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
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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)
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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
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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
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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
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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
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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
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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
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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?