93 research outputs found
Study of Black Holes with the ATLAS detector at the LHC
We evaluate the potential of the ATLAS detector for discovering black holes
produced at the LHC, as predicted in models with large extra dimensions where
quantum gravity is at the TeV scale. We assume that black holes decay by
Hawking evaporation to all Standard Model particles democratically. We comment
on the possibility to estimate the Planck scale.Comment: 27 page
QED and QCD helicity amplitudes in parton-shower gauge
We introduce photon and gluon propagators in which the scalar polarization
component is subtracted systematically by making use of the BRST invariance of
the off-shell vector boson created from physical on-shell states. The
propagator has the light-cone gauge form, where the spacial component of the
gauge vector points along the negative of the off-shell vector boson momentum.
We call the gauge as parton-shower gauge, since in collinear configurations the
absolute value squared of each Feynman amplitude reproduces all the singular
behaviors of the corresponding parton shower in this gauge. We introduce new
HELAS codes that can be used to calculate the tree-level helicity amplitudes of
arbitrary QED and QCD processes by using MadGraph. The absence of subtle gauge
cancellation among Feynman amplitudes allows numerical codes to evaluate
singular behaviors accurately, and helps us gaining physical insights on
interference patterns.Comment: 10 pages, 6 figures; v2: references added; v3: some issues clarified,
a reference added, version to appear in EPJ
Helicity amplitudes without gauge cancellation for electroweak processes
We introduce 5-component representation of weak bosons, W and Z bosons of the
standard model. The first four components make a Lorentz four vector,
representing the transverse and longitudinal polarizations excluding the scalar
component of the weak bosons. Its fifth component corresponds to the Goldstone
boson. We show that this description can be extended to off-shell weak bosons,
with the component propagators, and prove that exactly the same
scattering amplitudes are obtained by making use of the BRST
(Becchi-Rouet-Stora-Tyutin) identities among two sub-amplitudes connected by
one off-shell weak boson line in the unitary gauge. By replacing all weak boson
vertices with those among the 5-component wavefunctions, we arrive at the
expression of the electroweak scattering amplitudes, where the magnitude of
each Feynman amplitude has the correct on-shell limits for all internal
propagators, and hence with no artificial gauge cancellation among diagrams. We
implement the 5-component weak boson propagators and their vertices in the
numerical helicity amplitude calculation code HELAS (Helicity Amplitude
Subroutines), so that an automatic amplitude generation program such as
MadGraph can generate the scattering amplitudes without gauge cancellation. We
present results for several high-energy scattering processes where subtle
gauge-theory cancellation among diagrams takes place in all the other known
approaches.Comment: 32 pages, 14 figures, 9 tables; v2: references adde
Automatic generation of helicity amplitudes in Feynman-Diagram gauge
We develop a method to calculate helicity amplitudes of an arbitrary
tree-level process in Feynman-Diagram (FD) gauge for an arbitrary gauge model
with MadGraph5_aMC@NLO. We start from the 't Hooft-Feynman gauge Lagrangian in
FeynRules and generate scattering amplitudes by identifying the Goldstone boson
as the component of each weak boson. All the vertices of the
5-component weak bosons are then created automatically by assembling the
relevant weak boson and Goldstone boson vertices in the Feynman gauge. The
5-component weak boson vertices are then connected by the matrix
propagator in the FD gauge. As a demonstration of the method we calculate the
cross section for the process
with complex top Yukawa coupling, which can be obtained by adding a gauge
invariant dimension-6 operator to the Standard Model (SM) Lagrangian. The FD
gauge and the unitary (U) gauge amplitudes give exactly the same cross section,
and subtle gauge theory cancellation among diagrams in the U gauge at high
energies is absent in the FD gauge, as has been observed for various SM
processes. In addition, we find that the total cross sections at high energies
are dominated by a single, or a set of non-vanishing Feynman amplitudes with
the higher dimensional vertices in the FD gauge.Comment: 28 pages, 8 figures and 1 table; typos correcte
Helicity amplitudes in light-cone and Feynman-diagram gauges
Recently proposed Feynman-diagram (FD) gauge propagator for massless and
massive gauge bosons is obtained from a light-cone (LC) gauge propagator, by
choosing the gauge vector along the opposite direction of the gauge boson
three-momentum. We implement a general LC gauge propagator for all the gauge
bosons of the Standard Model (SM) in the HELicity Amplitude Subroutines (HELAS)
codes, such that all the SM helicity amplitudes can be evaluated at the tree
level in the LC gauge by using MadGraph. We confirm that our numerical codes
produce physical helicity amplitudes which are consistent among all gauge
choices. We then study interference patterns among Feynman amplitudes, for a
few scattering processes in QED and QCD, and the process
followed by the decays. We find that in a
generic LC gauge, where all the gauge boson propagators share a common gauge
vector, we cannot remove the off-shell current components which grow with their
energy systematically from all the Feynman amplitudes in processes. On
the other hand, the LC gauge propagator for the weak bosons removes
components which grow with energy due to the longitudinal polarization mode of
the external bi-fermion currents, and hence can give weak boson
scattering amplitudes which are free from subtle cancellation at high energies.
The particular choice of the FD gauge vector has advantages over generic LC
gauge, not only because all the terms which grow with energy of off-shell and
on-shell currents are removed systematically from all the diagrams, but also
because no artificial gauge vector direction dependence of individual
amplitudes appears.Comment: 20 pages, 14 figures; references adde
Potential for measuring the H^\pm W^\mp Z^0 vertex from WZ fusion at the Large Hadron Collider
We investigate the possibility of measuring the H^\pm W^\mp Z^0 vertex from
the single production process via WZ fusion at the CERN Large Hadron
Collider (LHC). This vertex strongly depends on the structure of the Higgs
sector in various new physics scenarios, so that its measurement can be useful
to distinguish the models. A signal and background simulation under the
expected detector performance at the LHC is done for the processes of pp \to
W^\pm Z^0 X \to H^\pm X \to tbX and pp \to W^\pm Z^0 X \to H^\pm X \to W^\pm
Z^0 X, and the required magnitudes of the H^\pm W^\mp Z^0 vertex for
observation are evaluated. It is found that although the loop induced H^\pm
W^\pm Z^0 vertex in multi-Higgs doublet models cannot be measurable, the latter
process can be useful to test the model with a real and a complex triplets.Comment: 23 pages, 6 figures, version accepted for publication in Physical
Review
Computing for Perturbative QCD - A Snowmass White Paper
We present a study on high-performance computing and large-scale distributed
computing for perturbative QCD calculations.Comment: 21 pages, 5 table
A Clinical Prospective Observational Cohort Study on the Prevalence and Primary Diagnostic Accuracy of Occult Vertebral Fractures in Aged Women with Acute Lower Back Pain Using Magnetic Resonance Imaging
Background. Elderly female patients complaints of acute low back pain (LBP) may involve vertebral fracture (VF), among which occult VF (OVF: early-stage VF without any morphological change) is often missed to be detected by primary X-ray examination. The current study aimed to investigate the prevalence of VF and OVF and the diagnostic accuracy of the initial X-ray in detecting OVF. Method. Subjects were elderly women (>70 years old) complaining of acute LBP with an accurate onset date. Subjects underwent lumbar X-ray, magnetic resonance imaging (MRI), and bone mineral density (BMD) measurement at their first visit. The distribution of radiological findings from X-ray and magnetic resonance imaging (MRI) as well as the calculation of the prevalence of VF and OVF are investigated. Results. The prevalence of VF among elderly women with LBP was 76.5% and L1 was the most commonly injured level. Among VF cases, the prevalence of OVF was 33.3%. Furthermore, osteoporotic patients tend to show increased prevalence of VF (87.5%). The predictive values in detecting VF on the initial plain X-ray were as follows: sensitivity, 51.3%; specificity, 75.0%; and accuracy rate, 56.7%. Conclusions. Acute LBP patients may suffer vertebral injury with almost no morphologic change in X-ray, which can be detected using MRI
Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector
A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements
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