James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection...

31
James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary Results Conclusions Transverse Single-spin Asymmetries from p + p jet + X and p + p jet + π ± +X at s = 500 GeV at RHIC STAR

Transcript of James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection...

Page 1: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 2: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 3: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 4: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

Transverse-spin Jet Asymmetries - Drachenberg 4

Transverse Jet Asymmetries

Φh

–pbeam

pbeamS⊥

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

Page 5: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

Transverse-spin Jet Asymmetries - Drachenberg 5

Transversity • Boer-Mulders • FF

Pretzelocity • Boer-Mulders • FF

Sivers • Boer-Mulders • Collins

Transverse Jet Asymmetries

Φh

–pbeam

pbeamS⊥

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)

Page 6: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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)

Page 7: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 8: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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)

Page 9: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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)

Page 10: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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)

Page 11: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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)

Page 12: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 13: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 14: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 15: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 16: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 17: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

Transverse-spin Jet Asymmetries - Drachenberg 17

Asymmetry Measurements

For uniform acceptance:

α and β yields are functions of integrated over

half of detector

Page 18: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 19: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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]

Page 20: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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]

Page 21: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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.

Page 22: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 23: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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.

Page 24: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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.

Page 25: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 26: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 27: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 28: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 29: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 30: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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

Page 31: James L. Drachenberg For the STAR Collaboration October 1, 2013 OUTLINE Introduction Event Selection Analysis Description Systematic Uncertainties Preliminary.

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