Search for possible dark matter effects in leptonic

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Search for possible dark matter effects in leptonic decays of light mesons measured in

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

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

Dark matter • Seen in galaxy rotation curves and in gravitational lensing • Not

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

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

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) •

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

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

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,

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

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

Forward detector

Forward detector

Central detector

Central detector

Pellet target • Frozen hydrogen or deuterium • Droplet diameter 25 -35 μm •

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

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

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

Results

Results

pp→ppπ° data • Collected with energy 550 Me. V (below threshold for π+π- production)

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 →

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

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

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

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-

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

Previous analysis in CELSIUS/WASA

Collected data and trigger system • • Experiment in Autumn of 2008 using pp→pp

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

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

→ 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

+ 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

+ 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 60 mm Data preliminary MCeeγ MCγγ

Photon conversion in the beam pipe + Data --- MC η→γγ --- MC η→e⁺e⁻γ

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

→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

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

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

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

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

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

Thank you for your attention