4,445 research outputs found

### The ${\bar c} c$ purity of $\psi(3770)$ and $\psi'$ challenged

It is suggested that the resonance $\psi(3770)$ may contain a sizeable
($O(10%)$ in terms of the probability weight factor) four-quark component with
the up- and down- quarks and antiquarks in addition to the $c {\bar c}$ pair,
which component in itself has a substantial part with the isospin I=1.
Furthermore such four-quark part of the wave function should also affect the
properties of the $\psi'$ charmonium resonance through the $\psi(3770) - \psi'$
mixing previously considered in the literature. It is argued that an admixture
of extra light quark pairs can explain a possible discrepancy between the
theoretical expectations and the recent data on the non-$D {\bar D}$ decay
width of the $\psi(3770)$ and the ratio of the yield of charged and neutral $D$
meson pairs in its decays, as well as on the extra rate of the $\psi'$ direct
decay into light hadrons and the rate of the decay $\psi' \to \pi^0 J/\psi$. It
is further argued that the suggested four-quark component of the wave function
of the $\psi(3770)$ should give rise to a measurable rate of the decays
$\psi(3770) \to \eta J/\psi$ and $\psi(3770) \to \pi^0 J/\psi$.Comment: 13 page

### Heavy quark spin symmetry breaking in near-threshold $J^{PC}=1^{--}$ quarkonium-like resonances

A mixing of near-threshold quarkonium-like resonances with heavy
meson-antimeson pairs results in an enhancement of heavy quark spin symmetry
breaking, since the meson pairs are not eigenstates of the heavy quark spin.
The decomposition of $P$-wave states of meson pairs in terms of the
heavy-quark-pair spin states is considered in the channel with $J^{PC}=1^{--}$,
which is directly produced in $e^+e^-$ annihilation. Specific processes are
suggested for experimental study of the effects of the mixing with heavy meson
pairs and of the internal spin structure of bottomonium and charmonium
resonances.Comment: 15 page

### Semiclassical Calculation of Photon-Stimulated Schwinger Pair Creation

We consider the electron-positron pair creation by a photon in an external
constant electric field. The presented treatment is based on a purely
quasiclassical calculation of the imaginary part of the on-shell photon
polarization operator. By using this approach we find the pair production rate
for photons with polarization parallel as well as orthogonal to the external
electric field in the leading order in the parameter $eE / m ^ 2$, which has
been recently found by other methods. For the orthogonal polarization we also
find a new contribution to the rate, which is leading in the ratio of the
photon energy to the electron mass $\omega/m$. We also reproduce by a purely
geometrical calculation the exponential factor in the probability of the
stimulated pair creation at arbitrary energy of the photon.Comment: 16 pages, 4 figure

### Bound States of the Heavy Flavor Vector Mesons and Y(4008) and $Z^{+}_1(4050)$

The $D^{*}\bar{D}^{*}$ and $B^{*}\bar{B}^{*}$ systems are studied dynamically
in the one boson exchange model, where $\pi$, $\eta$, $\sigma$, $\rho$ and
$\omega$ exchanges are taken into account. Ten allowed states with low spin
parity are considered. We suggest that the $1^{--}$, $2^{++}$, $0^{++}$ and
$0^{-+}$ $B^{*}\bar{B}^{*}$ molecules should exist, and the $D^{*}\bar{D}^{*}$
bound states with the same quantum numbers very likely exist as well. However,
the CP exotic ($1^{-+}$, $2^{+-}$) $B^{*}\bar{B}^{*}$ and $D^{*}\bar{D}^{*}$
states may not be bound by the one boson exchange potential. We find that the
I=0 configuration is more deeply bound than the I=1 configuration, hence
$Z^{+}_1(4050)$ may not be a $D^{*}\bar{D}^{*}$ molecule. Although Y(4008) is
close to the $D^{*}\bar{D}^{*}$ threshold, the interpretation of Y(4008) as a
$D^{*}\bar{D}^{*}$ molecule is not favored by its huge width. $1^{--}$
$D^{*}\bar{D}^{*}$ and $B^{*}\bar{B}^{*}$ states can be produced copiously in
$e^{+}e^{-}$ annihilation, detailed scanning of the $e^{+}e^{-}$ annihilation
data near the $D^{*}\bar{D}^{*}$ and $B^{*}\bar{B}^{*}$ threshold is an
important check to our predictions.Comment: 17 pages,6 figur

### Comment on "Summing One-Loop Graphs at Multi-Particle Threshold"

The propagator of a virtual $\phi$-field with emission of $n$ on-mass-shell
particles all being exactly at rest is calculated at the tree-level in $\lambda
\phi^4$ theory by directly solving recursion equations for the sum of Feynman
graphs. It is shown that the generating function for these propagators is
equivalent to a Fourier transform of the recently found Green's function within
the background-field technique for summing graphs at threshold suggested by
Lowell Brown. Also the derivation of the result that the tree-level
on-mass-shell scattering amplitudes of the processes $2 \to n$ are exactly
vanishing at threshold for $n > 4$ is thus given in the more conventional
Feynman diagram technique.Comment: 7 pages, LaTeX, TPI-MINN-92/46-

### Non-factorizable terms, heavy quark masses, and semileptonic decays of D and B mesons

The non-factorizable terms in the operator product expansion have been
recognized as one of theoretical obstacles for precision determination of the
mixing parameter $V_{ub}$ from semileptonic B decays. It is pointed out here
that the recent CLEO data on the parameters of the heavy quark expansion
$\lambda_1$ and ${\bar \Lambda}$, combined with a theoretical bound on
$\lambda_1$ strongly favor the existence of a sizeable contribution of
non-factorizable terms in semileptonic decays of $D$ mesons. Thus these terms
are likely to solve the long-standing problem of the deficit of semileptonic
decay rate of the $D$ mesons, and with better data their magnitude can be
determined and used in studies of the parameter $V_{ub}$.Comment: 7 pages, 1 figur

### Pionic transitions from X(3872) to chi_{cJ}

We consider transitions from the resonance X(3872) to the chi_{cJ} states of
charmonium with emission of one or two pions as a means of studying the
structure of the X resonance. We find that the relative rates for these
transitions to the final states with different J significantly depend on
whether the initial state is a pure charmonium state or a four-quark/molecular
state.Comment: 12 page

### Bottomonium dipion transitions

Dipion transitions of the subthreshold bottomonium levels $\Upsilon (nS)\to
\Upsilon (n'S) \pi\pi$ with $n>n', n=2,3,4, n'=1,2$ are studied in the
framework of the chiral decay Lagrangian, derived earlier. The channels $B\bar
B, B\bar B^*+ c.c, B^* \bar B^*$ are considered in the intermediate state and
realistic wave functions of $\Upsilon (n S),B$ and $B^*$ are used in the
overlap matrix elements. Imposing the Adler zero requirement on the transition
matrix element, one obtains 2d and 1d dipion spectra in reasonable agreement
with experiment.Comment: 34 pages, 18 figure

### Soliton-antisoliton pair production in particle collisions

We propose general semiclassical method for computing the probability of
soliton-antisoliton pair production in particle collisions. The method is
illustrated by explicit numerical calculations in (1+1)-dimensional scalar
field model. We find that the probability of the process is suppressed by an
exponentially small factor which is almost constant at high energies.Comment: 4 pages, 3 figures, journal versio

### Decays of b hadrons and a possible new four-quark interaction

A possibility is considered of explaining the low experimental value of the
ratio of the lifetimes tau(Lambda_b)/tau(B_d) by a new `centiweak' four-quark
interaction, i.e with a strength on the order of 10^{-2}G_F. It is noted that
the considered interaction can also improve agreement with the data on low
semileptonic branching ratio B_{sl}(B) in B meson decays with a simultaneous
slight decrease in the prediction for the average charm yield in those decays.
The proposed new interaction modifies within the present experimental limits
the predictions for differences of lifetimes among B mesons, and can thus be
probed by more precise data on these differences. A sample model is briefly
discussed, where the new interaction arises through a weak SU(2) singlet scalar
field with quantum numbers of a diquark.Comment: 10 page

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