Results for "J. R. Spencer"

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Aspects of Coulomb Dissociation and Interference in Peripheral Nucleus-Nucleus CollisionsMar 22 2002Coherent vector meson production in peripheral nucleus-nucleus collisions is discussed. These interactions may occur for impact parameters much larger than the sum of the nuclear radii. Since the vector meson production is always localized to one of the ... More
Coherent Vector Meson Photoproduction with Nuclear Breakup in Relativistic Heavy Ion CollisionsMay 10 2002Relativistic heavy ions are copious sources of virtual photons. The large photon flux gives rise to a substantial photonuclear interaction probability at impact parameters where no hadronic interactions can occur. Multiple photonuclear interactions in ... More
Two-Photon Interactions with Nuclear Breakup in Relativistic Heavy Ion CollisionsJul 07 2009Aug 26 2009Highly charged relativistic heavy ions have high cross-sections for two-photon interactions. The photon flux is high enough that two-photon interactions may be accompanied by additional photonuclear interactions. Except for the shared impact parameter, ... More
Cherenkov Radiation from $e^+e^-$ Pairs and Its Effect on $ν_e$ Induced ShowersJun 08 2005Nov 03 2005We calculate the Cherenkov radiation from an $e^+e^-$ pair at small separations, as occurs shortly after a pair conversion. The radiation is reduced (compared to that from two independent particles) when the pair separation is smaller than the wavelength ... More
Heavy ion beam loss mechanisms at an electron-ion colliderSep 18 2014There are currently several proposals to build a high-luminosity electron-ion collider, to study the spin structure of matter and measure parton densities in heavy nuclei, and to search for gluon saturation and new phenomena like the colored glass condensate. ... More
Localized Beampipe Heating due to $e^-$ Capture and Nuclear Excitation in Heavy Ion CollidersMay 11 2000Aug 04 2000At heavy ion colliders, two major sources of beam loss are expected to be $e^+e^-$ production, where the $e^-$ is bound to one of the nuclei, and photonuclear excitation and decay via neutron emission. Both processes alter the ions charged to mass ratio ... More
The LPM Effect: Comparing SLAC E-146 Data with ExperimentAug 05 1998The suppression of photon bremsstrahlung due to a variety of in-medium effects is discussed. Different electrodynamic suppression effects are discussed, and compared with the related color analogs. Higher order effects are considered, and found to be ... More
Bremsstrahlung and Pair Creation: Suppression Mechanisms and How They Affect EHE Air ShowersDec 16 1997Dec 11 2000Most calculations of air shower development have been based on the Bethe-Heitler cross sections for bremsstrahlung and pair production. However, for energetic enough particles, a number of different external factors can reduce these cross sections drastically, ... More
Ultra-peripheral Collisions at RHIC: An Experimental OverviewFeb 24 2015Ultra-peripheral collisions (UPCs) of ions allow us to study photonuclear and two-photon interactions at energies above those available at fixed target accelerators. For heavy ions, the couplings are large enough so that multi-photon interactions are ... More
Radiodetection of NeutrinosDec 07 2010Despite 100 years of effort, we still know very little about the origin of ultra-high energy cosmic rays. The observation of neutrinos produced when cosmic-ray protons with energies above $4\times 10^{19}$ eV interact with the cosmic microwave background ... More
Heavy Nuclei from RHIC to the CosmosNov 01 2002Ultra-relativistic heavy ion collisions produce a high-temperature, thermalized system that may mimic the conditions present shortly after the big bang. This writeup will given an overview of early results from the Relativistic Heavy Ion Collider (RHIC), ... More
Comment on "$η_c$ production in photon-induced interactions at the LHC"Aug 24 2018In "$\eta_c$ production in photon-induced interactions at the LHC," \cite{Goncalves:2018yxc} Goncalves and Moreira discuss inclusive and exclusive $\eta_c$ production at $pp$ and $pA$ collisions at LHC energies. The exclusive channels are via two-photon ... More
$e^+e^-$ Pair Production from 10 GeV to 10 ZeVFeb 16 2004Dec 07 2004At very high energies, pair production ($\gamma\to e^+e^-$) exhibits many interesting features. The momentum transfer from the target is very small, so the reaction probes the macroscopic properties of the target, rather than individual nuclei. Interference ... More
Muon Production in Relativistic Cosmic-Ray InteractionsJul 27 2009Sep 30 2009Cosmic-rays with energies up to $3\times10^{20}$ eV have been observed. The nuclear composition of these cosmic rays is unknown but if the incident nuclei are protons then the corresponding center of mass energy is $\sqrt{s_{nn}} = 700$ TeV. High energy ... More
Electrodynamics at the Highest EnergiesDec 13 2001At very high energies, the bremsstrahlung and pair production cross sections exhibit complex behavior due to the material in which the interactions occur. The cross sections in dense media can be dramatically different than for isolated atoms. This writeup ... More
Recent Results from RHIC & Some Lessons for Cosmic-Ray PhysicistsNov 29 2006The Relativistic Heavy Ion Collider (RHIC) studies nuclear matter under a variety of conditions. Cold nuclear matter is probed with deuteron-gold collisions, while hot nuclear matter(possibly a quark-gluon plasma (QGP)) is created in heavy-ion collisions. ... More
Particle Interactions in Matter at the Terascale: the Cosmic-Ray ExperienceDec 03 2012Cosmic-rays with energies up to $3\times10^{20}$ eV have been observed, as have as have astrophysical neutrinos with energies above 1 PeV. In this talk, I will discuss some of the unique phenomena that occur when particles with TeV energies and above ... More
Nonlinear QED Effects in Heavy Ion CollisionsDec 11 2000Peripheral collisions of relativistic heavy ions uniquely probe many aspects of QED. Examples include $e^+e^-$ pair production and nuclear excitation in strong fields. After discussing these reactions, I will draw parallels between $\gamma\to e^+e^-$ ... More
Suppression of Bremsstrahlung and Pair Production due to Environmental FactorsFeb 27 1998Mar 05 1999The environment in which bremsstrahlung and pair creation occurs can strongly affect cross sections for these processes. Because ultra-relativistic electromagnetic interactions involve very small longitudinal momentum transfers, the reactions occur gradually, ... More
Ultra-peripheral collisions and hadronic structureApr 16 2017Jul 11 2017Ultra-peripheral collisions are the energy frontier for photon-mediated interactions, reaching, at the Large Hadron Collider (LHC), $\gamma-p$ center of mass energies five to ten times higher than at HERA and reaching $\gamma\gamma$ energies higher than ... More
Studying High $p_T$ Muons in Cosmic-Ray Air ShowersDec 02 2006Most cosmic-ray air shower arrays have focused on detecting electromagnetic shower particles and low energy muons. A few groups (most notably MACRO + EASTOP and SPASE + AMANDA) have studied the high energy muon component of showers. However, these experiments ... More
Photoproduction at RHIC and the LHCOct 16 2008The strong electromagnetic fields carried by relativistic highly charged ions make heavy-ion colliders attractive places to study photonuclear interactions and two-photon interactions. At RHIC, three experiments have studied coherent photoproduction of ... More
Cascades from $ν_e$ above $10^{20}$ eVDec 21 2004At very high energies, the Landau-Pomeranchuk-Migdal effect reduces the cross sections for electron bremsstrahlung and photon $e^+e^-$ pair production. The fractional electron energy loss and pair production cross sections drop as the energy increases. ... More
The morphology of the emission line region of Compact Steep Spectrum radio sourcesJun 26 2000We present the results of HST narrow band imaging of eleven Compact Steep Spectrum (CSS) radio sources. Five of them (3C 48, 3C 147, 3C303.1, 3C 277.1 and 4C 12.50) were part of a dedicated ``pointed'' program of deep line imaging ([OIII]). For six additional ... More
CSSs in a sample of B2 radio sources of intermediate strengthJun 04 2002We present radio observations of 19 candidate compact steep-spectrum (CSS) objects selected from a well-defined, complete sample of 52 B2 radio sources of intermediate strength. These observations were made with the VLA A-array at 4.835 GHz. The radio ... More
High-resolution radio observations of GX 339-4Jan 24 1997We present radio observations of the black hole candidate X-ray binary GX 339-4 with the Australia Telescope compact array. Mapping of the highest resolution 3.5 cm data reveals a jet-like extension, which if confirmed would be the first detection of ... More
IceCube: An Instrument for Neutrino AstronomyJul 07 2010Aug 24 2010Neutrino astronomy beyond the Sun was first imagined in the late 1950s; by the 1970s, it was realized that kilometer-scale neutrino detectors were required. The first such instrument, IceCube, is near completion and taking data. The IceCube project transforms ... More
Electron and Photon Interactions in the Regime of Strong LPM SuppressionJun 30 2010Sep 22 2010Most searches for ultra-high energy (UHE) astrophysical neutrinos look for radio emission from the electromagnetic and hadronic showers produced in their interactions. The radio frequency spectrum and angular distribution depend on the shower development, ... More
Contracting orbits in Outer spaceFeb 13 2015Jul 22 2015We show that strongly contracting geodesics in Outer space project to parameterized quasigeodesics in the free factor complex. This result provides a converse to a theorem of Bestvina--Feighn, and is used to give conditions for when a subgroup of $\mathrm{Out}(\mathbb{F})$ ... More
Hyperbolic extensions of free groupsJun 10 2014Nov 29 2016Given a finitely generated subgroup $\Gamma \le \mathrm{Out}(\mathbb{F})$ of the outer automorphism group of the rank $r$ free group $\mathbb{F} = F_r$, there is a corresponding free group extension $1 \to \mathbb{F} \to E_{\Gamma} \to \Gamma \to 1$. ... More
Rank and Nielsen equivalence in hyperbolic extensionsJun 07 2017Dec 20 2018In this note, we generalize a theorem of Juan Souto on rank and Nielsen equivalence in the fundamental group of a hyperbolic fibered 3-manifold to a large class of hyperbolic group extensions. This includes all hyperbolic extensions of surfaces groups ... More
Compact steep-spectrum sources from the S4 sampleSep 12 2000We present the results of 5-GHz observations with the VLA A-array of a sample of candidate Compact Steep Spectrum sources (CSSs) selected from the S4 survey. We also estimate the symmetry parameters of high-luminosity CSSs selected from different samples ... More
Spontaneous symmetry breaking of a hyperbolic sigma model in three dimensionsOct 11 2004Non-linear sigma models that arise from the supersymmetric approach to disordered electron systems contain a non-compact bosonic sector. We study the model with target space H^2, the two-hyperboloid with isometry group SU(1,1), and prove that in three ... More
Neutrino Absorption in the Earth, Neutrino Cross-Sections, and New PhysicsApr 17 2013Large neutrino telescopes can measure the neutrino-nucleon cross-section by studying neutrino absorption in the Earth.
Photoproduction of Quarkonium in Proton-Proton and Nucleus-Nucleus CollisionsNov 13 2003We discuss the photoproduction of $\Upsilon$ and $J/\psi$ at high energy $\bar{p}p$, $pp$ and heavy ion colliders. We predict large rates in $\bar{p}p$ interactions at the Fermilab Tevatron %and in heavy-ion interactions at the CERN LHC. These reactions ... More
Does particle decay cause wave function collapse: An experimental testJun 11 2002Jan 08 2003We describe an experimental test of whether particle decay causes wave function collapse. The test uses interference between two well separated, but coherent, sources of vector mesons. The short-lived mesons decay before their wave functions can overlap, ... More
Interference in Exclusive Vector Meson Production in Heavy Ion CollisionsSep 03 1999Photons emitted from the electromagnetic fields of relativistic heavy ions can fluctuate into quark anti-quark pairs and scatter from a target nucleus, emerging as vector mesons. These coherent interactions are identifiable by final states consisting ... More
FIRST-based survey of Compact Steep Spectrum sources I. MERLIN images of arc-second scale objectsDec 21 2001Apr 11 2002Compact Steep Spectrum (CSS) sources are powerful extragalactic radio sources with angular dimensions of the order of a few arcseconds or less. Such a compactness is apparently linked to the youth of these objects. The majority of CSSs investigated so ... More
Multiple ejections during the 1975 outburst of A0620-00Feb 25 1999The well-known black-hole X-ray transient A0620-00 was a bright radio source during the first part of its outburst in 1975. We have revisited the available data and find for the first time evidence that the source exhibited multiple jet ejections. Rapid ... More
Radio flares and plasmon size in Cygnus X-3Apr 29 2001We have observed a number of minor radio flares in Cyg X-3 using the MERLIN array. Photometric observations show the system to be highly active with multiple flares on hourly timescales over the one month observing programme. Analysis of the source's ... More
The co-surface graph and the geometry of hyperbolic free group extensionsJan 01 2016We introduce the co-surface graph $\mathcal{CS}$ of a finitely generated free group $\mathbb{F}$ and use it to study the geometry of hyperbolic group extensions of $\mathbb{F}$. Among other things, we show that the Gromov boundary of the co-surface graph ... More
Hyperbolic extensions of free groupsJun 10 2014Given a finitely generated subgroup $\Gamma \le \mathrm{Out}(\mathbb{F})$ of the outer automorphism group of the rank $r$ free group $\mathbb{F} = F_r$, there is a corresponding free group extension $1 \to \mathbb{F} \to E_{\Gamma} \to \Gamma \to 1$. ... More
A Fokker-Planck reaction model for the epitaxial growth and shape transition of quantum dotsApr 11 2017We construct a Fokker-Planck reaction model to investigate the dynamics of the coupled epitaxial growth and shape transition process of an array of quantum dots. The Fokker-Planck reaction model is based on a coupled system of Fokker-Planck equations ... More
Prospects for Non-Standard Interactions at MiniBooNEOct 01 2006MiniBooNE is expected to soon report analysis of their closed data set, and we here consider what effect direct non-standard neutrino interactions would have on their observations. Current direct limits on non-standard interactions make interpretation ... More
The New Horizons Kuiper Belt Extended MissionJun 21 2018The central objective of the New Horizons prime mission was to make the first exploration of Pluto and its system of moons. Following that, New Horizons has been approved for its first extended mission, which has the objectives of extensively studying ... More
New Cooled Feeds for the Allen Telescope ArrayFeb 08 2017We have developed a new generation of low noise, broadband feeds for the Allen Telescope Array at the Hat Creek Observatory in Northern California. The new feeds operate over the frequency range 0.9 to 14 GHz. The noise temperatures of the feeds have ... More
The symbiotic star CH Cygni. III. A precessing radio jetSep 05 2002VLA, MERLIN and Hubble Space Telescope imaging observations of the extended regions of the symbiotic system CH Cygni are analysed. These extensions are evidence of a strong collimation mechanism, probably an accretion disk surrounding the hot component ... More
Symmetry Parameters of CSS Sources: Evidence of Fuelling?May 06 2003The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed to be young radiosources, with ages <10^6 yr. If the activity in the nucleus is fuelled by the supply of gas, one might find evidence of this gas by studying ... More
Evidence for deceleration in the radio jets of GRS1915+105?Dec 08 2006There is currently a clear discrepancy in the proper motions measured on different angular scales in the approaching radio jets of the black hole X-ray binary GRS1915+105. Lower velocities were measured with the Very Large Array (VLA) prior to 1996 than ... More
Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observationsApr 12 2007We present 0.8 to 2.4 micron spectral observations of uranian satellites, obtained at IRTF/SpeX on 17 nights during 2001-2005. The spectra reveal for the first time the presence of CO2 ice on the surfaces of Umbriel and Titania, by means of 3 narrow absorption ... More
Spectral evidence for a powerful compact jet from XTE J1118+480Jan 19 2001We present observations of the X-ray transient XTE J1118+480 during its Low/Hard X-ray state outburst in 2000, at radio and sub-millimetre wavelengths with the VLA, Ryle Telescope, MERLIN and JCMT. The high-resolution MERLIN observations reveal all the ... More
Heavy Quark Photoproduction in Ultra-peripheral Heavy Ion CollisionsJun 22 2002Aug 09 2002Heavy quarks are copiously produced in ultra-peripheral heavy ion collisions. In the strong electromagnetic fields, c c-bar and b b-bar are produced by photonuclear and two-photon interactions; hadroproduction can occur in grazing interactions. We present ... More
Photoproduction at Hadron CollidersJun 09 2005Photoproduction can be studied at hadron colliders by using the virtual photons associated with the hadron beams. The LHC will reach photonucleon energies 10 times higher than that available elsewhere. These reactions are already being studied at RHIC. ... More
Quantum Interferometry in $ρ^0$ Production in Ultra-Peripheral Heavy Ion CollisionsFeb 05 2004In $\rho^0$ photoproduction in ultra-peripheral heavy ion collisions, either ion can be the photon emitter or the target. The two possibilities are indistinguishable, and they should be able to interfere, reducing $rho^0$ production at low transverse ... More
Recent Highlights from IceCubeNov 26 2013The $\sim$1 km$^3$ IceCube neutrino observatory was completed in December, 2010 and is taking data on cosmic-ray muons and neutrinos, extra-terrestrial neutrinos, and setting limits on a variety of exotic phenomena. This proceeding will cover recent IceCube ... More
ARIANNA: A radio detector array for cosmic neutrinos on the Ross Ice ShelfJul 16 2012Nov 01 2012ARIANNA (The Antarctic Ross Ice Shelf Antenna Neutrino Array) is a proposed 100 km^3 detector for ultra-high energy (above 10^17 eV) astrophysical neutrinos. It will study the origins of ultra-high energy cosmic rays by searching for the neutrinos produced ... More
Dipping in Cygnus X-2 in a multi-wavelength campaign due to absorption of extended ADC emissionApr 09 2011We report results of one-day simultaneous multiwavelength observations of Cygnus X-2 using XMM, Chandra, the European VLBI Network and the XMM Optical Monitor. During the observations, the source did not exhibit Z-track movement, but remained in the vicinity ... More
IceCube: A Cubic Kilometer Radiation DetectorJun 30 2008Dec 01 2008IceCube is a 1 km^3 neutrino detector now being built at the Amundsen-Scott South Pole Station. It consists of 4800 Digital Optical Modules (DOMs) which detect Cherenkov radiation from the charged particles produced in neutrino interactions. IceCube will ... More
On the detectability of eccentric binary pulsarsFeb 20 2013Mar 27 2013By generalizing earlier work of Johnston & Kulkarni, we present a detailed description of the reduction in the signal-to-noise ratio for observations of binary pulsars. We present analytical expressions, and provide software, to calculate the sensitivity ... More
Manipulation of the spin helix in FeGe thin films and FeGe/Fe multilayersJun 04 2015Magnetic materials without structural inversion symmetry can display the Dzyaloshinskii-Moriya interaction, which manifests itself as chiral magnetic ground states. These chiral states can interact in complex ways with applied fields and boundary conditions ... More
Visible Cascade Higgs Decays to Four Photons at Hadron CollidersAug 29 2006The presence of a new singlet scalar particle a can open up new decay channels for the Higgs boson, through cascades of the form h -> 2a -> X, possibly making discovery through standard model channels impossible. If a is CP-odd, its decay products are ... More
Photoproduction of top in peripheral heavy ion collisionsMay 16 2000Jul 09 2001In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs\"acker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the ... More
First Results from IceCubeJan 12 2006IceCube is a 1 km$^3$ neutrino observatory being built to study neutrino production in active galactic nuclei, gamma-ray bursts, supernova remnants, and a host of other astrophysical sources. High-energy neutrinos may signal the sources of ultra-high ... More
A new contribution to the conventional atmospheric neutrino fluxSep 17 2014Dec 15 2014Atmospheric neutrinos are an important background to astrophysical neutrino searches, and are also of considerable interest in their own right. This paper points out that the contribution to conventional atmospheric $\nu_e$ of the rare semileptonic decay ... More
Loki, Io: New groundbased observations and a model describing the change from periodic overturnMay 09 2006Loki Patera is the most powerful volcano in the solar system. We have obtained measurements of Loki's 3.5 micron brightness from NASA's Infrared Telescope Facility (IRTF) and have witnessed a change from the periodic behavior previously noted. While Loki ... More
High energy cosmic-ray interactions with particles from the SunMar 25 2011Cosmic-ray protons with energies above $10^{16}$ eV passing near the Sun may interact with photons emitted by the Sun and be excited to a $\Delta^+$ resonance. When the $\Delta^+$ decays, it produces pions which further decay to muons and photons which ... More
Recent $ν$s from IceCubeOct 03 2008IceCube is a 1 km$^3$ neutrino detector now being built at the South Pole. Its 4800 optical modules will detect Cherenkov radiation from charged particles produced in neutrino interactions. IceCube will search for neutrinos of astrophysical origin, with ... More
The detectability of eccentric binary pulsarsJan 15 2014Studies of binary pulsars provide insight into various theories of physics. Detection of such systems is challenging due to the Doppler modulation of the pulsed signal caused by the orbital motion of the pulsar. We investigated the loss of sensitivity ... More
Photoproduction of charged final states in ultra-peripheral collisions and electroproduction at an electron-ion colliderMar 07 2019Ultra-peripheral collisions (UPCs) of relativistic ions are an important tool for studying photoproduction at high energies. Vector meson photoproduction is an important tool for nuclear structure measurements and other applications. A future electron-ion ... More
Supersymmetric and Kaluza-Klein Particles Multiple Scattering in the EarthMay 19 2009Neutrino telescopes with cubic kilometer volume have the potential to discover new particles. Among them are next to lightest supersymmetric (NLSPs) and next to lightest Kaluza-Klein (NLKPs) particles. Two NLSPs or NLKPs will transverse the detector simultaneously ... More
Quasi-diffusion in a 3D Supersymmetric Hyperbolic Sigma ModelJan 12 2009We study a lattice field model which qualitatively reflects the phenomenon of Anderson localization and delocalization for real symmetric band matrices. In this statistical mechanics model, the field takes values in a supermanifold based on the hyperbolic ... More
Double Neutrino Production and Detection in Neutrino DetectorsMay 22 2013Large, high-energy ($E>100$ GeV) cosmic neutrino telescopes are now quite mature. IceCube, for example, observes about 50,000 well-reconstructed single atmospheric neutrino events/year, with energies above 100 GeV. Although the neutrino detection probability ... More
Neutral Plasma Oscillations at Zero TemperatureApr 29 2002We use cold plasma theory to calculate the response of an ultracold neutral plasma to an applied rf field. The free oscillation of the system has a continuous spectrum and an associated damped quasimode. We show that this quasimode dominates the driven ... More
Ultra-Peripheral Collisions with gold ions in STARJun 08 2016In 2010, the STAR Collaboration collected 38 million low-multiplicity triggers for ultra-peripheral collision studies. We present new results involving photonuclear interactions in ultra-peripheral relativistic heavy ion collisions (UPCs), based on an ... More
In Situ Observations of Preferential Pickup Ion Heating at an Interplanetary ShockJul 13 2018Non-thermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed, that PUIs should be preferentially heated at quasi-perpendicular ... More
Universal Control of Ion Qubits in a Scalable Microfabricated Planar TrapSep 17 2015Feb 19 2016We demonstrate universal quantum control over chains of ions in a surface-electrode ion trap, including all the fundamental operations necessary to perform algorithms in a one-dimensional, nearest-neighbor quantum computing architecture. We realize both ... More
Modelling of nematic liquid crystal display devicesFeb 17 2006A lattice Boltzmann scheme is presented which recovers the dynamics of nematic and chiral liquid crystals; the method essentially gives solutions to the Qian-Sheng equations for the evolution of the velocity and tensor order-parameter fields. The resulting ... More
Endomorphisms, train track maps, and fully irreducible monodromiesJul 10 2015Aug 29 2017Any endomorphism of a finitely generated free group naturally descends to an injective endomorphism of its stable quotient. In this paper, we prove a geometric incarnation of this phenomenon: namely, that every expanding irreducible train track map inducing ... More
Developments in Stochastic Coupled Cluster Theory: The initiator approximation and application to the Uniform Electron GasNov 18 2015We describe further details of the Stochastic Coupled Cluster method and a diagnostic of such calculations, the shoulder height, akin to the plateau found in Full Configuration Interaction Quantum Monte Carlo. We describe an initiator modification to ... More
McMullen polynomials and Lipschitz flows for free-by-cyclic groupsOct 28 2013Sep 04 2016Consider a group G and an epimorphism u_0:G\to\Z inducing a splitting of G as a semidirect product ker(u_0)\rtimes_\varphi\Z with ker(u_0) a finitely generated free group and \varphi\in Out(ker(u_0)) representable by an expanding irreducible train track ... More
Unbounded asymmetry of stretch factorsMay 05 2014A result of Handel-Mosher guarantees that the ratio of logarithms of stretch factors of any fully irreducible automorphism of the free group $F_N$ and its inverse is bounded by a constant $C_N$. In this short note we show that this constant $C_N$ cannot ... More
Naturalness and Higgs Decays in the MSSM with a SingletNov 21 2005Aug 25 2006The simplest extension of the supersymmetric standard model - the addition of one singlet superfield - can have a profound impact on the Higgs and its decays. We perform a general operator analysis of this scenario, focusing on the phenomenologically ... More
Cannon-Thurston maps for hyperbolic free group extensionsJun 23 2015Dec 13 2015This paper gives a detailed analysis of the Cannon--Thurston maps associated to a general class of hyperbolic free group extensions. Let $F_N$ denote a free groups of finite rank $N\ge 3$ and consider a \emph{convex cocompact} subgroup $\Gamma\le Out(F_N)$, ... More
Dynamics on free-by-cyclic groupsJan 31 2013Jan 29 2015Given a free-by-cyclic group $G = F_N \rtimes_\varphi \mathbb{Z}$ determined by any outer automorphism $\varphi \in \mathrm{Out}(F_N)$ which is represented by an expanding irreducible train-track map $f$, we construct a $K(G,1)$ $2$-complex $X$ called ... More
Endomorphisms, train track maps, and fully irreducible monodromiesJul 10 2015Any endomorphism of a finitely generated free group naturally descends to an injective endomorphism of its stable quotient. In this paper, we prove a geometric incarnation of this phenomenon: namely, that every expanding irreducible train track map inducing ... More
A Supersymmetric Twin HiggsApr 07 2006We present a supersymmetric realization of the twin Higgs mechanism, which cancels off all contributions to the Higgs mass generated above a scale f. Radiative corrections induced by the top quark sector lead to a breaking of the twin sector electroweak ... More
The Discovery of Two New Satellites of PlutoJan 02 2006Pluto's first known moon, Charon, was discovered in 1978 (Christy 1978) and has a diameter about half that of Pluto (Buie 1992,Young 1994, Sicardy 2005), which makes it larger relative to its primary than any other moon in the Solar System. Previous searches ... More
Characteristics and Origin of the Quadruple System at PlutoDec 23 2005Our discovery of two new satellites of Pluto, designated S/2005 P 1 and S/2005 P 2 (henceforth, P1 and P2), combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, ... More
(42355) Typhon-Echidna: Scheduling Observations for Binary Orbit DeterminationApr 15 2008We describe a strategy for scheduling astrometric observations to minimize the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) ... More
Variable circular polarisation associated with relativistic ejections from GRS 1915+105Apr 26 2002We report the discovery of variable circularly polarised radio emission associated with relativistic ejections from GRS 1915+105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer ... More
Probing the polarization characteristics of SS433 on mas scalesDec 30 2004We present a status report of dedicated circular polarization (CP) global VLBI observations of SS433. The total intensity image of the source is presented. We do not detect linear polarization in the core region of the source. We estimate the errors in ... More
Steady jets from radiatively efficient hard states in GRS 1915+105Jan 25 2011Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS1915+105, using observations ... More
Multiple relativistic outbursts of GRS 1915+105: radio emission and internal shocksAug 10 2005We present 5-GHz MERLIN radio images of the microquasar GRS 1915+105 during two separate outbursts in 2001 March and 2001 July, following the evolution of the jet components as they move outwards from the core of the system. Proper motions constrain the ... More
MERLIN observations of GRS 1915+105 : a progress reportJun 03 1998We present a progress report on MERLIN radio imaging of a radio outburst from GRS 1915+105. The major ejection occurred at the end of an approximately 20-day `plateau' state, characterised by low/hard X-ray fluxes and a relatively strong flat-spectrum ... More
High Albedos of Low Inclination Classical Kuiper Belt ObjectsDec 22 2008Jan 01 2009We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope. Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, ... More
Silicates on Iapetus from Cassini's Composite Infrared SpectrometerOct 07 2015We present the first spectral features obtained from Cassini's Composite Infrared Spectrometer (CIRS) for any icy moon. The spectral region covered by CIRS focal planes (FP) 3 and 4 is rich in emissivity features, but previous studies at these wavelengths ... More
First e-VLBI observations of Cygnus X-3Nov 02 2006We report the results of the first two 5 GHz e-VLBI observations of the X-ray binary Cygnus X-3 using the European VLBI Network. Two successful observing sessions were held, on 2006 April 20, when the system was in a quasi-quiescent state several weeks ... More
MERLIN observations of relativistic ejections from GRS 1915+105Dec 08 1998We present high resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October / November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically-thin outbursts and several ... More
A transient relativistic radio jet from Cygnus X-1Feb 14 2006We report the first observation of a transient relativistic jet from the canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one of six epochs of MERLIN ... More
SS433, microquasars, and other transientsOct 03 2012X-ray binaries have been an important key in understanding the jet-disc symbiosis in accreting black holes on all mass scales, from stellar-mass to supermassive black holes. SS433 was the first Galactic XRB that has been extensively studied in the radio ... More