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

Support

ed b

y F

FE

pro

gra

m o

f th

e F

ors

chungszentr

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