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James L. DrachenbergFor the STAR Collaboration
October 1, 2013
OUTLINE• Introduction• Event Selection• Analysis Description• Systematic Uncertainties• Preliminary Results• Conclusions
Transverse Single-spin Asymmetries fromp↑ + p jet + X⟶ and p↑ + p jet + π⟶ ± +X
at √s = 500 GeV at RHIC
STAR
Transverse-spin Jet Asymmetries - Drachenberg 2
Contributions to Proton Spin Structure
Consider proton moving right
Δq(x)Δg(x)
Proton spin
Spin sum rule:
Polarized DIS: ~ 0.3
coming into focus:
h1(x)
Proton spin
Transversity – data over limited kinematic range: xBj ≤ 0.3
poorly constrained
arXiv:nucl-ex/1304.0079
Transverse-spin Jet Asymmetries - Drachenberg 3
Separate Sivers and Collins:Go beyond inclusive production - e.g. Jets, correlations, direct photons
Sivers ~ sin(φS)Collins ~ sin(φS-φh)φS—angle between spin and event plane φh—angle of hadron around jet axis
Sensitivity to Orbital Motion and Transversity?
⟨SP (⋅ p × kT,parton) ⟩ ≠ 0 ⟨Sq (⋅ p × kT,π) ⟩ ≠ 0
Sivers mechanism: asymmetry in the forward jet or γ production
SPkT,parton
p
p
Collins mechanism: asymmetry in the forward jet fragmentation
SP
p
p
Sq kT,π
Sensitive to proton spin– parton transverse motion correlations (needs Lz)
Sensitive to transversity (h1)
π± Kinematic Variablesz = pπ / pjet
jT (kT, π) = π pT relative to jet axis
Transverse-spin Jet Asymmetries - Drachenberg 4
Transverse Jet Asymmetries
Φh
–pbeam
pbeamS⊥
pπ
PJET
jT
ΦS
F. Yuan, PRL 100, 032003 (2008)D’Alesio et al., PRD 83, 034021 (2011)
Transverse Momentum Dependent (TMD) Approach
Asymmetry moments sensitive to various contributions
(analogous moments sensitive to gluon scattering)
AUT – Transverse single-spin asymmetry (also written AN)
Transversity • Boer-Mulders • FF
Pretzelocity • Boer-Mulders • FF
Sivers • Boer-Mulders • Collins
Transverse-spin Jet Asymmetries - Drachenberg 5
Transversity • Boer-Mulders • FF
Pretzelocity • Boer-Mulders • FF
Sivers • Boer-Mulders • Collins
Transverse Jet Asymmetries
Φh
–pbeam
pbeamS⊥
pπ
PJET
jT
ΦS
F. Yuan, PRL 100, 032003 (2008)D’Alesio et al., PRD 83, 034021 (2011)
Transverse Momentum Dependent (TMD) Approach
Asymmetry moments sensitive to various contributions
(analogous moments sensitive to gluon scattering)
AUT – Transverse single-spin asymmetry (also written AN)
Transverse-spin Jet Asymmetries - Drachenberg 6
Phys. Rev. D 86 32006 (2012)
STAR Transverse Jet Asymmetries at Central Pseudorapidity
STAR measured transverse single-spin asymmetries for inclusive jet production at central
pseudorapidity and √s = 200 GeV (2006)
: consistent with zero
: hints of non-zeroasymmetry with charge-sign dependence
Similarly, di-jet and π0 AN at central psuedorapidityand 200 GeV consistent with zero
PRL 99, 142003, PRL. 95, 202001, arXiv:hep-ex/1108.0911
R. Fersch, DNP2011
Collins Asymmetry A 2<sin(ϕ≣ S – ϕh)> vs. jTCollins Asymmetry A 2<sin(ϕ≣ S – ϕh)> vs. z
0.2 0.4 0.6 0.8z
-0.1
0.1
2<si
n(ϕ
S –
ϕh)>
10-1 1 10jT (GeV)
Transverse-spin Jet Asymmetries - Drachenberg 7
STAR Transverse Jet Asymmetries at Central Pseudorapidity
2012 STAR data provide opportunity for higher precision and greatly reduced systematic uncertainties at √s = 200 GeV
2011 STAR data provide opportunity for first measurements of central pseudorapidity
inclusive jet asymmetries at √s = 500 GeV Increased sensitivity to gluonic subprocesses
pT [GeV/c]Aschenauer, JLab UGM 2013
R. Fersch, DNP2011
Transverse-spin Jet Asymmetries - Drachenberg 8
Moments of Jet Asymmetries at 500 GeV
Various contributions to polarized jet+π cross section (TMD approach)
Phys. Rev. D 83, 034021 (2011)
Transverse-spin Jet Asymmetries - Drachenberg 9
Phys. Rev. D 83, 034021 (2011)
Moments of Jet Asymmetries at 500 GeV
Negligible under maximized scenario!
Various contributions to polarized jet+π cross section (TMD approach)
Transverse-spin Jet Asymmetries - Drachenberg 10
Phys. Rev. D 83, 034021 (2011)
Moments of Jet Asymmetries at 500 GeV
Possible non-zero contributions, expected to be quite small
e.g. Phys. Rev. Lett. 95, 202001 (2005);Phys. Rev. Lett 99, 142003 (2007);Phys. Rev. D 86, 032006 (2012);
arXiv:hep-ex/1108.0911 projections for √s = 500 GeV
D’Alesio et al.
Various contributions to polarized jet+π cross section (TMD approach)
Transverse-spin Jet Asymmetries - Drachenberg 11
Moments of Jet Asymmetries at 500 GeV
“Collins-like” asymmetry:Sensitive to linearly polarized gluons
Completely unconstrained!
Phys. Rev. D 83, 034021 (2011)
Maximized contributionfor √s = 500 GeV
D’Alesio et al.
Various contributions to polarized jet+π cross section (TMD approach)
Gluon helicity density matrix
Off-diagonal terms related to linear polarization in (xy) plane at angle φ to x-axis
Phys Rev. D 73, 014020 (2006)
Transverse-spin Jet Asymmetries - Drachenberg 12
Solenoidal Tracker at RHIC
Jet reconstruction utilizesTPC + Barrel + Endcap EMC
Minbias Trigger: coincidence invertex position detector (VPD)
- Pb+plastic scintillator with PMT readout- 570 cm from interaction point- 4.24 < |η| < 5.1,
J. Zhou, M.S. Thesis
Transverse-spin Jet Asymmetries - Drachenberg 13
Jet Reconstruction in STAR
Anti-kT algorithmJHEP 0804, 063 (2008)
Radius parameter R = 0.6
Use PYTHIA + GEANT to quantify detector response
STAR Di-jet event at detector-level
e-
e+
Det
ecto
r
GE
AN
TP
YT
HIA
gq,
Data jets MC jets
Par
ticl
eP
arto
n
π± Kinematic Variablesz – π momentum / jet momentumjT – π pT relative to jet axis
Transverse-spin Jet Asymmetries - Drachenberg 14
2011 STAR Event Selection
• Center of mass energy, √s = 500 GeV• Total integrated luminosity of 25 pb-1 for all transverse runs• Polarization for present analysis: 54±3%• Good event vertex with |zvertex| < 30 cm• Jet Reconstruction:
- Nhits, track > 12- Track association with event vertex- Minimum track pT > 0.2 GeV/c- Minimum tower energy > 0.2 GeV
• Jet requirements:- No charged particles with pT > 30 GeV/c- Total charged-particle pT > 0.5- Neutral-energy fraction < 0.98- |ηjet| < 1.0 (and within detector acceptance of BEMC+TPC)- 6 < pT,jet < 55 GeV/c
• Pion Requirements:- Restriction on dE/dx: -1 < nσ(π) < 2.5 (contamination < 3%)- (ηjet-ηch)2 + (φjet-φch)2 > (0.1)2
- Nhits, dE/dx > 5
Transverse-spin Jet Asymmetries - Drachenberg 15
Studies with Embedded Monte CarloSystematic shifts in reconstructed kinematic variables and azimuthal resolutions evaluated by comparing to
Monte Carlo embedded in real zero-bias datasimulate effects of real, out-of-time pile-up
Detector-level jets matched both toparton and particle jets with matched event vertex
Kinematic shifts evaluated at particle levelCorresponding parton-jet pT ~0.6-1.4 GeV/c lower
jet pT
track z
track jT
Transverse-spin Jet Asymmetries - Drachenberg 16
Sources of Systematic Uncertainty
Pion contamination < 3%
Dominant systematic uncertainty fromreconstructed jets unassociated with parton jets Take difference between measured AUT and AUT assuming dilution
Further systematic uncertainties:• Resolution of relevant azimuthal angles
corrected as dilution to measured asymmetry• “leak-through” of other moments of AUT• Pion contamination from K/p and e± (< 3%)• Polarization uncertainty (correlated vertical scale uncertainty, σ(P)/P = 5.6%)
Angular resolution corrected as dilution
Transverse-spin Jet Asymmetries - Drachenberg 17
Asymmetry Measurements
For uniform acceptance:
α and β yields are functions of integrated over
half of detector
Transverse-spin Jet Asymmetries - Drachenberg 18
Estimating Leak-through Systematic
Acceptance inefficiencies can allow non-zero effects to “leak through” and distort desired observable
Quantify distortion by event-weighting unpolarized data yields and calculating resulting asymmetries
Reasonable acceptance + small observed asymmetries Small upper-limits on “leak-through” systematic
0.2 < zreco < 0.3Assuming 10% Sivers asymmetry
For 10%, upper-limit ~ 0.0021For 2%, upper-limit ~ 0.0004
Transverse-spin Jet Asymmetries - Drachenberg 19
Sivers Asymmetries
No sign of sizable azimuthal asymmetry in jet production at √s = 500 GeVConsistent with expectation from
inclusive jets, di-jets, and neutral pions at √s = 200 GeV
Asymmetries shown as function of particle-jet pT
Corresponding parton-jet pT lower by 0.6-1.4 GeV/c
Horizontal errors include uncertainties from
statistics, calorimeter gains, efficiencies, track
momentum, and tracking efficiency
pT [GeV/c]
Transverse-spin Jet Asymmetries - Drachenberg 20
Sivers Asymmetries
Measured asymmetries shown in η-binsNo sign of sizable asymmetry
Asymmetries shown as function of particle-jet pT
Corresponding parton-jet pT lower by 0.6-1.4 GeV/c
Horizontal errors include uncertainties from
statistics, calorimeter gains, efficiencies, track
momentum, and tracking efficiency
pT [GeV/c]
Transverse-spin Jet Asymmetries - Drachenberg 21
Collins AsymmetriesIncreased gluonic subprocesses
at √s = 500 GeV lead to expectation of small Collins
asymmetry until larger z
Present data do not have sufficient statistics at high-z to observe Collins asymmetry of order 1%
D’Alesio et al.
Transverse-spin Jet Asymmetries - Drachenberg 22
Collins Asymmetries
Present model predictions expect negligible effects forAUT vs. jT integrated over z > 0.1
Measured asymmetries shown for xF > 0 (i.e. 0 < ηjet < 1) in z-binsNo sign of non-zero asymmetry as a function of jT or jet pTSimilarly, no sign of positive effect for backward region (xF < 0), as expected
Transverse-spin Jet Asymmetries - Drachenberg 23
Collins-like Asymmetries
Maximized contributionfor √s = 500 GeV
Model predictions shown for “maximized” effect, saturated to positivity boundUntil now, Collins-like asymmetries completely unconstrained
Sensitive to linearly polarized gluons
D’Alesio et al.
Transverse-spin Jet Asymmetries - Drachenberg 24
Collins-like Asymmetries
Measured asymmetries shown for -1 < η < 1 in z-binsConsistently below 2% maximum from modelD’Alesio et al.
Transverse-spin Jet Asymmetries - Drachenberg 25
Collins-like Asymmetries
Similarly, no large effect observed as a function of jet pT
Measured asymmetries shown for -1 < η < 1 in z-bins
Transverse-spin Jet Asymmetries - Drachenberg 26
Summary
• STAR data from 2011 provide first measurements of transverse spin asymmetries for jet production at central pseudorapidity and √s = 500 GeV
Transverse-spin Jet Asymmetries - Drachenberg 27
Summary
• STAR data from 2011 provide first measurements of transverse spin asymmetries for jet production at central pseudorapidity and √s = 500 GeV
• Increased gluonic subprocesses at √s = 500 GeV provide unique sensitivity to Collins-like asymmetry from linearly polarized gluons
Transverse-spin Jet Asymmetries - Drachenberg 28
Summary
• STAR data from 2011 provide first measurements of transverse spin asymmetries for jet production at central pseudorapidity and √s = 500 GeV
• Increased gluonic subprocesses at √s = 500 GeV provide unique sensitivity to Collins-like asymmetry from linearly polarized gluons
• No observation of large asymmetry from gluon Sivers effect, consistent with expectations from previous measurements of jet and π0 asymmetries at √s = 200 GeV
Transverse-spin Jet Asymmetries - Drachenberg 29
Summary
• STAR data from 2011 provide first measurements of transverse spin asymmetries for jet production at central pseudorapidity and √s = 500 GeV
• Increased gluonic subprocesses at √s = 500 GeV provide unique sensitivity to Collins-like asymmetry from linearly polarized gluons
• No observation of large asymmetry from gluon Sivers effect, consistent with expectations from previous measurements of jet and π0 asymmetries at √s = 200 GeV
• No observation of large asymmetry from Collins effect, consistent with expectations from decreased quark subprocesses at √s = 500 GeV
Transverse-spin Jet Asymmetries - Drachenberg 30
Summary
• STAR data from 2011 provide first measurements of transverse spin asymmetries for jet production at central pseudorapidity and √s = 500 GeV
• Increased gluonic subprocesses at √s = 500 GeV provide unique sensitivity to Collins-like asymmetry from linearly polarized gluons
• No observation of large asymmetry from gluon Sivers effect, consistent with expectations from previous measurements of jet and π0 asymmetries at √s = 200 GeV
• No observation of large asymmetry from Collins effect, consistent with expectations from decreased quark subprocesses at √s = 500 GeV
• First measurements of Collins-like asymmetries will help place constraints on model predictions beyond maximal projections from positivity bounds
Transverse-spin Jet Asymmetries - Drachenberg 31
Looking Forward
• Additional data from 2011 jet-patch triggers will allow much greater precision for higher jet pT (above 7 GeV/c)
• High precision measurements from 2012 at √s = 200 GeV will allow greater sensitivity to effects from quark subprocesses
STAR Preliminary Results at √s = 500 GeVSivers Asymmetries Collins Asymmetries Collins-like Asymmetries
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