exclusive vm electroproduction

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 1

Exclusive VM electroproduction

Aharon LevyTel Aviv University

on behalf of the H1 and ZEUS collaborations

* 0

0 , , , / ,

p V p

V J

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 2

Why are we measuring

IP

‘soft’

WW )(

‘hard’

gg||tbe

dt

d

• Expect to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q2)2)

• Expect b to decrease from soft (~10 GeV-2)

to hard (~4-5 GeV-2)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 3

s0.096

2( )2

QF x

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 4

Why are we measuring

IP

‘soft’

WW )(

‘hard’

gg||tbe

dt

d

• Expect to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q2)2)

• Expect b to decrease from soft (~10 GeV-2)

to hard (~4-5 GeV-2)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 5

Why are we measuring

Use QED for photon wave function. Study properties of VM wf and the gluon density in the proton.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 6

Mass distributions

KK /J

PHP

DIS 0

0

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 7

Photoproduction

W process becomes hard as scale (mass) becomes larger. real part of amplitude increases with hardness.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 8

(W) – ρ0

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 9

Proton dissociationMC: PYTHIA

pdiss: 19 ± 2(st) ± 3(sys) %

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 10

(W) – ρ0,

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 11

(W) - , J/,

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 12

(Q2+M2) - VM

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 13

Kroll + Goloskokov: = 0.4 + 0.24 ln (Q2/4) (close to the CTEQ6M gluon density, if parametrized as xg(x)~x-/4)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 14

(Q2) nMQ

22

Fit to whole Q2 range gives bad 2/df (~70)

VM n comments

ρ 2.44±0.09 Q2>10 GeV2

2.75±0.13 ±0.07

Q2>10 GeV2

J/ 2.486±0.080±0.068

All Q2

1.54±0.09 ±0.04

Q2>3 GeV2

101Q2(GeV2)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 15

(Q2)

* 2

2

2 2 ( )

1( )

( )p n QQ

Q M

2 2( ) 2.15 0.007n Q Q

(for Q2 > 1 GeV2)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 16

Proton vertex factorisation

photoproduction

proton vertex factorisation

Similar ratio within errors for and

proton vertex factorisation in DIS

IP IP

Yelastic p-dissociative

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 17

b(Q2) – ρ0, Fit

||tbedt

d

:

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 18

b(Q2+M2) - VM

2 2( )r b c

‘hard’

gg

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 19

Information on L and T

Use 0 decay angular distribution to get r0400 density matrix

element 04 04 200 00(cos ) (1 ) (3 1)cosh hf r r

04000400

1

1L

T

rR

r

- ratio of longitudinal- to transverse-photon fluxes ( <> = 0.996)

0400

L L

L T tot

r

using SCHC

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 20

R=L/T (Q2)When r00

04 close to 1, error on R large and asymmetric advantageous to use r00

04 rather than R.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 21

R=L/T (Q2)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 22

R=L/T (Mππ)

Why??

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 23

R=L/T (Mππ)

Possible explanation:

2

1R

M

example for =1.5

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 24

Photon configuration - sizessmall kT large

kTlarge config.

small config.

T: large size small size

strong color forces color screening

large cross section small cross section

*: *T, *L

*T – both sizes, *L – small size

Light VM: transverse size of ~ size of proton

Heavy VM: size small cross section much smaller (color transparency) but due to small size (scale given by mass of VM) ‘see’ gluons in the proton ~ (xg)2 large

qq

qq

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 25

L/tot(W)

L and T same W dependence

L in small configuration

T in small and large configurations

small configuration steep W dep

large configuration slow W dep

large configuration is suppressed

qq

qq

qq

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 26

L/tot(t)

TL bb

size of *L *T

large configuration suppressed

qq

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 27

(W) - DVCS* p p

Final state is real T

using SCHC initial * is *T

but W dep of steep

large *T configurations suppressed

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 28

Effective Pomeron trajectory

ρ0

photoproduction

Get effective Pomeron trajectory from d/dt(W) at fixed t

2[2 ( ) 2]( ) ( ) IP td

W F t Wdt

Regge:

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 29

Effective Pomeron trajectory

ρ0 electroproduction

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 30

Comparison to theory

• All theories use dipole picture

• Use QED for photon wave function

• Use models for VM wave function – usually take a Gaussian shape

• Use gluon density in the proton

• Some use saturation model, others take sum of nonperturbative + pQCD calculation, and some just start at higher Q2

• Most work in configuration space, MRT works in momentum space. Configuration space – puts emphasis on VM wave function. Momentum space – on the gluon distribution.

• W dependence – information on the gluon

• Q2 and R – properties of the wave function

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 31

ρ0 data (ZEUS) - Comparison to theory

• Martin-Ryskin-Teubner (MRT) – work in momentum space, use parton-hadron duality, put emphasis on gluon density determination. Phys. Rev. D 62, 014022 (2000).

• Forshaw-Sandapen-Shaw (FSS) – improved understanding of VM wf. Try Gaussian and DGKP (2-dim Gaussian with light-cone variables). Phys. Rev. D 69, 094013 (2004).

• Kowalski-Motyka-Watt (KMW) – add impact parameter dependence, Q2 evolution – DGLAP. Phys. Rev. D 74, 074016 (2006).

• Dosch-Ferreira (DF) – focusing on the dipole cross section using Wilson loops. Use soft+hard Pomeron for an effective evolution. Eur. Phys. J. C 51, 83 (2007).

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 32

ρ0 data (H1) - Comparison to theory

• Marquet-Peschanski-Soyez (MPS): Dipole cross section from fit to previous , and J/ data. Geometric scaling extended to non-forward amplitude. Saturation scale is t-dependent.

• Ivanov-Nikolaev-Savin (INS): Dipole cross section obtained from BFKL Pomeron. Use kt-unintegrated PDF and off-forward factor.

• Goloskokov-Kroll (GK): Factorisation of hard process and proton GPD. GPD constructed from standard PDF with skewing profile function.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 33

Q2

KMW – good for Q2>2GeV2 miss Q2=0

DF – miss most Q2

FSS – Gauss better than DGKP

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 34

Q2

Data seem to prefer

MRST99 and CTEQ6.5M

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 35

W dependence

KMW - close

FSS:

Sat-Gauss – right W-dep.

wrong norm.

MRT:

CTEQ6.5M – slightly better in W-dep.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 36

L/tot(Q2)

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 37

L/tot(W)

All models have mild W dependence. None describes all kinematic regions.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 38

L,T (Q2+M2)

• Different Q2+M2 dependence of L and T (L0 at Q2=0)

• Best description of L by GK; T not described.

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 39

Density matrix elements - ,

• Fair description by GK

• r500 violates SCHC

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 40

VM/tot

June 10, 2008 A. Levy: Exclusive VM, GPD08, Trento 41

Summary and conclusions• New high statistics measurements on ρ0,

electroproduction and on DVCS.• The Q2 dependence of (*pρp) cannot be described by a

simple propagator term.• The cross section is rising with W and its logarithmic

derivative in W increases with Q2.• The exponential slope of the t distribution decreases with Q2

and levels off at about b = 5 GeV-2.• Proton vertex factorisation observed also in DIS.• The ratio of cross sections induced by longitudinally and

transversely polarised virtual photons increases with Q2, but is independent of W and t.

• The effective Pomeron trajectory has a larger intercept and smaller slope than those extracted from soft interactions.

• All these features are compatible with expectations of perturbative QCD.

• None of the models which have been compared to the measurements are able to reproduce all the features of the data.