Search for a new light boson in  decays J.Stepaniak, M.Berłowski, NCBJ Warsaw For WASA-at-COSY Collaboration Meson2014,Krakow

Outline Hypothetical feebly-coupled sector, U vector boson, scalar boson. Search for new boson decaying to e+e- Search for  →  0 X( e+e-) with WASA detector at COSY Perspectives

Search for „dark” vector U boson in meson decays Natural extension of the SM Coupling to leptons possible, signals from astrophysics Low mass region (<1GeV) still not fully explored Exclusion plot (C.Bloise, Meson 2014) WASA -  o Dalitz decays

Search for new scalar boson in  → π 0 e + e - decay In the hypothetical new „dark” sector not only vector gauge boson may exist. The mass of U vector boson can be generated by scalar Higgs-like field Possible observation of such scalar in eta->  → π 0 e + e - decay channel. Small background from C conserving SM decay (~10 -8 ). Upper experimental limit for BR< Theoretical upper bound for  → π 0 (X → e + e - ) Branching Ratio with Yukawa –type coupling of the order of was given by Kozlov ( NuovoCimento 109A,1996). Leutwyler and Shifman (Nucl.Phys. B343 (1990) BR ~ |p π |/m   10 -6

Search for new scalar boson in  → π 0 e + e - decay with WASA Background from Standard Model is small. Calculated by Cheng(1967) in VMD Model, Ng and Peters (1992), In WASA the search of new boson X with lifetime short enough to be seen in the detector. The stringent limits exist in the small mass region for long-lived particle eg from beam dump experiments. Search for C-violating decay via one virtual photon was performed by WASA PhDstudents in the low mass (eg <40MeV in M.Zielinski PhD).

The coupling of the U vector boson to electically charged particles via „kinetic mixing” produces an effective parity conserving interaction ~U  J  EM The vector boson U generated in this scenario can not be responsible for the eta->eepi0.

Search for new scalar boson in  → π 0 e + e - decay with WASA An example of WASA-at-COSY analysis: data collected with WASA detector during 2 weeks run in 2008, pp → ppeta reaction at 1.4 GeV Trigger based on large energy deposit in left and right side of central part of Electromagnetic Calorimeter. About 59 mln of etas collected. Details of the analysis: correction for the trigger efficiency and external photon conversion reduction is described in Marcin Berlowski PhD(Warsaw,2013).

The  → ee  channel after external conversion rejection – well measured with WASA

Search for :selection of event candidates (1) First step : - Two charged particles with opposite charge, - At least two neutral clusters in electromagnetic calorimeter (E>10 MeV), - At least 2 tracks in FD (closest in time), MMpp ( ) GeV, - Identification of electrons P/Edep <1.2 for both charged tracks.

Selection of events (2) Second reduction: conversion removed Best pair of photons -> best pi0 mass 0.11 < Mgg < GeV

Selection of events (3) Third reduction: Momentum of each photon and lepton <500MeV/c, Total Missing Mass < 50MeV, IMeegg ( ) GeV, MMpp ( ) GeV,

The major background channels 1. pp→pp(  → ee  ) cascade split-off or photon from other event 2. pp→pp(  →  +  -  0 ) with pions misidentified as e+ and e-. 3. pp→pp  0  0 →pp  0 ee  Dalitz decay of one pion, one photon escaped or two overlapping photons. 4. pp→ pp(  →  )

MC  →  +  -  0 1mln generated ev ev 3. No events observed

Background MC pp→pp  0  0 →pp  0 ee  1mln of events generated with Dalitz decay of one pion ev ev → 3. 5 ev eff= We expect in the data : 324 mub/ 10mub x Neta x eff x BR D x 2 = ev

Signal simulation  →  0 e + e - (1 mln) Phace Space assumed to cover the whole range of M ee ev ev ev Average acceptance Acc=

Acceptance as a function of lepton pair invariant mass

63 events with Mee mass <0.1GeV consistant with background from pp  0 and  →ee  within 2sigma. No events with Mee > 0.1 GeV observed The average acceptance for the signal after all cuts and correction for the trigger efficiency: Preliminary 90% upper limit for the region of the mass >100 MeV: from the ratio to the observed eeg decay: ~ 2  The systematic errors under study.

Other decay channel:  → e + e - SM NMSSM(Phys.Lett.B676(2009) Present preliminary limit from WASA for BR (ee) 3.9  10-6 PhD M.Berlowski (NCBJ,Warsaw)

Summary Analysis presented here has been performed with a fraction of data collected by WASA Collaboration. The search with WASA for C-violated process with one virtual photon in low e+e- mass region was done by A.Winnemoller (PhD,Munster) and M.Zielinski (PhD, Krakow, arXiv: , upper limit 3.7  ). This limit was based on events from Mee<40MeV mass region. The 10 times larger eta sample from both pp->pp  and pd-> 3 He  reaction is under study.

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