1,243 research outputs found
Reconstruction of the Extended Gauge Structure from Observables at Future Colliders
The discovery of a new neutral gauge boson with a mass in the TeV region
would allow for determination of gauge couplings of the to ordinary quarks
and leptons in a model independent way. We show that these couplings in turn
would allow us to determine the nature of the extended gauge structure. As a
prime example we study the group. In this case two discrete constraints
on experimentally determined couplings have to be satisfied. If so, the
couplings would then uniquely determine the two parameters, and
, which fully specify the nature of the within . If the
is part of the gauge structure, then for TeV and
could be determined to around at the future colliders. The NLC
provides a unique determination of the two constraints as well as of and , though with slightly larger error bars than at the LHC. On
the other hand, since the LHC primarily determines three out of four normalized
couplings, it provides weaker constraints for the underlying gauge structure.Comment: 14 pages LaTeX using RevTeX and psfig.sty. TeX source and 3 PS
figures, tarred, compressed and uuencoded; also available via anonymous ftp
to ftp://dept.physics.upenn.edu/pub/Cvetic/UPR-636-T
Multilepton production via top flavour-changing neutral couplings at the CERN LHC
Zt and gamma t production with Z -> l+ l- and t -> Wb -> l nu b provides the
best determination of top flavour-changing neutral couplings at the LHC. The
bounds on tc couplings eventually derived from these processes are similar to
those expected from top decays, while the limits on tu couplings are better by
a factor of two. The other significant Z and W decay modes are also
investigated.Comment: 30 pages, 23 PS figures. Uses epsfig.sty and elsart.sty. Added some
references and corrected some typos. Added more comments about statistics.
Using elsart.sty reduces the size to 30 pages. Published in Nucl. Phys.
Lorentz Violation in Extra Dimensions
In theories with extra dimensions it is well known that the Lorentz
invariance of the -dimensional spacetime is lost due to the compactified
nature of the dimensions leaving invariance only in 4d. In such theories
other sources of Lorentz violation may exist associated with the physics that
initiated the compactification process at high scales. Here we consider the
possibility of capturing some of this physics by analyzing the higher
dimensional analog of the model of Colladay and Kostelecky. In that scenario a
complete set of Lorentz violating operators arising from spontaneous Lorentz
violation, that are not obviously Planck-scale suppressed, are added to the
Standard Model action. Here we consider the influence of the analogous set of
operators which break Lorentz invariance in 5d within the Universal Extra
Dimensions picture. We show that such operators can greatly alter the
anticipated Kaluza-Klein(KK) spectra, induce electroweak symmetry breaking at a
scale related to the inverse compactification radius, yield sources of parity
violation in, e.g., 4d QED/QCD and result in significant violations of
KK-parity conservation produced by fermion Yukawa couplings, thus destabilizing
the lightest KK particle. LV in 6d is briefly discussed.Comment: 26 pages, 2 figures; additional references and discussio
Constraining differential renormalization in abelian gauge theories
We present a procedure of differential renormalization at the one loop level
which avoids introducing unnecessary renormalization constants and
automatically preserves abelian gauge invariance. The amplitudes are expressed
in terms of a basis of singular functions. The local terms appearing in the
renormalization of these functions are determined by requiring consistency with
the propagator equation. Previous results in abelian theories, with and without
supersymmetry, are discussed in this context.Comment: 13 pages, LaTeX. Some equations corrected and a reference added.
Complete ps paper also available at http://www-ftae.ugr.es/papiros.html or
ftp://ftae3.ugr.es/pub/rmt/ugft73.p
Looking for signals beyond the neutrino Standard Model
Any new neutrino physics at the TeV scale must include a suppression
mechanism to keep its contribution to light neutrino masses small enough. We
review some seesaw model examples with weakly broken lepton number, and comment
on the expected effects at large colliders and in neutrino oscillations.Comment: LaTeX 10 pages, 9 PS figures. Contribution to the Proceedings of the
XXXI International School of Theoretical Physics "Matter To The Deepest"
Ustron, Poland, September 5-11, 2007. Typos correcte
Z' Decays into Four Fermions
If a new is discovered with a mass at LHC/SSC, its (rare)
decays into two charged leptons plus missing transverse energy will probe the
coupling to the lepton doublet and to , allowing
further discrimination among extended electroweak models.Comment: 9 pages plus 1 figure (not included but available), UG-FT-22/9
Electroweak scale seesaw and heavy Dirac neutrino signals at LHC
Models of type I seesaw can be implemented at the electroweak scale in a
natural way provided that the heavy neutrino singlets are quasi-Dirac
particles. In such case, their contribution to light neutrino masses has the
suppression of a small lepton number violating parameter, so that light
neutrino masses can arise naturally even if the seesaw scale is low and the
heavy neutrino mixing is large. We implement the same mechanism with fermionic
triplets in type III seesaw, deriving the interactions of the new quasi-Dirac
neutrinos and heavy charged leptons with the SM fermions. We then study the
observability of heavy Dirac neutrino singlets (seesaw I) and triplets (seesaw
III) at LHC. Contrarily to common wisdom, we find that heavy Dirac neutrino
singlets with a mass around 100 GeV are observable at the 5 sigma level with a
luminosity of 13 fb^-1. Indeed, in the final state with three charged leptons
l+- l+- l-+, not previously considered, Dirac neutrino signals can be
relatively large and backgrounds are small. In the triplet case, heavy
neutrinos can be discovered with a luminosity of 1.5 fb^-1 for a mass of 300
GeV in the same channel.Comment: LaTeX 19 pages, 22 PS figures. Enlarged discussion and added a
reference. Final version to appear in PL
Model-Independent Searches for New Quarks at the LHC
New vector-like quarks can have sizable couplings to first generation quarks
without conflicting with current experimental constraints. The coupling with
valence quarks and unique kinematics make single production the optimal
discovery process. We perform a model-independent analysis of the discovery
reach at the Large Hadron Collider for new vector-like quarks considering
single production and subsequent decays via electroweak interactions. An early
LHC run with 7 TeV center of mass energy and 1 fb-1 of integrated luminosity
can probe heavy quark masses up to 1 TeV and can be competitive with the
Tevatron reach of 10 fb-1. The LHC with 14 TeV center of mass energy and 100
fb-1 of integrated luminosity can probe heavy quark masses up to 3.7 TeV for
order one couplings.Comment: 37 pages, 11 figures, 7 table
Signature of heavy Majorana neutrinos at a linear collider: Enhanced charged Higgs pair production
A charged Higgs pair can be produced at an ee collider through a t-channel
exchange of a heavy neutrino (N) via e^+ e^- -> H^+ H^- and, if N is a Majorana
particle, also via the lepton number violating (LNV) like-sign reaction e^\pm
e^\pm \to H^\pm H^\pm. Assuming no a-priori relation between the effective
eNH^+ coupling (\xi) and light neutrino masses, we show that this interaction
vertex can give a striking enhancement to these charged Higgs pair production
processes. In particular, the LNV H^-H^- signal can probe N at the ILC in the
mass range 100 GeV < m_N < 10^4 TeV and with the effective mixing angle, \xi,
in the range 10^{-4} < \xi^2 < 10^{-8} - well within its perturbative unitarity
bound and the neutrinoless double beta decay (\beta\beta_{0\nu}) limit. The
lepton number conserving (LNC) e^+ e^- \to H^+ H^- mode can be sensitive to,
e.g., an O(10) TeV heavy Majorana neutrino at a 500 GeV International Linear
Collider (ILC), if \xi^2 > 0.001.Comment: Latex, 5 pages, 3 figures. V2 as published in PR
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