Luca Stanco - Padova Heavy Quarkonium, Fermilab 1

Charmonium Production at HERALuca Stanco – INFN Padova

Outline:• Introduction• J/ Production Mechanisms• Inelastic J/ and ’ photoproduction • Inelastic J/ electroproduction• Polarization Measurements• Conclusions and Outlook

International Workshop on Heavy QuarkoniumFERMILAB, September 20-22, 2003

H1 and ZEUS collaborations

Luca Stanco - Padova Heavy Quarkonium, Fermilab 2

Introduction - I

• photoproduction (Q2 < 1 GeV2) scattered e not seen in the main detector• electroproduction (2 < Q2 < 100 GeV2) scattered e detected in calorimeter • J/ detected through: +- , e+e- decay modes

Kinematic variables:

frame rest proton in

EE

PqPpz

qPW

kPs

kkqQ

p

/

/

)(

)(

)'(

2

2

222

Luca Stanco - Padova Heavy Quarkonium, Fermilab 3

Introduction - II

charmonium production ?

Color Singlet Model (CSM)

(orders of magnitude too low)

Non-Relativistic QCD (NRQCD)

Luca Stanco - Padova Heavy Quarkonium, Fermilab 4

Production Mechanisms - I

Direct photon gluon fusion: 2.0z

Color Singlet Modelcc must have J/ quantum numbersOnly one free parameter given by

ll

Non-Relativistic QCDcc also in colour octet stateAdditional free parameters

Long distance matrix elements (LDMEs)LDMEs not calculablefrom experiments

CSM CS+CO

Luca Stanco - Padova Heavy Quarkonium, Fermilab 5

Production Mechanisms - II

Resolved photon processes (gg fusion): 2.0z

Color Singlet Model Non-Relativistic QCD

suppressed with increasing Q2

CSM CS+CO

Luca Stanco - Padova Heavy Quarkonium, Fermilab 6

Production Mechanisms – III (background)

1. .Diffraction (subracted in ZEUS: 15%, not subtracted in H1: 2%)

2. Decay of diffractively or inelastically produced ’ mesons: ’ J/ not subtracted in data !

3. Decay of c mesons: ’ J/ (low z) not subtracted in data !4. Decay of B mesons: B J/ X (low z, high pt,) not subtracted in

data !

Suppressed by cuts z<0.9, p*t, (pt, in p), additional activity in the detector

ELASTIC diffraction: z = 1 Proton dissociation: z 1Proton dissociation: z 1ELASTIC diffraction: z = 1 Proton dissociation: z 1

Luca Stanco - Padova Heavy Quarkonium, Fermilab 7

Production Mechanisms and DATA

z > 0.450 < W < 180

From MC: 44% of diffractive proton-dissociation,(17% for z < 0.9)

J/ variables

27%17%

Luca Stanco - Padova Heavy Quarkonium, Fermilab 8

Photoproduction: DATA and CSM NLO

H1: 96-00, +-

ZEUS: 96-97, +-

(scaled to H1 phase space)

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

Luca Stanco - Padova Heavy Quarkonium, Fermilab 9

Photoproduction: DATA and NRQCD LOThe 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 10

Photoproduction: DATA and NRQCD LOCut on low pT to exclude low z (and to increase hard scale)

1. NRQCD LO calculation resumming soft contributions at high z (M.Beneke, G.A.Schuler and S.Wolf)

2. : energy loss of J/ due to soft gluon radiation

Resummation reduces discrepancies at high z

Luca Stanco - Padova Heavy Quarkonium, Fermilab 11

’ Photoproduction: DATA and CSM

1. Underlying production mechanisms for J/and’ are the same !

2. Integrated on DATA: 0.33±0.10+0.01

-0.02

3. .LO-CSM prediction : 24%

0.55 < z < 0.950 < W < 180

Luca Stanco - Padova Heavy Quarkonium, Fermilab 12

Electroproduction: Q2 and p*T, dependence

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,)2 missing higher orders ?

3. .CS+CO too high at low Q2 and (p*

T,)2 ( 2) CS+CO desription improves at high Q2 and (p*

T,)2 (smaller theoretical errors)

H1: 97-00, e+e- and +-

Luca Stanco - Padova Heavy Quarkonium, Fermilab 13

ZEUS: 99-00, +-

(not easyly scalable to H1 phase space)

Electroproduction: z and rapidity dependence

z: missing resummation of soft terms for CS+CO comparisons

CS below data but shape consistent

CS+CO above the data

Luca Stanco - Padova Heavy Quarkonium, Fermilab 14

Polarization Measurements

Polarization of J/ provides information on production processes independent of normalization uncertainties

Polarization is measured in decay angular distributions in J/ rest frame system:

* : angle + to z’ axis (opposite direction to that of the proton)* : angle + to plane determined by incoming photon and proton

**

*2*

2cos33

11

cos1cos

1

d

d

λd

d

=+1: transverse polarization, =-1: longitudinal polarization

Luca Stanco - Padova Heavy Quarkonium, Fermilab 15

H1: 96-00, +-

ZEUS: 96-00, +-

Polarization Measurements: photoproduction

1. variable more promising to disentangle

2. more data needed to decide on production mechanism(s)

Luca Stanco - Padova Heavy Quarkonium, Fermilab 16

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• ’ in line with expectations (but still large exper. errors)

Electroproduction: • CS (LO) alone too low, wrong (p*

T,)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)