Vector Meson Production

1
Vector Meson Production and DVCS from H1 at HERA
K.Hiller, DESY - for the H1 Collaboration

• Introduction
• r and J/y in Photo Electroproduction
• - W- dependence
• - t dependence
• - effective Regge trajectories
• - Q2 dependence
• - helicity studies
• Deeply Virtual Compton Scattering (DVCS)
• Conclusions

2
Introduction - Vector Mesons at HERA (1)
Vector mesons have JPC 1 - - as photon ?
no quantum number exchange necessary ? mainly
produced in diffractive processes
Elastic process, dominates at low t
Proton-dissociation, mainly at high t
Q2 g virtuality
0 lt Q2 lt 100 GeV2 Wgp energy of the
gp system 20 lt Wgp lt 200 GeV t
4-momentum tansfer at p-vertex 0 lt t
lt 30 GeV2 VM Vector mesons
r, w, f, J/y, y, U
HERA collides e-p at energies of 27 GeV
920 GeV
Large diffractive cross sections wide kinematic
range ? HERA is a good place for VM
studies
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Introduction Signatures (2)
Photoproduction
DIS
Mainly elastic and p-dissociative processes ,
2/3 track topologies
Clean samples with less backgrounds
J/y? mm-
F ? KK-
r ? pp-
4
Introduction Theoretical Concepts (3)
Regge Model
pQCD Models
hard processes by 2-gluons/ladder exchange
soft processes by Pomeron exchange
aP(t) a0 a t, a0 1.08, a 0.25GeV-2
Leading order s x g(x,Q2)2

• slow increasing total cross section
• s(W) W d , d 4(a0-1-a/b)
• t-dependence s(t) exp(-bt)
• with shrinkage b b0 4a ln(W/W0)
• s-channel helicity conservation (SCHC)

• steeper W-dependecnce according
• to gluon density at low x
• universal slope ds/dt t-n ,
• without shrinkage
• violation of SCHC

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r Photoproduction (1)
New 2005 data !!!

• Motivation
• replace 1993 r data 358 events
• 2005 data
• Fast Track Trigger , 23 prong events
• L 570 nb-1, 240.000 r candidates
• kinematic range
• Q2 lt 4 GeV2, ltQ2gt 0.01 GeV2
• 20 lt W lt 90 GeV
• t lt 3 GeV2
• Backgrounds r, w, f lt 2
• separation of elastic and p-dissociative
• processes using forward detectors

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r Photoproduction (2)
Large r statistics allows 2-dimensional analysis
in W-t-bins
Fit W-dependence in each t-bin s (W/W0)
4a 1 ? a(t)
H1 fit in good agreement with measurements of
ZEUS and OMEGA
First a(t) measurement in a single experiment
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r Photoproduction (3)

• Fitting a in t-bins by a linear trajectory
• a(t) a0 a t
• a0 1.093 0.003 0.008/0.007
• a (0.116 0.027 0.036/0.046) GeV-2

Note ZEUS values combine ZEUS H1
OMEGA

a differs significantly from classical value
a 0.25 GeV-2
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r Electroproduction (1)
What happens at larger photon masses ?
2000 data, L 42 pb-1 8 lt Q2 lt 60 GeV2 40 lt W
lt 180 GeV, t lt 0.5 GeV2

s W d
0.50 0.80 0.96 1.08
W-dependence steeper at large Q2 d(W) reflect
soft ? hard transition
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r Electroproduction (2)
What is the virtual photon polarization ?
Using 4 production decay angles to determine
spin density matrix element

• polar decay angle
• ds/dcosq ?
• 1 r0004 (3r0004-1) cos2Q

R sL/sT r0004 / e (1-r0004) ( e
1, SCHC )
Matrix elements based on no/single/double flip
amplitudes M,M -- / M0, M-0 / M-, M
- describe angular spectra
At larger Q2 longitudinal photon dominates pQCD
model works
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r Electroproduction (3)
Is the photon helicity conserved ? (at large Q2
r0004 gt 0 consistent with SCHC)
r azimuthal production angle ds/d f ? 1
?2e(1e) cosf(r0052r115) e cos2f (r001
2r111)
Clear violation of SCHC Increase with t
predicted by pQCD model
) Ivanov/Kirchner
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r Photoproduction at large t (1)
Published 2006
Continues SCHC violation at higher t ?
2000 data, L 20 pb-1 1.5 lt t lt 10 GeV2 Q2 lt
0.01 GeV2 , 75 lt W lt 95 GeV
r0004
Fitting r0004, r1004, r1-104 to s(Q, f) in
bins of t
r1-104
SCHC violation persists 2-gluon and BFKL models
clearly inconsistent with data
r1004
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J/y Photo Electroproduction (1)
Change the mass scale
19992000 data , L55pb-1 t lt 1.2 GeV2, MY lt
1.6 GeV p-diss. background 14
s Wd, with d 0.75 at low and high Q2, J/y
mass provides a hard scale for pQCD
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J/y Photo Electroproduction (2)
Describing W-t dependencies by an effective
Pomeron trajectory s ? exp(b0t) W d
with d 4(a(t)-1),
a(t) a0 a t
ltQ2gt 0 / ltQ2gt 9 GeV2 a0 1.22
/ 1.18 a 0.16 / 0.02
GeV-2
trajectories similar (errors !) large a0 1.2
reflects steep W-dependence of hard /
perturbative process
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J/y Photo Electroproduction (3)
What about helicity conservation ?
r0004 0 in photoproduction, but rises with
increasing Q2
For SCHC r1-11 (1 r0004)/2
Other spin density matrix elements 0
SCHC works
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J/y Photoproduction at large t (1)
2 lt t lt 30 GeV2 1996 2000 data, L 78
pb-1
Extension to higher t
d 0.77 1.29 1.30
s ? t-n n 3.00 0.08 0.05
d 1 confirms large t as hard scale, DGLAP
/ BFKL work in complementary t regions lt / gt
MJ/y2
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J/y Photoproduction at large t (2)
And helicity conservation
Untagged photoproduction 2 angles ? 3 matrix
elements
consistent with SCHC r0004 r1-104
Rer1004 0 (large errors at highest t)
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How J/y compares to r ?
J/y meson
r meson

• Low-Q2, low-t
• flat W-dependence ? W0.2,
• exponential s exp(-bt)
• a 0.1 GeV-2,
• no universal Pomeron
• High-Q2, low-t
• steeper W-dependence ? W1,
• effective Pomeron a0 1.2
• sL dominates at high Q2
  
• violation of SCHC
• Low-Q2, high-t
• power law s ? t-n
• violation of SCHC persists
  

• Low-Q2, low-t
• steeper W-dependence ? W1
• High-Q2, low-t
• SCHC valid
• Low-Q2, high-t
• SCHC valid

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Deeply Virtual Compton Scattering (1)
What can we learn about parton correlation ?

• Ideal process DVCS e p ? e p g
• massive g needs skewness ?,
• t-dependence for transverse motion
• H (x, ?, t, Q2)
• generalized parton density functions (GPDs)

LO
x - ?
x ?
NLO

• simple process similar to VM production, but
• no wave function needed
• - Bethe-Heitler background pure el.mag.process,
• precisely known

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DVCS Theoretical Tools (2)

• Generalized PDF Models
• hard process use factorized ansatz
• of hard scattering ME x GPDs
• NLO leading twist calculations
• using ansatz for gluon and q-singlet
• Hg (x, ?, t, µ2) x g(x,µ2) exp (-bt)
• Hq (x, ?, t, µ2) q(x,µ2) exp (-bt)
• with PDFs MRST2001 / CTEQ6
• Q2 and x dependence by evolution

Color Dipole Models
-

• g fluctuates into qq pair
• Color dipole interacts with proton
• qq pair converts to real g

-
) Freund, McDermott

• A ? d2Rdz Ygin sdipole Ygout
• sdipole model input

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DVCS W t dependences (3)
HERA-2 data
6.5 lt Q2 lt 80 GeV2, 30 lt W lt 140 GeV, t lt 1 GeV2
t - dependence
W - dependence
b 6 GeV-2
Wd, with d 1
Consistent with H1/HERA-1 data, ZEUS data at low
W a bit higher typical hard process at low t
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DVCS Models versus Q2 / W (4)
GPD - model
Dipole - model
Band for b-range 5.26 lt b lt 6.4 GeV-2, no Q2
dependence of b
Both models reproduce general trend quite
well, smaller errors needed for model
discrimination
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Summary

• New high-statistics r-measurement at Q2 0
• New publication of r-photoproduction at high t
  
• Soft to hard transition visible s ? W0.2 ?
  s ? W1
• Regge parametrization alive, but no universal
  Pomeron
• Violation of SCHC for r, not for J/y
• pQCD models describe the trend, need
  improvements in
• details ( helicity at high t)

Vector Mesons
DVCS

• Theoretical interest to extract generalized PDFs
• First HERA-2 measurement performed, confirms
  HERA-1 data
• Models based on GPDs and color dipoles
  reproduce trend

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Spare slides
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r Photoproduction (2)

• Data Monte Carlo
• Generator DIFFVM
• Pomeron exchange plus VM dominance
• ds/dMY2 MY -2(1e) with e 0.08
• low-t ds/dt exp(- bt)
• with b 11.4 / 6.5
  GeV-2
• for
  elastic/p-dissociative
• high-t ds/dt t-n
• with n 23 / 8.5
• for elastic
  /p-dissocitive

Monte Carlo reproduces well-known features of
diffractive vector meson production
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r Photoproduction at large t (2)
What about ds/dt ?
Standard low-texponential fails, ds/dt t-n
, with n 4.3

• QCD-Models
• 2-gluon model fixed/running as fails
• BFKL / gluon ladders better,
• but undershoots at highest t

High t range nice for tuning of QCD models
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J/y Photo Electroproduction (3)
Slope of the exp(-bt) in dependence on W
b(W) b0 4a ln(W/W0)
Photoproduction a (0.164 ) GeV-2
Electroproduction a (0.019 ) GeV-2
Photoproduction b increases with W
shrinkage Electroproduction b constant
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J/y Photo Electroproduction (5)
Use r0004 to separate sL and sT
(M2 J/y Q2) n n 2.486 0.080 0.068
stot sL sT
at low Q2 dominates sT, at large Q2 more
sL pQCD predictions at large Q2 sensitive to
gluon PDFs
) Martin, Ryskin, Teubner
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Vector Mesons pQCD
s ? x g(x,Q2)2 with x ? 1/W2
VM production sensitive to g(x,m2) At hard
scales, large MVM or Q2 or t, pQCD reproduces
general features
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DVCS Models versus W (4)
GPD - model
Dipole - model
Band for b-range 5.26 lt b lt 6.4 GeV-2, no Q2
dependence of b
Consistent with H1/HERA-1 data ZEUS data at low W
a bit higher Both models reproduce trend,
MRST2001 50 off
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DVCS Models versus Q2 (5)
Dipole - model
GPD - model
Band for b-range 5.26 lt b lt 6.4 GeV-2, no Q2
dependence of b
Again both models give reasonable
predictions, MRST2001 seems to overshoot the data