213 research outputs found
Surveillance of resistance in bacteria causing communityâacquired respiratory tract infections
Bacterial resistance to antibiotics in communityâacquired respiratory tract infections is a serious problem and is increasing in prevalence worldâwide at an alarming rate. Streptococcus pneumoniae, one of the main organisms implicated in respiratory tract infections, has developed multiple resistance mechanisms to combat the effects of most commonly used classes of antibiotics, particularly the ÎČâlactams (penicillin, aminopenicillins and cephalosporins) and macrolides. Furthermore, multidrugâresistant strains of S.âpneumoniae have spread to all regions of the world, often via resistant genetic clones. A similar spread of resistance has been reported for other major respiratory tract pathogens, including Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pyogenes. To develop and support resistance control strategies it is imperative to obtain accurate data on the prevalence, geographic distribution and antibiotic susceptibility of respiratory tract pathogens and how this relates to antibiotic prescribing patterns. In recent years, significant progress has been made in developing longitudinal national and international surveillance programs to monitor antibiotic resistance, such that the prevalence of resistance and underlying trends over time are now well documented for most parts of Europe, and many parts of Asia and the Americas. However, resistance surveillance data from parts of the developing world (regions of Central America, Africa, Asia and Central/Eastern Europe) remain poor. The quantity and quality of surveillance data is very heterogeneous; thus there is a clear need to standardize or validate the data collection, analysis and interpretative criteria used across studies. If disseminated effectively these data can be used to guide empiric antibiotic therapy, and to supportâand monitor the impact ofâinterventions on antibiotic resistance
Radiative corrections to all charge assignments of heavy quark baryon semileptonic decays
In semileptonic decays of spin-1/2 baryons containing heavy quarks up to six
charge assignments for the baryons and lepton are possible. We show that the
radiative corrections to four of these possibilities can be directly obtained
from the final results of the two possibilities previously studied. There is no
need to recalculate integrals over virtual or real photon momentum or any
traces.Comment: 15 pages, 2 figures, RevTex. Extended discussion. Final version to
appear in Physical Review
Electroweak Precision Constraints on the Littlest Higgs Model with T Parity
We compute the leading corrections to the properties of W and Z bosons
induced at the one-loop level in the SU(5)/SO(5) Littlest Higgs model with T
parity, and perform a global fit to precision electroweak data to determine the
constraints on the model parameters. We find that a large part of the model
parameter space is consistent with data. Values of the symmetry breaking scale
as low as 500 GeV are allowed, indicating that no significant fine tuning in
the Higgs potential is required. We identify a region within the allowed
parameter space in which the lightest T-odd particle, the partner of the
hypercharge gauge boson, has the correct relic abundance to play the role of
dark matter. In addition, we find that a consistent fit to data can be obtained
for large values of the Higgs mass, up to 800 GeV, due to the possibility of a
partial cancellation between the contributions to the T parameter from Higgs
loops and new physics.Comment: 23 pages, 9 figures. Minor correction
Little Hierarchy, Little Higgses, and a Little Symmetry
Little Higgs theories are an attempt to address the little hierarchy problem,
i.e., the tension between the naturalness of the electroweak scale and the
precision measurements showing no evidence for new physics up to 5-10 TeV. In
little Higgs theories, the Higgs mass-squareds are protected to the one-loop
order from the quadratic divergence. This allows the cutoff to be raised up to
\~10 TeV, beyond the scales probed by the precision data. However, strong
constraints can still arise from the contributions of the new TeV scale
particles and hence re-introduces the fine-tuning problem. In this paper we
show that a new symmetry, denoted as T-parity, under which all heavy gauge
bosons and scalar triplets are odd, can remove all the tree-level contributions
to the electroweak observables and therefore makes the little Higgs theories
completely natural. The T-parity can be manifestly implemented in a majority of
little Higgs models by following the most general construction of the low
energy effective theory a la Callan, Coleman, Wess and Zumino. In particular,
we discuss in detail how to implement the T-parity in the littlest Higgs model
based on SU(5)/SO(5). The symmetry breaking scale f can be even lower than 500
GeV if the contributions from the unknown UV physics at the cutoff are somewhat
small. The existence of -parity has drastic impacts on the phenomenology of
the little Higgs theories. The T-odd particles need to be pair-produced and
will cascade down to the lightest T-odd particle (LTP) which is stable. A
neutral LTP gives rise to missing energy signals at the colliders which can
mimic supersymmetry. It can also serve as a good dark matter candidate.Comment: 20 pages, 2 figures, RevTeX; v2: Yukawa sector in the SU(5)/SO(5)
model slightly modified. Also added comments on the Dirac mass term for the
fermionic doublet partner; v3: clarifying comments on the modified Yukawa
sector. version to appear on JHE
Littlest Higgs model and associated ZH production at high energy collider
In the context of the littlest Higgs (LH) model, we consider the Higgs
strahlung process . We find that the correction effects on
this process mainly come from the heavy photon . If we take the mixing
angle parameter in the range of 0.75 - 1, the contributions of the heavy
gauge boson is larger than 6%. In most of the parameter space, the
deviation of the total production cross section from its SM
value is larger than 5%, which may be detected in the future high energy
collider (LC) experiments. The future LC experiments could test
the LH model by measuring the cross section of the process .Comment: 13 pages, 3 figure
The Littlest Higgs in Anti-de Sitter Space
We implement the SU(5)/SO(5) littlest Higgs theory in a slice of 5D Anti-de
Sitter space bounded by a UV brane and an IR brane. In this model, there is a
bulk SU(5) gauge symmetry that is broken to SO(5) on the IR brane, and the
Higgs boson is contained in the Goldstones from this breaking. All of the
interactions on the IR brane preserve the global symmetries that protect the
Higgs mass, but a radiative potential is generated through loops that stretch
to the UV brane where there are explicit SU(5) violating boundary conditions.
Like the original littlest Higgs, this model exhibits collective breaking in
that two interactions must be turned on in order to generate a Higgs potential.
In AdS space, however, collective breaking does not appear in coupling
constants directly but rather in the choice of UV brane boundary conditions. We
match this AdS construction to the known low energy structure of the littlest
Higgs and comment on some of the tensions inherent in the AdS construction. We
calculate the 5D Coleman-Weinberg effective potential for the Higgs and find
that collective breaking is manifest. In a simplified model with only the SU(2)
gauge structure and the top quark, the physical Higgs mass can be of order 200
GeV with no considerable fine tuning (25%). We sketch a more realistic model
involving the entire gauge and fermion structure that also implements T-parity,
and we comment on the tension between T-parity and flavor structure.Comment: 42 pages, 7 figures, 3 tables; v2: minor rewording, JHEP format; v3:
to match JHEP versio
What Precision Electroweak Physics Says About the SU(6)/Sp(6) Little Higgs
We study precision electroweak constraints on the close cousin of the
Littlest Higgs, the SU(6)/Sp(6) model. We identify a near-oblique limit in
which the heavy W' and B' decouple from the light fermions, and then calculate
oblique corrections, including one-loop contributions from the extended top
sector and the two Higgs doublets. We find regions of parameter space that give
acceptably small precision electroweak corrections and only mild fine tuning in
the Higgs potential, and also find that the mass of the lightest Higgs boson is
relatively unconstrained by precision electroweak data. The fermions from the
extended top sector can be as light as 1 TeV, and the W' can be as light as 1.8
TeV. We include an independent breaking scale for the B', which can still have
a mass as low as a few hundred GeV.Comment: 52 pages, 16 figure
The Intermediate Higgs
Two paradigms for the origin of electroweak superconductivity are a weakly
coupled scalar condensate, and a strongly coupled fermion condensate. The
former suffers from a finetuning problem unless there are cancelations to
radiative corrections, while the latter presents potential discrepancies with
precision electroweak physics. Here we present a framework for electroweak
symmetry breaking which interpolates between these two paradigms, and mitigates
their faults. As in Little Higgs theories, the Higgs is a pseudo-Nambu
Goldstone boson, potentially composite. The cutoff sensitivity of the one loop
top quark contribution to the effective potential is canceled by contributions
from additional vector-like quarks, and the cutoff can naturally be higher than
in the minimal Standard Model. Unlike the Little Higgs models, the cutoff
sensitivity from one loop gauge contributions is not canceled. However, such
gauge contributions are naturally small as long as the cutoff is below 6 TeV.
Precision electroweak corrections are suppressed relative to those of
Technicolor or generic Little Higgs theories. In some versions of the
intermediate scenario, the Higgs mass is computable in terms of the masses of
these additional fermions and the Nambu-Goldstone Boson decay constant. In
addition to the Higgs, new scalar and pseudoscalar particles are typically
present at the weak scale
KK Parity in Warped Extra Dimension
We construct models with a Kaluza-Klein (KK) parity in a five- dimensional
warped geometry, in an attempt to address the little hierarchy problem present
in setups with bulk Standard Model fields. The lightest KK particle (LKP) is
stable and can play the role of dark matter. We consider the possibilities of
gluing two identical slices of 5D AdS in either the UV (IR-UV-IR model) or the
IR region (UV-IR-UV model) and discuss the model-building issues as well as
phenomenological properties in both cases. In particular, we find that the
UV-IR-UV model is not gravitationally stable and that additional mechanisms
might be required in the IR-UV-IR model in order to address flavor issues.
Collider signals of the warped KK parity are different from either the
conventional warped extra dimension without KK parity, in which the new
particles are not necessarily pair-produced, or the KK parity in flat universal
extra dimensions, where each KK level is nearly degenerate in mass. Dark matter
and collider properties of a TeV mass KK Z gauge boson as the LKP are
discussed.Comment: 35 pages, 11 figure
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