# Presentation: 268666

##  Meson Production in In-In Collisions and the  Puzzle

*Michele Floris**1** *
*for the NA60 Collaboration*
- Quark Matter 2008, Jaipur, India
- February 5th, 2008
*1**University and INFN, Cagliari, Italy*

## Outline

- Quark Matter 2008 - Jaipur

- The  puzzle in heavy ion collisions
- The NA60 experiment
-  yield: InIn vs PbPb
- yield in InIn
-  mT spectra: InIn vs PbPb
-  KK: a first look
- Summary

## The  puzzle

- Quark Matter 2008 - Jaipur

- The  puzzle

- Historic facts on
- NA49 KK vs NA50 
- 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

-   *puzzle:* in-medium effects on  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  puzzle?
- New NA50 analysis confirms previous results within 8% *(See D. Jouan, SessionXXIII)*

## The NA60 Experiment

- Quark Matter 2008 - Jaipur

- Fixed target dimuon experiment at the CERN SPS
- Apparatus composed of 4 main detectors

- Origin of muons can be accurately determined
- Improved dimuon mass resolution
- (~20 MeV/c2 at  instead of 80 MeV/c2)

- 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

- High luminosity  experiment: possible with radiation tolerant detectors and high speed DAQ

## Data Sample

- Quark Matter 2008 - Jaipur

- InIn collisions at 158 AGeV
- 5 weeks in Oct.-Nov. 2003
- ~ 4 ∙ 1012 ions delivered
- ~ 230 million dimuon triggers
- Data analysis
- Events with only one reconstructed vertex in target region (avoid re-interactions)
- Match muon tracks from Muon Spectrometer with charged tracks from Vertex
- Subtract combinatorial Background and Fake Matches

-  events:
- Subtract continuum below the  by means of side windows
- Effective statistics: 12000  mesons
- Statistics up to pT = 3 GeV
- Present results:
- 	up to pT = 2.6 GeV, 4000 A setup only

## Centrality Selection and Acceptance Correction

- Quark Matter 2008 - Jaipur

- Acceptance Correction
- Overlay Monte Carlo
-  kinematics tuned with an iterative procedure in the 5 centrality bins
- 	 1D Acceptance correction for
- 		differential spectra

- 5 Centrality Bins
- Selected using charged tracks density

- Acc. x Rec. Eff.

**Acceptance down to 0 p****T**

-   Acc x Rec. Eff.

*(within ~5%)*

## Rapidity and cos CS distributions

- Quark Matter 2008 - Jaipur

- Rapidity and cos CS distributions

- pT, rapidity and decay angular distributions (in the Collins-Soper, Gottfried-Jackson and Helicity frames) measured in the 5 centrality bins

-  full characterization of the  kinematics

*See also NA60@SQM07*

##  Cross Section and Yield

- Quark Matter 2008 - Jaipur

-  Cross Section and Yield

**Total Systematic Uncertainties ~ 10%**

-    multiplicity

*Eur. Phys. J. C13 69 (2000)*

##   Yield: Alternative Method

- Quark Matter 2008 - Jaipur

-   Yield: Alternative Method

-  yield defined as:

- Effective number of interactions
- estimated from observed counts
- of reference  process

- Alternative method with independent systematics!
- Total systematic error ~ 13%

##   Yield:  Results 

- Quark Matter 2008 - Jaipur

-   Yield:  Results

- Results in full phase space and corrected for BR = 2.86 · 10-4

**Centrality Dependence **(average of the 2 methods)

- Box: stat+syst. error

- Box: stat+syst. error

-  scales faster than Npart

##  yields In-In - comparison to  Pb-Pb

- Quark Matter 2008 - Jaipur

- NA60 In-In

- NA50 Pb-Pb

- Box: stat+syst. error

- NA49 Pb-Pb

- NA49 p-p

- NA60 In-In

- Box: stat+syst. error

- NA60 vs NA49 (full phase space):
- Also  in In-In higher than NA49 yield (like NA50)

- NA60 vs NA50 (NA50 kinematic range):
-  Npart scaling almost within errors
- 	- system size dependence
- 	- saturation in central collisions

- Difference in  and KK yields seems confirmed - related to a real physics mechanism (kaon absorption)?

-  yields In-In - comparison to  Pb-Pb

- Ceres PbPb
-   KK

**3 < y****lab**** < 4**
**|cos********CS****| < 0.5**
**m****T **** > 1.5 GeV**

## Further crosscheck: Yield

- Quark Matter 2008 - Jaipur

- Further crosscheck: Yield

-  yield: determined as the  yield

- Box: stat+syst. error

- The  yield scales with Npart

- Box: stat+syst. error

- Previously all NA60 results M < 1 GeV normalized to 
-  Now absolute calibration

##  yield: comparison to existing measurements 

- Quark Matter 2008 - Jaipur

-  yield: comparison to existing measurements

- Convert /Npart  ±± 0.007

- NA60 : follows the general trend

-  ratio insensitive to systematic uncertainties

- Factor 2 reduction in  
- Same change in 

- Completely out of systematics

- Absolute normalization robust

##  mT Distributions

- Quark Matter 2008 - Jaipur

-  mT Distributions

## T slopes: InIn vs PbPb

- Quark Matter 2008 - Jaipur

- NA60 In-In	(pT < 1.6 GeV)‏
- NA49 Pb-Pb	(pT < 1.6 GeV)‏
- NA50 Pb-Pb	(pT > 1.1 GeV)‏

- NA60 fits at low pT (NA49 range)

- Box: stat+syst. error

- NA60 In-In 	(pT > 1.1 GeV)‏
- NA49 Pb-Pb 	(pT < 1.6 GeV)‏
- NA50 Pb-Pb 	(pT > 1.1 GeV)‏

- 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?

- NA60 fits at high pT (NA50 range)

- Ceres Pb-Pb	(KK, pT > 0.75 GeV)

- Ceres Pb-Pb	(ee, pT < 1.5 GeV)

##  KK: Promising Signal

- Quark Matter 2008 - Jaipur

-  KK: Promising Signal

- No PID  Huge CB (S/B ~ 1/400 at the  peak)‏
- Event Mixing technique
- Effective statistics: ~500 events
- Analysis on-going
-       First results to come soon!

**Real**
**Mixed**

- Very Promising!

## Summary

- Quark Matter 2008 - Jaipur

- Measurement of  μμ in InIn confirms  puzzle:
- Yield in leptonic channel larger than in hadronic channel
- Dependence of T slope on pT suggests NA49 vs NA50 T difference larger than expected from radial flow
- Qualitatively consistent with expectations from kaon rescattering/absorption and/or in-medium effects on  and kaons
- Quantitative size of effect?
- New NA60 data on KK to come soon!
- PbPb measurements in a NA60-like experiment?