Feasibility of hyperon measurements @ SIS 100

1. Feasibility of hyperon measurements @ SIS 100 Evgeny Kryshen (PNPI) CBM @ SIS 100 Dubna, 20 May 2009 • Outline • Motivation • Framework • Acceptance • Cuts and significance optimization • Results

2. Exploring the "nuclear" EOS at 3ρ0 < ρ < 7ρ0 with (sub)threshold production of multistrange hyperons Measure the excitation function of (multi-strange) hyperon production in heavy-ion collisions from 2 - 15 AGeV: Direct production: NN  Λ0Λ0 NN (Ethr = 7.1 GeV) NN +- NN (Ethr = 9.0 GeV) NN +- NN (Ethr = 12.7 GeV) Production via multiple collisions: Hyperons (s quarks): 1. NN K+Λ0N,NN  K+K-NN, 2. Λ0K-- 0, 3.-K--- Antihyperons (anti-s quarks): 1. Λ0 K++0 , 2. +K+++. CBM at SIS100, 20 May 2009

3. Motivation - 2 Constituent quark scaling @ RHIC Hyperon resonances @ RHIC • Constituent quark scaling of elliptic flow @ SIS 100? • Hyperon resonances (Thermal model vs UrQMD), energy dependence • Dibaryons • Hyperon enhancement • Hyperon polarization CBM at SIS100, 20 May 2009

4. Hyperons at AGS • Threshold production of Xi measured in Au+Au collisions @ 6 GeV • Main detector: TPC with PID capabilities • ~250 Xi measured in 4 centrality bins • Results consistent with UrQMD • Neural network algorithm used for the bgd suppression CBM at SIS100, 20 May 2009

5. Hyperon properties QCD_CBM_PHYS-note-2005-002, July 2005. CBM at SIS100, 20 May 2009

6. Framework cbmroot trunk (May 2009) 100 • Current STS design : • no MVD in the STS setup • 8 STS stations • Strip design • Thickness: 400 µm • 60 um pitch • Still no clusters and charge sharing 95 75 60 50 40 35 30 • Features: • Background – central UrQMD • Signal for Lambda – central UrQMD • Signal for Xi and Omega - signal hyperon embedded into central UrQMD event • Event mixing for Xi and Omega for background estimates • Refit of secondary vertices • Multidimensional maximization of significance No PID CBM at SIS100, 20 May 2009

7. Acceptance @ 6 GeV Λ Λ Λ • Good acceptance coverage • Acceptance (including branching): • Lambda: 35.9% • Xi: 27.9% • Omega: 23.6% Ξ Ω CBM at SIS100, 20 May 2009

8. Scaling of magnetic field* • Momentum resolution drops down dramatically with the reduced magnetic field • No significant increase in acceptance • Decision: make simulations with the nominal magnetic field scale * Old simulation 2006 CBM at SIS100, 20 May 2009

9. Reconstruction efficiency Reco efficiency for sec. pions Accepted pions Reconstructed pions • Low momentum pions -> Pion reconstruction efficiency ~ 60% • Hyperon reconstruction efficiency: • Lambda: 60.5% • Xi: 35.0% • Omega: 48.8% CBM at SIS100, 20 May 2009

10. Background suppression strategy Cut variables: • Single track cuts: • impact parameter in the target plane for positive and negative tracks • Vertex quality cuts: • distance of closest approach, • chi2 of the fitted vertex • Additional topological cuts: • Position of the fitted decay vertex along the beam axis • Impact parameter of the reconstructed mother track • No particle identification is used The goal: maximum significance CBM at SIS100, 20 May 2009

11. 2-dimensional significance optimization Signal Background Significance • Significance as function of cut variables has very nontrivial shape, therefore two-dimensional analysis was developed. • Cut variables are grouped in pairs: • Single track cuts: impact parameters for positive and negative tracks • Vertex quality cuts: distance of closest approach, chi2 of the fitted vertex • Additional cuts: z-position of secondary vertex, impact parameter of the mother track CBM at SIS100, 20 May 2009

12. Λ analysis Statistics: 105 central UrQMD events @ 6 AGeV Λ • Total cut efficiency: 51.7% • S/B ratio: 43 • Significance: 364 • σΛ,[MeV/c2]: 1.53 • 1.5 reconstructed Λ/event after cuts CBM at SIS100, 20 May 2009

13. Ξ-analysis Statistics: 9.4 ∙105 central UrQMD events @ 6 AGeV • Total cut efficiency: 17.1% • S/B ratio: 8.31 • Significance: 36.9 • σΞ[MeV/c2]: 2.58 Ξ CBM at SIS100, 20 May 2009

14. Ω analysis Statistics: 1.4 ∙108 central UrQMD events @ 6 AGeV • Total cut efficiency: 15.5% • S/B ratio: 1.16 • Significance: 27.7 • σΩ[MeV/c2]: 2.43 Ω CBM at SIS100, 20 May 2009

15. Summary Ξ Λ Ω CBM at SIS100, 20 May 2009

16. Hyperons @ 6 GeV with PID* • - K-, main background comes from π- mistaken with K- PID hypothesis • Only 25% of K- tracks survive on the distance of 10 m, thus -acceptance with TOF is at least 4 times lower. Therefore the background π- rejection with TOF PID is preferred. • Semi-perfect PID: Pion tracks, which have MC points in TOF are rejected from combinatorics π- rejection No PID Ω Ω S/B=8.6 S/B=4.3 Preliminary conclusion: pion rejection does not help much, at least at 6 GeV * Old simulation 2006 CBM at SIS100, 20 May 2009

17. Conclusions • CBM is well-tuned for hyperons measurements @ SIS 100 • Good acceptance coverage (full phase space measurement is possible) • Differential measurements are possible • No PID is required • Omega trigger is desired CBM at SIS100, 20 May 2009

18. Backup slides CBM at SIS100, 20 May 2009

19. Particle multiplicities at 6 and 25 AGeV CBM at SIS100, 20 May 2009

20. ChiToVertex cut vs impact parameter cut CbmStsKFTrackFitter::GetChiToVertex(CbmStsTrack track) - Get impact parameter to primary vertex normalized to the xy uncertainty of the track extrapolation at the target plane • ChiToVertex also takes into account xy uncertainty of the primary vertex • ChiToVertex cut provides much better separation of secondary tracks if compared to ordinary impact parameter cut. • Example for Lambda hyperons at 25 GeV - the similar signal to background ratio is achieved with: ChiToVertex cut - 77% efficiency Impact parameter cut - 38% efficiency • ChiToVertex ~ 4σ is usually enough to separate secondary tracks • Requirement: correct estimation of the track extrapolation uncertainty CBM at SIS100, 20 May 2009

21. Illustration for the chi2vertex cut XY plane at primary vertex 1σ uncertainty region Impact parameter Primary vertex Track projection CBM at SIS100, 20 May 2009