Λ and Λ polarization at COMPASS

Transcription

1 and polarization at COMASS onghee Kang Johannes Gutenberg Universität Mainz On behalf of the COMASS collaboration IS XVIII International Workshop on eep Inelastic Scattering Firenze, 1 st April

2 Outline Introduction COMASS experiment Longitudinal & polarization ransversity from transverse & polarization Conclusion & outlook

3 Introduction Why polarization in deep inelastic scattering? Ideal probe to study spin effect Self analyzing weak decay Br 64% pπ Various spin aspects can be studied l ' l X with longitudinally polarized lepton-beam & longitudinally / transversely polarized target polarization can be used as a tool in the investigation of the spin of nucleon : Δ x, Δ x production should be a sensitive probe for sx

4 polarization Angular distribution of decay product of hyperon p ecay violates parity not isotropic d d cos 1 α ' cos π c.m.s. frame π Slope of the daughter baryon cos distribution is given by α If spin of is being perfectly in direction 1 & α.64 p c.m.s. frame Magnitude of asymmetry parameters are same for and no C violation! α ±.64 ±.13

5 COMASS spectrometer olarized μ beam Energy : 16 GeV Intensity : 8 μ/spill Luminosity : ~ 5 3 cm - s -1 olarization : ~ 8% Common Muon and roton Apparatus for Structure and Spectroscopy olarized H 3 6 Li arget Longitudinally / transversely hreetwo target cells olarization : ~ 855% ilution factor : ~ 1445% E/HCAL SM E/HCAL 6 Li/H 3 target SM1 MuonWall 5m RICH Spectrometer with stages μ Beam Large Angle SM1: 1. & Small Angle SM: 4.4 article identification: RICH, μf, ECAL, HCAL

6 Introduction Introduction COMASS experiment Longitudinal & polarization ransversity from transverse & polarization Conclusion & outlook

7 Longitudinal production μ s Quark-dist. function γ*q s μ ' Frag. function X In current fragmentation : x F > Study of the uark to baryon fragmentation processes spin transfer mechanism est of the strangeness uark and antiuark symmetry s x s x, Δs x Δs x Longitudinal polarization in IS dσ dσ r r l X r r l X dσ dσ r s l X r s l X B e y e x Δ x z z x unpolarized uark distribution function Δ z polarized fragmentation function z unpolarized fragmentation function B Beam polarization epolarization factor : 11 y y 1 1 y

8 Δ Δ Δ B B z x f y x e z x f x y e ] [ ] [ Longitudinal polarization In addition, the polarization can transfer from polarized uark Is there a dependence of the hyperon polarization on target polarization? olarized target Δ B Beam polarization arget polarization f ilution Factor epolarization factor : y y y Δx polarized uark distribution function x unpolarized uark distribution function Δ z polarized fragmentation function z unpolarized fragmentation function olarization asymmetry : no measurement exist Unpolarized target % Δ B LL z x e z x e y Spin transfer :

9 Identification of pπ, pπ, 4 AA Entries σ. MeV/c Sidebin subtraction method to obtain the number of s in each cos bins ~ 7, ~ 4, o particle I used in the selection Main background contributions Entries Mpπ MeV/c , 4 AA σ. MeV/c K γ Mπ p MeV/c Eur. hys. J. C kinematically indistinguishable K e e - pairs from γ conversion combinatorial background

10 Extraction of longitudinal polarization Spin analyzer for the measurement of longitudinal polarization Quantization axis along virtual photon γ* Angular distribution of the proton w.r.t γ* in helicity frame Extraction of polarization by bin-by-bin method subdivision of sample into bins of cos number of s from invariant mass distribution for each bin polarization from fit parameter of acceptance corrected cos distribution d d cos 1 α ' cos Acc

11 Kinematic distribution Acceptance correction function from MC simulations LEO of unpolarized & perfect agreement between data and MC in all kinematics.1 4 AA AA MC MC x F - -1 x x F x y -3 1 Q GeV/c y -3 1 Q GeV/c

12 Results of spin transfer Spin transfer :, LL, / y B LL.1±.47±.4 LL.49±.56±.49 spin transfer to increase significantly at high x F heory prediction by J.Ellis et al., Eur. hys. J. C CEQ5LF & SU6 model : LL < LL

13 Results of spin transfer LL.6.4 Influence of different Fs LL s CEQ5L, BJ model. GRV98LO, SU6 model LL s CEQ5L, BJ model COMASS x F s x > s x GRV98LO, SU6 model Sensitive to the strangeness distribution CEQ5L GRV98LO ata on the spin transfer to and could be used for the determination of s x s x

14 olarization asymmetry rediction for polarization asymmetry : Model calculation by A. IS9 x Bj.3 x Bj etermination of the spin transfer dependence on the target polarization allows to fix Δsx and Δsx o verify sign change of Δs for SSV, measure Δ in two bins at x Bj ~.3

15 ependence on the target polarization L, % 4 3 / 4,, - L, % 4 3 / 4,, - μ - COMASS reliminary x F - COMASS reliminary x F : target pol. is same to μ pol. L, % 4 3 / 4,, - L, % 4 3 / 4,, - : target pol. is opposite to μ pol. - COMASS reliminary - COMASS reliminary X X Averaged over full kinematics : Δ.1±.4 Δ.1±.5

16 Introduction Introduction COMASS experiment Longitudinal & polarization ransversity from transverse & polarization Conclusion & outlook

17 ransverse polarization μ μ ' IS Q > 1 GeV/c Fragmentation Δ z Δ x ransversity ransversityδ x can be measured on a transversely polarized target via ransverse polarization o measure chiral-odd Δ x, reuires another chiral-odd partner Δ z ransverse polarization from transversely polarized target dσ l dσ dσ l l X l X dσ l l l X l X f e Δ y e x Δ x z z Δ x transversely polarized uark distribution x unpolarized uark distribution function Δ z transversely polarized fragmentation z unpolarized fragmentation function B Beam polarization arget polarization f ilution Factor epolarization factor : 1 y y 1 1 y

18 Identification of pπ, pπ umber of 14 reliminary COMASS 7 transverse proton data GeV/c p ~ umber of mpπ - -mg[gev/c ] reliminary COMASS 7 transverse proton data ~ 5 3 GeV/c p p α p L L - p - p L - L mpπ -mg[gev/c ] p α p L L - p - p L - L

19 selection : RICH application article Identification by RICH Hadron masses calculated from the measured cherenkov angle ch Separation between π, K and p in the momentum range ~5 GeV/c π, K π -, K - veto for proton antiproton candidate hreshold momenta p π ~ GeV/c p K ~ 9 GeV/c p ~ 17 GeV/c Likelihood methods are used to reject π and K for proton candidate in the decay of pπ and pπ

20 selection : RICH application K w/o RICH γ Hadron masses calculated from the measured cherenkov angle ch Separation between π, K and p in the momentum range ~5 GeV/c π, K π -, K - veto for proton antiproton candidate RICH app. Likelihood methods are used to reject π and K for proton candidate in the decay of pπ and pπ

21 Extraction of transverse polarization Quantization axis for transverse polarization M. Anselmino & F. Murgia, hysics Letters B initial uak spin : component of target spin perpendicular to γ* final uark spin : symmetric of the w.r.t. the normal to the scattering plane If fragments into hyperon, the measurement of polarization w.r.t. reveals information about the initial uark polarization in the nucleon

22 Acceptance cancellation hree target H 3 cells with weekly reversal target polarizaiton in 7 : eriod 1. eriod. Acceptance cancellation by geometrical mean method : extraction from data itself using up-down symmetry of angular distribution unnecessary a MC to correct acceptance additional symmetries two target polarizations & two configurations

23 Asymmetry with recombination of data samples : 1 1 Acc Acc Acc Acc ] [ ] [ ] [ ] [ ε Assumption : constant acceptance & target polarization 1 hree target H 3 cells with weekly reversal target polarizaiton in 7 : eriod 1. eriod. Acceptance cancellation cos 1 ' α Acc # of s :

24 Acceptance cancellation hree target H 3 cells with weekly reversal target polarizaiton in 7 : eriod 1. eriod. Acceptance terms are canceled out in the asymmetry, leaving only the terms proportional to the polarization ε α cos need counting rate from 8 different data samples to extract polarization

25 ransverse & polarization reliminary COMASS 7 transverse proton data reliminary COMASS 7 transverse proton data x Bj x Bj reliminary COMASS 7 transverse proton data z reliminary COMASS 7 transverse proton data z.. 74σ stat Systematic errors have been estimated to be smaller than statistical errors : sys. σ

26 Interpretation of results upper limit global fit HERMES / COMASS / BELLE combined results for collins asymmetry For proton target a positive Δ x is expected : Δ ux 1 Δ dx Q.4 GeV/c Δ z seems to be small in < z <.5 : nearly no analyzing power x Bj Anselmino et al: hys. Rev. 75, 543 7

27 Interpretation of results x,z solid : z.6:1 dash : z.3:1 pqc Quark-diuark HERMES / COMASS / BELLE combined results for collins asymmetry For proton target a positive Δ x is expected : Δ ux 1 Δ dx > x,z pqc Quark-diuark x Bj Δ z seems to be small in < z <.5 : nearly no analyzing power pqc model : Δ dx / dx is positive Quark-diuark model : Δ dx / dx is negative solid : x Bj.6:1 dash : x Bj.3:1 Bo-Qiang. Ma et al: hys. Rev. 64, z eed extended kinematics : x Bj >.1 and z >.5

28 Conclusion & outlook Longitudinal polarization First measurement of dependence of the longitudinal polarization on the target polarization spin transfer : LL LL no significant dependence on target pol. is found for and work in progress with 6 proton data ransverse polarization First measurement of transversity via transverse polarization from transversely polarized proton target no clear x Bj and z dependence with proton target Δ z seems to have no analyzing power in COMASS kinematics data will allow to reduce the statistical error by factor