J/? suppression in p-A and In-In collisions at 158 GeV/nucleon

1
J/? suppression in p-A and In-In collisions at
158 GeV/nucleon
R. Arnaldi INFN Torino (Italy) for the NA60
Collaboration

• Introduction
• J/? suppression in In-In collisions
• New results from p-A collisions
• at 158 GeV
• ? study of the pT distributions and
• comparison with In-In
• v2 of the J/? in In-In
• Conclusions

2
J/? suppression at SPS energy

• Nuclear collisions results Pb-Pb (NA50) and
  In-In (NA60)
• p-A reference results from p-A collisions at
  400/450 GeV (NA50)

R. Arnaldi et al. (NA60), PRL99, 132302 (2007)

• Observed suppression exceeds nuclear absorption
• Onset of the suppression at Npart ? 80
• Comparison between different systems ? Npart
  scaling
• At RHIC (?s ? 10 ?sSPS) a very similar
  suppression pattern is observed

3
Nuclear absorption reference

• Measuring ?J/?/?DY in p-A collisions at 400/450
  GeV, NA50 extracts
• (Glauber analysis) ?absJ/? 4.20.5 mb, and
  (?J/?/?DY)pp 57.50.8

• The expected J/? yield for In-In and Pb-Pb
  collisions is calculated
• assuming ?absJ/? (158 GeV) ?absJ/? (400/450
  GeV)
• rescaling (?J/?/?DY)pp to 158 GeV with a
  semi-theoretical procedure

• Preliminary pA results from NA60 at 158GeV
• (averaged over nuclear targets) seem to
• validate the nuclear absorption normalization
• extracted from 400/450 GeV data

• Results on ?absJ/? will appear soon (HP08)
• ? crucial to confirm (or modify) the
  anomalous suppression pattern

Preliminary!
4
J/? transverse momentum
At SPS energy, previous results by NA50 seem to
indicate that the shape of the pT distributions
of the J/? are dominated by initial state effects
(multiple scattering of the incoming gluons, i.e.
Cronin effect)

• Main features

• ?pT2? (and T) linearly increase with L (mean
  thickness of nuclear matter)

• Phenomenological description with
• the expression

with an energy dependent ?pT2?pp and a common
slope ?gN 0.0810.002 (GeV/c)2/fm
pT distributions for pA and AA never studied in
the same energy/kinematical range
5
p-A collisions at 158 GeV

• A target system including 7 different nuclei
  (Be, Al, Cu, In, W, Pb, U) has been used
• Accurate target ID thanks to the NA60 vertex
  spectrometer (pixel)

pCu

• Mass resolution 100 MeV at the J/?, 40 MeV at
  the ?
• Under the J/?
• Combinatorial background is zero
• Drell-Yan contribution is small (lt2)

? A simple event counting technique can be used
to extract NJ/?
6
Study of pT distributions in pA at 158 GeV

• The pT distributions of the J/? have been
  obtained using a 1D acceptance
• correction method
• The input distributions for the other
  kinematical variables (y, cos?CS) have been
  obtained starting from a 3D correction algorithm
  and then adjusted iteratively on the data

• y distribution ? gaussian with ?y0.52
• cos?CS distribution ? flat
• (no J/? polarization)

• Rapidity coverage 0ltyCM lt1
• Same as in NA50(Pb)/NA60(In)
• ? extrapolation needed for
• upstream targets

7
pT distributions for 158 GeV pA
Distributions fitted with the function
0ltycmlt1
cos?lt0.5
in order to obtain the inverse slope Teff values
Teff values slightly increase with A
8
mT distributions for 158 GeV pA
In the explored mT-M range, no deviations from
the exponential behaviour can be appreciated
9
Dependence of ?pT2? on L

• Systematic errors are mainly
• due to the choice of the
• generated y and cos?
• distributions in the
• acceptance calculations

smaller than statistical errors

• Applying more severe event
• selection cuts there is no
• effect on the results

We observe a linear increase of ltpT2gt with L,
consistent with gluon scattering in the initial
state
10
Comparison p-A vs In-In (Pb-Pb)
For the first time we compare the transverse
momentum distributions of the J/? in p-A and A-A,
in the same energy/kinematical range
p-A
ltpT2gtpp1.13 0.04 (GeV/c)2 ?gN0.029 0.011
(GeV/c)2 / fm
In-In
ltpT2gtpp1.27 0.09 (GeV/c)2 ?gN0.058 0.014
(GeV/c)2 / fm
Pb-Pb
ltpT2gtpp1.19 0.04 (GeV/c)2 ?gN0.072
0.005(GeV/c)2 / fm

• ?pT2? increases linearly with L in both p-A,
  In-In and Pb-Pb
• However, the scaling of ?pT2? with L is broken
  moving from p-A to A-A
• On one hand comparing p-A and peripheral In-In
  the suppression scales with L
• On the other hand the J/? pT distributions do
  not scale with L !
• ?gNAA 2 ?gNpA ? does one simply add up
  projectile and target broadening ?

11
Comparison p-A In-In vs. Npart
We find an approximate scaling of ?pT2? with the
logarithm of the number of participant nucleons
12
Comparison pA 158 GeV vs pA 400 GeV

• NA60 has also taken p-A data at 400 GeV, i.e. in
  the same energy and kinematical
• domain covered by p-A data previously collected
  by NA50
• Compare (as a check) the results of the two
  experiments

• The slope of the p-A points at 400 GeV
• is compatible between NA50 and NA60
• (1.2 ?)

NA50 p-A 400 GeV ?gN0.087 0.004
NA60 p-A 400 GeV ?gN0.104 0.013
New 158 GeV data show that at SPS ?gN depends
on the energy of the collision
13
RAA for In-In at 158 AGeV

• We have not measured reference p-p collisions at
  158 GeV
• Build a J/? pT distribution using the functional
  form

with T obtained from the value of ?pT2? pp
coming from the fit of the p-A data
14
RAA for In-In at 158 AGeV (2)

• Clear rise at high pT consistent
• with the Cronin effect

• RAA much lower than 1 at low pT
• Effect seen dominated by nuclear absorption

• A systematic error (11) due to
• the data normalization is common
• to all points

15
RCP for In-In at 158 A GeV
Normalize pT distributions to the most peripheral
In-In bin, corresponding to Npart?50
We see that moving towards central collisions
there is an increasingly large suppression at
low pT (already seen in Pb-Pb)
16
Comparison with PHENIX
RAA similar at SPS and RHIC at low transverse
momentum (pT lt 1.5 GeV/c)
17
v2 measurements at NA60

• NA60 acceptance 0 lt ycm lt 1
• Use elliptic flow v2 to estimate the reaction
  plane (v1 0 at midrapidity)?
• Determination from charged particle tracks as
  measured in the vertex tracker

v2 for charged particles
18
J/? azimuthal anisotropy (1)

• Limited statistics (lt30000 J/? events) prevents
  a fine binning in centrality/pT
• Define 2 broad centrality classes

v2 consistent with zero for central events, v2 gt
0 (2.3?) for peripheral
19
J/? azimuthal anisotropy (2)

• Introduce a rough pT binning

Centrality ptlt1GeV/c ptgt1GeV/c
0.5 lt s/sgeo lt 28 0.000.03 -0.010.03
28 lt s/sgeo lt 83 0.030.05 0.110.05

• In spite of the relatively low statistics, we
  see an anisotropy for peripheral events,
• concentrated at high pT
• Hardly a signal of elliptic flow (charm
  collective motion), since at SPS
• Ncc is low (no recombination)
• Difficult to have charm thermalisation
• Effect likely to be connected with
  anisotropic absorption in QGP/nuclear matter

20
Conclusions

• First results on the J/? transverse momentum
  distributions in pA at 158 GeV
• We observe a linear increase of ltpT2gt with L,
  consistent with gluon scattering
• in the initial state
• However
• The slope is smaller than in In-In and Pb-Pb at
  the same energy
• Peripheral In-In and p-A collisions with
  approximately the same L
• have ltpT2gt different by 200 MeV

• The J/? suppression scales with L in p-A and
  peripheral In-In and Pb-Pb
• The L scaling is broken when looking at the pT
  distributions

• First results on the v2 of the J/? at SPS energy
• v2 significantly larger than zero for non
  central events at pT gt1 GeV/c
• Effect likely to be connected with anisotropic
  absorption
• in QGP/nuclear matter

• One key ingredient in the overall J/?
  suppression picture still missing
• ? ?absJ/? (158 GeV)
• Results are coming. stay tuned!

21
The NA60 Collaboration
http//cern.ch/na60
60 people 13 institutes8 countries
R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J.
Buytaert, J. Castor, B. Chaurand, W. Chen,
B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S.
Damjanovic, A. David, A. de Falco, N. de Marco,
A. Devaux, A. Drees, L. Ducroux, H. Enyo, A.
Ferretti, M. Floris, P. Force, A.A. Grigoryan,
J.Y. Grossiord, N. Guettet, A. Guichard,
H. Gulkanyan, J. Heuser, M. Keil, L. Kluberg, Z.
Li, C. Lourenço, J. Lozano, F. Manso, P. Martins,
A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano,
P. Parracho, P. Pillot, T. Poghosyan, G. Puddu,
E. Radermacher, P. Ramalhete, P. Rosinsky,
E. Scomparin, J. Seixas, S. Serci,
R. Shahoyan,P. Sonderegger, H.J. Specht, R.
Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R.
Veenhof and H. Wöhri
22
Backup
23
The experimental apparatus
Muon trigger and tracking (NA10/NA38/NA50
spectrometer)
24
Influence of Drell-Yan contamination

• The Drell-Yan contribution under the J/? is 2
  
• and its pT distribution is not known

p-Pb

• Describe the Drell-Yan pT shape with the
• same function used to fit the J/? distributions
  

• Assume the usual dependence of ?pT2? on L,
• ltpT2gt ltpT2gtpp ?qN L, with ?qN 4/9 ?gN

• Use ?pT2?pp 1.25 (GeV/c)2, as given by PYTHIA
• 2 assumptions for ?qN
• a) ?qN 0 (no dependence on L)
• b) ?qN 4/9 ? 0.081 (GeV/c)2/fm (dependence
  on L as measured by NA50 in PbPb)

Teff changes by less than 0.4 MeV ?pT2? changes
by less than 0.01 (GeV/c)2
25
ltpT2gt vs NPart
26
Comparison p-A and In-In
Comparison peripheral In-In vs p-Pb
27
ltpT2gt vs L different y ranges
p-A
ltpT2gtpp1.13 0.04 (GeV/c)2 ?gN0.029 0.011
(GeV/c)2 / fm
p-A
ltpT2gtpp1.14 0.04 (GeV/c)2 ?gN0.015 0.012
(GeV/c)2 / fm
28
RAA(pT) for In-In at 158 AGeV

• In p-A we find a good correlation between ?pT2?
  (and therefore T) and L
• Effects connected with Cronin and nuclear
  absorption should scale with L
• Extrapolate T to the L values reached in In-In
  and build the corresponding pT
• distribution, which is then used to normalize
  the measured In-In distributions

But we still observe an enhancement at high pT
(is the L extrapolation from p-A to A-A a good
one?)
The suppression effect is mainly present at low
pT
29
RAA
30
Systematic errors
31
Comparison with theoretical predictions
A. Capella, E. Ferreiro EPJ C42(2005) 419
R.Rapp, EPJ C43(2005) 91
S. Digal, S. Fortunato, H. Satz, EPJ C32(2004)
547
centrality dependent t0
fixed termalization time t0
Suppression by hadronic comovers (?co 0.65
mb, tuned for Pb-Pb collisions)
Dissociation and regeneration in QGP and hadron
gas
Percolation, with onset of suppression at
Npart140

• Size of the anomalous suppression reasonably
  reproduced
• Quantitative description not satisfactory

32
Maximum hadronic absorption

• Compare J/? yield to
• calculations assuming
• Nuclear absorption
• Maximum possible
• absorption in a
• hadron gas
• (T 180 MeV)

L. Maiani et al., Nucl.Phys. A748(2005) 209 F.
Becattini et al.,Phys. Lett. B632(2006) 233

• Both Pb-Pb and (to a lesser extent) In-In show
  extra-suppression

33
J/? suppression in In-In collisions
34
Smooth effect or sharp drop?
Step position Npart 86 8 ( ?Bj 1.6
GeV/fm3 ) A1 0.98 0.02 A2 0.84 0.01 ?2/dof
0.7

• Taking into account the EZDC resolution
• data are compatible with a sharp drop
• An onset smoother than our resolution on Npart
  (20) is disfavored

35
? in In-In collisions

• Use matching of muon spectrometer tracks
• Study limited by statistics (N? 300)
• Normalized to Drell-Yan yields

450, 400 and 200 GeV points rescaled to 158 GeV

• Most peripheral point
• (?Npart? 60) does not show
• an anomalous suppression

• Good agreement with
• Pb-Pb results

Preliminary
36
? in p-A collisions
450, 400 and 200 GeV points rescaled to 158 GeV
??/DY? 0.51 ? 0.07 ?L? 3.4 fm
Also the ? value measured by NA60 at 158 GeV is
in good agreement with the normal absorption
pattern, calculated from 450 (400) GeV data