International Workshop on Heavy Quarkonium
FERMILAB, September 20-22, 2003
Charmonium Production at HERA
Luca Stanco – INFN Padova
H1 and ZEUS collaborations
Outline:
• Introduction
•
•
•
•
•
J/y Production Mechanisms
Inelastic J/y and y’ photoproduction
Inelastic J/y electroproduction
Polarization Measurements
Conclusions and Outlook
Luca Stanco - Padova
Heavy Quarkonium, Fermilab
1
Introduction - I
Kinematic variables:
Q 2   q 2  ( k  k ' ) 2
s  ( P  k )2
Wp  ( P  q ) 2
z  py  P / q  P
 Ey / E
• photoproduction (Q2 < 1 GeV2)
scattered e not seen in the main detector
• electroproduction (2 < Q2 < 100 GeV2)
scattered e detected in calorimeter
• J/y detected through:
m+m- , e+e- decay modes
Luca Stanco - Padova
Heavy Quarkonium, Fermilab
in proton rest frame
2
Introduction - II
charmonium production ?
Color Singlet Model (CSM)
(orders of magnitude too low)
Non-Relativistic QCD
(NRQCD)
Luca Stanco - Padova
Heavy Quarkonium, Fermilab
3
Production Mechanisms - I
Direct photon gluon fusion:
z  0.2
CSM
Color Singlet Model
cc must have J/y quantum numbers
Only one free parameter given by y l  l 
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CS+CO
Non-Relativistic QCD
cc also in colour octet state
Additional free parameters
Long distance matrix elements (LDMEs)
LDMEs not calculablefrom experiments
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Production Mechanisms - II
Resolved photon processes (gg fusion): z
 0.2
suppressed with increasing Q2
CSM
Color Singlet Model
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CS+CO
Non-Relativistic QCD
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5
Production Mechanisms – III (background)
1. .Diffraction (subracted in ZEUS:  15%, not subtracted in H1:  2%)
ELASTIC diffraction: z = 1
ELASTIC diffraction: z = 1
Proton dissociation: z  1
Proton dissociation: z  1
Suppressed by cuts z<0.9, p*t,y (pt,y in p), additional activity in the detector
2. Decay of diffractively or inelastically produced y’ mesons: y’  J/y pp
not subtracted in data !
3. Decay of cc mesons: c’  J/y  (low z) not subtracted in data !
4. Decay of B mesons: B  J/y X (low z, high pt,y) not subtracted in data !
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Heavy Quarkonium, Fermilab
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Production Mechanisms and DATA
J/y variables
z > 0.4
50 < W < 180
From MC: 44% of
diffractive protondissociation,
(17% for z < 0.9)
17%
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27%
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7
Photoproduction: DATA and CSM NLO
H1: 96-00, m+m-
ZEUS: 96-97, m+m-
(scaled to H1 phase space)
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Heavy Quarkonium, Fermilab
1. Good agreement
between H1 and
ZEUS
2. full NLO of the
direct -g fusion
(M.Krämer)
3. theory in agreement
with data both in
shape and
normalization, within
the large theoretical
uncertainties
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Photoproduction: DATA and NRQCD LO
The measurements explore the low z region: z=0.05 (H1), z=0.1 (ZEUS)
1. Large uncertainties in
calculation due to
LDMEs from CDF
2. Large values of LDME
are here excluded
3. .NRQCD LO calculation
including direct and
resolved photon
(M.Krämer and
M.Cacciari) provide a
good description of
whole z range with
small LDMEs
Luca Stanco - Padova
Heavy Quarkonium, Fermilab
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Photoproduction: DATA and NRQCD LO
Cut on low pT to exclude low z (and to increase hard scale)
1.
2.
NRQCD LO calculation resumming soft contributions at high z (M.Beneke,
G.A.Schuler and S.Wolf)
L: energy loss of J/y due to soft gluon radiation
Resummation reduces discrepancies at high z
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Heavy Quarkonium, Fermilab
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y’ Photoproduction: DATA and CSM
0.55 < z < 0.9
50 < W < 180
1. Underlying production
mechanisms for J/y
and y’ are the same !
2. Integrated on DATA:
0.33±0.10+0.01-0.02
3. .LO-CSM prediction :
24%
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Heavy Quarkonium, Fermilab
11
Electroproduction: Q2 and p*T,y dependence
H1: 97-00, e+e- and m+mLuca Stanco - Padova
1. Comparisons with CS and
NRQCD LO contributions
(B.A.Kniehl and
L.Zwirner)
2. . CS contribution too low
by  2.7 and too step in
(p*T,y)2
missing higher orders ?
3. .CS+CO too high at low
Q2 and (p*T,y)2 (  2)
CS+CO desription
improves at high Q2 and
(p*T,y)2
(smaller theoretical
errors)
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Electroproduction: z and rapidity dependence
ZEUS: 99-00, m+m-
(not easyly scalable to H1 phase space)
z: missing resummation of soft
terms for CS+CO comparisons
Luca Stanco - Padova
CS below data but shape consistent
CS+CO above the data
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13
Polarization Measurements
Polarization of J/y provides information on production
processes independent of normalization uncertainties
Polarization is measured in decay angular distributions in J/y rest frame
system:
q* : angle m+ to z’ axis (opposite direction to that of the proton)
f* : angle m+ to plane determined by incoming photon and proton
d
2 *

1

λ
cos
q
*
 d cosq
1 d
 
*

1


cos
2
f
*
 df
3 3
1

n
=+1: transverse polarization, =-1: longitudinal polarization
Luca Stanco - Padova
Heavy Quarkonium, Fermilab
14
Polarization Measurements: photoproduction

n
Luca Stanco - Padova

n
Heavy Quarkonium, Fermilab
H1: 96-00, m+mZEUS: 96-00, m+m-
1. variable n more
promising to
disentangle
2. more data needed to
decide on production
mechanism(s)
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Conclusions and Outlook
Photoproduction:
• medium z: good agreement with CSM NLO calculations
• low z: resolved photon contributions improve agreement
• NRQCD with small LDMEs gives reasonable description
• y’ in line with expectations (but still large exper. errors)
Electroproduction:
• CS (LO) alone too low, wrong (p*T,y)2 dependence
• NRQCD with small LDMEs gives reasonable description
DATA:
• major improvements in data statistics needed for
conclusions from polarization measurements
 only possible with HERA II data
Theory:
• NLO calculation in NRQCD is needed !
(both for photo- and electro-production)
Luca Stanco - Padova
Heavy Quarkonium, Fermilab
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