arXiv subjects

James Keaveney

Publications and source records attributed to James Keaveney.

At least 19 recordsLinked to original sources

arXiv2026

Statistical Indications of Toponium Formation in Top Quark Pair Production

We present an analysis of six differential cross-section measurements of top-quark pair production in the dilepton channel from the ATLAS and CMS experiments. The data are compared to state-of-the-art QCD predictions with and without the inclusion of toponium formation effects. This contribution is modelled via a re-weighting of fixed-order matrix elements using the Green's function of the non-relativistic QCD Hamiltonian, and we employ a statistical model to quantify the preference of the data for the toponium hypothesis. All observables yield Bayes factors larger than unity, with two exceeding 20, yielding strong evidence for the toponium hypothesis in top-quark pair production at the LHC.

hep-ph↗

arXiv2025

Continuous time ultra-high frequency (UHF) sensing using ultra-cold Rydberg atoms

We present a technique for detecting ultra-high frequency (UHF) radio fields using a three-photon Rydberg excitation scheme in a continuously laser cooled sample of Rb-87 atoms. By measuring Autler-Townes splitting, we demonstrate resonant detection of UHF fields with frequency range 500-900 MHz through probing F -> G transitions, achieving a lowest minimum detectable field of 2.5(6) mV/cm at 899 MHz. We also demonstrate continuous time detection of a modulated RF signal, with a 3 dB bandwidth of 4.7(4) kHz at a carrier frequency of 899 MHz. Our approach employs atom loss spectroscopy rather than electromagnetically induced transparency (EIT), which enables detection whilst the atomic sample is simultaneously laser cooled. We investigate this operating regime to determine the feasibility of combining the benefits of reduced thermal dephasing (and subsequent increased sensitivity) of cold atoms with the continuous operation associated with thermal atoms. Our continuous time detection scheme provides an advantage over existing pulsed cold atom systems as we avoid the slow experimental duty cycles typically associated with replenishing the cold atom ensemble. We characterize the excitation scheme by varying laser detunings and analyze the impact and limitations due to various broadening mechanisms on the detection sensitivity.

physics.atom-ph↗

arXiv2023

Constraining the SMEFT with a differential cross section measurement of $tWZ$ production at the HL-LHC

A prospective measurement of the differential cross section of \tWZ production with respect to the transverse momentum of the \PZ boson using a general-purpose detector at the High-Luminosity Large Hadron Collider (HL-LHC) is described. The response of a general-purpose detector at the HL-LHC is simulated and used to estimate the uncertainties and covariances of the differential cross section measurement. Constraints on the Standard Model Effective Field Theory (SMEFT) enabled by the measurement are estimated. A parametric model of the differential cross section in the SMEFT is constructed and is used to determine the expected posterior probability function of six SMEFT Wilson coefficients and the expected 95\% Bayesian credible intervals for each coefficient and pair of coefficients. The intervals suggest that for all coefficients, the measurement will provide competitive but weaker constraints than those derived from other \mbox{HL-LHC} measurements involving top quarks and \PZ bosons. However, as the measurement is simultaneously sensitive to to a unique set of SMEFT coefficients, it will provide a useful input to a global SMEFT analysis that considers many operators.

hep-ph↗

arXiv2018

Simultaneous two-photon resonant optical laser locking (STROLLing) in the hyperfine Paschen--Back regime

We demonstrate a technique to lock simultaneously two laser frequencies to each step of a two-photon transition in the presence of a magnetic field sufficiently large to gain access to the hyperfine Paschen-Back regime. A ladder configuration with the 5S$_{1/2}$, 5P$_{3/2}$ and 5D$_{5/2}$ terms in a thermal vapour of $^{87}$Rb atoms is used. The two lasers remain locked for more than 24 hours. For the sum of the laser frequencies, which represents the stability of the two-photon lock, we measure a frequency instability of less than the Rb D$_2$ natural linewidth of 6 MHz for nearly all measured time scales

physics.atom-ph↗

arXiv2018

Self-similarity of optical rotation trajectories around the Poincare sphere with application to an ultra-narrow atomic bandpass filter

We present an investigation of magneto-optic rotation in both the Faraday and Voigt geometries. We show that more physical insight can be gained in a comparison of the Faraday and Voigt effects by visualising optical rotation trajectories on the Poincare sphere. This insight is applied to design and experimentally demonstrate an improved ultra-narrow optical bandpass filter based on combining optical rotation from two cascaded cells - one in the Faraday geometry and one in the Voigt geometry. Our optical filter has an equivalent noise bandwidth of 0.56 GHz, and a figure-of-merit value of 1.22(2) GHz$^{-1}$ which is higher than any previously demonstrated filter on the Rb D2 line.

physics.atom-ph↗

arXiv2018

Optimized ultra-narrow atomic bandpass filters via magneto-optic rotation in an unconstrained geometry

Atomic bandpass filters are widely used in a variety of applications, owing to their high peak transmission and narrow bandwidth. Much of the previous literature has used the Faraday effect to realize such filters, where an axial magnetic field is applied across the atomic medium. Here we show that by using a non-axial magnetic field, the performance of these filters can be improved in comparison to the Faraday geometry. We optimize the performance of these filters using a numerical model and verify their performance by direct quantitative comparison with experimental data. We find excellent agreement between experiment and theory. These optimized filters could find use in many of the areas where Faraday filters are currently used, with little modification to the optical setup, allowing for improved performance with relatively little change.

physics.atom-ph↗

arXiv2018

Automated translating beam profiler for in-situ laser beam spot-size and focal position measurements

We present a low-cost, high-resolution solution for automated laser-beam profiling with axial translation. The device is based on a Raspberry Pi, Pi Noir CMOS camera, stepper motor and commercial translation stage. We also provide software to run the device. The CMOS sensor is sensitive over a large wavelength range between 300-1100 nm, and can be translated over 25 mm along the beam axis. The sensor head can be reversed without changing its axial position, allowing for a quantitative estimate of beam overlap with counter-propagating laser beams. Although not limited to this application, the intended use for this device is the automated measurement of the focal position and spot-size of a Gaussian laser beam. We present example data of one such measurement to illustrate device performance.

physics.ins-det↗

arXiv2017

Four-wave mixing in a non-degenerate four-level diamond configuration in the hyperfine Paschen-Back regime

We present an experimental study of seeded four-wave mixing (4WM) using a diamond excitation scheme (with states from the 5S$_{1/2}$, 5P$_{1/2}$, 5P$_{3/2}$ and 5D$_{3/2}$ terms) in a thermal vapour of $^{87}$Rb atoms. We investigate the 4WM spectra under the application of a strong magnetic field (0.6 T). The Zeeman interaction is strong enough to realise the hyperfine Paschen-Back regime, which has the effect of separating the optical transitions by more than the Doppler width, thereby significantly simplifying the spectral features. We show that this facilitates a quantitative comparison, even in the regime of strong dressing, between experimental data and a simple theoretical model based only on four-level optical Bloch equations.

physics.atom-ph↗

arXiv2017

ElecSus: Extension to arbitrary geometry magneto-optics

We present a major update to ElecSus, a computer program and underlying model to calculate the electric susceptibility of an alkali-metal atomic vapour. Knowledge of the electric susceptibility of a medium is essential to predict its absorptive and dispersive properties. In this version we implement several changes which significantly extend the range of applications of ElecSus, the most important of which is support for non-axial magnetic fields (i.e. fields which are not aligned with the light propagation axis). Suporting this change requires a much more general approach to light propagation in the system, which we have now implemented. We exemplify many of these new applications by comparing ElecSus to experimental data. In addition, we have developed a graphical user interface front-end which makes the program much more accessible, and have improved on several other minor areas of the program structure.

physics.atom-ph↗

arXiv2017

Single-photon interference due to motion in an atomic collective excitation

We experimentally demonstrate the generation of heralded bi-chromatic single photons from an atomic collective spin excitation (CSE). The photon arrival times display collective quantum beats, a novel interference effect resulting from the relative motion of atoms in the CSE. A combination of velocity-selective excitation with strong laser dressing and the addition of a magnetic field allows for exquisite control of this collective beat phenomenon. The present experiment uses a diamond scheme with near-IR photons that can be extended to include telecommunications-wavelengths or modified to allow storage and retrieval in an inverted-Y scheme.

physics.atom-ph↗

arXiv2016

A single-mode external cavity diode laser using an intra-cavity atomic Faraday filter with short-term linewidth $<400$ kHz and long-term stability of $<1$ MHz

We report on the development of a diode laser system - the `Faraday laser' - using an atomic Faraday filter as the frequency-selective element. In contrast to typical external-cavity diode laser systems which offer tunable output frequency but require additional control systems in order to achieve a stable output frequency, our system only lases at a single frequency, set by the peak transmission frequency of the internal atomic Farady filter. Our system has both short-term and long-term stability of less than 1~MHz, which is less than the natural linewidth of alkali-atomic D-lines, making similar systems suitable for use as a `turn-key' solution for laser cooling experiments.

physics.atom-ph↗

arXiv2016

Direct measurement of excited-state dipole matrix elements using electromagnetically induced transparency in the hyperfine Paschen-Back regime

Applying large magnetic fields to gain access to the hyperfine Paschen-Back regime can isolate three-level systems in a hot alkali metal vapors, thereby simplifying usually complex atom-light interactions. We use this method to make the first direct measurement of the $\vert\langle 5P\vert\vert er\vert\vert 5D\rangle\vert$ matrix element in $^{87}$Rb. An analytic model with only three levels accurately models the experimental electromagnetically induced transparency spectra and extracted Rabi frequencies are used to determine the dipole matrix element. We measure $\vert\langle 5P_{3/2}\vert\vert er\vert\vert 5D_{5/2}\rangle\vert = (2.290\pm0.002_{\rm stat}\pm0.04_{\rm syst})~ea_{0}$ which is in excellent agreement with the theoretical calculations of Safronova, Williams, and Clark [Phys. Rev. A \textbf{69}, 022509 (2004)].

physics.atom-ph↗

arXiv2015

Probing top-philic sgluons with LHC Run I data

Many theories beyond the Standard Model predict the existence of colored scalar states, known as sgluons, lying in the adjoint representation of the QCD gauge group. In scenarios where they are top-philic, sgluons are expected to be copiously pair-produced at the LHC via strong interactions with decays into pairs of top quarks or gluons. Consequently, sgluons can be sought in multijet and multitop events at the LHC. We revisit two LHC Run I analyses in which events featuring either the same-sign dileptonic decay of a four-top-quark system or its single leptonic decay are probed. Adopting a simplified model approach, we show how this reinterpretation allows us to extract simultaneous bounds on the sgluon mass and couplings.

hep-ph↗

arXiv2015

Electromagnetically induced absorption in a non-degenerate three-level ladder system

We investigate, theoretically and experimentally, the transmission of light through a thermal vapour of three-level ladder-type atoms, in the presence of 2 counter-propagating control fields. A simple theoretical model predicts the presence of electromagnetically induced absorption (EIA) in this pure three-level system when the control field is resonant. Experimentally, we use $^{87}$Rb in a large magnetic field of 0.62~T to reach the hyperfine Paschen-Back regime and realise a non-degenerate three-level system. Experimental observations verify the predictions over a wide range of detunings.

physics.atom-ph↗

arXiv2015

Effect of line broadening on the performance of Faraday filters

We show that homogeneous line broadening drastically affects the performance of atomic Faraday filters. We use a computerized optimization algorithm to find the best magnetic field and temperature for Faraday filters with a range of cell lengths. The effect of self-broadening is found to be particularly important for short vapour cells, and for `wing-type' filters. Experimentally we realize a Faraday filter using a micro-fabricated $^{87}$Rb vapour cell. By modelling the filter spectrum using the ElecSus program we show that additional homogeneous line broadening due to the background buffer-gas pressure must also be included for an accurate fit.

physics.atom-ph↗

arXiv2015

Atomic Faraday filter with equivalent noise bandwidth less than 1 GHz

We demonstrate an atomic bandpass optical filter with an equivalent noise bandwidth less than 1 GHz using the D$_1$ line in a cesium vapor. We use the ElecSus computer program to find optimal experimental parameters, and find that for important quantities the cesium D$_1$ line clearly outperforms other alkali metals on either D-lines. The filter simultaneously achieves a peak transmission of 77%, a passband of 310 MHz and an equivalent noise bandwidth of 0.96 GHz, for a magnetic field of 45.3 gauss and a temperature of 68.0$\,^\circ$C. Experimentally, the prediction from the model is verified. The experiment and theoretical predictions show excellent agreement.

physics.atom-ph↗

arXiv2015

ElecSus: A program to calculate the electric susceptibility of an atomic ensemble

We present a computer program and underlying model to calculate the electric susceptibility of a gas, which is essential to predict its absorptive and dispersive properties. Our program focusses on alkali-metal vapours where we use a matrix representation of the atomic Hamiltonian in the completely uncoupled basis in order to calculate transition frequencies and strengths. The program calculates various spectra for a weak-probe laser beam in an atomic medium with an applied axial magnetic field. This allows many optical devices to be designed, such as Faraday rotators/filters, optical isolators and circular polarisation filters. Fitting routines are also provided with the program which allows the user to perform optical metrology by fitting to experimental data.

physics.atom-ph↗

arXiv2014

Search for standard model production of four top quarks in the lepton + jets channel in pp collisions at $\sqrt{s} = 8 $ TeV

A search is presented for standard model (SM) production of four top quarks ($t\bar{t}t\bar{t}$) in pp collisions in the lepton + jets channel. The data correspond to an integrated luminosity of 19.6 fb$^{-1}$ recorded at a centre-of-mass energy of 8 TeV with the CMS detector at the CERN LHC. A combination of kinematic reconstruction and multivariate techniques is used to distinguish between the small signal and large background. The data are consistent with expectations of the SM, and an upper limit of 32 fb is set at a 95\% confidence level on the cross section for producing four top quarks in the SM, where a limit of $32\pm{17}$ fb is expected.

hep-ex↗