7,340 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

### Anisotropic flow of identified particles in Au+Au collisions at AGS energy

Anisotropic flow of protons, $\pi^{\pm}$, $K^+$, deuterons, tritons, $^3$He,
and $^4$He is analyzed as a function of transverse momentum for different
centralities of the collision.Comment: RevTex, 4 pages, 4 figures included as eps files. To be published in
the Proceedings of QM97, Tsukuba, Japa

### Ultra-relativistic nuclear collisions: event shape engineering

The evolution of the system created in a high energy nuclear collision is
very sensitive to the fluctuations in the initial geometry of the system. In
this letter we show how one can utilize these large fluctuations to select
events corresponding to a specific initial shape. Such an "event shape
engineering" opens many new possibilities in quantitative test of the theory of
high energy nuclear collisions and understanding the properties of high density
hot QCD matter.Comment: 6 pages, 5 figure

### 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

### Anisotropic collective phenomena in ultra-relativistic nuclear collisions

For a detailed review of this subject I refer to a recent paper
\cite{Voloshin:2008dg}; in this talk I only very briefly comment on a few most
important questions: (a) Very recent significant progress in viscous
hydrodynamics calculations (b) Initial eccentricity/flow fluctuations, the
effect of which has been clarified recently (c) Initial conditions, in
particular the role of the gradients in the initial velocity field, (d)
Puzzling system size dependence of directed flow (e) Azimuthal correlations
that are sensitive to the strong parity violation (f) Future measurements at
RHIC and LHC, including pp-collisionsComment: 6 pages. Proceedings. Invited talk at PANIC 2008 conference, Eilat,
Israel, November 200

### Experimental study of local strong parity violation in relativistic nuclear collisions

Parity-odd domains, corresponding to non-trivial topological solutions of the
QCD vacuum, might be created in relativistic heavy ions collisions. These
domains are predicted to lead to charge separation along the system orbital
momentum of the system created in non-central collisions. Three-particle mixed
harmonics azimuthal correlator is a \P even observable but directly sensitive
to the charge separation effect. Using this observable to analyze Au+Au and
Cu+Cu collisions at $\sqrt{s_{NN}}=200$ and 62 GeV, STAR detects a signal
consistent with several of the theoretical expectations. Possible contributions
from effects not related to parity violation are studied with existing event
generators, which fail to describe the data. Future directions in studying the
effect are discussed.Comment: Proceedings, plenary invited talk at Quark Matter 2009 Conference,
Knoxville, Tennessee. One reference added. Final versio

### 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

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