The upper limit of the signal branching fraction is computed by Monte Carlo technique as described in Section4.4.3. The scan has been performed in the signal strength parameterµwith a step width of 0.1. For both the null and alternative hypothesis, 10 000 pseudo experiments have been evaluated at each scan step. We obtain an upper limit ofB B0→πτ ν<2.5×10−4 at the 90% confidence level and B B0 →πτ ν<2.8×10−4 at the 95% confidence level.
The search for the semileptonic decayB0→πτ ν at the Belle experiment has been presented in this thesis. The analysis and the results will be summarized shortly in the first section, followed by an outlook regarding further measurements of the decay.
7.1. Summary
This thesis presents the first published search for the decay B0 → πτ ν [123]. It has been performed on the huge Belle data sample, containing 772×106 BB pairs. The lifetime of theτ lepton is short enough for it to decay inside of the detector, such that it has to be reconstructed from its decay products. In this analysis, the τ lepton is reconstructed in its decays with one charged particle in the final state, namely τ+ → e+νeν¯τ, τ+ → µ+νµν¯τ, τ+ → π+ν¯τ, and τ+→ρ+ν¯τ. The final state contains 2-3 neutrinos, which makes it impossible to fully reconstruct theτ lepton or theBsigmeson. This analysis exploited the fact that the initial state of a collision at Belle is exactly known and defined by the sum four-momenta of the colliding leptons, as is the four-momentum of the final state. A complex, multivariate full reconstruction algorithm, based on NeuroBayes, is used to reconstruct one of the two B mesons in an event, the Btag. With the knowledge of all tracks that belong to the Btag, signal candidates can be selected. A very important event variable is the deposited energy in the ECL, which is not assigned to neither the reconstructedBtag, nor the two charged particles from theBsig. For the separation between signal and background, boosted decision trees are used, where one BDT classifier is trained for each τ reconstruction channel. While the full hadronic reconstruction allows to perform this analysis, it is also the dominant, limiting factor on the reconstruction efficiency.
While the decay B0 → π−τ+ντ has been proposed by theorists as a test of the SM for a long time, as has been shown in Section2.3, no analysis has been able to obtain a measurement, yet.
This first search for the decay has reached a significance of 2.4σ and has finally shown, that the decay is experimentally accessible. The analysis requires advanced reconstruction techniques, at the price of a very small reconstruction efficiency. While the systematic uncertainty is 8.3%, the statistical uncertainty on the branching fraction is roughly 47%. The numerical results of the analysis are summarized in Table7.1. More data or a combination of multiple analyses will be needed to obtain evidence for the decay, which will be the topic of Section7.2.
Number of signal events: 51.9±24.3 (stat. only)
Significance level 2.4σ
B B0 →πτ ν <2.5×10−4 @ 90% CL
<2.8×10−4 @ 95% CL Table 7.1.: Summary of the results of this analysis.
The results are well compatible with the SM prediction of B B0 →πτ ν= (0.935±0.038)× 10−4 [69]. First exclusions on tanβ and m+H in the MSSM, based on this result, have been published in Ref. [124] and are shown in Figure 7.1. The exclusion limits are to be taken with a word of warning, though. They are obtained using the branching fraction obtained in this analysis,B B0→πτ ν= (1.52±0.75)×10−4, to computeR(π) defined in Equation (2.31). As no evidence is obtained, the branching fraction result of this analysis is stated with the statistical uncertainty, only. Furthermore, the exclusion limits are obtained under the assumption, that the reconstruction efficiency and overall acceptance are the same for MSSM kinematics.
The obtained limit is lower than some of the values presented in Section2.3.2, which might result in the exclusion of certain parameter space regions of certain NP scenarios. Several input factors have been used in those calculations, though, and no explicit dependence of B B0→πτ ν on the input parameters are published. No constraints on other NP models are therefore published in this thesis.
(a) (b)
Figure 7.1.: Exclusion of MSSM parameter points under the assumption that the efficiencies and acceptances of the reconstruction are the same for MSSM scenarios. (a) The evolution of R(π) as a function of tanβ/m+H is shown in dark gray, the experi-mental 68% CL of R(π) is shown in light gray. (b) Excluded MSSM parameter points in the tanβ−m+H plane at the 68% CL (light gray) and 95% (dark gray).
Taken from [124]
7.2. Outlook
While this analysis was not able to obtain evidence for the decay on the full Belle data sample, it still shows that evidence is to be expected in the future. First of all, in the recent past, an al-gorithm for the semileptonic tagging has been developed at Belle. TheBtagis reconstructed into the semileptonic decays B →D(∗)`ν`, with light leptons ` [125]. The efficiency of this method is roughly two to three times higher than for the hadronic full reconstruction. A disadvantage is, however, the additional neutrino on the tag side, which means that the four-momentum of neither the tag, nor the signal side, can be determined. The search for B0 → πτ ν using the semileptonic tag has been started in Belle by two Master theses, one in Göttingen by Harrison Schreeck [126], and one in Bonn, by Stephan Duell. Due to time constraints, the analyses have been performed on simulated data, only, and no evidence but a similar significance level is
ex-pected. As different decay channels are used to reconstruct the tag side in both analyses, the events of both analyses can be expected to be statistically independent. Therefore, evidence and a measurement of the branching fraction could be obtained in a combination of both analyses.
Furthermore, Belle and BaBar recently published their first joint analysis, measuring the time-dependent CP violation in B0 → D(∗)CPh0 [127]. Combining the Belle and BaBar dataset for evidence of B0→πτ ν might be another option.
While LHCb is also specialized inB decays, it is not expected to be able to measure the decay B0 →πτ ν, though, as theBsig can not be constrained by fully reconstructing theBtag.
The upgrade of the Belle experiment, Belle II, will be able to obtain evidence for the decay, and shortly after, observe the decay. The Belle II experiment is scheduled to start taking data in 2018, and to collect a total integrated luminosity on the Υ(4S) resonance of 50 ab−1, roughly 70 times as much data as this analysis was performed on. Further improvements of the detector and tracking algorithms are expected to increase the tracking efficency. This would improve all full reconstruction methods, and of course, also the reconstruction of the signal decay.