1,282 research outputs found

### Moments of generalized parton distribution functions and the nucleon spin contents

It is shown that, based only on two empirically known facts besides two
reasonable theoretical postulates, we are inevitably led to a conclusion that
the quark orbital angular momentum carries nearly half of the total nucleon
spin. We also perform a model analysis to find that the quark spin fraction
$\Delta \Sigma$ is extremely sensitive to the pion mass, which may resolve the
discrepancy between the observation and the prediction of the recent lattice
QCD simulation carried out in the heavy pion region.Comment: LaTeX, 8 pages, 1 figur

### The Zero-Bin and Mode Factorization in Quantum Field Theory

We study a Lagrangian formalism that avoids double counting in effective
field theories where distinct fields are used to describe different infrared
momentum regions for the same particle. The formalism leads to extra
subtractions in certain diagrams and to a new way of thinking about
factorization of modes in quantum field theory. In non-relativistic field
theories, the subtractions remove unphysical pinch singularities in box type
diagrams, and give a derivation of the known pull-up mechanism between soft and
ultrasoft fields which is required by the renormalization group evolution. In a
field theory for energetic particles, the soft-collinear effective theory
(SCET), the subtractions allow the theory to be defined with different infrared
and ultraviolet regulators, remove double counting between soft, ultrasoft, and
collinear modes, and give results which reproduce the infrared divergences of
the full theory. Our analysis shows that convolution divergences in
factorization formul\ae occur due to an overlap of momentum regions. We propose
a method that avoids this double counting, which helps to resolve a long
standing puzzle with singularities in collinear factorization in QCD. The
analysis gives evidence for a factorization in rapidity space in exclusive
decays.Comment: 92 pages, v4- Journal version. Some improvements to language in
sections I, IIA, VI

### Factorization Structure of Gauge Theory Amplitudes and Application to Hard Scattering Processes at the LHC

Previous work on electroweak radiative corrections to high energy scattering
using soft-collinear effective theory (SCET) has been extended to include
external transverse and longitudinal gauge bosons and Higgs bosons. This allows
one to compute radiative corrections to all parton-level hard scattering
amplitudes in the standard model to NLL order, including QCD and electroweak
radiative corrections, mass effects, and Higgs exchange corrections, if the
high-scale matching, which is suppressed by two orders in the log counting, and
contains no large logs, is known. The factorization structure of the effective
theory places strong constraints on the form of gauge theory amplitudes at high
energy for massless and massive gauge theories, which are discussed in detail
in the paper. The radiative corrections can be written as the sum of
process-independent one-particle collinear functions, and a universal soft
function. We give plots for the radiative corrections to q qbar -> W_T W_T, Z_T
Z_T, W_L W_L, and Z_L H, and gg -> W_T W_T to illustrate our results. The
purely electroweak corrections are large, ranging from 12% at 500 GeV to 37% at
2 TeV for transverse W pair production, and increasing rapidly with energy. The
estimated theoretical uncertainty to the partonic (hard) cross-section in most
cases is below one percent, smaller than uncertainties in the parton
distribution functions (PDFs). We discuss the relation between SCET and other
factorization methods, and derive the Magnea-Sterman equations for the Sudakov
form factor using SCET, for massless and massive gauge theories, and for light
and heavy external particles.Comment: 44 pages, 30 figures. Refs added, typos fixed. ZL ZL plots removed
because of a possible subtlet

### Radiative quarkonium decays and the NMSSM Higgs interpretation of the HyperCP Sigma+ --> p mu+mu- events

We study the potential of radiative decays of the Upsilon(1S) and of the phi
mesons to search for a light pseudoscalar Higgs boson, proposed as a possible
interpretation of Sigma+ --> p mu+mu- events observed by the HyperCP
collaboration at Fermilab. We conclude that the detection of this signal should
certainly be possible with the current CLEO Upsilon(1S) data, and is within the
reach of KLOE in at least part of the range of couplings suggested by the
HyperCP findings.Comment: 6 pages, no figure

### QCD Flux Tubes as Sigma Model Relics

We describe flux tubes and their interactions in a low energy sigma model
induced by SU(\NF) \goto SO(\NF) flavor symmetry breaking in $SO(N_c)$ QCD.
Gauge confinement manifests itself in the low energy theory through flux tube
interactions with unscreened sources. The flux tubes which mediate confinement
also illustrate an interesting ambiguity in defining global Alice strings.Comment: 12 pages (REVTEX) plus one figur

### A Lattice Test of 1/N_c Baryon Mass Relations

1/N_c baryon mass relations are compared with lattice simulations of baryon
masses using different values of the light-quark masses, and hence different
values of SU(3) flavor-symmetry breaking. The lattice data clearly display both
the 1/N_c and SU(3) flavor-symmetry breaking hierarchies. The validity of 1/N_c
baryon mass relations derived without assuming approximate SU(3)
flavor-symmetry also can be tested by lattice data at very large values of the
strange quark mass. The 1/N_c expansion constrains the form of discretization
effects; these are suppressed by powers of 1/N_c by taking suitable
combinations of masses. This 1/N_c scaling is explicitly demonstrated in the
present work.Comment: 13 pages, 20 figures; v2 version to be published in PR

### Electroweak Corrections using Effective Field Theory: Applications to the LHC

Electroweak Sudakov logarithms at high energy, of the form alpha/sin^2
theta_W^n log^m s/M_{Z,W}^2, are summed using effective theory (EFT) methods.
The exponentiation of Sudakov logarithms and factorization is discussed in the
EFT formalism. Radiative corrections are computed to scattering processes in
the standard model involving an arbitrary number of external particles. The
computations include non-zero particle masses such as the t-quark mass,
electroweak mixing effects which lead to unequal W and Z masses and a massless
photon, and Higgs corrections proportional to the top quark Yukawa coupling.
The structure of the radiative corrections, and which terms are summed by the
EFT renormalization group is discussed in detail. The omitted terms are smaller
than 1%. We give numerical results for the corrections to dijet production,
dilepton production, t-\bar t production, and squark pair production. The
purely electroweak corrections are significant -- about 15% at 1 TeV,
increasing to 30% at 5 TeV, and they change both the scattering rate and
angular distribution. The QCD corrections (which are well-known) are also
computed with the EFT. They are much larger -- about a factor of four at 1 TeV,
increasing to a factor of thirty at 5 TeV. Mass effects are also significant;
the q \bar q -> t \bar t rate is enchanced relative to the light-quark
production rate by 40%.Comment: Additional details added on exponentiation, and the form of the
Sudakov series. Figures darkened to print better. 40 pages, 40 figure

### Model Independent Results for Heavy Quarkonium

We review a number of results for the spectrum and inclusive decays of heavy
quarkonium systems which can be derived from QCD under well controlled
approximations. They essentially follow from the hierarchy of scales in these
systems, which can be efficiently exploited using non-relativistic effective
field theories. In particular, we discuss under which conditions
non-relativistic potential models emerge as effective theories of QCD.Comment: 15 pages. Invited brief revie

### Baryon Masses in Partially Quenched Heavy Hadron Chiral Perturbation Theory

The masses of baryons containing a heavy quark are calculated to
next-to-leading order in partially quenched heavy hadron chiral perturbation
theory. Calculations are performed for three light flavors in the isospin limit
and additionally for two light non-degenerate flavors. The results presented
are necessary for extrapolating lattice QCD and partially quenched lattice QCD
calculations of the heavy hadron masses.Comment: 20 pages, 2 figures, RevTex

### A Renormalization Group Improved Calculation of Top Quark Production near Threshold

The top quark cross section close to threshold in $e^+e^-$ annihilation is
computed including the summation of logarithms of the velocity at
next-to-next-to-leading-logarithmic order in QCD. The remaining theoretical
uncertainty in the normalization of the total cross section is at the few
percent level, an order of magnitude smaller than in previous
next-to-next-to-leading order calculations. This uncertainty is smaller than
the effects of a light standard model Higgs boson.Comment: changed figures, added reference

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