Nicola Skidmore University of Southampton RAL On behalf
Nicola Skidmore (University of Southampton / RAL) On behalf of the LHCb collaboration 9 th April 2013 1
Contents 2
Theory 3
Theory There are two outcomes from an Rk measurement: 1. Rk compatible with SM 2. Rk is not compatible with the SM or MFV models 4
Results of Rk To Date • Belle and Ba. Bar are the only experiments to report results on Rk to date • Belle and Ba. Bar are limited by statistics • LHCb able to get a more precise result [ar. Xiv: 0904. 0770 v 2] [ar. Xiv: 1204. 3933 v 2] • No result for theoretically favoured dilepton invariant mass squared region (q 2 ) 1 -6 Ge. V 2/c 4 region • LHCb has enough events for a q 2 =1 -6 Ge. V 2/c 4 result 5
LHCb Detector LHCb is one of the four main experiments at the LHC. It is a forward-arm spectrometer with excellent vertex locating (VELO) and particle identification sub-detectors (RICH 1&2) 6
Data • This analysis uses the 1 fb-1 of data collected by LHCb in 2011 and the 2 fb-1 collected in 2012 • This will be the first analysis involving electrons to use the 3 fb-1 7
Analysis Strategy • 8
Bremsstrahlung Radiation • Electrons undergo significant Bremsstrahlung radiation in the detector material, unlike muons, due to their mass being of order 2 smaller • Power lost to Bremsstrahlung radiation is proportional to 1/mass 6 • The electron data has been corrected for Bremsstrahlung radiation, correction looks to recover energy when the radiation takes place before magnet • Bremsstrahlung before the magnet produces photon collinear trajectory and photon cluster in calorimeter can be added to the track to improve momentum/mass resolution • This correction is not perfect 9
Bremsstrahlung Radiation • The remaining effects of bremsstrahlung radiation can be clearly seen with the J/ψ resonance decays which dominate the data pre-selection • The plots below show the reconstructed B mass against dilepton invariant mass squared (q 2) LHCb Unofficial 2011 muon data 2012 electron data 10
Bremsstrahlung Radiation • LHCb Unofficial Muon line data (dominated by J/ψ resonance) LHCb Unofficial Electron line data (dominated by J/ψ resonance) 11
• Selection Process 12
Fitting B->Kμμ Fit: B->Kμμ has a clean signal and the partially reconstructed shoulder can be removed by requiring that m. K+μ+μ->5200 Me. V/c 2 Plots LHCb Unofficial 13
Fitting • LHCb Unofficial Partially reconstructed background ? Signal ? 14
Fitting • To find the B->Kμμ shoulder shape MC samples of Bu->J/ψ(μμ)X Bd->J/ψ(μμ)X Bs->J/ψ(μμ)X Lb->J/ψ(μμ)X have been through the selection process and combined in their relative production rates for LHCb. B->Kμμ data only has hadronic backgrounds where J/ψ is from the B LHCb Unofficial LHCb Simulation Fitted MC hadronic background The fit to the B->Kμμ data Partially reconstructed shoulder from hadronic bkgs 15
• Fitting LHCb Simulation Fitted MC hadronic and J/ψ background LHCb Unofficial The fit to the B->K J/ψ (->μμ) data Partially reconstructed shoulder from hadronic and J/ψ bkgs 16
Fitting LHCb Unofficial 17
Summary • 18
Backup Slides 19
Tighter PID cuts • Some tighter PID requirements were also introduced to further reduce background and were optimised by the same method. For electron line data: (plots with signal and background) Prob. NNk >0. 2 cut shown DLLeπ >1. 0 cut (same applied for muon chanel) • These cuts remove mostly background retaining high signal efficiency 20
Trigger Lines Muon line triggers : -BL 0 Di. Muon. Decision, BL 0 Hadron. Decision_TOS -BHlt 1 Track. All. L 0 Decision_TOS, BHlt 1 Track. Muon. Decision_TOS, BHlt 1 Di. Muon. Low. Mass. Decision_TOS -BHlt 2 Topo. Mu 2 Body. BBDTDecision_TOS BHlt 2 Di. Muon. Detached. Decision_TOS BHlt 2 Topo. Mu 3 Body. BBDTDecision_TOS • Electron line triggers : -B_L 0 Hadron. Decision_TOS, B_L 0 Electron. Decision_TOS, B_L 0 Photon_TIS, B_L 0 Muon_TIS, B_L 0 Hadron_TIS , B_L 0 Electronon_TIS -B_Hlt 1 Track. All. L 0 Decision_TOS, B_Hlt 1 Track. All. L 0 Decision_TIS -B_Hlt 2 Topo 2 Body. BBDTDecision_TOS, B_Hlt 2 Topo 3 Body. BBDTDecision_TOS, B_Hlt 2 Topo. E 2 Body. BBDTDecision_TOS B_Hlt 2 Topo. E 3 Body. BBDTDecision_TOS, B_Hlt 2 Topo 2 Body. BBDTDecision_TIS, B_Hlt 2 Topo 3 Body. BBDTDecision_TIS, B_Hlt 2 Topo. E 2 Body. BBDTDecision_TIS B_Hlt 2 Topo. E 3 Body. BBDTDecision_TIS 21
Specific Background calculation • 22
Pre-selection Some specific backgrounds peaking in m. K+e- mass where the electron has a pion mass were removed prior to selection. For the electron line data, in plots of m. K+e- where the electron has a pion mass, three regions can be seen coming from different specific backgrounds • Clear band at the D 0 mass and in low B mass due to the semileptonic decay B+->D 0(->K+ π -)e+ ν e • Region in D 0 band that is at the B mass due to the decay B+->D 0(->K+ π -) π+ • Region that extends down in B mass and m. K+e- due to semileptonic cascade decays 23
• A cut on all events with m. K+e- < 1885 Me. V/C 2 (where the electron has a pion mass was made). • This removes the peak at D mass and the semileptonic D background • This cut resulted in no significant signal loss as verified in MC data Cut on variable in electron line data Effect on B->ee. K MC data • In the muon line data events with m. K+μ- <1885 Me. V/C 2 where the muon has a pion mass were also removed 24
Specific Backgrounds • B->Kπ π Cannot be removed by way of a veto and expected yield for both channels must be fitted into final mass shapes • J/ψ decay with double mis-ID : K->l and l->K -For the muon data : if 3000<m. K+μ-<3200 or 3630<m. K+μ-<3740 the Kaon must be in the LHCb muon acceptance but not pass a tight muon PID cut -For the electron data an extra PID cut was made on the Kaon to remove ones that were electron like After this cut, using an MC sample of J/ψ events it was found that ~1 J/ψ double mis-ID event was expected in the electron line data in q 2 = 1 -6 Ge. V 2/c 4 25
• For the J/ψ control channel we are able to remove the effect of the bremsstrahlung radiation • The Full. Fit B mass variable constrains me+e- to the J/ψ mass • We can select a relatively clean sample of J/ψ events by taking the peak Electron line data without J/ψ mass constraint on dielectron invariant mass Electron line data with J /ψ mass constraint on dielectron invariant mass 26
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