Luminosity measurement at
√
s = 4.19 GeV
via Bhabha scattering
Andreas Pitka
Charmonium Meeting 17.1.17
Charmonium Meeting 17.1.17
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Event Selection
Good Tracks
Cut on interaction region: Rxy < 1 cm, Rz < 10 cm.
Cut on direction: | cos ϑ| < 0.93.
For loose electron identification: E/p > 0.7.
Event Topology
Number of positively charged particles: n+ = 1.
Number of positively charged particles: n− = 1.
No veto on photons.
Cuts similar to old BESII
R
Ldt measurement (arXiv:1503.03408)
α(e+ , e− ) > 175◦ .
√
pmin = 2 · ( 1c ) · s/4.26 = 1.97 GeV/c.
pmax = 4 GeV/c.
√
Eshower = 1.55 · s/4.26 = 1.55 GeV.
Charmonium Meeting 17.1.17
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Bhabha Cross Section and Monte Carlo Generation
Calculation of σBhabha
Numeric Monte Carlo Calculation with Babayaga3.5
Options of Babayaga3.5
Despite energy: use joboptions file from
√
s = 4180 MeV Monte Carlo
/besfs/groups/psip/psipgroup/public/bes3gen/workarea702p01/TestRelease/
TestRelease-00-00-84/4180MC/sim_ee.txt
Also used for Monte Carlo generation.
Ebeam = 2.095 GeV
θmin = 20◦
maximum acollinearity: 180◦
running alpha: true
θmax = 160◦
enable FSR: true
Emin = 1 MeV
Result (with 5 · 104 events)
→ unweighted σBhabha = (423.8 ± 1.6) nb
weighted σBhabha = (423.4 ± 0.6) nb
Charmonium Meeting 17.1.17
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Comparison of Data and Monte Carlo
Polar Angle Distribution
e+
MC
data
e−
MC
data
10−1
10−1
10
10−3
10−3
fraction/bin
fraction/bin
10−2
−2
10−4
10−4
10−5
10−5
−1
−0.5
0
0.5
cos(θ)
80 % of measured data at
√
1 −1
−0.5
0
0.5
1
cos(θ)
s = 4190 MeV (≈ 6600 from ≈ 8100 dst files).
3.1 · 105 Monte Carlo events.
Both distributions are normalized.
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Invariant Mass (e+ e− ) and opening Angle
MC
data
10−1
10−1
10−2
10−2
10−3
10−3
10−4
10−4
10−5
2
2.5
3
3.5
4
4.5
165◦
5
80 % of measured data at
175◦
10−5
180◦
θ(e+ , e− )
m(e+ e− ) (GeV/c2 )
√
170◦
fraction/bin
fraction/bin
MC
data
s = 4190 MeV (≈ 6600 from ≈ 8100 dst files).
3.1 · 105 Monte Carlo events.
Both distributions are normalized.
Charmonium Meeting 17.1.17
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Momentum Distribution
e−
MC
data
MC
data
10−1
10−1
10−2
10−2
10−3
10−3
10−4
10−4
fraction/bin
fraction/bin
e+
10−5
10−5
1
1.2
1.4
1.6
1.8
2
p (GeV/c)
80 % of measured data at
√
2.2
2.4 1
1.2
1.4
1.6
1.8
2
2.2
2.4
p (GeV/c)
s = 4190 MeV (≈ 6600 from ≈ 8100 dst files).
3.1 · 105 Monte Carlo events.
Both distributions are normalized.
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Shower Energy
e−
MC
data
MC
data
10−1
10−1
10−2
10−2
10−3
10−3
10−4
10−4
10−5
fraction/bin
fraction/bin
e+
10−5
1
1.2
1.4
1.6
1.8
2
EShower (GeV)
2.2
2.4 1
1.2
1.4
1.6
1.8
2
2.2
2.4
EShower (GeV)
Integrated efficiency determined by 3.1 · 105 Monte Carlo events after all cuts
ε = (40.2 ± 0.2) %.
Charmonium Meeting 17.1.17
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Result for integrated Luminosity
Integrated Luminosity
Z
Ldt =
Nrec
= (395.0 ± 1.3) pb−1
σBhabha · ε
Nrec = 6.7 · 107 ± 2.7 · 103
σBhabha = (423.8 ± 1.6)nb
ε = (40.2 ± 0.2) %
Only part of measured data used
Status from 12th January.
6630 of currently 8129 dst files.
≈ 82 % of measured data.
→ Current status later today.
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