Baryon-Antibaryon Pair
Production in Two-Photon Collisions at LEP
K. Freudenreich, ETH Zürich
DIS 2004, Štrbské Pleso, April 16, 2004
Overview
• Two-photon kinematics and theoretical models
• L3 subdetectors used in the analysis
• the data:
P. Achard et al., Λ and Σ0 pair production in twophoton collisions at LEP, Phys. Lett. B (2002) 24.
P. Achard et al., Proton-antiproton pair production in
two-photon collisions at LEP, Phys. Lett. B (2003) 11.
B. Echenard, PhD thesis in progress, university of Geneva.
• comparison to models
• summary
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
1/17
Kinematics
e+e− → e+e−BB̄ with B ≡ baryon = p, Λ or Σ0
e
-
-
e
P1 =
‘
‘
→‘
P1 = (E1 , p1)
θ1
→
→
(E1 , p1)
q1 = (ω1 , q1)
γ*
→
q2 = (ω2 , q2)
γ
e+
θ2
‘
→
P2 = (E2 , p2)
e+
‘
→‘
P2 = (E2 , p2)
Wγγ is the C.M.S. energy and Θ∗ is the polar angle in the γγ C.M.S.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
2/17
The Diquark Model
• Since perturbative QCD is inadequate at low pT , models are needed for that
region.
• Already M. Gell-Mann speculated in his 1964 quark paper about stable diquarks.
three quark model
K. Freudenreich,
quark-diquark model
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
3/17
The Handbag Model
In the handbag model the amplitude for the annihilation of two
photons into a baryon anti-baryon pair is factorized into a
hard γγ → qq̄ subprocess and into a soft BB̄ distribution amplitude:
H
H
H
H
d,p
F
A(BB̄) = rB (ρ) A(pp̄) with ρ = F u,p
M. Diehl, P. Kroll and C. Vogt, Eur. Phys. J., C26 (2003) 567.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
4/17
L3 subdetectors used in the analysis
RB24
Hadron Calorimeter Barrel
EG Calorimeter
SLUM
Quadrupole
Hadron Calorimeter
Endcaps HC1
BGO
BGO
HC3
HC2
Z chamber
TEC
SMD
FTC
Luminosity
Monitor
Active lead rings
VSAT
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
5/17
dE
dx
6
–
p, p
Measurement
6
–
Data
p, p
π ,π
arbitrary units
+
π ,π
-
+
4
4
2
2
0
0
0.5
MC
1
momentum
1.5
2
0
0
-
0.5
1
1.5
2
momentum
dE is used for the identification of protons and pions
dx
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
6/17
p p̄ selection
dE , Et + shower shape)
dx pt
Et
protons from dE
and
dx
pt < 0.6
2 tracks of opposite charge p̄ from neural net (p,
no photons (
P
p
~t (pair))2 < 0.1 GeV2 MC (e e → e e e e )
+ -
+ - + -
L3
Data
800
Events
600
400
cut
200
0
0
0.5
1
1.5
Et / pt
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
7/17
effective mass Wγγ of the p p̄ pair
10
3
cut
Data
L3
Events / 0.1 GeV
inclusive bkg
10
+ -
h h bkg
2
+ -
e e bkg
10
989 events found for L = 667 pb−1.
938 events selected with Wγγ > 2.1 GeV.
√
since very small s dependence →
√
combination into one s bin.
σ(e+e− → e+e−pp̄) =
( 26.7 ± 0.9 ± 2.7 ) pb
√
at < s > = 197 GeV
1
-1
10
2
3
4
Wγγ (GeV)
dσ(γγ → pp̄)/d|cosΘ∗| is obtained by dividing out the two-photon luminosity
function and extrapolation to Q2 = 0 with a GVDM form factor.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
8/17
σ(γγ → pp̄) for cos Θ∗ < 0.6
pp
10
10
L3
4
–
Data
pp
OPAL
Quark-diquark
CLEO
Three quark
VENUS
3
10
3
–
–
10
L3
σ(γγ→ pp) (pb)
σ(γγ→ pp) (GeV)
10
–
4
10
2
2
10
10
2
2.5
3
3.5
Wγγ (GeV)
4
1
2
2.5
3
3.5
4
Wγγ (GeV)
L3 data extend range in Wγγ , agreement with prev. experim. + diquark model.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP,
9/17
dσ/d|cosΘ∗| in three bins of Wγγ
L3 2.1 < Wγγ < 2.5
L3 2.5 < Wγγ < 3.0
3.0 < Wγγ < 4.5
CLEO 2.0 < Wγγ < 2.5
CLEO 2.5 < Wγγ < 3.0
Quark-diquark
Quark-diquark
2
dσ/d|cosθ*| (nb)
dσ/d|cosθ*| (nb)
Quark-diquark
10
0.15
dσ/d|cosθ*| (nb)
15
1
5
0
0.1
0.05
0
0.2
|cos θ*|
0.4
0.6
0
0
0.2
|cos θ*|
0.4
0.6
0
0
0.2
|cos θ*|
0.4
Diquark model starts to work for Wγγ > 2.5 GeV
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 10/17
0.6
CLEO
Belle
0.12
2.5 < Wγγ < 3.0 GeV/c
1.5
2
4
1+cos θ
----------C1× ----------2 *
1-cos θ
2 *
*
1.25
1
0.75
0.04
θ
1+cos
----------C1× ----------2 *
1-cos θ
0.25
1+cos θ
----------C1× ----------2 *
1-cos θ
2 *
0.02
χ /ndf=12.94
2
χ /ndf=443.7
2
0
χ /ndf=7.588
2
0
0.1
0.06
2 *
0.5
2
0
0.08
−
−
−
6
3.0 < Wγγ < 4.0 GeV/c2
0.1
*
*
dσ(γγ→pp)/d|cosθ |
2
dσ(γγ→pp)/d|cosθ |
2.0 < Wγγ < 2.5 GeV/c
8
CLEO
Belle
1.75
dσ(γγ→pp)/d|cosθ |
10
2
nb
nb
nb
dσ/d|cosΘ∗| in three bins of Wγγ
0.2
0.3
0.4
*
|cosθ |
0.5
0.6
0
0
0.1
0.2
0.3
0.4
*
|cosθ |
0.5
0.6
0
0.1
0.2
0.3
0.4
0.5
*
|cosθ |
Preliminary data from the Belle colaboration, presented at Photon
03, Frascati, Italy. They show a similar behaviour.
In the meantime their data have changed in the lowest mass bin
approaching both CLEO and L3.
Many thanks to Dr. C.-C. Kuo from Belle.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 11/17
0.6
σ(γγ → pp̄) vs. Wγγ in 2 bins of cos Θ∗
10
|cosθ*| < 0.3
4
10
4
0.3 < |cosθ*| <0.6
10
10
Quark-diquark
Quark-diquark
10
3
Fit
3
–
–
2
0.3 < |cosθ*| < 0.6
σ(γγ→ pp) (pb)
10
10
σ(γγ→ pp) (pb)
–
σ(γγ→ pp) (pb)
10
3
|cosθ*| < 0.3
2
10
2
10
10
L3
1
2
L3
1
2.5
3
3.5
Wγγ (GeV)
4
2
L3
2.5
3
3.5
Wγγ (GeV)
4
2
2.5
3
3.5
Wγγ (GeV)
Change in slope at Wγγ ∼ 3 GeV for |cosΘ∗| < 0.3
a fit with σ ∝ W−n in 0.3 < |cosΘ∗| < 0.6 yields n = 9.8 ± 0.3
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 12/17
4
Λ Λ̄ selection
2 secondary vertices with dca⊥ > 3 mm no photons no K0s
Et
p and π identification via dE
p̄
identification
via
dx
p > 0.7
t
L3
300
Data πcut
MC π–
tracks / 0.1
MC p
200
100
0
0
0.5
1
1.5
ET / p T
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 13/17
2
e+e− → ΛΛ̄, Σ0/Σ̄0 selection
Selection of decays Λ → pπ − Λ̄ → p̄π + Σ0 → Λγ
40
2
18
16
14
12
10
8
6
4
2
0
Entries / 20 MeV
Events/10x10MeV
L3
–0
Σ /Σ
0
m 1.2
(p _ 1.15
p+
1.2
1.15
)(G 1.1
eV 1.05 1.05 1.1 p- )(GeV)
)
m(p
30
20
10
0
100
200
300
m(pπγ) - m(pπ) (MeV)
likelihood fit yields 12.5 ± 6.1 ΛΛ̄ and 20.5 ± 6.5 Σ0Σ¯0 events
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 14/17
σ(γγ → ΛΛ̄, ΣΣ̄, Λ Σ̄0 + Λ̄ Σ0 )
–
10
4
ΛΛ
0 –0
Σ Σ
L3
CLEO
10
–0
2
10
2
10
10
0 –
3
L3 upper limit
95% CL
Diquark
2
0 –
0 –0
–
σ(γγ→ Σ Λ +Λ Σ ) (pb)
σ(γγ→ Σ Σ ) (pb)
σ(γγ→ Λ Λ) (pb)
10
10
10
Three quark
Three quark
3
–0
ΛΣ +Σ Λ
Diquark
3
Diquark
10
L3
10
1
1
1
-1
2.5
3
3.5
Wγγ (GeV)
10
2.5
3
Wγγ (GeV)
3.5
2.5
3
3.5
Wγγ (GeV)
Limited statistics. Disagreement with CLEO at low Wγγ .
Agreement with diquark model predictions.
Three quark model again much too low.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 15/17
σ(γγ → ΛΛ̄, ΣΣ̄, Λ Σ̄0 + Λ̄ Σ0 )
–
ΛΛ
CLEO
Handbag
Handbag
10
σ(γγ→ Σ Σ ) (pb)
3
0.25 < ρ < 0.75
3
10
2
0 –
2
L3 upper limit
95% CL
Handbag
0.25 < ρ < 0.75
10
0 –
–
10
10
0 –0
σ(γγ→ Λ Λ) (pb)
0.25 < ρ < 0.75
10
–0
ΛΣ +Σ Λ
–0
4
L3
σ(γγ→ Σ Λ +Λ Σ ) (pb)
10
0 –0
Σ Σ
L3
2
1
10
10
-1
2.5
3
3.5
Wγγ (GeV)
2.5
3
Wγγ (GeV)
3.5
10
2.5
3
3.5
Wγγ (GeV)
Agreement with the handbag model (M. Diehl et al.)
which uses a single model parameter ρ determined from
γγ → pp̄ data.
K. Freudenreich,
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 16/17
Summary
γγ → pp̄
model agrees with data for Wγγ > 2.5 GeV
• Three quark model predictions much too low
• Small and large angles have different Wγγ dependence
• Diquark
γγ → ΛΛ̄
γγ → ΣΣ̄
• Disagreement with three quark model
• Agreement with diquark and handbag models
• The Belle experiment will increase statistics
K. Freudenreich,
by a lot
Baryon-Antibaryon Pair Production in 2-γ Collisions at LEP, 17/17