SPS charmonium and bottomonium measurements

1
SPS charm(onium) and bottom(onium)measurements
E. Scomparin INFN 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

2
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 (_at_ ?s20 GeV,
  Bµµ?J/?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)

3
Charmonium productionnuclear collisions at
fixed target

• 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 observe in the data NOT due to
usual hadronic processes ?

• 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

4
The NA38/NA50/NA60 experiments
Based 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
5
pA collisions the reference

• Glauber fit to Bµµ?J/? 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

6
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/?DYpp _at_ 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)
7
?J/? /?DY in Pb-Pb collisions at 158 GeV

• Final NA50 set of data

New reference (only p-A collisions are used)
Old reference (include S-U in the determination)
8
Compatibility of data sets

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

9
Study of various centrality estimators

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

10
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

11
New charmonium studies NA60

• 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/? suppression pattern as a function
of different centrality variables, including
data from different collision systems

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

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

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

Study the nuclear dependence of ?c production in
p-A
12
NA60 In-In collisions

• 5-week long run in 2003 In-In _at_ 158
  GeV/nucleon
• 41012 ions on target
• 2108 dimuon triggers collected

• 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

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

13
The J/? / DY analysis (NA50-like)

• Combinatorial background from ?
• and K decays estimated from the
• measured like-sign pairs
• (lt3 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 2.94.5 GeV)

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

b) 2.2 lt M lt 2.5 GeV normalize the charm (with
DY fixed)
c) 2.9 lt M lt 4.2 GeV get the J/y yield (with DY
charm fixed)
14
Comparison with NA38/NA50 results
anomalous suppression present in Indium-Indium

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

How to get a more accurate suppression pattern ?
Do not use Drell-Yan
15
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

• Use matched J/? sample

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

• Main advantage
• ? Much smaller statistical errors

• Main drawback
• ? No intrinsic normalization, if
• absolute cross sections are not
• known

Work in progress to obtain d?J/?/dEZDC
16
Comparison with expected yield

• Data are compared with a calculated J/?
  centrality distribution
• Use ?J/?abs 4.18 ? 0.35 mb

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

Nuclear absorption

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

EZDC(TeV)
17
Comparison with previous results
The J/? suppression patterns are in fair
agreement in the Npart variable
The S-U, In-In and Pb-Pb data points do not
overlap in the L variable
S-U most central point ?
18
Other variables related to centrality
very preliminary
Bjorken energy density, estimated using VENUS

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

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

19
Comparison with theoretical models

• Good accuracy of NA60 data ? quantitative
  comparisons possible

• 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

• 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.

20
Comparison with theoretical models
No quantitative agreement with any model
21
One more model
L. Maiani _at_ QM2005
Maximum hadronic absorption (Hagedorn gas) not
enough to reproduce In-In and Pb-Pb
22
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 ?

• No final word from theory on the interpretation
  of the results
• SPSRHIC systematics ? great opportunity

• Other interesting results
• Suppression concentrated at low pT in PbPb (see
  NA50 _at_ QM05)

• Anomalous ? suppression identical in S-U and
  Pb-Pb (vs L)
• Already sets in for peripheral S-U collisions
  (see NA50 _at_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

23
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
24
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)

• Various possibilities

?(2S)
Study J/? suppressionvs. ?s (not possible
atpresent SPS energies)
?c(1P)
J/?
Study suppression of? states (depends on
available luminosity)
J/?
Needs NA60 upgrade ? first discussions are now
taking place
25
Heavy quark production

• Relatively comfortable cross section (?tot 20
  µb _at_ ?s20 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 (combinatorialDrell-Yan)
• Negligible contribution in the low-mass region
• Sizeable contribution in the intermediate mass
  region
• First studies by NA50, important progress with
  NA60

26
p-A shape analysis m, y, pT, cos? spectra
M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443

• Dimuon differential distributions in the
• region 0.5ltyCMlt0.5, ?cos?CS?lt0.5
• consistent with a superposition of
• Drell-Yan open charm

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

27
Extrapolation to A-A collisions

• Assumption DY and open charm behave as hard
  processes? A scaling

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

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

Enhancement of known sources
Nature of the excess
New sources appear
28
Enhancement of known sources
M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443
Factor 3 enhancement in central Pb-Pb

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

29
Thermal production?
R. Rapp and E. Shuryak, Phys. Lett. B473(2000) 13

• Explicit introduction of a
• QGP phase
• Initial temperature
• Ti192 MeV
• Critical temperature
• Tc175 MeV
• Fireball lifetime
• 14 fm/c
• (increasing to Ti221 MeV
• still good agreement)

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

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

Only way to solve the puzzle discriminate
between prompt and displaced dimuon sources
30
NA60 detector concept
2.5 T dipole magnet
NA50 spectrometer
beam tracker
vertex tracker
targets
Matching in coordinate and momentum space

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

31
Muon matching
Muons from muon spectrometer
Vertex spectrometer tracks
Compare slopes and momenta Define 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
Vary the cut on the matching ?2
32
Vertex resolution
?z 200 µm along beam axis
Good target ID (down to very peripheral events)
?x ?y 10- 20 µm in the transverse direction
(by comparing beam impact point on the target
and reconstructed interaction point)
33
Offset resolution
Resolution of the impact parameter of the track
at the vertex (offset)
40 50 µm
(studied using J/? events)
?vertex ? ?impact lt 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)

34
Weighted offset distribution of the expected
sources

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

35
Background subtraction

• Combinatorial background
• Dominant dimuon source for m??lt2 GeV/c2

• NA60 acceptance quite asymmetric ?

Cannot use

• Mixed event technique developed ? accurate to
  1

• 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
36
IMR is an excess present ?
Excess
Excess
NA50 norm.
World-aver. norm.
data prompt charm promptcharm

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

37
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
Results of fits reported in terms of DY and open
charm scaling factors needed to describe the
data
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)
38
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?

Kinematical domain 1.2 lt M lt 2.7 GeV/c20 lt yCM lt
1cos? lt 0.5
Answer No, the fit fails Charm is too flat to
describe the remaining spectrum
39
Alternative options

• 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)
40
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

NA50 p-A mm
world average
Dimuon weighted offsets
Answer No, both options require two times more
prompts than the expected Drell-Yan ! (the charm
contribution is too small to make a difference)
41
Mass shape of the excess

• Fix the DY and Charm contributions to expected
  yields

The mass spectrum of the excess dimuons is
steeper than DY(and flatter than Open Charm)
42
Centrality dependence of the excess
Relative excess(Data Sources) / Sources
very preliminary
Excess per participant (Data Sources) / Npart
Faster than linear increase with Npart
43
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 ?
44
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
phenomena occurring in various energy ranges is
an important challenge, 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 SPS Encouragement,
suggestions, participation are very welcome !