1. Recent results from NA60 and future plans
Gianluca Usai –University of Cagliari and INFN (Italy)
Hard Probes - Eilat 15/010/2010

2. Outline Thermal dimuons and hadrons:
r spectral function and thermal radiation in the IMR region
Transition form factors of h and w Dalitz decays in In-In and pA …collisions
Nuclear dependence of r, w, f cross sections vs A
Charmonia and open charm:
absorption cross section in pA collisions and anomalous suppression in AA collisions
J/y absolute cross sections in pA collisions
open charm production in pA collisions
Present ideas about a NA60-like future experiment

3. NA60 setup and data samples
2.5 T dipole magnet
hadron absorber
targets
beam tracker
Si-pixel tracker
muon trigger and tracking
magnetic field
Track matching in coordinate and momentum space  Improved dimuon ...mass resolution - Distinguish prompt from decay dimuons
Additional bend by the dipole field  acceptance coverage down to low pT
Radiation-hard silicon pixel detectors  high luminosity
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) 3

4. Excess dimuons in In-In collisions at the SPS
Phys. Rev. Lett. 96 (2006)
Peripheral data: well described by meson decay cocktail (η, η’, ρ, ω, f) and DD
More central data (shown): existence of excess dimuons
isolation of excess by subtraction of the measured decay cocktail (without r), based solely on local criteria for the major sources η Dalitz, ω and f

5. The r spectral function
Rapp-Wambach: hadronic model predicting strong broadening/no mass shift
Brown/Rho scaling: dropping mass due to dropping of chiral condensate
Predictions for In-In by Rapp et al (2003) for dNch/d = 140, covering all scenarios
Theoretical yields normalized to data in mass interval < 0.9 GeV
After acceptance filtering, data and predictions mirror the r spectral function, averaged over space-time and momenta
Only broadening of  (RW) observed, no mass shift (BR)

6. Excess mass spectrum up to 2.5 GeV
All known sources (hadro-cocktail, open charm, Drell-Yan) subtracted
Acceptance corrected spectrum (pT>0.2 GeV)
Absolute normalization → comparison to theory in absolute terms!
M<1 GeV: thermal pp r  mm
Strong r broadening – no mass shift
In-medium effects dominantly driven by baryons
M>1 GeV: thermal qq g  mm suggested to be dominant by Teff vs M (supported by R/R, D/Z)
and/or multipion processes (H/R)
Eur. Phys. J. C 59 (2009) 607
How to distinguish? 6 6 6

7. The rise and fall of radial flow of thermal dimuons
Phys. Rev. Lett. 100 (2008)
Teff: extracted from fits to mT=(Mmm2+pT2)1/2 with thermal function exp(-mT/Teff)
Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV
Rise consistent with radial flow of a hadronic source (here pp→r→mm) , taking the freeze-out ρ as the reference
Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→mm) 7

8. EM transition form factors for w and h Dalitz decays
EM decay of light unflavored mesons ABl+l-
Described by QED for point-like particles
A form factor
accounts for the more complex em structure
arising at the vertex of the transition
Believed to be dominated by photon-hadron
resonance interaction (vector meson dominance)
Old experimental results (Lepton-G experiment)
showed, for +-0, a deviation from VMD
at ~ 3 level
Use the high statistics In-In and pA data to
investigate this anomaly
VMD

9. Isolating the Dalitz region in the peripheral data
Phys. Lett. B 677 (2009) 260
subtraction of
- η, ω, ɸ resonance decays
also - η’ Dalitz decay (η’/η=0.12) - uncorr. μ+μ- from DDbar
(both nearly negligible ;  systematic errors)
correct for acceptance
fit remaining sources η, ω and ρ; χ2/ndf~1, globally and locally
anomaly of ω form factor directly visible in the spectrum 9

10. Results on form factors
Phys. Lett. B 677 (2009) 260
Perfect agreement of NA60 and Lepton G, confirming ω anomaly
Large improvement in accuracy; for ω, deviation from VMD 3  10 σ 10

11. NA60 results in the new edition of the PDG
First result from a heavy-ion experiment in the PDG ever 11 11

12. New results on form factors from the NA60 p-A data
400 GeVpA data: muon pairs without target selection
5.5104 -Dalitz, 1.2104 -Dalitz (600 and 60 for Lepton-G)
Further improvement of a factor 10 with respect to In-In peripheral analysis
preliminary
preliminary

13. r, w, f cross section and particle ratios in p-A at 400 GeV
Fit of the raw mass spectrum for each nuclear target
Cross section as a function of the mass number A, normalizing the cross sections to Be targets
Targets simultaneously exposed to the beam: uncertainties on the luminosity cancel out
 acceptance and recostruction efficiencies do not completely cancel out
(these quantities and their time evolution are computed for each target)

14. Fits of single targets Events per 20 MeV/c2 Events per 20 MeV/c2 p-Be
χ2/ndf = 111/55
p-Be
χ2/ndf = 87/55
p-Cu
χ2/ndf = 171/55
p-In
Events per 20 MeV/c2
χ2/ndf = 152/55
p-W
χ2/ndf = 116/55
p-Pb
χ2/ndf = 123/55
p-U

15. Cross section ratios Fit excluding Be target
αw (no Be) = 0.72 ± ± 0.017
αf (no Be) = 0.92 ±0.030 ±0.015
preliminary
preliminary
αr = ± 0.041± 0.020
αw = ± 0.013± 0.020
αf = ± 0.011± 0.013
Clear effect present when excluding Be: aw decreases towards 2/3 (full absorption) limit while af increases closer to 1

16. Particle ratios f meson has significantly higher alpha w.r.t the ω
preliminary
preliminary
NA27
Consistent with ρ/ω = 1
Clear evidence of a rising trend as a function of A
f meson has significantly higher alpha w.r.t the ω

17. r/w interference? Sides of the r/w peak not properly described
Suggestion of r/w coherent production? Statistics seems enough to be sensitive to a quantitative study

18. Charmonia suppression: pA, AA
Study of charmonium production/suppression in pA/AA collisions
THE hard probe at SPS energy
AA collisions
Color screening and charmonium suppression
> 20 year long history
pA collisions
Production models (CSM, NRQCD, CEM, ....)
Initial/final state nuclear effects (shadowing, dissociation,...)
Reference for understanding dissociation in a hot medium

19. A-dependence of relative cross sections
400 GeV
158 GeV
 increasing suppression of the J/ yield as a function of A
larger suppression at 158GeV
using a Glauber fit to extract abs
158 GeV:
abs J/ = 7.6 ± 0.7 ± 0.6 mb
400 GeV:
abs J/ = 4.3 ± 0.8 ± 0.6 mb
NA60 Coll., arXiv:
very good agreement with NA50 (4.6 ± 0.6mb)
shadowing neglected, as usually done at fixed target (but not correct!)
using
158 GeV:
= ± ± 0.008
400 GeV:
 = ± ± 0.009
systematic errors due to :
target thicknesses (<1.5%)‏
J/ y distribution (<1.5%)‏
reconstruction eff. calculation (<3%)‏

20. Anomalous suppression
In-In 158 GeV (NA60)
Pb-Pb 158 GeV (NA50)
Using the previously defined reference:
Central Pb-Pb:
 still anomalously suppressed
In-In:
almost no anomalous
suppression
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),

21. New result: J/y cross section in pA
J/ production cross sections for pA data
Preliminary
Systematic error on (absolute) luminosity estimation quite high
Relative luminosity estimate between 158 and 400 GeV
much better known (~2-3% systematic error)
Normalize NA GeV cross section ratios to NA50 results
158 GeV cross sections constrained by the relative normalization

22. New reference using J/y cross sections
Alternative approach for the normalization of the pA reference
curve based on the pA J/ absolute cross section
To fully profit from this approach, a measurement of the absolute J/
cross section in In-In would be needed. For the moment…
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
Preliminary
No practical consequence on anomalous J/Ψ suppression
difference with previous CNM reference ~1% well within errors

23. Anomalous suppression: SPS vs RHIC
SPS results compared with RHIC RAA results normalized to RAA(CNM)
Both Pb-Pb and Au-Au seem to
depart from the reference curve
at Npart~200
For central collisions more important suppression in Au-Au with respect to Pb-Pb
M. Leitch (E866), workshop on “Quarkonium in
Hot Media:From QCD to Experiment”, Seattle 2009
Agreement between SPS and RHIC
results as a function of the charged
particle multiplicity

24. Open charm production in pA collisions
Open charm share initial state effects with charmonium
 a measurement of open charm in p-A collisions may help
in understanding J/ suppression
Recent results from SELEX and E866 suggest rather strong
nuclear effects on open charm
E866/NuSea Preliminary
A. Blanco et al. (SELEX), EPJC64(2009) 637
M. Leitch (E866), workshop on “Heavy Quarkonia
Production in Heavy-Ion Collisions”, ECT* 2009

25. Open charm dimuons in pA: 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)
NA50 had to impose
a constant DD/DY vs A
(i.e. DD= DY ~1 )
M.C. Abreu et al., EPJC14(2000) 443
NA60 is much better placed, thanks to the muon matching
 S/B is ~60 times more favourable

26. Open charm dimuons in pA: results
Kinematic range: 3.2<ylab<3.7 (-0.17<ycm<0.33)
Study the ratio /DD (reduce systematic errors)
Preliminary
Systematic errors
Fit starting point
Track 2
/DY (norm. and J/)
Background normalization
Using the measured J/ value one gets DD = 0.960.03
Anti-shadowing would suggest DD >1

27. New Pb-Pb and/or other systems at lower energy
Future high precision dilepton measurements with an NA60-like setup
QCD phase diagram poorly known in the region of highest baryon densities and moderate temperatures – is there a critical point?
Marek’s “horn” in <K+>/<p+>: the critical point might be located within
√sNN~5-10 GeV
Heavy ion beams should be available at the SPS again in next years
New Pb-Pb and/or other systems at lower energy

28. Low mass dileptons: top to low SPS energies
Decrease of energy 160 to 40 AGeV:
predicted net r in-medium effects, in particular for M<0.4 GeV, increase by a factor 2 because of baryons!
Pioneering measurement by CERES at 40 AGeV: enhancement increases! Seems to confirm importance of baryonic effects
Might not be just coincidental with expectation of emergence of the critical point
Compelling to continue research into the regime of maximal baryon density experimentally accessible

29. QCD critical point search – experimental landscape
partly complementary programs
planned at CERN SPS 2011
BNL RHIC 2010
DUBNA NICA 2013
GSI SIS-CBM 2016
NA60-like experiment:
Dilepton measurements in region not covered by other experiments
High precision muon pair measurements:
- high luminosity → statistics
- very good mass resolution …- acceptance down to low pT
- background subtraction …. ….easier than in e+e-
Flexibility to change energy (and A)
Complementary to NA61

30. NA60-like apparatus layout for lower energies
Thoroidal magnet R=150 cm
Compress the spectrometer reducing the absorber
Enlarge transverse dimensions
Angular coverage: 240 mrad
Thoroidal magnet R=240 cm
Original NA60:Angular coverage: 160 mrad

31. Acceptance coverage of low energy setup
<y> AGeV
pT (GeV)
<y> AGeV
<y> AGeV y y
  +- 158 AGeV
  +- 30 AGeV
Standard NA60 setup y range 0-1 in the cms 158 GeV
Low energy setup

32. Combinatorial background: key point
Pions entering the absorber all interact within the first 1-2 interaction lengths
Additional muons produced in the absorber from secondary pions are all soft and with tracks not pointing to the interaction vertex
CB due to pions and kaons decaying into muons before the hadron
2.5 T dipole field
Muon
filter
~10 cm
vertex
, K  µ
offset
Beam Tracker
Vertex Detector
Detector concept: essential to keep distance from interaction point to absorber as small as possible

33. Summary
NA60 physics analysis still ongoing
pA low mass dimuons:
Transition form factors of h and w Dalitz decays: confirmation of Lepton G with significant improvement of errors
Light resonance cross sections vs A: ar,w close to 2/3, af closer to 1
pA high mass dimuons:
J/psi cross sections: new CNM reference with pA data taken in the same energy/kinematic domain of AA
Open charm in pA: DD close to 1
Ion beams will be circulating and might be available for experiments at the SPS in the incoming years
Feasibility of new dimuon experiment and setup of a future collaboration being pursued