Exclusive Production of Hadron Pairs in Two-Photon Interactions

Bertrand Echenard

University of Genevaon behalf of the LEP collaborations

Hadronic Physics Session

EPS, Aachen, 17-23 July 2003

contact: bertrand.echenard@cern.ch

B.Echenard EPS 2003, Aachen 17-23 july 2003

Outline

and K pair production and K meson pair production [ALEPH: CERN-EP:2003-030]

Baryon pair production pp [L3: CERN-EP:2003-014 OPAL: Eur. Phys. J. C28 (2003) 45 ]

and [L3: PLB 536 (2002) 24]

and pair production and pair production at Q20 [L3: submitted to EPS]

pair production at high Q2 [L3: CERN-EP:2003-020]

B.Echenard EPS 2003, Aachen 17-23 july 2003

Hadron pair production in collisions

and K pair production

Tests of QCD calculationsHandbag model

Amplitude HH factorized into:

Hard partqq scattering

Soft part: qq HH transition

(1) M. Diehl, P. Kroll and C. Vogt, PLB 532 (2002) 99, hep-ph/0112274.

(2) S. J. Brodsky and J. P. Lepage, PRD 22 (1980) 2157 .

Handbag model (1)

based on the Brodsky and Lepage approach (2)

B.Echenard EPS 2003, Aachen 17-23 july 2003

Hadron pair production in collisions

Baryon pair production

Tests of QCD calculations

Handbag model (1)

Three quark model (2)

Quark-diquark model (3)

Three quark model

Quark-diquark model

Diquark

Three quark & Diquark models

(1) M. Diehl, P. Kroll and C. Vogt, hep-ph/0206288.

(2) G. Farrar et al., Nucl Phys B 259 (1985) 702.

(3) C. F. Berger, B. Lechner and W. Schweiger, Fizika B8 (1999) 371; C. F. Berger and W. Schweiger hep-ph/0212066.

B.Echenard EPS 2003, Aachen 17-23 july 2003

and K pair selection

2 tracks of opposite charge

Electron rejection criteria

Muon rejection criteria

Cut on dE/dx to select pions and kaons

Exclusive event selection cut

ALEPH event selection:

B.Echenard EPS 2003, Aachen 17-23 july 2003

and cross sections

) = (within uncertainty

Shape well described by QCD predictions

Disagreement in normalization and ratio K/ (QCD: () 2 )

Fit with power law: AW-6

: A=200±40 nb•GeV6

: A=220±40 nb•GeV6

B.Echenard EPS 2003, Aachen 17-23 july 2003

Proton-antiproton pair selection

L3:

AntiprotonNeural network: P, dE/dx, Et/pt & shower shape

ProtondE/dx and Et/pt

OPAL: dE/dx and E/P

B.Echenard EPS 2003, Aachen 17-23 july 2003

pp cross section

Results compatible with previous measurements

Extend W range

Good agreement with quark-diquark model

Three quark model excluded

B.Echenard EPS 2003, Aachen 17-23 july 2003

pp angular distribution

Agreement in the intermediate and high W region

Disagreement with diquark model in the low W region

Low mass region well described by spherical harmonics : 92% Y20 and

8% Y00

Low mass Intermediate mass

High mass

B.Echenard EPS 2003, Aachen 17-23 july 2003

and pairselection

Decay channel: p p

B.Echenard EPS 2003, Aachen 17-23 july 2003

and cross sections

Good agreement with diquark model predictions

Three quark model excluded

B.Echenard EPS 2003, Aachen 17-23 july 2003

and cross sections

Agreement with the handbag model predictions using pp as input data

B.Echenard EPS 2003, Aachen 17-23 july 2003

m() spectrum mass spectrum

mass spectrum

m() spectrum

and pair production at Q20

channel

High statistics (~70000 events)

Clear peak

channel

Less statistics (~5000 events)

Clear peak

L3L3

L3L3

B.Echenard EPS 2003, Aachen 17-23 july 2003

and pair production at Q20

Broad enhancement near threshold of

Ratio of / cross sections incompatible with formation of one Isospin 0 or 1 resonance

Model with in different spin-parity-helicity states and isotropic 4

ABW12BW34 J ,Jz + permutations

A4=1 (JP,Jz) = (2+,2) dominant state

(0+,0) small contribution

(0-,0), (2+,0), (2-,0(1,2)) negligible

Partial Wave Analysis:

p

L3

B.Echenard EPS 2003, Aachen 17-23 july 2003

pair production at high Q2

Select 4 + tagged electron:

non-resonant

Separated by a box method

Q2 interval:

1.2 GeV2 < Q2 < 30 GeV2

m() spectrum

mass spectrumL3

0

00

B.Echenard EPS 2003, Aachen 17-23 july 2003

pair production at high Q2

Broad enhancement near threshold of as observed in Q20 measurement

1.2 GeV2 < Q2 < 8.5 GeV2

8.8 GeV2 < Q2 < 30 GeV2

NR

NR

B.Echenard EPS 2003, Aachen 17-23 july 2003

pair production at high Q2

Fit with 1/(Qm(Q2+W2)2):

QCD prediction(1) : m=2

Data: m=2.4 ± 0.3 (W=1.95)

(1) M. Diehl et al . PRD 62 073014 (2000); hep-ph/0003233

Low and high Q2 data are well described by GDVM form factor

-pole not sufficient

161-209 GeV

91 GeV

B.Echenard EPS 2003, Aachen 17-23 july 2003

Conclusions

and K pair production ) = (

Only W dependence agrees with QCD predictions

Normalization and K/ ratio disagree with QCD predictions

Baryon pair production Data can be described by diquark model and handbag model Three quark model excluded

and pair production Dominance of (JP,Jz) = (2+,2) for Q20

Broad enhancement near threshold of

Good agreement with QCD predictions for de+e-/dQ2

GVDM form factor also describes well the Q2 dependence of