Relative Phase between Strong and EM Decays at BESIII and CLEOc Marco Destefanis Università degli Studi di Torino 1 on behalf of the BESIII Collaboration Quarkonium 2013 The 9th International Workshop on Heavy Quarkonium IHEP, Beijing, April 22-26, 2013

Overview BESIII experiment Motivation CLEOc and SND results Investigated processes Summary 2

3  Charmonium Physics  D-Physics  Light Hadron Spectroscopy   -Physics ... Physics program The BESIII IHEP BEijing Spectrometer III e + e - collisions  S tuned depending on energy D.M. Asner et al, Physics at BES-III, arXiv: v1 [hep-ex] (2008)

4  Beam energy: GeV  Design Luminosity: 1×10 33 cm -2 s -1  Achieved Luminosity:  6.5×10 32 cm -2 s ψ(3770)  Optimum energy: 1.89 GeV  Energy spread: 5.16 ×10 -4  No. of bunches: 93  Bunch length: 1.5 cm  Total current: 0.91 A  Circumference: 237m e-e- e+e+ BEPCII Storage Rings Beijing Electron-Positron Collider II

5 MDC: small cell & He gas  xy = 130  m  p /p = dE/dx = 6% EMC: CsI crystals, 28 cm  E/E = GeV  z = 0.6 cm/  E TOF:  T = 80 ps Barrel 110 ps Endcap Magnet: 1T Superconducting Muon: 9 layer RPC Trigger: Tracks & Showers Pipelined; Latency = 2.4 ms Data Acquisition: Event rate = 3 kHz Thruput ~ 50 MB/s Zero Degree Detector (ISR) BESIII Detector

J/ψ Strong and Electromagnetic Decay Amplitudes Resonant contributions Γ J/ψ ~ 93KeV → pQCD pQCD: all amplitudes almost real [1,2] QCD -> Ф p ~ 10° [1] Non-resonant continuum pQCD regime A EM  6 [1] J. Bolz and P. Kroll, WU B [2] S.J. Brodsky, G.P. Lepage, S.F. Tuan, Phys. Rev. Lett. 59, 621 (1987). Strong → A 3g Electromagnetic → A γ Non-resonant Continuum → A EM hadrons

J/ψ Strong and Electromagnetic Decay Amplitudes If both real, they must interfere (Ф p ~ 0°/180°) On the contrary Ф p ~ 90° → No interference J/ψ → NN ( ½ + ½ - ) Ф p = 89° ± 15° [1] ; 89° ± 9° [2] J/ψ → VP ( ) Ф p = 106° ± 10° [3] J/ψ → PP ( ) Ф p = 89.6° ± 9.9° [4] J/ψ → VV ( ) Ф p = 138° ± 37° [4] Results are model dependent Model independent test: interference with the non resonant continuum [1] R. Baldini, C. Bini, E. Luppi, Phys. Lett. B404, 362 (1997); R. Baldini et al., Phys. Lett. B444, 111 (1998) [2] M. Ablikim et al., Phys. Rev. D 86, (2012). [3] L. Kopke and N. Wermes, Phys. Rep. 174, 67 (1989); J. Jousset et al., Phys. Rev. D41,1389 (1990). [4] M. Suzuki et al., Phys. Rev. D60, (1999). 7

8 J/ψ Strong and Electromagnetic Decay Amplitudes J/ψ → NN Favoured channel 3g match 3qq pairs Without EM contribution p = n, due to isospin EM contribution amplitudes have opposite sign, like magnetic moments BR nn expected ~ ½ BR pp But the BR are almost equal according to BESIII [1] : BR(J/ψ → pp) = (2.112 ± ± 0.027)10 -3 BR(J/ψ → nn) = (2.07 ± 0.01 ± 0.14)10 -3  Suggests 90° phase [1] M. Ablikim et al., Phys. Rev. D 86, (2012). A 3g,A   R<<1 A 3g  A  R  1

9 Cross section for e + e - -> π + π - π 0 Interference of φ(1020) SND experiment [1] Shape indicates full interference path phase ~ 180° φ decay in agreement with PQCD A 3g and A EM are both real [1] M.N. Achasov et al., PRD 63, (2001).

10 Phase CLEOc φ S. Dobbs et al., Phys. Rev. D 74, (2006).

11 Phase CLEOc E vis = event energy Charged π FF S. Dobbs et al., Phys. Rev. D 74, (2006).

Was an Interference Already Seen? e + e - → hadrons e + e - → µ + µ - e + e - → e + e - J.Z. Bai et al., Phys. Lett. B 355, (1995) Yes without the strong contribution 12

Investigated Processes  Inclusive scenario: does not see anything The phase is there, but the mean goes to 0 Interference Sum over all the final states Closure approximation But orthogonal states If we sum over all the channels, the interference ≈ 0 13

Investigated Processes  Exclusive scenario: could see interference effects e + e + -> J/ψ -> pp, nnNN BR ~ 2.17x10 -3 σ cont ~ 11 pb e + e - -> J/ψ -> ρπ VP BR ~ 1.69%σ cont ~ 20 pb e + e - -> J/ψ -> 2( π + π - ) π 0 BR ~ 5.5% σ cont ~ 500 pb 14

15 Investigated Processes  Exclusive scenario: could see interference effects also on e + e - -> J/ψ -> π + π - e + e - -> J/ψ -> K + K - e + e - -> J/ψ -> K 0 K 0 proposed and under study [1] All the other channels for free Even number of π : strong decay forbidden -> interference must be seen [1] H. Czyz, and J. Kühn, Phys. Rev. D80: (2009)

σ cont ~ 11 pb Continuum Cross Section 16 σ cont ~ 500 pb σ cont ~ 20 pb pp ρπ 5π5π5π5π V. Druzhinin et al., Rev. Mod. Phys. 83, 1545 (2011) ; B. Aubert et al., Phys. Rev. D73, (2006)

Phase Generator Event generator Monte-Carlo method ( iterations) Cross section evaluation at each point Beam spread gaussian (0.93 MeV) Radiative correction (simple model to be optimized) Max radiation 300 MeV (~20% E CM ) Cross section: 17

Simulated Yields for e + e - -> pp continuum reference σ ~ 11 pb beam energy spread + radiative corrections (to be optimized) no correctionsbeam energy spread (0.93 MeV) Δφ = 0° Δφ = 90° Δφ = 180° 18

Simulated Yields for pp -> µ + µ - continuum reference σ ~ 18 pb no correctionsbeam energy spread Δφ = 0° Δφ = 90° Δφ = 180° 19

Phase Sign * red: Δφ = -90° blue: Δφ = +90° Maximum differences at the 1% level 20 pp

Energy Points Choice ρπ 5π5π5π5πpp 21 Maximum interference: 0° Depends on the process

Energy Points Choice ρπ 5π5π5π5πpp 22  2 pts at low W fix the continuum fix the slope Maximum interference: 0° Depends on the process

Energy Points Choice ρπ 5π5π5π5πpp 23  2 pts at low W fix the continuum fix the slope  2 pts at deep positions Maximum interference: 0° Depends on the process

Energy Points Choice ρπ 5π5π5π5πpp 24  2 pts at low W fix the continuum fix the slope  2 pts at deep positions Maximum interference: 0° Depends on the process

Energy Points Choice ρπ 5π5π5π5πpp 25  2 pts at low W fix the continuum fix the slope  2 pts at deep positions  1 pt Beginning of the BW Maximum interference: 0° Depends on the process

Energy Points Choice  What happens at 90° Gradient calculation The deep corresponds roughly to the maximum gradient (σ 90 -σ i )/σ 90 i = 70 i = 100 i = 80 i = i = 70 i = 100 i = 80 i = 110 ppρπ

Energy Points Choice 3050 MeV 3060 MeV 3083 MeV 3090 MeV 3093 MeV 27

Luminosity Hypothesis 5 values of Luminosity: , 10 32, , , [cm -2 s -1 ] Time: 1 day = s Injection efficiency = 0.8 Reconstruction efficiency pp = 0.67 ρπ = π = 0.20 Rate = LTε injε rec σ Integrated Luminosity L int /day = L T ε inj , , , , [cm -2 ] 28

Precision of the Fit 29 10° 90° 170° Statistical error for: pp circle ρπtriangle 170° Lower sensitivity (No 0°-90° and 90°-180° symmetry) 2 parameters: φ and σ cont

30 5 days L int = 1.4x10 37 [cm -2 ] points: 3050,3060, 3083,3090, 3093 MeV ℓ 1 : ℓ 1 : ℓ 2 : ℓ 2 : ℓ 1 Fit results 10° 90° 170° Statistical error: pp circle ρπtriangle Open points: 1:1:0.5:0.5:2 Very low sensitivity to Luminosity ratios Best and simplest choice: 1:1:1:1:1

31 J/ ψ Scan 3 parameters: φ, σ cont and B out PointsParInj. eff.Δφ [°]Δσ [pb]ΔB out σ cont = 11 pbB out = parameters: needed (1 point more with high statistics) Δφ = +90° pp

32 J/ ψ Phase Energy requested [MeV] Energy collected [MeV] L int [pb -1 ]

33 J/ ψ Phase – Real Data Ecm(GeV)  (pb -1 ) ± ± ± ± ± ± ± ±0.010 Ecm(GeV)  (pb -1 ) ± ± ± ± ± ± ± ± B.X. Zhang, Luminosity measurement for J/psi phase and lineshape study.

34 e + e - -> μ + μ - Phase Reconstruction 2 good charged tracks: |Rxy|<1cm, |Rz|<10cm; |cos  |<0.8. No good neutral tracks in EMC: 0<T<14 (x50 ns) E  >25MeV (|cos  | 50 MeV (0.86<|cos  |<0.92)  ,charged<10 o. Vertex fit to impove the momentum resolution:  2 vertex <100. Veto e + e  : Each charged track has an energy deposit in EMC; E/p<0.25. Veto cosmic rays:  =|Tof(  + )-Tof(   )|<0.5 Momentum window cut: |p  ± -p the |<3  Leptonic decay Contributions from A γ and A EM

35 e + e - -> 2(π + π - ) Phase Reconstruction 4 good charged tracks: |Rxy|<1cm, |Rz|<10cm. Vertex fit to improve the momentum resolution. Veto bkg from  -conversion (2(e + e  )): All angles between   and  , 10 o <   +  <170 o. Veto events which have multi- tracks: Minimum angle between (     ) pairs:  (         )>170 o. G-Parity Contributions from A γ and A EM

36 e + e - -> 2(π + π - )π 0 Phase Reconstruction 4 good charged tracks: |Rxy|<1cm, |Rz|<10cm. At least 2 good neutral tracks in EMC: 0<T<14 (x50 ns); E  >25MeV (|cos  | 50 MeV (0.86<|cos  |<0.92)  ,charged<10 o. PID for each charged track: prob(  )>prob(K) Vertex fit:  2 vertex < C kinematic fit: Loop all photons, choose the combination with the minimum  2 3C (<200).  0 selection: |M(  )-0.135|<0.02 GeV/c2 Multi-combination from intermediate processes Contributions from A γ and A EM

37 2 good charged tracks: |Rxy| < 1 cm, |Rz| < 10 cm; back-to-back tracks: 178° < θ < 180°; p < 2 GeV/c; |cos| < 0.92 Analysis in Barrel + End Cap. ppbar Events Reconstruction M. Ablikim et al., Phys. Rev. D 86, (2012).

38 ISR Radiative Corrections Comparison of different generators KKMCPhase Space KKMC1 + cos 2 θ ConExc Babayaga ISR on/off Check at each energy point Reconstruction Efficiency and Systematic Errors

Summary J/ψ decay amplitude phase: 0° (theory) but 90° (data) Energy points collected: 3046, 3056, 3086, 3085, 3088 Statistical significance enough to discriminate between different theoretical predictions Precision of fit → Luminosity dependence Analysis is ongoing Next Steps Complete the presented analysis Analyze more final states More refined ISR evaluation 39