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

Transcription

1 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,

2 TMDs from Wigner-like Distributions Momentum Position (5 dimensions) (3 dimensions) (1 dimension) (0 dimensions) QCD-N16 Bilbao, Spain July 15,

3 Polarization Effects: TMDs and FFs nucleon quark U L T U f 1 h 1 L g 1 h 1L T f 1T g 1T h 1, h 1T Sivers Transversity Possible transverse asymmetries: f 1T f 1 D 1 h 1 h 1 D 1 h 1T h 1 D 1 h 1 f 1 H 1 f 1T h 1 H 1 h 1T f 1 H 1 initial state final state hadron quark U L T U D 1 H 1 L G 1L H 1L T H 1T G 1T H 1, H 1T Inclusive hadrons Direct photons Jets Jet structure Hadron correlations Interference fragmentation Drell-Yan W-bosons Collins 3

4 Initial state: TMDs and Twist-3 TMD Twist-3 A N Sivers Function. Λ QCD << Q T /P T << Q Q T /P T Efremov, Teryaev; Qiu, Sterman or Twist-3 FF Requires 2 scales: Hard scale Q 2 Soft scale p T SIDIS, Drell-Yan, W/Z, Access the full transverse momentum dynamics k T Single hard scale: p T Appropriate for inclusive A N (π 0, γ, jet) Access the average transverse momentum <k T > d 2 k k 2 M f 11 q x, k 2 SSSSS = T q,f (x, x) QCD-N16 Bilbao, Spain July 15,

5 Separating Initial- from Final-state Effects Sivers or Twist-3 Mechanisms: Collins or Twist-3/Novel FF Mechanisms: S P k T,q S P p p p p Sensitive to proton spin parton transverse motion correlations Sensitive to transversity S q j T,π Signatures: A N for jets or direct photons A N for W +/-, Z 0, Drell-Yan A N for heavy flavor (gluon) x, k T dependences in nucleon Sivers NOT universal Sign change from SIDIS to W, Z, and Drell-Yan QCD-N16 Bilbao, Spain July 15, 2016 Signatures: Collins effect Interference fragmentation functions (IFF) A N for pions novel FF z, j T dependence within jet Collins predicted to be universal 5

6 Color interactions in QCD Controlled non-universality of the Sivers function QCD: DIS Final-state interaction Drell-Yan, W or Z Initial-state interaction Sivers DIS = Sivers Drell-Yan or Sivers W or Sivers Z A N for direct photon has related sign change in Twist-3 Critical test of factorization Opportunity to visualize the repulsive interaction between like color charges Can explore all of these observables in 500 GeV pp collisions at RHIC QCD-N16 Bilbao, Spain July 15,

7 RHIC: Polarized Proton Collider Siberian Snakes Hydrogen Jet Polarimeter Carbon Polarimeters PHENIX STAR Spin Rotators Siberian Snakes Polarized Source Tune Jump Quads Helical Partial Snake 200 MeV Polarimeter LINAC BOOSTER AGS Strong Snake AGS pc Polarimeter 7

8 The STAR Detector Forward Meson Spectrometer EndCap EMCal Time Projection Chamber Barrel EMCal EMCal coverage -1<η<4 TPC Tracking/Particle ID coverage -1.3<η<+1.3 8

9 RHIC Contribution to TMD Physics QCD-N16 Bilbao, Spain July 15,

10 RHIC and TMDs Polarized TMDs: Mechanisms like Collins, Sivers, Twist-3, Transversity can be studied at RHIC Q 2 Evolution: Collisions at 200/500 GeV, W/Z mass; Goes beyond DGLAP evolution and gives insight into nature of confinement Check universality/factorizability by comparing functions extracted from two different processes, e.g., transversity from IFF or Collins or transversity from pp and SIDIS Can check sign change of Sivers function Polarized pa collisions can use asymmetries as a probe of cold nuclear matter to investigate saturation or nuclear modification effects QCD-N16 Bilbao, Spain July 15,

11 Mid-rapidity Di-hadron Production Extract proton transversity through its coupling with chiral-odd Interference Fragmentation Function Collinear factorization is preserved 11

12 Transversity Results at s = 200, 500 GeV Significant non-zero di-hadron asymmetries at s = 500 GeV Enhanced around ρ mass M. Skoby, Spin 2014 K. Landry, APS 2015 Consistent behavior observed in s = 200 GeV when scaled for 2 p T / s QCD-N16 Bilbao, Spain July 15,

13 Compare Transversity from RHIC/SIDIS PAVIA Group: Radici et al, arxiv: v1 Compare STAR data with transversity replicas extracted from HERMES/COMPASS SIDIS data and BELLE IFF Fragmentation Functions QCD-N16 Bilbao, Spain July 15,

14 SSA in pp sensitive to Transversity: Azimuthal distributions of pions in Jets φ S is defined as the angle between proton spin and reaction plane S ΦS pπ jt Φh pbeam j T defines particle transverse momentum in jet φ H defines angle between jet particle transverse momentum and reaction plane φ C = φ S - φ H (Collins Angle) pbeam PJET

15 Collins/Transversity Results at s = 200 GeV j T (GGG/c) particle jet p T (GGG/c) QCD-N16 Bilbao, Spain July 15,

16 Collins/Transversity Results at s = 500 GeV Non-zero Collins asymmetries observed at s = 500 GeV QCD-N16 Bilbao, Spain July 15,

17 Collins/Transversity Results at s = 500 GeV Non-zero Collins asymmetries observed at s = 500 GeV Consistent with s = 200 GeV results for consistent cuts and x T arxiv: Suggestive of slow TMD evolution/validity of factorization? QCD-N16 Bilbao, Spain July 15,

18 Transversity from pp IFF and SIDIS General agreement between SIDIS and IFF RHIC New STAR IFF measurements at 200 and 500 GeV are higher precision, functions of M pp, p T, η Consistency indicates slow Q 2 evolution Will shed light on transversity for x>0.1 New STAR Collins measurements at 200 and 500 GeV give non-zero Collins functions and check Q 2 evolution and validity of factorization. QCD-N16 Bilbao, Spain July 15,

19 A N for Weak Bosons Lepton s transverse momentum Boson s transverse momentum Asymmetry from lepton-decay is small Need full kinematic reconstruction of W/Z > Z 0 easy to reconstruct (but small cross-section) > W kinematics can be reconstructed from the hadronic recoil (first time at STAR) QCD-N16 Bilbao, Spain July 15,

20 Sivers Sign Change (No TMD Evolution) [Phys. Rev. Lett. 116, (2016)] A global fit to the (unevolved) KQ prediction was performed: solid line: assumption of a sign change in the Sivers function χ 2 /d.o.f. = 7.4/6 dashed line: assumption of no sign change in the Sivers function χ 2 /d.o.f. = 19.6/6 If there are no evolution effects (or cancellation of evolution in the ratio) our data favor the hypothesis of Sivers sign change QCD-N16 Bilbao, Spainl July 15,

21 Sivers Sign Change (Strong TMD Evolution) Size of the TMD evolution still uncertain -> terms calculable from QCD + non-perturbative terms (need data) A global fit to the EIKV prediction (largest predicted evolution effect): solid line: assumption of a sign change in the Sivers function χ 2 /d.o.f. = 10.26/6 dashed line: assumption of no sign change in the Sivers function χ 2 /d.o.f. = 11.93/6 Our uncertainties are still too high to compare with predictions QCD-N16 Bilbao, Spain July 15,

22 W/Z Transverse Asymmetries Reconstruction of W vastly improves ability to address Sivers sign change Data currently favors sign change Data agreement with sign change implies either slow Q 2 evolution or cancellation of Q 2 in polarized/unpolarized ratio Factor of 10 more data planned for 2017! QCD-N16 Bilbao, Spain July 15,

23 Forward Pion Asymmetries and Twist-3 s = 200 GeV H I FF switched off Kanazawa et al, PRD (2014) Collinear twist-3 factorization 2 H h/q z = z 2 d 2 k k 2 2M H h/q 1 z, z 2 2 k h s = 500 GeV and additional input from SIDIS and e + e h 1 h 2 X 23

24 Event Topology: Forward TSSAs E jjj = GeV GeV GeV s = 500 GeV 11 2γ 33 4γ 5γ 24

25 Event Topology Twist-3 + TMDs currently favored for generating large asymmetries in forward direction, but no simple interpretation Forward Asymmetries large for single pions Disappears rapidly with increasing amount of energy in event (including energy at mid-rapidity as well) Casts doubt on 2->2 parton interpretation of origin of asymmetries Roman Pot data taken in 2015/2017 will address diffractive nature of events with high asymmetry by tagging intact protons from collisions QCD-N16 Bilbao, Spain July 15,

26 Conclusions: TMDs and STAR RHIC/STAR can contribute much to our basic understanding of TMDs Early understanding of Q 2 evolution Examine factorization/sign change in W/Z & DY Measurements of Transversity in IFF and Collins/Transversity in Jets. Possible diffractive effects in Forward SSAs Can use asymmetries in pa/pp to explore nuclear modification/saturation Solid basis for detailed exploration by EIC QCD-N16 Bilbao, Spain July 15,

27 27

28 Original predictions: Polarized p + A collisions original π 0 predictions: Transverse asymmetries are sensitive to gluon saturation: Phys. Rev. D84, (2011) Q S A = A 1/3 Q S p Q S p = 1 GeV However! Hatta et al., arxiv v1 finds that nuclear effects cancel in forward direction and predict A N pa / A n pp ~ 1 28

29 Mid-rapidity di-hadron production Interference Fragmentation Function measured in e + e - annihilation at the Belle facility at KEK, Japan QCD-N16 Bilbao, Spain July 15,

30 Goal for 2015: PHENIX 40 pb 1, STAR 50 pb 1 30

31 S ΦS pbeam pπ pbeam jt Φh PJET 31

32 TMDs from polarized SIDIS and pp QCD-N16 Bilbao, Spain July 15,

33 Final state mechanism: Transversity x Collins 200 vs. 500 GeV Comparison: These measurements coupled with the interference fragmentation function (IFF) measurements at both 200 and 500 GeV are sensitive to the evolution and universality of TMD functions. These results could be sensible to the size of potential factorization-breaking in Collins in p+p. dependence of the Collins FF on pion transverse momentum (j T ) What we see Non-zero Collins asymmetry -> Access to transversity! Similar size asymm. in 200 and 500 GeV -> Small TMD evolution? -> Cancellation in num/denom may also be the key QCD-N16 Bilbao, Spain July 15,

34 Diffraction Effects as Source of Forward TSSA? A N for different # photons in EM-Jets 1-photon events, which include a large π 0 contribution in this analysis, 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 0 s A N decreases as the event complexity increases (i.e.,the "jettiness ) Several other Asym. for jettier events are also very small. Collins contribution is ~1% over the entire x F range Jettier events Sivers-type asymmetry in the jets is too small to explain π 0 asymmetry A N for π 0 may be dominated by hard diffraction: p +p π 0 + p +X Run 15 STAR has collected data using RPs to measure forward scattered protons QCD-N16 Bilbao, Spain July 15,

35 Unpolarized Example of TMD & 2 Scales Drell-Yan Hadroproduction of Z Bosons Scales are M Z and q T QCD-N16 Bilbao, Spain July 15,

36 IFF 200 GeV 1D Binning QCD-N16 Bilbao, Spain July 15,

37 Compare Transversity from RHIC/SIDIS PAVIA Group: Radici et al, arxiv: v1 Compare STAR data with transversity replicas extracted from HERMES/COMPASS SIDIS data and BELLE IFF Fragmentation Functions QCD-N16 Bilbao, Spain July 15,

38 Why are TMDs Important? Next step in direction of description of observed nucleon structure and observed scattering effects: explanation of Sivers, Collins, Transversity, etc. Study of nucleon structure at low x is becoming more and more important, e.g., for sum rules, sea/anti-quarks, and ultra-high energy processes: LHC and future EIC. At low x, the intrinsic transverse momentum of the nucleon becomes larger relative to the longitudinal momentum. pqcd using PDFs becomes less reliable and must be extended by re-summation over all orders of soft parton emission. These extensions are summarized as functions which depend explicitly on the intrinsic transverse momentum: TMDs. QCD-N16 Bilbao, Spain July 15,

39 Process Dependence of Sivers Function: Sign Change In semi-inclusive deep inelastic scattering (SIDIS), the Sivers function is associated with a final-state effect through gluon exchange between the struck parton and the target nucleon remnants [4]. In p+p collisions, on the other hand, the Sivers asymmetry originates from the initial state of the interaction for the DY process and W±/Z0 boson production. QCD-N16 Bilbao, Spain July 15,

40 TMD Evolution & Sea-quarks Sivers Z.-B. Kang & J.-W. Qui arxiv: before evolution Z.-B. Kang & J.-W. Qui Phys.Rev.D81:054020,2010 before evolution 200 GeV 500 GeV Size of the TMD evolution effect still under discussion in theory community Drell-Yan For details see J. Collins, T. Rogers, Phys.Rev. D91 (2015) 7, What is the sea-quark Sivers function? Sea quarks are mostly unconstrained from existing SIDIS data... but they can give a relevant contribution! W s ideal rapidity dependence of A N separates quarks from antiquarks W ± data can constrain the sea-quark Sivers function QCD-N16 Bilbao, Spain July 15,