charmonium production results from the NA60 experiment E Scomparin INFN Torino Italy NA60 c ollaboration Introduction experimental setup Charmonium suppression in pA and InIn ID: 460718
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Slide1
Open charm and charmonium production:results from the NA60 experiment
E. Scomparin (INFN – Torino, Italy), NA60 collaboration
Introduction, experimental set-up Charmonium suppression in p-A and In-In collisions (results, lessons from the learning process) Studying the Intermediate Mass Region (IMR) “Preliminary” results on the A-dependence of the open charm yield (from the dimuon mass spectrum) Conclusions
HICforFAIR
Workshop:
Heavy flavor physics withSlide2
SPS experiments
NA35
NA36NA49NA34(Helios-2)NA34/3(Helios-3)NA44NA45(Ceres)
NA38
NA50
NA60
WA80
WA98
WA85
WA97
NA57
NA52
WA94
HADRONS
LEPTONS, PHOTONS
S
multistrange
photons
electrons
1986
1994
2000
exotics
strangeness,
hadron spectra
strangeness
muons
2003
muons
strangeness,
hadron spectra
Pb
1
2
3
NA61
Long and glorious history, dating back to 1986
Third generation experiments: NA60, NA61Slide3
The NA60 experiment
NA60, the third generation experiment studying dimuon production at the CERN SPS
hadron absorber
Muon
Other
and tracking
Muon trigger
magnetic field
Iron wall
NA10/38/50 spectrometer
2.5 T dipole magnet
Matching in coordinate
and
momentum space
targets
beam tracker
vertex tracker
Data samples
In-In
collisions at 158
GeV
/nucleon
p-A
collisions at 158 and 400
GeV
9 nuclear targets, Al-U-W-Cu-In-Be1-Be2-Be3-Pb
(mixed A-order to limit possible z-dependent systematics)
ZDC
orSlide4
Performances
z-coordinate
of the reconstructed vertices 7 In targets 1.5 mm thick, 8 mm spacingVertex resolution~10 m (X), ~15 m (Y) Vertex tracker16 pixel planesALICE1LHCb readout chipsPixel size: 50 425 m210 MHz clockSlide5
A glimpse of low-mass results
20 MeV mass resolution at the Excess all along the spectrum NO
-mass shiftSlide6
Charmonia suppression: pA, AA
Study of charmonium production/suppression in pA/AA collisions
Production models (CSM, NRQCD, CEM, ....)Reference for understanding dissociation in a hot mediumInitial/final state nuclear effects (shadowing, dissociation,...)AA collisionsColor screening and charmonium suppression
>
25 year long history
pA collisions
THE hard probe at SPS energySlide7
7
J/ analysis: match vs no-match 2 event selections have been used for J/ analysis
1) No matching required Extrapolation of muon tracks must lie in the target region Higher statistics Poor vertex resolution (~1 cm)2) Matching between muon tracks and vertex spectrometer tracks Dimuon vertex in the most upstream interaction vertex (MC correction to account for centrality bias due to fragment reinteraction) Better control of systematics Good vertex resolution (~200 m) Lose 40% of the statistics
2 analyses
a) Use selection 1 and normalize to Drell-Yanb) Use selection 2 and normalize to calculated J/
nuclear absorption
After quality cuts
NJ/ ~ 45000 (1), 29000 (2)Slide8
8
J/ / DY analysis
Set 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) Tuning of quality cut for muon spectrometer tracks < 3%
Set B (
higher ACM current) Slide9
9
Data points have been normalized to an expected
yield which takes into account CNM effects, parameterized through deduced from p-A NA50 data at 400 and 450 GeVJ/ / DY vs. centrality (analysis a)
J/abs = 4.18 0.35 mb
Qualitative agreement with
NA50 results plotted as a function of
N
part
B. Alessandro et al., Eur. Phys. J. C39(2005) 335
3 centrality
bins, defined through E
ZDC
Anomalous suppression
present in Indium-Indium
WARNING: hypothesis on
s-independence
of CNM effects
NOT TESTED
at that timeSlide10
10
J/ yield vs nuclear absorption (analysis b)
Compare data to the expected J/ centrality distribution, calculated assuming CNM effects (parameterized through abs =4.18 mb) as the
only suppression source (see later)
require the ratio
measured/expected, integrated over centrality, to be equal to the same quantity from the (J/
)/DY analysis (0.87 ± 0.05)
Nuclear
absorption
Normalization
of the
CNM referenceSlide11
11Results and systematic errors
Small statistical errors
Careful study of systematicerrors is needed Sources Uncertainty on parameters which enter CNM calculation (abs(J/) and pp(J/)) Uncertainty on relative normalization between data and
CNM reference
Uncertainty on centrality determination (affects relative
position of data and abs. curve) Glauber model parameters
EZDC to Npart
~
10%
error centrality indep.
does not affect shape
of the
distributionSlide12
Moving to pA collisions
12Absence of pA data collected at the same energy of In-In (Pb-Pb) data considered as a serious issue obtained 3 days of primary SPS proton beam at 158 GeV
in 2004Slide13
s-dependence of CNM effects at SPS
abs J/ (400 GeV)= 4.3 ± 0.8 (stat) ± 0.6 (syst) mbUsing the Glauber model, we getUsing J/ = 0 A, we get
(400 GeV
) = 0.927 ± 0.013 (stat) ± 0.009 (syst
)
abs J/
(158 GeV)= 7.6 ± 0.7 (stat)
± 0.6
(
syst
)
mb
(158
GeV
)
=
0.882
±
0.009 (stat)
±
0.008 (
syst
)
(effective values, shadowing not corrected for)Slide14
Comparisons with other experiment: xF
Results on
vs xF from HERA-B, NA50, E866, NA3 (removed bias from use of p-p)In the region close to x
F = 0,
stronger deviation of from 1
when decreasing s
NA60400 GeV: very good
agreement with NA50 158 GeV:
smaller
Disagreement
with NA3
200
GeV
results
Systematics of fixed-target data still
difficult to interpret
roo
m for improvement on
theory
and
experiment
sideSlide15
Studying nuclear effects vs x2
The x2 acceptance of the NA60spectrometer
is ~ energy independentx2 is strongly correlated with sN expect same absorption at fixed x2Shadowing effects (21 approach) scale with x2If parton shadowing and final state absorption were the only two relevant mechanisms
should not depend on s at fixed x
2Slide16
x2-dependence of J/
Clearly
effects different from shadowing and final state absorption are presentNA60 can measure = (400) - (158)within the same experiment common systematics cancel reduced systematics on Slide17
CNM effects, evaluated in pA, can be extrapolated to AA, assuming a
scaling with the L variable and taking into account that:
absJ/ shows a dependence on energy/kinematics reference obtained from 158 GeV pA data (same energy/kinematics as the AA data)
in AA collisions, shadowing affects both projectile and target
proj
. and target
antishadowing taken into account in the reference determination
Use as
reference:
slope determined only from pA@158GeV
abs
J/
(158 GeV) = 7.6 ± 0.7 ± 0.6 mb
normalization to
J/
pp
determined from
pA@158
GeV
(J/
/DY point) and (to
reduce the overall error) SU@200GeV
SU has been included in the fit, since it has a slope similar to pA at 158 GeV
advantage:
small error on normalization (3%)
drawback:
hypothesis that SU is “normal”
Reference for AA dataSlide18
B. Alessandro et al., EPJC39 (2005) 335
R. Arnaldi et al., Nucl. Phys. A830 (2009) 345
R.Arnaldi, P. Cortese, E. Scomparin Phys. Rev. C 81 (2009), 014903 Using the previously defined reference:Central Pb-Pb: still anomalously suppressed
In-In:
almost no anomalous
suppression
In-In 158 GeV (NA60)
Pb-Pb 158 GeV (NA50)
Anomalous suppressionSlide19
Open charm production in p-A collisions
Open charm shares initial state effects with charmonium a measurement of open charm in p-A collisions
may help in understanding J/ suppression E866/NuSea Preliminary
Recent results from
SELEX and E866
suggest rather strong
nuclear effects on open charm
A. Blanco et al. (SELEX), EPJC64(2009) 637
M. Leitch (E866), workshop on “Heavy Quarkonia
Production in Heavy-Ion Collisions”, ECT* 2009Slide20
Open charm dimuons in p-A: NA60
NA50 tried to evaluate DD production studying the IMR in pA Large background levels (S/B ~0.05 at m
= 1.5 GeV/c2) NA60 is much better placed, thanks to the muon matching S/B is ~60 times more favourableNA50 had to imposea constant DD/DY vs A(i.e. DD= DY
~1
)
M.C. Abreu et al., EPJC14(2000) 443Slide21
Fit to the mass spectra
Simultaneous semi-muonic decays of DD pairs are the dominant source in the invariant mass region m<mJ
/High-mass DY statistics is low Drell-Yan cannot be directly constrained by the fitUse the ratios /DY from NA50 (EPJC 48 (2006)) 329 to fix DY Background evaluated with event mixing technique, remaining
muon
pairs come from open-charm decayNot possible to directly measure
the D decay length in
p-Ap-U 400 GeV
400 GeV: larger open
charm
signalSlide22
Open charm signal(s) in the mass spectra
Low background, small Drell-Yan contributionOpen charm is the dominant source
of dimuons in the IMRSlide23
Nuclear dependence of open charm
2/ndf = 0.4(stat.), 0.2 (tot.)
DD (400 GeV) = 0.948 ± 0.022 (stat) ± 0.018 (syst) Systematic errors include uncertainties on: target thickness, reconstruction efficiencies, fit inputs (/DY measured by NA50), background subtraction. They include also the effect of applying different fitting approaches and quality cutsSlide24
Influence of shadowing
Calculate the expected (pure shadowing) for J/
and DD pairs decaying into muons in the NA60 acceptance at 400 GeV To properly compare J/ with open-charm one has to take into account possible differences in shadowing effects due to the different x2 coverageSlide25
Nuclear dependence, J/ vs open charm
Shadowing effects quite similar for J/
and open-charmShadowing is not the origin of the measured < 1 for open-charmAnti-shadowing regionExperimentally we observe similar for J/ and open-charmSlide26
Outlook: open charm at 158 GeV
Possible presence of a strong nuclear dependence
to be further investigatedOpen-charm signal lower than at 400 GeV, need careful check of systematicsDY subtraction constrained by NA50 measurement at 158 GeV (EPJC 49 (2007) 559)direct fit on dataBackground subtractionSlide27
Conclusions
NA60 performed detailed studies of charmonium suppression in In-In collisions at 158 A GeV and in p-A collisions at 158 and 400 GeV
Nuclear effects stronger when decreasing s Lack of x2 scaling for J/ nuclear dependence shadowing + nuclear absorption scenario is ruled outAnomalous J/ suppression (beyond CNM effects) at SPS confirmed, significant only for Pb-Pb, beyond Npart~200-250
Measurement
of nuclear dependence of open-charm production at 400 GeV
Contrary to the expectation from shadowing an open-charm suppression is observed (
1.7 effect)
values similar for open-charm and J
/
Slide28
Future SPS charmonium measurements ?
Identify
thresholds for charmonium suppression via SPS energy scanDetailed study of c by detecting the decay photonStudies for a
NA60-like set-up
Scan feasible
(luminosity) down to 50-60
GeV
incident
Pb
energy
Feasible in a few weeks
at typical SPS beam intensities
Decreasing energySlide29
Feasibility studiesSlide30
158 GeV cross sections constrained by the relative normalization
Systematic error on (absolute) luminosity estimation quite high
Relative luminosity estimate between 158 and 400 GeV much better known (~2-3% systematic error) Normalize NA60 400 GeV cross section ratios to NA50 results
J/
production cross sections for pA dataNew result: J/
cross section in pASlide31
No practical consequence on anomalous J/
Ψ suppression
Preliminary Alternative approach for the normalization of the pA reference curve based on the pA J/ absolute cross section
J/
/
DY values are obtained rescaling the DY cross section measured at 450 GeV
by NA50 (not enough statistics at 158 GeV)
Main advantage: no assumption on SU, since it is not used
anymore in the fit
difference with previous CNM reference ~1% well within errors
To fully profit from this approach, a measurement of the
absolute J/
cross section in In-In would be needed. For the
moment…
New
reference
using J/
cross sections
Slide32
Target ID in the IMR
Low background in the IMR (matching)Good resolution on the
longitudinal position of the vertex in the IMR good target assignmentCross target contamination (0.5- 9%) has been corrected for1.5<m<2.4 GeV/c2