Semileptonic Branching Ratio Research Articles

The baffling semileptonic branching ratio of B mesons

The apparent gap between the measured and the expected value for the semileptonic branching ratio of B mesons has become more serious of r the last year. This is due to the improved quality of the data and to the increasing maturity of the theoretical treatment of nonperturbative corrections. We discuss various theoretical options to reduce the semileptonic B branching ratio; among the more spectacular resolutions of the apparent puzzle is the possibility of an unorthodox enhancement in nonperturbative corrections or even of an intervention by “New Physics”. Phenomenological implications of such scenarios are pointed out.

Extracting | Vbc|, mc and mb from inclusive D and B decays

Using recent results for nonperturbative contributions to the B and D meson inclusive semileptonic widths, a model independent extraction of | V bc |, m b and m b is made from the experimentally measured B and D lifetimes and semileptonic branching ratios. Constraining the parameters of HQET at O(1/m 2 Q) by the D semileptonic width, | V bc | is found to in the range 0.040 < | V bc | < 0.057. The c and b quark masses are not well constrained due to uncertainty in the relevant scale of α s. These results assume the validity of perturbative QCD at the low scales relevant to semileptonic charm decay. Without making this assumption, somewhat less stringent bounds on V bc from B decay be obtained.

Measurements of $$B^0 - \bar B^0 $$ mixing, $${{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern-\nulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}$$ and semileptonic branching ratios forb-flavoured hadrons in hadronicZ 0 decays

From a sample of about 450 000 hadronicZ 0 decays, measurements of the average\(B^0 - \bar B^0 \) mixing parameter, χ, the branching fraction ofZ 0 bosons into hadrons containing bottom quarks,\({{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern-\nulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}\), the average semileptonic branching ratios for such hadrons,B(b→l) andB(b→c→l), and the mean scaled energy of these hadrons, 〈xE〉, are presented. The measurements were obtained using a simultaneous fit to single-lepton and dilepton events collected with the OPAL detector at LEP, including both electrons and muons. The results are where the errors are statistical and systematic, respectively, in each case. The result forB)b→c→l) excludes decays of the typeb→→ℓ.

A study of B 0− [formula omitted]0 mixing using semileptonic decays of B hadrons produced from Z0

The B 0− B 0 mixing was studied by using about 250 000 hadronic decays of the Z 0, collected with the DELPHI detector at LEP. With 1665 dilepton events, the probability for a b quark to become a b before decaying was found to be χ = 0.121 −0.040 +0.044 ± 0.017. The semileptonic branching ratio of the b was measured from the dilepton and single lepton events and found to be Br(b → l) = (10.0 ± 0.7 ± 0.7)%.

Nonperturbative corrections to inclusive beauty and charm decays. QCD versus phenomenological models

We present a self-consistent method for treating nonperturbative effects in inclusive nonleptonic and semileptonic decays of heavy flavour hadrons. These effects give rise to powerlike corrections ∝ 1 m Q n with n ⩾ 2. The leading correction to the semileptonic branching ratio occurs for n = 2. It is expressed in terms of the vector-pseudoscalar mass splitting: δ BR sl BR sl ≅ BR nl · 6 [ (M v 2−M p 2) m Q 2 ](c + 2−c − 2) 2N c and yields a reduction of BR sl. This nonperturbative correction contributes to the nonleptonic width with a sign opposite to that of the perturbative terms that are non-leading in 1 N c . In beauty decays the former reduces the latter by 20% whereas in charm decays they more or less cancel. This leads to a reduction of BR sl by no more than 10% in beauty decays and by a factor of roughly two in charm decays. Phenomenological models of heavy flavour decays are unable to mimic these leading corrections by a specific choice of quark masses or by invoking Fermi motion.

Measurements of Z 0 → heavy flavours at LEP

The LEP experiments have studied the decays of the Z 0 to heavy quarks by exploiting their semi-leptonic decays, characteristic decay topology and lifetime, and by identifying inclusive and exclusive production of heavy flavoured hadrons. The emphasis here is placed on the measurement of the fraction of hadronic decays containing b or c quark decays to leptons, for which the LEP average values are: Γ b b Γ had × B(b→1)=0.0239±0.0007±0.0007 Γ c c Γ had × B(c→1)=0.0161±0.0015±0.0014 where the first error is statistical and the second systematic. The average partial widths derived from these measurements are: Γ b b =355±9±10±18( BR)MeV Γ c c =292±29±18( BR)MeV where systematic errors are dominated by the uncertainty in the semi-leptonic branching ratio. These results are in good agreement with the standard model expectations Γ b b = 378 MeV and Γ c c = 297 MeV , but the experimental accuracy is at present insufficient to confront detailed predictions such as the variation with top quark mass.

The semileptonic decay rates and fragmentation parameters of b and c states

From a fit to the p, p ⊥ distributions of single electron and muon candidates in a sample of 127 083 hadronic Z decays, collected using the ALEPH detector at L ep, measurements of the average semileptonic branching ratios B(b→ l) and B(c→ l) are obtained. In the fit Standard Model values are assumed for the branching ratios Z→b b/ZrmZ→q q=0.217 and Z→c c/ Z→q q=0.171 . In the fit the heavy quarks are assumed fragment according to the model of Peterson et al.. Within the framework of this model the mean values X = E hadron / E beam are obtained for both b and c states.

Measurements of the semileptonic branching ratio Br( b→ lvX), the lifetime of B-Hadrons and a determination of | Veb|

We present a study of the decay properties of Z 0→b b using inclusive muon and electron events. From a fit to the single and dilepton p and p ⊥ spectra, we determine Br( b→ℓ νX) = 0.119 ± 0.003( stat) ± 0.006( sys), assuming the Standard Model prediction of Γ b b = 378±3 MeV . From the ratio of the number of dipleton events to single lepton events, we find Br( b→ℓ νX) = 0.113 ± 0.010( stat) ± 0.006( sys), without assumptions on Γ b b . From a fit to the impact parameter distribution of lepton tracks from semileptonic b decay, the lifetime of B-hadrons is measured to be 1.32±0.08(stat)±0.09(sys) ps. Using these measurements, the CKM matrix element | V cb | is determined to be 0.046±0.002 (exp) −0.003 +0.004(theory).

Right-handed current effect on inclusive decays of theB meson

Recent experimental values for the inclusive semileptonic decay branching ratio of theB meson are low compared to the spectator model expectation. That branching ratio is investigated in the right-handed current model. The right-handed current could enhance the nonleptonic decay mode, thereby suppressing the semileptonic branching ratio. Then, the quark mixing matrix of the right-handed sector is restricted to be of one special type and the right-handed neutrino has to be the heavy Dirac one.

A measurement of the inclusive semileptonic decay fraction of charmed hadrons

Using the ARGUS detector at the DORIS-II storage ring at DESY, we have measured the inclusive semileptonic branching ratio of charmed hadrons produced in e +e − annihilition at =10 GeV to be BR(c→ev eX)=(9.8±0.9± 0.5 0.6)% and BR(c→ μv μ X)=(8.6±1.7± 0.7 0.8)%.

Solutions to the B meson semileptonic branching ratio puzzle within two-Higgs-doublet models

Recent experimental values for BR( B→ℓ ν ℓ X) seem low compared to spectator model expectations. Conventional explanations have to be stretched considerably to accommodate this, pointing towards the possibility of new physics explanations. In the two-Higgs-doublet models, the b→ cτ ν τ mode or the b→sg mode could get enhanced to the (10–20)% level, thereby suppressing BR( B→ℓ ν ℓ X) to the experimental value. The experimental consequences of these scenarios are discussed.

Measurements of [formula omitted] decays and the semileptonic branching ratio BR (b → ℓ + X)

We have measured the decay properties of Z 0 → b b from a study of inclusive muon and electron events. The average (e and μ) b-quark semileptonic branching ratio has been determined to be BR (b → ℓ + X) = 0.119 ± 0.003 (stat.) ± 0.006 (syst.), assuming the standard model prediction of Γ bb = 378 ± 3 MeV. From the ratio of the number of dilepton events to single lepton events, we find BR (b → ℓ + X) = 0.113 ± 0.010 (stat.) ± 0.006 (syst.), without assumptions on Λ bb. The partial decay width of the Z 0 into b b has been measured to be Λ bb = 385 ± 7 (stat.) ± 11 (syst.) MeV with an additional 19 MeV error from the uncertainty in BR (b → ℓ + X). The average fractional energy of bottom hadrons in Z 0 → b b events has been determined to be 〈 x E 〉 = 0.686 ± 0.006 (stat.) ± 0.016 (syst.).

Measurement of the relative branching fraction Gamma (D0-->K micro nu )/ Gamma (D0--> microX).

The fraction f of D0 semimuonic decays which occur through the Kμν mode has been measured in a hybrid emulsion spectrometer. Analysis of 124 semimuonic D0-decay candidates gives f=0.32±0.05 (stat) ±0.05 (syst). From this measurement and existing data on the D0 semileptonic branching ratio and lifetime, we obtain the branching ratio R(D0→K−μ+ν)=(2.4±0.4±0.5)% and partial decay rate Γ(D0→K−μ+ν)=(5.6±0.9±1.2)×1010 s−1.Received 14 January 1991DOI:https://doi.org/10.1103/PhysRevLett.66.1819©1991 American Physical Society

Investigation of quark-hadron duality in semileptonicB-meson decays

This paper is devoted to the analytical calculation of the lepton spectrum with a restriction on the mass of produced hadron system in semileptonicB-meson decays. Using this analytical dependence, it is shown that the quark-hadron duality in semileptonicB-meson decays could be violated up to 25–30%, and this fact has influence on the semileptonic branching ratio.

Measurement of heavy quark charge asymmetries at 35 and 43 GeV and heavy flavour physics with CELLO

Production and decays of heavy quarks in e + e − annihilations have been investigated with the CELLO detector in the range from 35 GeV up to the highest PETRA energies. The semileptonic branching ratios and the charge asymmetries of b and c quarks at 35 and 43 GeV have been simultaneously measured using, in addition to the lepton transverse momentum, correlated jet variables for quark flavour separation. We determined the charge asymmetries to be A FB b = −(22.2 ± 8.1)%, A FB c = −(12.9 ± 8.8)% at 35 GeV and A FB b = −(49.1 ± 16.5)% A FB c = +(7.7 ± 14.0)% at 43 GeV, respectively. The combined measurements result in values for the axial vector couplings of the heavy quarks of a c = +(0.28 ± 0.45) and a b = −(1.26 ± 0.38) taking B 0 B 0 mixing into account. We also measured the b b and c c production cross section at both energies and give an upper limit for the B 0 meson mixing parameter of χl < 0.16 (90% C.L.) and lower limits in the 1–2 TeV range for the scale of a possible common electron-heavy quark compositeness from the measured asymmetries.

Recent ARGUS results on B meson decays

Recent ARGUS results on B meson decays are briefly presented: Masses, semileptonic branching ratios to D ∗− and D −, lifetime ratio of B 0 and B +, update of B 0 mixing and upper limits on b→ s decays.

Heavy flavour production in Z decays

From an analysis of inclusive leptons in data collected by the ALEPH detector at LEP, we measure the fractions of b b and c c events in hadronic Z decays. The b b fraction times semileptonic branching ratio is measured to be Br( b → e)·Γ b b /Γ had = 0.0224 ± 0.0016 ± 0.0010 . Assuming a b semileptonic branching ratio of 0.102 ± 0.010 gives Γ b b /Γ had = 0.220 ± 0.016 ± 0.024 , in good agreement with the standard model prediction of 0.217. The c c fraction times semileptonic branching ratio is measured to be Br( c → e)·Γ c c /Γ had = 0.0133 ± 0.0040 −0.0031 +0.0038 . Assuming a c semileptonic branching ratio of 0.090 ± 0.013 gives Γ c c /Γ had = 0.148 ± 0.044 −0.038 +0.045 , in agreement with the standard model prediction of 0.171.

Remarks on non [formula omitted] decays of the ϒ(4S)

We consider the recent experimental observation of non B B decays of ϒ(4S) into final states containing J/ ψ mesons in the context of a model which assumes an admixture of gluons and/or light quark pairs in the ϒ(4S). We emphasize the necessity of a normalization-independent measurement of the B meson semileptonic branching ratio as a means of finding the total fraction of the non B B decays of ϒ(4S). We also propose a search for an enchancement in the spectrum of inclusive photons from ϒ(4S) in the energy range 4–5 GeV at a level of a few tenths of a percent of the total width as an indicator of the presence of a colored b b heavy quark pair in the ϒ(4S).

ANALYSIS OF THE DATA IN HEAVY FLAVOUR DECAYS

The charmed and bottom hadron decays have been analyzed phenomenologically, keeping both the spectator and nonspectator contributions to the total decay rates. A sum rule relating the total decay widths of the four low-lying charmed baryons is given. It is pointed out that all the data which have become recently available can be understood in all cases with negligible Pauli interference effects but with important exchange- mechanism contributions. Predictions for the lifetimes of [Formula: see text] and [Formula: see text] together with the expected lifetime hierarchy for the bottom hadrons are given. The corresponding semileptonic branching ratios, where possible, are also predicted.

Heavy quark decay

Exclusive D and B meson semileptonic branching ratio measurements are summarized and compared with models. In the D meson sector study of the ratio of rates of Kℓ +ν K ∗ℓ +ν and the K ∗ polarization are in disagreement with most models, while in the B sector studies of similar final states are in agreement with the models. By comparing the D ∗0ℓ − ν and D ∗+ℓ − ν rates, the lifetime ratio of neutral and charged B's is shown to be equal to 1.18±0.27 +0.22 −0.30. Using this lifetime ratio, coupled with a new measurement of the fraction of neutral B's on the T(4S), the B 0− B 0 mixing parameter r is determined to be in the range 0.15 > r > 0.39. A hint of a positive signal for the b→u transition using the endpoint of the lepton momentum spectrum is shown to be consistent in the standard model with the mixing measurement and CP violation in the kaon sector. Finally indirect measurements of the absolute branching ratios for Λ c→pK − π + and D s→ Θπ + yield 4.1±1.5% and 1.5±0.8%, respectively.