1,907 research outputs found

### Effective Field Theory, Past and Future

This is a written version of the opening talk at the 6th International
Workshop on Chiral Dynamics, at the University of Bern, Switzerland, July 6,
2009, to be published in the proceedings of the Workshop. In it, I reminisce
about the early development of effective field theories of the strong
interactions, comment briefly on some other applications of effective field
theories, and then take up the idea that the Standard Model and General
Relativity are the leading terms in an effective field theory. Finally, I cite
recent calculations that suggest that the effective field theory of gravitation
and matter is asymptotically safe.Comment: Correction to footnote 29 and associated text. 21 page

### Soft Bremsstrahlung

Simple analytic formulas are considered for the energy radiated in low
frequency bremsstrahlung from fully ionized gases. A formula that has been
frequently cited over many years turns out to have only a limited range of
validity, more narrow than for a formula derived using the Born approximation.
In an attempt to find a more widely valid simple formula, a soft photon theorem
is employed, which in this context implies that the differential rate of photon
emission in an electron-ion collision with definite initial and final electron
momenta is correctly given for sufficiently soft photons by the Born
approximation, to all orders in the Coulomb potential. Corrections to the Born
approximation arise because the upper limit on photon energy for this theorem
to apply to a given collision becomes increasingly stringent as the scattering
approaches the forward direction. A general formula is suggested that takes
this into account.Comment: 14 pages, no figure

### The Cosmological Constant Problems (Talk given at Dark Matter 2000, February, 2000)

The old cosmological constant problem is to understand why the vacuum energy
is so small; the new problem is to understand why it is comparable to the
present mass density. Several approaches to these problems are reviewed.
Quintessence does not help with either; anthropic considerations offer a
possibility of solving both. In theories with a scalar field that takes random
initial values, the anthropic principle may apply to the cosmological constant,
but probably to nothing else.Comment: 10 pages, Late

### Living in the Multiverse

This is the written version of the opening talk at the symposium
"Expectations of a Final Theory," at Trinity College, Cambridge, on September
2, 2005. It is to be published in Universe or Multiverse?, ed. B. Carr
(Cambridge University Press).Comment: 13 page

### Tetraquark Mesons in Large $N$ Quantum Chromodynamics

It is argued that exotic mesons consisting of two quarks and two antiquarks
are not ruled out in quantum chromodynamics with a large number $N$ of colors,
as generally thought. They can come in two varieties: short-lived tetraquarks
with decay rates proportional to $N$, which would be unobservable if $N$ were
sufficiently large, and long-lived tetraquarks with decay rates proportional to
1/N. The $f_0(500)$ and $f_0(980)$ may be examples of these two varieties of
exotic mesons.Comment: 6 page

### What is Quantum Field Theory, and What Did We Think It Is?

This is a talk presented at the conference ``Historical and Philosophical
Reflections on the Foundations of Quantum Field Theory,'' at Boston University,
March 1996. It will be published in the proceedings of this conference.Comment: 17 pages, LaTeX, no macros needed, no figure

### Minimal fields of canonical dimensionality are free

It is shown that in a scale-invariant relativistic field theory, any field
$\psi_n$ belonging to the $(j,0)$ or $(0,j)$ representations of the Lorentz
group and with dimensionality $d=j+1$ is a free field. For other field types
there is no value of the dimensionality that guarantees that the field is free.
Conformal invariance is not used in the proof of these results, but it gives
them a special interest; as already known and as shown here in an appendix, the
only fields in a conformal field theory that can describe massless particles
belong to the $(j,0)$ or $(0,j)$ representations of the Lorentz group and have
dimensionality $d=j+1$. Hence in conformal field theories massless particles
are free.Comment: 14 page

### Quantum Mechanics Without State Vectors

It is proposed to give up the description of physical states in terms of
ensembles of state vectors with various probabilities, relying instead solely
on the density matrix as the description of reality. With this definition of a
physical state, even in entangled states nothing that is done in one isolated
system can instantaneously effect the physical state of a distant isolated
system. This change in the description of physical states opens up a large
variety of new ways that the density matrix may transform under various
symmetries, different from the unitary transformations of ordinary quantum
mechanics. Such new transformation properties have been explored before, but so
far only for the symmetry of time translations into the future, treated as a
semi-group. Here new transformation properties are studied for general symmetry
transformations forming groups, rather than semi-groups. Arguments are given
that such symmetries should act on the density matrix as in ordinary quantum
mechanics, but loopholes are found for all of these arguments.Comment: 28 page

### Three-Body Interactions Among Nucleons and Pions

A chiral invariant effective Lagrangian may be used to calculate the
three-body interactions among low-energy pions and nucleons in terms of known
parameters. This method is illustrated by the calculation of the pion-nucleus
scattering length.Comment: 16 pages plus four figures, LATEX, Texas preprint UTTG-11-9

### Conference Summary

Talk presented at the XXVI International Conference on High Energy Physics,
Dallas, Texas, August, 12, 1992.Comment: 40 page

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