# Presentation: 396661

## mid-rapidity kaon and proton ratios at sqrt(s)=200 GeV

-  Motivation
-  pbar/p ratio Analysis
-  K-/K+ ratio Analysis
-  Summary

## Motivation

- Energy const:
- Smooth dependence of baryon ratios on |S|
- Anti-proton over proton ratio detemines the chemical potential of the colliding system
- With pbar/p & K-/K+ ratio known -> other particle ratios can be estimated

- To understand the baryon physics and chemical properties, pt and y
- distributions of protons,kaons are needed.

## pbar/p and K-/K+ form Au+Au at 200 GeV

-  P02gc, P02gd production, FullField + ReversedFullField
-  |zVertex| < 25 cm, -> 890k, 470k minbias events pass.
-  nHits >= 23
-  9 centrality bins {510, 431,312,217,146,94,56,30,14}
-  Proton, anti-proton ID by dE/dx
-  Kaon ID by dE/dx
-  Proton momentum energy loss corrected
-  Anti-proton, K- absorption corrected

## Proton ID using dE/dx

- pt increasing:
- proton dE/dx merges with dE/dx from other particle species.

##  Proton dca distributions

-  Proton dca distributions

- Primary particles interact with detector material generate background protons -> secondaries
- Large background at pt » 400MeV/c
- Steeply decreasing at higher pt

-  red = pos
- blue = neg
- green=bkrd

## Proton and pbar backround

-  Assume:		 		dcaP = C * dcapbar + dcaback
-  Fit with:
- fback(x)= A * (1 - exp(-x / k) )
-  c2 / ndf = 1.6
- dca < 1cm minimizes background

- Hijing, GEANT simulations of dca distributions for 2001 geometry have been studied.

## pbar/p, Au+Au at 200 GeV

- _vs. pt, minbias:_ 0 < |y| < 0.3			pbar/p= 0.81 ± 0.002(stat.) ± 0.05 (syst.)
- _vs. y, minbias:_ 0.6 < pt < 0.8 GeV/cpbar/p= 0.81 ± 0.002 (stat.) ± 0.05(syst.)
- _vs. nCharge:_ 								0 < |y| < 0.3, 0.6 < pt < 0.8 GeV/chighest bin: top 5%:					pbar/p = 0.78 ± 0.002(stat.) ±  0.05(syst.)  lowest bin: 70-80%:					pbar/p = 0.83 ± 0.002(stat.) ±  0.05(syst.)

- no strong dependence on: pt, y

## pbar/p: Systematic Errors

- Au+Au @ 200 GeV, minbias, |y| < 0.3, 0.6 < pt < 0.8 GeV/c

- _Study__	__	__	__	__effect on pbar/p ratio_
- NFitPts: 23 -> 28:		< 0.005
- Backround:			< 0.03
- 10 < |VertexZ| < 25		< 0.005
- h * VertexZ > 0			< 0.010
- run #					< 0.01
- 5% minbias,5% central: 0.019
- e/w TPC side 			check
- Systematic error estimated as ± 0.05(syst.)
- minbias, 0 < |y| < 0.3, 0.4 < pt < 1.0 GeV :
- pbar/p= 0.81 ± 0.002(stat.) ± 0.05 (syst.)

## K-/K+, Au+Au at 200 GeV

- minbias, 0<|y|<0.4, 0.15<pt<0.65 GeV/c
- K- absorption corrected
- Electron contamination corr.
- K-/K+ =  0.953 ± 0.0012(stat.) ± 0.08(syst.)
- no strong dependence: pt, y, centrality

## pbar/p from p+p collisions at 200 GeV

- analysis:
-  P02ge production, FullField + ReversedFullField, 8M events
-  events with >=4, >= 20 tracks in TPC
-  |zVertex| < 25 cm, 5M events pass
-  nHits >= 23
-  Proton, anti-proton ID by dE/dx
-  Proton momentum energy loss corrected
-  Anti-proton absorption corrected

##  p+p: proton dca distributions

-  p+p: proton dca distributions

- Primary particles interact with detector material generate background protons -> secondaries
- Large background at pt » 400MeV/c
- Steeply decreasing at higher pt
- 2 < Bump < 4 cm

-  red = pos
- blue = neg
- green=bkrd

- p+p events with vertex and >= 4 tracks / event

##  p+p: proton dca distributions

-  p+p: proton dca distributions

- _p+p:_
- vertex found
- >= 20 tracks / event
- Bump at 2-4 cm diappears
- _Bump caused by:_
- vertex finder res. ?
- low mult pile up ?

-  red = pos
- blue = neg
- green=bkrd

- p+p events with vertex and >= 20 tracks / event

- Extrapolate tracks
- to CTB!

- Under Investigation!

## pbar/p, p+p at 200 GeV

- events with >= 20 tracks in TPC
- _vs. pt:_ |y| = 0.0 - 0.4					pbar/p= 0.81 ± 0.01 (stat.) ± 0.1 (syst.)
- _vs. y:_ pt = 0.6 - 0.8 GeV/cpbar/p= 0.81 ± 0.01 (stat.) ± 0.1 (syst.)
- 		analysis in progress !!!

## Ratios vs. Energy

- Au+Au: pbar/p = 0.81 ± 0.002(stat.) ± 0.05(syst.) 5% most central
- p+p: pbar/p = 0.81 ± 0.01(stat.) ± 0.10(syst.)
- events >= 20 tracks

- Au+Au: K-/K+ =
- 0.953 ± 0.0012(stat.) ± 0.08(syst.)
- Not net baryon-free at RHIC

## Summary

## Conclusions & Outlook

- 1) pbar/p and K-/K+ ratios measured at the RHIC top energy:
-     Au+Au, minbias:	pbar/p = 0.81 +- 0.002(stat.) ± 0.05(syst.)
-     p+p,event>=20tr: 	pbar/p = 0.81 +- 0.010(stat.) ± 0.10(syst.)
-     Au+Au, minbias:	K-/K=  0.953±0.0012(stat.)±0.08(syst.)
- 2) No strong dependence on transverse momentum and rapidity
-     in the ratios.
- 3) Systematic studies on the pbar/p from Au+Au have been done.
- -NfitPoints,
- -Backround
- -Vertex Z
- -Tracks crossing central menbrane
- -Run#
- 4) Working on a ratiopaper draft.

- p+p Analysis still in progress!

## STAR proton acceptance

- _analyze at:_	0.4 < pt < 1.0 GeV/c-0.3 < y < 0.3
- _PID limit:_1) low momentum:
- absorption in detector material + backround protons2) large rapidity:
- particles miss TPC volume
- 3) momentum > 1GeV:
- no separation in dE/dx from other particle species.

## p	= a /  * 1 / sin 

- energy loss of protons traversing through detector material.
- global tracks!
- year2001
- b: relativistic velocity
- q: angle between beam axis and particle trajectory

***p**	**= a / **** * 1 / sin** ***
*a **	**= -0.0032888*
*c **	**= 2.43938*

## Au+Au: proton dca distributions II

-  red = pos
- blue = neg
- green=bkrd

## proton dca distibution 2000, 2001

- Year 2000:	 	beam pipe,	 	inner fieldcage, 		SVT support
- Year 2001: 	+ SVT leaders
- dca distributions 	normalized on same number of events.
-  Year 2001: more secondaries at higher dca's than in 2000.

## proton dca full, half B field

- Width of primary peak is B field dependent (slightly)
- full field ->  narrower peak.

## Where are pbars absorbed?

- Year2000	Year2001

-  year2000:	Beam Pipe		Inner field cage		SVT supports
- Year2001:	+ 3SVT ladders

- Beam pipe

- Inner field cage

- SVT supports

- SVT ladders

## pbar-absorption correction

- -0.3 < y < 0.3

- absorption of pbar at
- 0.45 < pt < 0.65 GeV/c:
- 1) Pbar2000	:0.970
- 2) Pbar2001	:0.925
- 3) Proton2001:0.990

## Kaon ID using dE/dx

-  kaon identification using fits
-  electron contamination > 500 MeV

## electron contamination in kaon yields

- kaons, electrons are seperated to pt<450 MeV
- use yields from 	          pt <450 MeV to extrapolate to higher pt