1,635,867 research outputs found
Top-Quark Production and Decay in the MSSM
We review the features of top-quark decays and loop-induced effects in the
production cross section and CP-violating observables of e+e- -> t t-bar which
are specific to the R-parity conserving Minimal Supersymmetric Standard Model
(MSSM).Comment: LaTeX, 28 pages, 10 figures included, uses cite.sty. Contribution to
the proceedings of the 2nd Joint ECFA/DESY Workshop on Physics and Detectors
for a Linear Electron-Positron Collider. References adde
High-precision determination of the electric and magnetic form factors of the proton
New precise results of a measurement of the elastic electron-proton
scattering cross section performed at the Mainz Microtron MAMI are presented.
About 1400 cross sections were measured with negative four-momentum transfers
squared up to Q^2=1 (GeV/c)^2 with statistical errors below 0.2%. The electric
and magnetic form factors of the proton were extracted by fits of a large
variety of form factor models directly to the cross sections. The form factors
show some features at the scale of the pion cloud. The charge and magnetic
radii are determined to be r_E=0.879(5)(stat.)(4)(syst.)(2)(model)(4)(group) fm
and r_M=0.777(13)(stat.)(9)(syst.)(5)(model)(2)(group) fm.Comment: 5 pages, 2 figures, published in Phys. Rev. Lett. v3: added
references, updated text, color figure
The Direct Detection of Boosted Dark Matter at High Energies and PeV events at IceCube
We study the possibility of detecting dark matter directly via a small but
energetic component that is allowed within present-day constraints. Drawing
closely upon the fact that neutral current neutrino nucleon interactions are
indistinguishable from DM-nucleon interactions at low energies, we extend this
feature to high energies for a small, non-thermal but highly energetic
population of DM particle , created via the decay of a significantly more
massive and long-lived non-thermal relic , which forms the bulk of DM. If
interacts with nucleons, its cross-section, like the neutrino-nucleus
coherent cross-section, can rise sharply with energy leading to deep inelastic
scattering, similar to neutral current neutrino-nucleon interactions at high
energies. Thus, its direct detection may be possible via cascades in very large
neutrino detectors. As a specific example, we apply this notion to the recently
reported three ultra-high energy PeV cascade events clustered around PeV
at IceCube (IC). We discuss the features which may help discriminate this
scenario from one in which only astrophysical neutrinos constitute the event
sample in detectors like IC.Comment: v1: 6 pages, 4 figures; v2: More references added, minor text changes
for clarification; v3: Title change, major revision, updated references; v4:
Corrected Fig. 1b, Version published in JCA
The Elliptic Double-Box Integral: Massless Amplitudes Beyond Polylogarithms
We derive an analytic representation of the ten-particle, two-loop double-box
integral as an elliptic integral over weight-three polylogarithms. To obtain
this form, we first derive a four-fold, rational (Feynman-)parametric
representation for the integral, expressed directly in terms of
dual-conformally invariant cross-ratios; from this, the desired form is easily
obtained. The essential features of this integral are illustrated by means of a
simplified toy model, and we attach the relevant expressions for both integrals
in ancillary files. We propose a normalization for such integrals that renders
all of their polylogarithmic degenerations pure, and we discuss the need for a
new 'symbology' of iterated elliptic/polylogarithmic integrals in order to
bring them to a more canonical form.Comment: 4+2 pages, 2 figures. Explicit results are included as ancillary
files. v2: minor changes made for clarification; references adde
Soft gluon corrections to double transverse-spin asymmetries for small- dilepton production at RHIC and J-PARC
We calculate the double transverse-spin asymmetries, \aqt, in transversely
polarized Drell-Yan process at the transverse-momentum of the produced
lepton pair. We perform all-order resummation of the logarithmically enhanced
contributions in the relevant Drell-Yan cross sections at small , which
are due to multiple soft gluon emission in QCD, up to the next-to-leading
logarithmic accuracy. The asymmetries \aqt to be observed in polarized
experiments at RHIC and J-PARC are studied numerically as a function of .
We show that the effects of the soft gluon resummation to the polarized and
unpolarized cross sections largely cancel in \aqt, but the significant
corrections still remain and are crucial for making a reliable QCD prediction
of \aqt. In particular, the soft gluon corrections enhance \aqt
considerably in the small region compared with the asymmetry in the
fixed-order perturbation theory. We also derive a novel asymptotic
formula which embodies those remarkable features of \aqt at small in a
compact analytic form and is useful to extract the transversity
from the experimental data.Comment: 26 pages, 12 figures, typos corrected, references added, version to
appear in Nucl. Phys.
Indirect searches of dark matter via polynomial spectral features
We derive the spectra arising from non-relativistic dark matter annihilations
or decays into intermediary particles with arbitrary spin, which subsequently
produce neutrinos or photons via two-body decays. Our approach is model
independent and predicts spectral features restricted to a kinematic box. The
overall shape within that box is a polynomial determined by the polarization of
the decaying particle. We illustrate our findings with two examples. First,
with the neutrino spectra arising from dark matter annihilations into the
massive Standard Model gauge bosons. Second, with the gamma-ray and neutrino
spectra generated by dark matter annihilations into hypothetical massive spin-2
particles. Our results are in particular applicable to the 750 GeV diphoton
excess observed at the LHC if interpreted as a spin-0 or spin-2 particle
coupled to dark matter. We also derive limits on the dark matter annihilation
cross section into this resonance from the non-observation of the associated
gamma-ray spectral features by the H.E.S.S. telescope.Comment: 19 pages, modified title, added references, minor changes. To appear
in JCA
Unstable superheavy relic particles as a source of neutrinos responsible for the ultrahigh-energy cosmic rays
Decays of superheavy relic particles may produce extremely energetic
neutrinos. Their annihilations on the relic neutrinos can be the origin of the
cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin cutoff. The red
shift acts as a cosmological filter selecting the sources at some particular
value z_e, for which the present neutrino energy is close to the Z pole of the
annihilation cross section. We predict no directional correlation of the
ultrahigh-energy cosmic rays with the galactic halo. At the same time, there
can be some directional correlations in the data, reflecting the distribution
of matter at red shift z=z_e. Both of these features are manifest in the
existing data. Our scenario is consistent with the neutrino mass reported by
Super-Kamiokande and requires no lepton asymmetry or clustering of the
background neutrinos.Comment: 3 pages, revtex; references adde
The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations
We discuss the theoretical bases that underpin the automation of the
computations of tree-level and next-to-leading order cross sections, of their
matching to parton shower simulations, and of the merging of matched samples
that differ by light-parton multiplicities. We present a computer program,
MadGraph5_aMC@NLO, capable of handling all these computations -- parton-level
fixed order, shower-matched, merged -- in a unified framework whose defining
features are flexibility, high level of parallelisation, and human intervention
limited to input physics quantities. We demonstrate the potential of the
program by presenting selected phenomenological applications relevant to the
LHC and to a 1-TeV collider. While next-to-leading order results are
restricted to QCD corrections to SM processes in the first public version, we
show that from the user viewpoint no changes have to be expected in the case of
corrections due to any given renormalisable Lagrangian, and that the
implementation of these are well under way.Comment: 158 pages, 27 figures; a few references have been adde
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