Strange and multi-strange particle ratios in p+p reactions at √s = 200

GeV

Anja Billmeier(Wayne State University)

for the STAR Collaboration

Strange Quark Matter, Atlantic Beach, March 2003

Talk Outline

• Motivation

• The STAR Experiment:

- Event Selection / Data Sets

- Reconstruction / Analysis Procedure

• STAR p+p Results -> Anti-Baryon/Baryon Ratios

• Comparison to SPS data and Model calculations

• Mixed Hadron Ratios -> Excitation Functions

• Summary and Outlook

Strangeness production in p+p• interpretation of HI results requires knowledge of reaction characteristics in p+p interactions

• discussion of strangeness enhancement as QGP signature:

strangeness/‹Npart› increases p+p -> A+A

-> data of both collision systems at same energy needed (reference data) -> very few measurements of multi-strange hyperons in elementary collisions

• enhancement expected to increase with strangeness content of particle -> multi-strange baryons (and anti-baryons) most sensitive probes

• RHIC: first high-statistics measurement of multi-strange hyperons

• evolution with beam-energy -> comparison to SPS p+p data

• model predictions for Anti-baryon/baryon ratios in p+p vary significantly

• STAR measures p+p and A+A collisions with same experimental set-up -> most systematic effects cancel out

(QGP/HG distinction: J. Rafelski and B. Müller PRL48 (1982) 1066)

2 m

4 m

The STAR Detector

Barrel EM Calorimeter

RICH

ZDC

Time Projection Chamber

(TPC)Magnet

Coils

TPC Endcap

MWPC

ZDC

Central Trigger

Barrel

FTPCs

Silicon Vertex Tracker

+

Silicon Strip Detector

Vertex Position

DetectorsEndcap Calorimeter

Raw Multiplicity

STAR preliminary

STAR Vertex Finder:• loses low multiplicity events• spectra shifted to higher pT

Beam-Beam Counters (BBC):• used for p+p triggering (coincidence)• ± 3.74 m from interaction region• 3.3 < |η| < 5.0, 0 < φ < 2π• 70 cm Vertex resolution

-> to be considered when analyzing p+p data!

beam pipe

BBC (West)

BBC (East)

Schematic side view of the detector

FPD

~7.5 m

STAR Magnet

+ TPC2 m

Pile-up effect in p+p collisions:• rejection by matching of tracks to CTB hits • using SVT to suppress pile-up events• ongoing studies… -> Poster by Mark Heinz!

Event selection in p+p

Observed hyperon decay channels in STAR:• Λ -> p + π- m = 1.116 GeV/c2 BR: 63.9%• Ξ -> Λ + π- m = 1.321 GeV/c2 BR: 99.9%• Ω-> Λ + K- m = 1.672 GeV/c2 BR: 67.8%

Identical algorithm for p+p and Au+Au data!

Selection techniques: • PID via specific ionisation (dE/dx)• geometric and kinematic cuts -> invariant mass

B

Analysis Procedure

Ξ-Λ

π-π-

p

Reconstruction via decay topology of charged daughter tracks in TPC

STAR p+p preliminary

p (GeV/c)

dE/d

x (

keV

/cm

)

15

110-10

e

p

π

d

K

K0s

Λ

Λ

K*(892)

Λ*(1520)

Λ*(1520)

Ω- + Ω+

Ξ-Ξ+

Σ*(1385)~ 8.3 M min. bias events

STAR p+p at √s = 200 GeV

all plots: STAR p+p preliminary!

(Multi-)strange Particle Ratios• particle ratios instead of absolute yields: simplify comparison of different experiments -> systematic errors in measurement of yields reduced

• measurement of ratios excellent tool to distinguish between various model scenarios!

• STAR azimuthally symmetric: same efficiencies for particles and anti-particles -> Anti-Baryon/Baryon ratios can be extracted w/o any efficiency correction

• useful probe of initial environment created in ultra-relativistic collisions

• hyperon ratios as function of transverse momentum might yield information of dynamics driven by pQCD processes

• ratios approach 1 at RHIC energies due to decreasing net-baryon number -> statistical sensitivity reduced to small differences from unity -> systematic errors limit precision

• mixed hadron ratios (i. e. strange/non-strange) allow quantitative evaluation of strangeness production and probe strangeness enhancement p+p -> A+A

Anti-Ξ/Ξ ratio: 0.97 ± 0.04 (sys. error ~ 10%)

p+p Spectra:

Parallel talk by

Richard Witt!

STAR preliminary

STAR preliminary

Λ

Λ

_Ξ+

_

Λ analysis in p+p:

Poster by J. Adams

Anti-Λ/Λ ratio: 0.89 ± 0.03 (sys. error ~ 7%)

STAR preliminary

Fits:

• powerlaw

• exponential

pt spectra |y| < 0.5

Ξ analysis in p+p:

Poster by M. Estienne

(Multi-)strange hyperons in p+p

STAR preliminary

powerlaw fitted pt spectra

Λ

Ξ-

Ξ-

Ω Analysis in p+p

Ω-

Anti-Ω+

Ω signal visible in p+p data!

Anti-Ω/Ω ratio: 0.90 ± 0.19 (sys. error ~ 15%)

S/N: 1.1σ ~ 5.5 MeV

STAR preliminary

however: statistics not enough for Ω spectra!

130 ± 16 Ω + Anti-Ω in mass peak (8 M events)

STAR preliminary

Comparison to SPS data

C. Blume, SPSC Meeting CERN, January 2003 - SQM Talk by M. Mitrovski

NA49 Anti-Omega/Omega ratio:

0.67 ± 0.26 (stat.)

p+p at √s = 17.3 GeV

NA49 preliminary NA49 preliminary

Strangeness content per particle

An

ti-B

ary

on

/Bary

on

Rati

o

all calculations:

√s = 17.3 GeV, |y| < 1

Model predictions for p+p collisions

Statistical Models:F. Becattini, G. Passaleva, Eur. Phys. J. C23 (2002) 551M. Gorenstein, M. Gazdzicki, Phys. Lett. B483 (2000) 60

String Fragmentation Models:M. Bleicher et al., PRL 88 (2002) 202501F.M. Liu et al., Phys. Rev. D67 (2003) 034011

Statistical Models: • Ω ratio ~ 0.6 at SPS• slight increase with √s• ratios always < 1

String Models: • Ω ratio > 1 for all √s• SPS ~ 1.7, RHIC ~ 1.3

New NeXuS 3.0: • Ξ ratio = 0.56• Ω ratio = 0.71• ratios -> 1 for higher √s

preliminary

String Fragmentation Models Statistical Models

preliminary

STAR 200 GeVpreliminary

An

ti-B

aryo

n/B

aryo

n R

atio

Strangeness content per particle

0 1 2 3

Model predictions for RHIC energies

p+p √s = 200 GeV, |y|<1

STAR p+p data:

p ratio: 0.82 ± 0.01

Λ ratio: 0.89 ± 0.03

Ξ ratio: 0.97 ± 0.04

Ω ratio: 0.90 ± 0.19

STAR preliminary

J. Aichelin, K. Werner,

private communication

NeXuS 3.97:

p ratio: 0.810

Λ ratio: 0.895

Ξ ratio: 0.945

Ω ratio: 0.940

NeXuS 3.97

-> NeXuS-Talk by K. Werner!

STAR B/B Ratios: p+p vs. Au+Au

Au+AuSTAR p+p √s = 200 GeV

Integrated raw ratios at mid-rapidity:

• p+p ratios similar to A+A

• slow increase with strangeness content

• ratios approach 1 -> zero net-baryon number

-> not yet baryon-free!STAR preliminary

All data:• mid-rapidity• ratios from raw yields• statistical errors only• Au+Au: most central data

-

Systematics of p+p Ratios

-> Ratios in p+p flat over wide pT range!

STAR preliminary

Anti-Λ/Λ ratio

Anti-Ξ/Ξ ratio

• absorption corrected

• sys. errors included

• stat. errors only

STAR preliminary

p/p soft+quenchp/p soft+quench

-> Ratios vs. pT in Au+Au -> Talk by Ben Norman!

• absorption corrected

• sys. errors included

Au+Au

p+p Spectra at 200 GeVd

2N

/(2π

mTd

mTd

y)

(c4/G

eV

2)

mT-m0 (GeV/c2)

STAR preliminary

π-

π-

K-

K-

p_

p_

STAR preliminary

Au+Au 200 GeV central

T = 180 ± 1 MeV

T = 182 ± 3 MeV

T = 190 ± 2 MeV

Spectra shapes/slopes:

• similar in p+p

• different in Au+Au

• Particle identification via dE/dx• Kaons: dominate strangeness production• pions: measure of particle production (entropy)

-> Talk by Richard Witt

Mixed Hadron Ratios – Excitation Function

STAR preliminary

SPS RHIC

√sNN (GeV)

K/π

K+/K

-

• K-/π- monotonic increase - same trend for p+p and A+A • p+p: K/π ratios lower than A+A• K+/K- decreases -> reduced net-baryon density at RHIC• similar K-/K+ ratios for p+p and A+A• net-baryon density constant

AGS

STAR preliminary

K- /K

+

200 GeV

SPS/NA49

AGS/E866

p+p

●

NeXuS 3.97 K-/K+: 0.95 for p+p@200 GeV

●

dNπ-/dy

A+A: K+/π+

A+A: K-/π-

STAR p+p: K-/π- K+/π+

p+p

A+A: K+/K-

K/π Excitation Function

Data collected in:

G. Bocquet et al., Phys. Lett. B336 (1996) 447

Fit: f(√s) = a+b · ln(√s)K

/π r

ati

o

√sNN (GeV)

p + p collisions-

STAR p+p 200 GeV:

K±/π± preliminary

STAR p+p data:

• K+/π+: 0.102 ± 10%

• K-/π-: 0.105 ± 10%

-> in good agreement!

all data: stat.+sys. errors

Summary and Outlook

• strange hyperon spectra in p+p: Λ, Ξ over wide pT range (~ 4 GeV/c)• Ω signal observed: ~130 Ω + Anti-Ω in 8M p+p events• observation of both, Ω and Anti-Ω separately• uncorrected Anti-Baryon/Baryon ratios for (multi-)strange hyperons in p+p at √s = 200 GeV: - significantly higher than at SPS - much slower increase with |S| - similar to Au+Au ratios - approach 1 for RHIC data -> not yet baryon-free! - no observed pT dependence over wide range

- very good agreement with NeXuS 3.97 calculations• K/π ratios agree with data of other experiments - increase with √s• production of additional 5M p+p events currently running: statistics gain -> smaller errors on (multi-)strange ratios

Outlook

STAR preliminary

TPC onlyTPC+SVT

STAR preliminary

TPC only

TPC+SVT

~35-40% greater yield in K0s region mostly at low pt

Including SVT contribution:• increase in yields• smaller errors on ratios• important for multi-strange hyperons

Next:Back-up Slides

The STAR Collaboration Russia:MEPHI - MoscowLPP/LHE JINR - DubnaIHEP - Protvino

U.S. Laboratories:ArgonneBerkeleyBrookhaven

U.S. Universities: Arkansas UniversityUC BerkeleyUC DavisUC Los AngelesCarnegie Mellon UniversityCreighton UniversityIndiana UniversityKent State UniversityMichigan State UniversityCity College of New YorkOhio State UniversityPenn. State UniversityPurdue UniversityRice UniversityTexas A&MUT AustinWashington UniversityWayne State UniversityYale University

Brazil: Universidade de Sao Paulo

China: IHEP – BeijingIMP - LanzouIPP – WuhanUSTCSINR – ShanghaiTsinghua University

Great Britain: University of Birmingham

France:IReS StrasbourgSUBATECH - Nantes

Germany: MPI – MunichUniversity of Frankfurt

India:IOP - BhubaneswarVECC - CalcuttaPanjab UniversityUniversity of RajasthanJammu UniversityIIT - BombayVECC – Kolcata

Poland:Warsaw University of Tech.

p

Observed decay channels in STAR: p + - m = 1.116 GeV/c2 BR: 63.9% + - m = 1.321 GeV/c2 BR: 99.9% + K- m = 1.672 GeV/c2 BR: 67.8%Reconstruction via decay topology of charged daughter tracks in TPC

Identical algorithm for p+p and Au+Au data

Selection techniques: • PID via specific ionisation (dE/dx)• geometric and kinematic cuts (e.g. dca, decay length) to reduce combinatorial background invariant mass

B

PionsProtons

Kaons

Analysis Procedure

CTB Matching Studies

lower luminosity (early in run, RFF) higher luminosity (late in run, FF)

track matches CTB hit

track projects to CTBmatching efficiency =

good measure of pile-uplower matching efficiency -> higher pile-up

Azimuth0 180 360

STAR Preliminary

Azimuth

1

0.5

00 180 360

STAR Preliminary

• match at least 1 daughter track to a CTB hit

(studies by Jon Gans)

-2

Z

Po

siti

on

2STAR Preliminary STAR Preliminary

using SVT matched tracks -> rejection of pile-up?

DC

A C

ascad

e t

o P

rim

ary

Vert

ex

DCA < 1cm

Noise

• CTB matching: loss in statistics, gain in S/N

(studies by Mark Heinz)

Cuts for Ω analysis:

• cut on correct Triggerword

• dE/dx cut: 3σ for all charged decay tracks

• TPC hits > 15 for all charged decay tracks

• Λ mass within ± 7 MeV window

• rejection of Ξ mass ± 10 MeV

• pT > 0.5 GeV/c

• DCA Ω and Λ daughters < 0.8 cm

• decay length Ω < 20 cm (2-20 cm)

• decay length Λ > 1.0 cm

• DCA K and Λ to primary Vertex > 0.1 cm

• DCA Ω to primary Vertex < 0.8 cm

Cut optimization

dependent on PrimVtx.Z!

Ξ Analysis in p+p

Invariant mass for Ξ … …and Anti-Ξ8M p+p events

Anti-Ξ/Ξ ratio: 1.01 ± 0.04 (Richard’s analysis) 0.90 ± 0.05 (Hui’s old analysis) 0.90 ± 0.07 (Sevil’s analysis)

Parallel talk by

Richard Witt

STAR preliminary

Ξ spectra: 6 pT bins up to 4.0 GeV/c possible

STAR preliminary

Omega

Anti-Omega

Ω Analysis in p+pS/N: 1.1σ ~ 5.5 MeV

S/N: 1.2σ ~ 6.0 MeV

STAR preliminaryAnti-Ω/Ω ratio:0.90 ± 0.19 (all cuts)0.89 ± 0.20 (no PrimVtx.Z cuts)

no PrimVtx.Z cuts all cuts

sys. error (7%) included, absorption corr.

• Anti-Λ/Λ ratio for p+p and Au+Au:

flat over wide pt range (~3 GeV)

• possible decrease at higher pt

for most central Au+Au data

A Comparison: Ratios Au+Au vs. p+p

Hui Long

John Adams

STAR PreliminaryK- /π

dNπ-/dy

Mixed Hadron Ratios - Systematics

Mid-rapidity ratios at RHIC: • smooth increase p+p -> A+A• little centrality dependence -> similar freeze-out conditions • linear increase at AGS and SPS -> change in production mechanism at most central collisions RHIC?• saturation of strangeness production?

STAR data: including syst. errors

AGS/SPS data: stat. errors only

Mass dependence in p+p?

• p+p collisions at lower energy (√s < 23 GeV):

all particle species exhibit same inverse slope

• deviation of this behavior in A+A attributed to flow

NA44: Phys. Rev. Lett. 78 (1997) 2080 NA49: A.B., PhD thesis (2001)

p+p data at SPS

Λ ΛK0s

√s = 17.3 GeV

-

p+p collisions at high √s

A. Dumitru, C. Spieles, Phys. Lett. B446 (1999) 326 (Inverse slopes at mid-rapidity)

PYTHIA Model:

• multiple mini-jet production and fragmentation

-> color strings that are not ONLY longitudinal

• agree with results from UA1/5 experiments

(p + p-bar at √s = 540 GeV)

• T strongly dependent on hadron mass

• HYDRO: not flow effect -> flat for p+p

increase in Au+Au

• SPS data: T ~ 150-180 MeV for all particle species

More complicated picture at high √s

√s = 200 GeV

STAR data points: very preliminary!

Put in T for Lambda and Xi if available!

Statistical model and particle ratiosat RHIC

Au+Au Ratios: STAR vs. Models

Data: 130 GeV Au+Au central

Very good agreement between model and data!

STAR Preliminary

σ

Statistical Models:Different implementations (Kaneta/Xu, PBM et al., …): • all work well for A+A collisions at RHIC

M. Kaneta and N. Xu, J. Phys. G27 (2001) 589

particle generation process well understood

assuming chem. equilibration and f.o. before thermal f.o. and after hadronization

STAR B/B Ratios in Au+AuAll data:• feed-down corrected• mid-rapidity, central data• ratios from raw yields

WA97

STAR preliminary

Integrated raw ratios at mid-rapidity:• ratios increase with strangeness content and energy• higher anti-baryon production at RHIC• approaching zero net baryon number → not yet baryon-free!

Latest News: d+Au data!

50k events

STAR running since January

√s = 200 GeV

already > 20 M events taken!

Latest News: d+Au data!

• ~ 50k events

• dE/dx and TPC hits cut only

• including SVT contribution