Denys Wilkinson Building

Oxford, United Kingdom

Denys Wilkinson Building

Oxford, United Kingdom
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Barr A.J.,Denys Wilkinson Building | Dolan M.J.,SLAC | Englert C.,University of Glasgow | de Lima D.E.F.,University of Glasgow | And 2 more authors.
Journal of High Energy Physics | Year: 2015

Abstract: An important physics goal of a possible next-generation high-energy hadron collider will be precision characterisation of the Higgs sector and electroweak symmetry breaking. A crucial part of understanding the nature of electroweak symmetry breaking is measuring the Higgs self-interactions. We study dihiggs production in proton-proton collisions at 100 TeV centre of mass energy in order to estimate the sensitivity such a machine would have to variations in the trilinear Higgs coupling around the Standard Model expectation. We focus on the (Formula presented.) final state, including possible enhancements in sensitivity by exploiting dihiggs recoils against a hard jet. We find that it should be possible to measure the trilinear self-coupling with 40% accuracy given 3/ab and 12% with 30/ab of data. © 2015, The Author(s).


Barr A.J.,Denys Wilkinson Building | Dolan M.J.,Durham University | Englert C.,University of Glasgow | Spannowsky M.,Durham University
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

Higgs boson self-interactions can be investigated via di-Higgs (pp → ± hh + X) production at the LHC. With a small O(30) fb Standard Model production cross section, and a large tt background, this measurement has been considered challenging, even at a luminosity-upgraded LHC. We demonstrate that by using simple kinematic bounding variables, of the sort already employed in existing LHC searches, the dominant tt background can be largely eliminated. Simulations of the signal and the dominant background demonstrate the prospect for measurement of the di-Higgs production cross section at the 30% level using 3 ab~1 of integrated luminosity at a high luminosity LHC. This corresponds to a Higgs self-coupling determination with 60% accuracy in the bbr+r~ mode, with potential for further improvements from e.g. subjet technologies and from additional di-Higgs decay channels. © 2013 The Authors. Published by Elsevier B.V.


Barr A.J.,Denys Wilkinson Building | Khoo T.J.,University of Cambridge | Konar P.,Theoretical Physics Group | Kong K.,University of Kansas | And 3 more authors.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

This paper seeks to demonstrate that many of the existing mass-measurement variables proposed for hadron colliders (mT, meff, m T2, missing p→T, hT, √s min, etc.) are far more closely related to each other than is widely appreciated, and indeed can all be viewed as a common mass-bound specialized for a variety of purposes. A consequence of this is that one may understand better the strengths and weaknesses of each variable, and the circumstances in which each can be used to best effect. In order to achieve this, we find it necessary first to revisit the seemingly empty and infertile wilderness populated by the subscript "T" (as in "pT") in order to remind ourselves what this process of transversification actually means. We note that, far from being simple, transversification can mean quite different things to different people. Those readers who manage to battle through the barrage of transverse notation distinguishing "T" from or from and "early projection" from "late projection," will find their efforts rewarded towards the end of the paper with (i)a better understanding of how collider mass variables fit together, (ii)an appreciation of how these variables could be generalized to search for things more complicated than supersymmetry, (iii)will depart with an aversion to thoughtless or naïve use of the so-called transverse methods of any of the popular computer Lorentz-vector libraries, and (iv)will take care in their subsequent papers to be explicit about which of the 61 identified variants of the "transverse mass" they are employing. © 2011 American Physical Society.


Bremer M.N.,Hh Wills Physics Laboratory | Silk J.,Denys Wilkinson Building | Davies L.J.M.,Hh Wills Physics Laboratory | Lehnert M.D.,Laboratoire dEtudes des Galaxies
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

We have carried out a redshift survey using the VIMOS spectrograph on the VLT towards the cosmic microwave background cold spot. A possible cause of the cold spot is the Integrated Sachs-Wolfe effect imprinted by an extremely large void (hundreds of Mpc in linear dimension) at intermediate or low redshifts. The redshift distribution of over seven hundred z < 1 emission-line galaxies drawn from an I-band flux limited sample of galaxies in the direction of the cold spot shows no evidence of a gap on scales of Δz ≳ 0.05 as would be expected if such a void existed at 0.35 < z < 1. There are troughs in the redshift distribution on smaller scales (Δz ≈ 0.01) indicating that smaller scale voids may connect regions separated by several degrees towards the cold spot. A comparison of this distribution with that generated from similarly sized subsamples drawn from widely spaced pointings of the VVDS survey does not indicate that the redshift distribution towards the cold spot is anomalous or that these small gaps can be uniquely attributed to real voids. © 2010 The Authors. Journal compilation © 2010 RAS.


Powell A.,Denys Wilkinson Building
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2011

Particle identification in the LHCb experiment is provided by two RICH detectors, covering a momentum range of approximately 1100 GeV/c. To maintain the integrity of the LHCb physics performance, it is essential to measure and monitor the particle identification efficiency and misidentification fraction over time. To achieve this, the unique decay kinematics associated with K S 0, Λ and φ decays are exploited to obtain high purity samples of pions, protons and kaons through the use of tracking information alone. Such calibration samples then allow for an unbiased assessment of the performance of the RICH detectors. Given the high production rates of K S 0, Λ and φ mesons at the LHC, monitoring of the RICH detectors performance is possible both on-line during running and off-line after full-reconstruction of the data. Following analysis of the first LHC collisions, the performance of the LHCb RICH detectors will be presented based on a preliminary alignment of the detector. Identification and mis-identification probabilities as a function of track momentum will be shown. © 2010 Elsevier B.V. All rights reserved.


Khochfar S.,Max Planck Institute for Extraterrestrial Physics | Silk J.,Denys Wilkinson Building
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We study the specific star formation rate (SSFR) and its evolution at z ≳ 4, in models of galaxy formation, where the star formation is driven by cold accretion flows. We show that constant star formation and feedback efficiencies cannot reproduce the observed trend of SSFR with stellar mass and its observed lack of evolution at z > 4. Model galaxies with log(M*) ≲ 9.5 M⊙ show systematically lower SSFRs by orders of magnitudes, while massive galaxies with M* ≳ 5 × 1010 M⊙ have up to an order of magnitude larger SSFRs, compared to recent observations by Stark et al. To recover these observations we apply an empirical star formation efficiency in galaxies that scales with the host halo velocity dispersion as ∝ 1/σ3 during galaxy mergers. We find that this modification needs to be of stochastic nature to reproduce the observations, i.e. only applied during mergers and not during accretion driven star formation phases. Our choice of star formation efficiency during mergers allows us to capture both, the boost in star formation at low masses and the quenching at high masses, and at the same time produce a constant SSFR-stellar mass relation at z ≳ 4 under the assumption that most of the observed galaxies are in a merger-triggered star formation phase. Our results suggest that observed high-z low-mass galaxies with high SSFRs are likely to be frequently interacting systems, which experienced bursts in their star formation rate and efficiency (mode 1), in contrast to low redshift z ≲ 3 galaxies which are cold accretion-regulated star forming systems with lower star formation efficiencies (mode 2). © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.


Weidberg R.,Denys Wilkinson Building
Journal of Instrumentation | Year: 2012

Vertical Cavity Surface Emitting Lasers (VCSELs) are used for optical transmission for all ATLAS detectors. There have been various problems with the reliability of the VCSELs used in some but not all of the sub-systems. This paper briefly reviews the design of VCSELs and the potential reliability issues. Some of the techniques used in Failure Analysis (FA) are explained with examples from studies of devices that failed during ATLAS operation. Estimates of the failure rates from the different systems are given and the implications for detector operations are discussed. The attempts to diagnose the causes of the problems are described. The mitigation strategies for the two sub-systems most seriously affected are described. Finally some conclusions about VCSEL reliability and the outlook for ATLAS operation will be given. © 2012 CERN.


Barr A.J.,Denys Wilkinson Building | Lester C.G.,University of Cambridge
Journal of Physics G: Nuclear and Particle Physics | Year: 2010

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. © 2010 IOP Publishing Ltd.


Barr A.J.,Denys Wilkinson Building | Gripaios B.,University of Cambridge | Lester C.G.,University of Cambridge
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012

We reconsider observables for discovering a heavy Higgs boson (with m h>2m W) via its dileptonic decays h→WW→ℓνℓν. We show that observables generalizing the transverse mass that take into account the fact that both of the intermediate W-bosons are likely to be on-shell give a significant improvement over the variables used in existing searches. We also comment on the application of these observables to other decays which proceed via narrow-width intermediates. © 2012 Elsevier B.V.


Powell A.,Denys Wilkinson Building
AIP Conference Proceedings | Year: 2012

We present studies from the LHCb experiment of decays of the type H b → HcX, where Hb represents a beauty hadron (B±, B0, Bs 0 or Λb) and Hc a charmed hadron (D0, D*+, Ds or Λc). Such decays are important for the determination of the CKM angle γ, a key goal of the LHCb physics programme. We report on the observation of new decay modes, and first measurements on the road to a precise determination of γ. We also present measurements of decays to hadronic final states without charm, H b→h+h-′. These charmless modes have a rich phenomenology and high sensitivity to possible contributions from New Physics. We exploit the data accumulated in 2010, and in the first half of the 2011 run. © 2012 American Institute of Physics.

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