68 research outputs found
Electroweak precision constraints on the Lee-Wick Standard Model
We perform an analysis of the electroweak precision observables in the
Lee-Wick Standard Model. The most stringent restrictions come from the S and T
parameters that receive important tree level and one loop contributions. In
general the model predicts a large positive S and a negative T. To reproduce
the electroweak data, if all the Lee-Wick masses are of the same order, the
Lee-Wick scale is of order 5 TeV. We show that it is possible to find some
regions in the parameter space with a fermionic state as light as 2.4-3.5 TeV,
at the price of rising all the other masses to be larger than 5-8 TeV. To
obtain a light Higgs with such heavy resonances a fine-tuning of order a few
per cent, at least, is needed. We also propose a simple extension of the model
including a fourth generation of Standard Model fermions with their Lee-Wick
partners. We show that in this case it is possible to pass the electroweak
constraints with Lee-Wick fermionic masses of order 0.4-1.5 TeV and Lee-Wick
gauge masses of order 3 TeV.Comment: 24 pages, 7 figure
Direct Higgs production and jet veto at the Tevatron and the LHC in NNLO QCD
We consider Higgs boson production through gluon--gluon fusion in hadron
collisions, when a veto is applied on the transverse momenta of the
accompanying hard jets. We compute the QCD radiative corrections to this
process at NLO and NNLO. The NLO calculation is complete. The NNLO calculation
uses the recently evaluated NNLO soft and virtual QCD contributions to the
inclusive cross section. We find that the jet veto reduces the impact of the
NLO and NNLO contributions, the reduction being more sizeable at the LHC than
at the Tevatron.Comment: 22 pages, 12 postscript figure
Running into New Territory in SUSY Parameter Space
The LEP-II bound on the light Higgs mass rules out the vast majority of
parameter space left to the Minimal Supersymmetric Standard Model (MSSM) with
weak-scale soft-masses. This suggests the importance of exploring extensions of
the MSSM with non-minimal Higgs physics. In this article, we explore a theory
with an additional singlet superfield and an extended gauge sector. The theory
has a number of novel features compared to both the MSSM and Next-to-MSSM,
including easily realizing a light CP-even Higgs mass consistent with LEP-II
limits, tan(beta) < 1, and a lightest Higgs which is charged. These features
are achieved while remaining consistent with perturbative unification and
without large stop-masses. Discovery modes at the Tevatron and LHC are
discussed.Comment: 15 pages, 5 figures; Typo in equation (4.5) corrected; submitted to
JHE
Hadronic Contributions to the Photon Vacuum Polarization and their Role in Precision Physics
I review recent evaluations of the hadronic contribution to the shift in the
fine structure constant and to the anomalous magnetic moment of the muon.
Substantial progress in a precise determination of these important observables
is a consequence of substantially improved total cross section measurement by
the CMD-2 and BES II collaborations and an improved theoretical understanding.
Prospects for further possible progress is discussed.Comment: 17 pages 7 figures 2 tables, update: incl. CMD-2 data, reference
Natural Little Hierarchy from a Partially Goldstone Twin Higgs
We construct a simple theory in which the fine-tuning of the standard model
is significantly reduced. Radiative corrections to the quadratic part of the
scalar potential are constrained to be symmetric under a global U(4) x U(4)'
symmetry due to a discrete Z_2 "twin" parity, while the quartic part does not
possess this symmetry. As a consequence, when the global symmetry is broken the
Higgs fields emerge as light pseudo-Goldstone bosons, but with sizable quartic
self-interactions. This structure allows the cutoff scale, \Lambda, to be
raised to the multi-TeV region without significant fine-tuning. In the minimal
version of the theory, the amount of fine-tuning is about 15% for \Lambda = 5
TeV, while it is about 30% in an extended model. This provides a solution to
the little hierarchy problem. In the minimal model, the "visible" particle
content is exactly that of the two Higgs doublet standard model, while the
extended model also contains extra vector-like fermions with masses ~(1-2)TeV.
At the LHC, our minimal model may appear exactly as the two Higgs doublet
standard model, and new physics responsible for cutting off the divergences of
the Higgs mass-squared parameter may not be discovered. Several possible
processes that may be used to discriminate our model from the simple two Higgs
doublet model are discussed for the LHC and for a linear collider.Comment: 22 page
Electroweak Physics, Experimental Aspects
Collider measurements on electroweak physics are summarised. Although the
precision on some observables is very high, no deviation from the Standard
Model of electroweak interactions is observed. The data allow to set stringent
limits on some models for new physics.Comment: Plenary Talk at the UK Phenomenology Workshop on Collider Physics,
Durham, 199
Uncertainties of the Inclusive Higgs Production Cross Section at the Tevatron and the LHC
We study uncertainties of the predicted inclusive Higgs production cross
section due to the uncertainties of parton distribution functions (PDF).
Particular attention is given to bbH Yukawa coupling enhanced production
mechanisms in beyond SM scenarios, such as MSSM. The PDF uncertainties are
determined by the robust Lagrange Multiplier method within the CTEQ global
analysis framework. We show that PDF uncertainties dominate over theoretical
uncertainties of the perturbative calculation (usually estimated by the scale
dependence of the calculated cross sections), except for low Higgs masses at
LHC. Thus for the proper interpretation of any Higgs signal, and for better
understanding of the underlying electroweak symmetry breaking mechanism, it is
important to gain better control of the uncertainties of the PDFs.Comment: LaTeX, JHEP, 19 pages, 14 figure
Neutralino relic density in supersymmetric GUTs with no-scale boundary conditions above the unification scale
We investigate SU(5) and SO(10) GUTs with vanishing scalar masses and
trilinear scalar couplings at a scale higher than the unification scale. The
parameter space of the models, further constrained by b-\tau Yukawa coupling
unification, consists of a common gaugino mass and of \tan\beta. We analyze the
low energy phenomenology, finding that A-pole annihilations of neutralinos
and/or coannihilations with the lightest stau drive the relic density within
the cosmologically preferred range in a significant region of the allowed
parameter space. Implications for neutralino direct detection and for CERN LHC
experiments are also discussed.Comment: 14 pages, 5 figures, JHEP style. Version accepted for publication in
JHE
Neutralino, axion and axino cold dark matter in minimal, hypercharged and gaugino AMSB
Supersymmetric models based on anomaly-mediated SUSY breaking (AMSB)
generally give rise to a neutral wino as a WIMP cold dark matter (CDM)
candidate, whose thermal abundance is well below measured values. Here, we
investigate four scenarios to reconcile AMSB dark matter with the measured
abundance: 1. non-thermal wino production due to decays of scalar fields ({\it
e.g} moduli), 2. non-thermal wino production due to decays of gravitinos, 3.
non-thermal wino production due to heavy axino decays, and 4. the case of an
axino LSP, where the bulk of CDM is made up of axions and thermally produced
axinos. In cases 1 and 2, we expect wino CDM to constitute the entire measured
DM abundance, and we investigate wino-like WIMP direct and indirect detection
rates. Wino direct detection rates can be large, and more importantly, are
bounded from below, so that ton-scale noble liquid detectors should access all
of parameter space for m_{\tz_1}\alt 500 GeV. Indirect wino detection rates via
neutrino telescopes and space-based cosmic ray detectors can also be large. In
case 3, the DM would consist of an axion plus wino admixture, whose exact
proportions are very model dependent. In this case, it is possible that both an
axion and a wino-like WIMP could be detected experimentally. In case 4., we
calculate the re-heat temperature of the universe after inflation. In this
case, no direct or indirect WIMP signals should be seen, although direct
detection of relic axions may be possible. For each DM scenario, we show
results for the minimal AMSB model, as well as for the hypercharged and gaugino
AMSB models.Comment: 29 pages including 13 figure
Quantum chromodynamics with advanced computing
We survey results in lattice quantum chromodynamics from groups in the USQCD
Collaboration. The main focus is on physics, but many aspects of the discussion
are aimed at an audience of computational physicists.Comment: 17 pp. Featured presentation at Scientific Discovery with Advanced
Computing, July 13-17, Seattl
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