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SPIN2016, UIUC, Jincheng Mei
Measurement of Transverse Spin Transfer of Λ and Λin Transversely Polarized Proton+Proton Collisions
at RHIC-STAR
Jincheng Mei Shandong University & Brookhaven National Laboratory
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SPIN2016, UIUC, Jincheng Mei
Outline
✦ Motivation
✦ ( ) Reconstruction
✦ Measurement Method
✦ DTT measurement results
✦ Summary
⇤ ⇤̄
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SPIN2016, UIUC, Jincheng Mei
Motivation✦ The transverse spin transfer of ( ) can provide insights into transversely polarized
fragmentation function and transversity distribution.
✦ Transversity is one of the important pdfs to describe the transverse spin structure of nucleon.
✦ ( ) polarization can be extracted from the angular distribution of its decay product in its rest frame.
transversity distribution
transverse spin transfer
transversely polarized fragmentation function
pQCD calculation
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Sp
π- ! rest frame
θ*
θ* : the angle between decay particle (proton) momentum in !’s rest frame and the direction of ! polarization
⇤ ⇤̄
⇤ ⇤̄
α: decay parameter 0.642 for and -0.642 forP!: polarization of ( )⇤ ⇤̄
⇤ ⇤̄
Xu, Qing-hua et al.Phys.Rev. D73 (2006) 077503
SPIN2016, UIUC, Jincheng Mei
✦ DTT : the spin transfer along the polarization of outgoing quark considering the rotation in scattering plane. In this analysis, reconstructed jet axis is used as substitute of the outgoing quark momentum direction
✦ Previous measurement of transverse spin transfer. -Only DNN spin transfer w.r.t. production plane (Fermilab E704 Collaboration,1997). DNN : spin transfer along normal direction of Λ production plane.
Definition of Transverse Spin Transfer
DTT
J.Collins et al, NPB420 (1994)565
Significant spin transfer was found at large xF.
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SPIN2016, UIUC, Jincheng Mei
RHIC - Relativistic Heavy Ion Collider
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✦ The Relativistic Heavy Ion Collider(RHIC) is the first and only polarized proton collider in the world.
✦ Data sample: transversely polarized P+P collision at 200GeV at STAR taken in 2012.✦ RHIC Beam polarization: blue beam: 64%, yellow beam: 58% (2012_PP200GeV)
SPIN2016, UIUC, Jincheng Mei
STAR Detectors
✦ TPC : Time Projection Chamber " ~ (-1.2, 1.2) Track reconstruction of charged particles and charged particle identification.
✦ BEMC: Barrel Electromagnetic Calorimeter " ~ (-1.0, 1.0) For triggering.
STAR experiment detectors
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SPIN2016, UIUC, Jincheng Mei
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
20
40
60
80
100
120
140
310×h_im_L_J1_pT0
Fitted mean: 1.1154
Cand.: 489832Λ
Bkg Frac.: 0.079 Signal m : 1.111 ~ 1.119
h_im_L_J1_pT0
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
10000
20000
30000
40000
50000
60000
70000
h_im_L_J1_pT1
Fitted mean: 1.1156
Cand.: 322603Λ
Bkg Frac.: 0.085 Signal m : 1.111 ~ 1.121
h_im_L_J1_pT1
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
5000
10000
15000
20000
25000
30000
h_im_L_J1_pT2
Fitted mean: 1.1158
Cand.: 182517Λ
Bkg Frac.: 0.073 Signal m : 1.109 ~ 1.123
h_im_L_J1_pT2
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
2000
4000
6000
8000
10000
12000
h_im_L_J1_pT3
Fitted mean: 1.1159
Cand.: 78022Λ
Bkg Frac.: 0.066 Signal m : 1.108 ~ 1.124
h_im_L_J1_pT3
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
500
1000
1500
2000
2500
3000
3500
4000
h_im_L_J1_pT4
Fitted mean: 1.1162
Cand.: 32480Λ
Bkg Frac.: 0.071 Signal m : 1.106 ~ 1.126
h_im_L_J1_pT4
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
h_im_L_J1_pT5
Fitted mean: 1.1166
Cand.: 20272Λ
Bkg Frac.: 0.084 Signal m : 1.104 ~ 1.128
h_im_L_J1_pT5
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
20
40
60
80
100
120
140
310×h_im_L_J1_pT0
Fitted mean: 1.1154
Cand.: 489832Λ
Bkg Frac.: 0.079 Signal m : 1.111 ~ 1.119
h_im_L_J1_pT0
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
10000
20000
30000
40000
50000
60000
70000
h_im_L_J1_pT1
Fitted mean: 1.1156
Cand.: 322603Λ
Bkg Frac.: 0.085 Signal m : 1.111 ~ 1.121
h_im_L_J1_pT1
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
5000
10000
15000
20000
25000
30000
h_im_L_J1_pT2
Fitted mean: 1.1158
Cand.: 182517Λ
Bkg Frac.: 0.073 Signal m : 1.109 ~ 1.123
h_im_L_J1_pT2
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
2000
4000
6000
8000
10000
12000
h_im_L_J1_pT3
Fitted mean: 1.1159
Cand.: 78022Λ
Bkg Frac.: 0.066 Signal m : 1.108 ~ 1.124
h_im_L_J1_pT3
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
500
1000
1500
2000
2500
3000
3500
4000
h_im_L_J1_pT4
Fitted mean: 1.1162
Cand.: 32480Λ
Bkg Frac.: 0.071 Signal m : 1.106 ~ 1.126
h_im_L_J1_pT4
mass[GeV]1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.160
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
h_im_L_J1_pT5
Fitted mean: 1.1166
Cand.: 20272Λ
Bkg Frac.: 0.084 Signal m : 1.104 ~ 1.128
h_im_L_J1_pT5✦ Applied topological cuts to reconstruct and via their decay channels: and
and Reconstruction⇤̄ ! p̄⇡+⇤ ! p⇡�⇤ ⇤̄
⇤ ⇤̄
Raw yields of and after selection
PT [GeV/c]1~2 489832 5442122~3 322603 381872
3~4 182517 2014134~5 78022 743125~6 32480 262796~8 20272 13773
Total 1125726 1241861
Λ :PT 1~2 GeV/c Λ :PT 4~5 GeV/c
Background fraction is estimated by side bands.The obtained Draw and their statistical uncertainties were corrected for the residual background:
DTT =Draw
TT � rDbkgTT
1� r
�DTT =
r(�Draw
TT )2 +⇣r�Dbkg
TT
⌘2
1� r
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⇤ ⇤̄
⇤ ⇤̄
r : residual background fraction
Draw
Dbkg
SPIN2016, UIUC, Jincheng Mei
Jet Correlation✦ Use anti-Kt algorithm to reconstruct jet then make correlation to a ( )by calculating #R.✦ Request ηjet ~ (-0.7, 0.9), PT > 5.0 GeV/c, neutral fraction of Jet < 0.95.
If #R < 0.6 (near side), we use the jet momentum direction as outgoing quark direction to obtain quark’s polarization direction.
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⇤ ⇤̄
Λ :PT 2~3 GeV/c Λ :PT 4~5 GeV/c
SPIN2016, UIUC, Jincheng Mei
Measurement Method
✦ Based on the relationship between polarization of ( ) and the angular distribution of its decay product in its rest frame. In this analysis, DTT is extracted from the asymmetry of ! counts with opposite beam polarization in a small cosθ* bin:Acceptance of reverse beam polarization is expected to be the same in each cosθ* bin, and thus cancelled.
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Pbeam : polarization of beam<cosθ*> : mean in each cosθ* bin : ( ) counts with positive beam polarization : ( ) counts with negative beam polarization R: relative luminosity
N"
N#⇤ ⇤̄⇤ ⇤̄
⇤ ⇤̄
DTT =
1
↵Pbeam < cos ✓⇤ >
N" � RN#
N"+ RN#
SPIN2016, UIUC, Jincheng Mei
Extraction of Spin Transfer DTT
✦ Separate the whole range of cosθ* into 20 bins.✦ Obtain a DTT in each bin. ✦ Fit the 20 DTT with a constant.✦ Fitted result as the DTT in the pT bin.
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✦ DTT extraction in each cosθ*
*θcos1− 0.5− 0 0.5 1
TTD
0.4−
0.2−
0
0.2
0.4
0.6 / ndf 2χ 21.91 / 19
p0 0.008131±0.008204 −
/ ndf 2χ 21.91 / 19
p0 0.008131±0.008204 −
>0|ALL )η:1~2GeV|phy. T
( L |PTTDDTT for Λ :PT 1~2 GeV/c
statistical uncertainty only
✦ The method passed the null check with
DTT =
1
↵Pbeam < cos ✓⇤ >
N" � RN#
N"+ RN#
Λ : pT 1~2 GeV/c, $ > 0positive beam polarization
N"
Λ : pT 1~2 GeV/c, $ > 0negative beam polarization
N#
SPIN2016, UIUC, Jincheng Mei
✦ 1.2% scale uncertainty from RHIC beam polarization measurement. ✦ 2% from decay parameter (0.642 ± 0.013). ✦ 0.012 from relative luminosity measurement.✦ Trigger bias estimated from MC simulation.✦ Residual background fraction estimation.✦ Pileup effect.
Systematic Uncertainty
Systematic Uncertainty Summary$ > 0 Trigger Bias Residual Background
fractionPileup effect Total Systematic
Uncertainty
pT[GeV/c]
1-2 1.98E-04 2.07E-03 1.05E-05 8.44E-06 8.22E-04 9.57E-04 1.20E-02 1.22E-02
2-3 3.47E-03 1.11E-04 9.45E-06 0.00E+00 1.13E-03 7.07E-04 1.25E-02 1.20E-02
3-4 6.62E-03 1.95E-03 1.49E-05 2.41E-05 1.08E-03 3.09E-04 1.38E-02 1.22E-02
4-5 1.87E-03 7.58E-03 1.12E-05 1.42E-04 2.99E-03 5.78E-04 1.25E-02 1.42E-02
5-6 2.07E-03 8.01E-03 9.61E-04 1.25E-04 3.11E-03 1.05E-03 1.26E-02 1.45E-02
6-8 4.39E-03 2.28E-02 3.08E-03 1.91E-03 3.42E-03 1.89E-03 1.36E-02 2.59E-02
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⇤ ⇤̄ ⇤ ⇤̄ ⇤ ⇤̄ ⇤ ⇤̄
SPIN2016, UIUC, Jincheng Mei
DTT Results from STAR measurement
$ < 0$ > 0
✦ Most precise measurement on ( ) polarization in P+P collision at RHIC, which reach pT ~8 GeV/c with statistical uncertainty of 0.04 .
✦ The dominant source of systematic uncertainty is from relative luminosity.✦ DTT of and are consistent with each other and consistent with zero within uncertainty.
✦ Results of transverse spin transfer DTT of and in P+P collision at 200GeV.
STAR Preliminary STAR Preliminary
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⇤ ⇤̄
⇤ ⇤̄
⇤ ⇤̄
feed-down contribution included feed-down contribution included
SPIN2016, UIUC, Jincheng Mei
✦ First measurement on transverse spin transfer of ( ) in P+P collision, 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 the precision of DTT is ~0.04 at ( ), which reach pT ~8 GeV/c.
✦ DTT of and are consistent with each other and consistent with zero within uncertainty.
Summary
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⇤ ⇤̄
⇤ ⇤̄
⇤ ⇤̄⇤ ⇤̄
Thanks for Your Attention!
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SPIN2016, UIUC, Jincheng Mei
Backup
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SPIN2016, UIUC, Jincheng Mei
DTT comparison two axises
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/[GeV]T
p1 2 3 4 5 6 7 8
TTD
0.3−
0.2−
0.1−
0
0.1
0.2
> 0η , physical Λ for TTD
Jet Axis
) AxisΛ(Λ
> 0η , physical Λ for TTD
/[GeV]T
p1 2 3 4 5 6 7 8
TTD
0.3−
0.2−
0.1−
0
0.1
0.2
> 0η , physical Λ for TTD
Jet Axis
) AxisΛ(Λ
> 0η , physical Λ for TTD
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SPIN2016, UIUC, Jincheng Mei
Theory Predict
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Transverse polarization for transverse momentum pT ≧ 8 GeV/c in PP collisions at 200 GeV with one transversely polarized beam versus pseudorapidity " of the Positive " is taken along the polarized beam direction.
⇤̄
⇤̄Xu, Qing-hua et al.Phys.Rev. D73 (2006) 077503