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

Size: px

Start display at page:

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

Dorothy Ryan
6 years ago
Views:

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

2 The η-meson! " " " # #, (" + " - ) l + Hadronic Radiative l - (Semi-)Leptonic #, (" + " - ), (l + l - ) m η = GeV/c 2, τ 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 2

3 2 The data sets Events per 0.65 MeV/c N(+q,-q) 1 and N(γ) 2 GeV c 2 = m η 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 Duration of beam time 4 weeks 8 weeks 2 weeks 7 weeks 8 weeks η-mesons detected / produced Daniel Lersch for the WASA-at-COSY collaboration Slide 3

Wide Angle Shower Apparatus - WASA Example: pp ppη[η π + π γ] 31.

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

5 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 5

6 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)( ) 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), 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 6

7 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: , 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., , 2014 Photon energy distribution E γ: (d) (d) WASA-at-COSY coll. Phys. Lett., B707: , 2012 [a.u.] de d Γ γ α = 1.89 GeV α = 0 WASA-at-COSY: + - pd 3 Heη[η π π γ] A η π + - π γ 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] Daniel Lersch for the WASA-at-COSY collaboration Slide 7

8 η π + π γ : Status in pp ppη 2 Events per 0.65 MeV/c Analysis of η π + π - γ Data MC cocktail η π + π γ η π + π - π pp ppπ + π - π 0 - pp ppπ + π Preliminary [a.u.] de d Γ γ Preliminary GeV Missing mass (p,p,p,p ) c 2 beam target 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 8

9 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, , 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/ v1, 2001 (c) KLOE coll. Phys. Lett., B702: , Daniel Lersch for the WASA-at-COSY collaboration Slide 9

10 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) ± 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 10

11 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 11

12 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, , 2014 entries / bin p r e l i m i n a r y invariant mass of lepton pair / GeV/c Daniel Lersch for the WASA-at-COSY collaboration Slide 12

13 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 13

14 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 14

15 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 15

16 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 < C-violation η e + e < Physics beyond the SM i) pd 3 Heη[η...] ii) pp ppη[η...] (a): PDG, Chin. Phys., , 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: , 2012 (e): Publication in preparation Daniel Lersch for the WASA-at-COSY collaboration Slide 16

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 ) a) B.R. Holstein, Phys. Scripta, T99:55-67, 2002 [a.u.] de d Γ γ E γ in η-rest frame [GeV] Daniel Lersch for the WASA-at-COSY collaboration Slide 17

18 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: , E γ in η-rest frame [GeV] Daniel Lersch for the WASA-at-COSY collaboration Slide 17

19 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: , 2003 c) J.Bijnens et al., Phys. Lett., B237: , E γ in η-rest frame [GeV] Daniel Lersch for the WASA-at-COSY collaboration Slide 17

20 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: , 2003 c) J.Bijnens et al., Phys. Lett., B237: , 1990 d) F.Stollenwerk et al., Phys. Lett., B707: , E γ in η-rest frame [GeV] Daniel Lersch for the WASA-at-COSY collaboration Slide 17

21 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 ± ± 0.4 Thaler et al ± Layter et al ± 0.1 GAMS ± 0.1 CRYSTAL BARREL ± 0.53 CLEO ± WASA-at-COSY Preliminary: ± ± 0.86 KLOE ± ± 0.2 CLAS Analysis ongoing - N/D ± ± 0.02 HLS ± ± 0.01 (O(p 6 ) + 1 loop) ± ± 0.1 Box anomaly ± ± 0.02 Measured η π + π γ Daniel Lersch for the WASA-at-COSY collaboration Slide 18

22 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 19

23 η π + π γ: Analysis ii) Kinematic fit Counts per bin reject + - Fit hypothesis: pp ppπ π γ Data accept MC cocktail + - η π π γ η π + π - π η e e γ pp ppπ + π - π pp ppπ π 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 Daniel Lersch for the WASA-at-COSY collaboration Slide 20

24 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 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.

25 Conversion events!"#"$! "# $%%$! "&!"#"$! "# $%%$! "& Radius of closest approach (xy-plane) [mm] non-conversion events GeV + Invariant mass (e - e ) at beam pipe c 2 conversion events '()*$ +,+( '()*$ +,+( %&'()*+,-./*+0.1*232, 4+,5'*-2+,.%5',) 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:

26 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: , 2002 (b) KLOE coll. Phys. Lett., B675: , 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

27 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) Daniel Lersch for the WASA-at-COSY collaboration Slide 24

28 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, Daniel Lersch for the WASA-at-COSY collaboration Slide 25

29 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 ± 0.3 pd 3 Heπ + π ± 0.5 pp ppπ + π π ± 1.5 pp ppπ 0 π ± 30 pp ppπ + π ± 100 a) R. Bilger et al., Phys. Rev., C65(044608), 2002 b) CELSIUS/WASA coll.., Phys. Lett., B649: , 2007 c) M. Bashkanov et al.., Phys. Lett., B637: , Daniel Lersch for the WASA-at-COSY collaboration Slide 26

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

Mitglied der Helmholtz-Gemeinschaft The η-meson Decay Program at WASA-at-COSY 15.09.2015 Daniel Lersch for the WASA-at-COSY collaboration XVI International Conference on Hadron Spectroscopy The η-meson!