1,783 research outputs found
A Review of the Mass Measurement Techniques proposed for the Large Hadron Collider
We review the methods which have been proposed for measuring masses of new
particles at the Large Hadron Collider paying particular attention to the
kinematical techniques suitable for extracting mass information when invisible
particles are expected.Comment: 72 pages - in form to be published in JPhys
Constrained invariant mass distributions in cascade decays. The shape of the "-threshold" and similar distributions
Considering the cascade decay in which
are massive particles and are massless particles, we
determine for the first time the shape of the distribution of the invariant
mass of the three massless particles for the sub-set of decays in
which the invariant mass of the last two particles in the chain is
(optionally) constrained to lie inside an arbitrary interval, . An example of an experimentally
important distribution of this kind is the `` threshold'' -- which is
the distribution of the combined invariant mass of the visible standard model
particles radiated from the hypothesised decay of a squark to the lightest
neutralino via successive two body decay,: \squark \to q \ntlinoTwo \to q l
\slepton \to q l l \ntlinoOne , in which the experimenter requires
additionally that be greater than . The
location of the ``foot'' of this distribution is often used to constrain
sparticle mass scales. The new results presented here permit the location of
this foot to be better understood as the shape of the distribution is derived.
The effects of varying the position of the cut(s) may now be seen more
easily.Comment: 12 pages, 3 figure
Improving estimates of the number of fake leptons and other mis-reconstructed objects in hadron collider events: BoB's your UNCLE. (Previously "The Matrix Method Reloaded")
We consider current and alternative approaches to setting limits on new
physics signals having backgrounds from misidentified objects; for example jets
misidentified as leptons, b-jets or photons. Many ATLAS and CMS analyses have
used a heuristic matrix method for estimating the background contribution from
such sources. We demonstrate that the matrix method suffers from statistical
shortcomings that can adversely affect its ability to set robust limits. A
rigorous alternative method is discussed, and is seen to produce fake rate
estimates and limits with better qualities, but is found to be too costly to
use. Having investigated the nature of the approximations used to derive the
matrix method, we propose a third strategy that is seen to marry the speed of
the matrix method to the performance and physicality of the more rigorous
approach.Comment: v1 :11 pages, 5 figures. v2: title change requested by referee, and
other corrections/clarifications found during review. v3: final tweaks
suggested during review + move from revtex to jhep styl
Efficient simulation techniques for biochemical reaction networks
Discrete-state, continuous-time Markov models are becoming commonplace in the
modelling of biochemical processes. The mathematical formulations that such
models lead to are opaque, and, due to their complexity, are often considered
analytically intractable. As such, a variety of Monte Carlo simulation
algorithms have been developed to explore model dynamics empirically. Whilst
well-known methods, such as the Gillespie Algorithm, can be implemented to
investigate a given model, the computational demands of traditional simulation
techniques remain a significant barrier to modern research.
In order to further develop and explore biologically relevant stochastic
models, new and efficient computational methods are required. In this thesis,
high-performance simulation algorithms are developed to estimate summary
statistics that characterise a chosen reaction network. The algorithms make use
of variance reduction techniques, which exploit statistical properties of the
model dynamics, to improve performance.
The multi-level method is an example of a variance reduction technique. The
method estimates summary statistics of well-mixed, spatially homogeneous models
by using estimates from multiple ensembles of sample paths of different
accuracies. In this thesis, the multi-level method is developed in three
directions: firstly, a nuanced implementation framework is described; secondly,
a reformulated method is applied to stiff reaction systems; and, finally,
different approaches to variance reduction are implemented and compared.
The variance reduction methods that underpin the multi-level method are then
re-purposed to understand how the dynamics of a spatially-extended Markov model
are affected by changes in its input parameters. By exploiting the inherent
dynamics of spatially-extended models, an efficient finite difference scheme is
used to estimate parametric sensitivities robustly.Comment: Doctor of Philosophy thesis submitted at the University of Oxford.
This research was supervised by Prof Ruth E. Baker and Dr Christian A. Yate
Re-weighing the evidence for a Higgs boson in dileptonic W-boson decays
We reconsider observables for discovering and measuring the mass of a Higgs
boson via its di-leptonic decays: H --> WW* --> l nu l nu. We define an
observable generalizing the transverse mass that takes into account the fact
that one of the intermediate W-bosons is likely to be on-shell. We compare this
new variable with existing ones and argue that it gives a significant
improvement for discovery in the region m_h < 2 m_W.Comment: 3 pages, 2 figures. Changes in v2: (i) implemented a model of
detector smearing, (ii) switched LHC simulation from 14 TeV to 7 TeV running,
(iii) presenting results for 10 rather than 3 inverse femtobarns, (iv)
corrected a typo in Fig 2 legend. Changes in v3: included published erratu
A Search for the Higgs Boson Produced in Association With Top Quarks in Multilepton Final States at Atlas
This thesis presents preliminary results of a search for Higgs boson production
in association with top quarks in multilepton final states. The search
was conducted in the 2012 dataset of proton-proton
collisions delivered by the CERN Large Hadron Collider at a center-of-mass energy of 8 TeV and collected by
the ATLAS experiment. The dataset corresponds to an integrated luminosity of
20.3 inverse femtobarns. The analysis is conducted by measuring event counts in
signal regions distinguished by the number of leptons (2 same-sign,
3, and 4), jets and b-tagged jets present in the reconstructed events. The observed
events in the signal regions constitute an excess over
the expected number of background events. The results
are evaluated using a frequentist statistical model.
The observed exclusion upper limit at the 95 % confidence
level is 5.50 times the predicted Standard Model production cross section for Higgs production in association with top quarks. The fitted value of the ratio of the observed production rate to the expected Standard Model
production rate is 2.83 1.58 1.35
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Ring identification and pattern recognition in ring imaging Cerenkov (RICH) detectors.
An algorithm for identifying rings in Ring Imaging Cherenkov (RICH) detectors is
described. The algorithm is necessarily Bayesian and makes use of a Metropolis-
Hastings Markov chain Monte Carlo sampler to locate the rings. In particular, the
sampler employs a novel proposal function whose form is responsible for significant
speed improvements over similar methods. The method is optimised for finding
multiple overlapping rings in detectors which can be modelled well by the LHbC
RICH toy model described herein
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Constrained invariant mass distributions in cascade decays: The Shape of the 'm(qll)-threshold' and similar distributions.
Considering the cascade decay D â cC â cbB â cbaA in which D,C,B,A are
massive particles and c, b, a are massless particles, we determine for the shape of the
distribution of the invariant mass of the three massless particles mabc for the sub-set
of decays in which the invariant mass mab of the last two particles in the chain is (optionally)
constrained to lie inside an arbitrary interval, mab â [mcut min
ab ,mcut max
ab ].
An example of an experimentally important distribution of this kind is the âmqll
thresholdâ â which is the distribution of the combined invariant mass of the visible
standard model particles radiated from the hypothesised decay of a squark to
the lightest neutralino via successive two body decay,: Ëq â q Ë 02
â qlËl â qll Ë 01
, in
which the experimenter requires additionally that mll be greater than mmax
ll /â2.
The location of the âfootâ of this distribution is often used to constrain sparticle
mass scales. The new results presented here permit the location of this foot to be
better understood as the shape of the distribution is derived. The effects of varying
the position of the mll cut(s) may now be seen more easily
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