68 research outputs found
Neutrino Physics (theory)
Nonzero neutrino masses are the first definitive need to extend the standard
model. After reviewing the basic framework, I describe the status of some of
the major issues, including tests of the basic framework of neutrino masses and
mixings; the question of Majorana vs. Dirac; the spectrum, mixings, and number
of neutrinos; models, with special emphasis on constraints from typical
superstring constructions (which are not consistent with popular bottom-up
assumptions); and other implications.Comment: 13 pages, 6 figures, invited plenary talk at ICHEP200
Phenomenological Implications of Supersymmetric Family Non-universal U(1)-prime Models
We construct a class of anomaly-free supersymmetric U(1)' models that are
characterized by family non-universal U(1)' charges motivated from E_6
embeddings. The family non-universality arises from an interchange of the
standard roles of the two SU(5) 5* representations within the 27 of E_6 for the
third generation. We analyze U(1)' and electroweak symmetry breaking and
present the particle mass spectrum. The models, which include additional Higgs
multiplets and exotic quarks at the TeV scale, result in specific patterns of
flavor-changing neutral currents in the b to s transitions that can accommodate
the presently observed deviations inthis sector from the SM predictions.Comment: 25 pages, 3 figure
Electroweak interactions
The theory and phenomenology of electroweak interactions and their role in establishing the standard model and searching for new physics will be described. Topics will include the structure of the standard model, spontaneous symmetry breaking, renormalization and radiative corrections, QED, the weak charged current, flavor changing effects, the weak neutral current and Z-pole physics. The implications of these for searches for new physics, including grand unification, supersymmetry, compositeness, dynamical symmetry-breaking, extended gauge structure and exotic particles will be developed
Recent Developments in Precision Electroweak Physics
Developments in precision electroweak physics in the two years since the
symposium are briefly summarized.Comment: Update on recent developments, prepared for the publication of the
Proceedings of Alberto Sirlin Symposium, New York University, October 2000.
10 pages, 1 figur
Massive Neutrinos and (Heterotic) String Theory
String theories in principle address the origin and values of the quark and
lepton masses. Perhaps the small values of neutrino masses could be explained
generically in string theory even if it is more difficult to calculate
individual values, or perhaps some string constructions could be favored by
generating small neutrino masses. We examine this issue in the context of the
well-known three-family standard-like Z_3 heterotic orbifolds, where the theory
is well enough known to construct the corresponding operators allowed by string
selection rules, and analyze the D- and F-flatness conditions. Surprisingly, we
find that a simple see-saw mechanism does not arise. It is not clear whether
this is a property of this construction, or of orbifolds more generally, or of
string theory itself. Extended see-saw mechanisms may be allowed; more analysis
will be needed to settle that issue. We briefly speculate on their form if
allowed and on the possibility of alternatives, such as small Dirac masses and
triplet see-saws. The smallness of neutrino masses may be a powerful probe of
string constructions in general. We also find further evidence that there are
only 20 inequivalent models in this class, which affects the counting of string
vacua.Comment: 18 pages in RevTeX format. Single-column postscript version available
at http://sage.hep.upenn.edu/~bnelson/singpre.p
The Z-Z' Mass Hierarchy in a Supersymmetric Model with a Secluded U(1)'-Breaking Sector
We consider the Z'/Z mass hierarchy in a supersymmetric model in which the
U(1)' is broken in a secluded sector coupled to the ordinary sector only by
gauge and possibly soft terms. A large mass hierarchy can be achieved while
maintaining the normal sparticle spectra if there is a direction in which the
tree level potential becomes flat when a particular Yukawa coupling vanishes.
We describe the conditions needed for the desired breaking pattern, to avoid
unwanted global symmetries, and for an acceptable effective mu parameter. The
electroweak breaking is dominated by A terms rather than scalar masses, leading
to tan beta ~ 1. The spectrum of the symmetry breaking sector is displayed.
There is significant mixing between the MSSM particles and new standard model
singlets, for both the Higgs scalars and the neutralinos. A larger Yukawa
coupling for the effective mu parameter is allowed than in the NMSSM because of
the U(1)' contribution to the running from a high scale. The upper bound on the
tree-level mass of the lightest CP even Higgs doublet mass is about c x 174
GeV, where c is of order unity, but the actual mass eigenvalues are generally
smaller because of singlet mixing.Comment: Latex, 12 Tables, 22 page
Higgs Sector in Extensions of the MSSM
Extensions of the Minimal Supersymmetric Standard Model (MSSM) with
additional singlet scalar fields solve the important mu-parameter fine tuning
problem of the MSSM. We compute and compare the neutral Higgs boson mass
spectra, including one-loop corrections, of the following MSSM extensions:
Next-to-Minimal Supersymmetric Standard Model (NMSSM), the nearly-Minimal
Supersymmetric Standard Model (nMSSM), and the U(1)'-extended Minimal
Supersymmetric Standard Model (UMSSM) by performing scans over model
parameters. We find that the Secluded U(1)'-extended Minimal Supersymmetric
Standard Model (sMSSM) is identical to the nMSSM if three of the additional
scalars decouple. The dominant part of the one-loop corrections are
model-independent since the singlet field does not couple to MSSM particles
other than the Higgs doublets. Thus, model-dependent parameters enter the
masses only at tree-level. We apply constraints from LEP bounds on the Standard
Model and MSSM Higgs boson masses and the MSSM chargino mass, the invisible Z
decay width, and the Z-Z' mixing angle. Some extended models permit a Higgs
boson with mass substantially below the SM LEP limit or above theoretical
limits in the MSSM. Ways to differentiate the models via masses, couplings,
decays and production of the Higgs bosons are discussed.Comment: 65 pages, 15 figures. Figure replaced and typos corrected. Version to
appear in Phys. Rev.
Indications for an Extra Neutral Gauge Boson in Electroweak Precision Data
A new analysis of the hadronic peak cross section at LEP 1 implies a small
amount of missing invisible width in Z decays, while the effective weak charge
in atomic parity violation has been determined recently to 0.6% accuracy,
indicating a significantly negative S parameter. As a consequence of these two
deviations, the data are described well if the presence of an additional Z'
boson, such as predicted in Grand Unified Theories, is assumed. Moreover, the
data are now rich enough to study an arbitrary extra Z' boson and to determine
its couplings in a model independent way. An excellent best fit to the data is
obtained in this case, suggesting the possibility of a family non-universal Z'
with properties similar to ones predicted in a class of superstring theories.Comment: 5 pages of ReVTeX, 2 figure
Phenomenology of A Three-Family Standard-like String Model
We discuss the phenomenology of a three-family supersymmetric Standard-like
Model derived from the orientifold construction, in which the ordinary chiral
states are localized at the intersection of branes at angles. In addition to
the Standard Model group, there are two additional U(1)' symmetries, one of
which has family non-universal and therefore flavor changing couplings, and a
quasi-hidden non-abelian sector which becomes strongly coupled above the
electroweak scale. The perturbative spectrum contains a fourth family of exotic
(SU(2)- singlet) quarks and leptons, in which, however, the left-chiral states
have unphysical electric charges. It is argued that these decouple from the low
energy spectrum due to hidden sector charge confinement, and that anomaly
matching requires the physical left-chiral states to be composites. The model
has multiple Higgs doublets and additional exotic states. The moduli-dependent
predictions for the gauge couplings are discussed. The strong coupling agrees
with experiment for reasonable moduli, but the electroweak couplings are too
small.Comment: 22 pages, 4 figure
TeV physics and the Planck scale
Supersymmetry is one of the best motivated possibilities for new physics at
the TeV scale. However, both concrete string constructions and phenomenological
considerations suggest the possibility that the physics at the TeV scale could
be more complicated than the Minimal Supersymmetric Standard Model (MSSM),
e.g., due to extended gauge symmetries, new vector-like supermultiplets with
non-standard SU(2)xU(1) assignments, and extended Higgs sectors. We briefly
comment on some of these possibilities, and discuss in more detail the class of
extensions of the MSSM involving an additional standard model singlet field.
The latter provides a solution to the problem, and allows significant
modifications of the MSSM in the Higgs and neutralino sectors, with important
consequences for collider physics, cold dark matter, and electroweak
baryogenesis.Comment: 17 pages, 5 figures. To appear in New Journal of Physic
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