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Collinear hard processes, e.g. DIST-odd phenomena

Content

- Spin structure & transversity
- Transverse momenta & azimuthal asymmetries
- T-odd phenomena & single spin asymmetries

Collinear hard processes, e.g. DIS

- Structure functions (observables) are identified with distribution functions (matrix elements of quark-quark correlators along lightcone)
- Longitudinal gluons (A+, not seen in LC gauge) are part of DF’s, rendering them gauge inv.
- There are three ‘leading’ DF’s

f1q(x) = q(x), g1q(x) = Dq(x), h1q(x) = dq(x)

- Transverse gluons appear at 1/Q and are contained in (higher twist) qqG-correlators
- DF’s are quark densities that are directly linked to lightcone wave functions squared
- Perturbative QCD evolution (reflecting the large pT-behavior of the correlator proportional to as/pT2 in the pT-integration)

Ellis, Furmanski, Petronzio

Efremov, Radyushkin

A+ gluons

gauge link

Leadingorder DIS- In limit of large Q2 the result

of ‘handbag diagram’ survives

- … + contributions from A+ gluons

ensuring color gauge invariance

Collinear hard processes, e.g. DIS

- Structure functions (observables) are identified with distribution functions (matrix elements of quark-quark correlators along lightcone)
- Longitudinal gluons (A+, not seen in LC gauge) are part of DF’s, rendering them gauge inv.
- There are three ‘leading’ DF’s

f1q(x) = q(x), g1q(x) = Dq(x), h1q(x) = dq(x)

- Transverse gluons appear at 1/Q and are contained in (higher twist) qqG-correlators
- DF’s are quark densities that are directly linked to lightcone wave functions squared
- Perturbative QCD evolution (reflecting the large pT-behavior of the correlator proportional to as/pT2 in the pT-integration)

Parametrization of lightcone correlator

- M/P+ parts appear as M/Q terms in s
- T-odd part vanishes for distributions
- but is important for fragmentation

Jaffe & Ji NP B 375 (1992) 527

Jaffe & Ji PRL 71 (1993) 2547

Collinear hard processes, e.g. DIS

- Structure functions (observables) are identified with distribution functions (matrix elements of quark-quark correlators along lightcone)
- Longitudinal gluons (A+, not seen in LC gauge) are part of DF’s, rendering them gauge inv.
- There are three ‘leading’ DF’s

f1q(x) = q(x), g1q(x) = Dq(x), h1q(x) = dq(x)

- Transverse gluons appear at 1/Q and are contained in (higher twist) qqG-correlators
- DF’s are quark densities that are directly linked to lightcone wave functions squared
- Perturbative QCD evolution (reflecting the large pT-behavior of the correlator proportional to as/pT2 in the pT-integration)

Bacchetta, Boglione, Henneman & Mulders

PRL 85 (2000) 712

Matrix representationfor M = [F(x)g+]TRelated to the

helicity formalism

Anselmino et al.

- Off-diagonal elements (RL or LR) are chiral-odd functions
- Chiral-odd soft parts must appear with partner in e.g. SIDIS, DY

- Longitudinal gluons (A+, not seen in LC gauge) are part of DF’s, rendering them gauge inv.
- There are three ‘leading’ DF’s

f1q(x) = q(x), g1q(x) = Dq(x), h1q(x) = dq(x)

- Transverse gluons appear at 1/Q and are contained in (higher twist) qqG-correlators
- DF’s are quark densities that are directly linked to lightcone wave functions squared

Non-collinear processes, e.g. SIDIS

- Relevant in electroweak processes with two hadrons (SIDIS, DY)
- Beyond just extending DIS by tagging quarks …
- Transverse momenta of partons become relevant, appearing in azimuthal asymmetries
- DF’s and FF’s depend on two variables,

F(x,pT) and D(z,kT)

- Gauge link structure is process dependent ( [])
- pT-dependent distribution functions and (in general) fragmentation functions are not constrained by time-reversal invariance
- This allows T-odd functions h1^ and f1T^ (H1^ and D1T^) appearing in single spin asymmetries

Leading order SIDIS

- In limit of large Q2 only result

of ‘handbag diagram’ survives

- Isolating parts encoding soft physics

?

?

Lightfront correlators

Collins & Soper

NP B 194 (1982) 445

no T-constraint

T|Ph,X>out =|Ph,X>in

Jaffe & Ji,

PRL 71 (1993) 2547;

PRD 57 (1998) 3057

Non-collinear processes, e.g. SIDIS

- Relevant in electroweak processes with two hadrons (SIDIS, DY)
- Beyond just extending DIS by tagging quarks …
- Transverse momenta of partons become relevant, appearing in azimuthal asymmetries
- DF’s and FF’s depend on two variables,

F[](x,pT) and D[](z,kT)

- Gauge link structure is process dependent ( [])
- pT-dependent distribution functions and (in general) fragmentation functions are not constrained by time-reversal invariance
- This allows T-odd functions h1^ and f1T^ (H1^ and D1T^) appearing in single spin asymmetries

Distribution

including the gauge link (in SIDIS)

A+

One needs also AT

G+a = +ATa

ATa(x)= ATa(∞) + dh G+a

Belitsky, Ji, Yuan, hep-ph/0208038

Boer, M, Pijlman, hep-ph/0303034

From <y(0)AT()y(x)> m.e.

Non-collinear processes, e.g. SIDIS

- Relevant in electroweak processes with two hadrons (SIDIS, DY)
- Beyond just extending DIS by tagging quarks …
- Transverse momenta of partons become relevant, appearing in azimuthal asymmetries
- DF’s and FF’s depend on two variables,

F[](x,pT) and D[](z,kT)

- Gauge link structure is process dependent ( [])
- pT-dependent distribution functions and (in general) fragmentation functions are not constrained by time-reversal invariance
- This allows T-odd functions h1^ and f1T^ (H1^ and D1T^) appearing in single spin asymmetries

Parametrization of F(x,pT)

- Link dependence allows also T-odd distribution functions since T U[0,] T = U[0,-]
- Functions h1^ and f1T^ (Sivers) nonzero!
- These functions (of course) exist as fragmentation functions (no T-symmetry) H1^ (Collins) and D1T^

Interpretation

unpolarized quark

distribution

need pT

T-odd

helicity or chirality

distribution

need pT

T-odd

need pT

transverse spin distr.

or transversity

need pT

need pT

functions

Matrix representationfor M = [F[±](x,pT)g+]TT-odd: g1T g1T – i f1T^ and h1L^ h1L^ + i h1^

Bacchetta, Boglione, Henneman & Mulders

PRL 85 (2000) 712

pT-dependent DF’stwist structure

- For integrated correlator F(x) the (M/P+)t-2 expansion corresponds with definite twist assignments a la OPE
- For unintegrated correlators F[](x,pT) the (M/P+)t-2 does not correspond with definite twist assignments; they contain operators of twist t
- Transverse momentsFa(x,pT) d2pTpTa F(x,pT) project out the parts in F[](x,pT) proportional to pT. They correspond to matrix elements with operators of twist t and t+1 (quark-quark and quark-quark-gluon)
- Transverse moments are measured in azimuthal asymmetries with in addition to angular averaging also require an explicit (transverse) momentum
- The difference between F[+]a(x) and F[-]a(x) is a gluonic pole matrix element (soft gluon pole), which for distribution functions is T-odd
- The socalled Lorentz invariance relations based on general structure of quark-quark correlators need to be augmented because of gauge link
- Factorization of explicit pT-dependent functions requires ‘soft factors’
- Universality of subleading functions (appearing at M/P+ level) in F[+]a(x) seems ok, but this seems not the case for subleading functions in F[+]a(x) [so f1T(1) ok but f(1) problematic: in cos fh asymmetry pTaf at order 1/Q gets pQCD correction as f1/|pT|, which shows up as as f1 at order 1]

Difference between F[+] and F[-] upon integration

Back to the lightcone

integrated quark

distributions

transverse

moments

measured in azimuthal asymmetries

±

pT-dependent DF’stwist structure

- For integrated correlator F(x) the (M/P+)t-2 expansion corresponds with definite twist assignments a la OPE
- For unintegrated correlators F[](x,pT) the (M/P+)t-2 does not correspond with definite twist assignments; they contain operators of twist t
- Transverse momentsFa(x,pT) d2pTpTa F(x,pT) project out the parts in F[](x,pT) proportional to pT. They correspond to matrix elements with operators of twist t and t+1 (quark-quark and quark-quark-gluon)
- Transverse moments are measured in azimuthal asymmetries with in addition to angular averaging also require an explicit (transverse) momentum
- The difference between F[+]a(x) and F[-]a(x) is a gluonic pole matrix element (soft gluon pole), which for distribution functions is T-odd
- The socalled Lorentz invariance relations based on general structure of quark-quark correlators need to be augmented because of gauge link
- Factorization of explicit pT-dependent functions requires ‘soft factors’
- Universality of subleading functions (appearing at M/P+ level) in F[+]a(x) seems ok, but this seems not the case for subleading functions in F[+]a(x) [so f1T(1) ok but f(1) problematic: in cos fh asymmetry pTaf at order 1/Q gets pQCD correction as f1/|pT|, which shows up as as f1 at order 1]

pT-dependent DF’stwist structure

- For integrated correlator F(x) the (M/P+)t-2 expansion corresponds with definite twist assignments a la OPE
- For unintegrated correlators F[](x,pT) the (M/P+)t-2 does not correspond with definite twist assignments; they contain operators of twist t
- Transverse momentsFa(x,pT) d2pTpTa F(x,pT) project out the parts in F[](x,pT) proportional to pT. They correspond to matrix elements with operators of twist t and t+1 (quark-quark and quark-quark-gluon)
- Transverse moments are measured in azimuthal asymmetries with in addition to angular averaging also require an explicit (transverse) momentum
- The difference between F[+]a(x) and F[-]a(x) is a gluonic pole matrix element (soft gluon pole), which for distribution functions is T-odd
- The socalled Lorentz invariance relations based on general structure of quark-quark correlators need to be augmented because of gauge link
- Factorization of explicit pT-dependent functions requires ‘soft factors’
- Universality of subleading functions (appearing at M/P+ level) in F[+]a(x) seems ok, but this seems not the case for subleading functions in F[+]a(x) [so f1T(1) ok but f(1) problematic: in cos fh asymmetry pTaf at order 1/Q gets pQCD correction as f1/|pT|, which shows up as as f1 at order 1]

T-odd phenomena

- T-odd phenomena appear in single spin asymmetries
- T-odd parts for distribution functions are in the gluonic pole part, hence in F[+]a(x) and F[-]a(x) they have opposite signs
- T-odd parts for fragmentation functions in D[+]a(x) and D[-]a(x) are not related. This needs to be considered including QCD corrections, because of the interplay between T-behavior of hadronic states and gauge links
- Contributions in other hard processes, such as pp pX involving three hadrons require a careful analysis

Wmn(q;P,S;Ph,Sh) = -Wnm(-q;P,S;Ph,Sh)

- Wmn(q;P,S;Ph,Sh) = Wnm(q;P,S;Ph,Sh)
- Wmn(q;P,S;Ph,Sh) = Wmn(q;P, -S;Ph, -Sh)
- Wmn(q;P,S;Ph,Sh) = Wmn(q;P,S;Ph,Sh)

_

_

_

_

_

_

_

_

_

_

_

_

T-oddsingle spin asymmetrysymmetry

structure

hermiticity

*

parity

- with time reversal constraint only even-spin asymmetries
- the time reversal constraint cannot be applied in DY or in 1-particle inclusive DIS or e+e-
- In those cases single spin asymmetries can be used to select T-odd quantities

time

reversal

*

T-odd phenomena

- T-odd phenomena appear in single spin asymmetries
- T-odd parts for distribution functions are in the gluonic pole part, hence in F[+]a(x) and F[-]a(x) they have opposite signs
- T-odd parts for fragmentation functions in D[+]a(x) and D[-]a(x) are not related. This needs to be considered including QCD corrections, because of the interplay between T-behavior of hadronic states and gauge links
- Contributions in other hard processes, such as pp pX involving three hadrons require a careful analysis

Time reversal constraints for distribution functions

T-odd

(imaginary)

Time reversal: F[+](x,pT) F[-](x,pT)

pFG

F[+]

F

T-even

(real)

Conclusion:

T-odd effects in SIDIS and DY have opposite signs

F[-]

T-odd phenomena

- T-odd phenomena appear in single spin asymmetries
- T-odd parts for distribution functions are in the gluonic pole part, hence in F[+]a(x) and F[-]a(x) they have opposite signs
- T-odd parts for fragmentation functions in D[+]a(x) and D[-]a(x) are not related. This needs to be considered including QCD corrections, because of the interplay between T-behavior of hadronic states and gauge links
- Contributions in other hard processes, such as pp pX involving three hadrons require a careful analysis

Time reversal constraints for fragmentation functions

T-odd

(imaginary)

Time reversal: D[+]out(z,pT) D[-]in(z,pT)

pDG

D[+]

D

T-even

(real)

D[-]

Time reversal constraints for fragmentation functions

T-odd

(imaginary)

Time reversal: D[+]out(z,pT) D[-]in(z,pT)

D[+]out

pDG out

D out

T-even

(real)

D[-]out

Conclusion:

T-odd effects in SIDIS and e+e- are not related

- T-odd phenomena appear in single spin asymmetries

other hard processes

- qq-scattering as hard subprocess
- insertions of gluons collinear with parton 1 are possible at many places
- this leads for ‘external’ parton fields to gauge link to lightcone infinity

other hard processes

- qq-scattering as hard subprocess
- insertions of gluons collinear with parton 1 are possible at many places
- this leads for ‘external’ parton fields to gauge link to lightcone infinity
- The correlator F(x,pT) enters for each contributing term in squared amplitude with specific link
- The link may enhance the effect of the gluonic pole contribution involving also specific color factors

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