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

Transcription

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

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: 1 pd 3 Heη[η...] σ(η) = (0.412 ± 0.016) µb at T beam = 1 GeV 2 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 Reconstruct η-meson via missing mass: P in P out 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 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

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 analysis 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, ˆm = 1 m d 2 m2 2 (mu + m d) 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 Parameter: a b d f Theo. ChPT (NNLO) (b) 1.271(75) 0.394(102) 0.055(57) 0.025(160) NREFT (c) 1.213(14) 0.308(23) 0.050(3) 0.083(19) Exp. KLOE (a) 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) KLOE coll., JHEP, 05, (2008) (b) J. Bijnens and K. Ghorbani., JHEP, 11, (2007) (c) S- P. Schneider et al., JHEP, 028, (2011) (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] = 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) and determine α Γ(η π + π π 0 ) Daniel Lersch for the WASA-at-COSY collaboration Slide 8

9 η π + π e + e 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 CP-Violation η π + π γ [γ e + e ] Look at asymmetry A Φ (a) 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% Results found by KLOE: (b) 1.) A Φ = ( 0.6 ± 2.5 stat ± 1.8 sys) ) Γ(η π + π e + e ) Γ η = (2.68 ± 0.09 stat ± 0.07 sys) 10 4 (a) D. Gao. Mod. Phys. Lett., A17: , 2002 (b) KLOE coll. Phys. Lett., B675: , Daniel Lersch for the WASA-at-COSY collaboration Slide 9

10 η π + π e + e Results from pd 3 Heη Preliminary Entries / (0.04) η π + π e + e events (includes background) Flat reference distribution Prelimiary sin φ cos φ 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: 1.) A Φ = ( 1.1 ± 6.6 stat ± 0.2 sys) 10 2 Γ(η π + π e + e ) Γ η 2.) = (2.7 ± 0.2 stat ± 0.2 sys) 10 4 Need more statistics ppη data set Daniel Lersch for the WASA-at-COSY collaboration Slide 10

11 η π + π e + e Status in pp ppη Analysis procedure shown above done for a fraction of 2010 pp ppη data set: (c) 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 (c) D. Coderre,PhD Thesis, Daniel Lersch for the WASA-at-COSY collaboration Slide 11

12 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 and Dilepton mass 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 12

13 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 13

14 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 i) PID Reduction of pions ii) Rejection of conversion events Suppression of η γγ iii) Check for energy and momentum conservation In preparation: 1.) Calculation of Γ(η e+ e γ) Γ η 2.) 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 14

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

16 Summary and Outlook Decay mode Γ(η...)/Γ η (a) Issue η π 0 π 0 π 0(b) (32.68 ± 0.23)% Dalitz plot analysis η π + π π 0(c) (22.92 ± 0.28)% Dalitz plot analysis 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 1 pd 3 Heη[η...] 2 pp ppη[η...] Analysis ongoing (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 More fun with η-decays: See talk by M. Kunkel (1F1) Daniel Lersch for the WASA-at-COSY collaboration Slide 16

17 Contents (2) The η-meson (3) The data set (4) WASA η π + π π 0 (5) Dalitz plot analysis (6) Results from pd 3 Heη η π + π γ (7) The box anomaly and π + π FSI (8) Status in pp ppη η π + π e + e (9) CP-Violation (10) Results from pd 3 Heη (11) Status in pp ppη η e + e γ and η e + e e + e (12) Dalitz decays (13) Results from pd 3 Heη (14) Status in pp ppη η π 0 e + e (15) C-Violation (16) Summary and Outlook Daniel Lersch for the WASA-at-COSY collaboration Slide 17

18 Backup η π + π γ Theoretical Models Theoretical Predictions and Recent Measurements Analysis (Split-off rejection) Analysis (Kinematic fit) Determining the E γ -distribution η e + e γ and η e + e e + e Form factor F (q 2 ) Theoretical predictions for Γ(η e + e e + e )/Γ(η) Conversion events η Production mechanisms Preselection of the pp ppη data set PID at WASA Daniel Lersch for the WASA-at-COSY collaboration Slide 18

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 ) 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 19

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: [ 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 19

21 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 2mρ 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 19

22 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 2mρ 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 19

23 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 20

24 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 ] Phys. Rev.,D2: , ± ± 0.4 Phys. Rev.,D7: , ± Phys. Rev.,D7: , ± 0.1 Phys.,C50: , ± 0.1 Phys. Lett.,B402:195, ± 0.53 Phys. Rev. Lett.,99(122001), ± Phys. Rev. Lett.,B707: , ± 0.86 Phys. Lett.,B718: , ± ± 0.2 Chin. Phys., , ± Phys. Scripta, T99:55-67, ± ± 0.02 Europ. Phys. Journal, C31: , ± ± 0.01 Phys. Lett., B237: , ± ± 0.1 Phys. Scripta, T99:55-67, ± ± 0.02 Measured η π + π γ Daniel Lersch for the WASA-at-COSY collaboration Slide 20

25 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 21

26 ii) Kinematic fit η π + π γ Analysis 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 22

27 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 23

28 η e + e γ Form factor F (q 2 ) a) Result of the CB/TAPS b) Results of the Lepton-G (open circles) Single-pole measurement formula: of thef η e + e P (q 2 ) = (1 γ bpq 2 and 2 ) 1 the, b P NA60 1 (triangles) measurements of the η µ + µ γ decay. The Λ P decay [28] solid and dashed-dotted lines are fits to Daniel Lersch for the WASA-at-COSY collaboration Slide 24

29 η e + e e + e Theoretical predictions for Γ(η e + e e + e )/Γ(η) Double transition form factor F(q 2 1, q 2 2) Different approaches for calculation of F (a) : F (q 2 1, q 2 2) Γ(η e + e e + e )/Γ(η) [10 5 ] ± 0.02 m 4 ρ (m 2 ρ q2 1 )(m2 ρ q2 2 ) 2.65 ± 0.02 m 2 ρ (m 2 ρ q2 1 q2 2 ) 2.64 ± 0.02 m 4 ρ 4π2 F 2 π N C (q 2 1 +q2 2 ) (m 2 ρ q2 1 )(m2 ρ q2 2 ) 2.61 ± 0.02 (a) J. Bijnens et al. arxiv:hep-ph/ v1, Daniel Lersch for the WASA-at-COSY collaboration Slide 25

30 Conversion events! "# $%%$! "&! "# $%%$! "&!"#"$!"#"$ '()*$ '()*$ +,+( +,+( %&'()*+,-./*+0.1*232, 4+,5'*-2+,.%5',) Radius of closest approach (xy-plane) [mm] conversion events non-conversion events Invariant mass (e e ) at beam pipe GeV c Conversion events: small opening angle and origin at beam pipe Non-Conversion events: large opening angle and origin at reaction vertex Daniel Lersch for the WASA-at-COSY collaboration Slide 26

31 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 27

32 2 Preselection of the pp ppη data set Counts per 0.01 GeV/c Before cut on missing mass After cut on missing mass MC: pp ppη Cut on missing mass accept event Counts per bin accept event Missing mass (p,p,p,p ) GeV beam target 1 2 c Number of charged particle pairs in CD with Q Q = Preselection done in two steps: i) Condition on missing mass Rejection of multi-pion background ii) Condition on charged tracks in the Central Detector Selection of charged η decay modes Daniel Lersch for the WASA-at-COSY collaboration Slide 28

33 E FRH(Layer1) [GeV] PID at WASA Counts per 10-3 GeV/c E FRH(Layer2) [GeV] Data MC: η π+π- π Invariant mass (γ 1,γ 2) GeV c2 Daniel Lersch for the WASA-at-COSY collaboration Slide 29