Imaging in Exclusive Processes Tanja Horn INT10-03 “Imaging QCD Matter”, Institute for Nuclear...

Imaging in Exclusive Processes

Tanja Horn

INT10-03 “Imaging QCD Matter”, Institute for Nuclear Theory, UW, Seattle

12 November 2010

Tanja Horn, CUA ColloquiumTanja Horn, Imaging in Exclusive Processes, INT10-3, Seattle 1

Nucleon Structure: landscapeNucleon Structure: landscape

Tanja Horn, Imaging in Exclusive Processes, INT10-3, Seattle

• Hadrons in QCD are relativistic many-body systems

– Fluctuating number of elementary quark/gluon constituents

– Rich structure of the wave function

• Components probed in ep scattering:– JLab 12 GeV: valence region– EIC: sea quarks, gluons, Q2

dependence

• Physical properties– Transverse imaging– Correlations: transverse, longitudinal,

and nuclear modifications– Tests of reaction mechanism

valence quarks/gluons

non-pert. sea quarks/gluons

radiative gluons/sea

2

[Weiss 09]

Nucleon Structure: exclusive processesNucleon Structure: exclusive processes


• Physical interest in GPDs– Transverse spatial distribution of partons with

longitudinal momentum x: transverse imaging of nucleon [Burkhardt 00]

– Correlations in wave function– Moment xn-1 Form factor of local twist-2 spin-n

operator: EM tensor, angular momentum [Ji 96, Polyakov 02]

• Tests of reaction mechanism– Model-independent features of small-size regime– Finite-size corrections – [Frankfurt et al. 96, Kroll, Goloskokov >05]

π, K, γ, etc.

hardpointlike

GPD ΔT

Q2

N N’

ee’

Transverse Fourier x-x’

• Exclusive processes at sufficiently high Q2 should be understandable in terms of the “handbag” diagram

– The non-perturbative (soft) physics is represented by the GPDs

• Shown to factorize from QCD perturbative processes for longitudinal photons [Collins, Frankfurt, Strikman 97]

Q2>>R-2

ξ=0

3

Valence Quark Imaging Example: DVCS at Valence Quark Imaging Example: DVCS at JLab 12 GeVJLab 12 GeV


Interference with BH gives access to DVCS amplitude

Transverse spatial image of proton obtained by Fourier transforming the measured GPD

Projected results for GPD H(ξ,x=ξ,t) extracted from beam spin asymmetry

x=0.25

x=0.35

x=0.45

xb (fm)|t| (GeV2)

H(x

=ξ,

t)/F

1(t

)

t- dependence allows Fourier transform in ξ=0 limit

4

DVCS: future facilitiesDVCS: future facilities

Tanja Horn, Imaging in Exclusive Processes, INT10-3, Seattle 5

2ppt

2pqs

• EIC: great opportunities for sea quarks and gluons

• JLab 12 GeV: Valence quark GPDs– Spin observables, p/D– Re DVCS from TCS with (and ?)

e

• COMPASS: DVCS at 0.01<x<0.1– Re DVCS from BCA with

[Stepanyan 09]

[Schoeffel 09]

[Munoz 09]

[Stepanyan et al. 09]

TCS kinematics projection at an EIC for 6 GeV electrons and 60 GeV protons

Deep Virtual Meson Production (DVMP)Deep Virtual Meson Production (DVMP)

Nucleon GPDs: spin-flavor

long. only

• Need good understanding of reaction mechanism– QCD factorization for mesons is complex (additional interaction of the

produced meson)

Tanja Horn, Imaging in Exclusive Processes, INT10-3, Seattle 6

• Nucleon structure described by 4 GPDs: – H, E (unpolarized), , (polarized)H

~E~

• Quantum numbers probe individual GPD components more selectively– Vector: ρº/ρ+/K* select H, E– Pseudoscalar: π,η,K select the polarized GPDs, and H

~ E~

Meson Reaction Mechanism: JLab 12 GeVMeson Reaction Mechanism: JLab 12 GeV


non-pole

+

π° has no pole

contribution!

VGG GPD-based calculation

pole

°

• Understanding of reaction mechanism– Role of qqbar pair knockout– Finite-size corrections

• Feature: pole term in GPD• Understand relative importance of “pole

and “non-pole” contributions

Vector Mesons Pseudoscalar Mesons

Q2 [GeV2]d

σ/d

t (t

=t m

in)

[nb

/Ge

V2 ]

“pole”

7

ρ˚ data

EIC: Quark Imaging through Meson Production

• Mesons select definite charge, spin, flavor component of GPD


valence quarks

Non-perturbative sea quarks

radiative sea

x

• Exclusive meson production

– Requires Q2~10GeV2 for dominance of “pointlike” configurations pQCD

BMN *

• Physics interest– Transverse imaging of

nonperturbative sea quarks and gluons

– Information about meson wave function: spin/flavor structure

8

EIC: Gluon Imaging with J/Ψ

• Physics interest– Valence gluons, dynamical origin

– Chiral dynamics at b~1/Mπ

[Strikman, Weiss 03/09, Miller 07]

– Diffusion in QCD radiation


• Transverse spatial distributions from exclusive J/ψ, and φ at Q2>10 GeV2

– Transverse distribution directly from ΔT dependence

– Reaction mechanism, QCD description studied at HERA

• Existing data– Transverse area x<0.01 [HERA]

– Larger x poorly known [FNAL]

9

Gluon Imaging: Valence Gluons

• Imaging requires– Full t-distribution for Fourier transform– Non-exponential? Power-like at |t|>1

GeV2?– Electroproduction with Q2>10 GeV2:

test reaction mechanism, compare different channels, control systematics


• Transverse imaging of valence gluons through exclusive J/ψ, φ

• Experimentally need:– Recoil detection for exclusivity, wide

coverage in t with high resolution– Luminosity ~ 1034, electroproduction,

high-t

First gluon images of the nucleon at large x!Hyde, Weiss ‘09

10

Gluon imaging: gluon vs. singlet quark size


Detailed differential image of nucleon’s partonic structure

• EIC: gluon size from J/ψ, singlet quark size from DVCS

– x-dependence: quark vs. gluon diffusion in wave function

– Detailed analysis: LO NLO [Mueller et al.]

• Do singlet quarks and gluons have the same transverse distribution?

– Hints from HERA:

– Dynamical models predict difference: pion cloud, constituent quark picture [Strikman, Weiss 09]

– No difference assumed in present pp MC generators for LHC!

)(gAreaqqArea

11

[Tanja Horn, Antje Bruell, Christian Weiss]

• New territory for collider!

• Spatial structure of non-perturbative sea– Closely related to JLab 12 GeV

o Quark spin/flavor separationso Nucleon/meson structure

• Simulation for π+ production assuming 100 days at a luminosity of 1034 with 5 on 50 GeV (s=1000 GeV2)

– V. Guzey, C. Weiss: Regge model– T. Horn: empirical π+ parameterization


EIC: Transverse sea quark imagingEIC: Transverse sea quark imaging

ep → e'π+n

• Lower and more symmetric energies essential to ensure exclusivity

12

Transverse spatial structure of non-perturbative sea quarks!

Pushes luminosity towards > 1034, also at lower energy


EIC: Transverse strange sea quark imaging

• Do strange and non-strange sea quarks have the same spatial distribution?

– πN or KΛ components in nucleon– QCD vacuum fluctuations– Nucleon/meson structure

ep → e‘K+n

• Lower and more symmetric energies essential to ensure exclusivity

• Rate estimate for KΛ using an empirical fit to kaon electroproduction data from DESY and JLab assuming 100 days at a luminosity of 1034 with 5 on 50 GeV (s=1000 GeV2)

– Consistent with back-of-the-envelope scaling arguments

13

Imaging of strange sea quarks!

[Tanja Horn, David Cooper]

Transverse polarization example

• Deformation of transverse distribution by transverse polarization of nucleon

– Helicity flip GPD E, cf. Pauli ff

• EIC: exclusive ρ and φ production with transversely polarized beam

– Excellent statistics at Q2>10 GeV2

– Transverse polarization natural for collider


[Horn, Weiss 09]

Transverse spin

x

slower

quarks move faster

x

Asy

mm

etry

14

Beyond transverse imaging


• Longitudinal correlations in nucleon– GPDs at x’≠x: correlated qqbar pairs in nucleon

– QCD vacuum structure, relativistic nature of nucleon

– EIC: reveal correlations through exclusive meson, γ at x>0.1, Q2 dependence

• Orbital motion of quarks/gluons– TMD and orbital motion from SIDIS

– Major component of the EIC program

– Connection with GPDs– Unintegrated distributions, Ji sum rule

…needs kinematic coverage way beyond JLab 12 GeV

…should be discussed together

15

L/T separations in exclusive K+/π+ production

• Virtual photon polarization, ε, goes to unity at high √s

Requires special low energies for at least one ε point

Q2=10 GeV2, x=0.1, -t=0.1

4 on 2505 on 50

3 on 17

Δε~0.22


[Horn 08]

• L/T separated cross sections require:– Data taken at different beam

energies (Rosenbluth)

– Sufficiently large Δε (to avoid magnification of the systematic uncertainty in the separation)

16

L/T separation examples

EIC: Ee=5 GeV, Ep=50 GeV

Δε~0.22

stat: ΔσL/σL ~5%, syst: 6%/Δε

Horn, Huber, Bruell, Weiss [EICC 2008 HU]

Excellent potential to study the QCD transition nearly over the whole range from the strong QCD regime to the hard QCD regime.

Fπ,K

π, K, etc

φ

φ

Hard Scattering

GPD

• In exclusive reactions we can study both nucleon GPDs and meson form factors

s=1000 GeV2

100 days

Luminosity 1034

ep → e'π+n Pion form factorPion factorization

1/Q6

1/Q8

1/Q4

Q2 [GeV2]Q2 [GeV2]

17

Lower-energy, and symmetric kinematics allow for wider π+ angular distribution:

Facilitates detection

Better angular and momentum resolution

4 on 12

10 on 250

Deep Exclusive - meson kinematics

[Tanja Horn]Tanja Horn, Imaging in Exclusive Processes,

INT10-3, Seattle

P (GeV/c)

5 on 50 10 on 50

4 on 250

Q2>10GeV2

High momentum over full angular range

Moderate momentum over large angular range

Physics interest x>0.01:

Non-perturbative sea quarks

ep → e'π+n

18

= 5 = 1.3 = 1.3

= 0.3 = 0.3 t/t ~ t/Ep

Wider recoil neutron distribution at lower Ep

Better t- resolution

(Tanja Horn)

Want 0 < t < 1 GeV

4 on 12 5 on 50 10 on 50

4 on 250 10 on 250

Deep Exclusive – recoil baryon kinematics

[Tanja Horn]

ep → e'π+n

Tanja Horn, Imaging in Exclusive Processes, INT10-3, SeattleExclusive processes at x>0.01: better prospect with lower-energy and more

symmetric kinematics19

Exclusive Meson Production Exclusive Meson Production PerspectivesPerspectives


• Luminosity– Non-diffractive proceses (exclusive π and K production) require high

luminosity for low rates, differential measurements in x, t, Q2

– Kaons push luminosity >1034

• Kinematic reach– Need Q2>10 GeV2 (pointlike configurations)– x range between 0.001 and 0.1 overlapping with HERA and JLab12 GeV– s-range between 200 and 1000 GeV2

• Energies– More symmetric energies favorable, 5 on 50 seems to be a sweet spot

for exclusive meson production– Lower energies essential for ε range in pseudoscalar L/T separations

(pion form factor)

• Detection– Recoil detection for exclusivity, t-range

20

Summary

• The EIC is an excellent tool to access nucleon structure

Tanja Horn, EIC@JLab - taking nucleon structure beyond the valence region, INT09-43W


• JLab 12 GeV

– Main focus: valence quark imaging with DVCS

– Also initial deep exclusive meson production studies

• EIC: gluon and sea quarks

– Transverse gluon and sea quark imaging through deep exclusive meson production

21

Imaging in Exclusive Processes Tanja Horn INT10-03 “Imaging QCD Matter”, Institute for Nuclear...

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