# Presentation: 313057

## Anti-particle to particle ratios in p+p, Cu+Cu and Au+Au collisions at RHIC

**Anti-particle to particle ratios in p+p, Cu+Cu and Au+Au collisions at RHIC**

- Gábor I. Veres
*E**ötvös Loránd** University, Budapest*
*Massachusetts Institute of Technology, Cambridge*

**Quark Matter 2006 Conference, Shanghai, November 19, 2006**

##                     Collaboration

- Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel,
-  Wit Busza (Spokesperson), Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova,  Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey,
- Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov,
- Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs,  Barbara Wosiek, Krzysztof Woźniak, Bolek Wysłouch
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

- 9 ongoing PhD thesis studies

## Outline

-  	**Particle ratios:**

- 	PHOBOS detector and PID methods

**     ****Particle spectra: **

- 	• Au+Au PID spectra at 62.4 GeV
- 	• low-pT particle yields
-  	• baryon/hadron ratios
- 	• mT scaling
- 	• centrality dependence of the net proton yield

-      Summary

- 		• ratios in central Au+Au vs. s
- 		• Cu+Cu ratios vs. centrality at 200 and 62 GeV
- 		• comparison of p+p, Cu+Cu and Au+Au ratios vs. Npart

## Motivation

- _Particle ratios:_
-  • are influenced by baryon pair production,
- 			baryon number transport, and
- 			final state interactions
-  • show how close the system is
-     to ideal baryon-free plasma
-  • provide input to quantitative
-     analysis of chemical equilibrium
-     in thermal models

- z

**-x**

**y**

## The                 detector

**1m**

**Spectrometer**

**Time of Flight**

## PHOBOS PID Capabilities

**p****T ****(GeV/c)**

**0.03**

**0.3**

**3.0**

**Stopping**
**particles**

**dE/dx**

**TOF**

**E****loss**** ****(MeV)**

**1/v (ps/cm)**

**p+p**

******+****+********-**

**p (GeV/c)**

## Analysis techniques

-  Particles identified via dE/dx
- 	in the spectrometer
-  Monte Carlo studies
- 	to estimate corrections

-  *a*cceptance in y and pT :

*a*cceptance

- fficiency,
- corrections

-    0 < y < 1.5
- 0.2 < pT< 0.8 GeV/c

**p [GeV/c]**

## Controlling systematic uncertainties

- Geometrical *a*cceptance drops out of the ratios:

- Ratios measured independently for different:				 • bending directions				 • spectrometer arms

- by changing the polarity of the magnetic field

## Corrections

- GEANT Monte Carlo
- (FLUKA+Geisha)/2

- _Absorption_	• GEANT used
- 	• 4% for p/p	• 1.5% for K–/K+

- _

- _Secondary particles_
- 	• GEANT used	• 1% for p/p, 2% for –/+

- _Feed-down_
- 	• daughters of weakly decaying particles (,, –)	• HIJING particle yields are used	• track DCA cut (3.5 mm!) keeps correction small	• 2% for p/p, <1% for K–/K+ and –/+

- _

- _

## Errors on the ratios

-  Statistical errors: 1% (tens of millions of events analyzed)

-  Systematic errors (90% C.L.):

| beam orbit variations | 2% | 2% | 2% |
| --- | --- | --- | --- |
| PID selection cuts | 4% | 4% | 1% |
| track quality cuts | 3% | 3% | 3% |
| vertex cuts | 1% | 1% | 1% |
| DCA cuts | 2% | 2% | 1% |
| trigger selection cuts | 1% | 1% | 1% |
| absorption correction | 3% | 1%  | 1%  |
| feed-down correction | 3% | 0% | 0% |
| correction for secondaries | 1.5% | 0% | 2% |
| Total: | 7.5% | 6% | 4.5% |

- p         K        

## Results: energy dependence

- New results at 62.4 GeV fit smoothly into the energy evolution of antiparticle/particle ratios

- arXiv:nucl-ex/0610001submitted to PRC

- stat.errors only

## Cu+Cu ratios, all 3 species vs. Npart

- Cu+Cu ratios, all 3 species vs. Npart

-  Integrated over pT and y

-  Similarly to Au+Au, strong
-    collision energy    dependence

**200 GeV**

**62.4 GeV**

- y0

## Comparison of systems at 200 GeV

-  p/p ratios do not
-   depend strongly on
- 	- system size
- 	- centrality

- _

-  The study is being   currently extended   to 62.4 GeV

- y0

- p+p: Phys.Rev. C71, 021901 (2005)

## Ratios of different species

- Ratios of different species (like p/) are of significant interest:
- they are used to study the *baryon excess* observed
- at higher RHIC energies in the 2 – 4 GeV/c pT range.
-  For that, *identified spectra* of all species were analyzed.

## Identified particle pT -spectra, Au+Au at 62.4 GeV

- the meson spectra

- Time-of-Flight measurement
- extends the pT reach to 3 GeV/c
- for protons

- y0

- Smooth evolution with centrality

- arXiv:nucl-ex/0610001submitted to PRC

- Identified particle pT -spectra, Au+Au at 62.4 GeV

## Particle production at very low pT

- Low pT spectra carry information about radial flow, possible anomalous
- enhancement at very low pT, quantum mechanical effects, scaling laws

-  Unique low-pT coverage of PHOBOS

- T        = 1016 MeV
- surface = 0.720.02

- T        = 1026 MeV
- surface = 0.760.02

- T        = 1036 MeV
- surface = 0.780.02

## mT -scaling in d+Au vs. Au+Au at 200 GeV

- mT -scaling in d+Au vs. Au+Au at 200 GeV

- Lack of mT scaling in heavy ion collisions

## (Anti)proton/hadron ratios

- for the two charge signs

- Baryon excess
- approached at about pT3 GeV/c

- y0

- p>++K+

- arXiv:nucl-ex/0610001submitted to PRC

- PHOBOS

## Net protons (p–p)

- Net protons (p–p)

- _

## Summary

- _New results from PHOBOS:_
-  antiparticle/particle ratios in Cu+Cu collisions
-  p(p)/h± ratios, identified spectra in 62.4 GeV Au+Au collisions
- _What have we learned?_
-  Centrality-independence:		• p/p ratios		• net proton, p and p yield per participant       	   (still not fully understood theoretically)
-  baryon excess in Au+Au collisions at 62.4 GeV
-  mT-scaling holds in d+Au but violated in Au+Au

- _

- _

- _