1. National Centre for Nuclear Research
Search for possible dark matter effects in leptonic decays of light mesons measured in the WASA-at-COSY experiment
Marcin Berłowski
National Centre for Nuclear Research
Uppsala, Sweden 24.IV.2014 1

2. Outline
Motivation
WASA-at-COSY
Experimental setup
Results
The future

3. Dark matter
Seen in galaxy rotation curves and in gravitational lensing
Not in the Standard Model
Interacting by a gravitational force

4. Dark matter
We are looking for a hypothetical U boson that could mediate the annihilation of dark matter particles:
Simple extension of the Standard Model with U’ symmetry with ε coupling constant (also called mixing parameter ε2=α’/α)
511 keV line from around the Galaxy center measured by the INTEGRAL experiment
The assumption that such a particle exists could provide an explanation for the results seen by many experiments (PAMELA, ATIC, …)‏
Mass U= MeV
[Phys.Rev.D75:115017,2007]

5. Dark matter Vector boson (dark photon)‏
It’s signature should be visible in Dalitz decays of pseudoscalar mesons
Two methods to explore: e+e- mass spectrum and photon mass spectrum in this decay

6. γcτ~1mm(γ/10)(10-4/ε)2·(100MeV/mU)‏
Dark matter
Lifetime and decay length:
γcτ~1mm(γ/10)(10-4/ε)2·(100MeV/mU)‏
Constrains on ε (coupling constant) vs mU space set by the results of many different experiments – astrophysics, beam-dump experiments, particle decays, and measurement of magnetic moments of leptons (g-2)‏
arXiv:

7. WASA-at-COSY Collaboration
26 institutes from all around the world
Nearly 200 people involved
COoler SYnchrotron localized in Forshungszentrum Juelich, Germany

8. WASA-at-COSY experiment
Storage ring with circumference of 184 m for protons and deuterons
Maximum beam energy 3.7 GeV

9. WASA-at-COSY physics program
Meson (π°, η, ω) decays and production – symmetry breaking, rare decays
Search for physics beyond the Standard Model
Resonance effects in multi pion production
Eta-mesic nuclei
Isospin violation process in dd→4Heπ°

10. WASA setup
WASA – Wide Angle Shower Apparatus
Three main parts: pellet target, central detector and forward detector

11. Forward detector

12. Central detector

13. Pellet target Frozen hydrogen or deuterium Frequency 8-10 kHz
Droplet diameter μm
Velocities around 80 m/s
COSY

14. Drift chamber 1738 tubes in 17 layers Diameter of tubes 4 to 8 mm
Filled with CO2 i argon
Magnetic field of 1 Tesla
Working principle:

15. Electromagnetic calorimeter
Covers most of the angular space
1012 CsI(Na) crystals varying from 20 to 30 cm (~16 X0)‏

16. Results

17. pp→ppπ° data
Collected with energy 550 MeV (below threshold for π+π- production)‏
Two weeks of data taking (effective 4 days)
Biggest sample of Dalitz decay events
5·105 π°→γe+e- (SINDRUM ~105)‏
We were looking for U→e⁺e⁻ in e+e- mass spectrum from π°→γe+e- decay
The results were published in Phys. Lett. B 726 (2013), 187
Carl-Oscar Gullström & Uppsala Univ.

18. Dalitz decay of π° meson
· Data
--- MC sum
--- MC π⁰ → e⁺e⁻γ (BR=1.2%)‏
--- MC external conversion
--- MC π⁰ → e⁺e⁻γ plus false e⁺ from π⁺
„Search for a dark photon in π⁰ → e⁺e⁻γ decay” [arXiv: ]

19. ε2 coupling constant vs U boson mass
„Search for a dark photon in π⁰ → e⁺e⁻γ decay” [arXiv: ]

20.  meson
Mass ~550 MeV/c2
Big mass (in comparison to  mesons) connected to admixture of strange quarks
Long lifetime due to the fact, that all of its decay channels are somehow forbidden

21. BRexp=(7.48±0.29±0.25)x10-8; BRtheo=(6.2±0.1)x10-8
Dark matter
KTeV collaboration results showing 3.3σ deviation from a very precise theoretical calculations for a Branching Ratio for π°e+e-
BRexp=(7.48±0.29±0.25)x10-8; BRtheo=(6.2±0.1)x10-8
Proposed explanation by a vector boson U coupling both to quarks and electrons
Similar effect could be seen in ηe+e-, but here we have much smaller statistic
[Nucl.Phys.B683:219,2004]
[Phys.Rev.D78:115002,2008]

22. Leptonic decays of  meson
Branching ratios:
→ % exp
→e+e- ~0.7% exp
→e+e- ~?10-9? theo
We have some theoretical predictions within the frames of the Standard Model, but enhanced BR for the decay can be a sign of an unconventional process.

23. Previous analysis in CELSIUS/WASA

24. Collected data and trigger system
Experiment in Autumn of 2008 using pp→pp 1.4 GeV
Proton run chosen because of higher  production rate
During 2 weeks of data taking we collected ~150 million events
Trigger system used a special reaction property demanding high energy deposits in both halves of electromagnetic calorimeter
Trigger worked nearly the same for each leptonic decay channel
Trigger simulation
preliminary

25. Data pp→pp(→)‏ IMγγ MMpp Two photon events Used for normalizationπº η
Data
MMpp η 25

26. → preliminary ~5.9·107 η πº Numer of eta mesons produced:
With systematic error of 15% 26

27. MCeeγ e+e- pairs in data e+e- pairs Data at the beginning of analysis
The invariant mass of e+e- pairs produced by a virtual photon conversion is usually small
MCeeγ
Where MP is in this case η meson mass, q2 is Mee mass 27

28. Events with small e+e- mass
pp→pp(→e+e-)‏ πº η η
Data
Events with small e+e- mass

29. Photon conversion in the beam pipe
60 mm
Data
preliminary
MCeeγ
MCγγ

30. Photon conversion in the beam pipe
+ Data
--- MC η→γγ
--- MC η→e⁺e⁻γ
--- MC sum
preliminary
e+e- pairs mass, before and after conversion reduction
conversion to Dalitz ratio - before: ~1:2, after: ~1:20
signal reduction after conversion reduction ~20%

31. →e+e- conclusions
Good agreement between the number of observed Dalitz decays and the number of eta meson calculated from the normalization → channel
It served as a testing field for developing needed analysis methods such as:
identification and measurement of electrons
understanding photon conversion in the beam pipe
investigating the detector response for electrons with various energies

32. In search for →e+e- - background reactions
pp→pp+-
100 times greater cross section than for  meson production
Two charged particles in the final state
→e+e-
Photons with small energy can be undetected
pp→p∆(1232)→pp(*→e+e-)‏
The same final state as in our reaction
Other physical processes were found to be negligible when compared to those mentioned above

33. In search for →e+e- - reaction signature (simulations)‏X MC
pp→pp+-
pp→pp(→e+e-)‏
An example how to distinguish between pions and electrons based on their energy deposits in the electromagnetic calorimeter

34. Analysis – final results
The complete lack of signal events allowed us to set the BR limit for →e+e- equal to <2.1·10-6 CL90%
If we take into account Poisson statistics for a small number of events and dominating systematical error from normalization the result transforms to the preliminary result:
3.9·10-6 with confidence level 90%
(todays best limit in PDG 5.6·10-6 Eur.Phys.J. A48 (2012) 64)‏
Part of my PhD Thesis

35. Data: η Dalitz decay Data analysis (by Damian Pszczel) is in progress
At least 10⁸ η mesons produced in 1.4 GeV
Lower statistics than in π⁰ case but up to higher masses
We expect around η Dalitz events in the full data set
Small part of data

36. Summary and something about future
Analysed sample of ~6·107  mesons and ~109 π°
Several channels of  i π° decays involving leptons were observed and measured
Better limit for BR of →e+e- was established as well as the limit for ε parameter for U boson
The analysis performed in order to search U boson in π⁰→e⁺e⁻γ has been published, the article summarizing the search for →e+e- is in preparation
Analysis of a bigger statistic (x8) of  meson decays and π⁰ (x4) is in progress

37. Thank you for your attention