495 research outputs found
Multiplicative renormalizability and quark propagator
The renormalized Dyson-Schwinger equation for the quark propagator is
studied, in Landau gauge, in a novel truncation which preserves multiplicative
renormalizability. The renormalization constants are formally eliminated from
the integral equations, and the running coupling explicitly enters the kernels
of the new equations. To construct a truncation which preserves multiplicative
renormalizability, and reproduces the correct leading order perturbative
behavior, non-trivial cancellations involving the full quark-gluon vertex are
assumed in the quark self-energy loop. A model for the running coupling is
introduced, with infrared fixed point in agreement with previous
Dyson-Schwinger studies of the gauge sector, and with correct logarithmic tail.
Dynamical chiral symmetry breaking is investigated, and the generated quark
mass is of the order of the extension of the infrared plateau of the coupling,
and about three times larger than in the Abelian approximation, which violates
multiplicative renormalizability. The generated scale is of the right size for
hadronic phenomenology, without requiring an infrared enhancement of the
running coupling.Comment: 17 pages; minor corrections, comparison to lattice results added;
accepted for publication in Phys. Rev.
Analysis of a quenched lattice-QCD dressed-quark propagator
Quenched lattice-QCD data on the dressed-quark Schwinger function can be
correlated with dressed-gluon data via a rainbow gap equation so long as that
equation's kernel possesses enhancement at infrared momenta above that
exhibited by the gluon alone. The required enhancement can be ascribed to a
dressing of the quark-gluon vertex. The solutions of the rainbow gap equation
exhibit dynamical chiral symmetry breaking and are consistent with confinement.
The gap equation and related, symmetry-preserving ladder Bethe-Salpeter
equation yield estimates for chiral and physical pion observables that suggest
these quantities are materially underestimated in the quenched theory: |<bar-q
q>| by a factor of two and f_pi by 30%.Comment: 9 pages, LaTeX2e, REVTEX4, 6 figure
Survey of nucleon electromagnetic form factors
A dressed-quark core contribution to nucleon electromagnetic form factors is
calculated. It is defined by the solution of a Poincare' covariant Faddeev
equation in which dressed-quarks provide the elementary degree of freedom and
correlations between them are expressed via diquarks. The nucleon-photon vertex
involves a single parameter; i.e., a diquark charge radius. It is argued to be
commensurate with the pion's charge radius. A comprehensive analysis and
explanation of the form factors is built upon this foundation. A particular
feature of the study is a separation of form factor contributions into those
from different diagram types and correlation sectors, and subsequently a
flavour separation for each of these. Amongst the extensive body of results
that one could highlight are: r_1^{n,u}>r_1^{n,d}, owing to the presence of
axial-vector quark-quark correlations; and for both the neutron and proton the
ratio of Sachs electric and magnetic form factors possesses a zero.Comment: 43 pages, 17 figures, 12 tables, 5 appendice
Analytic properties of the Landau gauge gluon and quark propagators
We explore the analytic structure of the gluon and quark propagators of
Landau gauge QCD from numerical solutions of the coupled system of renormalized
Dyson--Schwinger equations and from fits to lattice data. We find sizable
negative norm contributions in the transverse gluon propagator indicating the
absence of the transverse gluon from the physical spectrum. A simple analytic
structure for the gluon propagator is proposed. For the quark propagator we
find evidence for a mass-like singularity on the real timelike momentum axis,
with a mass of 350 to 500 MeV. Within the employed Green's functions approach
we identify a crucial term in the quark-gluon vertex that leads to a positive
definite Schwinger function for the quark propagator.Comment: 42 pages, 16 figures, revtex; version to be published in Phys Rev
Digital Quantum Simulation with Rydberg Atoms
We discuss in detail the implementation of an open-system quantum simulator
with Rydberg states of neutral atoms held in an optical lattice. Our scheme
allows one to realize both coherent as well as dissipative dynamics of complex
spin models involving many-body interactions and constraints. The central
building block of the simulation scheme is constituted by a mesoscopic Rydberg
gate that permits the entanglement of several atoms in an efficient, robust and
quick protocol. In addition, optical pumping on ancillary atoms provides the
dissipative ingredient for engineering the coupling between the system and a
tailored environment. As an illustration, we discuss how the simulator enables
the simulation of coherent evolution of quantum spin models such as the
two-dimensional Heisenberg model and Kitaev's toric code, which involves
four-body spin interactions. We moreover show that in principle also the
simulation of lattice fermions can be achieved. As an example for controlled
dissipative dynamics, we discuss ground state cooling of frustration-free spin
Hamiltonians.Comment: submitted to special issue "Quantum Information with Neutral
Particles" of "Quantum Information Processing
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
A New Omomyid Primate from the Earliest Eocene of Southern England: First Phase of Microchoerine Evolution
Hooker, J.J. 2012. A new omomyid primate from the earliest Eocene of southern England: First phase of microchoerin
Updated precision measurement of the average lifetime of B hadrons
The measurement of the average lifetime of B hadrons using inclusively reconstructed secondary vertices has been updated using both an improved processing of previous data and additional statistics from new data. This has reduced the statistical and systematic uncertainties and gives \tau_{\mathrm{B}} = 1.582 \pm 0.011\ \mathrm{(stat.)} \pm 0.027\ \mathrm{(syst.)}\ \mathrm{ps.} Combining this result with the previous result based on charged particle impact parameter distributions yields \tau_{\mathrm{B}} = 1.575 \pm 0.010\ \mathrm{(stat.)} \pm 0.026\ \mathrm{(syst.)}\ \mathrm{ps.
Limits on the production of scalar leptoquarks from Z (0) decays at LEP
A search has been made for pairs and for single production of scalar leptoquarks of the first and second generations using a data sample of 392000 Z0 decays from the DELPHI detector at LEP 1. No signal was found and limits on the leptoquark mass, production cross section and branching ratio were set. A mass limit at 95% confidence level of 45.5 GeV/c2 was obtained for leptoquark pair production. The search for the production of a single leptoquark probed the mass region above this limit and its results exclude first and second generation leptoquarks D0 with masses below 65 GeV/c2 and 73 GeV/c2 respectively, at 95% confidence level, assuming that the D0lq Yukawa coupling alpha(lambda) is equal to the electromagnetic one. An upper limit is also given on the coupling alpha(lambda) as a function of the leptoquark mass m(D0)
Search for strong gravity in multijet final states produced in pp collisions at √s=13 TeV using the ATLAS detector at the LHC
A search is conducted for new physics in multijet final states using 3.6 inverse femtobarns of data from proton-proton collisions at √s = 13TeV taken at the CERN Large Hadron Collider with the ATLAS detector. Events are selected containing at least three jets with scalar sum of jet transverse momenta (HT) greater than 1TeV. No excess is seen at large HT and limits are presented on new physics: models which produce final states containing at least three jets and having cross sections larger than 1.6 fb with HT > 5.8 TeV are excluded. Limits are also given in terms of new physics models of strong gravity that hypothesize additional space-time dimensions
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