630 research outputs found
Excited Baryons in Large N_c QCD Revisited: The Resonance Picture Versus Single-Quark Excitations
We analyze excited baryon properties via a 1/N_c expansion from two
perspectives: as resonances in meson-nucleon scattering, and as single-quark
excitations in the context of a simple quark model. For both types of analysis
one can derive novel patterns of degeneracy that emerge as N_c --> \infty, and
that are shown to be compatible with one another. This helps justify the
single-quark excitation picture and may give some insight into its successes.
We also find that in the large N_c limit one of the S_{11} baryons does not
couple to the pi-N channel but couples to the eta-N channel. This is
empirically observed in the N(1535), which couples very weakly to the pi-N
channel and quite strongly to the eta-N channel. The comparatively strong
coupling of the N(1650) to the pi-N channel and weak coupling to eta-N channel
is also predicted. In the context of the simple quark model picture we
reproduce expressions for mixing angles that are accurate up to O(1/N_c)
corrections and are in good agreement with mixing angles extracted
phenomenologically.Comment: 13 pages, ReVTeX
Decays of Baryons --- Quark Model versus Large-
We study nonleptonic decays of the orbitally excited, \su6 \rep{70}-plet
baryons in order to test the hypothesis that the successes of the
nonrelativistic quark model have a natural explanation in the large- limit
of QCD. By working in a Hartree approximation, we isolate a specific set of
operators that contribute to the observed s- and d-wave decays in leading order
in . We fit our results to the current experimental decay data, and make
predictions for a number of allowed but unobserved modes. Our tentative
conclusion is that there is more to the nonrelativistic quark model of baryons
than large-.Comment: LaTeX 49pp. (38 pp. landscape), PicTex, PrePicTex, PostPicTex
required for 3 figures, Harvard Preprint HUTP-94/A008. (Two additional
operators are included, but conclusions are unchanged.
Is the Lightest Kaluza-Klein Particle a Viable Dark Matter Candidate?
In models with universal extra dimensions (i.e. in which all Standard Model
fields, including fermions, propagate into compact extra dimensions) momentum
conservation in the extra dimensions leads to the conservation of Kaluza--Klein
(KK) number at each vertex. KK number is violated by loop effects because of
the orbifold imposed to reproduce the chiral Standard Model with zero modes,
however, a KK parity remains at any order in perturbation theory which leads to
the existence of a stable lightest KK particle (LKP). In addition, the
degeneracy in the KK spectrum is lifted by radiative corrections so that all
other KK particles eventually decay into the LKP. We investigate cases where
the Standard Model lives in five or six dimensions with compactification radius
of TeV size and the LKP is the first massive state in the KK tower of
either the photon or the neutrino. We derive the relic density of the LKP under
a variety of assumptions about the spectrum of first tier KK modes. We find
that both the KK photon and the KK neutrino, with masses at the TeV scale, may
have appropriate annihilation cross sections to account for the dark matter,
.Comment: 31 pages, 12 figures; v2: A couple of references added, a few minor
clarifications (on KK parity and on the 6d case
Chasing the identification of ASCA Galactic Objects (ChIcAGO): An X-ray survey of unidentified sources in the galactic plane. I : Source sample and initial results
We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the Fx 10-13 to 10-11 erg cm -2 s-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multiwavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3′ of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the Fx 10-13 to 10-11 erg cm -2 s-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding wind binaries, X-ray binaries, and magnetars. There is also another primary population that is still unidentified but, on the basis of its X-ray and infrared properties, likely comprises partly Galactic sources and partly active galactic nuclei.Peer reviewedSubmitted Versio
Running into New Territory in SUSY Parameter Space
The LEP-II bound on the light Higgs mass rules out the vast majority of
parameter space left to the Minimal Supersymmetric Standard Model (MSSM) with
weak-scale soft-masses. This suggests the importance of exploring extensions of
the MSSM with non-minimal Higgs physics. In this article, we explore a theory
with an additional singlet superfield and an extended gauge sector. The theory
has a number of novel features compared to both the MSSM and Next-to-MSSM,
including easily realizing a light CP-even Higgs mass consistent with LEP-II
limits, tan(beta) < 1, and a lightest Higgs which is charged. These features
are achieved while remaining consistent with perturbative unification and
without large stop-masses. Discovery modes at the Tevatron and LHC are
discussed.Comment: 15 pages, 5 figures; Typo in equation (4.5) corrected; submitted to
JHE
Two-Boson Exchange Physics: A Brief Review
Current status of the two-boson exchange contributions to elastic
electron-proton scattering, both for parity conserving and parity-violating, is
briefly reviewed. How the discrepancy in the extraction of elastic nucleon form
factors between unpolarized Rosenbluth and polarization transfer experiments
can be understood, in large part, by the two-photon exchange corrections is
discussed. We also illustrate how the measurement of the ratio between
positron-proton and electron-proton scattering can be used to differentiate
different models of two-photon exchange. For the parity-violating
electron-proton scattering, the interest is on how the two-boson exchange
(TBE), \gamma Z-exchange in particular, could affect the extraction of the
long-sought strangeness form factors. Various calculations all indicate that
the magnitudes of effect of TBE on the extraction of strangeness form factors
is small, though can be large percentage-wise in certain kinematics.Comment: 6 pages, 5 figures, prepared for Proceedings of the fifth
Asia-Pacific Conference on Few-Body Problems in Physics (APFB2011), Seoul,
Korea, August 22-26, 2011, to appear in Few-Body Systems, November 201
Detection and Localization Sensor Assignment with Exact and Fuzzy Locations
Sensor networks introduce new resource allocation problems in which sensors need to be assigned to the tasks they best help. Such problems have been previously studied in simplified models in which utility from multiple sensors
is assumed to combine additively. In this paper we study more complex utility models, focusing on two particular applications: event detection and target localization.
We develop distributed algorithms to assign directional sensors of different types to multiple simultaneous tasks using exact location information. We extend our algorithms by introducing the concept of fuzzy location which may
be desirable to reduce computational overhead and/or to preserve location privacy. We show that our schemes perform well using both exact or fuzzy location information
Polarization observables in the reaction
We study the reaction slightly above the threshold within an
extended one-boson exchange model which also accounts for knock-out. It
is shown that polarization observables, like the beam-target asymmetry, are
sensible quantities for identifying a admixture in the nucleon wave
function on the few per cent level.Comment: 11 LaTeX pages including 4 ps figure
Physics searches at the LHC
With the LHC up and running, the focus of experimental and theoretical high
energy physics will soon turn to an interpretation of LHC data in terms of the
physics of electroweak symmetry breaking and the TeV scale. We present here a
broad review of models for new TeV-scale physics and their LHC signatures. In
addition, we discuss possible new physics signatures and describe how they can
be linked to specific models of physics beyond the Standard Model. Finally, we
illustrate how the LHC era could culminate in a detailed understanding of the
underlying principles of TeV-scale physics.Comment: 184 pages, 55 figures, 14 tables, hundreds of references; scientific
feedback is welcome and encouraged. v2: text, references and Overview Table
added; feedback still welcom
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