The η-meson Decay Program at WASA-at-COSY

Mitglied der Helmholtz-Gemeinschaft The η-meson Decay Program at WASA-at-COSY 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Institute for Nuclear Physics

The η-meson! " " " # #, (" + " - ) l + Hadronic Radiative l - (Semi-)Leptonic #, (" + " - ), (l + l - ) m η = 0.5478 GeV/c 2, τ 5 10 19 s (Γ η = (1.31 ± 0.05) kev) J PC = 0 + = η-meson is: C-, P-, G- and CP- eigenstate All strong and electromagnetic decays are forbidden to first order Access to rare decay processes: Explore anomalous sector of QCD Determine electromagnetic transition form factors Study of symmetry-breaking-phenomena η-meson production at WASA-at-COSY: i) pd 3 Heη[η...] σ(η) = (0.412 ± 0.016) µb at T beam = 1 GeV ii) pp ppη[η...] σ(η) = (9.8 ± 1) µb at T beam = 1.4 GeV 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 2

2 The data sets Events per 0.65 MeV/c 450 400 350 300 250 200 150 100 50 3 10 N(+q,-q) 1 and N(γ) 2 GeV c 2 = 0.5478 m η 0 0.35 0.4 0.45 0.5 0.55 0.6 0.65 Missing mass (p, p, p, p ) GeV beam target 1 2 c 2 Background contributions from direct pion production reactions: pd 3 HeX, pp ppx X = π + π, X = π 0 π 0 and X = π + π π 0 Reconstruct η-meson via missing mass: e.g. P beam + P target (P proton1 + P proton2 ) pd 3 Heη pp ppη Data taken in 2008 2009 2008 2010 2012 Duration of beam time 4 weeks 8 weeks 2 weeks 7 weeks 8 weeks η-mesons detected / produced 1 10 7 2 10 7 1 10 8 4 10 8 5 10 8 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 3

Wide Angle Shower Apparatus - WASA Example: pp ppη[η π + π γ] 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 4

η π + π π 0 : Dalitz plot Dalitz plot variables: X = 3 Y = T π + T π T π + +T π +T π 0 3T π 0 T π + +T π +T π 0 (a) KLOE coll., JHEP, 05, (2008) Decay via strong isospin violation: Γ meas = ( QD Q ) 4 Γ Q 2 = m2 s ˆm2 with ˆm = 0.5 (m m d 2 u + m d ) m2 u Γ calculated with ChPT at Dashen limit, Q D = 24.2 dγ Dalitz plot analysis: (1 + ay + by 2 + dx 2 + fy 3 + gx 2 Y +...) dxdy c, e and h would imply C-violation 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 5

η π + π π 0 : Results from pd 3 Heη Exp. / Theo. a b d f ChPT (NNLO) (a) 1.271(75) 0.394(102) 0.055(57) 0.025(160) NREFT (b) 1.213(14) 0.308(23) 0.050(3) 0.083(19) KLOE (c) 1.090(5)( +8 19 ) 0.124(6)(10) 0.057(6)(+7 16 ) 0.14(1)(2) WASA (d) 1.144(18) 0.219(19)(47) 0.086(18)(15) 0.115(37) (a): J. Bijnens and K. Ghorbani., JHEP, 11, (2007) (b): S- P. Schneider et al., JHEP, 028, (2011) (c): KLOE coll., JHEP, 05, (2008) (d): WASA-at-COSY coll., Phys. Rev., C90(045207), 2014 1.2 10 5 η π + π π 0 events selected via a kinematic fit for the final event sample Dalitz plot analysis for pp ppη[η π + π π 0 ] in progress 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 6

η π + π γ: The box anomaly and π + π FSI Beyond chiral limit: ρ π + π + Γ Theory (η π + π γ) = 35.7 ev (b) Γ Exp. (η π + π γ) = (55.3 ± 2.4) ev (c) - π (b): B.R. Holstein, Phys. Scripta, T99:55-67, 2002 π 0,η,η' L WZW = + +... γ η,η' Include π + π Final State Interactions - π Modification of decay amplitude: (a) (a) F.Stollenwerk et al., Phys. Lett., B707:184-190, 2012 A η π + π γ [F PV (s ππ) (1 + αs ππ)] Description of FSI: *Input from theory { by F PV α = 0 reaction specific α 0 γ (c): PDG, Chin. Phys., 090001, 2014 Photon energy distribution E γ: (d) (d) WASA-at-COSY coll. Phys. Lett., B707:243-249, 2012 [a.u.] de d Γ γ 9000 8000 7000 6000 5000 4000 3000 2000 1000-2 α = 1.89 GeV α = 0 WASA-at-COSY: + - pd 3 Heη[η π π γ] A η π + - π γ 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 E γ in η-rest frame [GeV] Goal:Determine Γ(η π+ π γ) Γ(η π + π π 0 ) and α via Eγ-distribution in pp ppη[η π+ π γ] 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 7

η π + π γ : Status in pp ppη 2 Events per 0.65 MeV/c 22000 Analysis of η π + π - γ 20000 Data 18000 MC cocktail - 16000 η π + π γ 14000 η π + π - π 0 12000 pp ppπ + π - π 0 - pp ppπ + π 10000 8000 Preliminary [a.u.] de d Γ γ 14000 Preliminary 12000 10000 8000 6000 6000 4000 4000 2000 2000 0 0.35 0.4 0.45 0.5 0.55 0.6 0.65 GeV Missing mass (p,p,p,p ) c 2 beam target 1 2 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 E γ in η-rest frame [GeV] 209 k η π + π γ events reconstructed i) Rejection of wrongly reconstructed photons Reduce background from pp ppπ + π ii) Kinematic fit Further reduction of multi pion background and improvement of E γ resolution E γ distribution after background correction from direct pion production Next steps (ongoing): 1.) Include efficiency corrections for different assumptions on α 2.) Calculate Γ(η π+ π γ) Γ(η π + π π 0 ) and α 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 8

η e + e γ and η e + e e + e : Dalitz decays Single off-shell transition form factor F (q 2 ) [ ] dγ dq = dγ 2 dq F 2 QED (q2 ) 2 Observables to test: Γ(η e+ e γ) Γ η invariant mass distribution and Dilepton Recent result: Γ(η e+ e γ) = (6.9 ± 0.4) 10 Γ 3(a) η (a) K. Olive et al. Chin. Phys., C38, 090001, 2014 Double off-shell transition form factor F (q 2 1, q2 2 ) Different approaches for calculation of F (b) Observable to test: Γ(η e+ e e + e ) Γ η Current result measured by KLOE: (c) Γ(η e + e e + e ) =(2.4±0.2 Γη stat ±0.1 sys) 10 5 (b) J. Bijnens et al. arxiv:hep-ph/0106130v1, 2001 (c) KLOE coll. Phys. Lett., B702:324-328, 2011 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 9

η e + e γ and η e + e e + e : Results from pd 3 Heη Preliminary Preliminary 14, 040 ± 120 events η e + e γ events reconstructed Preliminary: Γ(η e+ e γ) Γ η = (6.72 ± 0.07 stat ± 0.31 sys) 10 3 18 ± 5 η e + e e + e events reconstructed Preliminary: Γ(η e+ e e + e ) Γ η = (3.2 ± 0.9 stat ± 0.5 sys) 10 5 Need more statistics ppη data set Challenges in analysis: i) Dilepton pairs from conversion events at the beam pipe ii) Handling of large pion background 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 10

η e + e γ: Status in pp ppη Preliminary Reduce uncertainty of F(q 2 ) at high q 2 : Increase statistics ppη data set 29 k η e + e γ events reconstructed in current pp ppη[η e + e γ] analysis In preparation: i) Calculation of Γ(η e+ e γ) Γ η ii) Dilepton invariant mass distribution F(q 2 ) Analysis of pp ppη[η e + e e + e ] in preparation 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 11

η π 0 e + e : C-Violation Forbidden by SM: BR(η π 0 e + e 5 (a) ) < 4 10 Different models for C-violation (e.g. VMD) Investigate existing upper limit BR with hight statistics pp ppη data set (a) K. Olive et al. Chin. Phys., C38, 090001, 2014 entries / bin 15 10 5 p r e l i m i n a r y 0 0 0.1 0.2 0.3 0.4 0.5 0.6 2 invariant mass of lepton pair / GeV/c 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 12

Summary and Outlook η-decays measured in pd 3 Heη with WASA-at-COSY Published results for: η πππ (Dalitz plot) and η π + π γ (E γ-distribution) Publication for BR of: η π + π γ, η e + e γ, η e + e e + e (and η π + π e + e ) in progress Large statistics pp ppη data set taken with WASA-at-COSY Start analysis of decays presented above for the 2010 pp ppη data set: BR and E γ-distribution for η π + π γ Determination of form factor F(q 2 ) and BR for η e + e γ Upper limit for BR of η π 0 e + e Analysis of pp ppη[η e + e e + e ] in preperation Analysis of pp ppη[η π + π e + e ] Analysis of remaining pp ppη data set 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 13

Contents (2) The η-meson (3) The data set (4) WASA η π + π π 0 (5) Dalitz plot (6) Results from pd 3 Heη η π + π γ (7) The box anomaly and π + π FSI (8) Status in pp ppη η e + e γ and η e + e e + e (9) Dalitz decays (10) Results from pd 3 Heη (11) Status in pp ppη η π 0 e + e (12) C-Violation (13) Summary and Outlook 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 14

Backup η-meson Decays accessible with WASA-at-COSY η π + π γ Theoretical Models Theoretical Predictions and Recent Measurements Analysis (Split-off rejection) Analysis (Kinematic fit) Determining the E γ-distribution Conversion events η π + π e + e CP-Violation Results from pd 3 Heη Outlook for pp ppη η Production mechanisms 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 15

η-meson Decays accessible with WASA-at-COSY Decay mode Γ(η...)/Γ η (a) Issue η π 0 π 0 π 0(b) (32.68 ± 0.23)% Dalitz plot parameter, quark masses η π + π π 0(c) (22.92 ± 0.28)% Dalitz plot parameter, quark masses η π + π γ (d) (4.22 ± 0.08)% Box-anomaly, π + π -FSI η e + e γ (e) (0.69 ± 0.11)% Single-off-shell transition form factor η π 0 γγ (2.7 ± 0.5) 10 4 Test of ChPT η π + π e + e (e) (2.68 ± 0.11) 10 4 CP-violation η e + e e + e (e) (2.40 ± 0.22) 10 5 Double-off-shell transition form factor η π 0 e + e < 4 10 5 C-violation η e + e < 5.6 10 6 Physics beyond the SM i) pd 3 Heη[η...] ii) pp ppη[η...] (a): PDG, Chin. Phys., 090001, 2014 (b): WASA-at-COSY coll., Phys. Lett., B677:24-29, 2009 (c): WASA-at-COSY coll., Phys. Rev., C90(045207), 2014 (d): WASA-at-COSY coll., Phys. Lett., B707:243-249, 2012 (e): Publication in preparation 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 16

η π + π γ:theoretical Models N/D-Model: a) One-loop chiral corrections and VMD [ 1+0.5m 2 ] Modify A η π + π γ with: ρ sππ D 1 (sππ ) a) B.R. Holstein, Phys. Scripta, T99:55-67, 2002 [a.u.] de d Γ γ 50000 40000 30000 20000 10000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 E γ in η-rest frame [GeV] 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 17

η π + π γ:theoretical Models N/D-Model: a) One-loop chiral corrections and VMD [ 1+0.5m 2 ] Modify A η π + π γ with: ρ sππ D 1 (sππ ) HLS (Hidden Local Symmetries)-Model: b) γ V Transitions Modify A η π + π γ with: [ 1 + 3m2 ] ρ Dρ(sππ ) [a.u.] de d Γ γ a) B.R. Holstein, Phys. Scripta, T99:55-67, 2002 b) M.Benayoun et al., Europ. Phys. Journal, C31:525-547, 2003 50000 40000 30000 20000 10000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 E γ in η-rest frame [GeV] 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 17

η π + π γ:theoretical Models N/D-Model: a) One-loop chiral corrections and VMD [ 1+0.5m 2 ] Modify A η π + π γ with: ρ sππ D 1 (sππ ) HLS (Hidden Local Symmetries)-Model: b) γ V Transitions Modify A η π + π γ with: O(p 6 ) + 1 loop-modell: c) [ 1 + 3m2 ] ρ Dρ(sππ ) Higher momentum orders O(p 6 ) and one loop chiral corrections Modify A η π + π γ with: [ 1 + C loops + 3 2m ρ 2 (p π + + p π ) 2] [a.u.] de d Γ γ a) B.R. Holstein, Phys. Scripta, T99:55-67, 2002 b) M.Benayoun et al., Europ. Phys. Journal, C31:525-547, 2003 c) J.Bijnens et al., Phys. Lett., B237:488-494, 1990 50000 40000 30000 20000 10000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 E γ in η-rest frame [GeV] 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 17

η π + π γ:theoretical Models N/D-Model: a) One-loop chiral corrections and VMD [ 1+0.5m 2 ] Modify A η π + π γ with: ρ sππ D 1 (sππ ) HLS (Hidden Local Symmetries)-Model: b) γ V Transitions Modify A η π + π γ with: O(p 6 ) + 1 loop-modell: c) [ 1 + 3m2 ] ρ Dρ(sππ ) Higher momentum orders O(p 6 ) and one loop chiral corrections Modify A η π + π γ with: [ 1 + C loops + 3 2m ρ 2 (p π + + p π ) 2] Pion-Vektor-Formfactor: d) π + π -interactions (universal) Modify A η π + π γ with: F PV (s ππ) a s 3 ππ + b s2 ππ + c sππ + d [a.u.] de d Γ γ a) B.R. Holstein, Phys. Scripta, T99:55-67, 2002 b) M.Benayoun et al., Europ. Phys. Journal, C31:525-547, 2003 c) J.Bijnens et al., Phys. Lett., B237:488-494, 1990 d) F.Stollenwerk et al., Phys. Lett., B707:184-190, 2012 50000 40000 30000 20000 10000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 E γ in η-rest frame [GeV] 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 17

η π + π γ: Theoretical Predictions and Recent Measurements Experiment Theory Γ(η π + π γ)/γ(η π + π π 0 ) α [GeV 2 ] Gormley et al. 0.202 ± 0.006 1.8 ± 0.4 Thaler et al. 0.209 ± 0.004 - Layter et al. - 0.9 ± 0.1 GAMS-200-2.7 ± 0.1 CRYSTAL BARREL - 1.8 ± 0.53 CLEO 0.175 ± 0.013 - WASA-at-COSY Preliminary: 0.206 ± 0.011 1.89 ± 0.86 KLOE 0.1856 ± 0.003 1.32 ± 0.2 CLAS Analysis ongoing - N/D 0.2188 ± 0.0088 0.64 ± 0.02 HLS 0.1875 ± 0.0094 0.23 ± 0.01 (O(p 6 ) + 1 loop) 0.1565 ± 0.0063 0.7 ± 0.1 Box anomaly 0.119 ± 0.0048 1.7 ± 0.02 Measured η π + π γ 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 18

η π + π γ: Analysis i) Rejection of split-offs One (charged) particle in the calorimeter No split-off split-off Edep = 0 Edep! 0 (charged) particle (charged) particle Hit in calorimeter is assigned to a cluster Split-off: Satellite cluster with close distance to primary cluster low energy fake photon Predominant background: pp ppπ + π (γ) Reject low energy fake photons with close distance to primary cluster 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 19

η π + π γ: Analysis ii) Kinematic fit Counts per bin 10 7 6 10 5 10 10 4 3 10 10 2 10 reject + - Fit hypothesis: pp ppπ π γ Data accept MC cocktail + - η π π γ η π + π - π 0 + - η e e γ pp ppπ + π - π 0 + - pp ppπ π 1 0 0.2 0.4 0.6 0.8 1 Probability P(χ 2 pp ppπ,4) + π - γ Least squares fit: χ 2 Np Nv ( v fit v ij meas ) ij 2 = +2 σ i=1 j=1 ij meas λ µf µ(v11 fit,..., vfit µ Np Nv ) F µ: energy and momentum conservation 4 constraints P(χ 2 1, N) = e t 2 t 1 2 N Γ( 1 2 N 1 dt 2 N) χ 2 Use kinematic fit to: a) Improve resolution b) Suppress background 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 20

η π + π γ: Determining the E γ -distribution Scan two proton missing mass distribution in E γ-intervals Subtract background for each E γ-interval Obtain number of η π + π γ events 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 21

Conversion events!"#"$! "# $%%$! "&!"#"$! "# $%%$! "& Radius of closest approach (xy-plane) [mm] 90 80 70 60 50 40 30 non-conversion events 20 10 0 0 0.02 0.04 0.06 0.08 0.1 GeV + Invariant mass (e - e ) at beam pipe c 2 conversion events 2 10 10 1 '()*$ +,+( '()*$ +,+( %&'()*+,-./*+0.1*232, 4+,5'*-2+,.%5',) 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 22

η π + π e + e : CP-Violation Decay η π + π γ: CP-conserving for M 1 and E 2 transitions Access to CP-violation measure E 1 transition Need information about polarisation of single photon Alternative: η π + π γ, γ e + e (a) Look at asymmetry A Φ of angle Φ between decay planes of electrons and pions A Φ = N(sin[Φ] cos[φ]>0) N(sin[Φ] cos[φ]<0) N(sin[Φ] cos[φ]>0)+n(sin[φ] cos[φ]<0) Upper limit predicted by theory (a) : 1% Result found by KLOE (b) : A Φ = ( 0.6 ± 2.5 stat ± 1.8 sys) 10 2 (a) D. Gao. Mod. Phys. Lett., A17:1583-1588, 2002 (b) KLOE coll. Phys. Lett., B675:283-288-914, 2009 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 23

η π + π e + e : Results from pd 3 Heη Preliminary Preliminary 251 ± 17 η π + π e + e events in the final sample i) Rejection of photon conversion events Reduce contributions from η π + π γ / η π + π π 0 ii) Identification of π ± and e ± in the mixed final state Crucial for the determination of A Φ iii) Kinematic fit Reduction of η π + π π 0 [π 0 e + e γ] Preliminary: A Φ = ( 1.1 ± 6.6 stat ± 0.2 sys) 10 2 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 24

η π + π e + e : Outlook for pp ppη Analysis procedure shown above done for a fraction of 2010 pp ppη data set: (a) 220 η π + π e + e events reconstructed 1, 000 events expected for full pp ppη data sample Analysis in pp ppη needs to be continued (a) D. Coderre,PhD Thesis, 2012 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 25

η Production mechanisms pd 3 Heη pp ppη T beam 1 GeV 1.4 GeV σ(η) a),b) (0.412 ± 0.016) µb (9.8 ± 1) µb Suited for study of not-so-rare η decays study of (not-so-) rare η decays Background low multi-pion background high multi-pion background Reaction T beam [GeV] σ[µb] b),c) pd 3 Heπ 0 π 0 0.893 2.8 ± 0.3 pd 3 Heπ + π 0.893 5.1 ± 0.5 pp ppπ + π π 0 1.36 4.6 ± 1.5 pp ppπ 0 π 0 1.36 200 ± 30 pp ppπ + π 1.36 660 ± 100 a) R. Bilger et al., Phys. Rev., C65(044608), 2002 b) CELSIUS/WASA coll.., Phys. Lett., B649:122-127, 2007 c) M. Bashkanov et al.., Phys. Lett., B637:223-228, 2006 31.08.2015 Daniel Lersch for the WASA-at-COSY collaboration Slide 26