B→ D (∗) τν decays in the aligned two-Higgs-doublet model
IIFIC/14-65FTUV/14-1007July 2, 2018 B → D ( ∗ ) τ ν decays in the aligned two-Higgs-doublet model Alejandro Celis IFIC, Universitat de Val`encia – CSICApt. Correus 22085, E-46071 Val`encia, SPAIN
In this talk I review the status of B → D ( ∗ ) τ ν decays within theframework of the aligned two-Higgs-doublet model.PRESENTED AT The 8th International Workshop on the CKM UnitarityTriangle (CKM 2014)Vienna, Austria, September 8-12, 2014 Work supported by the Spanish Government and ERDF funds from the EU Commission [GrantsFPA2011-23778 and CSD2007-00042 (Consolider Project CPAN)]. a r X i v : . [ h e p - ph ] O c t Introduction
The BaBar collaboration has reported an excess with respect to the Standard Model(SM) in exclusive semileptonic transitions of the type b → cτ − ν τ [1]. More specifically,they have measured the ratios R ( D ) ≡ Br( B → Dτ − ν τ )Br(B → D (cid:96) − ν (cid:96) ) BaBar = 0 . ± . ± . avg . = 0 . ± . ,R ( D ∗ ) ≡ Br( B → D ∗ τ − ν τ )Br( B → D ∗ (cid:96) − ν (cid:96) ) BaBar = 0 . ± . ± . avg . = 0 . ± . , (1)normalized by the corresponding light lepton modes (cid:96) = e, µ . The second valuesare the averages with previous measurements by the Belle collaboration [2, 3]. TheBaBar measurements show an excess of 2 . σ ( R ( D )) and 2 . σ ( R ( D ∗ )) with respectto the SM [1]. We consider here the possibility that the observed excess in R ( D ( ∗ ) )is due to a charged Higgs contribution entering at tree level. The analysis presentedis done within the framework of the aligned two-Higgs-doublet model (A2HDM) [4],see Refs. [5, 6] for details. Other attempts to explain the excess in these observablesusing different models can be found for example in Refs. [7–17]. B → D ( ∗ ) τ ν decays in the A2HDM The full set of experimental observables considered in our analysis and their respectiveSM predictions is given in Table 1. We only consider processes mediated at tree-levelby the charged Higgs, loop-mediated processes have in general a higher UV sensitivity.It is worth pointing out: • The analysis presented here does not include the latest measurement of Br( B + → τ + ν τ ) with the semileptonic tagging method using the full Belle data sam-ple [18]. • Our SM prediction for R ( D ) agrees with that in Ref. [19]. More recent es-timations of R ( D ) have reduced the discrepancy in this observable to about1 σ [20, 21].The inclusion of these points would not make a qualitative difference in our analysisof the A2HDM since R ( D ∗ ) is the problematic observable at the moment.Charged Higgs interactions with fermions are parametrized in the A2HDM by [4] L Y = − √ v H + (cid:110) u [ ς d V CKM M d P R − ς u M u V CKM P L ] d + ς l νM l P R l (cid:111) + h . c . . (2)1able 1: SM predictions for the various semileptonic and leptonic decays considered in theanalysis, together with their corresponding experimental values. The first uncertainty givenalways corresponds to the statistical error, and the second, when given, to the theoreticalone.
Observable SM Prediction Exp. Value R ( D ) 0 . +0 . − . ± .
015 0 . ± . R ( D ∗ ) 0 . ± . ± .
003 0 . ± . B → τ ν τ ) (0 . +0 . − . ± . × − (1 . ± . × − Br( D s → τ ν τ ) (5 . ± . ± . × − (5 . ± . × − Br( D s → µν ) (5 . ± . ± . × − (5 . ± . × − Br( D → µν ) (4 . +0 . − . ± . × − (3 . ± . × − Γ( K → µν ) / Γ( π → µν ) 1 . ± . ± .
026 1 . ± . τ → Kν τ ) / Γ( τ → πν τ ) (6 . ± . ± . × − (6 . ± . × − Here v (cid:39) ( √ G F ) − / (cid:39)
246 GeV, M u,d,l are the diagonal fermion mass matrices while V CKM is the CKM matrix. Chiral projectors P L,R = (1 ∓ γ ) / ς f ( f = u, d, l ) are independent complexquantities in general. The different versions of the 2HDM with natural flavour conser-vation are recovered in specific limits of the A2HDM [4]. The 95% CL allowed regionsby the different observables are shown in Figure 1. The constraints are shown in thecomplex planes ς d ς ∗ l /M H ± and ς u ς ∗ l /M H ± . We observe that R ( D ∗ ) + B → τ ν pre-fer large and negative values for Re( ς u ς ∗ l ) /M H ± , entering in conflict with constraintsfrom leptonic meson decays. There is no allowed region when all the observables areconsidered, though there is if R ( D ∗ ) is excluded from the fit. To explain the currentexcess in R ( D ∗ ) within the framework of 2HDMs one therefore needs a departurefrom the family universality of the Yukawa couplings, see for example Refs. [7, 9].If the observed excess in R ( D ( ∗ ) ) persists, we would like to gain as much infor-mation as possible about the underlying new physics. Three-body decays like theones at hand offer considerable information in the differential distributions, see forexample Ref. [8]. Interestingly, one can build observables which are not sensitive tocharged scalar contributions. Any deviation from the SM in these observables wouldindicate unequivocally the presence of non scalar new physics. One observable of thiskind is [6] X ( q ) ≡ R D ∗ ( q ) − R ∗ L ( q ) , (3)with R D ( ∗ ) ( q ) = d Γ( B → D ( ∗ ) τ − ν τ ) /dq d Γ( B → D ( ∗ ) (cid:96) − ν (cid:96) ) /dq , R ∗ L ( q ) = d Γ Lτ /dq d Γ L(cid:96) /dq . (4)This observable is not sensitive to charged scalar contributions because a chargedHiggs does not contribute to the transverse helicity amplitudes. Other observables2igure 1: CL allowed regions in the parameter space of the A2HDM by the differentobservables considered. sharing this feature are [6]: X D ( q ) ≡ R D ( q ) (cid:0) A Dλ ( q ) + 1 (cid:1) , X D ∗ ( q ) ≡ R D ∗ ( q ) (cid:0) A D ∗ λ ( q ) + 1 (cid:1) . (5)Here A D ( ∗ ) λ ( q ) represents the τ -spin asymmetry defined in the center-of-mass frameof the leptonic system, A D ( ∗ ) λ ( q ) = d Γ D ( ∗ ) [ λ τ = − / /dq − d Γ D ( ∗ ) [ λ τ = +1 / /dq d Γ D ( ∗ ) [ λ τ = − / /dq + d Γ D ( ∗ ) [ λ τ = +1 / /dq . (6)CP violating observables which are not sensitive to charged scalar contributions havebeen defined in Ref. [22].So far we have discussed constraints coming from flavour processes alone. Directand indirect searches for a charged Higgs at colliders place stringent bounds for a lightcharged Higgs, being complementary to flavour processes. Precision measurements ofthe Z -width at LEP imply a robust lower bound on the charged Higgs mass M H ± > . M H ± (cid:38)
80 GeV was set at LEP from directcharged Higgs searches in the e + e − → H + H − channel, assuming that the chargedHiggs decays dominantly into fermions [23]. LHC searches for a charged Higgs via topdecays t → H + b have been interpreted within the CP-conserving A2HDM in Ref. [24],putting a limit | ς u ς l | /M H ± (cid:46) − GeV − in the mass range M H ± ∈ [90 , The BaBar collaboration has observed hints for lepton universality violations in exclu-sive semileptonic transitions of the type b → cτ − ν τ [1]. The present excess in R ( D ∗ )3an not be accommodated within the A2HDM taking into account leptonic mesondecays in which the charged Higgs also enters at tree level. None of the 2HDMs withnatural flavour conservation can explain the excess in R ( D ∗ ) either, being particularcases of the A2HDM. If the current excess in R ( D ( ∗ ) ) persists in the future, the studyof differential distributions in these processes will play a crucial role in discriminatingbetween different new physics scenarios. ACKNOWLEDGEMENTS
I am grateful to the organizers of the conference for the pleasant atmosphere.
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