9,753 research outputs found
The Evolution of Quantum Field Theory, From QED to Grand Unification
In the early 1970s, after a slow start, and lots of hurdles, Quantum Field
Theory emerged as the superior doctrine for understanding the interactions
between relativistic sub-atomic particles. After the conditions for a
relativistic field theoretical model to be renormalizable were established,
there were two other developments that quickly accelerated acceptance of this
approach: first the Brout-Englert-Higgs mechanism, and then asymptotic freedom.
Together, these gave us a complete understanding of the perturbative sector of
the theory, enough to give us a detailed picture of what is now usually called
the Standard Model. Crucial for this understanding were the strong indications
and encouragements provided by numerous experimental findings. Subsequently,
non-perturbative features of the quantum field theories were addressed, and the
first proposals for completely unified quantum field theories were launched.
Since the use of continuous symmetries of all sorts, together with other topics
of advanced mathematics, were recognised to be of crucial importance, many new
predictions were pointed out, such as the Higgs particle, supersymmetry and
baryon number violation. There are still many challenges ahead.Comment: 25 pages in total. A contribution to: The Standard Theory up to the
Higgs discovery - 60 years of CERN - L. Maiani and G. Rolandi, ed
Minimal Gauge Invariant Classes of Tree Diagrams in Gauge Theories
We describe the explicit construction of groves, the smallest gauge invariant
classes of tree Feynman diagrams in gauge theories. The construction is valid
for gauge theories with any number of group factors which may be mixed. It
requires no summation over a complete gauge group multiplet of external matter
fields. The method is therefore suitable for defining gauge invariant classes
of Feynman diagrams for processes with many observed final state particles in
the standard model and its extensions.Comment: 13 pages, RevTeX (EPS figures
Colored Resonant Signals at the LHC: Largest Rate and Simplest Topology
We study the colored resonance production at the LHC in a most general
approach. We classify the possible colored resonances based on group theory
decomposition, and construct their effective interactions with light partons.
The production cross section from annihilation of valence quarks or gluons may
be on the order of 400 - 1000 pb at LHC energies for a mass of 1 TeV with
nominal couplings, leading to the largest production rates for new physics at
the TeV scale, and simplest event topology with dijet final states. We apply
the new dijet data from the LHC experiments to put bounds on various possible
colored resonant states. The current bounds range from 0.9 to 2.7 TeV. The
formulation is readily applicable for future searches including other decay
modes.Comment: 29 pages, 9 figures. References updated and additional K-factors
include
The U(1)A anomaly in noncommutative SU(N) theories
We work out the one-loop anomaly for noncommutative SU(N) gauge
theories up to second order in the noncommutative parameter .
We set and conclude that there is no breaking of the classical
symmetry of the theory coming from the contributions that are either
linear or quadratic in . Of course, the ordinary anomalous
contributions will be still with us. We also show that the one-loop
conservation of the nonsinglet currents holds at least up to second order in
. We adapt our results to noncommutative gauge theories with
SO(N) and U(1) gauge groups.Comment: 50 pages, 5 figures in eps files. Some comments and references adde
Limitations on the superposition principle: superselection rules in non-relativistic quantum mechanics
The superposition principle is a very basic ingredient of quantum theory.
What may come as a surprise to many students, and even to many practitioners of
the quantum craft, is tha superposition has limitations imposed by certain
requirements of the theory. The discussion of such limitations arising from the
so-called superselection rules is the main purpose of this paper. Some of their
principal consequences are also discussed. The univalence, mass and particle
number superselection rules of non-relativistic quantum mechanics are also
derived using rather simple methods.Comment: 22 pages, no figure
History of Supersymmetric Extensions of the Standard Model
We recall the many obstacles which seemed, long ago, to prevent supersymmetry
from possibly being a fundamental symmetry of Nature. We also present their
solutions, leading to the construction of the supersymmetric extensions of the
Standard Model. Finally we discuss briefly the early experimental searches for
Supersymmetry.Comment: 40 pages, 10 figures, 3 Table
Quantum field theory with varying couplings
A quantum scalar field theory with spacetime-dependent coupling is studied.
Surprisingly, while translation invariance is explicitly broken in the
classical theory, momentum conservation is recovered at the quantum level for
some specific choice of the coupling's profile for any finite-order
perturbative expansion. For one of these cases, some tree and one-loop diagrams
are calculated. This is an example of a theory where violation of Lorentz
symmetry is not enhanced at the quantum level. We draw some consequences for
the renormalization properties of certain classes of fractional field theories.Comment: 12 pages. v2: discussion improved, minor typos correcte
The Glorious Days of Physics - Renormalization of Gauge theories
This is an account of the author's recollections of the turbulent days
preceding the establishment of the Standard Model as an accurate description of
all known elementary particles and forces.Comment: 21 pages plain TeX, 4 figures PostScript. Small cosmetic - yet
important - changes were made in the original manuscrip
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