1. Resonances production from the NA60 experiment
Alessandro De Falco (University and INFN Cagliari)
Resonance Workshop at UT Austin
March
Results for the fm+m- channel in In-In collisions
Results for the fK+K- channel in In-In collisions
Comparison between the hadronic and leptonic channels
Comparison between In-In and other systems
r production in In-In collisions
Eur.Phys.J.C64:1 (2009)
Phys.Lett.B699:325 (2011)
05-07/03/2012
Alessandro De Falco

2. The f puzzle Historic facts on NA49 fKK vs NA50 fmm
Yields in hadronic channel lower than in leptonic channel by factor ~4 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?
CERES hadronic yield and inverse slope similar to NA49.
Within large errors leptonic yield also compatible with NA49  No f puzzle?
05-07/03/2012
Alessandro De Falco

3. The NA60 detector layout
Origin of muons can be accurately determined
Improved dimuon mass resolution
(~20 MeV/c2 at  instead of 80 MeV/c2)
Additional bend by the dipole field extends the
dimuon coverage down to low pt
High luminosity mm experiment: possible with radiation tolerant detectors and high speed DAQ
2.5 T dipole magnet
hadron absorber
targets
beam tracker
vertex tracker
muon trigger and tracking (NA50)
magnetic field
>10m
<1m
Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them (in both angles and momentum) to the tracks measured in the spectrometer
05-07/03/2012
Alessandro De Falco

4. Data sample In-In collisions at 158 AGeV Incident beam energy
5 weeks in Oct.-Nov. 2003
~ 4 ∙ 1012 ions delivered
~ 230 million dimuon triggers
Data analysis for dimuons
Select events with
only one reconstructed vertex
in target region
(avoid re-interactions)
Match muon tracks from
Muon Spectrometer with
charged tracks from
Vertex Tracker (candidates
selected using weighted distance
squared  matching c2)
Subtract Background
05-07/03/2012
Alessandro De Falco

5. Extraction of differential spectra
Mmm (GeV/c2)
4000 A data set only
We select the events on the f peak and use two side mass windows to estimate the pT,y and decay angle distribution of the continuum under the peak
5 centrality bins
Systematic error: variation of analysis cuts and parameters
Acceptance: Overlay Monte Carlo tuned to
data with an iterative process
05-07/03/2012
Alessandro De Falco

6. fmm: Multiplicity Centrality Dependence (average of the 2 methods)
Multiplicity determined either directly with the cross section measurement or
extracted using the J/y (corrected for the nuclear and anomalous suppression)
Results in full phase space and corrected for BRmm = 2.86 · 10-4
Yield integrated in centrality:
Direct method:
J/y Calibration:
Centrality Dependence (average of the 2 methods)
f scales faster than Npart
Box: stat+syst. error
Box: stat+syst. error
05-07/03/2012
Alessandro De Falco

7. fmm: transverse mass distributions
Box: stat+syst. error
Spectra fitted with the function:
Depends on the fit range in presence of radial flow
Effective temperature (larger T at low pT)
05-07/03/2012
Alessandro De Falco

8. T slopes in In-In and Pb-Pb collisions
NA60 fits at low pT (NA49 range)
NA60 fits at high pT (NA50 range)
NA60 In-In (mm, pT < 1.6 GeV)‏
NA49 Pb-Pb (KK, pT < 1.6 GeV)‏
NA50 Pb-Pb (mm, pT > 1.1 GeV)‏
NA60 In-In (mm, pT > 1.1 GeV)‏
NA49 Pb-Pb (KK, pT < 1.6 GeV)‏
NA50 Pb-Pb (mm, pT > 1.1 GeV)‏
Ceres Pb-Pb (KK, pT > 0.75 GeV)
Ceres Pb-Pb (ee, pT < 1.5 GeV)
Box: stat+syst. error
Box: stat+syst. error
NA60 low vs high pT: maximal difference in T slopes only ~ 15 MeV presumably related to radial flow
well below difference between NA50 and NA49 (~ 70 MeV) in the most central bin  significant extra hardening of hadronic channel beyond radial flow?
05-07/03/2012
Alessandro De Falco

9. fKK in In-In collisions
No PID: all tracks assumed to be kaons Huge combinatorial background
Events are accepted if there is one and only one vertex
All triggers are taken (dominated by dimuon trigger)
Event mixing technique
Cuts on tracks: c2<3, pT > 0.2 GeV/c, < y < 3.7
Cut on pairs: < qKK < 0.15
Acceptance correction evaluated with an overlay Monte Carlo
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
05-07/03/2012
Alessandro De Falco

10. fKK in In-In collisions
Residual background present in spectra
Mass spectra fitted with a function that describes the MC f mass peak + the BKG Several functions used to describe the residual background. Differences taken into account in the systematic error
Different selections are applied to check the consistency of the results
Peak position and width correctly reproduced when left as free parameters, independent of centrality
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
05-07/03/2012
Alessandro De Falco

11. fKK: mT distributions
pT and mT distributions are obtained fitting the mass spectra in pT slices
High background at low pT: fit limited to pT>0.9 GeV/c
Spectra normalized to the multiplicity in 4p (discussed in next slides)
Due to limited statistics in the peripheral data, the pT distributions can be extracted only in the two most central bins
Systematic error on T of ~ 5 MeV
Clear agreement between the mT distributions measured in In-In collisions in the hadronic and dileptonic channels
Phys.Lett.B699:325 (2011)
Spectra normalized
to multiplicity
05-07/03/2012
Alessandro De Falco

12. Yield vs Npart in KK compared to mm
Multiplicity obtained for pT>0.9 GeV/c for each centrality bin
Data corrected for the integrated acceptance in 4p and branching ratio Teff from KK in (semi)central collisions, from mm in (semi)peripheral collisions
f multiplicity vs Npart in the kaon channel in agreement with the results in muons
05-07/03/2012
Alessandro De Falco

13. Sensitivity to rescattering-absorptionKK mm
Models like AMPT predict effects due to rescattering/absorption concentrated at low pT
Common coverage of the NA60 mm and KK chanels for mT – m0> 0.34 GeV
The local Teff value in this region would differ by MeV in the two channels
Fractional loss in kaons would be 35-50% Not seen in NA60 data
For lower mT: Extreme hypothesis: assume Teff value measured by NA49 in Pb-Pb (330 MeV) for for mT – m0< 0.34 GeV
This variation would lead to a reduction of the f yield in kaons by 12%
RESCATTERING-ABSORPTION MODEST OR ABSENT IN In-In
05-07/03/2012
Alessandro De Falco

14. T and <f>/Npart in central collisions
Inverse slope: increase fast at low Npart, less pronounced at high Npart lower value observed by NA50
Yield per participant: in In-In collisions the dilepton and hadron channels, differences within 22% in full pT <f>/Npart in In-In exceeds the one measured in Pb-Pb (KK) by about 30% NA50 point higher by a factor of 2
Suppression of the kaon channel in Pb-Pb below experimental sensitivity in In-In?
05-07/03/2012
Alessandro De Falco

15. Peripheral (Nch<30) In-In collisions
Well described by meson decay ‘cocktail’: η, η’, ρ, ω, f and contributions (Genesis generator developed within CERES and adapted for dimuons by NA60).
A. Uras, QM2011
Eur.Phys.J.C 49 (2007) 235
p-A
Similar cocktail describes NA60 p-A collisions, 400 GeV data
05-07/03/2012
Alessandro De Falco

16. More central In-In collisions
Clear excess of data above decay ‘cocktail’ describing peripheral events
Excess isolated subtracting the measured decay cocktail (without r) independently for each centrality bin, based on local criteria for h,w,f (2-3% accuracy)
Advantages:
no need for reference data (pA, peripheral collisions, models)
Less uncertainties (e.g. strangeness enhancement: h,f)
Phys. Rev. Lett. 96 (2006)
05-07/03/2012
Alessandro De Falco 16

17. NA60 LMR excess dimuons monotonic broadening with centrality
Evolution of the excess with centrality studied with precision with a rather fine binning in multiplicity
Eur.Phys.J.C 49 (2007) 235
data – cocktail (all pT)
No cocktail r and no DD subtracted
Clear excess above the cocktail , centered at the nominal r pole and rising with centrality
monotonic broadening with centrality
“melting” of the r

18. Centrality dependence of LMR excess
NA60, In-In 158A GeV
Eur.Phys.J.C 49 (2007) 235 r
peak: R=C-1/2(L+U) continuum: 3/2(L+U) cocktail ρ is fixed by ρ/ω=1.0
Excess rises faster than linear with multiplicity: compatible with emission from annihilation processes
rapid initial increase of total - already 3 at dNch/dh=Npart=50
strong increase of continuum (by a factor of >10)
Precision measurement of excess yield (r-clock):
-the most precise constraint in the fireball lifetime (6.5±0.5 fm/c) in heavy ion collisions to date!
05-07/03/2012
Alessandro De Falco

19. LMR excess: r dropping mass vs broadening
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 display spectral functions, averaged over space-time and momenta
Only broadening of  (RW) observed, no mass shift (BR)

20. Excess mass spectrum up to 2.5 GeV
All known sources (hadro-cocktail, open charm, DY) subtracted
Acceptance corrected spectrum (pT>0.2 GeV)
Absolute normalization → comparison to theory in absolute terms!
thermal pp g  mm (M<1 GeV)
&& thermal qq g  mm (M >1 GeV) suggested dominant by Teff vs M (supported by R/R, D/Z)
also multipion processes (H/R)
Eur. Phys. J. C 59 (2009) 607
Planck-like mass spectrum; falling exponentially
Agreement with theoretical models up to 2.5 GeV! 20
05-07/03/2012
Alessandro De Falco 20 20 20 20

21. Dimuon mT distributions
Phys. Rev. Lett. 100 (2008)
Eur. Phys. J. C 59 (2009) 607
all mT spectra exponential for mT-M > 0.1 GeV; <0.1 GeV ??
IMR
LMR
fit with 1/mT dN/mT ~ exp(-mT/Teff)

22. Evolution of inverse slope Teff with mass
Phys. Rev. Lett. 100 (2008)
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)
05-07/03/2012
Alessandro De Falco

23. Low mass dimuons in the LHC era: ALICE
arXiv:
Dimuon mass spectrum in p-p collisions at 7 TeV described by hadronic cocktail + open charm Mass resolution at the f peak of about 60 MeV
05-07/03/2012
Alessandro De Falco

24. w,f pT-differential cross sections
arXiv: f w
05-07/03/2012
Alessandro De Falco

25. Summary
f production in In-In (kaons vs muons): results in the KK channel agree with the ones in mm both for T and multiplicity
Compared to Pb-Pb central collisions: -TEFF values in In-In significantly lower than kaon points in Pb-Pb (NA49/ CERES) higher than NA50(mm) points in Pb-Pb - <f>/NPART in In-In much lower than the NA50 result slightly higher than results in kaon pairs and dielectrons - mm vs KK in Pb-Pb still leave room for a physical effect However, CERES measurements in kaons and dielectrons agree
Excess in dilepton emission: Good agreement with models of thermal emission Faster than linear scaling with Ncharged Planck-like mass spectra Teff rising with mass in LMR and, after sharp drop, flat in IMR
No f puzzle in In-In collisions
05-07/03/2012
Alessandro De Falco