arXiv subjects

Ken Kiers

Publications and source records attributed to Ken Kiers.

At least 19 recordsLinked to original sources

arXiv2023

Disentangling the Seesaw in the Left-Right Model -- An Algorithm for the General Case

Senjanovic and Tello have analyzed how one could determine the neutrino Dirac mass matrix in the minimal left-right model, assuming that the mass matrices for the light and heavy neutrinos could be taken as inputs. They have provided an analytical solution for the Dirac mass matrix in the case that the left-right symmetry is implemented via a generalized parity symmetry and that this symmetry remains unbroken in the Dirac Yukawa sector. We extend the work of Senjanovic and Tello to the case in which the generalized parity symmetry is broken in the Dirac Yukawa sector. In this case the elegant method outlined by Senjanovic and Tello breaks down and we need to adopt a numerical approach. Several iterative approaches are described; these are found to work in some cases but to be highly unstable in others. A stable, prescriptive numerical algorithm is described that works in all but a vanishingly small number of cases. We apply this algorithm to numerical data sets that are consistent with current experimental constraints on neutrino masses and mixings. We also provide some additional context and supporting explanations for the case in which the parity symmetry is unbroken.

hep-ph↗

arXiv2020

Weyl's problem: A computational approach

The distribution of eigenvalues of the wave equation in a bounded domain is known as Weyl's problem. We describe several computational projects related to the cumulative state number, defined as the number of states having wavenumber up to a maximum value. This quantity and its derivative, the density of states, have important applications in nuclear physics, degenerate Fermi gases, blackbody radiation, Bose-Einstein condensation and the Casimir effect. Weyl's theorem states that, in the limit of large wavenumbers, the cumulative state number depends only on the volume of the bounding domain and not on its shape. Corrections to this behavior are well known and depend on the surface area of the bounding domain, its curvature and other features. We describe several projects that allow readers to investigate this dependence for three bounding domains - a rectangular box, a sphere, and a circular cylinder. Quasi-one- and two-dimensional systems can be analyzed by considering various limits. The projects have applications in statistical mechanics, but can also be integrated into quantum mechanics, nuclear physics, or computational physics courses.

physics.comp-ph↗

arXiv2018

Single-top production and rare top interactions

The study of the top quark's properties is an important part of the LHC programme. In earlier work, we have studied the rare decay $t \to b \bar b c$, using effective operators to capture the effects of physics beyond the Standard Model. However top decay is primarily sensitive to new physics in the sub-TeV energy regime. If this new physics resides at a higher energy scale, then one needs to turn to single-top production. In this paper, we use the $s$-channel and $t$-channel single-top production measurements to constrain the new physics parameter space associated with such contact interactions. We also study the net top polarization as a means to distinguish between contributions from operators involving different fermion chiralities and Lorentz structures.

hep-ph↗

arXiv2016

Pseudoscalar top-Higgs coupling: Exploration of $\mathrm{CP}$-odd observables to resolve the sign ambiguity

We present a collection of $\mathrm{CP}$-odd observables for the process $pp\to t\,\left(\rightarrow b {\ell}^+ ν_{\ell}\right) \bar{t}\,\left(\rightarrow \bar{b} {\ell}^-{\barν}_{\ell}\right)\,H$ that are linearly dependent on the scalar ($κ_t$) and pseudoscalar ($\tildeκ_t$) top-Higgs coupling and hence sensitive to the corresponding relative sign. The proposed observables are based on triple product (TP) correlations that we extract from the expression for the differential cross section in terms of the spin vectors of the top and antitop quarks. In order to explore other possibilities, we progressively modify these TPs, first by combining them, and then by replacing the spin vectors by the lepton momenta or the $t$ and $\bar{t}$ momenta by their visible parts. We generate Monte Carlo data sets for several benchmark scenarios, including the Standard Model ($κ_t=1$, $\tildeκ_t=0$) and two scenarios with mixed $\mathrm{CP}$ properties ($κ_t=1$, $\tildeκ_t=\pm 1$). Assuming an integrated luminosity that is consistent with that envisioned for the High Luminosity Large Hadron Collider, using Monte Carlo-truth and taking into account only statistical uncertainties, we find that the most promising observable can disentangle the "$\mathrm{CP}$-mixed" scenarios with an effective separation of $\sim 19σ$. In the case of observables that do not require the reconstruction of the $t$ and $\bar{t}$ momenta, the power of discrimination is up to $\sim 13σ$ for the same number of events. We also show that the most promising observables can still disentangle the $\mathrm{CP}$-mixed scenarios when the number of events is reduced to values consistent with expectations for the Large Hadron Collider in the near term.

hep-ph↗

arXiv2016

Measuring CP-Violating Observables in Rare Top Decays at the LHC

In this paper we consider CP-violating new-physics contributions to the decay $t \to b \bar b c$. We examine the prospects for detecting such new physics at the LHC, which requires studying the process $gg \to t (\to b \bar b c) \bar t (\to \bar b \ell \bar ν)$. We find two observables that can be used to reveal the presence of CP-violating new physics in $t \to b \bar b c$. They are (i) the partial-rate asymmetry and (ii) the triple-product correlations involving the momenta of various particles associated with the interaction. A Monte Carlo analysis is performed to determine how well these observables can be used to detect the presence of new physics, and to measure its parameters. We find that there is little difficulty in extracting the value of the relevant new-physics parameter from the partial-rate asymmetry. For the triple-product correlations, we test multiple strategies that can be used for the extraction of the corresponding combination of new-physics parameters.

hep-ph↗

arXiv2015

Search for New Physics in Rare Top Decays: $t \bar t$ Spin Correlations and Other Observables

In this paper we study new-physics contributions to the top-quark decay $t \to b \bar b c$. We search for ways of detecting such new physics via measurements at the LHC. As top quarks are mainly produced at the LHC in $t \bar t$ production via gluon fusion, we analyze the process $gg \to t \bar t \to (b \bar b c) (\bar b \ell \bar ν)$. We find six observables that can be used to reveal the presence of new physics in $t \to b \bar b c$. Three are invariant mass-squared distributions involving two of the final-state particles in the top decay, and three are angular correlations between the final-state quarks coming from the $t$ decay and the $\ell^-$ coming from the $\bar t$ decay. The angular correlations are related to the $t \bar t$ spin correlation.

hep-ph↗

arXiv2015

Detecting New Physics in Rare Top Decays at the LHC

In the companion paper it was shown that there are six observables in $gg\to t \bar t \to (b \bar b c) (\bar b \ell \bar ν)$ that can be used to reveal the presence of new physics (NP) in $t \to b \bar b c$. In the present paper we examine the prospects for detecting and identifying such NP at the LHC, in both the short term and long term. To this end, we develop an algorithm for extracting the NP parameters from measurements of the observables. In the short term, depending on what measurements have been made, there are several different ways of detecting the presence of NP. It may even be possible to approximately determine the values of certain NP parameters. In the long term, it is expected that all six observables will be measured. The values of the NP parameters can then be determined reasonably precisely from a fit to these measurements, which will provide good information about the type of NP present in $t \to b \bar b c$.

hep-ph↗

arXiv2015

Probing sensitivity to charged scalars through partial differential widths: $τ\rightarrow Kππν_τ$ decays

We define and test $\mathrm{CP}$-even and $\mathrm{CP}$-odd partial differential widths for the process $τ\rightarrow Kππν_τ$ assuming that an intermediate heavy charged scalar contributes to the decay amplitude. Adopting a model-independent approach, we use a Monte Carlo simulation in order to study the number of events needed to recover information on the new physics from these observables. Our analysis of the $\mathrm{CP}$-odd observables indicates that the magnitude of $f_H η_P$, which is related to the new-physics contribution, can be recovered with an uncertainty smaller than $3%$ for $3\times 10^6$ events. This number of events would also allow one to retrieve certain parameters appearing in the SM amplitude at the percent level. In addition, we discuss the possibility of using the proposed observables to study specific models involving two Higgs doublets, such as the aligned two-Higgs-doublet model (A2HDM). This analysis is undertaken within the context of the upcoming Super B-factories, which are expected to provide a considerably larger number of events than that which was supplied by the B-factories. Moreover, a similar set of observables could be employed to study other decay modes such as $τ\rightarrowπππν_τ,\,τ\rightarrow KKπν_τ$ and $τ\rightarrow KKKν_τ$.

hep-ph↗

arXiv2012

CP violation in hadronic tau decays

We consider CP-violating effects in tau -> 3 pi nu_ tau and 4 pi nu_ tau (Delta S = 0), and tau -> K pi pi nu_tau (Delta S = 1), assuming that the usual Standard Model amplitudes for these processes interfere with analogous amplitudes mediated by a charged Higgs boson. In the Delta S = 0 case we focus specifically on the intermediate resonant processes tau -> V pi nu_tau (with V=omega, rho, and a_1), and consider three CP-odd observables -- the partial rate asymmetry, a polarization-dependent asymmetry and a triple-product asymmetry. In the Delta S = 1 case we examine the partial rate asymmetry, two "modified" rate asymmetries, and a triple-product asymmetry. The partial rate asymmetry is expected to be small for both the Delta S = 0 and Delta S = 1 cases. Evaluation of the other asymmetries indicates that they could potentially be measurable.

hep-ph↗

arXiv2011

Using t -> b \bar{b} c to Search for New Physics

We consider new-physics (NP) contributions to the decay t -> b \bar{b} c. We parameterize the NP couplings by an effective Lagrangian consisting of 10 Lorentz structures. We show that the presence of NP can be detected through the measurement of the partial width. A partial identification of the NP can be achieved through the measurements of a forward-backward-like asymmetry, a top-quark-spin-dependent asymmetry, the partial rate asymmetry, and a triple-product asymmetry. These observables, which vanish in the standard model, can all take values in the 10-20% range in the presence of NP. Since |V_{tb}V_{cb}| \simeq |V_{ts} V_{cs}|, most of our results also hold, with small changes, for t -> s \bar{s} c.

hep-ph↗

arXiv2009

CP Violation in Three-Body Chargino Decays

CP violation in supersymmetry can give rise to rate asymmetries in the decays of supersymmetric particles. In this work we compute the rate asymmetries for the three-body chargino decays \tildeχ^\pm_2 \to \tildeχ^\pm_1 HH, \tildeχ^\pm_2 \to \tildeχ^\pm_1 ZZ, \tildeχ^\pm_2 \to \tildeχ^\pm_1 W^+ W^- and \tildeχ^\pm_2 \to tildeχ^\pm_1 ZH. Each of the decays contains contributions mediated by neutral Higgs bosons that can possibly go on shell. Such contributions receive a resonant enhancement; furthermore, the strong phases required for the CP asymmetries come from the widths of the exchanged Higgs bosons. Our results indicate that the rate asymmetries can be relatively large in some cases, while still respecting a number of important low-energy bounds such as those coming from B meson observables and electric dipole moments. For the parameters that we consider, rate asymmetries of order 10% are possible in some cases.

hep-ph↗

arXiv2008

CP violation in hadronic tau decays

We examine CP violation in the Delta S=0 decays tau -> omega pi nu_tau and tau -> a_1 pi nu_tau and the Delta S=1 decay tau -> K pi pi nu_tau. We assume that the new physics is a charged Higgs. We show that sizeable CP-violating effects are possible in tau -> a_1 pi nu_tau (polarization-dependent rate asymmetry) and tau -> omega pi nu_ tau (triple-product asymmetry). The Delta S=1 decay tau -> K pi pi nu_tau can proceed via several resonances. We construct two modified rate asymmetries and a triple product asymmetry for this decay and discuss the potential sensitivities of these asymmetries.

hep-ph↗

arXiv2008

CP violation in tau -> K pi pi nu_tau

We consider CP-violating effects in tau->K pi pi nu_tau, assuming that a charged Higgs boson provides a new amplitude that can interfere with the usual Standard Model amplitude. We consider four CP-odd observables -- the regular rate asymmetry, two modified rate asymmetries and a triple-product asymmetry. The regular rate asymmetry is expected to be small because it requires the interference of the new physics amplitude with the standard model amplitude containing the hadronic scalar form factor. The other CP asymmetries may be more promising in terms of their new physics reach. Numerical estimates indicate that the maximum obtainable values for the modified and triple-product asymmetries are on the order of a percent.

hep-ph↗

arXiv2007

CP Violation in Supersymmetric Theories: stop2->stop1 H H, stop2->stop1 Z Z, stop2->stop1 W+ W-, stop2->stop1 Z H

We study the decays stop2->stop1 H H, stop2->stop1 Z Z, stop2->stop1 W+ W- and stop2->stop1 Z H, with an eye towards measuring the CP-violating supersymmetric parameters contained in these processes. We find that stop2->stop1 H H tends to have the largest CP asymmetry and width, and is perhaps the most favourable experimentally. These decays are sensitive primarily to phi_At, the phase of the trilinear coupling A_t.

hep-ph↗

arXiv2007

CP Violation in Hadronic tau Decays

We re-examine CP violation in the Delta S = 0 decays tau -> N pi nu_tau (N=2,3,4). We assume that the new physics (NP) is a charged Higgs. We show that there is no NP contribution to tau -> pi pi nu_tau, which means that no CP violation is expected in this decay. On the other hand, NP can contribute to tau -> N pi nu_tau (N=3,4). These are dominated by the intermediate resonant decays tau -> omega pi nu_tau, tau -> rho pi nu_tau and tau -> a_1 pi nu_tau. We show that the only sizeable CP-violating effects which are possible are in tau -> a_1 pi nu_tau -> 4 pi nu_tau (polarization-dependent rate asymmetry) and tau -> omega pi nu_tau (triple-product asymmetry).

hep-ph↗

arXiv2006

CP Violation in Supersymmetric Theories: stop2 -> stop1 tau- tau+

Supersymmetric (SUSY) theories include many new parameters, some of which are CP-violating. Assuming that SUSY is found at a future high-energy collider, we examine the decay stop2 -> stop1 tau- tau+ with an eye to obtaining information about the new CP phases. We show that there are two CP-violating asymmetries which can be large in some regions of the SUSY parameter space. These involve measuring one or both of the tau spins. These asymmetries are particularly sensitive to phi_At, the phase of the trilinear coupling A_t.

hep-ph↗

arXiv2005

Neutrinos in a left-right model with a horizontal symmetry

We analyze the lepton sector of a Left-Right Model based on the gauge group SU(2)_L x SU(2)_R x U(1), concentrating mainly on neutrino properties. Using the seesaw mechanism and a horizontal symmetry, we keep the right-handed symmetry breaking scale relatively low, while simultaneously satisfying phenomenological constraints on the light neutrino masses. We take the right-handed scale to be of order 10's of TeV and perform a full numerical analysis of the model's parameter space, subject to experimental constraints on neutrino masses and mixings. The numerical procedure yields results for the right-handed neutrino masses and mixings and the various CP-violating phases. We also discuss phenomenological applications of the model to neutrinoless double beta decay, lepton-flavor-violating decays (including decays such as τ\to 3μ) and leptogenesis.

hep-ph↗

arXiv2005

Higgs Sector of the Left-Right Model with Explicit CP Violation

We explore the Higgs sector of the Minimal Left-Right (LR) Model based on the gauge group SU(2)_L x SU(2)_R x U(1)_{B-L} with explicit CP violation in the Higgs potential. Since flavour-changing neutral current experiments and the small scale of neutrino masses both place stringent constraints on the Higgs potential, we seek to determine whether minima of the Higgs potential exist that are consistent with current experimental bounds. We focus on the case in which the right-handed symmetry-breaking scale is only ``moderately'' large, of order 15-50 TeV. Unlike the case in which the Higgs potential is CP-invariant, the CP noninvariant case does yield viable scenarios, although these require a small amount of fine-tuning. We consider a LR model supplemented by an additional U(1) horizontal symmetry, which results in a Higgs sector consistent with current experimental constraints and a realistic spectrum of neutrino masses.

hep-ph↗