arXiv subjects

Michael Luke

Publications and source records attributed to Michael Luke.

52 records · Page 3Linked to original sources

arXiv1993

Extracting $|V_{bc}|$, $m_c$ and $m_b$ 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 $\vbc$, $m_c$ and $m_b$ is made from the experimentally measured $B$ and $D$ lifetimes and semileptonic branching ratios. Constraining the parameters of the HQET at $\CO(1/m_Q^2)$ by the $D$ semileptonic width, $\vbc$ is found to lie in the range $.040<\vbc< 0.057$. 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 alone may be obtained.

hep-ph↗

arXiv1993

Nonperturbative Contributions to the Inclusive Rare Decays $B\to X_sγ$ and $B\to X_s\ell^+\ell^-$

We discuss nonperturbative contributions to the inclusive rare $B$ decays $B\to X_sγ$ and $B\to X_s\ell^+\ell^-$. We employ an operator product expansion and the heavy quark effective theory to compute the leading corrections to the decay rate found in the free quark decay model, which is exact in the limit $m_b\to\infty$. These corrections are of relative order $1/m_b^2$, and may be parameterised in terms of two low-energy parameters. We also discuss the corrections to other observables, such as the average photon energy in $B\to X_sγ$ and the lepton invariant mass spectrum in $B\to X_s\ell^+\ell^-$.

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arXiv1993

Heavy Quark Fragmentation to Baryons Containing Two Heavy Quarks

We discuss the fragmentation of a heavy quark to a baryon containing two heavy quarks of mass $m_Q\ggΛ_{\rm QCD}$. In this limit the heavy quarks first combine perturbatively into a compact diquark with a radius small compared to $1/Λ_{\rm QCD}$, which interacts with the light hadronic degrees of freedom exactly as does a heavy antiquark. The subsequent evolution of this $QQ$ diquark to a $QQq$ baryon is identical to the fragmentation of a heavy antiquark to a meson. We apply this analysis to the production of baryons of the form $ccq$, $bbq$, and $bcq$.

hep-ph↗

arXiv1993

Flavour Changing Neutral Currents, Weak-Scale Scalars and Rare Top Decays

We examine the decays $t\rightarrow cγ$ and $c Z^0$ in the Standard Model with an extra scalar doublet and no discrete symmetry preventing tree-level flavour changing neutral currents. The Yukawa couplings of the new scalars are assumed to be proportional to fermion masses, evading bounds on FCNC's from the light quark sector. These rare top decays may be visible at the SSC.

hep-ph↗

arXiv1993

Heavy Quark Fragmentation to Polarised Quarkonium

We calculate the polarisation of $ψ$'s and $Υ$'s produced by the fragmentation of heavy quarks. We find that fragmentation to transversely aligned quarkonium is slightly enhanced relative to longitudinally polarised. This net alignment corresponds to a $\sim 5\%$ asymmetry in the angular distribution of leptons produced in the subsequent decay of the quarkonium. We point out that the leading gluon contribution to $ψ$ and $Υ$ production arises from the evolution of the charm quark fragmentation function, rather than from direct gluon fragmentation.

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arXiv1992

Chiral Perturbation Theory Analysis of the Baryon Magnetic Moments

Nonanalytic $m_q^{1/2}$ and $m_q\ln m_q$ chiral corrections to the baryon magnetic moments are computed. The calculation includes contributions from both intermediate octet and decuplet baryon states. Unlike the one-loop contributions to the baryon axial currents and masses, the contribution from decuplet intermediate states does not partially cancel that from octet intermediate states. The fit to the observed magnetic moments including $m_q^{1/2}$ corrections is found to be much worse than the tree level SU(3) fit if values for the baryon-pion axial coupling constants obtained from a tree level extraction are used. Using the axial coupling constant values extracted at one loop results in a better fit to the magnetic moments than the tree level SU(3) fit. There are three linear relations amongst the magnetic moments when $m_q^{1/2}$ corrections are included, and one relation including $m_q^{1/2}$, $m_q\ln m_q$ and $m_q$ corrections. These relations are independent of the axial coupling constants of the baryons and agree well with experiment.

hep-ph↗

arXiv1992

Analyticity and the Isgur-Wise Function

We reconsider the recent derivation by de Rafael and Taron of bounds on the slope of the Isgur-Wise function. We argue that one must be careful to include cuts starting below the heavy meson pair production threshold, arising from heavy quark-antiquark bound states, and that if such cuts are properly accounted for then no constraints may be derived.

hep-ph↗

arXiv1992

A Comment on the Strong Interactions of Color-Neutral Technibaryons

We estimate the cross section for the scattering of a slow, color-neutral technibaryon made of colored constituents with nuclei. We find a cross section of order $A^2\ 10^{-45}$ cm$^2$, where $A$ is the atomic number of the nucleus. Even if technibaryons constitute the dark matter in the galactic halo, this is too small to be detected in future underground detectors.

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arXiv1992

Weak Radiative Hyperon Decays in Chiral Perturbation Theory

The parity-conserving $a$ and parity-violating $b$ amplitudes for weak radiative hyperon decay are studied using chiral perturbation theory. The imaginary parts of $a$ and $b$ are computed using unitarity. The real part of $b$ is dominated by a one-loop infrared divergent graph which is computed. The real part of $a$ has a large theoretical uncertainty and cannot be calculated reliably. Counterterms for the $a$ and $b$ amplitudes are classified using $CPS$ symmetry. The experimental values for decay widths and asymmetries are consistent with theory, with the exception of the asymmetry parameter for the $Σ^+ \rightarrow p γ$ decay.

hep-ph↗

arXiv1992

Properties of Baryons Containing a Heavy Quark in the Skyrme Model

The properties of baryons containing one heavy quark are studied in the Skyrme model, where they are treated as bound states of a heavy meson with an $SU(2)$ chiral soliton. In the large $N_c$ limit, the baryon spectrum is an infinite tower of degenerate states with isospin and spin of the light degrees of freedom $I=s_\ell=0,1,2,\dots$, and total spin $s=\abs{s_\ell\pm1/2}$. Exotic states with no quark model analogues in the large $N_c$ limit are found to be unbound. The $Σ_c-Λ_c$ mass difference is computed and found to be in good agreement with experiment. The $Σ_c\,Σ_c\,π$ and $Σ_c\,Λ_c\,π$ coupling constants are calculated to leading order in $N_c$ in terms of $g_A$, the axial coupling constant of the nucleon. We discuss the extension of our results to $SU(3)$ chiral solitons.

hep-ph↗

arXiv1992

The Rare Decays $\piee$, $\etaee$ and \etamumu$ in Chiral Perturbation Theory

We calculate the decay rates for $\piee$, $\etaee$ and $\etamumu$ in chiral perturbation theory. The linear combination of counterterms necessary to render these amplitudes finite is fixed by the recently measured branching fraction for $\etamumu$. We find $\Br(\piee ) = 7\pm 1\times 10^{-8}$ and $\Br(\etaee )=5\pm 1\times 10^{-9}$.

hep-ph↗

arXiv1992

Strong Decays of Excited Heavy Mesons In Chiral Perturbation Theory

We construct an effective Lagrangian describing the interaction of soft pions and kaons with mesons containing a heavy quark and light degrees of freedom in an orbital $p$ wave. The formalism is easily extended to heavy mesons and baryons in arbitrary excited states. We calculate the leading contributions to the strong decays $\dtwo\to\dπ$, $\dtwo\to\dstarπ$ and $\done\to\dstarπ$. We confirm the relations between the rates previously obtained by Isgur and Wise using heavy quark symmetry, and find that the absolute widths are consistent with na\"ıve power counting. We also estimate the branching ratios for the two pion decays $\dtwo\to\dstarππ$, $\done\to\dstarππ$ and $\done\to\dππ$, which are dominated by pole graphs. Our predictions depend on the masses and widths of the as yet unseen scalar-pseudovector $p$-wave doublet. Heavy quark spin symmetry predicts $Γ(\dtwo\to\dstarππ): Γ(\done\to\dstarππ):Γ(\done\to\dππ)=3:1:2$, but this relation is badly violated in practice because $1/M$ effects arising purely from kinematics are large.

hep-ph↗

arXiv1992

Reparameterisation Invariance Constraints on Heavy Particle Effective Field Theories

Since fields in the heavy quark effective theory are described by both a velocity and a residual momentum, there is redundancy in the theory: small shifts in velocity may be absorbed into a redefinition of the residual momentum. We demonstrate that this trivial reparameterisation invariance has non-trivial consequences: it relates coefficients of terms of different orders in the $1/m$ expansion and requires linear combinations of these operators to be multiplicatively renormalised. For example, the operator $-D^2/2m$ in the effective lagrangian has zero anomalous dimension, coefficient one, and does not receive any non-perturbative contributions from matching conditions. We also demonstrate that this invariance severely restricts the forms of operators which may appear in chiral lagrangians for heavy particles.

hep-ph↗

arXiv1992

Semileptonic $B_c$ Decay and Heavy Quark Spin Symmetry

Semileptonic decay of the $B_c$ meson is studied in the heavy quark limit. The six possible form factors for $B_c \rightarrow B_s (B^0),B_s^* (B^{*0})$ semileptonic decay are determined by two invariant functions. Only one of these functions contributes at zero recoil, where it is calculable to lowest order in an operator product expansion in terms of the meson decay constant $f_B$ and the $B_c$ wavefunction. A similar result is found for $B_c \rightarrow D^0,D^{*0}$ and for $B_c\rightarrowη_c,J/ψ$ semileptonic decay for a restricted kinematic region. Semileptonic $B_c$ decay provides a means for determining the KM mixing angle $|V_{ub}|$.

hep-ph↗

arXiv1992

The Residual Mass Term in the Heavy Quark Effective Theory

We reformulate the heavy quark effective theory in the presence of a residual mass term, which has been taken to vanish in previous analyses. While such a convention is permitted, the inclusion of a residual mass allows us to resolve a potential ambiguity in the choice of the expansion parameter which defines the effective theory. We show to subleading order in the mass expansion that physical quantities computed in the effective theory do not depend on the expansion parameter.

hep-ph↗

arXiv1992

A QCD Calculation of the Interaction of Quarkonium with Nuclei

The interaction of quarkonium with nuclei is studied in the $m_Q\rightarrow \infty$ limit of QCD, where the binding energy is found to be exactly computable. The dominant contribution to the interaction is from two-gluon operators. The forward matrix elements of these two-gluon operators can be determined from the QCD scale anomaly, and from deep inelastic scattering. We apply our results to the $Υ$ and $J/ψ$, treating the $\qqbar$ interaction as purely Coulombic. We find the $Υ$ binds in nuclear matter with a binding energy of a few $\mev$, while for the $J/ψ$ binding is of order 10 $\mev$. For the $J/ψ$ in particular we expect confinement effects to produce large corrections to this result.

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