Cross section measurement of ee bar at 2
Cross section measurement of e+e- ΛΛbar at 2. 40, 2. 80 and 3. 08 Ge. V Xiaorong Zhou 1
Boss version & data sets • Boss version: 6. 6. 4. p 01 • Data sets – 3. 42 pb-1 @ 2. 40 Ge. V – 3. 75 pb-1 @ 2. 80 Ge. V – 30. 73 pb-1 @ 3. 08 Ge. V • MC samples – e+e- ΛΛbar, Λ pπ-, Λbar pbarπ+ in conexc – exclusive background channels (40 k for each channel at each c. m. e) • • e+e- γ(FSR)ΛΛbar e+e- Σ 0Σ 0 bar e+e- ΛΣ 0 bar e+e- Ξ 0Ξ 0 bar 2
Event selections • Charged track – |Vr|<30 cm, |Vz|<10 cm, |cosθ|<0. 93 – Ngood>=4 • Particle identification – Np=Npbar=Nπ+=Nπ-=1 • Second vertex fitting for pπ- and pbarπ+ • Mass window cut – |MΛ-1. 115|<0. 01 Ge. V & |MΛbar-1. 115|<0. 01 Ge. V • Angle cut between Λ and Λbar candidate – θΛΛbar >170 o at 2. 40 Ge. V – θΛΛbar >176 o at 2. 80 Ge. V – θΛΛbar >178 o at 2. 40 Ge. V 3
Reconstruction of e+e- ΛΛbar at 2. 40 Ge. V • No cuts on the decay length L/Lerr. • Back-to-back angle between ΛΛbar larger than 170 o. 4
Reconstruction of e+e- ΛΛbar at 2. 40 Ge. V • Mass window cut on Λ, Λbar. • No cuts on the momentum of Λ, Λbar. 5
Reconstruction of e+e- ΛΛbar at 2. 40 Ge. V • Ratio of invariant mass of ΛΛbar over c. m. energy, MΛΛ/Ecm. • The number of signal is obtained by counting. 6
Reconstruction of e+e- ΛΛbar at 2. 40 Ge. V 7
Background analysis (non-MΛ peaking bkg) • Signal region • • |MΛ-1. 115|<0. 01 Ge. V & |MΛbar-1. 115|<0. 01 Ge. V Sideband region: • 1. 084<MΛ<1. 104, 1. 105<MΛbar<1. 126 • 1. 105<MΛ<1. 126, 1. 084<MΛbar<1. 104 • 1. 084<MΛ<1. 104, 1. 084<MΛbar<1. 104 8
Background analysis (MΛ peaking bkg) • • The MΛ peaking background may from the baryonic process contain Λ in the final states. For each exclusive background channel, 40 k MC events are generated. The cross section of e+e- γ(FSR)ΛΛbar is about 1/α of e+e- ΛΛbar. The MΛ peaking background can be neglected. Ecm Source 2. 40 Ge. V εMCsel 2. 80 Ge. V 3. 08 Ge. V σ(pb) NMCnor εMCsel σ(pb) NMCnor e+e- γ(FSR)ΛΛbar 1. 6% <1. 3 0. 1 0. 5% <0. 16 0 0. 2% <0. 04 0 e+e- Σ 0Σ 0 bar 30 0 0. 2% 17 0. 1 0. 2% 3. 4 0. 2 0 e+e- ΛΣ 0 bar e+e- Ξ 0Ξ 0 bar Sum 32 - - 2. 9 0. 1 0 - <8. 7 0 0. 1 0 - 0 0. 2 9
Reconstruction of e+e- ΛΛbar at 2. 80 Ge. V • • Back-to-back angle between ΛΛbar larger than 176 o. Mass window cut on Λ, Λbar. 10
Reconstruction of e+e- ΛΛbar at 2. 80 Ge. V • Zero non- MΛ peaking bkg from 2 D MΛ sideband study • Zero MΛ peaking bkg from exclusive background study 11
Reconstruction of e+e- ΛΛbar at 3. 08 Ge. V • • Back-to-back angle between ΛΛbar larger than 176 o. Mass window cut on Λ, Λbar. 12
Reconstruction of e+e- ΛΛbar at 3. 08 Ge. V • Zero non- MΛ peaking bkg from 2 D MΛ sideband study • Zero MΛ peaking bkg from exclusive background study 13
Calculation of cross section Ecm (Ge. V) Nsig Nbkg L (pb-1) efficiency (1+δ) σBorn (pb) 2. 40 46± 7 1 3. 42 23. 08% 0. 99 131. 03± 21. 02 2. 80 8± 3 0 3. 75 10. 63% 3. 07 16. 01± 5. 66 3. 08 13± 4 0 30. 73 7. 85% 3. 15 4. 19± 1. 16 14
Calculation of cross section _ Phokhara Ecm (Ge. V) Nsig Nbkg L (pb-1) efficiency (1+δ) σBorn (pb) 2. 40 46± 7 1 3. 42 9. 62% 0. 96 142. 48± 21. 24 2. 80 8± 3 0 3. 75 7. 96% 1. 70 15. 77± 5. 57 3. 08 13± 4 0 30. 73 4. 23% 3. 26 3. 07± 0. 85 15
Systematic uncertainty Source 2. 40 Ge. V 2. 80 Ge. V 3. 08 Ge. V Reconstruction of Λ 3. 8% Reconstruction of Λbar 3. 4% Mass window cut of Λ 2. 49% Mass window cut of Λbar 2. 96% Open angle cut data: 99. 89% 99. 0% 96. 94% MC model not finished Sum 16
Reconstruction of Λ/Λbar • Control sample J/ψ p. K-Λbar+c. c • Event selection – |Vr|<1 cm, |Vz|<10 cm, |cosθ|<0. 93, Ngood>=2 – PID, Np=NK-=1 – Missing one Λbar, 1 C kinematic fit, χ21 c<20 • Reconstruction of Λbar – |Vr|<10 cm, |Vz|<30 cm, |cosθ|<0. 93 – particle identification – second vertex fit • Purity: 93. 9%, no peaking background • Efficiency: Nreconstructed/Nsample 17
Reconstruction of Λ/Λbar • Comparison of the distributions between data and MC. 18
Reconstruction of Λ/Λbar • • The overall reconstruction efficiency of Λbar is 32. 8% for data and 33. 9 for MC. The overall reconstruction efficiency of Λ is 36. 4% for data and 35. 0 for MC. 19
Uncertainty of mass window cut MΛ Efficiency of mass cut Uncertainty MΛbar MC data 96. 00% 93. 67% 96. 01% 93. 25% 2. 49% 2. 96% 20
Uncertainty of MC model • • • The MC efficiency dependent on the MC model. efficiency 1: angular distribution of Λ set to be (1 -cos 2θ) efficiency 2: angular distribution of Λ set to be (1+cos 2θ) Ecm efficiency 1 efficiency 2 2. 40 Ge. V 21. 333 27. 85 2. 80 Ge. V 10. 06 12. 52 3. 08 Ge. V 7. 37 9. 37 Uncertainty 21
Crosscheck with another method @ 2. 40 Ge. V • • • Np=Nπ-=Npbar=1 Secondvertex fit of p πdecaylength/lengtherror>2 Yields: 54. 0 MC efficiency: 32. 5% Cross section: 54. 0/(3. 42*32. 4*0. 99*0. 639)=120. 58 pb 22
@ 2. 80 Ge. V 23
Crosscheck with Yan Liang’s method @ 2. 40 Ge. V • Np=Nπ-=Npbar=Nπ+=1 • Second vertex • 4 C kinematic fitting Yields: 18 MC efficiency: 6. 36% Cross section: 18. 0/(3. 42*6. 36%*0. 639)=129. 51 pb 24
Summary • The process e+e- ΛΛbar at 2. 40, 2. 80 and 3. 08 Ge. V is studied. The results consistent well with previous measurements. • Systematic uncertainty of e+e- ΛΛbar will be finished soon. • The results will be added in the BAM-00139. 25
2015 -1 -18 26
Reconstruction of e+e- ΛΛbar(γISR) • To retrieve the ISR process, no requirement on the back-toback angle. • To remove the main background from e+e- ΛΛbarγ / ΛΛbarγγ / ΛΛbarπ0 processes, we only select the ISR photon along the beam axis. • If requiring pxymiss<0. 05 Ge. V, which is about 10 o<θmiss<π-10 o, the probability of ISR photon in this region is about 80%. 27
M 2 miss : The mass-squared of the missing particle • The resolution of M 2 miss of process e+e- ΛΛbar is larger than e+e ΛΣ 0 bar at 2. 80 and 3. 08 Ge. V • The resolution of M 2 miss of process e+e- ΛΛbar is small at 2. 40 Ge. V because of the small ISR process. @2. 40 Ge. V red: signal MC blue: e+e- ΛΣ 0 bar @2. 80 Ge. V @3. 08 Ge. V green: e+e- Σ 0Σ 0 bar pink: e+e- Ξ 0Ξ 0 bar 28
M 2 miss versus Nγ • When no ISR photon, the Emiss and Pmiss are both small resolution gaussian with 0 mean value. • When there is ISR photon, the resolution of M 2 miss is large since the transverse momentum of final states is smaller. No ISR photon With ISR photon 29
M 2 miss versus pxy • Check the transverse momentum of proton. e+e- ΛΛbar e+e- ΛΣ 0 bar 30
@ 3. 08 Ge. V • |M 2 miss | < 0. 02 Ge. V 2/c 4 31
Comparison of other Variables • MrecΛ: invariant mass of recoil vector of Λ or Λbar. • MΛΛbar/Ecm: ratio of invariant mass ΛΛbar to c. m energy. • Angle: opening angle between Λ and Λbar. 32
pxymiss & cosθmiss • pxymiss: The transverse momentum of missing particle recoiling against the ΛΛbar system. • cosθmiss: Angular distribution of missing particle recoiling against the ΛΛbar system. If we cut on the cosθmiss, the born process will be eliminated. Therefore, pxymiss<0. 05 Ge. V || pxymiss>0. 2 Ge. V is applied. 33
MrecΛ : invariant mass of recoil vector of Λ or Λbar Σ(1385) ? • |MrecΛ-1. 193|>0. 03 Ge. V • |MrecΛbar-1. 193|>0. 03 Ge. V 34
Summary of the selection • |M 2 miss | < 0. 02 Ge. V 2/c 4. • pxymiss<0. 05 || pxymiss>0. 2 Ge. V. • |MrecΛ-1. 193|>0. 03 Ge. V &|MrecΛbar-1. 193|>0. 03 Ge. V. 35
Background from sideband 36
Results @ 3. 08 Ge. V Selection of ΛΛbar(γISR) Selection of ΛΛbar Nsig 37 13 Nbkg 2 0 Efficiency 20. 6% 7. 85% (1+δ) 3. 15 Born cross section 4. 30± 0. 77 4. 19± 1. 16 Although consistent, they are questionable, due to the pxy miss problem 37
Problems on pxymiss • If we require pxymiss<0. 05 Ge. V without include pxymiss>0. 2 Ge. V. 38
Problems on pxymiss • pxymiss>0. 2 Ge. V ~ 16 o<θmiss<π-16 o • From MC, 18. 2% of all the survived events locates in this region. • But from data, 13 events locates in this region, which is 35. 1% 39
Compare the MC truth between conexc and phokhara 40
phokhara • 24 • 22. 8% 41
phokhara’s problem phokhara conexc 42
Fit on FF • p. QCD prediction: , Λ=0. 3 Ge. V is the QCD scale parameter. A is a free parameter. • A=61. 85± 2. 59. 43
Efficiency difference Phokhara Conexc • This is not due to the selection of good charged track, but the number of all the charged track in collector. 44
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How the lineshape affects the results 46
Method 1 • Angle cut between Λ and Λbar candidate – θΛΛbar >170 o at 2. 40 Ge. V – θΛΛbar >176 o at 2. 80 Ge. V – θΛΛbar >178 o at 3. 08 Ge. V 47
Calculation of cross section_method 1 angular distribution 1+cos 2θ Ecm (Ge. V) 2. 40 2. 80 3. 08 Nsig-Nbkg 45± 7 8± 3 13± 4 L (pb-1) 3. 42 3. 75 30. 73 Line 1 Line 2 Efficiency (%) 19. 17 19. 47 10. 12 22. 11 7. 41 16. 16 1+δ*fvacuum 0. 99 0. 97 3. 07 1. 34 3. 14 1. 48 Efficiency‘ (%) 18. 98 18. 89 31. 07 29. 63 23. 27 23. 92 Difference (%) 0. 48 4. 86 2. 72 48
Calculation of cross section_method 1 angular distribution 1 -cos 2θ Ecm (Ge. V) 2. 40 2. 80 3. 08 Nsig-Nbkg 45± 7 8± 3 13± 4 L (pb-1) 3. 42 3. 75 30. 73 Line 1 Line 2 Efficiency (%) 24. 14 25. 11 12. 46 27. 46 9. 48 20. 30 1+δ *fvaccum 0. 99 0. 97 3. 07 1. 34 3. 14 1. 48 23. 90 24. 36 38. 25 36. 80 29. 77 30. 04 Efficiency‘ (%) Difference (%) 1. 89 3. 94 0. 90 The difference of results for different line-shape is less than 5. 0%! 49
Method 2 • |M 2 miss | < 0. 02 Ge. V 2/c 4. • pxymiss<0. 05 Ge. V/c. • |MrecΛ-1. 193|>0. 03 Ge. V &|MrecΛbar-1. 193|>0. 03 Ge. V. 50
Calculation of cross section_method 2 angular distribution 1+cos 2θ Ecm (Ge. V) 2. 40 2. 80 3. 08 Nsig-Nbkg 42± 8± 24± L (pb-1) 3. 42 3. 75 30. 73 Line 1 Line 2 Efficiency (%) 18. 55 18. 88 18. 02 25. 26 15. 14 23. 29 1+δ*fvacuum 0. 99 0. 97 3. 07 1. 34 3. 14 1. 48 Efficiency ‘(%) 18. 36 18. 31 55. 32 33. 85 47. 54 34. 47 Difference (%) 0. 27 63. 42 37. 92 51
Calculation of cross section_method 2 angular distribution 1 -cos 2θ Ecm (Ge. V) 2. 40 2. 80 3. 08 Nsig-Nbkg 42± 8± 24± L (pb-1) 3. 42 3. 75 30. 73 Line 1 Line 2 Efficiency (%) 23. 43 24. 43 23. 59 32. 24 20. 37 30. 01 1+δ *fvaccum 0. 99 0. 97 3. 07 1. 34 3. 14 1. 48 Efficiency ‘(%) 23. 20 23. 70 72. 42 43. 20 69. 96 44. 41 difference (%) 2. 16 67. 60 Huge difference for different line-shape ! 57. 53 52
Calculation of cross section_method 1 angular distribution 1 -cos 2θ (another line-shape with non-0 threshold cross section) Ecm (Ge. V) 2. 40 2. 80 3. 08 Nsig-Nbkg 45± 7 8± 3 13± 4 L (pb-1) 3. 42 3. 75 30. 73 Line 2 Line 1 Line 2 Efficiency (%) 25. 11 24. 97 27. 46 27. 56 20. 30 20. 35 1+δ *fvaccum 0. 97 Efficiency‘ (%) Difference (%) 24. 36 24. 22 0. 57 1. 34 36. 80 36. 93 1. 48 30. 04 0. 35 The uncertainty of MC efficiency is 0. 09% 30. 12 0. 27 53
Calculation of cross section Conexc Ecm (Ge. V) Nsig Nbkg L (pb-1) efficiency (1+δ) σBorn (pb) 2. 40 42± 3. 42 22. 36% 0. 99 135. 87± 21. 02 2. 80 8± 3. 75 % 3. 07 16. 01± 5. 66 3. 08 24± 30. 73 16. 89% 3. 15 4. 19± 1. 16 Phokhara Ecm (Ge. V) Nsig Nbkg L (pb-1) efficiency (1+δ) σBorn (pb) 2. 40 42± 3. 42 9. 48% 0. 96 131. 03± 21. 02 2. 80 8± 3. 75 % 1. 70 16. 01± 5. 66 3. 08 24± 30. 73 7. 77% 3. 26 4. 19± 1. 16 54
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