1,150 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
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
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 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
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
Baryon Exotics in the Quark Model, the Skyrme Model and QCD
We identify the quantum numbers of baryon exotics in the Quark Model, the
Skyrme Model and QCD, and show that they agree for arbitrary colors and
flavors. We define exoticness, E, which can be used to classify the states. The
exotic baryons include the recently discovered qqqq qbar pentaquarks (E=1), as
well as exotic baryons with additional q qbar pairs (E >=1). The mass formula
for non-exotic and exotic baryons is given as an expansion in 1/N, and allows
one to relate the moment of inertia of the Skyrme soliton to the mass of a
constituent quark
Power Counting and Perturbative One Pion Exchange in Heavy Meson Molecules
We discuss the possible power counting schemes that can be applied in the
effective field theory description of heavy meson molecules, such as the
X(3872) or the recently discovered Zb(10610) and Zb(10650) states. We argue
that the effect of coupled channels is suppressed by at least two orders in the
effective field theory expansion, meaning that they can be safely ignored at
lowest order. The role of the one pion exchange potential between the heavy
mesons, and in particular the tensor force, is also analyzed. By using
techniques developed in atomic physics for handling power-law singular
potentials, which have been also successfully employed in nuclear physics, we
determine the range of center-of-mass momenta for which the tensor piece of the
one pion exchange potential is perturbative. In this momentum range, the one
pion exchange potential can be considered a subleading order correction,
leaving at lowest order a very simple effective field theory consisting only on
contact-range interactions.Comment: 21 pages, 1 figur
Chiral Perturbation Theory Analysis of the Baryon Magnetic Moments
Nonanalytic and chiral corrections to the baryon
magnetic moments are computed. The calculation includes contributions from both
intermediate octet and decuplet baryon states. Unlike the one-loop
contributions to the baryon axial currents and masses, the contribution from
decuplet intermediate states does not partially cancel that from octet
intermediate states. The fit to the observed magnetic moments including
corrections is found to be much worse than the tree level SU(3) fit
if values for the baryon-pion axial coupling constants obtained from a tree
level extraction are used. Using the axial coupling constant values extracted
at one loop results in a better fit to the magnetic moments than the tree level
SU(3) fit. There are three linear relations amongst the magnetic moments when
corrections are included, and one relation including ,
and corrections. These relations are independent of the
axial coupling constants of the baryons and agree well with experiment.Comment: (16 pages, 2 figures; uses harvmac and uufiles), CERN-TH.6735/92,
UCSD/PTH 92-3
Analysis of General Power Counting Rules in Effective Field Theory
We derive the general counting rules for a quantum effective field theory
(EFT) in dimensions. The rules are valid for strongly and weakly
coupled theories, and predict that all kinetic energy terms are canonically
normalized. They determine the energy dependence of scattering cross sections
in the range of validity of the EFT expansion. We show that the size of cross
sections is controlled by the power counting of EFT, not by chiral
counting, even for chiral perturbation theory (PT). The relation between
and is generalized to dimensions. We show that the
naive dimensional analysis counting is related to counting. The
EFT counting rules are applied to PT, low-energy weak interactions,
Standard Model EFT and the non-trivial case of Higgs EFT.Comment: V2: more details and examples added; version published in journal. 17
pages, 4 figures, 2 table
The Heavy Quark Spin Symmetry Partners of the X(3872)
We explore the consequences of heavy quark spin symmetry for the charmed
meson-antimeson system in a contact-range (or pionless) effective field theory.
As a trivial consequence, we theorize the existence of a heavy quark spin
symmetry partner of the X(3872), with , which we call X(4012) in
reference to its predicted mass. If we additionally assume that the X(3915) is
a heavy spin symmetry partner of the X(3872), we end up predicting a
total of six molecular states. We also discuss the error
induced by higher order effects such as finite heavy quark mass corrections,
pion exchanges and coupled channels, allowing us to estimate the expected
theoretical uncertainties in the position of these new states.Comment: 18 pages; final version accepted for publicatio
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
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