Measurement of Rk Using LHCb Spectrometer Data Analysis

1. Measurement of from LHCbIOP Conference 2013 Nicola Skidmore (University of Southampton / RAL) On behalf of the LHCb collaboration 9th April 2013

2. Contents • Theory of • Current results • LHCbdetector • Data used in analysis • Analysis strategy • Bremsstrahlung radiation • Fitting procedure

3. Theory • and are rare decays with Branching Fractions of order 10-7. • In the SM decays proceed via supressed one-loop processes at lowest order  Electroweak Penguin and Box processes • New Physics from non-SM particles which can enter the loops e.g charged Higgs • Charged Higgs couples to electrons and muons with different strengths making deviate from unity • The ratio of branching fractions in the SM [arXiv:0310219v2] as the SM decay processes do not distinguish between leptons

4. Theory Rkis complementary to the result from LHCb in 2012 [arXiv:1211.2674] Yellow region: SM Rkprediction Green region: Current result for from LHCb MFV→ There are two outcomes from an Rk measurement: Rk compatible with SM 2. Rk is not compatible with the SM or MFV models • This makes an ideal probe of New Physics

5. Results of Rk To Date • Belle and BaBar are the only experiments to report results on Rk to date • Belle and BaBar are limited by statistics • LHCb able to get a more precise result [arXiv:0904.0770v2] [arXiv:1204.3933v2] • No result for the theoretically favoured dilepton invariant mass squared region (q2 ) 1-6 GeV2/c4 region • LHCb has enough events for a q2 =1-6 GeV2/c4 result

6. 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 (RICH1&2)

7. 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

8. Analysis Strategy Rk = = (where εa and εb are the ratios of efficiencies between the control channels and the rare decays and N stands for number) • Double ratio using J/ψ resonance decays (->l+ l-) as high statistics control channels • In the double ratio efficiencies cancel

9. 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/mass6 • 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

10. Bremsstrahlung Radiation • The remaining effects of bremsstrahlung radiation can be clearly seen with the J/ψresonance decays which dominate the data pre-selection • The plots below show the reconstructed B mass against dilepton invariant mass squared (q2) LHCb Unofficial LHCb Unofficial 2011 muon data 2012 electron data

11. Bremsstrahlung Radiation • Partially reconstructed backgrounds are decays where one or more particles are not reconstructed e.gψ X. It is at low B mass • The muon data has a distinct signal and partially reconstructed shoulder • Due to the Bremsstrahlung in the electron data the two regions are merged • The plots below show the reconstructed B mass for the muon and electron data pre-selection LHCb Unofficial LHCb Unofficial Muon line data (dominated by J/ψ resonance) Electron line data (dominated by J/ψ resonance)

12. Selection Process • Pre-selection in the electron and muon data to remove semileptonic B decays • Multivariate selection methods (BDT) created using TMVA to remove combinatorial background for the electron and muon data • Tight Particle Identification (PID) criteria were required on the kaonin both the electron and muon data and a tight PID cut was made on the electrons, taking advantage of LHCb’s PID sub-detectors • Specific backgrounds B->Kππ (->l+ l-) with a double mis-ID (K->l and l->K) These cannot be removed by a veto and will be fitted into the final mass plots

13. Fitting B->Kμμ Fit: B->Kμμ has a clean signal and the partially reconstructed shoulder can be removed by requiring that mK+μ+μ->5200 MeV/c2 Plots LHCb Unofficial (->μ+ μ-) fit with : Double Crystal Ball shape for signalExponential background B->μμK fit with: Double Crystal Ball shape with parameters from (->μ+ μ-) Floating exponential background (Peaking backgrounds not included here-of order 1%) LHCb Unofficial

14. Fitting LHCb Unofficial Fit: • The fit for the is far more complex due to the merging of the signal and partially reconstructed shoulder. Partially reconstructed background ? Signal ? • The shape of the partially reconstructed shoulder for will be modelled by the shoulder of B->Kμμconvolved with the B mass of the (->e+ e-) control channel to model Bremsstrahlung

15. 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 hadronicbkgs

16. Fitting (->μ+ μ-) data has both the Hadronic and J/ψ backgrounds where J/ψ not from B but from higher resonances e.gψ(2S)) LHCb Simulation LHCb Unofficial Fitted MC hadronic and J/ψ background The fit to the B->K J/ψ (->μμ) data Partially reconstructed shoulder from hadronic and J/ψbkgs

17. Fitting • Work on the background shape is ongoing • In preliminary fittings: • The partially reconstructed shoulder and signal shape were modelled by that of the (->e+ e-) control channel • There are ̴170 events LHCb Unofficial Part reco shape from (->e+ e-) Signal shape from (->e+ e-)

18. Summary • Rk is sensitive to New Physics and is complementary to the LHCb B(Bs-> μμ ) result • A selection model has been developed for and fit models are being investigated • Preliminarily ̴170 events, statistically most precise result • Hope is to release result this summer

19. Backup Slides

20. 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) ProbNNk >0.2 cut shown DLLeπ >1.0 cut (same applied for muon chanel) • These cuts remove mostly background retaining high signal efficiency

21. Trigger Lines • Muon line triggers : -BL0DiMuonDecision, BL0HadronDecision_TOS -BHlt1TrackAllL0Decision_TOS, BHlt1TrackMuonDecision_TOS, BHlt1DiMuonLowMassDecision_TOS-BHlt2TopoMu2BodyBBDTDecision_TOS BHlt2DiMuonDetachedDecision_TOS BHlt2TopoMu3BodyBBDTDecision_TOSElectron line triggers :-B_L0HadronDecision_TOS, B_L0ElectronDecision_TOS, B_L0Photon_TIS, B_L0Muon_TIS, B_L0Hadron_TIS ,B_L0Electronon_TIS -B_Hlt1TrackAllL0Decision_TOS, B_Hlt1TrackAllL0Decision_TIS -B_Hlt2Topo2BodyBBDTDecision_TOS, B_Hlt2Topo3BodyBBDTDecision_TOS, B_Hlt2TopoE2BodyBBDTDecision_TOS B_Hlt2TopoE3BodyBBDTDecision_TOS, B_Hlt2Topo2BodyBBDTDecision_TIS, B_Hlt2Topo3BodyBBDTDecision_TIS, B_Hlt2TopoE2BodyBBDTDecision_TIS B_Hlt2TopoE3BodyBBDTDecision_TIS

22. Specific Background calculation • Using the efficiency of selecting the B+->K+J/ψ(->l+l-) (where l= e, μ) decays also found with MC data: (The fits for finding the number of events is shown on slides 15 and 17)

23. Pre-selection Some specific backgrounds peaking in mK+e- mass where the electron has a pion mass were removed prior to selection. • For the electron line data, in plots of mK+e- where the electron has a pion mass, three regions can be seen coming from different specific backgrounds • Clear band at the D͞0 mass and in low B mass due to the semileptonic decay • B+->D͞0(->K+ π-)e+ ν͞e • Region in D͞0 band that is at the B mass due to the decay • B+->D͞0(->K+ π-) π+ • Region that extends down in B mass and mK+e- due to semileptonic cascade decays

24. A cut on all events with mK+e- < 1885 MeV/C2(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->eeK MC data • In the muon line data events with mK+μ-<1885 MeV/C2 where the muon has a pion mass were also removed

25. Specific Backgrounds • B->Kππ Cannot be removed by way of a veto and expected yield for both channels must be fitted into final mass shapes • J/ψdecay with double mis-ID : K->l and l->K -For the muon data : if 3000<mK+μ-<3200 or 3630<mK+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 q2 = 1-6 GeV2/c4

26. For the J/ψ control channel we are able to remove the effect of the bremsstrahlung radiation • The FullFit 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