84 research outputs found
Factorization Approach for Inclusive Production of Doubly Heavy Baryon
We study inclusive production of doubly heavy baryon at a collider
and at hadron colliders through fragmentation. We study the production by
factorizing nonpertubative- and perturbative effects. In our approach the
production can be thought as a two-step process: A pair of heavy quarks can be
produced perturbatively and then the pair is transformed into the baryon. The
transformation is nonperturbative. Since a heavy quark moves with a small
velocity in the baryon in its rest frame, we can use NRQCD to describe the
transformation and perform a systematic expansion in the small velocity. At the
leading order we find that the baryon can be formed from two states of the
heavy-quark pair, one state is with the pair in state and in color
, another is with the pair in state and in color . Two matrix elements are defined for the transformation from the two
states, their perturbative coefficients in the contribution to the
cross-section at a collider and to the function of heavy quark
fragmentation are calculated. Our approach is different than previous
approaches where only the pair in state and in color is
taken into account. Numerical results for colliders at the two
-factories and for hadronic colliders LHC and Tevatron are given.Comment: Add results for large p_t, minor change
Corrections For Two Photon Decays of chi_{c0} and chi_{c2} and Color Octet Contributions
Using the fact that the c-quark inside a charmonium moves with a small
velocity v in the charmonium rest-frame, one can employ an expansion in v to
study decays of charmonia and results at the leading order for chi_{c0,2} -->
gamma gamma exist in the literature. We study corrections at the
next-to-leading order in the framework of nonrelativistic QCD(NRQCD)
factorization. The study presented here is different than previous approaches
where chi_{c0,2} is taken as a bound-state of a c cbar pair and a
nonrelativistic wave-function is used for the pair. We find that the
corrections are consist not only of relativistic corrections, but also of
corrections from Fock state components of chi_{c0,2} in which the c cbar pair
is in a color-octet state. For chi_{c2} there is also a contribution from a
Fock state component in which the pair is in a F-wave state. We determine the
factorization formula for decay widths in the form of NRQCD matrix elements
representing nonperturbative effects related to chi_{c0,2}, and calculate the
perturbative coefficients at tree-level. Because the NRQCD matrix elements are
unknown, a detailed prediction for the decay chi_{c0,2} --> gamma gamma can not
be made, but the effect of these corrections can be determined at certain
level. Estimations show that the effect is significant and can not be
neglected.Comment: 8 pages, 2 figures; typo in Eq.(24) and Eq.(26) are corrected. add
one referenc
Non-perturbatively Renormalized Light-Quark Masses with the Alpha Action
We have computed the light quark masses using the O(a^2) improved Alpha
action, in the quenched approximation. The renormalized masses have been
obtained non-perturbatively. By eliminating the systematic error coming from
the truncation of the perturbative series, our procedure removes the
discrepancies, observed in previous calculations, between the results obtained
using the vector and the axial-vector Ward identities. It also gives values of
the quark masses larger than those obtained by computing the renormalization
constants using (boosted) perturbation theory. Our main results, in the RI
(MOM) scheme and at a renormalization scale \mu=2 GeV, are m^{RI}_s= 138(15)
MeV and m^{RI}_l= 5.6(5) MeV, where m^{RI}_s is the mass of the strange quark
and m^{RI}_l=(m^{RI}_u+m^{RI}_d)/2 the average mass of the up-down quarks. From
these results, which have been obtained non-perturbatively, by using continuum
perturbation theory we derive the \bar{MS} masses, at the same scale, and the
renormalization group invariant (m^{RGI}) masses. We find m^{NLO \bar{MS}}_s=
121(13)$ MeV and m^{NLO\bar{MS}}_l= 4.9(4) MeV at the next-to-leading order;
m^{N^2LO \bar{MS}}_s= 111(12) MeV, m^{N^2LO \bar{MS}}_l= 4.5(4) MeV, m_s^{RGI}=
177(19) MeV and m^{RGI}_l= 7.2(6) MeV at the next-to-next-to-leading order.Comment: 13 pages, 1 figur
Transverse Momentum Dependent Light-Cone Wave Function of B-Meson and Relation to the Momentum Integrated One
A direct generalization of the transverse momentum integrated(TMI) light-cone
wave function to define a transverse momentum dependent(TMD) light-cone wave
function will cause light-cone singularities and they spoil TMD factorization.
We motivate a definition in which the light-cone singularities are regularized
with non-light like Wilson lines. The defined TMD light-cone wave function has
some interesting relations to the corresponding TMI one. When the transverse
momentum is very large, the TMD light-cone wave function is determined
perturbatively in term of the TMI one. In the impact -space with a small
, the TMD light-cone wave function can be factorized in terms of the TMI
one. In this letter we study these relations. By-products of our study are the
renormalization evolution of the TMI light-cone wave function and the
Collins-Soper equation of the TMD light-cone wave function, the later will be
useful for resumming Sudakov logarithms.Comment: Minor change in text, 7 pages, two figure
Calculating the I=2 Pion Scattering Length Using Tadpole Improved Clover Wilson Action on Coarse Anisotropic Lattices
In an exploratory study, using the tadpole improved clover Wilson quark
action on small, coarse and anisotropic lattices, the scattering
length in the I=2 channel is calculated within quenched approximation. A new
method is proposed which enables us to make chiral extrapolation of our lattice
results without calculating the decay constant on the lattice. Finite volume
and finite lattice spacing errors are analyzed and the results are extrapolated
towards the infinite volume and continuum limit. Comparisons of our lattice
results with the new experiment and the results from Chiral Perturbation Theory
are made. Good agreements are found.Comment: 21 pages, 8 figures, latex file typeset with elsart.cls, minor
change
On Transverse-Momentum Dependent Light-Cone Wave Functions of Light Mesons
Transverse-momentum dependent (TMD) light-cone wave functions of a light
meson are important ingredients in the TMD QCD factorization of exclusive
processes. This factorization allows one conveniently resum Sudakov logarithms
appearing in collinear factorization. The TMD light-cone wave functions are not
simply related to the standard light-cone wave functions in collinear
factorization by integrating them over the transverse momentum. We explore
relations between TMD light-cone wave functions and those in the collinear
factorization. Two factorized relations can be found. One is helpful for
constructing models for TMD light-cone wave functions, and the other can be
used for resummation. These relations will be useful to establish a link
between two types of factorization.Comment: add more discussions and reference
Diffractive Photoproduction of Eta_c
Diffractive photoproduction of is an important process to study the
effect of Odderon, whose existence is still not confirmed in experiment. A
detailed interpretation of Odderon in QCD, i.e., in terms of gluons is also
unclear.Taking charm quarks as heavy quarks, we can use NRQCD and take
as a bound state. Hence, in the production of a free pair is first produced and this pair is transformed into
subsequently.In the forward region of the kinematics, the pair
interacts with initial hadron through exchanges of soft gluons. This
interaction can be studied with HQET, which provides a systematic expansion in
the inverse of the -quark mass . We find that the calculation of the
-matrix element in the forward region can be formulated as the problem of
solving a wave function of a -quark propagating in a background field of
soft gluons. At leading order we find that the differential cross-section can
be expressed with four functions, which are defined with a twist-3 operator of
gluons. The effect of exchanging a Odderon can be identified with this operator
in our case. We discuss our results in detail and compare them with those
obtained in previous studies. Our results and those from other studies show
that the differential cross-section is very small in the forward region. We
also show that the production through photon exchange is dominant in the
extremely forward region, hence the effect of Odderon exchange can not be
identified in this region.For completeness we also give results for diffractive
photoproduction of .Comment: 20 pages with 3 figures. Text improve
Brain Plasticity and Intellectual Ability Are Influenced by Shared Genes
Although the adult brain is considered to be fully developed and stable until senescence when its size steadily decreases, such stability seems at odds with continued human (intellectual) development throughout life. Moreover, although variation in human brain size is highly heritable, we do not know the extent to which genes contribute to individual differences in brain plasticity. In this longitudinal magnetic resonance imaging study in twins, we report considerable thinning of the frontal cortex and thickening of the medial temporal cortex with increasing age and find this change to be heritable and partly related to cognitive ability. Specifically, adults with higher intelligence show attenuated cortical thinning and more pronounced cortical thickening over time than do subjects with average or below average IQ. Genes influencing variability in both intelligence and brain plasticity partly drive these associations. Thus, not only does the brain continue to change well into adulthood, these changes are functionally relevant because they are related to intelligence. Copyright©2010 the authors
QCD Form Factors and Hadron Helicity Non-Conservation
Recent data for the ratio shocked the
community by disobeying expectations held for 50 years. We examine the status
of perturbative QCD predictions for helicity-flip form factors. Contrary to
common belief, we find there is no rule of hadron helicity conservation for
form factors. Instead the analysis yields an inequality that the leading power
of helicity-flip processes may equal or exceed the power of helicity conserving
processes. Numerical calculations support the rule, and extend the result to
the regime of laboratory momentum transfer . Quark orbital angular
momentum, an important feature of the helicity flip processes, may play a role
in all form factors at large , depending on the quark wave functions.Comment: 25 pages, 5 figure
Consistent OPE Description of Gluon Two- and Three-point Green Function?
We perform an OPE analysis of the flavorless non-perturbative gluon
propagator and the symmetric three-gluon vertex in the Landau gauge. The first
subdominant operator is which can condensate in the Landau gauge
``vacuum'' although being a non-gauge invariant operator. We neglect all higher
dimension operators. Then the gluon propagator and the symmetric three gluon
vertex only depend on one common unknown condensate. We propose a consistency
check from lattice data. At two loops for the leading coefficient and with
corrections at tree-level order the two fitted values for the
condensate do not agree. At three loops we argue that the today unknown
should be equal to to fulfill the OPE relation. Inclusion of the power
corrections' anomalous dimensions should improve further the agreement. We show
that these techniques cannot be applied to the asymmetric three gluon vertex
with one vanishing momentum.Comment: latex-file,10 figs.,13 pg
- âŠ