78,593 research outputs found
Collective Quartics and Dangerous Singlets in Little Higgs
Any extension of the standard model that aims to describe TeV-scale physics
without fine-tuning must have a radiatively-stable Higgs potential. In little
Higgs theories, radiative stability is achieved through so-called collective
symmetry breaking. In this letter, we focus on the necessary conditions for a
little Higgs to have a collective Higgs quartic coupling. In one-Higgs doublet
models, a collective quartic requires an electroweak triplet scalar. In
two-Higgs doublet models, a collective quartic requires a triplet or singlet
scalar. As a corollary of this study, we show that some little Higgs theories
have dangerous singlets, a pathology where collective symmetry breaking does
not suppress quadratically-divergent corrections to the Higgs mass.Comment: 4 pages; v2: clarified the existing literature; v3: version to appear
in JHE
Probing the SUSY breaking scale at an collider
If supersymmetry is spontaneously at a low energy scale then the resulting
gravitino would be very light. The interaction strength of the longitudinal
components of such a light gravitino to electron-selectron pair then becomes
comparable to that of electroweak interactions. Such a light gravitino could
modify the cross-section for e^_L e^_R-->\tilde {e}_L\tilde {e}_R from its
MSSM value. Precision measurement of this cross-section could therefore be used
to probe the low energy SUSY breaking scale.Comment: Plain Tex, 7 pages, No figure
One Loop Renormalization of the Littlest Higgs Model
In Little Higgs models a collective symmetry prevents the Higgs from
acquiring a quadratically divergent mass at one loop. This collective symmetry
is broken by weakly gauged interactions. Terms, like Yukawa couplings, that
display collective symmetry in the bare Lagrangian are generically renormalized
into a sum of terms that do not respect the collective symmetry except possibly
at one renormalization point where the couplings are related so that the
symmetry is restored. We study here the one loop renormalization of a
prototypical example, the Littlest Higgs Model. Some features of the
renormalization of this model are novel, unfamiliar form similar chiral
Lagrangian studies.Comment: 23 pages, 17 eps figure
Colossal negative magnetoresistance in dilute fluorinated graphene
Adatoms offer an effective route to modify and engineer the properties of
graphene. In this work, we create dilute fluorinated graphene using a clean,
controlled and reversible approach. At low carrier densities, the system is
strongly localized and exhibits an unexpected, colossal negative
magnetoresistance. The zero-field resistance is reduced by a factor of 40 at
the highest field of 9 T and shows no sign of saturation. Unusual "staircase"
field dependence is observed below 5 K. The magnetoresistance is highly
anisotropic. We discuss possible origins, considering quantum interference
effects and adatom-induced magnetism in graphene.Comment: 21 pages, 4 figures, including supplementary informatio
Elastic Scattering and Direct Detection of Kaluza-Klein Dark Matter
Recently a new dark matter candidate has been proposed as a consequence of
universal compact extra dimensions. It was found that to account for
cosmological observations, the masses of the first Kaluza-Klein modes (and thus
the approximate size of the extra dimension) should be in the range 600-1200
GeV when the lightest Kaluza-Klein particle (LKP) corresponds to the
hypercharge boson and in the range 1 - 1.8 TeV when it corresponds to a
neutrino. In this article, we compute the elastic scattering cross sections
between Kaluza-Klein dark matter and nuclei both when the lightest Kaluza-Klein
particle is a KK mode of a weak gauge boson, and when it is a neutrino. We
include nuclear form factor effects which are important to take into account
due to the large LKP masses favored by estimates of the relic density. We
present both differential and integrated rates for present and proposed
Germanium, NaI and Xenon detectors. Observable rates at current detectors are
typically less than one event per year, but the next generation of detectors
can probe a significant fraction of the relevant parameter space.Comment: 23 pages, 11 figures; v2,v3: Ref. added, discussion improved,
conclusions unchanged. v4: Introduction was expanded to be more appropriate
for non experts. Various clarifications added in the text. Version to be
published in New Journal of Physic
Phenomenology of the Littlest Higgs with T-Parity
Little Higgs models offer an interesting approach to weakly coupled
electroweak symmetry breaking without fine tuning. The original little Higgs
models were plagued by strong constraints from electroweak precision data which
required a fine tuning to be reintroduced. An economical solution to this
problem is to introduce a discrete symmetry (analogous to R-parity of SUSY)
called T-parity. T-parity not only eliminates most constraints from electroweak
precision data, but it also leads to a promising dark matter candidate. In this
paper we investigate the dark matter candidate in the littlest Higgs model with
T-parity. We find bounds on the symmetry breaking scale f as a function of the
Higgs mass by calculating the relic density. We begin the study of the LHC
phenomenology of the littlest Higgs model with T-parity. We find that the model
offers an interesting collider signature that has a generic missing energy
signal which could "fake" SUSY at the LHC. We also investigate the properties
of the heavy partner of the top quark which is common to all littlest Higgs
models, and how its properties are modified with the introduction of T-parity.
We include an appendix with a list of Feynman rules specific to the littlest
Higgs with T-parity to facilitate further study.Comment: 32 pages, 8 figures; dark matter bounds revised; comphep model files
made publicly available at http://www.lns.cornell.edu/public/theory/tparity
Measuring Invisible Particle Masses Using a Single Short Decay Chain
We consider the mass measurement at hadron colliders for a decay chain of two
steps, which ends with a missing particle. Such a topology appears as a
subprocess of signal events of many new physics models which contain a dark
matter candidate. From the two visible particles coming from the decay chain,
only one invariant mass combination can be formed and hence it is na\"ively
expected that the masses of the three invisible particles in the decay chain
cannot be determined from a single end point of the invariant mass
distribution. We show that the event distribution in the
vs. invariant mass-squared plane, where , are the transverse
energies of the two visible particles, contains the information of all three
invisible particle masses and allows them to be extracted individually. The
experimental smearing and combinatorial issues pose challenges to the mass
measurements. However, in many cases the three invisible particle masses in the
decay chain can be determined with reasonable accuracies.Comment: 45 pages, 32 figure
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