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Proposed answeres to physics related issuesFrom: CommentsDear Colleagues, During the Physics meeting on Tuesday, we came to the following conclusions for the referees' questions on physics related issues. Please let me know your comments. yours Tatsuya
Physics related questions >From Andrey Rostovtsev --------------------------- 1) Would it be possible to utilize slow charged hadrons for tagging in the present tracker configuration? answer: Same side flavour tag with slow pions coming from B** or b quark fragmentations is difficult at LHCb; not due to the tracker configuration but due to its forward geometry. Both b and anti-b quark jets are strongly boosted in the forward direction in the pp centre of mass system and they are very close in phase space. Since the boost factor is not a priori known, we cannot boost them back to the quark-antiquark centre of mass system. Therefore, those slow pions cannot be well isolated. Plots which demonstrate this argument will be prepared for the July meeting if necessary. ---------------------------- 2) Kaon tagging. Could you please give the numbers for expected dilution for electrons, muons and kaons separately to see the weight of the Kaons answer With our "optimal cuts" for the tagging particle, we have Muon triggered B-> pi+ pi- events: Muon tag Kaon tag Purity ~0.7 ~0.7 Efficiency 0.4 ~0.2 Hadron triggered B-> pi+ pi- events: Muon tag Kaon tag Purity ~0.7 ~0.7 Efficiency 0.04 ~0.3 The muon tagging efficiency for muon triggered events, ~0.3, can be understood due to the following reasons: 1) Muon trigger Pt threshold is 1 GeV (this also has be sensitive to J/psi->mu mu decays) while for tag, Pt > 1.5 GeV is required to reduce charm background. 2) As seen from the B->mu+X trigger efficiency and background suppression in Fig.12.12 in TP, roughly 2/3 of the muon triggered B->pi pi events are triggered by muons from the underlying events. ------------------------------ 3) K+pi channel. It doesn't seem to be statistically limited if you even require only lepton tagged events. Isn't? B0->K+ pi- and c.c. can be used to study CP violation in the decay amplitude. The large measured branching fraction for B->K pi suggests that the contribution from the penguin diagram is substantial. Thus a CP violation effect of few % could be generated which requires 10**5 events to be observed. Since these decay modes are self-tagging, no additional flavour tag is needed. As seen from Tab.15.12 of TP, 3.3*10**4 B0->K+pi +c.c. can be reconstructed at LHCb, i.e. three years of running. With lepton trigger alone, only 10**4 events can be reconstructed in a year. ------------------------------ 4) D+K* channel. Is it possible here to use the gamma-trigger to exploit D0-kpipi0 channel or pi0 from K*? Gammas from Pi0's, in particular one from D0->K pi pi0, do not have high-Pt. In addition, K*0->Ks pi0 decays are not useful since the flavour of K*0 cannot be used to identify B0 and anti-B0 which is essential in studying CP violation in B0->DK*0 and anti-B0->D anti-K*0 decays. In conclusion, the Gamma trigger is not useful for these decay modes. ------------------------------ 5) Have I forgotten something else, where the hadronic trigger gives a unique opportunity for physics reaches of the experiment? Statistics of the pure hadronic final states used for CP violation studies, such as B-> pi pi, D* pi, Ds p, Ds K, DK* etc., are increased by a factor of two to three depending on the final states and whether it is self-tagging or not. All of those decay modes are necessary to extract angles of the unitarity triangles in unambiguous way.
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