Recent transverse spin results from STAR

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1 1 STAR! Recent transverse spin results from STAR Qinghua Xu, Shandong University PanSpin215, Taipei, October 8, 215!

2 Outline 2 Introduction Single spin asymmetries in the forward region Mid-rapidity hadron-jet correlations (Transversity+Collins) Di-hadron spin asymmetries (Transversity+IFF) Single spin asymmetries for weak bosons (Sivers) Summary & outlook

3 3 RHIC- the first polarized pp collider in the world Absolute Polarimeter (H jet) RHIC pc Polarimeters Siberian Snakes PHENIX 6% to 55% polarization s low to high (51 GeV) STAR Siberian Snakes Spin Rotators (longitudinal polarization) Pol. H - Source Solenoid Partial Siberian Snake LINAC 2 MeV Polarimeter BOOSTER Rf Dipole AGS Spin Rotators (longitudinal polarization) Helical Partial Siberian Snake AGS Internal Polarimeter AGS pc Polarimeters Strong Helical AGS Snake Spin flipper Data sets for transversely polarized pp

4 STAR - Solenoid Tracker At RHIC Magnet.5 T Solenoid Triggering & Luminosity Monitor Beam-Beam Counters 3.4 < η < 5. Zero Degree Calorimeters Vertex Position Detector Central Tracking Large-volume TPC η < 1.3 Calorimetry Barrel EMC (Pb/Scintilator) η < 1. Endcap EMC (Pb/Scintillator) 1. < η < 2. Forward Meson Spectrometer(Pb/Glass) 2.5 < η < 4. Tai Sakuma, Thesis, MIT (21) FMS 4

5 5 Mechanisms for Transverse Single-spin Asymmetries Anomalously large A N observed for nearly 4 years: Sivers effect (Sivers 9): parton spin and k correlation in initial state (related to orbital angular momentum) Collins effect (Collins 93): quark spin and k T correlation in fragmentation process (related to transversity) Need two scales (Q and p T ), Q>>p T Twist-3 mechanism (Efremov-Teryaev 82, Qiu-Sterman 91): Collinear/twist-3 quark-gluon correlation + fragmentation functions Need one scale (Q or p T ), Q, p T >>Λ QCD Both mechanisms apply when Q>>p T >>Λ QCD Ji-Qiu-Vogelsang-Yuan,26

6 π A N in the forward region Rising A N with X F A N nearly independent of s A N persist at high p T, no falling evidence. CIPANP212, S. Heppelmann 6

7 Detailed structure for forward π A N A N for forward EM events: dependence on jettiness : 1-photon events, which include a large π contribution, are similar to 2-photon events Three-photon jet-like

7 7 Detailed structure for forward π A N A N for forward EM events: dependence on jettiness : 1-photon events, which include a large π contribution, are similar to 2-photon events Three-photon jet-like events have a clear non-zero asymmetry, but substantially smaller than that for isolated π s A N decreases as the event complexity increases(i.e., the "jettiness ) M. Mondal DIS data, 22 pb at 5 GeV, anti-k T jet algorithm on FMS photons, R =.7

8 8 Detailed study for forward π A N EM Jets with isolated π have larger asymmetry. Y. Pan, SPIN 214

$A N of isolated π are lower when there is a correlated away -side jet M. Mondal DIS 214 How much of the large forward π A N comes from partonic scattering? ->diffractive effect?$

9 Detailed study for forward π A N EM Jets with isolated π have larger asymmetry. Y. Pan, SPIN 214 A N for correlated central jets and no central jet cases. A N of isolated π are lower when there is a correlated away -side jet M. Mondal DIS 214 How much of the large forward π A N comes from partonic scattering? ->diffractive effect? New Roman pots at STAR 9

h 1 (x) " H 1 # (z, j T ) Collins angle: Φ c =Φ s -Φ h Collins modulation: sin(φ s -Φ h

10 1 Mid-rapidity hadron-jet correlations (Collins) Study proton transversity through its coupling to Collins function: Collins asymmetries: A UT! h 1 (x) " H 1 # (z, j T ) Collins angle: Φ c =Φ s -Φ h Collins modulation: sin(φ s -Φ h ) j T transverse momentum in jet Φ s : azimuthal angle of beam spin Φ h : azimuthal angle of hadron -F.Yuan, PRL1,323

11 11 Mid-rapidity hadron-jet correlations (Collins) Non-zero Collins asymmetries observed from run212 2 GeV J.K. Adkins, SPIN data, 2 pb at 2 GeV, Pb=61%, anti-k T jet algorithm, R =.7

12 Collins asymmetries at 5 GeV 12 Zero asymmetry at 5 GeV? High Q 2? J. Drachenberg, PANIC data, 25 pb at 5 GeV, anti-k T jet algorithm, R =.7

Collins asymmetries at 5 GeV 13 Comparison of Collins asymmetries at 2 & 5 GeV: Kinematics are dictated by the ΔR min cut: arxiv:151.

13 Collins asymmetries at 5 GeV 13 Comparison of Collins asymmetries at 2 & 5 GeV: Kinematics are dictated by the ΔR min cut: arxiv: Collins asymmetry depends on j T and z correlation. Consistent results at both 2 and 5 GeV with same j T! The higher j T asymmetries (bottom) both go away. Higher Q 2 and same x?

14 14 Collins assymmetry for π in forward EM Jets Collins asymmetries for π with EM jets in 5 GeV pp: Hints of possible non-zero Collins asymmetries!

15 Collins-like Asymmetries at 5 GeV sin(! Collins-like asymmetry A s "2! H ) UT, provide sensitivity to gluon linear polarization. First measurement on sin(! A s "2! H ) UT J. Drachenberg, PANIC 214 z U. D Alesio, F. Murgia, and C. Pisano, PRD 83, 3421 (211); z

16 16 Di-hadron spin asymmetries at STAR Di-hadron correlation provides access to IFF& transversity: Interference Fragmention Function Bacchetta, Radici, Phys.Rev. D7 (24)

17 17 Di-hadron spin asymmetries at STAR Significantly non-zero di-hadron asymmetries from run6 data: STAR, arxiv: STAR, arxiv: Sign of non-zero signal for di-hadron transverse single spin asymmetries-> constraints on transversity!

18 18 Di-hadron spin asymmetries at STAR Significant, high-precision di-hadron asymmetries with 212 data: Significant non-zero IFF observed also at 5 GeV:

of TMD s and TMD factorization Advantages of weak boson production Low background High Q 2 -scale (~ W/Z boson

19 Single spin asymmetry of weak boson Sivers sign change in DIS and DY/W/Z process: DIS: γq scattering attractive FSI pp: q q annihilation repulsive ISI Sivers DIS = - Sivers (DY or W or Z) -Critical test for our understanding of TMD s and TMD factorization Advantages of weak boson production Low background High Q 2 -scale (~ W/Z boson mass) STAR goal: measure sign change and pin down TMD -evolution by measuring A N for all the processes: γ, W ±, Z, DY 19

20 The effects from TMD evolution Z.-B. Kang & J.-W. Qiu, PRL13, 1721(29) before evolution A N.2 1x smaller Very strong TMD evolution effects size of the asymmetry, still under discussion in theory community. Strong dependence on y W -> need fully reconstruction of W kinematics! M. Echevarria et al, PRD89, arxiv: W - after evolution y A N after evolution before evolution W y 2

Strategy to reconstruct W kinematics at STAR 21 W boson momentum reconstruction technique in h-h collision, has been well tested at Fermi Lab and LHC CDF: PRD 7, 324

21 Strategy to reconstruct W kinematics at STAR 21 W boson momentum reconstruction technique in h-h collision, has been well tested at Fermi Lab and LHC CDF: PRD 7, 324 (24); ATLAS: JHEP 112 (21) 6 Select events with the W-signature (Step 1) Isolated high P T electron Neutrino transverse momentum is reconstructed from missing P T (Step 2)! P W T = P! e T + P!" T = # P! recoil T The STAR detector Recoil reconstructed using tracks and towers, MC correction applied to incorporate those outside of STAR acceptance. Neutrino s longitudinal momentum is reconstructed from the decay kinematics (Step 3) ( ) 2 # ( p! e + p! " ) 2 M W 2 = E e + E " TPC ( η < 1.3) Barrel EMCAL ( η < 1)

22 First W, Z A N results at 5 GeV from STAR Data: STAR 211 transverse 5 GeV, integrated luminosity ~25 pb Average beam polarization P = 53% Systematics estimated based Monte Carlo package Left-right cross ratio formula for A N : A N for W ± and Z : A N " 1 N # R N $ L % N # $ L N R P N # R N $ L + N # $ L N R A N syst.(%) STAR preliminary W STAR p-p 5 GeV L = 25 pb y < 1 3.4% beam pol. uncertainty not shown + - W + l - l % W 1 P T A N rel. syst.(%) STAR Preliminary S. Fazio and D. Smirnov, DIS214 W STAR p-p 5 GeV L = 25 pb.5 < P T < 7 GeV 3.4% beam pol. uncertainty not shown + - W + l - l y W.6 A N Z l l STAR preliminary STAR p-p 5 GeV L = 25 pb < P T < 25 GeV 3.4% beam pol. uncertainty not shown y Z

23 23 Future measurements of W/Z A N at STAR STAR plans to collect ~4 pb transverse pp in 217: A N rel. syst.(%) W - + W - - W - - W - L(del.) = 9 pb L(del.) = 4 pb L(del.) = 9 pb L(del.) = 4 pb % W P T Goal: A N rel. syst.(%) W - + W - - W - - W - L(del.) = 9 pb L(del.) = 4 pb L(del.) = 9 pb L(del.) = 4 pb y W.6 S. Fazio - DIS 215 Constrain TMD evolution sea-quark Sivers function A N Z - L(del.) = 9 pb Z - L(del.) = 4 pb y Z Test sign-change if TMD-evolution suppression factor ~5 or less

Future measurements of photon at STAR 24 Projection for direct asymmetry with 4 pb pp data at STAR: arxiv:151.

24 Future measurements of photon at STAR 24 Projection for direct asymmetry with 4 pb pp data at STAR: arxiv: Observables with DY for Sivers sign-change: Z.-B. Kang & J.-W. Qiu PRD81,542(21) before evolution 5 GeV 2 GeV M. Echevarria et al, PRD89, arxiv: after evolution DY 4 < Q < 9 GeV < q T <1 GeV

Accessing the gluon GPD in pau Unique opportunity to measure A N for exclusive J/ψ in ultra peripheral p Au collisions: arxiv:151.122 S. Klein, J.

25 Accessing the gluon GPD in pau Unique opportunity to measure A N for exclusive J/ψ in ultra peripheral p Au collisions: arxiv: S. Klein, J.Nystrand, hep-ph/ possible process MC simulation Detect the scattered proton in Roman Pots and veto the break-up of Au A nonzero asymmetry would be the first signature of a nonzero GPD E for gluons. 25

26 Transverse pa run in Successful pa run at STAR in 215 Why pa? Saturation effects-cgc Nuclear effect of A N Distinguish different mechanisms for A N Exclusive J/Ψ for GPD... 17M ultra-peripheral pau collisions with Roman Pots pau data with FMS FoM: P 2 L~16nb pal with FMS FoM: P 2 L~27nb

27 Summary & Outlook Exploration of transverse spin asymmetries at STAR: Single spin asymmetries in the forward region observed, π asymmetry with different event topology studied. Observation of non-zero Collins asymmetries at 2GeV & 5GeV. Non-zero di-hadron spin asymmetries (IFF) observed. First measurements of weak boson A N, gain insights into TMD evolution and sea quark Sivers function. Transversely polarized pa data taken for the first time in 215 A N asymmetries in p+a Exclusive J/φ in ultra peripheral pau-> gluon GPD Long transverse run planned for 217 at 5 GeV A N for W,Z, gamma-> TMD evolution, Sivers sign change. Future measurements for DY, forward di-jet in 22+ with dedicated detector upgrades. 27

28 28

Contributions to our Understanding of TMDs from Polarized Proton Collisions at STAR

Contributions to our Understanding of TMDs from Polarized Proton Collisions at STAR Stephen Trentalange University of California at Los Angeles, for the STAR Collaboration QCD-N16 Bilbao, Spain July 15,