615 research outputs found
Odd Tracks at Hadron Colliders
New physics that exhibits irregular tracks such as kinks, intermittent hits
or decay in flight may easily be missed at hadron colliders. We demonstrate
this by studying viable models of light, O(10 GeV), colored particles that
decay predominantly inside the tracker. Such particles can be produced at
staggering rates, and yet may not be identified or even triggered on at the
LHC, unless specifically searched for. In addition, the models we study provide
an explanation for the original measurement of the anomalous charged track
distribution by CDF. The presence of irregular tracks in these models reconcile
that measurement with the subsequent reanalysis and the null results of ATLAS
and CMS. Our study clearly illustrates the need for a comprehensive study of
irregular tracks at the LHC.Comment: 6 pages, 1 figur
Flavorful Supersymmetry
Weak scale supersymmetry provides elegant solutions to many of the problems
of the standard model, but it also generically gives rise to excessive flavor
and CP violation. We show that if the mechanism that suppresses the Yukawa
couplings also suppresses flavor changing interactions in the supersymmetry
breaking parameters, essentially all the low energy flavor and CP constraints
can be satisfied. The standard assumption of flavor universality in the
supersymmetry breaking sector is not necessary. We study signatures of this
framework at the LHC. The mass splitting among different generations of squarks
and sleptons can be much larger than in conventional scenarios, and even the
mass ordering can be changed. We find that there is a plausible scenario in
which the NLSP is a long-lived right-handed selectron or smuon decaying into
the LSP gravitino. This leads to the spectacular signature of monochromatic
electrons or muons in a stopper detector, providing strong evidence for the
framework.Comment: 20 pages; typos corrected, comments added, to appear in PR
Monojet versus rest of the world I: t-channel Models
Monojet searches using Effective Field Theory (EFT) operators are usually
interpreted as a robust and model independent constraint on direct detection
(DD) scattering cross-sections. At the same time, a mediator particle must be
present to produce the dark matter (DM) at the LHC. This mediator particle may
be produced on shell, so that direct searches for the mediating particle can
constrain the effective operator being applied to monojet constraints. In this
first paper, we do a case study on t-channel models in monojet searches, where
the (Standard Model singlet) DM is pair produced via a t-channel mediating
particle, whose supersymmetric analogue is the squark. We compare monojet
constraints to direct constraints on single or pair production of the mediator
from multi-jets plus missing energy searches and we identify the regions where
the latter dominate over the former. We show that computing bounds using
supersymmetric simplified models and in the narrow width approximation, as done
in previous work in the literature, misses important quantitative effects. We
perform a full event simulation and statistical analysis, and we compute the
effects of both on- and off-shell production of the mediating particle, showing
that for both the monojet and multi-jets plus missing energy searches,
previously derived bounds provided more conservative bounds than what can be
extracted by including all relevant processes in the simulation. Monojets and
searches for supersymmetry (SUSY) provide comparable bounds on a wide range of
the parameter space, with SUSY searches usually providing stronger bounds,
except in the regions where the DM particle and the mediator are very mass
degenerate. The EFT approximation rarely is able to reproduce the actual
limits. In a second paper to follow, we consider the case of s-channel
mediators.Comment: 22 pages + appendices, 10 figure
Fastlim: a fast LHC limit calculator
Fastlim is a tool to calculate conservative limits on extensions of the
Standard Model from direct LHC searches without performing any Monte Carlo
event generation. The program reconstructs the visible cross sections from
pre-calculated efficiency tables and cross section tables for simplified event
topologies. As a proof of concept of the approach, we have implemented searches
relevant for supersymmetric models with R-parity conservation. Fastlim takes
the spectrum and coupling information of a given model point and provides, for
each signal region of the implemented analyses, the visible cross sections
normalised to the corresponding upper limit, reported by the experiments, as
well as the exclusion -value. To demonstrate the utility of the program we
study the sensitivity of the recent ATLAS missing energy searches to the
parameter space of natural SUSY models. The program structure allows the
straight-forward inclusion of external efficiency tables and can be generalised
to R-parity violating scenarios and non-SUSY models. This paper serves as a
self-contained user guide, and indicates the conventions and approximations
used.Comment: 39 pages, 43 figure
Constraining the dipole moments of the top quark
We investigate the direct and indirect bounds on dipole operators involving
the top quark. A careful analysis shows that the experimental upper limit on
the neutron electric dipole moment strongly constrains the chromo-electric
dipole of the top. We improve previous bounds by two orders of magnitude. This
has significant implications for new physics models and it also means that CP
violation in top pair production mediated by dipole operators will not be
accessible at the LHC. The CP conserving chromo-magnetic dipole moments are
constrained by recent measurements of the t\bar t spectrum by the ATLAS
collaboration. We also update the indirect constraints on electric and magnetic
dipole moments from radiative b -> s transitions, finding that they can be
considerably larger than their colored counterparts.Comment: 6 pages, 3 figures; extended and updated discussion of constraints on
top EDM and MDM from rare B decays; typos correcte
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Study of position effect as a mechanism arising from chromosomal translocations in leukaemia
This thesis was submitted for the award of Master of Philosophy and was awarded by Brunel University London.The chromosomal translocation of t(14;18)(14q32;18q21) is a characteristic aberration of follicular lymphoma and Diffuse Large B cells lymphoma. By PCR, it was proved that the rearrangement of chromosomes 14 and 18 leads to an overexpression of BCL2, an anti-apoptotic protein, which is one of the factors responsible for the maturation of the diseases. The translocation involves the promoter region of IGH gene and the transcriptional unit of BCL2 gene. Previous studies carried out in Dr Tosi’s lab showed a looping out of the BCL2 gene from its chromosome territory in 15% of the nuclei analysed. This looping out could be possibly responsible for the transcriptional activity of the gene. A further relevant finding concerns the spatial distribution of the genes involved in the translocation in the interphase nuclei. In the Pfeiffer cell line, harbouring the t(14;18) rearrangement, the translocated BCL2 gene was positioned in the cell nuclei according to a bimodal distribution. One could speculate that the distribution in the periphery and in the centre of the nuclei could divide the Pfeiffer cell line in two different subpopulations, consequently from the transcriptional activity. These preliminary data set the ground for more experimental work to test whether genes associated with the nuclear interior were transcriptionally active as opposed to the genes positioned towards the nuclear periphery, transcriptionally inactive. The work here presented focuses on this investigation using RNA-DNA FISH (Fluorescence in situ hybridization). My work enabled the detection of IGH, BCL2 and t(14;18) genes along with their transcripts inside of the nuclei of Pfeiffer cell line. Contrary to what had been hinted by previous work, my results showed multiple nuclear positions of transcriptionally active IGH/BCL2 translocation. The result will need to be further supported by software analysis in order to define its specific nuclear position and to ensure the perfect localization of the genes inside each nucleus
Sensitivity of a small matter-wave interferometer to gravitational waves
We study the possibility of using matter wave interferometry techniques to
build a gravitational wave detector. We derive the response function and find
that it contains a term proportional to the derivative of the gravitational
wave, a point which has been disputed recently. We then study in detail the
sensitivity that can be reached by such a detector and find that, if it is
operated near resonance, it can reach potentially interesting values in the
high frequency regime. The correlation between two or more of such devices can
further improve the sensitivity for a stochastic signal.Comment: Revtex4, 19 pages, 3 figures. Several changes in the calculation of
the response function. Accepted by PR
Implications of the dimuon CP asymmetry in B_{d,s} decays
The D0 Collaboration reported a 3.2sigma deviation from the standard model
prediction in the like-sign dimuon asymmetry. Assuming that new physics
contributes only to B_{d,s} mixing, we show that the data can be analyzed
without using the theoretical calculation of \Delta\Gamma_s, allowing for
robust interpretations. We find that this framework gives a good fit to all
measurements, including the recent CDF S_{\psi\phi} result. The data allow
universal new physics with similar contributions relative to the SM in the B_d
and B_s systems, but favors a larger deviation in B_s than in B_d mixing. The
general minimal flavor violation framework with flavor diagonal CP violating
phases can account for the former and remarkably even for the latter case. This
observation makes it simpler to speculate about which extensions with general
flavor structure may also fit the data.Comment: 5 pages, 4 figures; final version accepted in PR
Electroweak Symmetry Breaking with a compact extra dimension
[Excerpt from the preface]: During my Ph.D. course, under the supervision of Riccardo Barbieri, I started
working on Theoretical High Energy Physics. In particular I focused on the problem
of Electroweak Symmetry Breaking and in this context I investigated the
bene ts of introducing a compact extra dimension.
In a series of papers, together with Riccardo Barbieri, Guido Marandella,
Lawrence Hall, Yasunori Nomura, Takemichi Okui and Steven Oliver I built supersymmetric
models in 5D and studied their phenomenology in detail [1, 2, 3, 4].
In this models Supersymmetry is broken by boundary conditions in the fth dimension
and the ElectroWeak Symmetry Breaking is triggered by Supersymmetry
breaking via the top/stop radiative corrections. The phenomenology of these models
is signi cantly di erent than the one of the Minimal Supersymmetric Standard
Model scenarios extensively studied in the literature.
These papers constitute the subject of this Thesis and their results will be
presented in the following Chapters
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