322 research outputs found
Two-photon exchange and elastic electron-proton scattering
Two-photon exchange contributions to elastic electron-proton scattering cross
sections are evaluated in a simple hadronic model including the finite size of
the proton. The corrections are found to be small in magnitude, but with a
strong angular dependence at fixed . This is significant for the
Rosenbluth technique for determining the ratio of the electric and magnetic
form factors of the proton at high , and partly reconciles the apparent
discrepancy with the results of the polarization transfer technique.Comment: 4 pages, 4 figures, v2: additional references and minor
clarifications in text, accepted in Physical Review Letter
How unusual was autumn 2006 in Europe?
The temperatures in large parts of Europe have been record high during the
meteorological autumn of 2006. Compared to 1961-1990, the 2m temperature was
more than three degrees Celsius above normal from the North side of the Alps to
southern Norway. This made it by far the warmest autumn on record in the United
Kingdom, Belgium, the Netherlands, Denmark, Germany and Switzerland, with the
records in Central England going back to 1659, in the Netherlands to 1706 and
in Denmark to 1768.
Assuming that the mean of the temperature distribution changes proportional
to the global mean temperature, but the shape remains the same includes to
first order the effects of global warming. Even under this assumption the
autumn temperatures were very unusual, with estimates of the return time of 200
to 2000 years in this region. The lower bound of the 95% confidence interval is
more than 100 to 300 years.
Climate models that simulate the current atmospheric circulation well
underestimate the observed mean rise in autumn temperatures. They do not
simulate a change in the shape of the distribution that would increase the
probability of warm events under global warming. This implies that the warm
autumn 2006 either was a very rare coincidence, or the local temperature rise
is much stronger than modelled, or non-linear physics that is missing from
these models increases the probability of warm extremes
Large-scale atmospheric circulation biases and changes in global climate model simulations and their importance for climate change in Central Europe
The quality of global sea level pressure patterns has been assessed for simulations by 23 coupled climate models. Most models showed high pattern correlations. With respect to the explained spatial variance, many models showed serious large-scale deficiencies, especially at mid-latitudes. Five models performed well at all latitudes and for each month of the year. Three models had a reasonable skill. <p style='line-height: 20px;'> We selected the five models with the best pressure patterns for a more detailed assessment of their simulations of the climate in Central Europe. We analysed observations and simulations of monthly mean geostrophic flow indices and of monthly mean temperature and precipitation. We used three geostrophic flow indices: the west component and south component of the geostrophic wind at the surface and the geostrophic vorticity. We found that circulation biases were important, and affected precipitation in particular. Apart from these circulation biases, the models showed other biases in temperature and precipitation, which were for some models larger than the circulation induced biases. <p style='line-height: 20px;'> For the 21st century the five models simulated quite different changes in circulation, precipitation and temperature. Precipitation changes appear to be primarily caused by circulation changes. Since the models show widely different circulation changes, especially in late summer, precipitation changes vary widely between the models as well. Some models simulate severe drying in late summer, while one model simulates significant precipitation increases in late summer. With respect to the mean temperature the circulation changes were important, but not dominant. However, changes in the distribution of monthly mean temperatures, do show large indirect influences of circulation changes. Especially in late summer, two models simulate very strong warming of warm months, which can be attributed to severe summer drying in the simulations by these models. The models differ also significantly in the simulated warming of cold winter months. Finally, the models simulate rather different changes in North Atlantic sea surface temperature, which is likely to impact on changes in temperature and precipitation. These results imply that several important aspects of climate change in Central Europe are highly uncertain. Other aspects of the simulated climate change appear to be more robust. All models simulate significant warming all year round and an increase in precipitation in the winter half-year
New Supersymmetric Contributions to
We calculate the electroweak-like one-loop supersymmetric contributions to
the rare and flavor-violating decay of the top quark into a charm quark and a
gauge boson: , with . We consider loops of both charginos
and down-like squarks (where we identify and correct an error in the
literature) and neutralinos and up-like squarks (which have not been calculated
before). We also account for left-right and generational squark mixing. Our
numerical results indicate that supersymmetric contributions to can be
upto 5 orders of magnitude larger than their Standard Model counterparts.
However, they still fall short of the sensitivity expected at the
next-generation top-quark factories.Comment: 13 pages, LaTex, 1 figure included. Final version to appear in
Physical Review D. Chargino contribution dealt with in greater detail. Minor
revisions in tex
Scaling and trends of hourly precipitation extremes in two different climate zones â Hong Kong and the Netherlands
Hourly precipitation extremes in very long time series from the Hong Kong Observatory and the Netherlands are investigated. Using the 2 m dew point temperature from 4 h before the rainfall event as a measure of near surface absolute humidity, hourly precipitation extremes closely follow a 14% per degree dependency â a scaling twice as large as following from the Clausius-Clapeyron relation. However, for dew point temperatures above 23 °C no significant dependency on humidity was found. Strikingly, in spite of the large difference in climate, results are almost identical in Hong Kong and the Netherlands for the dew point temperature range where both observational sets have sufficient data. Trends in hourly precipitation extremes show substantial increases over the last century for both De Bilt (the Netherlands) and Hong Kong. For De Bilt, not only the long term trend, but also variations in hourly precipitation extremes on an inter-decadal timescale of 30 yr and longer, can be linked very well to the above scaling; there is a very close resemblance between variations in dew point temperature and precipitation intensity with an inferred dependency of hourly precipitation extremes of 10 to 14% per degree. For Hong Kong there is no connection between variations in humidity and those in precipitation intensity in the wet season, May to September. This is consistent with the found zero-dependency of precipitation intensity on humidity for dew points above 23 °C. Yet, outside the wet season humidity changes do appear to explain the positive trend in hourly precipitation extremes, again following a dependency close to twice the Clausius-Clapeyron relation
QCD Corrections to Associated Higgs Boson-Heavy Quark Production
We compute the O(alpha_s) QCD corrections to the inclusive process e^+e^- -->
t \bar t h. Although the total rate is small, it has a distinctive experimental
signature and can potentially be used to measure the top quark-Higgs boson
Yukawa coupling. The QCD corrections increase the rate by a factor of roughly
1.5 for e^+e^- --> t \bar t h at sqrt(s)=500 GeV and Mh=100 GeV. At sqrt(s)=1
TeV, the corrections are small.Comment: 18 pages, 11 figures, version to appear in Phys. Rev.
CP Violation In Single Top Production And Decay Via p p-bar -> t b-bar +X -> W^+ b b-bar +X Within The MSSM: A Possible Application For Measuring \arg(A_t) At Hadron Colliders
CP-nonconserving effects in the reaction p p-bar -> t b-bar +X -> W^+ b b-bar
+X, driven by the supersymmetric CP-odd phase of the stop trilinear soft
breaking term, \arg(A_t), are studied. We discuss the CP-nonconserving effects
in both production and the associated decay amplitudes of the top. We find
that, within a plausible low energy scenario of the MSSM and keeping the
neutron electric dipole moment below its current limit, a CP-violating
cross-section asymmetry as large as 2-3% can arise if some of the parameters
lie in a favorable range. A partial rate asymmetry originating only in the top
decay t -> W^+ b is found to be, in general, below the 0.1% level which is
somewhat smaller than previous claims. For a low \tan\beta of order one the
decay asymmetry can reach at the most ~0.3%. This (few) percent level overall
CP-violating signal in p p-bar -> t b-bar +X -> W^+ b b-bar +X might be within
the reach of the future 2(4) TeV pp-bar Tevatron collider that may be able to
produce ~10000(~30000) such tb-bar events with an integrated luminosity of 30
fb^{-1}. In particular, it may be used to place an upper bound on \arg(A_t) if
indeed \arg(\mu) -> 0, as implied from the present experimental limit on the
neutron electric dipole moment. The partial rate asymmetry in the top decay
(~few \times 10^{-3}) may also be within the reach of the LHC with ~10^7 pairs
of tt-bar produced, provided detector systematics are sufficiently small. We
also show that if the GUT-scale universality of the soft breaking trilinear
terms is relaxed, then the phases associated with \arg A_u and \arg A_d can
take values up to ~few \times 10^{-1} even with squarks and gluino masses of
several hundred GeV's without contradicting the experimental limit on the
neutron electric dipole moment.Comment: 39 pages, plain latex, 20 figures embadded in the text using epsfi
Quark-Loop Amplitudes for W^+- H^-+ Associated Hadroproduction
In this addendum to our paper entitled "W^+- H^-+ Associated Production at
the Large Hadron Collider" [Phys. Rev. D 59, 015009 (1999)], we list analytic
results for the helicity amplitudes of the partonic subprocess gg -> W^-H^+
induced by virtual quarks.Comment: 6 pages (Latex
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