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Analysis of the C-violating Decay
η → π0 + γ∗ → π0 + e+ + e− with WASA-at-COSY
Analysis
Florian Bergmann* for the WASA-at-COSY Collaboration
*Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Germany
Motivation • Decay η → π0 + γ∗ → π0 + e+ + e− is forbidden since it violates the 𝐶-parity conservation
• Current upper limit assuming a phase space distribution:
• BR η → π0 + e+ + e− < 4 ∙ 10−5 [1]
• Aim: Lowering the existing upper limit by high statistics measurements at WASA-at-COSY
to test the 𝐶-parity conservation with increased sensitivity
• Different model assumptions relevant for the analysis: Decay via a virtual photon, Vector
meson dominance, Phase space distribution, …
WASA-at-COSY
Experiment COSY – Cooler Synchrotron
• Provides an (un)polarized proton or
deuterium beam with momentum up to
3.7 GeV/c [2]
WASA – Wide Angle Shower Apparatus
• Frozen hydrogen or deuterium pellets
as an internal target
• The central part can detect neutral as
well as charged particles with a near
4𝜋-acceptance
• The forward detector is used for detecting charged particles like protons, deuterons and
He nuclei [3]
Preliminary Results • 2008 data analyzed by A. Winnemöller, PhD thesis “Analyse des verbotenen η-Meson Zerfalls
η → π0 + e+ + e− am Experimentaufbau WASA-at-COSY”, 2011:
• Overall detection efficiency: After applying the optimal set of cuts 9920 out of 106 simulated
η → π0 + γ∗ → γ + γ + e+ + e− events remain
• According to simulations the signal to background ratio is 1/1.7 ∙ 106 and only one background event
is expected to pass all cuts
• Experiment: After applying all cuts on the measured data one out of 1.7 ∙ 108 events remains
Central Detector Forward Detector
π0
η
e+
e–
γ*
( , , )ρ ω φ *
π0
η
e+
e–
γ*
= + + ⋯
• ~30 ∙ 106 p + d → He3 + η events on disc
from two beam times:
• 2008: ~10 ∙ 106 η-events
• 2009: ~20 ∙ 106 η-events
• Preselection on p + d → He3 + X events via
He3 identification in Δ𝐸-𝐸 plot (energy loss in
Forward Trigger Hodoscope versus energy
loss in Forward Range Hodoscope)
• Preselection on the signature of the decay
η → π0 + e+ + e− → γ + γ + e+ + e−:
• 2 neutral particles in central detector (2γ)
• 1 positively and 1 negatively charged
particle in central detector (e+,e−)
en
erg
y l
os
s 1
. la
ye
r F
TH
/ G
eV
energy loss 1. layer FRH / GeV
• Determination of further selection conditions by simulating all
possible background reactions like direct (multi-)pion production and
other η-decay channels
• Discrimination of electrons and pions by energy versus momentum
correlation
• Invariant mass distribution of the lepton pair is model dependent
momentum / (GeV/c)
Ed
ep
ca
lori
me
ter
/ G
eV
momentum / (GeV/c)
Ed
ep
ca
lori
me
ter
/ G
eV
MC: η → π0e+e− MC: η → π0π+π−
invariant mass / (GeV/c²)
MC: η → π0γ∗ → π0e+e−
invariant mass / (GeV/c²)0 0.1 0.2 0.3 0.4 0.5 0.6
-1d
N/d
m /
(4
Me
V/c
²)
0
200
400
600
800
1000
MC: η → π0e+e− phase space
• Fitting of the measured data (2008) with a Monte Carlo cocktail
using the same fit parameters for all differential spectra
• Result: Accurate description of the observed data
• Determination of the optimal selection conditions via an evaluation
function for all cuts:
𝑆𝑅: relative signal
𝐵𝑅: relative background
• Use Monte Carlo data and vary cuts to find the maximum of the
evaluation function 𝐺
π0
η
e+
e–?
Outlook • Analysis of ~20 ∙ 106 p + d → He3 + η events from 2009 in progress
• Decay model independent analysis
• Significantly more p + p → p + p + η events on tape for analysis
New, model independent upper limit for the branching ratio of the η-decay
channel η → π0 + e+ + e− can be achieved
[2] R. Maier, Nucl. Instrum. Meth., doi:10.1016/S0168-9002(97)00324-0
[3] H.-H. Adam et al. (WASA-at-COSY Collaboration), arXiv:nucl-ex/0411038 [1] K. Nakamura et al.: Review of Particle Physics. Journal of Physics G: Nuclear and Particle Physics, 37(075021):186, 2010
)2missing mass / (GeV/c0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
-1 )2
dN
/dm
/ (
4 M
eV
/c
0
5000
10000
15000
20000
25000
data-
+
-
+
-
+
0
0
0
- e+ e
0
0-
+
-
+
0
-
+
MC sum
Missing mass
)2invariant mass / (GeV/c0 0.2 0.4 0.6 0.8 1
-1 )2
dN
/dm
/ (
4 M
eV
/c
0
500
1000
1500
2000
2500
3000
3500
Inv. mass
2 ch., 2 neutr.
𝐺 = 𝑆𝑅
𝑆𝑅
𝐵𝑅
pre
limin
ary
pre
limin
ary
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ed b
y F
FE
pro
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