Factorization, Evolution and Soft factors

Transcription

1 Factorization, Evolution and Soft factors Jianwei Qiu Brookhaven National Laboratory INT Workshop: Perturbative and nonperturbative aspects of QCD at collider energies University of Washington, Seattle, WA, September 20, 2010

2 Outline Collinear factorization, PDFs, correlation functions Evolution of PDFs and correlation functions TMD factorization: TMD distributions, and soft factors Evolution of TMD distributions Phenomenology Summary

3 Connecting hadrons to partons Experiments measure hadrons and leptons, not partons Large momentum transfer sensitive to partons: Sensitive to partonic dynamics (Diagrams with more active partons from each hadron!) Connection between hadron and parton QCD factorization connecting partons to hadrons: Hadronic matrix elements of parton fields: p, s O(ψ,A α ) p, s : p, s ψ(0)γ + ψ(y) p, s, p, s F +α (0)F +β (y) p, s( g αβ ) Isolate pqcd calculable short-distance partonic dynamics

4 Factorization and partonic distribution Factorization of a physical observable: Short-distance probability Power corrections parton correlations Measured Long-distance probability Factorization is an approximation! Difficulty: long-range soft and collinear gluon interaction Complication: difference in observed physical scales Parton distribution: φ(1/r) p, s O p, s Explicit form of the operator: O used to derive the factorization arises from the approximation

5 Inclusive DIS Color flow, final-state interaction, gauge link: Collinear factorization: σh DIS (x B,Q) f Leading power Collinear quark distribution cut vertex: dx φ f (x, µ 2 F )ˆσ f/h (x B /x, Q 2 /µ 2 F, α s (µ)) + O(1/Q) d 4 k 1 γ n δ(x k n/p n) (2π) 4 2P n +UVCT(µ 2 ) q(x, µ 2 )= dy 2π eixp Artifact of collinear factorization + y P ψ(0) γ+ 2 L n(0, + )L n (y, + )ψ(y ) P

6 Evolution equation Evolution equation is a consequence of factorization: Evolution (differential-integral) equation for PDFs PDFs and coefficient functions share the same logarithms PDFs: Coefficient functions: DGLAP evolution equation: Evolution equation is a consequence of factorization!

7 Evolution kernels of PDFs Factorization scale dependence of PDFs: d 4 k 1 γ n δ(x k n/p n) (2π) 4 2P n +UVCT(µ 2 ) from the UVCT renormalization of the composite operator used to define the PDFs Evolution kernels process independent: extract from the PDFs of a parton state: e.g. quark distribution of a quark state µ 2 µ 2 φ(1) q/q (x, µ2 )=P (1) qq (x) φ (0) q/q (x, µ2 ) Extract from the partonic cross section: Q 2 Q 2 σ(1) q (x, Q 2 )=P (1) qq (x) σ (0) q (x, Q 2 ) OPE and renormalization of composite operators:

8 Evolution of twist-3 correlation functions Almost all existing calculations of SSA are at LO: Strong dependence on renormalization and factorization scales Artifact of the lowest order calculation Improve QCD predictions needed: Complete set of twist-3 correlation functions relevant to SSA LO evolution for the universal twist-3 correlation functions NLO partonic hard parts for various observables NLO evolution for the correlation functions, Current status: Two sets of twist-3 correlation functions LO evolution kernel for and NLO hard part for SSA of p T weighted Drell-Yan Kang, Qiu, 2009 Braun, et al 2009 Vogelsang, Yuan, 2009

9 Twist-3 distributions relevant to SSA Two-sets Twist-3 correlation functions: No probability interpretation! Twist-2 distributions: Unpolarized PDFs: Polarized PDFs: Evolution equations: See Kang, Qiu, 2009, and Braun, et al 2009

10 Evolution equations and evolution 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 Calculate UV renormalization of the operators Braun et al, 2009

11 Evolution equations I Feynman diagram representation of twist-3 distributions: Kang, Qiu, 2009 Different twist-3 distributions diagrams with different cut vertices Collinear factorization of twist-3 distributions: Cut vertex and projection operator in LC gauge:

12 Evolution equations II Closed set of evolution equations (spin-dependent): Plus two more equations for: and

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

14 Evolution kernels Feynman diagrams: Kang, Qiu, PRD, 2009 LO for flavor non-singlet channel:

15 Quark: Leading order evolution equations I Kang, Qiu, PRD, 2009 Antiquark: 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 Leading order evolution equations II Gluons: Kang, Qiu, PRD, 2009 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

17 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:

18 Scale dependence of twist-3 correlations Follow DGLAP at large x Large deviation at low x (stronger correlation) Kang, Qiu, PRD, 2009

19 TMD factorization Need processes with two observed momentum scales: necessary for pqcd factorization to have a chance sensitive to parton s transverse motion Example semi-inclusive DIS: p Both p and p are observed q k p p T probes the parton s k T Effect of k T is not suppressed by Q Limited processes with valid TMD factorization Collins, Qiu, 2007, Vogelsang, Yuan, 2007 Collins, 2007, Rogers, Mulder, 2010 TMD distribution is more fundamental, more information Semi-inclusive DIS is an ideal place to probe the TMD distributions As well as generalized parton distributions (GPD s)

20 TMD factorization in QCD I SIDIS as an example Parton model LO QCD: Parton distribution function in a hadron parton-to-hadron fragmentation function QCD interaction and leading power regions: Collinear regions: k P A, k P B soft regions k µ 0

21 TMD factorization in QCD II Leading pinch surface: hard region collinear to P region collinear to P region soft region Factorization: TMD fragmentation TMD parton distribution Soft factors

22 TMD factorization in QCD III Factorization formalism: W µν f d 2 k at d 2 k bt S(q ht k at k bt )Tr P A J f/a (x, k at )P A Hf ν (Q)P B J f B (z,k bt )P B H µ f (Q) The soft factors: S = Z s S (0) S (0) (k st )= 1 N c dk + s dk s (2π) 4 2 The soft factors in collinear factorization: S (0) (k st ) S (0) (k st = 0) 1

23 TMD distributions commonly used version TMD parton distribution: φ (0) f/a (x, k at )=Tr color Tr Direc γ + 2 dk a 2π TMD fragmentation function: D (0) f B (z,k bt )= Tr color N c Tr Direc 4 γ + z dk b 2π Note: Without integration over k T, TMD distributions do not have the UV divergence from dk T, the one responsible for the DGLAP

24 Going to the b-space Factorization in b-space: Collins et al, 1995 W µν f W µν (0)f d 2 2 b T (2π) 2 2 e iq ht b T S(bT ) φ f/a (x, b T ) D f D (z,b T ) Universality of the soft factor: Wilson line: Evolution of TMD distributions: from the wave function renormalization and the renormalization of the soft factors

25 Modified TMD distributions Modified TMD distribution remove the soft factor: The TMD distribution:

26 Evolution of TMD s: Evolution of TMD s Evolved TMD s:

27 CSS b-space resummation formalism Leading order K T -factorized cross section: resummed No large log s The Q T -distribution is determined by the b-space function: June 28, 2005 Jianwei Qiu, ISU 27

28 The b-space distribution: Phenomenology June 28, 2005 Jianwei Qiu, ISU 28

29 Resummed x-section: Calculation in the b-space Features:

30 Remove the divergence: Resummed p T distribution Features:

31 Works for heavy boson production Upsilon at Tevatron: Dominated by perturbative small-b contribution in its Fourier conjugate space Berger, Qiu, Wang, 2005 Works better for W/Z, also works for Drell-Yan, Qiu, Zhang, 2001 Brock et al,

32 Comparison with Fermilab Drell-Yan data Low energy data: Qiu, Zhang, 2001 E288 data P beam = 400 GeV Much larger power corrections

33 Factorization: QCD factorization for SIDIS Collins, Soper Ji, Ma, Yuan Low P ht TMD factorization: High P ht Collinear factorization: σ SIDIS (Q, P h,x B,z h )=Ĥ(Q, P h, α s ) φ f D f h + O P ht Integrated - Collinear factorization: σ SIDIS (Q, x B,z h )= H(Q, α s ) φ f D f h + O 1 Q 1, 1 P h Q

34 Also works for HERA data Nadolsky, et al, 1999, 2001 It will be interested to see if it works for Jlab energies and future EIC hadron s transverse momentum distributions

35 Summary Parton distributions and correlation functions are NOT direct physical observables could be defined differently Right definition arises from the approximation used in deriving the factorization! The evolution equation of the parton distributions and correlation functions is the consequence of the factorization The soft factors are universal and could be absorbed into the definition of TMD distributions Thank you!

36 What the twist-3 distribution can tell us? 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