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
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SPS charm(onium) and bottom(onium) measurements E. Scomparin INFN Torino (Italy) Introduction Past heavy quark and quarkonium measurements: NA38/NA50 (Helios-3)
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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:
•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
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
• 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 !