Search for possible dark matter effects in leptonic

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

Outline • • • Motivation WASA-at-COSY Experimental setup Results The future

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

Dark matter • We are looking for a hypothetical U boson that could mediate the annihilation of dark matter particles: [Phys. Rev. D 75: 115017, 2007] Mass U=10 -200 Me. V • Simple extension of the Standard Model with U’ symmetry with ε coupling constant (also called mixing parameter ε 2=α’/α) • 511 ke. V 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, …)

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

Dark matter • Lifetime and decay length: γcτ~1 mm(γ/10)(10 -4/ε)2·(100 Me. V/m. U) • Constrains on ε (coupling constant) vs m. U space set by the results of many different experiments – astrophysics, beam-dump experiments, particle decays, and measurement of magnetic moments of leptons (g-2) ar. Xiv: 1104. 2747

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

WASA-at-COSY experiment Storage ring with circumference of 184 m for protons and deuterons Maximum beam energy 3. 7 Ge. V

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→ 4 Heπ°

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

Forward detector

Central detector

Pellet target • Frozen hydrogen or deuterium • Droplet diameter 25 -35 μm • Frequency 8 -10 k. Hz • Velocities around 80 m/s COSY

Drift chamber • • • 1738 tubes in 17 layers Diameter of tubes 4 to 8 mm Filled with CO 2 i argon Magnetic field of 1 Tesla Working principle:

Electromagnetic calorimeter • Covers most of the angular space • 1012 Cs. I(Na) crystals varying from 20 to 30 cm (~16 X 0)

Results

pp→ppπ° data • Collected with energy 550 Me. V (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.

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” [ar. Xiv: 1304. 0671]

ε 2 coupling constant vs U boson mass „Search for a dark photon in π⁰ → e⁺e⁻γ decay” [ar. Xiv: 1304. 0671]

meson • Mass ~550 Me. V/c 2 • 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

Dark matter • KTe. V 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)x 10 -8; BRtheo=(6. 2± 0. 1)x 10 -8 • Proposed explanation by a vector boson U coupling both to quarks and electrons [Nucl. Phys. B 683: 219, 2004] [Phys. Rev. D 78: 115002, 2008] • Similar effect could be seen in η e+e-, but here we have much smaller statistic

Leptonic decays of meson Branching ratios: → 39% 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.

Previous analysis in CELSIUS/WASA

Collected data and trigger system • • Experiment in Autumn of 2008 using pp→pp reaction @ 1. 4 Ge. V 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 simulation preliminary • Trigger worked nearly the same for each leptonic decay channel

pp→pp( → ) IMγγ πº • Two photon events • Used for normalization η Data MMpp η

→ preliminary πº η Numer of eta mesons produced: ~5. 9· 107 With systematic error of 15% 26

+ ee The invariant mass of e+epairs produced by a virtual photon conversion is usually small pairs in data e+e- pairs Data at the beginning of analysis π+π- pairs MCeeγ Where MP is in this case η meson mass, q 2 is Mee mass

+ pp→pp( →e e ) πº η η Data Events with small e+e- mass

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

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%

→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

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

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

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 CL 90% • 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. A 48 (2012) 64) Part of my Ph. D Thesis

Data: η Dalitz decay Data analysis (by Damian Pszczel) is in progress At least 10⁸ η mesons produced in pp @ 1. 4 Ge. V Lower statistics than in π⁰ case but up to higher masses We expect around 40000 η Dalitz events in the full data set Small part of data

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 (x 8) of meson decays and π⁰ (x 4) is in progress

Thank you for your attention