Decays Of Heavy Quarks Research Articles

Heavy flavour production and decay with prompt leptons in the ALEPH detector

In 431 000 hadronicZ decays recorded with the ALEPH detector at LEP, the yields of electrons and muons in events with one or more prompt leptons have been analysed to give information on the production and decay of heavy quarks. The fractions of $$b\bar b$$ and $$c\bar c$$ events are measured to be 0.219±0.006±0.005 and 0.165±0.005±0.020, and the corresponding forward-backward asymmetries at theZ mass are measured to be 0.090±0.013±0.003 and 0.111±0.021±0.018, after QED and QCD corrections. Measurements for the semileptonic branching ratios BR $$(b \to \ell ^ - \bar vX)$$ and BR (b→cl+ vX) yield 0.114±0.003±0.004 and 0.082±0.003±0.012, respectively. The dilepton events enable measurement of theb mixing parameter, Χ=0.114±0.014±0.008. Results are also presented for the energy variation of the $$b\bar b$$ asymmetry and the parameters required to describe heavy quark fragmentation. From the asymmetry measurements, the effective electroweak mixing angle is sin2θ eff =0.2333±0.0022.

CP violation in top pair production at an e +e − collider

We investigate possible CP violating effects in e +e − annilation into top quark pairs. One of the interesting observable effects is the difference in production rates between the two CP conjugate polarized states t L t L and t R t R . The result is an asymmetry in the energy spectra of the lepton and the anti-lepton from the heavy quark decays. Another CP-odd observable is the up-down asymmetry of the leptons with respect to the reaction plane. These two asymmetries measure complementarily the absorptive and dispersive form factors of the electric dipole moment. Finally, as an illustration, we calculate the size of the CP violating form factors in a model where the CP nonconservation originates from the Yukawa couplings of a neutral Higgs boson.

A study of differences between quark and gluon jets using vertex tagging of quark jets

Quark and gluon jets with equal energies are identified in three-jet hadronicZ 0 events, using reconstructed secondary vertices from heavy quark decay in conjunction with energy ordering of the jets to anti-tag the gluon jets. Selection of jets from a symmetric event topology allows their properties to be compared in a simple and direct manner. The jets under study have an energy of about 24 GeV. It is observed that gluon jets have a larger angular width than quark jets and yield a softer particle energy spectrum. Correspondingly, the mean particle multiplicity is found to be larger for gluon than for quark jets. Correcting the distributions for residual misidentification of the quark and gluon jets, the ratio of mean particle multiplicty of gluon relative to quark jets is measured to be $$\frac{{\left\langle n \right\rangle _{gluon} }}{{\left\langle n \right\rangle _{quark} }} = 1.27 \pm 0.04(stat.) \pm 0.06(syst.),$$ where the jets are defined using thek ⊥ jet finder. The numerical value of this ratio is found to be sensitive to the choice of the jet algorithm. The experimental results are compared to Monte Carlo calculations which incorporate perturbative QCD along with different assumptions about the hadronization process.

End-point region of the electron spectrum in inclusive semileptonic heavy-quark decay.

I examine the relationship between the free-quark-decay and sum-over-exclusive-resonances pictures for the inclusive electron spectrum of semileptonic heavy-quark decay. The analysis shows that one should [ital expect] the electron spectrum to be suppressed near the end point relative to the prediction obtained from a free-quark-decay-type model [ital which] [ital has] [ital had] [ital its] [ital end] [ital point] [ital adjusted] [ital to] [ital the] [ital physical] [ital end] [ital point]. This conclusion is consistent with arguments that (up to small radiative corrections) free-quark-decay-type models are, in principle, dual'' to a sum over resonant channels.

The Isgur-Wise limit on the lattice

We construct the Isgur-Wise limit of QCD in a form appropriate to lattice gauge theory techniques. The formulation permits a calculation of heavy quark processes even when the momentum transfers are much larger than the inverse lattice spacing. Applications include semi-leptonic heavy quark decay and scattering processes, including the computation of the nonperturbative part of the Isgur-Wise universal function.

Inconsistency of the effective theory for energetic light quarks

For the analysis of exclusive nonleptonic decays of heavy quarks, such as B→ π + D, B→ ϱ + D, an effective theory based on QCD has recently been formulated and applied; the main result is that it implies factorization of the decay amplitude. This theory describes sources with very high momentum moving with the velocity of light, whose motion is not affected by finite momentum transfers. In general, it can be applied to any process involving light quarks with large energy and soft interactions. Despite its clear physical meaning, this effective theory is shown to be unstable. This phenomenon arises because the positivity of the energy is not preserved in the expansion leading to the effective hamiltonian. The energy spectrum is unbounded from below and transitions of the “effective quark” into states of arbitrarily negative energies are kinematically possible and have nonzero amplitudes. These processes have no counterpart in the real world and spoil the effective theory of physical meaning.

The endpoint region of the electron spectrum in inclusive semileptonic heavy-quark decay

The endpoint region of the electron spectrum induced by the quark-level semileptonic decay b to ue nu /sup /e is potentially important for the measurement of the Cabibbo-Kobayashi-Maskawa (CKM) parameter Vub. The endpoint region is crucial to this measurement since, with Vub<<Vcb, only electron energies Ee with (mB2-mD2)/2mB<or approximately=Ee<or=(mB2-mpi 2)/2mB are useful for this purpose: for Ee much below this interval the spectrum is dominated by decays induced by b to ce nu e. However, the extraction of Vub requires that theory be able to calculate the rate d Gamma /dEe in this region apart from the overall factor of mod Vub mod 2 arising from the CKM matrix.

New approach to the universal form factors in decays of heavy quarks

New results on current-induced transitions between heavy mesons are presented. In the heavy-quark limit of QCD, consistency relations fixing the relative q 2−dependence of different hadronic form factors arise. With the help of these relations, we investigate to what extent the spin-flavour symmetry that QCD exhibits in the limit of infinite quark masses is broken in B → D (∗) and D → K (∗) transitions. We develop a method to express the universal functions, that appear in leading and next-to-leading order in the 1/m Q expansion of current matrix elements, in terms of hadronic form factors evaluated at maximum recoil. In turn, the heavy-quark symmetries impose constraints on these form factors. To illustrate the application of our formalism and the kind of constraints it yields, the general results are evaluated within the framework of a particular model. Some phenomenological applications are discussed in detail.

Lattice implementation of the Isgur-Wise limit.

We construct the Isgur-Wise limit of QCD in a form appropriate for measurement by lattice-gauge-theory techniques. The coordinate-space fermion propagator in the presence of a background gauge field has a systematic expansion in powers of the inverse quark mass about the Isgur-Wise limit. The formulation permits a calculation of heavy-quark processes even when the momentum transfers are much larger than the inverse lattice spacing. Applications include semileptonic heavy-quark decay and scattering processes, including the computation of the nonperturbative part of the Isgur-Wise universal function

A lattice implementation of the Isgur-Wise limit

We construct the Isgur-Wise limit of QCD in a form appropriate for lattice gauge theory. The formulation permits a calculation of heavy quark processes even when the momentum transfers are much larger than the inverse lattice spacing. Applications include semi-leptonic heavy quark decay and scattering processes, including the computation of the nonperturbative part of the Isgur-Wise universal function.

Heavy top quark decay into a charged Higgs scalar and a photon (gluon)

We consider the decay of a heavy top quark into a charged Higgs scalar and a hard photon (gluon)t→H+bγ(g). We give the general result for fermions of arbitrary mass. Our results for the differential decay rate for these two processes are remarkably simple. The radiation amplitude zero (RAZ) factorization is explicit but there is no RAZ for these processes because of the presence of opposite-sign charges. We obtain a branching ratio of 0.063% for bH+γ and 5.6% for bH+g. Thus both decays should be relatively easy to see at the SSC.

Rare decays of the top quark in the standard and two-Higgs-doublet models.

The decays $t\ensuremath{\rightarrow}\mathrm{cV}$, where $V=\ensuremath{\gamma}, g, or Z$, and $t\ensuremath{\rightarrow}\mathrm{cH}$ are calculated, keeping all masses, in the standard model (SM) for all possible values of the top-quark mass that are consistent with present data. We find that the branching fractions for these processes are small, the largest being $B(t\ensuremath{\rightarrow}cH)\ensuremath{\sim}{10}^{\ensuremath{-}7}$ and $B(t\ensuremath{\rightarrow}cg)\ensuremath{\sim}{10}^{\ensuremath{-}10}$. Our calculations are then extended to include the possible contributions of charged Higgs bosons to the transitions $t\ensuremath{\rightarrow}\mathrm{cV}$ in the context of general two-Higgs-doublet models. These new contributions can enhance the SM branching fractions by as much as 3-4 orders of magnitude, e.g., $B(t\ensuremath{\rightarrow}cg)\ensuremath{\sim}{10}^{\ensuremath{-}7}\ensuremath{-}{10}^{\ensuremath{-}6}$, for various values of the parameters. General expressions for flavor-changing neutral-current decays of heavy quarks including all masses and momenta, in the SM and in the two-Higgs-doublet models, are also given.

Three-jet distributions ine + e − annihilation: electromagnetic and weak production

Hard gluon bremsstrahlung off quarks ine+e− annihilation is expected to produce three-jet events. The angular distributions of the jets are calculated for electromagnetic and weak production in the high energy region (Z resonance decay included). Numerical estimates are presented for the vectoraxialvector interference terms within the Weinberg-Salam theory. We discuss (i) angular correlations between jets, (ii) semiinclusive pion distributions and (iii) we comment on angular asymmetries of leptons originating from heavy quark decays.

Power corrections to leading logs and their application to heavy quark decays

When a heavy virtual particle of mass M contributes to a process at low energies E ⪡ M, the anomalous dependence on M of the amplitude can be obtained by integrating out the heavy particle and constructing an effective hamiltonian. If there are two heavy particles of masses E ⪡ m ⪡ M one proceeds analogously, integrating them out in succession. But if m and M are comparable, the full dependence on both masses can produce a substantial correction. We consider the problem of keeping in the effective hamiltonian the full anomalous dependence on both heavy masses. We solve it by expanding perturbatively both in the gauge coupling constant and in the ratio of masses m/ M. The method we present has application in the static quark effective theory, in the context of which we compute corrections of order m c / m b to the form factors for B-meson to D-meson transitions.

Vector symmetry and heavy quark decays

We discuss the consequences of the vector symmetry recently suggested by one of us for the decays of heavy quarks, b and c.

Simulation of QCD coherence in heavy quark production and decay

We present an approximate method for including coherent soft gluon emission in Monte Carlo simulations of heavy quark production and decay. This involves two main new features in comparison with the method developed earlier for gluonic and light quark processes: (i) a treatment of the soft radiation distribution analogous to the angular-ordering approximation but taking account of the screening of collinear singularities by heavy quark masses; and (ii) an analysis of the colour structure of hard scattering cross sections including quark mass terms. We explain how these developments are implemented in the Monte Carlo program HERWIG, comparing the numerical results with analytical calculations where available. We illustrate the expected differences between heavy and light quark jets and show that these should be visible in hadron distributions.

Possible structure in cosmic gamma rays from dark matter particle annihilation

The possibilities to observe structures in the cosmic ray gamma flux are discussed. It is shown that nearly monoenergetic photon lines coming from the annihilation of supersymmetric dark matter particles (neutralinos) in the galactic halo giving two photons or a photon and a heavy vector meson may be observed experimentally in recently proposed experiments for the Space Station. A novel signature of dark matter annihilation is pointed out, namely, breaks or very sharp steps in the photon flux as a function of energy. Such steps may originate from quasi-two-body decays of heavy quarks produced in the main annihilation modes of the neutralinos. An example is provided by the decay b→s+ γ or b′→b+ γ, where b′ is a hypothetical fourth generation charge − 1 3 quark. Estimates of cosmic ray photon backgrounds are given, and the possible enhancements of annihilation in the galactic centre are investigated. Some new results on the relic abundance and annihilation cross sections are presented for the case when the neutralino is a heavy higgsino. The possible significance of novel hadronic and photonic decay modes is also discussed.

Transition Radiation Detector with Uniform π/e Rejection Ability

We have developed a planar transition radiation detector to be used for electron tagging in the experiment of heavy quark decay. It consists of a radiator of multilayer polypropylene foils and a Xe gas multiwire proportional chamber for X-ray detection. The performance was tested in the mixed π/e beam of several GeV/c range. With four modules of this type, the pion contamination less than 3.5% was achieved at the electron detection efficiency of 90% over the effective area of 55×55 cm2. Some important feature for the stable operation are also discussed.

Effect of the W-boson propagator in an analytic expression for heavy-quark semileptonic decay rate.

We find an expression for heavy-quark semileptonic decay which is analytic and takes into account the effects of the {ital W}-boson propagator. The result is given for {ital m}{sub {ital b}}, {ital m}{sub {ital t}}, and {ital m}{sub {ital W}} each finite and different from zero.

Lepton pairs from heavy quarks produced in hadron colliders.

Distributions with respect to rapidity and transverse momenta are studied for lepton pairs arising from decays of heavy quarks produced in hadron-hadron colliders. Particular attention is paid to the merits of complete angular coverage and low threshold in transverse momentum.