Lepton Energy Research Articles

Subleading light-cone distribution functions in the decays Λb→Xsγ and [formula omitted

In this Letter we investigate the subleading twist corrections to the photon energy spectrum in the decay Λb→Xsγ and to the lepton energy spectrum in Λb→Xuℓν̄ℓ. As a first step we rederive the matching coefficients of the subleading twist using a much simpler method. Parametrizing the matrix elements for Λb and using reparametrization invariance, we show that the energy spectra close to the endpoints are given in terms of only a single universal function.

Reducing theoretical uncertainties in mb, λ1

I present a recent calculation of generalized lepton energy moments in inclusive semileptonic decays. These moments allow to reduce the uncertainties in m b , λ 1 and | V cd |, as well as experimentally test the underlying assumptions of the operator product expansion.

The bound state corrections to the semileptonic decays of the heavy baryons

We present an investigation of the lepton energy distributions in the inclusive semileptonic weak decays of heavy baryons performed within a relativistic quark model formulated on the light front (LF). Using the heavy-quark LF distribution functions related to the equal time momentum wave functions taken from the Kalman-Tran-D'Souza model we compute the electron energy spectra and the total semileptonic widths of the Λ Q and Ξ Q ( Q = c, b) baryons and confront the results with existing data.

Heavy quark parameters and |Vcb| from spectral moments in semileptonic B decays

We extract the heavy quark masses and non-perturbative parameters from the Delphi preliminary measurements of the first three moments of the charged lepton energy and hadronic mass distributions in semileptonic B decays, using a multi-parameter fit. We adopt two formalisms, one of which does not rely on a 1/mc expansion and makes use of running quark masses. The data are consistent and the level of accuracy of the experimental inputs largely determines the present sensitivity. The results allow to improve on the uncertainty in the extraction of |Vcb|.

Reducing theoretical uncertainties inmbandλ1

We calculate general moments of the lepton energy spectrum in inclusive semileptonic B -> X_c l \nu decay. Moments which allow the determination of mb^{1S} and lambda1 with theoretical uncertainties Delta(mb^{1S}) ~ 0.04 GeV and Delta(lambda1) ~ 0.05 GeV^2 are presented. The short distance 1S mass is used to extract a mass parameter free of renormalon ambiguities. Moments which are insensitive to mb and lambda1 and therefore test the size of the 1/mb^3 matrix elements and the validity of the OPE are also presented. Finally, we give an expression for the total branching ratio with a lower cut on the lepton energy, which allows one to eliminate a source of model dependence in current determinations of |Vcb| from B -> X_c l \nu decay.

Measurement of the mass of the W boson in ee collisions using the fully leptonic channel

A novel method of determining the mass of the W boson in the channel is presented and applied to 667 pb-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics.

Subleading shape functions in [formula omitted] and the determination of |Vub|

We calculate the subleading twist contributions to the endpoint of the inclusive lepton energy spectrum in B→Xuℓν̄. We show that the same two subleading twist functions that appear in the decay B→Xsγ govern the subleading effects in this decay. Using these results we find large O(ΛQCD/mB) corrections to the determination of |Vub| from the endpoint of the charged lepton spectrum. Using a simple model for the relevant subleading shape functions, we estimate the uncertainty in |Vub| from ΛQCD/mb corrections to be at the ∼15% level for a lower lepton energy cut of 2.2 GeV.

Enhanced subleading structure functions in semileptonic B decay

The charged lepton spectrum in semileptonic B→X uℓ ν ̄ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of Λ QCD/ m b but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width ΔE ℓ∼ Λ QCD near the endpoint the contributions proportional to δ( E ℓ− m b /2) times (i) the matrix element of the chromomagnetic operator, and (ii) four-quark operators. These contribute to the total rate at the few percent level, but affect the endpoint region much more significantly. Implications for the determination of | V ub | are discussed.

Semileptonic b-->u decays: lepton invariant mass spectrum.

We compute O(alpha(2)(s)) QCD corrections to the lepton invariant mass spectrum in the decay b-->ulnu(l), relevant for the determination of the Cabibbo-Kobayashi-Maskawa matrix element /V(ub)/. Our method can also be used to evaluate moments of the lepton energy distribution with an O(alpha(2)(s)) accuracy. The Abelian part of our result gives the neutrino invariant mass spectrum in the muon decay and, upon integration, the O(alpha(2)) correction to the muon lifetime.

Propagation of muons and taus at high energies

The photonuclear contribution to charged lepton energy loss has been reevaluated taking into account DESY HERA results on real and virtual photon interactions with nucleons. With large ${Q}^{2}$ processes incorporated, the average muon range in rock for energies of ${10}^{9} \mathrm{GeV}$ is reduced by only 5% compared with the standard treatment. We have calculated the tau energy loss for energies up to ${10}^{9} \mathrm{GeV}$ taking into consideration the decay of tau. A Monte Carlo evaluation of tau survival probability and range shows that at energies below ${10}^{7}--{10}^{8} \mathrm{GeV},$ depending on the material, only tau decays are important. At higher energies the tau energy losses are significant, reducing the survival probability of the tau. We show that the average range for tau is shorter than its decay length and reduces to 17 km in water for an incident tau energy of ${10}^{9} \mathrm{GeV},$ as compared with its decay length of 49 km at that energy. In iron, the average tau range is 4.7 km for the same incident energy.

Probing the violation of equivalence principle at a muon storage ring via neutrino oscillation

We examine the possible tests of violation of the gravitational equivalence principle (VEP) at a muon storage ring via neutrino oscillation experiments. If the gravitational interactions of the neutrinos are not diagonal in the flavour basis and the gravitational interaction eigenstates have different couplings to the gravitational field, this leads to the neutrino oscillation. If one starts with μ + beam then appearance of τ ±, e + and μ − in the final state are the signals for neutrino oscillation. We have estimated the number of μ − events in this scenario in ν μ – N deep inelastic scattering. Final state lepton energy distribution can be used to distinguish the VEP scenario from the others. A large area of VEP parameter space can be explored at a future muon storage ring facility with moderate beam energy.

Hadronic invariant mass spectrum inB→Xulνdecay with a lepton energy cut

We discuss the implications of charged lepton energy cut to the hadronic invariant mass spectrum in charmless semileptonic B decays. Charged-lepton energy cut is inevitable in order to remove secondary leptonic events such as b -> c, tau -> l, and to identify the chaged leptons at detectors experimentally. We consider three possible lepton energy cuts, E_l^{cuts} = 0.6,1.5,2.3 GeV, and found that with the most probable cuts E_l^{cut} = 1.5 GeV and M_X^{max} = 1.5~(1.86) GeV, 45 ~ 60 % ~(58 ~ 67 %) of decay events survive. Therefore, B -> X_u l nu decay events can be efficiently distinguished from B -> X_c l nu decay events. We also discuss the possible model dependence on the results.

Lepton energy asymmetry and precision supersymmetry study at hadron colliders

We study the distribution of lepton pairs from the second lightest neutralino decay \tilde{\chi}^0_2-->\tilde{l}l followed by \tilde{l}\to \tilde{\chi}^0_1 l. The distribution of the ratio of lepton transverse momenta A_T shows peak structure if m_{ll}< m^{max}_{ll}/2 is required. The peak position A_T^{peak} is described by a simple function of the ino and slepton masses in the m_{ll}\sim 0 limit. When a moderate m_{ll} cut is applied, A_T^{peak} depends on the \tilde{\chi}^0_2 velocity distribution, but the dependence would be corrected by studying the lepton P_T distribution. A_T^{peak} and the edge of m_{ll} distributions are used to determine the mass parameters involved in the decay for parameters of interest to LHC experiments. For some cases the ino and slepton masses may be determined within 10% by the lepton distribution only independent of model assumptions. Correct combinations of A_{T}^{peak} and m_{ll}^{edge} would be identified even if different \tilde{\chi}^0_2 decay chains are co-existing. The analysis could be extended to the Tevatron energy scale or other cascade decays.

DEPENDENCE OF |Vcb| ON FERMI MOMENTUM

The Fermi momentum Pf for the Bc meson has been estimated in the framework of the statistical model. The matrix element |Vcb| has been extracted from the lepton energy spectrum using the estimated value of the Fermi momentum, in the framework of the ACCMM model. The value of Pf for Bc meson has been estimated as Pf=1.14 GeV which is somewhat large compared to the existing estimates.

Effect of anomalous tbW vertex on decay-lepton distributions in e + e − → tt and CP-violating asymmetries

We obtain analytic expressions for the energy and polar-angle double differential distributions of a secondary lepton l + (l −) arising from the decay of t(t) in e + e − → tt with an anomalous tbW decay vertex. We also obtain analytic expressions for the various differential cross-sections with the lepton energy integrated over. In this case, we find that the angular distributions of the secondary lepton do not depend on the anomalous coupling in the decay, regardless of possible anomalous couplings occurring in the production amplitude for e + e − → tt. Our study includes the effect of longitudinal e − and e + beam polarization. We also study the lepton energy and beam polarization dependence of certain CP-violating lepton angular asymmetries arising from an anomalous tbW decay vertex and compare them with the asymmetries arising due to CP-violation in the production process due to the top electric or weak dipole moment.

Probe of the Wtb coupling in $t \bar{t}$ pair production at linear colliders

The Wtb vertex can be probed on future colliders in the processes of single top production (LHC, pp mode, NLC, $\gamma e$ mode) and of top pair production (NLC, e^+ e^- mode). We analyse observables sensitive to anomalous Wtb couplings in the top pair production process of e^+ e^- collisions. In particular, forward-backward and spin-spin asymmetries of the top decay products and the asymmetry of the lepton energy spectum are considered. Possible bounds on anomalous couplings obtained are competitive to those expected from the upgraded Tevatron and LHC. The validity of the infinitely small width approximation for the three-body top decay is also studied in detail.

Determination of | Vub|/| Vcb| with DELPHI at LEP

The ratio of the CKM quark-mixing matrix elements | V ub |/| V cb | has been measured using B hadron semileptonic decays. The analysis uses the reconstructed mass M X of the secondary hadronic system produced in association with an identified lepton. Since B→X uℓ ν ̄ transitions are characterised by hadronic masses below those of the D mesons produced in B→X cℓ ν ̄ transitions, events with a reconstructed value of M X significantly below the D mass are selected. Further signal enrichments are obtained using the topology of reconstructed decays and hadron identification. A fit to the numbers of decays in the b→ u enriched and depleted samples with M X above and below 1.6 GeV/ c 2 and to the shapes of the lepton energy distribution in the B rest frame gives |V ub|/|V cb|= 0.103 +0.011 −0.012 (stat.)±0.016 (syst.)±0.010 (model) and, correspondingly, a charmless semileptonic B decay branching fraction of BR( B→X uℓ ν ̄ )= (1.57±0.35 (stat.)±0.48 (syst.)±0.27 (model))×10 −3 .

A model independent determination of | Vub|

It is shown that measuring the lepton invariant mass spectrum in inclusive semileptonic B ̄ →X uℓ ν ̄ decay yields a model independent determination of | V ub |. Unlike the lepton energy and hadronic invariant mass spectra, nonperturbative effects are only important in the resonance region, and play a parametrically suppressed role when d Γ/d q 2 is integrated over q 2>( m B − m D ) 2, which is required to eliminate the charm background. Perturbative and nonperturbative corrections are presented to order α s 2 β 0 and Λ QCD 2/ m b 2, and the Λ QCD 3/ m b 3 corrections are used to estimate the uncertainty in our results. The utility of the B ̄ →X sℓ +ℓ − decay rate above the ψ(2 S) resonance is discussed.

B-quark decay in the collinear approximation

The semileptonic decay of a b quark, $\stackrel{\ensuremath{\rightarrow}}{b}\mathrm{cl}\ensuremath{\nu},$ is considered in the relativistic limit where the decay products are approximately collinear. Analytic results for the double differential lepton energy distributions are given for finite charm-quark mass. Their use for the fast simulation of isolated lepton backgrounds from heavy quark decays is discussed.

Λb polarization in Z 0 decays at LEP

The longitudinal polarization of the Λ b baryon is measured at the LEP e +e − collider by DELPHI. It is determined from the charged lepton and neutrino energy spectra in 249±19 Λ b semileptonic decays reconstructed in ≈3.5 million hadronic Z 0 decays using Λ 0-lepton correlations. The measured polarization is: P Λ b =−0.49 −0.30 +0.32(stat.)±0.17(syst.)