38,982 research outputs found
New Precision Electroweak Tests of SU(5) x U(1) Supergravity
We explore the one-loop electroweak radiative corrections in supergravity via explicit calculation of vacuum-polarization and
vertex-correction contributions to the and
parameters. Experimentally, these parameters are obtained from a global fit to
the set of observables , and . We
include -dependent effects, which induce a large systematic negative shift
on for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). The
(non-oblique) supersymmetric vertex corrections to \Zbb, which define the
parameter, show a significant positive shift for light chargino
masses, which for can be nearly compensated by a negative
shift from the charged Higgs contribution. We conclude that at the 90\%CL, for
m_t\lsim160\GeV the present experimental values of and
do not constrain in any way supergravity in
both no-scale and dilaton scenarios. On the other hand, for m_t\gsim160\GeV
the constraints on the parameter space become increasingly stricter. We
demonstrate this trend with a study of the m_t=170\GeV case, where only a
small region of parameter space, with \tan\beta\gsim4, remains allowed and
corresponds to light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). Thus
supergravity combined with high-precision LEP data would
suggest the presence of light charginos if the top quark is not detected at the
Tevatron.Comment: LaTeX, 11 Pages+4 Figures(not included), the figures available upon
request as an uuencoded file(0.4MB) or 4 PS files from [email protected],
CERN-TH.7078/93, CTP-TAMU-68/93, ACT-24/9
NuSTAR Observations of G11.2–0.3
We present in this paper the hard X-ray view of the pulsar wind nebula in G11.2−0.3 and its central pulsar powered pulsar J1811−1925 as seen by NuSTAR. We complement the data with Chandra for a more complete picture and confirm the existence of a hard, power-law component in the shell with photon index Γ = 2.1 ± 0.1, which we attribute to synchrotron emission. Our imaging observations of the shell show a slightly smaller radius at higher energies, consistent with Chandra results, and we find shrinkage as a function of increased energy along the jet direction, indicating that the electron outflow in the PWN may be simpler than that seen in other young PWNe. Combining NuSTAR with INTEGRAL, we find that the pulsar spectrum can be fit by a power law with Γ = 1.32 ± 0.07 up to 300 keV without evidence of curvature
Singlet fermion dark matter and electroweak baryogenesis with radiative neutrino mass
The model of radiative neutrino mass with dark matter proposed by one of us
is extended to include a real singlet scalar field. There are then two
important new consequences. One is the realistic possibility of having the
lightest neutral singlet fermion (instead of the lightest neutral component of
the dark scalar doublet) as the dark matter of the Universe. The other is a
modification of the effective Higgs potential of the Standard Model, consistent
with electroweak baryogenesis.Comment: 9 pages, no figure
Supersymmetric photonic signals at LEP
We explore and contrast the single-photon and diphoton signals expected at
LEP 2, that arise from neutralino-gravitino (e^+ e^- -> chi + gravitino ->
gamma + E_miss) and neutralino-neutralino (e^+ e^- -> chi + chi -> gamma +
gamma + E_miss) production in supersymmetric models with a light gravitino. LEP
1 limits imply that one may observe either one, but not both, of these signals
at LEP 2, depending on the values of the neutralino and gravitino masses:
single-photons for m_chi > Mz and m_gravitino < 3 x 10^-5 eV; diphotons for
m_chi < Mz and all allowed values of m_gravitino.Comment: 11 pages, LaTeX, 4 figures (included). Shortened version to appear in
Physical Review Letter
Weak positive cloud-to-ground flashes in Northeastern Colorado
The frequency distributions of the peak magnetic field associated with the first detected return stroke of positive and negative cloud-to-ground (CG) flashes were studied using lightning data from northeastern Colorado. These data were obtained during 1985 with a medium-to-high gain network of three direction finders (DF's). The median signal strength of positive flashes was almost two times that of the negatives for flashes within 300 km of the DF's, which have an inherent detection-threshold bias that tends to discriminate against weak signals. This bias increases with range, and affects the detection of positive and negative flashes in different ways, because of the differing character of their distributions. Positive flashes appear to have a large percentage of signals clustered around very weak values that are lost to the medium-to-high gain Colorado Detection System very quickly with increasing range. The resulting median for positive signals could thus appear to be much larger than the median for negative signals, which are more clustered around intermediate values. When only flashes very close to the DF's are considered, however, the two distributions have almost identical medians. The large percentage of weak positive signals detected close to the DF's has not been explored previously. They have been suggested to come from intracloud discharges and thus are improperly classified as CG flashes. Evidence in hand, points to their being real positive, albeit weak CG flashes. Whether or not they are real positive ground flashes, it is important to be aware of their presence in data from magnetic DF networks
Experimental aspects of SU(5)xU(1) supergravity
We study various aspects of supergravity as they relate to
the experimental verification or falsification of this model. We consider two
string-inspired, universal, one-parameter, no-scale soft-supersymmetry-breaking
scenarios, driven by the -terms of the moduli and dilaton fields. The model
is described in terms of the supersymmetry mass scale (\ie, the chargino mass
), , and the top-quark mass. We first determine the
combined effect on the parameter space of all presently available direct and
indirect experimental constraints, including the LEP lower bounds on sparticle
and Higgs-boson masses, the rate, the anomalous magnetic moment
of the muon, the high-precision electroweak parameters
(which imply m_t\lsim180\GeV), and the muon fluxes in underground detectors
(neutrino telescopes). For the still-allowed points in
parameter space, we re-evaluate the experimental
situation at the Tevatron, LEPII, and HERA. In the 1994 run, the Tevatron could
probe chargino masses as high as 100 GeV. At LEPII the parameter space could be
explored with probes of different resolutions: Higgs boson searches, selectron
searches, and chargino searches. Moreover, for m_t\lsim150\GeV, these
Higgs-boson searches could explore all of the allowed parameter space with
\sqrt{s}\lsim210\GeV.Comment: latex, 36 pages, 25 figures (not included). Figures are available via
anonymous ftp from hplaa02.cern.ch (/pub/lopez) as either 33 ps files
(Easpects*.ps, 8.1MB) or one uuencoded file (AllFigures.uu, 3.7MB
High speed single photon detection in the near-infrared
InGaAs avalanche photodiodes (APDs) are convenient for single photon
detection in the near-infrared (NIR) including the fibre communication bands
(1.31/1.55 m). However, to suppress afterpulse noise due to trapped
avalanche charge, they must be gated with MHz repetition frequencies, thereby
severely limiting the count rate in NIR applications. Here we show gating
frequencies for InGaAs-APDs well beyond 1 GHz. Using a self-differencing
technique to sense much weaker avalanches, we reduce drastically afterpulse
noise. At 1.25 GHz, we obtain a detection efficiency of 10.8% with an
afterpulse probability of 6.16%. In addition, the detector features low jitter
(55 ps) and a count rate of 100 MHz
Precision Electroweak Tests of the Minimal and Flipped SU(5) Supergravity Models
We explore the one-loop electroweak radiative corrections in the minimal
and the no-scale flipped supergravity models via explicit
calculation of vacuum polarization contributions to the
parameters. Experimentally, are obtained from a global fit
to the LEP observables, and measurements. We include -dependent
effects which have been neglected in most previous ``model-independent"
analyses of this type. These effects induce a large systematic negative shift
on for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV).
In agreement with previous general arguments, we find that for increasingly
large sparticle masses, the heavy sector of both models rapidly decouples, \ie,
the values for quickly asymptote to the Standard Model
values with a {\it light} Higgs (m_{H_{SM}}\sim100\GeV). Specifically, at
present the CL upper limit on the top-quark mass is m_t\lsim175\GeV in
the no-scale flipped supergravity model. These bounds can be
strengthened for increasing chargino masses in the 50-100\GeV interval. In
particular, for m_t\gsim160\GeV, the Tevatron may be able to probe through
gluino() and squark() production up to m_{\tilde g}\approx
m_{\tilde q}\approx250\GeV, exploring at least half of the parameter space in
this model.Comment: 15 pages,(6 ps figures available upon request), TeX(harvmac),
CTP-TAMU-19/93, ACT-07/9
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