Baryon Antibaryon Pair Production
in Two-Photon collisions at LEP
Bertrand Echenard
University of Geneva
Photon 2003, 7-11 April 2003
Outline:
➨ Introduction
➨ γγ → pp production
0
0
➨ γγ → ΛΛ and γγ → Σ Σ channels
➨ γγ → ∆++ ∆−− analysis (ongoing)
➨ Summary
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Motivation
Measure the cross section γγ → HH (H = p, Λ, Σ0 )
➪ compare with three-quark predictions
➪ compare with quark-diquark predictions
Three quark
Diquark
QUARK
DIQUARK
10
4
–
pp
Diquark
10
2
3
ΛΛ
Diquark
(2)
(1)
Three quark
10
(2)
2
–
–
σ(γγ→ pp) (pb)
10
10
σ(γγ→ ΛΛ) (pb)
Three quark
3
–
(1)
10
10
1
-1
1
2.5
3
3.5
4
10
Wγγ (GeV)
2.5
3
3.5
Wγγ (GeV)
(1) C. Berger, B. Lechner and W. Schweiger, Fizika B 8 (1999) 371;
C. Berger and W. Schweiger, hep-ph/0212066.
(2) G. Farrar et al., Nucl. Phys. B 259 (1985) 702.
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 2)
Betrand ECHENARD
University of Geneva
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Motivation bis
Also test the handbag model(1) . In this model, the
amplitude of the process γγ → HH is factorized into:
➪ hard part: γγ → qq scattering.
➪ soft part: qq → HH transition described by
form factors (which represents moment of
time-like generalized parton distribution).
H
H
H
H
In this framework:
➪ assume that (anti)quark carries almost the
full momentum of (anti)baryon.
➪ use γγ → pp measurement to fit the
parameters of the model.
➪ predictions for all other baryon octet members
depend only on one parameter ρ, a ratio of
form factors of the proton.
(1) M. Diehl, P. Kroll and C. Vogt, hep-ph/0206288.
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 3)
Betrand ECHENARD
University of Geneva
e+ e− → e+ e− pp
+ - + -
1
pt2 | cut.
1.5
(page 4)
Typical event
e+ e− → e+ e− pp event selection
❑ 2 tracks of opposite charge.
❑ Antiproton identification Neural networks,
use P, dE/dx, Et/pt, shower shape.
+ -
MC (e e → e e e e )
Data
–
pp selection
cut
0.5
Et / pt (proton)
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
L3
❑ Proton identification dE/dx and Et/pt.
800
600
400
200
0
0
❑ Exclusive event no photons and |
7-11 April 2003
Events
L3
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3
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3
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3333
Betrand ECHENARD
University of Geneva
Λ → pπ −
Σ0 → γΛ
–
pt2 | cut.
–
–
Data p
-
MC p
MC π
3
0
(page 5)
Typical event
e+ e− → e+ e− ΛΛ, Σ0 Σ event selection
0
Λ → pπ +
e+ e− → e+ e− ΛΛ, Σ0 Σ
❑ Decays
❑ Antiproton identification Et/pt .
L3
ΛΛ selection
1
ET/pT
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
2
❑ Proton and pion identification dE/dx.
400
200
0
0
❑ Exclusive event |
7-11 April 2003
tracks / 0.1
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Betrand ECHENARD
University of Geneva
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γγ → pp results
L3
–
pp
Data
inclusive bkg
Events / 0.1 GeV
10
+ -
2
h h bkg
+ -
e e bkg
10
1
-1
10
2
3
4
Wγγ (GeV)
❑ 989 events found (L = 667 pb−1 ).
❑ Wγγ range : 2.0 < Wγγ < 4.5 GeV.
❑ Background level: between 4% and 69%.
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 6)
Betrand ECHENARD
University of Geneva
–
10
10
4
3
2
2.5
–
pp
3
L3
OPAL
CLEO
4
VENUS
3.5
Wγγ (GeV)
2
3
4
γγ → pp results
10
10
10
10
1
2
L3
–
3
Data
4
Quark-diquark
Three quark
3.5
Wγγ (GeV)
pp
2.5
(page 7)
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❑ Data globally described by the
diquark model.
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
❑ Good agreement with previous
experiments
7-11 April 2003
❑ Three-quark model excluded.
2
–
σ(γγ→ pp) (pb)
❑ Extend Wγγ range
10
10
σ(γγ→ pp) (GeV)
7-11 April 2003
dσ/d|cosθ*| (nb)
15
10
5
0
0
L3 2.1 <
Wγγ
CLEO 2.0 <
0.4
< 2.5
< 2.5
Wγγ
|cos θ*|
Quark-diquark
0.2
2
1
0
0
L3 2.5 <
Wγγ
CLEO 2.5 <
0.4
< 3.0
< 3.0
Wγγ
|cos θ*|
Quark-diquark
0.2
0.6
0
0.05
0.1
0.15
Differential cross sections dσ/d| cos θ ∗ |
0.6
dσ/d|cosθ*| (nb)
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
(page 8)
0
3.0 < Wγγ < 4.5
|cos θ*|
Quark-diquark
0.2
0.4
0.6
L3
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3
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Betrand ECHENARD
Striking disagreement with the quark-diquark predictions in
the low Wγγ region !!
dσ/d|cosθ*| (nb)
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Differential cross sections dσ/d| cos θ ∗ |
2.1 < Wγγ < 2.5
10
5
5
χ / dof = 51 / 5
2
0
0.2
2
0.4
0
0.6
0
2.1 < Wγγ < 2.5
Y12
0.2
Fit
10
5
|cos θ*|
0.4
0.6
2.1 < Wγγ < 2.5
Y22
dσ/d|cosθ*| (nb)
dσ/d|cosθ*| (nb)
|cos θ*|
Fit
10
χ / dof = 132 / 5
0
2.1 < Wγγ < 2.5
Y02
Fit
dσ/d|cosθ*| (nb)
dσ/d|cosθ*| (nb)
Y00
Fit
10
5
χ2 / dof = 561 / 5
χ2 / dof = 35 / 5
0
0
0.2
|cos θ*|
0.4
0
0.6
0
0.2
|cos θ*|
0.4
0.6
A single spherical harmonic is not sufficient
to describe the data !!
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 9)
Betrand ECHENARD
University of Geneva
7-11 April 2003
10
5
0
0
All
Wγγ
< 2.5
0.4
0.6
10
5
0
0
0.2
0.4
2.1 < Wγγ < 2.5
Fit
|cos θ*|
χ2 / dof = 7.35 / 4
Y20 + Y00
Differential cross sections dσ/d| cos θ ∗ |
2.1 <
Fit
χ2 / dof = 7.34 / 2
0.2
|cos θ*|
OR
Good fit with all harmonics (left)
dσ/d|cosθ*| (nb)
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
(page 10)
0.6
Good fit with 92% Y20 and 8% Y00 (right).
dσ/d|cosθ*| (nb)
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Betrand ECHENARD
University of Geneva
–
10
10
10
4
3
2
L3
3
|cosθ*| < 0.3
4
0.3 < |cosθ*| <0.6
3.5
Wγγ (GeV)
4
3
2
L3
2.5
3
10
10
2
Fit
Wγγ (GeV)
3.5
4
0.3 < |cosθ*| < 0.6
3
L3
2.5
|cosθ*| < 0.3
10
2
Quark-diquark
4
Quark-diquark
3.5
Wγγ (GeV)
3
γγ → pp cross section
10
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1
2
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
(page 11)
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Betrand ECHENARD
Large angle
7-11 April 2003
Clear change of shape at Wγγ ∼ 3 GeV and the quarkdiquark model fails to reproduce the data.
2.5
–
σ(γγ→ pp) (pb)
Small angle
2
–
σ(γγ→ pp) (pb)
Data are reproduced by the quark-diquark model, fit with
a power law σ ∝ W −n gives n = 9.8 ± 0.3.
1
10
σ(γγ→ pp) (pb)
2
7-11 April 2003
18
16
14
12
10
8
6
4
2
0
1.1
1.05
1.1
1.2
)
)(GeV
1.15
-
0
40
30
20
10
0
–
(page 12)
200
L3
300
❑ Clear Σ0 /Σ peak.
0
m(pπγ) - m(pπ) (MeV)
100
Σ0 / Σ0
γγ → ΛΛ and γγ → Σ0 Σ results
1.05
m(p Entries / 20 MeV
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
❑ Estimated background < 1%.
❑ Clean peak, 33 events selected.
V)
() G
e
(p _ 1.15
+
m 1.2
Events/10x10MeV
L3
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Betrand ECHENARD
University of Geneva
L3
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3
L
L
0
0
ΛΛ, ΛΣ + ΛΣ0 and Σ0 Σ separation
0
0
0
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L
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333
0
ΛΛ, ΛΣ + ΛΣ and Σ Σ all mixed
➪ Separate the three contributions
Procedure:
1. Classify events:
0
0
ΣΣ
0
ΛΣ + ΛΣ0
ΛΛ
0
0
if a Σ and Σ are found
0
if a Σ0 or Σ is found
otherwise
2. Use a maximum extended likelihood fit to find
the fractions ri of different channels.
3. Results:
rΛΛ
= 0.38 ± 0.18
(rΛΣ0 +ΛΣ0 = 0)
rΣ 0 Σ 0
= 0.62 ± 0.18
(rΛΣ0 +ΛΣ0 = 0)
rΛΣ0 +ΛΣ0
7-11 April 2003
compatible with 0
< 0.58 at 95% CL
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 13)
Betrand ECHENARD
University of Geneva
–
4
3
–
ΛΛ
L3
CLEO
Diquark
Three quark
3
10
10
10
1
10
3
2
-1
0 –0
Σ Σ
2.5
L3
Diquark
Three quark
3
0
3.5
γγ → ΛΛ and γγ → Σ0 Σ results
3.5
Wγγ (GeV)
10
10
10
1
3
2
–0
0 –
ΛΣ +Σ Λ
2.5
3.5
L3 upper limit
95% CL
Diquark
3
Wγγ (GeV)
❑ Good agreement with diquark
0
predictions for ΛΛ and Σ0 Σ .
(page 14)
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Betrand ECHENARD
❑ Diquark predictions compatible
0
with σ(ΛΣ + ΛΣ0 )
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
❑ Three quark model excluded.
7-11 April 2003
–
σ(γγ→ Σ Λ +Λ Σ0) (pb)
0 –
10
10
2
2.5
Wγγ (GeV)
–
σ(γγ→ Σ0 Σ0) (pb)
❑ Disagreement with CLEO
at low Wγγ .
❑ Limited statistics.
1
10
10
σ(γγ→ Λ Λ) (pb)
–
4
3
–
ΛΛ
L3
CLEO
Handbag
0.25 < ρ < 0.75
3
10
10
10
3
2
0 –0
Σ Σ
2.5
L3
Handbag
0
0.25 < ρ < 0.75
3
3.5
γγ → ΛΛ and γγ → Σ0 Σ results
3.5
Wγγ (GeV)
–
10
10
1
10
σ(γγ→ Σ0 Λ +Λ Σ0) (pb)
–
10
10
2
2.5
Wγγ (GeV)
–
σ(γγ→ Σ0 Σ0) (pb)
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
2
-1
(page 15)
0
–0
0 –
ΛΣ +Σ Λ
2.5
L3 upper limit
95% CL
Handbag
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Betrand ECHENARD
3.5
0.25 < ρ < 0.75
3
Wγγ (GeV)
❑ Handbag predictions compatible with σ(ΛΣ + ΛΣ0 )
❑ Good agreement with ΛΛ and Σ0 Σ .
0
❑ Predictions for the range 0.25 < ρ < 0.75.
10
10
σ(γγ→ Λ Λ) (pb)
L3
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3
L
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L
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333
∆++ ∆−− channel
Can we measure the γγ → ∆++ ∆−− cross section ?
➪ With the naive three quark picture:
σ(γγ → ∆++ ∆−− ) = 16 · σ(γγ → pp)
BUT upper limit measurement1 excludes this model:
σ(γγ → ∆++ ∆−− )/σ(γγ → pp) = 1 at 95% CL for
2.6 GeV < Wγγ < 2.8 GeV.
➪ With the diquark model, calculations have not
been done yet, but a rough estimation1 gives for
Wγγ 2.8 GeV:
σ(γγ → ∆++ ∆−− ) 0.1 · σ(γγ → pp)
Interesting challenge and can provide
information to improve models
(1) ARGUS, H. Albrecht et al., Z. Phys. C 42 (1989) 543.
(2) M. Anselmino et al., Int. J. Mod. Phys. A 4 (1989) 5213.
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 16)
Betrand ECHENARD
University of Geneva
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Event Selection
Decays:
∆++ → pπ +
∆−− → pπ −
Selection:
❑ 4 tracks from interaction point.
❑ Proton (antiproton) is identified as the positive
(negative) track with the largest momentum.
❑ dE/dx identification: CL > 0.05 for p, p and
CL > 0.01 for π ± .
At this point:
Configuration correctly identified: 98.5%
Main background: γγ → π + π − π + π − .
➪ Antiproton identification:
dE/dx : CL(p) / ( CL(p) + CL(K) + CL(π) ) > 0.95
p annihilation in calorimeter : Et/pt > 0.8
Finally, apply cut to select exclusive events :
2
No photons and | pt | <0.02 GeV2 .
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 17)
Betrand ECHENARD
University of Geneva
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Data Results
60
Data
Entries / 0.03 GeV
Entries / 0.03 GeV
Data
40
20
0
1
1.2
1.4
+
1.6
1.8
40
20
0
2
m(pπ ) (GeV)
2
–
1.2
1.4
–
1.6
m(pπ-) (GeV)
1.8
2
Data
1.8
m(pπ-) (GeV)
1
1.6
L dt = 667 pb−1
1.4
369 events selected
1.2
1
1
1.2
1.4
+
1.6
1.8
2
m(pπ ) (GeV)
Seems there is something...
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 18)
Betrand ECHENARD
University of Geneva
7-11 April 2003
arbitrary units
6
4
2
0
0
-
momentum
1
1.5
Data
Background?
–
p, p
+
π ,π
0.5
2
Next step:
❑ γγ → ∆0 ∆
❑ γγ → ppπ + π − non resonant
0
❑ γγ → ∆++∆−−
In data:
γγ → ppπ + π − candidates
∼ 100% ppπ + π −
(page 19)
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Betrand ECHENARD
➪ Separate the three contributions and extract cross
sections σ(γγ → ∆++ ∆−− )
0
and σ(γγ → ∆0 ∆ ).
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
University of Geneva
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Summary
γγ → pp:
➪ Striking disagreement with the three quark
model.
➪ Data globally described by the diquark model.
➪ Differential cross section is not reproduced by
the diquark model at low Wγγ .
➪ Large angle and small angle cross sections
have different Wγγ dependence.
0
0
γγ → ΛΛ, γγ → ΛΣ + Σ0 Λ and γγ → Σ0 Σ :
➪ Striking disagreement with the three quark
model.
➪ Good agreement with the diquark predictions.
➪ Good agreement with the handbag
calculations.
γγ → ∆++ ∆−− :
➪ Ongoing analysis with promising results.
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 20)
Betrand ECHENARD
University of Geneva
Selection
cuts (%)
7.3
7.3
7.3
7.3
7.3
14.3
30.5
30.5
30.5
Back(%)
1.0
1.3
1.7
2.4
3.8
5.8
12.4
12.8
15.0
Total
(%)
10.3
9.5
9.6
9.7
10.2
16.5
33.5
33.7
34.5
γγ → pp systematics
Other
ground(%)
7
6
6
6
6
6
6
6
6
Systematic uncertainties
Wγγ
(GeV)
2.1 − 2.2
2.2 − 2.3
2.3 − 2.4
2.4 − 2.5
2.5 − 2.6
2.6 − 2.8
2.8 − 3.1
3.1 − 3.6
3.6 − 4.5
Background subtraction:
Wγγ
(GeV)
2.1 − 2.2
2.2 − 2.3
2.3 − 2.4
2.4 − 2.5
2.5 − 2.6
2.6 − 2.8
2.8 − 3.2
3.2 − 3.6
3.6 − 4.5
h+ h−
(%)
3.4
4.2
5.8
8.1
11.9
17.5
29.3
23.9
25.5
Background
inclusive
(%)
0.3
0.4
0.4
0.4
0.6
1.1
2.6
4.2
6.3
➪ uncertainty of 50% on γγ → π + π − and γγ → K+ K− cross sections.
(page 21)
L3
L
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3
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e+ e−
(%)
0
0
0
0.4
2.8
4.6
17.2
20.8
25.1
Betrand ECHENARD
➪ uncertainty of 30% on the e+ e− → e+ e− e+ e− cross section due to MC statistic.
Baryon Antibaryon Pair Production in Two-Photon collisions at LEP
➪ 50% on the inclusive background due to the fitting procedure.
7-11 April 2003
University of Geneva
L3
L
3
L
L
0
γγ → ΛΛ and γγ → Σ0 Σ results
L3
L
33
L
L
L
L
L
L
333
➪ Fit to determine the Σ0 contamination.
0
0
➪ Assume σ(ΛΣ + ΛΣ0 ) σ(Σ0 Σ ).
0
➪ 22% (Σ0 /Σ ) contamination
7-11 April 2003
Baryon Antibaryon Pair Production in Two-Photon collisions
at LEP
(page 22)
Betrand ECHENARD
University of Geneva