First limit on inclusive decay and constraints on new physics

Abstract

In a recent article [1] we presented the derivation of the first limit on the inclusive B → X s νν decay rate. Our work stemmed from the observation that a published ALEPH bound on BR(B → τν) [2], inferred from the absence of large missing energy events in B decays, implies also a limit on BR(B → X s νν). We estimated this limit by comparing the missing energy spectra in the two decay modes. Theoretically, B → X s νν is a very clean process, which is also sensitive to several possible sources of new physics [1]. Therefore, a dedicated experimental search is important. The result of such an analysis by the ALEPH Collaboration, using the full LEP-I data sample, is to appear soon [3].As a result of discussions with members of the ALEPH Collaboration [4], we found two errors in our numerical results. The corrected analysis yields a limit weaker than our original result [1] by about a factor of three. The corrections included here are formally beyond the free quark decay result at tree level (of order α s , Λ QCD /m b , and higher). However, they affect the final limit significantly, due to the specific details of the experimental analysis.First, due to a mistake in our Monte Carlo code, the energy of the X s hadronic final state in the B rest frame was evaluated as a fraction of the b quark mass, rather than as a fraction of the B meson mass. This resulted in a harder missing energy spectrum than the true one. Correcting for this weakens the limit by about 35%.Second, we neglected the effects of the invariant mass distribution of the final hadrons, 1 which we (implicitly) assumed not to extend to high mass states. However, the invariant mass of the X s system can be well above 1 GeV [5]. Because of the large boost into the LEP laboratory frame and the high missing energy range (> 35 GeV) used in the analysis[2] (which is large compared to the average B meson energy at LEP), neglecting this effect yielded a missing energy spectrum in the laboratory frame considerably harder than the correct one.It is not straightforward to include properly this second effect into the analysis. However, an estimate can be obtained by approximating the invariant mass spectrum of the X s system with a Gaussian distribution with mean (µ) and variance (σ) fitted to the averages M 2 X and M 4 X , as given in [5]. Such a fit yields µ ≃ 1.35 GeV and σ ≃ 0.6 GeV. There is about a 10% uncertainty in these values, due to their dependences on the heavy quark effective theory parameterΛ (≃ m B − m b ). Treating the X s mass distribution as independent of the energy spectrum, and including a theoretical uncertainty of about 15% related to the fitted values of µ and σ, we find that the original limit given in [1] is weakened by about a factor of three. While small values of E X favor small values of M X (beyond the trivial kinematic constraint E X > M X ), the correlation between E X and M X only slightly improve the bound.In conclusion, the bound on BR(B → X s νν) given in the Abstract and in Eqs. (1.2) and (6.1) of [1] is weaker by about a facto...

Right-handed currents in rare exclusive decays

“…The free-quark prediction for the missing-energy spectrum of the B → X s νν decay can be read off from, e.g., Ref. [5]. While the differential decay rate is directly proportional to ǫ 2 , its dependence upon η is due to interference effects between the LH and RH currents.…”

“…Under the only assumption of purely left-handed neutrinos (possible neutrino mass effects are expected to be negligible [5]) NP effects in the b → sνν transitions can be a modification of the SM value of the coefficient c L and/or the introduction of a new right-handed (RH) operator, viz.…”

“…Ref. [5]). Therefore, a full determination of the coefficients c L and c R requires at least to consider also decay processes other than the inclusive B → X s νν one.…”

Rare decay beyond standard model

“…B → K * νν decay has been extensively investigated in framework of the SM and its various minimal extensions [16,17]. Note that this mode was studied in [18] in a similar way to our analysis, but in that work scalar and tensor type interactions were introduced via violation of the lepton number since neutrino was assumed to be massless.…”

The potential for neutrino physics at muon colliders and dedicated high current muon storage rings

“…The FCNC vertices uusb of Eq. (6.4) will be extensively studied in exclusive and inclusive B-decays B + X,uG at the B-factories as well as at CERN [107]. The inclusive processes such as B + X,UP and u,,N + U,bXB,o have considerable advantage over the corresponding FCNC processes involving charged leptons simply because these processes are mostly dominated by the short-distance physics.…”