Scale dependence of Twist-3 correlation functions

Transcription

1 Scale dependence of Twist-3 correlation functions Jianwei Qiu Brookhaven National Laboratory Based on work with Z. Kang QCD Evolution Workshop: from collinear to non collinear case Thomas Jefferson National Accelerator Facility, April 8-9, 2011 Newport News, Virginia, USA

2 Outline of my talk Go beyond leading power collinear pqcd Single transverse spin asymmetry Twist-3 correlation functions, fragmentation functions Evolution equations and evolution kernels Global QCD analysis of SSAs Summary

3 Transverse spin phenomena in QCD Left-right asymmetry: A(p A,s )+B(p B ) π(p)+x Vanish without parton s transverse motion: A direct probe for parton s transverse motion A direct probe of QCD quantum interference

4 Single transverse spin asymmetry SSA corresponds to a naively T-odd triple product: A N i s p (p ) i µναβ p µ s ν α p β Novanish A N requires a phase, enough vectors to fix a scattering plan, and a spin flip at the partonic scattering Leading power in QCD: Kane, Pumplin, Repko, PRL, 1978 σ AB (p T, s) = α s m q p T A N connects to parton s transverse motion!

5 Collinear factorization Cross section with one large momentum transfer: Q >> Λ QCD p, s k σ(q, s) t 1/Q 2 = σ LP (Q, s)+ Q s Q σnlp (Q, s)+... Q 2 s k 2, kt 2 H LP f 2 f 2 + Q s Q HNLP f 2 f pqcd factorization Single transverse spin asymmetry: A N σ(q, S ) σ(q, S ) H(Q) [p, S O(ψ,A µ ) p, S p, S O(ψ,A µ ) p, S ] Parity and time-reversal invariance: Not all operators contribute to SSA!

6 Inclusive DIS Inclusive DIS cross section: Leptonic tensor is symmetric: L µν = L νµ Hadronic tensor: The difference of two cross sections: P and T invariance: P,s j µ(0)j ν (y) P,s = P, s j ν(0)j µ (y) P, s W µν (s )=W νµ ( s ) A N =0

7 Single hadron inclusive One large scale: A(p A,S )+B(p B ) h(p)+x with p T >> Λ QCD A N σ(p T,S ) σ(p T, S ) T (3) (x, x, S ) ˆσ T D(z)+δq(x, S ) ˆσ D D (3) (z,z)+... Leading power contribution to cross section cancels! Twist-3 three-parton correlation functions: Qiu, Sterman, 1991, T (3) (x, x, S ) Moment of Sivers function? Single jet inclusive Twist-3 three-parton fragmentation functions: D (3) (z,z) Kang, Yuan, Zhou, 2010 Moment of Collins function? No probability interpretation!

8 Twist-3 correlation functions Twist-2 parton distributions: Kang, Qiu, PRD, 2009 Unpolarized PDFs: Polarized PDFs: Two-sets Twist-3 correlation functions:

9 Evolution equations and kernels Evolution equation is a consequence of factorization: Factorization: DGLAP for f 2 : Evolution for f 3 : Evolution kernel is process independent: Calculate directly from the variation of process independent twist-3 distributions Extract from the scale dependence of the NLO hard part of any physical process Kang, Qiu, 2009 Yuan, Zhou, 2009 Vogelsang, Yuan, 2009 Renormalization of the twist-3 operators Braun et al, 2009

10 The Feynman diagram representation Feynman diagram representation of twist-3 distributions: Different twist-3 distributions diagrams with different cut vertices Collinear factorization of twist-3 distributions: Cut vertex and projection operator in LC gauge: Kernel

11 Variation of twist-3 correlation functions Closed set of evolution equations (spin-dependent): Plus two more equations for: and

12 Evolution equations Distributions relevant to SSA: Important symmetry property: These two correlation functions do not give the gluonic pole contribution directly

13 Feynman diagrams: Evolution kernels LO for flavor non-singlet channel:

14 Quark: Leading order evolution equations - I Antiquark: All kernels are infrared safe Diagonal contribution is the same as that of DGLAP Quark and antiquark evolve differently caused by tri-gluon

15 Quark: Leading order evolution equations - I Antiquark: Missing a term Braun et al, 2009 All kernels are infrared safe Diagonal contribution is the same as that of DGLAP Quark and antiquark evolve differently caused by tri-gluon

16 Three-gluon correlation and evolution Two possible color contributions: LO diagrams: d abc, if abc = T (d) G,F (x 1,x 2 ), T (f) G,F (x 1,x 2 )

17 Gluons: Leading order evolution equations - II Similar expression for Kernels are also infrared safe diagonal contribution is the same as that of DGLAP Two tri-gluon distributions evolve slightly different has no connection to TMD distribution Evolution can generate as long as

18 Leading order evolution equations - III Evolution equations for diagonal correlation functions are not closed! Model for the off-diagonal correlation functions: For the symmetric correlation functions:

19 Scale dependence Follow DGLAP at large x Large deviation at low x (stronger correlation)

20 QCD global analysis of SSAs Factorization for physical observables: A(p A,S )+B(p B ) h(p)+x A(p A,S )+B(p B ) jet(p)+x A(p A,S )+B(p B ) γ(p)+x... Urgently needed NLO hard parts: A N σ(q, S ) T (3) f (x, x) Ĥf... Beyond LO! Only NLO calculation SSA for p T weighted Drell-Yan Vogelsang, Yuan, 2009 A completely new domain to test QCD! From paton s transverse motion to direct QCD quantum interference

21 Interpretation of twist-3 correlation functions Measurement of direct QCD quantum interference: T (3) (x, x, S ) Qiu, Sterman, 1991, Interference between a single active parton state and an active two-parton composite state Expectation value of QCD operators: P, s ψ(0)γ + ψ(y ) P, s P, s ψ(0)γ + γ 5 ψ(y ) P, s P, s ψ(0)γ + P, s ψ(0)γ + αβ s T α ig αβ s T α dy ψ(y 2 F + ) P, s β (y 2 ) dy 2 F + β (y 2 ) ψ(y ) P, s How to interpret the expectation value of the operators in RED?

22 A simple example The operator in Red a classical Abelian case: Change of transverse momentum: In the c.m. frame: The total change: Net quark transverse momentum imbalance caused by color Lorentz force inside a transversely polarized proton

23 Collinear vs TMD factorization Cover two different kinematic regions: Collinear: TMD: Q 1 Q n >> Λ QCD Q 1 >> Q 2 > Λ QCD Twist-3 correlation functions: Integrated effect of parton k T TMDs: direct information on parton k T more interesting if we can measure them Consisitent in the overlap (perturbative) region: A N (Q 2,p T ) p T Q p T Q Ji,Qiu,Vogelsang,Yuan, Koike, Vogelsang, Yuan Q s p T

24 Summary QCD factorization/calculation have been very successful in interpreting HEP scattering data What about the hadron structure? Not much! < 1/10 fm Experiments with a polarized hadron beam opened up new ways to test QCD and to study hadron structure Parton s transverse motion and hadron s transverse structure Collinear and TMD factorization give complementary descriptions of QCD dynamics Thank you!

25 Backup slices

26 SSA from quark-gluon correlation (FermiLab E704) (RHIC STAR) Kouvaris,Qiu,Vogelsang,Yuan, 2006 Nonvanish twist-3 function Nonvanish transverse motion

27 First hint of tri-gluon correlation PHENIX data on J/psi: Collinear factorization: tri-gluon correlation direct quantum interference Challenges: PHENIX data on open charm: J/psi production mechanism Initial- vs final-state effect Connection to Gluon Sivers function Collins, Qiu, Vogelsang, Yuan, Rogers, Mulder,