214 research outputs found
Weyl-van-der-Waerden formalism for helicity amplitudes of massive particles
The Weyl-van-der-Waerden spinor technique for calculating helicity amplitudes
of massive and massless particles is presented in a form that is particularly
well suited to a direct implementation in computer algebra. Moreover, we
explain how to exploit discrete symmetries and how to avoid unphysical poles in
amplitudes in practice. The efficiency of the formalism is demonstrated by
giving explicit compact results for the helicity amplitudes of the processes
gamma gamma -> f fbar, f fbar -> gamma gamma gamma, mu^- mu^+ -> f fbar gamma.Comment: 24 pages, late
Multiphoton Production at High Energies in the Standard Model I
We examine multiphoton production in the electroweak sector of the Standard
Model in the high energy limit using the equivalence theorem in combination
with spinor helicity techniques. We obtain recursion relations for currents
consisting of a charged scalar, spinor, or vector line that radiates
photons. Closed form solutions to these recursion relations for arbitrary
are presented for the cases of like-helicity and one unlike-helicity photon
production. We apply the currents singly and in pairs to obtain amplitudes for
processes involving the production of photons with up to two unlike
helicities from a pair of charged particles. The replacement of one or more
photons by transversely polarized Z$-bosons is also discussed.Comment: 75 pages, CLNS 91/111
Multi-gluon helicity amplitudes with one off-shell leg within high energy factorization
Basing on the Slavnov-Taylor identities, we derive a new prescription to
obtain gauge invariant tree-level scattering amplitudes for the process g*g->Ng
within high energy factorization. Using the helicity method, we check the
formalism up to several final state gluons, and we present analytical formulas
for the the helicity amplitudes for N=2. We also compare the method with
Lipatov's effective action approach.Comment: 25 pages, quite a few figures, an appendix added, typos correcte
A New Approach for Analytic Amplitude Calculations
We present a method for symbolic calculation of Feynman amplitudes for
processes involving both massless and massive fermions. With this approach
fermion strings in a specific amplitude can be easily evaluated and expressed
as basic Lorentz scalars. The new approach renders the symbolic calculation of
some complicated physical processes more feasible and easier, especially with
the assistance of algebra manipulating codes for computer.Comment: LaTex, no figure, to appear in PR
Quantum Extremism: Effective Potential and Extremal Paths
The reality and convexity of the effective potential in quantum field
theories has been studied extensively in the context of Euclidean space-time.
It has been shown that canonical and path-integral approaches may yield
different results, thus resolving the `convexity problem'. We discuss the
transferral of these treatments to Minkowskian space-time, which also
necessitates a careful discussion of precisely which field configurations give
the dominant contributions to the path integral. In particular, we study the
effective potential for the N=1 linear sigma model.Comment: 11 pages, 4 figure
One-loop corrections to the D3 brane action
We calculate one-loop corrections to the effective Lagrangian for the D3
brane. We perform the gauge-fixing of the kappa-symmetric Born-Infeld D3 brane
action in the flat background using Killing gauge. The linearized supersymmetry
of the gauge-fixed action coincides with that of the N=4 Yang-Mills theory. We
use the helicity amplitude and unitarity technique to calculate the one-loop
amplitudes at order alpha^4. The counterterms and the finite 1-loop corrections
are of the form (dF)^4 and their supersymmetric generalization. This is to be
contrasted with the Born-Infeld action which contains (F)^4 and other terms
which do not depend on derivatives of the vector field strength.Comment: 21 pages, LaTex with Axodraw figures. In the revised version new
references have been adde
Leptonic - and -decays: mass effects, polarization effects and radiative corrections
We calculate the radiative corrections to the unpolarized and the four
polarized spectrum and rate functions in the leptonic decay of a polarized into a polarized electron. The new feature of our calculation is that we
keep the mass of the final state electron finite which is mandatory if one
wants to investigate the threshold region of the decay. Analytical results are
given for the energy spectrum and the polar angle distribution of the final
state electron whose longitudinal and transverse polarization is calculated. We
also provide analytical results on the integrated spectrum functions. We
analyze the limit of our general results and investigate the
quality of the approximation. In the case we
discuss in some detail the role of the anomalous helicity flip
contribution of the final electron which survives the limit. The
results presented in this 0203048 also apply to the leptonic decays of
polarized -leptons for which we provide numerical results.Comment: 39 pages, 11 postscript figures added. Updated version. Four
references added. A few text improvements. Final version to appear in
Phys.Rev.
Laser Doppler Flowmetry Combined with Spectroscopy to Determine Peripheral Tissue Perfusion and Oxygen Saturation:A Pilot Study in Healthy Volunteers and Patients with Peripheral Arterial Disease
Background: In this study, we assessed the ability of the EPOS system (Perimed AB, Jarfalla, Stockholm, Sweden) to detect differences in tissue perfusion between healthy volunteers and patients with peripheral arterial disease (PAD) with different severity of disease. Methods: This singlecenter prospective pilot study included 10 healthy volunteers and 20 patients with PAD scheduled for endovascular therapy (EVT). EPOS measurements were performed at rest at 32 degrees C and 44 degrees C, followed by transcutaneous oxygen pressure (TcPo2) measurements. The measurements were performed on the dorsal and medial side of the foot, as well as the lateral side of the calf. EPOS parameters included hemoglobin oxygen saturation (HbSO(2)) and speed-resolved red blood cell (RBC) perfusion. Results: HbSO(2) at 44 degrees C was significantly different between the three groups for all measurement locations. The overall speed-resolved RBC perfusion at 44 degrees C was statistically significant between the groups on the dorsal and medial side of the foot but not on the calf. TcPO2 values were not significantly different between the three groups. Conclusions: This study demonstrates that the EPOS system can depict differences in tissue perfusion between healthy volunteers, patients with Fontaine class IIb PAD, and those with Fontaine class III or IV PAD but only after heating to 44 degrees C
The Ratio of W + N jets To Z/gamma + N jets As a Precision Test of the Standard Model
We suggest replacing measurements of the individual cross-sections for the
production of W + N jets and Z/gamma + N jets in searches for new high-energy
phenomena at hadron colliders by the precision measurement of the ratios (W+0
jet)/(Z+0 jet), (W+1 jet)/(Z+1 jet), (W+2 jets)/(Z+2 jets),... (W+N jets)/(Z+N
jets), with N as large as 6 (the number of jets in ttbarH). These ratios can
also be formed for the case where one or more of the jets is tagged as a b or c
quark. Existing measurements of the individual cross sections for Wenu + N jets
at the Tevatron have systematic uncertainties that grow rapidly with N, being
dominated by uncertainties in the identification of jets and the jet energy
scale. These systematics, and also those associated with the luminosity, parton
distribution functions (PDF's), detector acceptance and efficiencies, and
systematics of jet finding and b-tagging, are expected to substantially cancel
in calculating the ratio of W to Z production in each N-jet channel, allowing a
greater sensitivity to new contributions in these channels in Run II at the
Tevatron and at the LHC.Comment: 10 pages, 8 figures, added reference
- …