Measurement of Transverse Spin Transfer of and in Transversely - PowerPoint PPT Presentation

Measurement of Transverse Spin Transfer of Λ and Λ 
 in Transversely Polarized Proton+Proton Collisions at RHIC-STAR Jincheng Mei (for STAR Collaboration) Shandong University 1 DIS17, Jincheng Mei

Outline ✦ Motivation ✦ Experiment Setup ✦ ( ) Reconstruction Λ ¯ Λ ✦ Measurement Method ✦ D TT Measurement Results ✦ Summary 2 DIS17, Jincheng Mei


 
 
 Motivation ✦ Transversely polarized cross section of hyperons could be parameterized into three parts: 
 Xu, Liang, Sichtermann. 
 Phys.Rev. D73 (2006) 077503 transversity pQCD transversely polarized distribution calculation fragmentation function Crucial elements for the study of nucleon spin structure: 
 transversity distribution and transversely polarized fragmentation ✦ The transverse spin transfer is defined as: ✦ The transverse spin transfer provides insights into transversely polarized fragmentation function and transversity distribution function. 3 DIS17, Jincheng Mei


 Frames and Azimuthal Distributions ✦ ( ) polarization can be extracted from the 
 Λ ¯ Λ S angular distribution of its decay product in its rest p θ * frame. π - 𝛭 rest frame α : decay parameter 
 θ * : the angle between decay particle 
 ¯ Λ Λ 0.642 for and -0.642 for (proton) momentum in 𝛭 ’s rest frame 
 Λ ¯ Λ P 𝛭 : polarization of ( ) and the direction of 𝛭 polarization In transverse case, azimuthal angle of polarization needs to be determined which is not needed in longitudinal case. ✦ D TT : the spin transfer along the polarization of D TT outgoing quark considering the rotation in scattering plane. 
 In this analysis, reconstructed jet axis is used as 
 substitute of the outgoing quark momentum direction 
 J.Collins et al, NPB420 (1994)565 4 DIS17, Jincheng Mei

RHIC - Relativistic Heavy Ion Collider ✦ The Relativistic Heavy Ion Collider (RHIC) is the first and only polarized proton collider in the world. ✦ Data sample: transversely polarized p+p collisions at 200GeV taken with STAR detector in 2012. ✦ RHIC Beam polarization: blue beam: 64%, yellow beam: 58%. 5 DIS17, Jincheng Mei

STAR Detectors STAR experiment detectors BEMC TPC Sub detectors involved in this analysis: ✦ TPC : Time Projection Chamber 𝞱 ~ (-1.2, 1.2) 
 Track reconstruction of charged particles and charged particle identification. ✦ BEMC: Barrel Electromagnetic Calorimeter 𝞱 ~ (-1.0, 1.0) 
 For triggering. 6 DIS17, Jincheng Mei

and Reconstruction ¯ Λ Λ ✦ Applied topological cuts to reconstruct and via their decay channels: and ¯ ¯ p π + Λ Λ Λ → p π − Λ → ¯ × 3 10 200 Central : 1.1160 GeV Central : 1.1157 GeV Λ Λ , p :4~5 GeV/c Width : 0.0034 GeV , p :2~3 GeV/c Width : 0.0021 GeV 16000 180 T Bkg Frac. : 0.0660 T Bkg Frac. : 0.0716 14000 160 140 12000 120 10000 100 8000 80 6000 60 4000 40 2000 20 0 0 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 mass[GeV] mass[GeV] × 3 10 200 Central : 1.1157 GeV Λ , p :2~3 GeV/c Width : 0.0020 GeV 180 T Bkg Frac. : 0.0812 160 140 120 100 80 60 40 20 0 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 mass[GeV] 7 DIS17, Jincheng Mei

and Reconstruction ¯ Λ Λ ✦ Applied topological cuts to reconstruct and via their decay channels: and ¯ ¯ p π + Λ Λ Λ → p π − Λ → ¯ × 3 ¯ Λ Λ 10 Raw yields of and after selection 200 Central : 1.1157 GeV Λ , p :2~3 GeV/c Width : 0.0021 GeV P T [GeV/c] ¯ 180 Λ Λ T Bkg Frac. : 0.0716 160 1~2 490 k 544 k 140 2~3 323 k 382 k 120 3~4 183 k 201 k 100 4~5 78 k 74 k 80 5~6 32 k 26 k 60 6~8 20 k 14 k 40 Total 1126 k 1242 k 20 0 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 mass[GeV] × 3 10 200 Central : 1.1157 GeV Λ , p :2~3 GeV/c Width : 0.0020 GeV 180 T Bkg Frac. : 0.0812 160 140 120 100 80 60 40 20 0 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 mass[GeV] 8 DIS17, Jincheng Mei

and Reconstruction ¯ Λ Λ ✦ Applied topological cuts to reconstruct and via their decay channels: and ¯ ¯ p π + Λ Λ Λ → p π − Λ → ¯ × 3 ¯ 10 Raw yields of and after selection Λ Λ 200 Central : 1.1157 GeV Λ , p :2~3 GeV/c Width : 0.0021 GeV P T [GeV/c] ¯ 180 Λ Λ T Bkg Frac. : 0.0716 160 1~2 490 k 544 k D raw 140 2~3 323 k 382 k 120 3~4 183 k 201 k 100 D bkg 4~5 78 k 74 k 80 5~6 32 k 26 k 60 6~8 20 k 14 k 40 Total 1126 k 1242 k 20 0 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 mass[GeV] × 3 10 Background fraction is estimated by side bands. 200 Central : 1.1157 GeV Λ , p :2~3 GeV/c Width : 0.0020 GeV 180 T Bkg Frac. : 0.0812 The obtained D raw and their statistical uncertainties 160 were corrected for the residual background: 140 − rD bkg 120 D T T = D raw T T T T 100 1 − r 80 r ⌘ 2 T T ) 2 + ⇣ r δ D bkg ( δ D raw 60 T T δ D T T = 40 1 − r 20 r : residual background fraction 0 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 mass[GeV] 9 DIS17, Jincheng Mei

Jet Correlation ✦ Use anti-Kt algorithm to reconstruct jet then make correlation to a ( )by calculating 𝛦 R. Λ ¯ Λ ✦ Request η jet ~ (-0.7, 0.9), P T > 5.0 GeV/c, neutral fraction of Jet < 0.95. 
 If 𝛦 R < 0.6 (near side), we use the jet momentum direction as outgoing quark direction to obtain quark’s polarization direction. Λ : P T 2~3 GeV/c Λ : P T 4~5 GeV/c 10 DIS17, Jincheng Mei

Measurement Method ✦ Based on the relationship between polarization of ( ) and the angular distribution Λ ¯ Λ of its decay product in its rest frame. ✦ In this analysis, D TT is extracted from the asymmetry of 𝛭 counts with opposite beam polarization in a small cos θ * bin: P beam : polarization of beam 
 <cos θ *> : mean in each cos θ * bin 
 N ↑ − RN ↓ 1 N ↑ Λ ¯ D T T = : ( ) counts with positive beam polarization 
 Λ α P beam < cos θ ∗ > N ↑ + RN ↓ N ↓ : ( ) counts with negative beam polarization Λ ¯ Λ R: relative luminosity ✦ Acceptance of reverse beam polarization is expected to be the same in each cos θ * bin, and thus cancelled. 11 DIS17, Jincheng Mei

Extraction of Spin Transfer D TT N ↓ N ↑ counts versus cos θ * Λ with opposite beam polarization. Separate the whole range of cos θ * into 20 bins. Λ : p T 1~2 GeV/c, 𝜃 > 0 Λ : p T 1~2 GeV/c, 𝜃 > 0 negative beam polarization positive beam polarization statistical uncertainty only statistical uncertainty only D TT extraction in each cos θ * D TT for Λ : P T 1~2 GeV/c η D ( L |P :1~2GeV|phy. >0|ALL ) TT N ↑ − RN ↓ T 1 D T T = TT 0.6 χ χ 2 2 / ndf / ndf 21.91 / 19 21.91 / 19 α P beam < cos θ ∗ > N ↑ + RN ↓ D − − ± ± p0 p0 0.008204 0.008204 0.008131 0.008131 0.4 0.2 0 Fit the 20 D TT with a constant. − 0.2 Fitted result as the D TT in the p T bin. − 0.4 statistical uncertainty only − − 1 0.5 0 0.5 1 The method passed the null check with θ cos * 12 DIS17, Jincheng Mei

Systematic Uncertainty ✦ 3.4% scale uncertainty from RHIC beam polarization measurement. 
 —obtained from the polarimetry group of RHIC ✦ 2% from decay parameter (0.642 ± 0.013). 
 —PDG ✦ 0.012 from relative luminosity measurement. ✦ Residual background fraction estimation. 
 —Compare two methods of residual background fraction ( fit and side-band ) ✦ Trigger bias estimated from MC simulation. ✦ Pileup effect. ✦ The sum of systematic uncertainty is range of 0.012 ~ 0.026 13 DIS17, Jincheng Mei

Results of D TT ✦ Results of transverse spin transfer D TT of and in p+p collision at 200 GeV. ¯ Λ Λ STAR Preliminary STAR Preliminary 𝜃 > 0 𝜃 < 0 3.4% beam polarization uncertainty included 3.4% beam polarization uncertainty included feed-down contribution included feed-down contribution included ✦ Most precise measurement on ( ) polarization in p+p collision at RHIC, which reach p T Λ ¯ Λ ~8 GeV/c with statistical uncertainty of 0.04 . ✦ The dominant source of systematic uncertainty is from relative luminosity in low p T . ✦ D TT of and are consistent with each other and consistent with zero at the current ¯ Λ Λ precision. 14 DIS17, Jincheng Mei

Summary ✦ First measurement on transverse spin transfer of ( ) in p+p Λ ¯ Λ collisions, which can provide insights into transversely polarized fragmentation function and nucleon transversity distribution. ✦ The ( ) sample is the largest so far in p+p collision at RHIC and Λ ¯ Λ the precision of D TT is ~0.04 at p T ~8 GeV/c for ( ) Λ ¯ Λ ✦ D TT of and are consistent with each other and consistent with ¯ Λ Λ zero at the presently available precision. 15 DIS17, Jincheng Mei

Thanks for Your Attention! 16

Backup 17 DIS17, Jincheng Mei


 Previous Results ✦ Previous measurement of transverse spin transfer. 
 -Only D NN spin transfer w.r.t. production plane (Fermilab E704 Collaboration,1997). 
 D NN : spin transfer along normal direction of 
 Λ production plane. Significant spin transfer was found at large x F . 18 DIS17, Jincheng Mei