1,238 research outputs found
Multiloop String-Like Formulas for QED
Multiloop gauge-theory amplitudes written in the Feynman-parameter
representation are poised to take advantage of two important developments of
the last decade: the spinor-helicity technique and the superstring
reorganization. The former has been considered in a previous article; the
latter will be elaborated in this paper. We show here how to write multiloop
string-like formulas in the Feynman-parameter representation for any process in
QED, including those involving other non-electromagnetic interactions. The
general connection between the Feynman-parameter approach and the
superstring/first-quantized approach is discussed. In the special case of a
one-loop multi-photon amplitude, these formulas reduce to the ones obtained by
the superstring and the first quantized methods. The string-like formulas
exhibits a simple gauge structure which makes the Ward-Takahashi identity
apparent, and enables the integration-by-parts technique of Bern and Kosower to
be applied, so that gauge-invariant parts can be extracted diagram-by-diagram
with the seagull vertex neglected.Comment: 25 pages in Plain Tex, plus four figures in a postscript file;
McGill/92-5
Sideband cooling while preserving coherences in the nuclear spin state in group-II-like atoms
We propose a method for laser cooling group-II-like atoms without changing
the quantum state of their nuclear spins, thus preserving coherences that are
usually destroyed by optical pumping. As group-II-like atoms have a
closed-shell ground state, nuclear spin and electronic degrees of freedom are
decoupled, allowing for independent manipulation. The hyperfine interaction
that couples these degrees of freedom in excited states can be suppressed
through the application of external magnetic fields. Our protocol employs
resolved-sideband cooling on the forbidden clock transition, , with quenching via coupling to the rapidly decaying state,
deep in the Paschen-Back regime. This makes it possible to laser cool neutral
atomic qubits without destroying the quantum information stored in their
nuclear spins, as shown in two examples, Yb and Sr.Comment: 4 pages, 3 figures v4: minor changes in text, changes in the
references, published versio
Generalized Gluon Currents and Applications in QCD
We consider the process containing two quark lines and an arbitrary number of
gluons in a spinor helicity framework. A current with two off-shell gluons
appears in the amplitude. We first study this modified gluon current using
recursion relations. The recursion relation for the modified gluon current is
solved for the case of like-helicity gluons. We apply the modified gluon
current to compute the amplitude for in the like-helicity gluon case.Comment: 80 pages, 2 figures (appended in pictex), CLNS 91/112
One Loop Multiphoton Helicity Amplitudes
We use the solutions to the recursion relations for double-off-shell fermion
currents to compute helicity amplitudes for -photon scattering and
electron-positron annihilation to photons in the massless limit of QED. The
form of these solutions is simple enough to allow {\it all}\ of the
integrations to be performed explicitly. For -photon scattering, we find
that unless , the amplitudes for the helicity configurations (+++...+) and
(-++...+) vanish to one-loop order.Comment: 27 pages + 4 uuencoded figures (included), Fermilab-Pub-93/327-T,
RevTe
Multigluon Helicity Amplitudes Involving a Quark Loop
We apply the solution to the recursion relation for the double-off-shell
quark current to the problem of computing one loop amplitudes with an arbitrary
number of gluons. We are able to compute amplitudes for photon-gluon
scattering, electron-positron annihilation to gluons, and gluon-gluon
scattering via a quark loop in the case of like-helicity gluons. In addition,
we present the result for the one-loop gluon-gluon scattering amplitude when
one of the gluons has opposite helicity from the others.Comment: 31 pages (RevTeX) + 2 uuencoded figures (included),
Fermilab-Pub-93/389-
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
The Analysis of Multijet Events Produced at High Energy Hadron Colliders
We define and discuss a set of (4N - 4) parameters that can be used to
analyse events in which N jets have been produced in high energy hadron-hadron
collisions. These multijet variables are the multijet mass and (4N - 5)
independent dimensionless parameters. To illustrate the use of the variables
QCD predictions are presented for events with up to five jets produced at the
Fermilab Tevatron Proton-Antiproton Collider. These QCD predictions are
compared with the predictions of a model in which multijet events uniformly
populate the N-body phase-space
Detection of Neutral MSSM Higgs Bosons at LEP-II and NLC
We study the possibility of detecting the neutral Higgs bosons predicted in
the Minimal Supersymmetric Standard Model (h0, H0, A0), with the reactions e+
e- --> b b h0 (H0, A0), using the helicity formalism. We analyze the region of
parameter space (m_A0-tan beta) where h0(H0, A0) could be detected in the limit
when tan beta is large. The numerical computation is done for the energy which
is expected to be available at LEP-II (sqrt{s} = 200 GeV) and for a possible
Next Linear e+ e- Collider (sqrt{s}=500 GeV).Comment: To be published in Phys.Rev.
Tree-Level Unitarity Constraints on the Gravitational Couplings of Higher-Spin Massive Fields
We analyse the high-energy behavior of tree-level graviton Compton amplitudes
for particles of mass m and arbitrary spin, concentrating on a combination of
forward amplitudes that will be unaffected by eventual cross- couplings to
other, higher spins. We first show that for any spin larger than 2, tree-level
unitarity is already violated at energies well below the Planck scale M, if m
<< M. We then restore unitarity to this amplitude up to M by adding non-minimal
couplings that depend on the curvature and its derivatives, and modify the
minimal description - including particle gravitational quadrupole moments - at
scales O(1/m).Comment: 12 pages (Latex file, needs FEYNMAN macros), IASSNS-HEP-94/63,
NYU-TH-94/05/01, CERN-TH.7388/9
On the Cause of the Mid‐Pleistocene Transition
The Mid-Pleistocene Transition (MPT), where the Pleistocene glacial cycles changed from 41 to ∼100 kyr periodicity, is one of the most intriguing unsolved issues in the field of paleoclimatology. Over the course of over four decades of research, several different physical mechanisms have been proposed to explain the MPT, involving non-linear feedbacks between ice sheets and the global climate, the solid Earth, ocean circulation, and the carbon cycle. Here, we review these different mechanisms, comparing how each of them relates to the others, and to the currently available observational evidence. Based on this discussion, we identify the most important gaps in our current understanding of the MPT. We discuss how new model experiments, which focus on the quantitative differences between the different physical mechanisms, could help fill these gaps. The results of those experiments could help interpret available proxy evidence, as well as new evidence that is expected to become available
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