Transcript ppt - Desy

Results on Diffractive Vector Meson
Production in ZEUS
Joachim Tandler
Bonn University
DIS 03
St. Petersburg, 23-27 March 2003
on behalf of
Motivation
Experimental results:
different VMs,
different kinematic ranges
Summary
HERA: Elastic VM Production
Clean experimental signature!
M
MVM
VM mass, VM = r, w, f, J/Y
0.8 – 3.1 GeV
Q2
= -q2 = (k-k‘)2
Virtuality of exchanged g*
0 – 100 GeV2
W
= (q+p)2
g*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
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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Models for VM Production
Vector Dominance  Regge
soft regime
g*  VM oscillation (before interaction)
Soft pomeron exchange
a(t) = a0 + a‘ t, „trajectory“
(a0 = 1.08, a‘ = 0.25 GeV-2)
ds/dt  eb0t (W/W0)4 (a(t) - 1)
Predictions:
 Slow rise s(W)  Wd, d  0.22
Shrinkage: b(W) = b0 + 4a‘ ln(W/W0)
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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Models for VM Prodution
Perturbative QCD
_
qq
In proton rest frame
_
• g*  qq oscillation
_
• qq scatters off the proton
•
VM is formed (after interaction)
_
qq lifetime t  1/(xMp): small x  long lifetime
sL (x,Q2) =  d2rdz |YL(r,z,Q2)|2 · sqq_2(x,r)
_
_
YL: g*L  qq wave function
r: transverse separation of qq
_
_
sqq: qq-proton cross-section
r = [z(1-z)Q2 +mq2]-½ is
small,
z = Eq/Eg*
if Q2 large
or_ quarks heavy:
_
VM=cc or bb
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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Models for VM Production
M
hard regime
pQCD
Two gluon exchange (LO) in colour
singlet state
sL  as2 (Qeff2)/Q6 |xG(x,Qeff2)|2
Qeff2  7 GeV2
Predictions:
Questions:
 Fast rise s(W)  [x -0.2]2, (x  Q2/W2)
 Wd, d  0.8
•
 Universal t dependence ds/dt  ebt
b = b2g  4 GeV-2 and a‘  0
• At
 Naively: r : w : f : J/y = 9 : 1 : 2 : 8
25 March 2003
Do these models describe HERA
data?
which scale, Qeff2, has xG to be
evaluated?
I.e. which combination of Q2, MVM
and t?
Ryskin: Qeff2 = ¼ (Q2 + MVM2 + t)
Joachim Tandler, DIS 03, St. Petersburg
5
2
Q
as scale
Fit s(W)  Wd
soft
Q
No saturation yet
2
hard
r electroproduction
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
soft
hard
6
MVM as scale
optical theorem
soft
MVM
hard
Vector Meson
Photoproduction
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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J/y Photoproduction
Fast rise with W:
d = 0.69 ± 0.02
Described by pQCD
with MJ/y as hard
scale
Sensitivity to the
gluon density
sL  |xG(x,Q2)|2
25 March 2003
J/y Photoproduction
Joachim Tandler, DIS 03, St. Petersburg
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Two scales: MVM and Q2
Fit s(W)  Wd
Q2
J/y Electroproduction
No change of
W dependence with Q2
25 March 2003
 d is flat in Q2
Joachim Tandler, DIS 03, St. Petersburg
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Two scales: MVM and Q2
W dependence in tbins
Fit with
ds/dt  (W/W0)4 (a(t) - 1)
Extract the
pomeron trajectory
J/y Electroproduction
Compatible with photoproduction?
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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Two scales: MVM and Q2
J/y Electroproduction
Pomeron trajectory in J/y DIS
 Yes, it is compatible with photoproduction
 Hard process in photoproduction and DIS
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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J/y Electroproduction
J/y
r
sL
sT
25 March 2003
Fit s(Q2)  (Q2+MJ/y2)-n
Q2 sets the
size r of the colour dipole
Cross section well
described in shape and
normalisation (MRT) by
pQCD models
 sqq(r) is well modelled
different n for sL and sT
as expected
r, J/y Electroproduction
Joachim Tandler, DIS 03, St. Petersburg
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Size of the proton?
ds/dt  ebt, b is related to the
transverse sizes
of the scattering objects
r
„b = b2g + bVM“
pQCD: universal b slope
b = b2g  4 GeV-2
r p-diss
J/y
25 March 2003
(gluon could of the proton)
 J/y and r at high Q2 are
point-like
r, J/y Electroproduction
Joachim Tandler, DIS 03, St. Petersburg
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VMs at High t
Expectations:
Soft: ds/dt  ebt
Hard: power-like
(BFKL)
(naïve: t4)
 Indeed power-like behavior
g p  VM Y,
VM = r,f,J/y
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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Q2+M2 universal scale?
r, f, J/y
Q2+M2
Naïve SU(4) approach
Use scale Q2 + M2
25 March 2003

Fails for the J/y

VM wave function
effects?
Joachim Tandler, DIS 03, St. Petersburg
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Ratio sV/stot
J/y
Q2+M2
4 GeV2
r
pQCD:
sV  aS/Q6
|xG(x,Q2)|2
sV/stot  W2l/b
Regge:
sV  W4 (a(0) - 1)
sV/stot  W2 (a(0) - 1) /b
W independence for r cannot be explained by pQCD or Regge
 still unknown soft physics?
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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Summary
New Pomeron trajectory in high-Q2 J/y
confirms J/y 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 r production at high Q2 still soft?
25 March 2003
Joachim Tandler, DIS 03, St. Petersburg
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