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/‹N part › 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 Detectors Endcap Calorimeter

Raw Multiplicity STAR preliminary STAR Vertex Finder: loses low multiplicity events spectra shifted to higher p T 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 + TPC 2 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 = GeV/c 2 BR: 63.9% Ξ -> Λ + π - m = GeV/c 2 BR: 99.9% Ω-> Λ + K - m = GeV/c 2 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-Ξ-  Λ π-π-  π-π- p Reconstruction via decay topology of charged daughter tracks in TPC STAR p+p preliminary p (GeV/c) dE/dx (keV/cm) e p π d K

K0sK0s Λ Λ K*(892) Λ*(1520) Ω - + Ω + Ξ-Ξ- 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 Λ Λ _ Ξ+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 p t spectra |y| < 0.5 Ξ analysis in p+p: Poster by M. Estienne (Multi-)strange hyperons in p+p STAR preliminary powerlaw fitted p t spectra Λ Ξ-Ξ- Ξ-Ξ-

Ω Analysis in p+p Ω- Ω- Anti- Ω+ Ω signal visible in p+p data! Anti-Ω/Ω ratio: 0.90 ± 0.19 (sys. error ~ 15%) S/N: 1.1 σ ~ 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 SQM Talk by M. Mitrovski NA49 Anti-Omega/Omega ratio: 0.67 ± 0.26 (stat.) p+p at √s = 17.3 GeV NA49 preliminary

Strangeness content per particle Anti-Baryon/Baryon Ratio 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) 551 M. Gorenstein, M. Gazdzicki, Phys. Lett. B483 (2000) 60 String Fragmentation Models: M. Bleicher et al., PRL 88 (2002) F.M. Liu et al., Phys. Rev. D67 (2003) 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 ModelsStatistical Models preliminary STAR 200 GeV preliminary

Anti-Baryon/Baryon Ratio Strangeness content per particle 0123 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: Λ ratio: Ξ ratio: Ω ratio: NeXuS > NeXuS-Talk by K. Werner!

STAR B/B Ratios: p+p vs. Au+Au Au+Au STAR 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 p T range! STAR preliminary Anti-Λ/Λ ratio Anti-Ξ/Ξ ratio absorption corrected sys. errors included stat. errors only STAR preliminary p/p soft+quench -> Ratios vs. p T in Au+Au -> Talk by Ben Norman! absorption corrected sys. errors included Au+Au

p+p Spectra at 200 GeV d 2 N/(2πm T dm T dy) (c 4 /GeV 2 ) m T -m 0 (GeV/c 2 ) STAR preliminary π-π- π-π- K-K- K-K- p _ p _ 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 SPSRHIC √s NN (GeV) K/πK/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 GeV SPS/NA49 AGS/E866 p+p ● NeXuS 3.97 K - /K + : 0.95 for GeV ● dN π- /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 ratio √s NN (GeV) p + p collisions - STAR p+p 200 GeV: K ± /π ± preliminary STAR p+p data: K + /π + : ± 10% K - /π - : ± 10% -> in good agreement! all data: stat.+sys. errors

Summary and Outlook strange hyperon spectra in p+p: Λ, Ξ over wide p T 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 p T 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 only TPC+SVT STAR preliminary TPC only TPC+SVT ~35-40% greater yield in K 0 s region mostly at low p t Including SVT contribution: increase in yields smaller errors on ratios important for multi-strange hyperons

Next: Back-up Slides

The STAR Collaboration Russia: MEPHI - Moscow LPP/LHE JINR - Dubna IHEP - Protvino U.S. Laboratories: Argonne Berkeley Brookhaven U.S. Universities: Arkansas University UC Berkeley UC Davis UC Los Angeles Carnegie Mellon University Creighton University Indiana University Kent State University Michigan State University City College of New York Ohio State University Penn. State University Purdue University Rice University Texas A&M UT Austin Washington University Wayne State University Yale University Brazil: Universidade de Sao Paulo China: IHEP – Beijing IMP - Lanzou IPP – Wuhan USTC SINR – Shanghai Tsinghua University Great Britain: University of Birmingham France: IReS Strasbourg SUBATECH - Nantes Germany: MPI – Munich University of Frankfurt India: IOP - Bhubaneswar VECC - Calcutta Panjab University University of Rajasthan Jammu University IIT - Bombay VECC – Kolcata Poland: Warsaw University of Tech.

       p Observed decay channels in STAR :   p +  - m = GeV/c 2 BR: 63.9%    +  - m = GeV/c 2 BR: 99.9%    + K - m = GeV/c 2 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 Pions Protons 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 CTB matching efficiency = good measure of pile-up lower matching efficiency -> higher pile-up Azimuth STAR Preliminary Azimuth STAR Preliminary match at least 1 daughter track to a CTB hit (studies by Jon Gans) -2 Z Position 2 STAR Preliminary using SVT matched tracks -> rejection of pile-up?

DCA Cascade to Primary Vertex 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 p T > 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-Ξ 8M 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 p T bins up to 4.0 GeV/c possible

STAR preliminary Omega Anti-Omega Ω Analysis in p+p S/N: 1.1 σ ~ 5.5 MeV S/N: 1.2 σ ~ 6.0 MeV STAR preliminary Anti-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 Preliminary K-/πK-/π dN π- /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 ΛΛ K0sK0s √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 ~ 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 ratios at 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+Au All 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