144 research outputs found

### Decays rates for S- and P-wave bottomonium

We use the Bodwin-Braaten-Lepage factorization scheme to separate the long-
and short-distance factors that contribute to the decay rates of $\Upsilon$,
$\eta_b$ (S-wave) and $\chi_b$,$h_b$ (P-wave). The long distance matrix
elements are calculated on the lattice in the quenched approximation using a
non-relativistic formulation of the $b$ quark dynamics.Comment: 3 pages Latex using espcrc2.sty and epsf.sty + 2 postscript figure

### Prompt J/psi Polarization at the Tevatron

The polarization of prompt J/psi at the Tevatron is calculated within the
nonrelativistic QCD (NRQCD) factorization framework. The contribution from
radiative decays of P-wave charmonium states decreases, but does not eliminate,
the transverse polarization at large transverse momentum. The prediction agrees
with measurements from the CDF collaboration at intermediate values of p_T, but
disagrees at the large values of p_T measured.Comment: 5 pages, 2 figures, Talk given at 5th Workshop on QCD, 3-7 January
2000, Villefranche-sur-Mer, Franc

### Resummation of QCD Corrections to the eta_c Decay Rate

We examine the ratio of the decay rate of the eta_c into light hadrons to the
decay rate into photons and find that most of the large next-to-leading-order
(NLO) correction is associated with running of the strong coupling alpha_s. We
resum such contributions by analyzing final-state chains of vacuum-polarization
bubbles. We show that the nonperturbative parts of the bubble chains can be
absorbed into a color-octet matrix element, once one has used contour
deformations of the phase-space integrals to cancel certain contributions. We
argue that these contributions are incompatible with the uncertainty principle.
We also argue that perturbation theory is reliable only if one carries out the
phase-space integrations before the perturbation summation. Our results are in
good agreement with experiment and differ considerably from those that one
obtains by applying the scale-setting method of Brodsky, Lepage, and Mackenzie
to the NLO result.Comment: 41 pages, REVTEX, corrected minor typos in tex

### $\chi_{QJ} \to\ell^+\ell^-$ within and beyond the Standard Model

We revisit $\chi_{QJ}\to \ell^+\ell^-$ (with $J=0,1,2$ and $Q=b,c$) within
the Standard Model (SM). The electro-magnetic contributions are given in
color-singlet model with non-vanishing lepton masses at the leading order of
$v$. Numerically, the branching ratios of $\chi_{QJ}\to\ell^{+}\ell^{-}$
predicted within the SM are so small that such decays are barely possible to be
detected at future BESIII and SuperB experiments, but may be possible to be
observed at the LHC. We investigate $\chi_{b0}\to\ell^+\ell^-$ in Type-II 2HDM
with large $\tan\beta$, and $\chi_{b2}\to\ell^+\ell^-$ in the Randall-Sundrum
model, to see their chance to be observed in future experiments.Comment: 11 pages, 8 figures. To match the published versio

### $\Upsilon$ radiative decays to light quark jets and color octet mechanism

We study radiative decays of $\Upsilon$ to light quark jets in
nonrelativistic QCD by taking both the color singlet and color octet $b\bar b$
operators into consideration. The cut for quark jet energy and cut for the
angle between two quark jets are introduced. The sensitivity to the soft and
collinear singularities in the loop integrals are greatly reduced by these
cuts. With the jet energy cut of about 1 GeV, and the jet angle cut of about
$36^\circ$, the branching ratio for $\Upsilon\to\gamma q\bar q$ is found to be
$8.2\times 10^{-4}$ from color singlet contributions. The color octet
contributions could be much larger than that of color singlet, depending on the
estimate of the color octet matrix elements. This process may provide a new
test for the color octet mechanism in nonrelativistic QCD.Comment: journal version; a few references adde

### Fragmentation Function in Non-Equilibrium QCD Using Closed-Time Path Integral Formalism

In this paper we implement Schwinger-Keldysh closed-time path integral
formalism in non-equilibrium QCD to the definition of Collins-Soper
fragmentation function. We consider a high p_T parton in QCD medium at initial
time t_0 with arbitrary non-equilibrium (non-isotropic) distribution function
f(\vec{p}) fragmenting to hadron. We formulate parton to hadron fragmentation
function in non-equilibrium QCD in the light-cone quantization formalism. It
may be possible to include final state interactions with the medium via
modification of the Wilson lines in this definition of the non-equilibrium
fragmentation function. This may be relevant to study hadron production from
quark-gluon plasma at RHIC and LHC.Comment: 15 pages latex, Accepted for Publication in European Physical Journal

### Inclusive $S-$wave charmonium productions in $B$ decays

The inclusive $S-$wave charmonium production rates in $B$ decays are
considered using the Bodwin-Braaten-Lepage (BBL) approach, including the
relativistic corrections and the color-octet mechanism suggested as a possible
solution to the $\psi^{'}$ puzzle at the Tevatron. We first consider
relativistic and radiative corrections to $J/\psi \rightarrow e^{+} e^{-}$ and
$J/\psi \rightarrow$ Light Hadrons (LH), in order to determine two
nonperturbative parameters, $\langle J/\psi | O_{1} (^{3}S_{1}) | J/\psi
\rangle$, $\langle J/\psi | P_{1} (^{3}S_{1}) | J/\psi \rangle$, in the
factorization formulae for these decays. Using these two matrix elements and
including the color-octet $c\bar{c}(^{3}S_{1})$ state contribution, we get a
moderate increase in the decay rates for $B$ decays into $J/\psi ~({\rm
or}~\psi^{'}) ~+ X$. Our results, $B(B \rightarrow J/\psi~({\rm or}~\psi^{'}) +
X) = 0.58~(0.23) \%$ for $M_{b} = 5.3$ GeV, get closer to the recent CLEO data.
As a byproduct, we prefer a larger decay rate for $\eta_{c} \rightarrow$ LH
compared to the present data.Comment: 12 page

### Power counting and effective field theory for charmonium

We hypothesize that the correct power counting for charmonia is in the
parameter Lambda_QCD/m_c, but is not based purely on dimensional analysis (as
is HQET). This power counting leads to predictions which differ from those
resulting from the usual velocity power counting rules of NRQCD. In particular,
we show that while Lambda_QCD/m_c power counting preserves the empirically
verified predictions of spin symmetry in decays, it also leads to new
predictions which include: A hierarchy between spin singlet and triplet octet
matrix elements in the J/psi system. A quenching of the net polarization in
production at large transverse momentum. No end point enhancement in radiative
decays. We discuss explicit tests which can differentiate between the
traditional and new theories of NRQCD.Comment: 18 pages, 1 figure Replaced plot of the psi polarization parameter
alpha as a function of transverse momentum. Alpha is now closer to zero for
large transverse moment

### Fragmentation Functions for Lepton Pairs

We calculate the fragmentation function for a light quark to decay into a
lepton pair to leading order in the QCD coupling constant. In the formal
definition of the fragmentation function, a QED phase must be included in the
eikonal factor to guarantee QED gauge invariance. We find that the longitudinal
polarization fraction is a decreasing function of the factorization scale, in
accord with the intuitive expectation that the virtual photon should behave
more and more like a real photon as the transverse momomentum of the
fragmenting quark increases.Comment: 13 pages, 4 figures, normalization corrected, text abbreviate

### Resumming the color-octet contribution to e+ e- -> J/psi + X

Recent observations of the spectrum of J/psi produced in e+ e- collisions at
the Upsilon(4S) resonance are in conflict with fixed-order calculations using
the Non-Relativistic QCD (NRQCD) effective field theory. One problem is that
leading order color-octet mechanisms predict an enhancement of the cross
section for J/psi with maximal energy that is not observed in the data.
However, in this region of phase space large perturbative corrections (Sudakov
logarithms) as well as enhanced nonperturbative effects are important. In this
paper we use the newly developed Soft-Collinear Effective Theory (SCET) to
systematically include these effects. We find that these corrections
significantly broaden the color-octet contribution to the J/psi spectrum. Our
calculation employs a one-stage renormalization group evolution rather than the
two-stage evolution used in previous SCET calculations. We give a simple
argument for why the two methods yield identical results to lowest order in the
SCET power counting.Comment: 27 pages, 7 figure

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