B→ D(∗)τν at BaBar and Belle

Elisa Manoni

Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, via A. Pascoli 06123 Perugia (Italy)

Abstract

We will discuss the most recent results on the B → D(∗)τν searches from the BaBar and Belle experiments. Acomparison among the experimental results and with the expectations from theoretical models, which motivates themeasurements, will also be shown.

Keywords: Bottom mesons, Leptonic and semileptonic decays, Supersymmetric Higgs bosons

1. Theoretical Motivations and Experimenta Status

In the Standard Model (SM), the quark level b→ cτνtransition happens through a W-boson exchange (Fig 1).In New Physics (NP) models, which predict the exis-tence of a charged Higgs sector, such as the type II2-Higgs Doublet Model (2HDM-II) [1], the W± maybe replaced by a charged Higgs and the B → D(∗)τνdecay turns out to be sensitive to physics beyond theSM. Since the Higgs-lepton coupling is proportionalto the charged lepton mass, final states with τ’s aremore suitable to detect NP effects with respect to de-cay modes with lighter leptons (� = e, μ). In addition toB → D(∗)τν, there are other B decays which are sen-sitive to the H± contributions, such as B → τν andB → Xsγ. Among them, the processes under discus-sion present several advantages, such as: quite preciseprediction of measurable quantities within the SM (i.e.σ(B)/B � 5 − 7% [1], with B being the branching frac-tion), H± contribution at tree level, quite high decay rate(B � 0.7−1.5% [1]), and experimental constraints fromthe presence of a D(∗) in the final state. Three exper-imental measurements will be discussed in this contri-bution: the BaBar 2012 [2], the Belle 2009 [3] and theBelle 2010 [4] results. In the latter, B(B → D(∗)τν) ismeasured while the first two report results on the ratioR(D(∗)) defined as:

R(D(∗)) ≡ B(B→ D(∗)τν)B(B→ D(∗)�ν)

=ND(∗)τν

ND(∗)�ν· εD(∗)�ν

εD(∗)τν, (1)

Figure 1: B→ D(∗)τν quark level diagram.

with N being the yield of a given final state and ε theestimated efficiency. If one considers only leptonic τdecays, the signal mode (B → D(∗)τν) and the normal-ization one (B → D(∗)�ν) are reconstructed by meansof the same final state particles. In this way, some ofthe experimental uncertainties cancel in the ratio. Thesame happens for theoretical uncertainties on quanti-ties entering the two B in Eq. 1, such as Vcb and theform factors. Such cancellations, facilitate the compar-ison between theoretical predictions and experimentalresults in order to confirm the SM framework or findhints of NP. Reference [2] quotes the SM expectationsfor the ratios defined in Eq. 1: R(D) = 0.297 ± 0.017and R(D∗) = 0.252±0.003. In the 2HDM-II framework,studies on the dependence of R(D(∗)) on the ratio of tan βand mH ( with β and mH being the ratio of the vacuumexpectation values and the mass of the charged Higgs,respectively) have also been made, as documented in

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Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 111–114

0920-5632/© 2014 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/npbps

http://dx.doi.org/10.1016/j.nuclphysbps.2014.09.027

Figure 2: Experimental results for R(D(∗)) from BaBar and Belle: the SM prediction as in Ref. [2] and the experimental average, prior to the 2012BaBar measurements, are reported, along with the Belle 2007 [6], BaBar 2008 [7], Belle 2009 [3], Belle 2010 [4], and BaBar 2012 [2] results.

Ref. [5].Before the measurements discussed in this contribu-

tion, B → D(∗)τν studies were published by Belle in2007 [6] and by BaBar in 2008 [7]. Those are shown inFig. 1 and are in agreement between them and with theSM prediction at 1 − 2 σ level.

2. Belle and BaBar most recent results

At the B-factory experiments, pairs of BB̄ mesonswithout any other accompanying particle are produced.An event of this kind can be divided into tag side andsignal side. The latter is the one containing the decay ofa B to the final state of interest (Bsig). Since in the analy-ses under discussion a τ decaying to lepton final states isreconstructed in the signal side, three undetectable neu-trinos are produced. In order to suppress contaminationfrom mis-reconstructed events, kinematic constraints inthe tag side B (Btag) are applied.

In the Belle 2010 analysis, an inclusive method isused: after having identified the Bsig signature, all theremaining tracks and neutrals are associated with theBtag and are required to be consistent with comingfrom a B meson decay. In the Belle 2009 and BaBar2012 studies, an exclusive reconstruction is performed:hadronic Btag final states are searched for and the com-patibility of the remaining particles with a B → D(∗)τνdecay is checked. Comparing the two methods, thefirst has a higher efficiency, the second a higher pu-rity. In all these analysis, the main contamination comesfrom mis-reconstructed semileptonic decays with lightleptons and excited states of charmed mesons, such asB→ D∗∗�ν.

2.1. Belle measurementsIn 2009, Belle presented a measurement of R(D∗) [3]

using a sample of 657×106 BB̄ pairs. An exclusive Btag

reconstruction method is used: B+tag (B0tag) candidates

are reconstructed with an efficiency of 0.15% (0.09%)and a purity of 58% (51%). In the signal side, τ’s arereconstructed in leptonic final states. A selection basedon kinematics and event shape variables is applied tothe sample of reconstructed Btag − Bsig pairs. Selec-tion criteria are chosen in order to maximize the ra-tio between the number of expected signal events andthe square root of the total number of expected events.Among the variables used at this stage, the most relevantare: the missing mass squared, the lepton momentum inthe center-of-mass frame, and Eextra, the energy mea-sured in the electromagnetic calorimeter not assignedto the Btag nor to the Bsig. Events surviving this stepof the analysis are used to extract the signal yield bymeans of a two-dimensional unbinned maximum likeli-hood fit to the missing mass squared and the Eextra dis-tributions. The yield of B → D(∗)�ν events is estimatedusing a D(∗)�ν-enriched sample. In Tab. 1 the mea-sured R(D(∗)) values and the significance of the signalare reported. It can be noticed that the measurementsare statistically limited and in agreement with the SMprediction at 2 σ level.

R Σ(Σstat)B0 → D−τ+ν 0.476+0.216+0.063

−0.193−0.054 2.6(2.8)B+ → D̄0τ+ν 0.702+0.189+0.110

−0.180−0.091 3.8(4.4)B0 → D∗−τ+ν 0.481+0.140+0.058

−0.123−0.041 4.7(5.9)B+ → D̄∗0τ+ν 0.468+0.106+0.062

−0.102−0.072 3.9(5.2)

Table 1: Summary of the 2009 Belle measurement: the R(D(∗)) ratiosand the significance Σ of the signal, incorporating statistical and sys-tematic effects (and statistical error only). In the ratios, the first erroris statistical and the second systematic.

In 2010, Belle presented a measurement [4] of theB+ → D̄(∗)0τ+ν branching fraction using a sample of

E. Manoni / Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 111–114112

657 × 106 BB̄ pairs. For the event reconstruction, theinclusive method is adopted. Other than leptonic fi-nal states, also τ → πν are reconstructed in the signalside. Moreover, in order to reduce the number of mis-reconstructed events, only the cleanest D final states areused in the signal side and a tighter selection, with re-spect to the 2009 analysis, is applied. The signal yieldis extracted by a two-dimensional unbinned maximumlikelihood fit to the Btag mass and the momentum of theD produced in the B → D(∗)τν decay. Belle publishedthe branching fraction, shown in Tab. 2, and reportedthe first evidence of the B+ → D̄0τ+ν channel. Using theB → D(∗)�ν branching fractions, R(D(∗)) can be com-puted: R(D) = 0.35± 0.11 and R(D∗) = 0.43± 0.08. Ascan be noticed from Fig. 1, these are in agreement withthe SM predictions at 1 σ level.

B(%) Σ

B+ → D̄0τ+ν 0.77 ± 0.22 ± 0.12 3.6B+ → D̄∗0τ+ν 2.12+0.28

−0.27 ± 0.29 8.8

Table 2: Summary of the 2010 Belle measurement: the branchingfractions and the significance Σ of the signal, incorporating statisticaland systematic effects. In the branching fractions, the first error isstatistical and the second systematic.

2.2. BaBar measurementsThe most recent and precise measurements on R(D∗)

have been published in 2012 by BaBar [2]. The fulldata sample, consisting of 471 × 106 BB̄ pairs, hasbeen analyzed. The exclusive reconstruction method isused: 1680 decay chains are considered to identify theBtag and this results in a purity and a reconstruction effi-ciency, in the Btag kinematic signal region, of 0.40% and75%. This represents a big improvement with respect tothe previous BaBar hadronic reconstruction algorithm,with a reconstruction efficiency that is doubled with re-spect to the past. It can be noticed that the performancesof the BaBar hadronic reconstruction method are bet-ter then the Belle ones, although Belle is in progress ofpublishing results with an improved algorithm. Once aBtag is reconstructed, in its recoil a charmed meson anda light lepton are selected to identify a Bsig candidate.Thanks to an improved particle identification method,low momentum electrons and muons are used. No ex-tra tracks in the event are allowed and selection criteriaon the momentum transfer and on the missing momen-tum are applied in order to reduce the contaminationfrom combinatorial mis-reconstructed and B → D(∗)�νcandidates. To further suppress non-signal event, themost discriminant kinematic and event shape variables

are combined in a Boosted Decision Tree. At this stageof the selection, the sample in the squared missing masssignal region (> 1 GeV2/c4) has the following compo-sition: 39% of Bsig → D(∗)τν, 10% of Bsig → D(∗)�ν,19% of e+e− → qq̄, 13% of Bsig → D∗∗�ν, and19% of other BB̄ events. The next step of the anal-ysis is a two-dimensional unbinned maximum likeli-hood fit to the distributions of the missing mass squaredand the lepton momentum in the center-of-mass frame.Eight samples are simultaneously fitted: 4 D(∗)� sam-ples (which account for the signal and the normaliza-tion channels) and 4 D(∗)�π0 samples (which serve todescribe D∗∗�ν events). Several yields are free param-eters in the fit, among which the signal and the nor-malization, while others are fixed from Monte Carlo(MC) studies (charge cross-feed, BB̄ combinatorial andqq̄(γ) contamination). Fifty-six fully two dimensionalprobability density functions, approximated using non-parametric kernel estimators, are used in the fit. Theirparameterization is derived from MC samples and keptfixed in the final fit. With the fitted yield and the se-lection efficiency estimated from MC, the R(D∗) ratiocan be computed. Other than statistical error, also sys-tematic effects are accounted for: the largest uncertaintyon the ratio related to the yield estimation is due to theknowledge of D∗∗�ν and other backgrounds. The mainsystematic error on R(D∗) related to the efficiency is dueto the limited MC size, which is anyway small with re-spect to background-related errors. Fit stability has beenchecked by toy MC tests while result robustness is ver-ified by fitting subsamples according to data taking pe-riod and by varying selection criteria. The results of theanalysis are listed in Tab. and discussed in the nextsection. It can be noticed that the first 5σ observationfor the B→ Dτν channel is reported by this BaBar mea-surement.

R Σ(Σstat)B0 → D−τ+ν 0.469 ± 0.084 ± 0.053 5.2(6.1)B+ → D̄0τ+ν 0.429 ± 0.082 ± 0.052 4.7(5.5)B0 → D∗−τ+ν 0.355 ± 0.039 ± 0.021 10.4(11.6)B+ → D̄∗0τ+ν 0.322 ± 0.032 ± 0.022 9.4(11.3)B→ D̄τν 0.440 ± 0.058 ± 0.042 6.8(8.4)B→ D̄∗τν 0.332 ± 0.024 ± 0.018 13.2(16.4)

Table 3: Summary of the 2012 BaBar measurement: the R(D(∗)) ra-tios and the significance Σ of the signal, incorporating statistical andsystematic effects (and statistical error only). In the ratios, the firsterror is statistical and the second systematic. In the last two rows, theresults from an isospin-constrained fit are reported.

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3. Discussion of the results

In Fig. 1, a comparison between all the experimentalresults collected so far is presented. The gray band, rep-resenting the average of the measurements, does not in-corporate the latest BaBar results, which has improvedthe R(D(∗)) precision. The experiments are consistentamong them.

The BaBar measurements have been compared to thetheoretical predictions from the SM and the 2HDM-II.In Fig. 3 the experimental result and the SM predictionare shown as a white and black cross, respectively, in theR(D)-R(D∗) plane. Accounting for the R(D) and R(D∗)correlation, the experiment deviates from the theory by3.4 σ.

The effect of the charged Higgs boson presence, inthe 2HDM-II framework, has been also evaluated bythe BaBar collaboration. This is done by reweight-ing the simulated events at the matrix element level,considering several values of the tan β/mH+ ratio. Re-peating the fit with updated probability density func-tions and signal efficiencies and correctly accountingfor statistical and systematic uncertainties, a scan ofthe R(D(∗)) values as a function of tan β/mH+ is per-formed and the results are shown by the blue band inFig. 4. Those are compared with the prediction in the2HDM-II represented by the red band in the same plot.It can be noticed that the BaBar results and the theo-retical predictions overlaps at two different tan β/mH+

values for R(D) (tan β/mH+ = 0.44 ± 0.22) and R(D∗)(tan β/mH+ = 0.75 ± 0.04). As a result, the combina-tion of the R(D) and R(D∗) measurements exclude the2HDM-II at 99.8% confidence level. The only assump-tion made in the scan is mH+ > 10 GeV , since the lowermass region was already excluded by B → Xsγ mea-surements [8].

Figure 3: Comparison between the SM predictions and the BaBar2012 measurements for R(D(∗)).

Figure 4: Comparison between the BaBar measurements (blue) andthe 2HDM-II predictions (red) as a function of t = tan β/mH+ .

4. Conclusions

The potential of NP discovery in the B → D(∗)τνsearches has been discussed, in particular for modelspredicting the existence of a charged Higgs sector. BothBaBar and Belle have presented searches for these chan-nels, measuring the branching fractions and the ratiosR(D(∗)) and their results are in good agreement. BaBarhas recently presented the most precise measurementswith 16% and 10% uncertainties on R(D) and R(D∗), re-spectively, which are dominated by statistics. BaBar re-sults deviate from the SM expectations at 3.4σ level andthe 2HDM-II cannot accommodate these experimentalresults. As a result, this NP model is excluded at 99.8% confidence level. BaBar and Belle will update theirresults: both of them will consider additional τ decaymodes and Belle will update on the full statistics.

References

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