1. Thermal dimuons and f meson production in In-In collisions at the SPS
Outline
The NA60 Experiment at CERN
Thermal dimuon production
f meson production in the mm and KK channels
Michele Floris (CERN) WISH 2010, Sep. 2010, Catania
10/09/2010
Michele Floris - WISH 2010

2. or ? Measuring dimuons Energy loss Multiple scattering
Other
muon trigger and tracking
magnetic field
hadron absorber
target
beam
Energy loss
Multiple scattering or ?
Cannot distinguish prompt muons from decay muons
Degraded dimuon mass resolution
10/09/2010
Michele Floris - WISH 2010

3. Measuring dimuons in NA60
Other
muon trigger and tracking
magnetic field
2.5 T dipole magnet
beam tracker
vertex tracker
hadron absorber
targets
Matching in coordinate and momentum space or !
Origin of muons can be accurately determined
Improved dimuon mass resolution (80  20 MeV at the f)
Background from p/K reduced
Fake (wrong) matches: new source of BG
10/09/2010
Michele Floris - WISH 2010

4. Data Sample In-In collisions at 158 AGeV incident beam energy
5 weeks in 2003
~ 4 ∙ 1012 ions delivered
~ 230 million mm triggers
Data analysis for dimuons
Only one reconstructed vertex
(avoid re-interactions)
Subtract Background
Combinatorial Background
Fake Matches
5 Centrality bins
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Michele Floris - WISH 2010

5. Thermal dileptons
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Michele Floris - WISH 2010

6. Thermal dileptons in High Energy Nuclear Collisions
Dilepton emitted throughout the whole life-time of the fireball
Production rate convoluted with space time evolution
LMR: M<1 GeV
Thermal dileptons mainly via: pp → r* → ℓℓ in-medium r modifications: probe of chiral symmetry restoration? (R. Pisarski, PLB ‘82)
IMR: M>1 GeV
Thermal dimuons from hadronic or partonic phase? Controversial (hadron-parton duality)!
probe of deconfinement? (Kajantie, McLerran, al. ‘82 ff)
10/09/2010
Michele Floris - WISH 2010

7. Previous SPS results on excess dileptons
LMR: mostly NA45/CERES IMR: NA34, NA38, NA50
Previous SPS results on excess dileptons
Rapp-Wambach Brown/Rho
meson cocktail
2000 data
Statistical accuracy and resolution insufficient to unambiguously determine the in-medium spectral properties of the ρ
No experimental capability to distinguish a prompt thermal dileptons from decay dileptons due to open charm
10/09/2010
Michele Floris - WISH 2010

8. Excess dimuons Peripheral data (not shown):
Phys. Rev. Lett. 96 (2006)
Peripheral data (not shown):
well described by meson decay cocktail (η, η’, ρ, ω, ɸ) and DD
existence of excess dimuons
More central data (shown):
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 ɸ
10/09/2010
Michele Floris - WISH 2010

9. Comparison of data to RW, BR and vacuum r
Phys. Rev. Lett. 96 (2006)
Predictions for all scenarios (Rapp 2003)
Theoretical yields normalized to data for M<0.9 GeV
Only broadening of r (RW) observed, no mass shift (BR)
Data and predictions as shown, after acceptance filtering, roughly mirror the r spectral function, averaged over space-time and momenta. (Eur.Phys.J.C 49 (2007) 235)
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Michele Floris - WISH 2010 9

10. Extension to intermediate mass region
EPJC 59, (2009)
Measurement of muon offsets Dm: distance between interaction vertex and track impact point
Isolation of excess by subtraction of open charm and Drell-Yan
Charm not enhanced; excess: prompt, 2.4 × DY
Excess similar to open charm steeper than Drell-Yan
10/09/2010
Michele Floris - WISH 2010

11. Hadron-Parton Duality for M >1 GeV
Hees/Rapp Phys.Rev.Lett. (2006)
Renk/Ruppert, hep-ph/
Do we see
in-medium f modifications?
Mass region above 1 GeV described in terms of hadronic processes, 4p …
Mass region above 1 GeV described in terms of partonic processes, qq…
How to distinguish?
Transverse Momentum Spectra
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Michele Floris - WISH 2010 11

12. Centrality-integrated excess mT spectra
Phys. Rev. Lett. 100 (2008)
transverse mass: mT = (pT2 + M2)1/2
proper relative normalization
steepening at low mT; not observed for hadrons (like f)
signs for mass-dependent radial flow?
monotonic flattening of spectra with mass up to M=1 GeV, followed by a steepening above
fit mT spectra for pT>0.4 GeV with
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Michele Floris - WISH 2010 12

13. Dilepton transverse momentum spectra
Two main contributions to pT spectra
T - dependence of thermal distribution of “mother” hadrons/partons
M - dependent radial flow (bT) of “mother” hadrons/partons
Dilepton pT spectra superposition from all fireball stages
early emission: high T, low bT late emission: low T, high bT
Hadron pT spectra determined at freeze-out Tf (restricted information)
note: final-state lepton pairs themselves only weakly coupled
→ handle on emission region
Final spectra from space-time folding over T-bT history from Ti → Tf (including low-flow partonic phase)
10/09/2010
Michele Floris - WISH 2010

14. The rise and fall of radial flow of thermal dimuons
Phys. Rev. Lett. 100 (2008)
Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV
Combining M and pT of dileptons seems to overcome hadron-parton duality
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)
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Michele Floris - WISH 2010 14

15. f production
10/09/2010
Michele Floris - WISH 2010

16. The f puzzle Historic facts on NA49 fKK (low pT) NA50 fmm (high pT)vs
NA50 fmm (high pT)
Yields in hadronic channel lower than in leptonic channel by factor >2 in central collisions
Inverse slopes in central collisions Hadronic (low pT) ~ 300 MeV Leptonic (high pT) ~ 230 MeV
 f puzzle: in-medium effects on f and kaons + kaon absorption and rescattering leading to reduced yield and hardened pT spectrum in hadronic channel?
More recently:
CERES hadronic yield and inverse slope similar to NA49.
Within large errors leptonic yield also compatible with NA49  No f puzzle?
Recet NA50 analysis confirms previous results within 8%
10/09/2010
Michele Floris - WISH 2010

17. f  mm, Analysis Details f events: Acceptance Correction
Subtract continuum below the f by means of side windows
Effective statistics: f mesons
Statistics up to pT = 3 GeV
Acceptance Correction
Overlay Monte Carlo
f kinematics tuned with an iterative procedure in the 5 centrality bins
 1D Acceptance correction
Acc. x Rec. Eff.
Acceptance down to 0 pT
03/04/2009
10/09/2010
Michele Floris - WISH 2010
Michele Floris - WISH 2010 17

18. fKK, Analysis Details
No PID: all tracks assumed to be K Huge combinatorial background
All triggers used (dominated by dimuon trigger)
Event mixing technique
Bins: same target, multiplicity, reaction plane
Cuts on tracks: c2<3, pT > 0.2 GeV/c, < ylab < 3.7
Cut on pairs: < qKK < 0.15
Single tracks distributions are reproduced within ~ 0.1% PT y f
Real / Mixed
10/09/2010
Michele Floris - WISH 2010

19. fKK, Acceptance and resolution
Acceptance correction: overlay Monte Carlo where the f kinematic distributions tuned according to the results in the dimuon channel
The expected mass resolution is 7.8 MeV (20 MeV in muons)
MC f mass
spectrum
MKK (GeV/c2)
dN/dMKK
s=7.8 MeV
10/09/2010
Michele Floris - WISH 2010

20. fKK Invariant mass fits
Residual background present in spectra
fitted with MC f mass peak + BG
Several BG functions
Different selections are applied to
check consistency of the results
Peak position and width correctly reproduced when left as free parameters
Npart
NMB(*106)
NfRAW(*103)
S/B 39
1.47
14  5
1 / 167 75
4.40
120  15
1 / 212
132
9.59
616  40
1 / 271
183
7.11
631  50
1 / 412
0.8 < pT < 1.8 GeV/c
<Npart> = 75
<Npart> = 132
<Npart> = 183

21. f  mm, mT Distributions EPJ C 64, 1-18 (2009)
Spectra fitted with the function:
Depends on the fit range in presence of radial flow
Effective temperature (larger T at low pT)
Difference is ~15 MeV when fitting in the
NA49/NA50 range
10/09/2010
Michele Floris - WISH 2010

22. fKK: mT distributions
High background at low pT: fit limited to pT>0.8 GeV/c
pT distributions only in the two most central bins (Statistics)
Systematic error on T of ~ 5 MeV
Clear agreement between the mT distributions measured in In-In collisions in the hadronic and dileptonic channels
T=252  11 MeV (stat)
T=258  14 MeV (stat)
Spectra normalized
to multiplicity
Spectra normalized
to multiplicity
10/09/2010
Michele Floris - WISH 2010

23. Teff vs other measurements
In-In collisions the measurements in kaon and muon pairs are in agreement
Central collisions: NA49 and CERES (KK) in Pb-Pb above In-In by ~30 MeV
Difference NA50/NA49 ~ 70 MeV
Radial flow: differences up to 15 MeV (cannot describe the difference)
central collisions
10/09/2010
Michele Floris - WISH 2010

24. f  mm yield: 2 methods (independent syst)
J/y as a reference
f yield defined as:
J/y
Measured in same exp
Once corrected for nuclear/anomalous abs
scales with Ncoll
Effective number of interactions
estimated from observed counts
of reference process
Direct s measurement
Beam Counters
Effective Target Length
Beam Absorption
Transverse Size
Monte Carlo
Dedicated Runs
(DAQ-vetoed)

25. Yield vs Npart: KK and mm
Determine Nf (KK) integrated for pT>0.8 GeV/c for each centrality bin
Correct for the integrated acceptance in 4p and for the branching ratio
(Teff from KK in (semi)central collisions, from mm in (semi)peripheral collisions)
Nf is then divided by the number of events in that centrality bin
<f> vs Npart in the kaon and muon channels agree
Teff taken
from fmm
10/09/2010
Michele Floris - WISH 2010

26. <f>/Npart compared with other measurements
Comparison with NA50 in full phase space: ambiguous extrapolation (T slope)
Extreme hypotheses T=220 MeV (NA50) and T=300 MeV (NA49)
 values significantly higher than In-In
f/Npart in central In-In collisions exceeds the Pb-Pb values in KK and ee
Central
collisions
10/09/2010
Michele Floris - WISH 2010

27. f  mm, Mass and Width – MC tuning
Uncertainties in MC (material, alignment, B field maps, Eloss) lead to differences with data
Data driven approach  MC tuned to data in peripheral events, using full spectrum below 1 GeV
All Centralities
All Centralities
4% smearing
10/09/2010
Michele Floris - WISH 2010

28. f  mm, Low pT and central events
Excess more likely to appear in central events and at low pT
Fit shown using RH (RR yields identical conclusions)
Central + semicentral,
pT < 0.6
All centralities,
pT < 0.2 GeV
No evidence for the presence of an excess (local c2/NDF ~ 1)
10/09/2010
Michele Floris - WISH 2010

29. fmm, Mass and Width vs Centrality
f mass peak fitted with a
Relativistic Breit-Wigner × detector response
Mass and width compatible with PDG at any centrality
EPJ C 64, 1-18 (2009)
Semi Central
fKK: mass and width compatible with simulation
10/09/2010
Michele Floris - WISH 2010

30. Summary NA60: 3rd generation dilepton experiment at the SPS
Rad tolerant silicon pixels  unprecedented data quality
Potential for continuation at lower energy
Thermal dimuon production:
pp annihilations major contribution M < 1 GeV
Broadening (no mass shift) of the r: chiral simmetry?
mT spectra: low flow source at M > 1 GeV: Partonic?
f production and the f puzzle
Dependence of T slope on pT suggests NA49 vs NA50 T difference larger than expected from radial flow
Yield in leptonic channel in InIn larger than in hadronic channel in PbPb
No effect in InIn: mm and KK channels compatible
Hard to reconcile all observation in a consistent framework
No modifications of mass and width
PbPb measurements in a NA60-like experiment?
10/09/2010
Michele Floris - WISH 2010

31. The NA60 Experiment http://cern.ch/na60 Lisbon CERN Bern Torino
Yerevan
Cagliari
Lyon
Clermont
Riken
Stony Brook
Palaiseau
Heidelberg
BNL
~ 60 people
13 institutes 8 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. Damjanović, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord, N. Guettet, A. Guichard, H. Gulkanian, 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, 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
10/09/2010
Michele Floris - WISH 2010