# Presentation: 661887

## Study of the mT spectra of strange particles in Pb-Pb collisions at the SPS 

- Giuseppe E. Bruno
- Università degli Studi di Bari and INFN, Bari, Italy
*for the NA57 Collaboration*

## Contents

- The NA57 experiment
- Inverse slopes in Pb-Pb, p-Be, p-Pb
- Blast wave analysis of the spectra in Pb-Pb collisions
- 160 A GeV/c (_[J. ](http://stacks.iop.org/0954-3899/30/823)__[Phys](http://stacks.iop.org/0954-3899/30/823)__[. ](http://stacks.iop.org/0954-3899/30/823)__[G 30](http://stacks.iop.org/0954-3899/30/823)__[ (2004) 823-840](http://stacks.iop.org/0954-3899/30/823)_ )
- 40 A GeV/c    (new at *Hot Quarks*)

## Aim of the experiment

- and multi-strange baryons in heavy ion
- interactions at mid-rapidity as a function of
-  collision centrality
-  collision energy

**Silicon telescope**

- target

-     + + -  ( BR = 68.6 % )
-     - + p       ( BR = 63.9 % )
- -    + -      ( BR = 99.9 % )
- -    + K-     ( BR = 67.8 % )

- s

- 1M channels

- beam

## Transverse mass spectra in Pb-Pb at 160 A GeV

- events

|  |  |
| --- | --- |
| K0 | 237±4±24 |
|  | 289±7±29 |
|  | 287±6±29 |
|  | 297±5±30 |
|  | 316±11±30 |
| +  | 271±16±27 |
|  | 264±19±27 |
|  | 284±28±27 |

- Inverse slopes (MeV)

- s

## Centrality of  the collision

- Centrality determination from charged particle multiplicity measurement
- Nwound from trigger cross section (Glauber+WNM calculations)

|  |  | <Nwound> |  |
| --- | --- | --- | --- |
| bin | % of tot | 160 | 40 |
| 0 | 40-53 % | 62 | 57 |
| 1 | 23-40 % | 121 | 119 |
| 2 | 11-23 % | 209 | 208 |
| 3 | 5-11 % | 290 | 292 |
| 4 | 0-5 % | 349 | 346 |

## Transverse mass spectra in Pb-Pb at 160 A GeV 

## Inverse slopes at 160 A GeV/c

|  | p-Be | p-Pb | 0 | 1 | 2 | 3 | 4 |
| --- | --- | --- | --- | --- | --- | --- | --- |
| K0 | 197±4 | 217±6 | 239±15 | 239±8 | 233±7 | 244±8 | 234±9 |
|  | 180±2 | 196±6 | 237±19 | 274±13 | 282±12 | 315±14 | 305±15 |
|  | 157±2 | 183±11 | 277±19 | 264±11 | 283±10 | 313±14 | 295±14 |
|  | 202±13 | 235±14 | 290±20 | 290±11 | 295±9 | 304±11 | 299±12 |
|  | 182±17 | 224±21 | 232±29 | 311±23 | 294±18 | 346±28 | 356±31 |
|  | 169 ±40 | 334±99 | 274±34 |  | 274±28 | 268±23 |  |
|  |  |  | 264±19 |  |  |  |  |
|  |  |  | 284±28 |  |  |  |  |

- In central and semi-central Pb-Pb collisions (bin 1,2,3,4)
-    slopes are compatible for  particle and its anti-particle
- No such symmetry in p-Be

- s

##  Hydro picture:

-  the mT spectra are sensitive to the transverse flow

- Blast-wave description of the spectra

*Ref: *E Schnedermann, J Sollfrank and U Heinz, **Phys. Rev. C48 (**1993) 2462** **

## Blast wave fit 

- Blast wave fit

- 53%  most
- central events

-  T and >  depend weakly on  **n**
-  **n**=2 case disfavoured by data (bad )

-  statistical error are highly anti-correlated
-  systematic errors:  T   10% ,  (correlated)

- n=1

- 		     T  = 144 ± 7(stat) ± 14(syst)  MeV
-  = 0.381 ±0.013(stat) ± 0.012(syst)

## Freeze-out parameters:  multi- vs. singly- strange particles

- Fit to singly strange particles

-  Fit driven by singly strange particles
-   and can be described by same parameters

- Freeze-out parameters:  multi- vs. singly- strange particles

- n=1

## Blast wave description of the inverse slope values

-  Inverse slope depends on the
-   mT range used to fit the spectrum

## Centrality dependence of the thermal freeze-out in Pb-Pb at 160 A GeV

- 	With increasing centrality:
- Transverse flow velocity increases
- Freeze-out temperature decreases
-  Earlier decoupling for
-   peripheral collisions ?

- n=1

- 1contours

## v2 at SPS: far from hydro limit ?

- some discrepancy between NA45 and NA49
- v2 not at hydro limit?
- v2 well below hydro limit for     Tf = 120 MeV
- but NA57: Tf increases for peripheral events

**[Snellings et al.: nucl-ex/0305001]**

**[NA45: Phys. Rev. Lett. 92 (2004) 032301]**

- Tf = 146 ± 17 MeV for (11-23)% centrality; NA45 data (13-26)%
- so hydro limit perhaps not too far...

## Blast fit for most central collisions

|  | n | T (MeV) |  | /ndf |
| --- | --- | --- | --- | --- |
| NA57 | 1 | 11813 | 0.450.02 | 53/43  |
| NA49 (a) | 0 | 1271 | 0.480.01 | 120/43 |
| NA49 (b) | 0 | 1142 | 0.500.01 | 91/41 |

- (a)  K+, p, , , 

- (b)  K-, p, , , , 

- NA49 centrality:  5% for K± , 
- 10% for p, , ; 20% for  

*Ref: *M van Leeuwen, **Nucl. Phys. A715 (**2003) 161c** **

## Blast-wave analysis in Pb-Pb at 40 A GeV/c

## Global fit with different profiles

|  | n=0 | n=1/2 | n=1 | n=2 |
| --- | --- | --- | --- | --- |
| T | 128±4 | 123±3 | 118±5 | 123±7 |
| <  > | 0.432±0.012 | 0.423±0.008 | 0.398±0.010 | 0.325±0.011 |
|  /ndf | 62/34 | 65/34 | 79/34 | 150/34 |

- 40 GeV

## Global fit to all strange particle spectralinear profile

- Full centrality range: 53%

## Multi- vs. singly- strange particles  (i)

|  | T | <  > |  /ndf |
| --- | --- | --- | --- |
| global | 118±5 | 0.398±0.010 | 79/34 |
| K0,  | 116±5 | 0.404±0.009 | 59/19 |
| -+ | 170 |   0.24 | 7.9/11 |

- + 60

- - 70

- - 0.24

- + 0.13

- Early freeze-out at 40 GeV ?
- Let’s look at the inverse slopes !

- Multi- vs. singly- strange particles  (i)

## Blast wave description of the inverse slope values

- Indication of an  early decoupling of the 

## Centrality dependence

-   increases with
-    increasing Npart
-  Larger T for bin 1

## Conclusions  (i)

- Transverse mass spectra at 160 A GeV/c:
- Symmetry between hyperon and anti-hyperon in central and semi-central Pb-Pb collisions (bins 1,2,3,4), not in p-Be
- Common blast-wave fit describes the data but, from inverse slopes, there is indication of an early freeze-out of the 
- _Evidence for a centrality dependence of the thermal freeze-out parameters_

## Conclusions  (ii)

- Blast wave analysis in Pb-Pb at 40 A GeV/c:
- Thermal freeze-out temperature is smaller  than at 160 GeV, similar transverse flow velocity
- From inverse slopes: early freeze-out of 
-  Global behaviour similar as at 160 GeV/c

## The NA57 Collaboration

## The NA57 Collaboration: age < 45 !

## Layout of the NA57 experiment at CERN SPS

**Target:**** 1% Pb , 8% Be**
**Scintillator Petals:**** **
**           ****centrality trigger**
**MSD:**** Multiplicity Silicon Detector**
**Tracking device:**
**           ****silicon pixel planes (5 x 5 cm****2 ****)**
**           ****pixel size: 50********500 ********m****2**
**Lever arm:**
**           ****pixel planes & double side ********strips**

- 1M channels

- Pb-Pb only

## Selection of hyperons and KS at 40 A GeV

- 0

-  Strange  particle signals extracted
-   by geometrical and  kinematical cuts

## Selected signals

**160 A GeV**

**40 A GeV**

## Hyperon d2N/dmTdy measurement

- Data corrected for acceptance and also for detector and reconstruction efficiency by Monte Carlo simulation
- In the acceptance window:
- Yield
- (i.e. particle per event)
- Transverse mass spectra (Tapp)
- Extrapolation to a common window:
- one unit of rapidity about ycm
- full range of pT