6,278 research outputs found
The heavy top quark and supersymmetry
Three aspects of supersymmetric theories are discussed: electroweak symmetry
breaking, the issues of flavor, and gauge unification. The heavy top quark
plays an important, sometimes dominant, role in each case. Additional
symmetries lead to extensions of the standard model which can provide an
understanding for many of the outstanding problems of particle physics. A
broken supersymmetric extension of spacetime allows electroweak symmetry
breaking to follow from the dynamics of the heavy top quark; an extension of
isospin provides a constrained framework for understanding the pattern of quark
and lepton masses; and a grand unified extension of the standard model gauge
group provides an elegant understanding of the gauge quantum numbers of the
components of a generation. Experimental signatures for each of these
additional symmetries are discussed.Comment: 60 pages, 1 ps file; lectures delivered at the 1995 SLAC Summer
Institut
Higgs Parity Grand Unification
The vanishing of the Higgs quartic coupling of the Standard Model at high
energies may be explained by spontaneous breaking of Higgs Parity. Taking Higgs
Parity to originate from the Left-Right symmetry of the gauge group,
leads to a new scheme for precision gauge coupling unification that is
consistent with proton decay. We compute the relevant running of couplings and
threshold corrections to allow a precise correlation among Standard Model
parameters. The scheme has a built-in solution for obtaining a realistic value
for , which further improves the precision from gauge coupling
unification, allowing the QCD coupling constant to be predicted to the level of
1 % or, alternatively, the top quark mass to 0.2 %. Future measurements of
these parameters may significantly constrain the detailed structure of the
theory. We also study an embedding of quark and lepton masses, showing
how large neutrino mixing is compatible with small quark mixing, and predict a
normal neutrino mass hierarchy. The strong CP problem may be explained by
combining Higgs Parity with space-time parity.Comment: 39 pages, 9 figure
Why Are Neutrinos Light? -- An Alternative
We review the recent proposal that neutrinos are light because their masses
are proportional to a low scale, f, of lepton flavor symmetry breaking. This
mechanism is testable because the resulting pseudo-Goldstone bosons, of mass
m_G, couple strongly with the neutrinos, affecting the acoustic oscillations
during the eV era of the early universe that generate the peaks in the CMB
radiation. Characteristic signals result over a very wide range of (f, m_G)
because of a change in the total relativistic energy density and because the
neutrinos scatter rather than free-stream. Thermodynamics allows a precise
calculation of the signal, so that observations would not only confirm the
late-time neutrino mass mechanism, but could also determine whether the
neutrino spectrum is degenerate, inverted or hierarchical and whether the
neutrinos are Dirac or Majorana.
The flavor symmetries could also give light sterile states. If the masses of
the sterile neutrinos turn on after the MeV era, the LSND oscillations can be
explained without upsetting big bang nucleosynthesis, and, since the sterile
states decay to lighter neutrinos and pseudo-Goldstones, without giving too
much hot dark matter.Comment: Talk given by LJH at the Fujihara Seminar on Neutrino Mass and Seesaw
Mechanism held at KEK, Japan, February 2004. 11 pages, 1 figure, 3 table
Unification of Weak and Hypercharge Interactions at the TeV Scale
A realistic SU(3)_C x SU(3)_W unified theory is constructed with a TeV sized
extra dimension compactified on the orbifold S_1/Z_2, leaving only the standard
model gauge group SU(3)_C x SU(2)_L x U(1)_Y unbroken in the low energy 4D
theory. The Higgs doublets are zero modes of bulk SU(3)_W triplets and serve to
normalize the hypercharge generator, apparently giving a tree-level prediction
for the weak mixing angle: \sin^2\theta = 1/4. The orbifold boundary conditions
imply a restricted set of SU(3)_W gauge transformations: at an orbifold fixed
point only the transformations of SU(2)_L x U(1)_Y are operative. This allows
quarks to be located at this fixed point, overcoming the longstanding problem
of how to incorporate matter in a unified SU(3)_W theory. However, in general
this local, explicit breaking of SU(3)_W symmetry, necessary for including
quarks into the theory, destroys the tree-level prediction for the weak mixing
angle. This apparent contradiction is reconciled by making the volume of the
extra dimension large, diluting the effects of the local SU(3)_W violation. In
the case that the electroweak theory is strongly coupled at the cutoff scale of
the effective theory, radiative corrections to the weak mixing angle can be
reliably computed, and used to predict the scale of compactification: 1 - 2 TeV
without supersymmetry, and in the region of 3 - 6 TeV for a supersymmetric
theory. The experimental signature of electroweak unification into SU(3)_W is a
set of ``weak partners'' of mass 1/2R, which are all electrically charged and
are expected to be accessible at LHC. These include weak doublets of gauge
particles of electric charge (++,+), and a charged scalar. When pair produced,
they yield events containing multiple charged leptons, missing large transverse
energy and possibly Higgs and electroweak gauge bosons.Comment: 13 pages, LaTeX, note added on charge quantizatio
Grand Unification and Intermediate Scale Supersymmetry
With minimal field content and for an interesting range of the supersymmetric
Higgs mixing parameter, 0.5 < tan^2 \beta < 2, the superpartner mass scale,
\tilde{m}, is found to be at the intermediate scale, ~ 10^{10 \pm 1} GeV, near
where the Standard Model Higgs quartic coupling passes through zero. For any 4d
supersymmetric grand unified symmetry spontaneously broken by a vacuum
expectation value , if superpotential interactions for \Sigma are
forbidden e.g. by R symmetries, the uneaten color octet, \Sigma_8, and weak
triplet, \Sigma_3, have masses of order \tilde{m}. The combination of
superpartner and \Sigma_{8,3} states leads to successful gauge coupling
unification, removing the disastrously high proton decay rate of minimal
Standard Model unification. Proton decay could be seen in future experiments if
\tilde{m} ~ 10^{11} GeV, but not if it is lower. If the reheating temperature
after inflation, T_R, is less than \tilde{m} dark matter may be axions. If T_R
> \tilde{m}, thermal LSP dark matter may lead to the environmental selection of
a TeV-scale LSP, either wino or Higgsino, which could comprise all or just one
component of dark matter. In the Higgsino case, the dark matter is found to
behave inelastically in direct detection experiments, and gauge coupling
unification occurs accurately without the need of any threshold corrections.Comment: 14 pages, 3 figures; version to appear in JHE
Implications of Higgs Discovery for the Strong CP Problem and Unification
A symmetry that extends the weak interaction, , and the Higgs sector, , yields a Standard Model quartic coupling that vanishes at scale . Near , theories either have a "prime" sector, or possess
"Left-Right" (LR) symmetry with . If the symmetry
incorporates spacetime parity, these theories can solve the strong CP problem.
The LR theories have all quark and lepton masses arising from operators of
dimension 5 or more, requiring Froggatt-Nielsen structures. Two-loop
contributions to are estimated and typically lead to a neutron
electric dipole moment of order e cm that can be observed in future
experiments. Minimal models, with gauge group , have precise gauge coupling unification for GeV, successfully correlating gauge unification with the observed
Higgs mass of GeV. With embedded in ,
the central value of the unification scale is reduced from GeV to
below GeV, improving the likelihood of proton decay discovery.
Unified theories based on are constructed that have
in a or and generate higher-dimensional flavor
operators, while maintaining perturbative gauge couplings.Comment: 36 pages, 5 figure
Study of Inclusive Multi-Ring Events from Atmospheric Neutrinos
The current analysis of atmospheric neutrinos by the Super-Kamiokande
Collaboration is based only on fully-contained one-ring events and partially
contained events. We show that the up-down ratio of fully-contained, inclusive,
multi-ring events gives an independent test of the atmospheric neutrino
anomaly, without the need for particle identification. Moreover, this class of
events is rich in neutral current events and hence gives crucial information
for discriminating between oscillations of \nu_\mu into \nu_{e, \tau} and
\nu_s.Comment: 12 pages, 3 figures, LaTeX2e, psfig.st
Squarks in Tevatron Dilepton Events ?
We consider unusual events in the CDF and D0 dilepton+jets sample with very
high ET(lepton) and ET(missing). It is possible, but very unlikely, that these
events originate from top quark pair production; however, they have
characteristics that are better accounted for by decays of supersymmetric
quarks with mass in the region of 300 GeV.Comment: 5 pages, 2 postscript (eps) figures, uses sprocl.sty (included
Radiative PQ Breaking and the Higgs Boson Mass
The small and negative value of the Standard Model Higgs quartic coupling at
high scales can be understood in terms of anthropic selection on a landscape
where large and negative values are favored: most universes have a very
short-lived electroweak vacuum and typical observers are in universes close to
the corresponding metastability boundary. We provide a simple example of such a
landscape with a Peccei-Quinn symmetry breaking scale generated through
dimensional transmutation and supersymmetry softly broken at an intermediate
scale. Large and negative contributions to the Higgs quartic are typically
generated on integrating out the saxion field. Cancellations among these
contributions are forced by the anthropic requirement of a sufficiently
long-lived electroweak vacuum, determining the multiverse distribution for the
Higgs quartic in a similar way to that of the cosmological constant. This leads
to a statistical prediction of the Higgs boson mass that, for a wide range of
parameters, yields the observed value within the 1 statistical
uncertainty of 5 GeV originating from the multiverse distribution. The
strong CP problem is solved and single-component axion dark matter is
predicted, with an abundance that can be understood from environmental
selection. A more general setting for the Higgs mass prediction is discussed.Comment: 30 pages, 10 figures; v2, JHEP versio
Minimal Mirror Twin Higgs
In a Mirror Twin World with a maximally symmetric Higgs sector the little
hierarchy of the Standard Model can be significantly mitigated, perhaps
displacing the cutoff scale above the LHC reach. We show that consistency with
observations requires that the Z2 parity exchanging the Standard Model with its
mirror be broken in the Yukawa couplings. A minimal such effective field
theory, with this sole Z2 breaking, can generate the Z2 breaking in the Higgs
sector necessary for the Twin Higgs mechanism. The theory has constrained and
correlated signals in Higgs decays, direct Dark Matter Detection and Dark
Radiation, all within reach of foreseen experiments, over a region of parameter
space where the fine-tuning for the electroweak scale is 10-50%. For dark
matter, both mirror neutrons and a variety of self-interacting mirror atoms are
considered. Neutrino mass signals and the effects of a possible additional Z2
breaking from the vacuum expectation values of B-L breaking fields are also
discussed.Comment: 42 pages, 8 figures; additional references and discussion on higgs
physics and dark matte
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