nuclear collisions at fixed target

•Study carried out by NA38/NA50/NA60 at the SPS from 1986 until today•Basic facts

•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 •HERA-B

•The question to be answered by studying charmonium in heavy-ion collisions at the SPS

Is (at least part of the) suppression of charmonia that we observein the data NOT due to usual hadronic processes ?

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

• As it is well known, 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)

J/ /DY in Pb-Pb collisions at 158 GeV

• Final NA50 set of data

Old reference(include S-U in the determination)

New reference (only p-A collisions are used)

Compatibility of data sets

• Older data sets considered not as reliable as recent ones• 1996 high statistic data: biased by reinteractions (thick target)

Study of various centrality estimators

• Pattern consistent with ET-based analysis• Departure from normal nuclear absorption at mid-centrality• Suppression increases with centrality

What about S-U ?

•Absorption curve calculated using p-A data only

• S-U data found to be in agreement (once rescaling are performed) with

• p-A extrapolation• Peripheral Pb-Pb collisions

• No indication for a sizeable extra-absorption in S-U wrt p-A

Study the J/ suppression patternas a function of different centrality variables, including data from different collision systems

Study collisions between other systems, such as Indium-Indium

• Which is the variable driving the suppression?

• Is the anomalous suppression also present in lighter nuclear systems?

Study the nuclear dependence of c production in p-A

• What is the impact of the c feed-down on the observed J/ suppression pattern?

New charmonium studies : NA60

Study J/ production in p-A collisions at 158 GeV

• What is the normal nuclear absorption cross-section at the energy of the heavy ion data?

S-U

In-In

Pb-Pb

Npart

L (f

m)

pure Glauber calculation

NA60: In-In collisions

• 5-week long run in 2003 – In-In @ 158 GeV/nucleon• ~ 4×1012 ions on target• ~ 2×108 dimuon triggers collected

Set ASet B

Raw +-

invariant massspectrum

mµµ (GeV/c2)

Events

/50

MeV

• Centrality selection: use• spectator energy in the ZDC• charged multiplicity in the vertex spectrometer

• Two muon spectrometer settings• Set A (low ACM current)

• Good acceptance at low mass• Used for LMR and IMR analysis

• Set B (high ACM current)• Good resolution at high mass• Used for J/ suppression, together with set A

• Multi-step fit a) M > 4.2 GeV : normalize DY

b) 2.2 < M < 2.5 GeV: normalize the charm (with DY fixed)

c) 2.9 < M < 4.2 GeV: get the J/y yield (with DY & charm fixed)

• Combinatorial background from and K decays estimated from the measured like-sign pairs (<3% contribution under the J/)

• Signal mass shapes from MC• PYTHIA and GRV 94 LO p.d.f.• GEANT 3.21 for detector simulation • reconstructed as the measured data

• Acceptances from Monte Carlo simulation: J/ : 12.4 % (setB); 13.8 % (setA) DY : 13.2 % (setB); 14.1 % (setA) (in mass window 2.9–4.5 GeV)

J/

’

DY

Background

Charm

without matching6500 data setno centrality selection

The J/ / DY analysis (NA50-like)

• Normal absorption curve based on the NA50 results• Uncertainty (~ 8%) at 158 GeV dominated by the extrapolation from the 400 and 450 GeV data

Comparison with NA38/NA50 results

“anomalous suppression” present in Indium-Indium

How to get a more accurate suppression pattern ? Do not use Drell-Yan

Study of the J/ centrality distribution

• Compare the centrality distribution of the measured J/ sample with the distribution expected in case of pure nuclear absorption

• Main advantage Much smaller statistical errors

• Main drawback No intrinsic normalization, if

absolute cross sections are not known

• Use matched J/ sample

EZDC (GeV)

dNJ/

/d

EZ

DC

εvertex dimuon > 99.5 %

ε ver

tex

• Inefficiencies introduced by the cuts, used in the event selection, affect in a negligible way the J/ sample (or are not centrality dependent)

Work in progress to obtain dJ//dEZDC

Comparison with expected yield• Data are compared with a calculated J/ centrality distribution• Use J/

abs= 4.18 0.35 mb

• Onset of anomalous suppression in the range 80 < Npart < 100• Saturation at large Npart

Nuclearabsorption

• Ratio (Measured / Expected) normalized to the standard analysis (~7% error)

EZDC(TeV)

Comparison with previous results

The S-U, In-In and Pb-Pb data points do not overlap

in the L variable

The J/ suppression patterns are in fair agreement in the

Npart variable

S-U most central point ?

Other variables related to centrality

NA50 Pb-PbNA60 In-In

NA50 Pb-PbNA60 In-In

• A more significant comparison requires Pb-Pb points with reduced errors

very

preliminaryBjorken energy density, estimated using VENUS

• Work in progress inside NA50 to have a non-DY analysis for the 2000 data Results expected soon

Comparison with theoretical models

• Good accuracy of NA60 data quantitative comparisons possible

• J/ absorption by produced hadrons (comovers) Capella and Ferreiro, hep-ph/0505032

• J/ suppression in the QGP and hadronic phases (including thermal regeneration and in-medium properties of open charm and charmonium states) Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; hep-ph/0403204

• c suppression by deconfined partons when geometrical percolation sets in Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

• Consider models • formulating specific predictions for In-In collisions• previously tuned on the p-A, S-U and Pb-Pb suppression patterns obtained by NA38 and NA50

Comparison with theoretical models

Satz, Digal, FortunatoRapp, Grandchamp, BrownCapella, Ferreiro

No quantitative agreement with any model

One more model

Maximum hadronic absorption (Hagedorn gas)not enough to reproduce In-In and Pb-Pb

L. Maiani @ QM2005

Summary on charmonium at the SPS

• Anomalous J/ suppression• Established fact in Pb-Pb (NA50) and, more recently, in In-In (NA60)• Not present in S-U collisions (NA38)

• Onset around Npart = 100• Does S-U show an incompatibility with Pb-Pb and In-In ?

• Other interesting results• Suppression concentrated at low pT in PbPb (see NA50 @ QM05)• Anomalous ’ suppression identical in S-U and Pb-Pb (vs L)

• Already sets in for peripheral S-U collisions (see NA50 @QM05)

• News to be exepcted in the near future• NA50: non-DY analysis more meaningful comparison with NA60• NA60: use full statistics for analysis ~ factor 2 more

• No final word from theory on the interpretation of the results• SPS+RHIC systematics great opportunity

Can SPS go beyond charmonium ?

NA50 measured A-dependence in p-A at 450 GeV

= 0.98 0.08

production not accessible in A-A at present SPS, s too low

Bottomonium in A-A at the SPS ?

• In the framework of the upgrade of CERN machines the SPS+ concept is presently under discussion Availability of ~1 TeV protons from ~2014 onwards

Pb ions at ~ 400 GeV/nucleon(s ~ 28 GeV)

Study J/ suppressionvs. s (not possible atpresent SPS energies)

Study suppression of states (depends on available luminosity) J/

(2S)c(1P)

J/

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

b(1P)

(1S)

• Various possibilities:

Needs NA60 upgrade first discussions are now taking place

Heavy quark production• Relatively comfortable cross section (tot~ 20 µb @ s=20 GeV)• However

• D0 K • Difficult to single out in the high hadronic multiplicity

(attempt by NA49,no signal, nucl-ex/0507031)• D0 µX

• Full reconstruction of the decay topology impossible• Important background (combinatorial+Drell-Yan)• Negligible contribution in the low-mass region• Sizeable contribution in the intermediate mass region• First studies by NA50, important progress with NA60

Pb-PbNpart=110

Pb-PbNpart=381

p-A shape analysis: m, y, pT, cos spectra

• Dimuon differential distributions in the region –0.5<yCM<0.5, cosCS<0.5 consistent with a superposition of Drell-Yan + open charm

M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443

• Absolute cross sections found to be consistent with direct measurements of open charm production

Extrapolation to A-A collisions

•Assumption: DY and open charm behave as hard processes A scalingPb-PbNpart=110

Pb-PbNpart=381

• Excess of dimuon yield: Data/Sources ~1.3 in S-U, ~1.7 in Pb-Pb• Smoothly growing with centrality

Nature of the excess

Enhancement of known sources

New sources appear

M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443

Enhancement of known sources

• Excess not compatible with background shape• Compatible with an an enhancement of open charm (m,pT spectra)

Factor 3 enhancementin central Pb-Pb

M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443

Thermal production?

• Good description of the mass spectra in the two approaches for central Pb-Pb events

R. Rapp and E. Shuryak,Phys. Lett. B473(2000) 13

•Explicit introduction of a QGP phase•Initial temperature: Ti=192 MeV•Critical temperature: Tc=175 MeV•Fireball lifetime: 14 fm/c(increasing to Ti=221 MeV still good agreement)

L. Capelli et al.,NA50, Nucl. Phys. A698(2002) 539c

Only way to solve the puzzle: discriminate between prompt and displaced dimuon sources

hadron absorberMuon

Other

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

Muon matching

Vary the cut on the matching 2

Muons from muon spectrometer Vertex spectrometer tracks

Compare slopes and momentaDefine a matching 2

Re-fit matched tracks• With this procedure

• Combinatorial background can be reduced• A certain level of fake matches is present (new kind of background)

improve the signal/background ratio

Vertex resolution

z ~ 200 µm along beam axis

Good target ID (down to very peripheral events)

Dispersion between beam track andVT vertex

Vertex resolution (assuming BT=20 m)

10

20

30

0

(

m)

Number of tracks

x ~ y ~ 10- 20 µmin the transverse direction

(by comparing beam impactpoint on the target andreconstructed interaction point)

Offset resolution

J/

Weighted Offset () 100

Offs

et r

esol

utio

n (

m)

Resolution of the impact parameter of the track at the vertex (offset)

40 – 50 µm

(studied using J/ events)

vertex impact < c (D+ : 312 m, Do : 123 m)

Prompt dimuons can be separated from open charm decays

• Define weighted offset to eliminate momentum dependence of offset resolution (offset wighted by error matrix of the fit)

Weighted offset distribution of the expected sources

• Prompt contribution average of the J/ and measured offsets• Open charm contribution MC distribution, after smearing

Background subtraction

• Combinatorial background • Dominant dimuon source for m<2 GeV/c2

• Mixed event technique developed accurate to ~ 1%

• NA60 acceptance quite asymmetric Cannot use NN2Nbck

• Fake matches background also rejected with a mixed event approach • Less important in the intermediate mass region

1% error in the comb. background

estimate

10% error on the signal

IMR: is an excess present ?

• Answer: Yes, an excess in the IMR is clearly present(same order of magnitude of the NA50 result)

NA50 norm.

• Open charm and Drell-Yan generated with PYTHIA• Drell-Yan normalization fixed using the high mass region• Open charm normalization: use

NA50 p-A result (better control of systematics related to channel) World-average cc cross section (based on direct charm measurements)

(differ by a factor ~ 2)

World-aver. norm.

datapromptcharmprompt+charm

Excess Excess

Is the excess compatible with the NA50 observation?

• Can we describe the measured mass spectrum by leaving the open charm normalization as a free parameter, as done by NA50?

~ 2 in terms of NA50p-A normalization

Answer: Yes, we can describe the In-In data with a “charm enhancement” factor around 2 in “NA50 units”

(to be compared with ~ 3 for PbPb in NA50)

Results of fits reported in terms of DY and open charm scaling factors needed to describe the data

Check NA50 hypothesis using muon offsets

• Fix the prompt contribution to the expected DY• Can the offset distribution be described with an enhanced charm yield?

Dimuon weighted offsets

Answer: No, the fit fails

Charm is too flat to describe the remaining spectrum…

Kinematical domain1.2 < M < 2.7 GeV/c2

0 < yCM < 1|cos| < 0.5

Alternative options

Dimuon weighted offsets

•Try to describe the offset distribution leaving both contributions free

Answer: Two times more prompts than the expected Drell-Yan provides a good fit

(and the charm yield is as expected from the NA50 p-A dimuon data)

Is the prompt yield sensitive to the charm level?

• Fix the charm contribution to either of the two references, and see how the level of prompts changes

Answer: No, both options require two times more prompts than the expected Drell-Yan !

(the charm contribution is too small to make a difference)

Dimuon weighted offsets

“world average” “NA50 p-A ”

Mass shape of the excess

The mass spectrum of the excess dimuons is steeper than DY(and flatter than Open Charm)

• Fix the DY and Charm contributions to expected yields

Relative excess:(Data – Sources) / Sources

Excess per participant:(Data – Sources) / Npart

Faster than linear increase with Npart

Centrality dependence of the excess

very

preliminary

Summary on open charm at the SPS

• Serious study much delayed with respect to charmonia investigations

• First generation experiments• Excess in the intermediate mass region• Connession with open charm possible (NA50)• Could not be proved

• Second generation experiment (NA60)• Equipped with accurate vertex detector• Present understanding: open charm yield in A-A follows Ncoll scaling

• What next ?• Update NA60 results (full statistics, more accurate alignment)• Run NA60 with PbPb (after 2010)

If the IMR excess is not charm, then what can it be ?

Conclusions

• Long and fascinating history (started 19 years ago!)• Many interesting results, both recent and (relatively) ancient• Still interesting now, when higher energy domains are opening up ?

Surely yes!Finding a consistent

description of phenomenaoccurring in various energy

ranges is an importantchallenge, that deserves

being investigated

• Future of heavy-ions at SPS ?• Still not defined, but

• Heavy-ions can be available once LHC has been commissioned• SPS+ will be built in case LHC luminosity upgrade is approved

Some of us are starting to think about a new dimuon experiment at SPSEncouragement, suggestions, participation are very welcome !