Results on Diffractive Vector Meson Production in ZEUS

Results on Diffractive Vector Meson Production in ZEUS

Joachim TandlerBonn University

DIS 03St. Petersburg, 23-27 March 2003

Motivation

Experimental results: different VMs, different kinematic ranges

Summary

on behalf of

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 2

Clean experimental signature!

HERA: Elastic VM Production

MVM VM mass, 0.8 – 3.1 GeV

Q2 = -q2 = (k-k‘)2 Virtuality of exchanged * 0 – 100 GeV2

W = (q+p)2 *p centre of mass energy 20 – 290 GeV

x = Q2/(p·q) = 1/(1+(W/Q)2)

x Bjørken variable 10-4 – 0.9

t = (P-P‘)2 4-momentum transfer squared at p vertex

-20 – 0 GeV2

VM = J/

M

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 3

Models for VM Production

* VM oscillation (before interaction)

Soft pomeron exchange

t‘ t, „trajectory“

(‘ = 0.25 GeV-2)

ddt eb0t (W/W0)4 (t1)

Vector Dominance Regge

soft regime

Predictions:

Slow rise W) W 0.22

Shrinkage: b(W) = b0 + 4‘ ln(W/W0)

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 4

Models for VM ProdutionPerturbative QCD

In proton rest frame

• * qq oscillation

• qq scatters off the proton

• VM is formed (after interaction)

qq lifetime 1/(xMp): small x long lifetime

L (x,Q2) = d2rdz |L(r,z,Q2)|2 · qq2(x,r)

L: *L qq wave function

qq: qq-proton cross-section

z = Eq/E*

r: transverse separation of qq

r = [z(1-z)Q2 +mq2]½ is small,

if Q2 large or quarks heavy: VM=cc or bb

qq

_

_

_

_

_

_

_

_

_

_ _

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 5

Predictions:

Fast rise W) [x 0.2]2, (x Q2/W2) W 0.8

Universal t dependence d/dt ebt

b = b2g 4 GeV-2 and ‘ 0

Naively: J=9 : 1 : 2 : 8

Two gluon exchange (LO) in colour singlet state

L s2

(Qeff2)/Q6 |xG(x,Qeff

2)|2

Models for VM Production

Qeff2 7 GeV2

hard regime

pQCD

Questions:

• Do these models describe HERA data?

• At which scale, Qeff2, has xG to be

evaluated? I.e. which combination of Q2, MVM and t?Ryskin: Qeff

2 = ¼ (Q2 + MVM2 + t)

M

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Q2 as scale

electroproduction

Q2

softhard

Fit W) W

soft hard

No saturation yet

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MVM as scale

Vector Meson

Photoproduction

MVMsoft

hard

optical theorem

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J/ Photoproduction

Fast rise with W: = 0.69 ± 0.02

Described by pQCD with MJ/ as hard scale

Sensitivity to the

gluon density

L |xG(x,Q2)|2

J/ Photoproduction

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Two scales: MVM and Q2

J/ Electroproduction

is flat in Q2

Q2

No change of W dependence with Q2

Fit W) W

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 10

Two scales: MVM and Q2

Compatible with photoproduction?

New!

W dependence in t-bins

Fit with ddt (W/W0)4 (t1)

Extract thepomeron trajectory

J/ Electroproduction

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 11

Two scales: MVM and Q2

Pomeron trajectory in J/ DIS Yes, it is compatible with photoproduction Hard process in photoproduction and DIS

New!

J/ Electroproduction

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J/ Electroproduction

Q2 sets the size r of the colour dipoleCross section well described in shape and normalisation (MRT) by pQCD models

qq(r) is well modelled

different n for L andTas expected

, J/ Electroproduction

J/

Fit Q2) (Q2+MJ/2)-n

L

T

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 13

Size of the proton?

J/ and at high Q2 are point-like

J/

p-diss

d/dt ebt, b is related to the transverse sizes of the scattering objects

„b = b2g + bVM“

pQCD: universal b slope

b = b2g 4 GeV-2

(gluon could of the proton)

, J/ Electroproduction

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VMs at High t

Expectations:Soft: d/dt ebt

Hard: power-like (BFKL) (naïve: t4)

p VMY, VM = J/

Indeed power-like behavior

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 15

Q2+M2 universal scale?

Naïve SU(4) approachUse scale Q2 + M2

Fails for the J/

VM wave function effects?

J/

Q2+M2

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Ratio V/tot

W independence for cannot be explained by pQCD or Regge

still unknown soft physics?

J/ pQCD:

V S/Q6 |xG(x,Q2)|2

V/tot W2/b

Regge:

V W4 (01)

V/tot W2 (01) /b

Q2+M2

4 GeV2

25 March 2003 Joachim Tandler, DIS 03, St. Petersburg 17

Summary New Pomeron trajectory in high-Q2 J/

confirms J/ to be hard alredy in photoproduction

Various kinematic regions

Validity range of pQCD in terms of MVM, Q2, t

Generally, pQCD and the colour dipole model

are able to describe the hard regime

Still not clear, what is finally the scale to use

Is elastic production at high Q2 still soft?