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: [email protected]
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