Feasibility of hyperon measurements SIS 100 Evgeny Kryshen

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 Evgeny Kryshen CBM at SIS 100, 20 May 2009

Exploring the "nuclear" EOS at 3ρ0 < ρ < 7ρ0 with (sub)threshold production of multistrange hyperons Measure the excitation function of (multistrange) hyperon production in heavy-ion collisions from 2 - 15 AGe. V: Direct production: NN Λ 0Λ 0 NN (Ethr = 7. 1 Ge. V) NN + - NN (Ethr = 9. 0 Ge. V) NN + - NN (Ethr = 12. 7 Ge. V) Production via multiple collisions: Hyperons (s quarks): 1. NN K+Λ 0 N, NN K+K-NN, 2. Λ 0 K- - 0, 3. -K- - Antihyperons (anti-s quarks): 1. Λ 0 K+ + 0 , 2. + K+ + +. Evgeny Kryshen CBM at SIS 100, 20 May 2009 2

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

Hyperons at AGS • • • Threshold production of Xi measured in Au+Au collisions @ 6 Ge. V Main detector: TPC with PID capabilities ~250 Xi measured in 4 centrality bins Results consistent with Ur. QMD Neural network algorithm used for the bgd suppression Evgeny Kryshen CBM at SIS 100, 20 May 2009 4

Hyperon properties Particle Quark content Mass (Ge. V/c 2) Lifetime c (cm) Multiplicity at 6 Ge. V (Ur. QMD) Decay channel BR uds 1. 116 7. 89 12. 8 p - 63. 9% - dss 1. 321 4. 91 ~0. 12 - 99. 9% - sss 1. 672 2. 46 ~7. 2 10 -4 K- 67. 8% QCD_CBM_PHYS-note-2005 -002, July 2005. Evgeny Kryshen CBM at SIS 100, 20 May 2009 5

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

Acceptance @ 6 Ge. V Λ Λ Λ Good acceptance coverage Ξ Ω Acceptance (including branching): • Lambda: 35. 9% • Xi: 27. 9% • Omega: 23. 6% Evgeny Kryshen CBM at SIS 100, 20 May 2009 7

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 Evgeny Kryshen CBM at SIS 100, 20 May 2009 8

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% Evgeny Kryshen CBM at SIS 100, 20 May 2009 9

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, – chi 2 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 Evgeny Kryshen CBM at SIS 100, 20 May 2009 10

2 -dimensional significance optimization Background • • Signal 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, chi 2 of the fitted vertex – Additional cuts: z-position of secondary vertex, impact parameter of the mother track Evgeny Kryshen CBM at SIS 100, 20 May 2009 11

Λ analysis Statistics: 105 central Ur. QMD events @ 6 AGe. V Λ • • • Evgeny Kryshen Total cut efficiency: 51. 7% S/B ratio: 43 Significance: 364 σΛ, [Me. V/c 2]: 1. 53 1. 5 reconstructed Λ/event after cuts CBM at SIS 100, 20 May 2009 12

Ξ- analysis Statistics: 9. 4 ∙ 105 central Ur. QMD events @ 6 AGe. V Ξ Evgeny Kryshen • • Total cut efficiency: S/B ratio: Significance: σΞ [Me. V/c 2]: CBM at SIS 100, 20 May 2009 17. 1% 8. 31 36. 9 2. 58 13

Ω analysis Statistics: 1. 4 ∙ 108 central Ur. QMD events @ 6 AGe. V Ω Evgeny Kryshen • • Total cut efficiency: S/B ratio: Significance: σΩ [Me. V/c 2]: CBM at SIS 100, 20 May 2009 15. 5% 1. 16 27. 7 2. 43 14

Summary Λ Evgeny Kryshen Ξ CBM at SIS 100, 20 May 2009 Ω 15

Hyperons @ 6 Ge. V 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 S/B=4. 3 Ω S/B=8. 6 Ω Preliminary conclusion: pion rejection does not help much, at least at 6 Ge. V * Old simulation 2006 Evgeny Kryshen CBM at SIS 100, 20 May 2009 16

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 Evgeny Kryshen CBM at SIS 100, 20 May 2009 17

Backup slides Evgeny Kryshen CBM at SIS 100, 20 May 2009 18

Particle multiplicities at 6 and 25 AGe. V Evgeny Kryshen CBM at SIS 100, 20 May 2009 19

Chi. To. Vertex cut vs impact parameter cut Cbm. Sts. KFTrack. Fitter: : Get. Chi. To. Vertex(Cbm. Sts. Track track) - Get impact parameter to primary vertex normalized to the xy uncertainty of the track extrapolation at the target plane • Chi. To. Vertex also takes into account xy uncertainty of the primary vertex • Chi. To. Vertex cut provides much better separation of secondary tracks if compared to ordinary impact parameter cut. • Example for Lambda hyperons at 25 Ge. V - the similar signal to background ratio is achieved with: Chi. To. Vertex cut - 77% efficiency Impact parameter cut - 38% efficiency • Chi. To. Vertex ~ 4σ is usually enough to separate secondary tracks • Requirement: correct estimation of the track extrapolation uncertainty Evgeny Kryshen CBM at SIS 100, 20 May 2009 20

Illustration for the chi 2 vertex cut XY plane at primary vertex 1σ uncertainty region Impact parameter Primary vertex Track projection Evgeny Kryshen CBM at SIS 100, 20 May 2009 21