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David H. Evans

Publications and source records attributed to David H. Evans.

3 recordsLinked to original sources
arXiv2000

Short Distance Analysis of $\bar B -> D^{(*)0} e+e-$ and $\bar B -> J/psi e+e-$

Over a large fraction of phase space a combination of an operator product and heavy quark expansions effectively turn the decay $\bar B -> D^{(*)0} e+e-$ into a ``short distance'' process, i.e., one in which the weak and electromagnetic interactions occur through single local operators. These processes have an underlying W-exchange quark diagram topology and are therefore Cabibbo allowed but suppressed by combinatoric factors and short distance QCD corrections. Our technique allows a clearer exploration of these effects. For the decay $\bar B_{d,s} -> J/psi(η_c) e+e-$ one must use a non-relativistic (NRQCD) expansion, in addition to an operator product expansion and a heavy quark effective theory expansion. We estimate the decay rates for $\bar B_{d,s} -> J/psi e+e-$, $\bar B_{d,s} -> eta_c e+e-$, $\bar B_{d,s} -> D^{*0} e+e-$ and $\bar B_{d,s} -> D^{0} e+e-$.

hep-ph↗
arXiv2000

Operator Product Expansion for Exclusive Decays: B^+ ->Ds^+ e+e- and B^+ -> Ds^{*+} e+e-

The decays $B^+\to D_{s,d}^+e^+e^-$ and $B^+\to D_{s,d}^{*+}e^+e^-$ proceed through a weak and an electromagnetic interaction. This is a typical ``long distance'' process, usually difficult to compute systematically. We propose that over a large fraction of phase space a combination of an operator product and heavy quark expansions effectively turns this process into one in which the weak and electromagnetic interactions occur through a local operator. Moreover, we use heavy quark spin symmetry to relate all the local operators that appear in leading order of the operator expansion to two basic ones. We use this operator expansion to estimate the decay rates for $B^+\to D_{s,d}^{(*)+}e^+e^-$.

hep-ph↗
arXiv1999

Determining V(ub) from B+ --> D*+(s) e+ e- and B+ --> D*+ e+ e-

It was recently pointed out that the decays B^+ --> D^{*+}_s gamma and B^+ --> D^{*+} gamma can be used for an extraction of |V_{ub}|. The theory of these decays is poorly understood. It was shown that in a world of almost degenerate b and c-quarks the decay would be computable. The severe difficulties that are encountered in the realistic calculation stem primarily from the very hard photon produced in the two body decay. We point out that in the decays B^+ --> D^{*+}_s e^+e^- and B^+ --> D^{*+} e^+e^- the photon vertex is soft when the charmed meson is nearly at rest (in the B^+ rest frame). This allows us to compute with some confidence the decay rate in a restricted but interesting kinematic regime. Given enough data the extraction of V_{ub} with reasonably small uncertainties could proceed through an analysis of these exclusive decays much as is done in the determination of V_{cb}.

hep-ph↗