arXiv subjects

German Valencia

Publications and source records attributed to German Valencia.

At least 37 records · Page 2Linked to original sources

arXiv2023

FCNC $B$ and $K$ Meson Decays with Light Bosonic Dark Matter

We consider decays of $B$ and $K$ mesons into a pseudo-scalar or vector meson plus missing energy. Within the SM, these modes originate from flavor changing neutral current (FCNC) processes with two neutrinos in the final state. In this paper we consider the experimental upper bounds on these modes and interpret the difference between these bounds and the SM prediction as a window into new light invisible particles. In particular we consider the case where some new symmetry requires the new particles to be produced in pairs. We first construct the general low energy effective Lagrangian coupling an FCNC with two dark sector particles of spin zero, one-half and one. We then present numerical estimates for the constraints that can be placed on these interactions, finding that an effective new physics scale from ${\cal O}(10)$-${\cal O}(10^{11})$ GeV can be probed, with the exact value strongly depending on the interaction structure as well as the mass of the invisible particle. For $K^+\to π^+ \displaystyle{\not} E$ we incorporate into our constraints the effect of using only the signal regions of NA62, and for $B^+\to K^+ \displaystyle{\not} E$ the $q^2$-dependent efficiency of Belle II.

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arXiv2022

New and simplified manual controls for projection and slice tours, with application to exploring classification boundaries in high dimensions

This paper describes new user controls for examining high-dimensional data using low-dimensional linear projections and slices. A user can interactively change the contribution of a given variable to a low-dimensional projection, which is useful for exploring the sensitivity of structure to particular variables. The user can also interactively shift the center of a slice, for example, to explore how structure changes in local subspaces. The Mathematica package as well as example notebooks are provided, which contain functions enabling the user to experiment with these new manual controls, with one specifically for exploring regions and boundaries produced by classification models. The advantage of Mathematica is its linear algebra capabilities, and interactive cursor location controls. Some limited implementation has also been made available in the R package tourr.

stat.CO↗

arXiv2022

Pursuit of $CP$ violation in hyperon decays at $e^+e^-$ colliders

We present a concise overview on $CP$ violation in hyperon decays, including past, present, and future efforts to search for it. We highlight in particular the most recent results of the BESIII experiment using a quantum-entangled pair of hyperon and antihyperon produced by a charmonium resonance formed in $e^+e^-$ annihilation. The subsequent nonleptonic decays of these hyperon pairs allow for the simultaneous determination of various parameters pertaining to $CP$ violation in the decays. We compare the reported data with the corresponding current predictions for $CP$ asymmetries within the standard model of particle physics. Moreover, we touch on how much these asymmetries might be enhanced by possible new physics beyond the standard model. We also comment briefly on the importance of the proposed super tau-charm factories in future quests for hyperon $CP$-violation.

hep-ph↗

arXiv2022

Hole or grain? A Section Pursuit Index for Finding Hidden Structure in Multiple Dimensions

Multivariate data is often visualized using linear projections, produced by techniques such as principal component analysis, linear discriminant analysis, and projection pursuit. A problem with projections is that they obscure low and high density regions near the center of the distribution. Sections, or slices, can help to reveal them. This paper develops a section pursuit method, building on the extensive work in projection pursuit, to search for interesting slices of the data. Linear projections are used to define sections of the parameter space, and to calculate interestingness by comparing the distribution of observations, inside and outside a section. By optimizing this index, it is possible to reveal features such as holes (low density) or grains (high density). The optimization is incorporated into a guided tour so that the search for structure can be dynamic. The approach can be useful for problems when data distributions depart from uniform or normal, as in visually exploring nonlinear manifolds, and functions in multivariate space. Two applications of section pursuit are shown: exploring decision boundaries from classification models, and exploring subspaces induced by complex inequality conditions from multiple parameter model. The new methods are available in R, in the tourr package.

stat.CO↗

arXiv2022

$R^ν_{K^{(*)}}$ and non-standard neutrino interactions

We discuss the modes $B\to K^{(*)}ν\barν$ in the context of non-standard neutrino interactions that add incoherently to the SM rates. We consider two scenarios: an additional light neutrino; and neutrino lepton flavour violation. We find that an additional light neutrino that interacts with SM fields via a non-universal $Z^\prime$ can increase $R^ν_{K^{(*)}}$ by up to a factor of two without conflicting with $B_s-\bar B_s$ mixing. This model then predicts rates for $B_s\to τ^+τ^-$ up to six times larger than the SM. In the context of neutrino lepton flavour violation mediated by leptoquarks we find that the current experimental upper bounds on $R^ν_{K^{(*)}}$ are already more constraining than direct bounds from $B_s\to τ\ell$ and $B\to K^{(*)}τ\ell$ modes for $\ell=e,μ$.

hep-ph↗

arXiv2021

Pandemonium: a clustering tool to partition parameter space -- application to the B anomalies

We introduce the interactive tool pandemonium to cluster model predictions that depend on a set of parameters. The model predictions are used to define the coordinates in observable space which go into the clustering. The results of this partitioning are then visualized in both observable and parameter space to study correlations between them. The tool offers multiple choices for coordinates, distance functions and linkage methods within hierarchical clustering. It provides a set of diagnostic statistics and visualization methods to study the clustering results in order to interpret the outcome. The methods are most useful in an interactive environment that enables exploration, and we have implemented them with a graphical user interface in R. We demonstrate the concepts with an application to phenomenological studies in flavor physics in the context of the so-called B anomalies.

physics.data-an↗

arXiv2021

On long-lived electroweak-singlet coloured scalars

There has been much recent interest in long-lived massive particles at the LHC, understood as those with lifetimes between tens of micrometers and several meters. In this context we consider the possibility of long-lived electroweak singlet scalars charged under colour $\mathrm{SU}(3)$ with masses near a TeV. The shortest lifetime of interest is already longer than typical hadronisation scales. These exotic new particles would therefore appear as colour singlet bound states of the new scalars with quarks and gluons and it is their colour charge that prevents them from decaying. In particular we consider colour representations consistent with maintaining asymptotic freedom, those with dimensionality $d_R \leq 15$. We find that only the octets can decay, and they do so into multi-jet final states through the two-gluon channel. The other representations are stable and form fractionally charged colour singlets, with the decuplet being the only one that can form electrically neutral colour singlets.

hep-ph↗

arXiv2020

Color-octet scalar decays to a gluon and an electroweak gauge boson in the Manohar-Wise model

We present one loop results for the amplitudes giving rise to couplings between a color octet scalar, a gluon, and an electroweak gauge boson. These amplitudes could signal new physics in $γ$ jet, $Z$ jet and $W$ jet production at the LHC. We compute the relevant branching ratios and identify regions of parameter space where these decay modes become important. This can happen for scalar masses below the threshold for decay into heavy quark pairs ($t\bar t$ and $t\bar b$); or for small Yukawa couplings in which case the colored scalars are fermiophobic. In the case of light scalars, ${\cal B}(S\to γg)$ can reach up to 10\% whereas ${\cal B}(S\to Z g)$ can reach a few percent. In the fermiophobic region of parameter space, ${\cal B}(S\to γg)$ and ${\cal B}(S\to Z g)$ can reach up to 72\% and 28\% respectively, whereas ${\cal B}(S\to g g)$ can be 100\%. For the charged scalar, the decay mode ${\cal B}(S^\pm \to W^\pm g)$ can become dominant in both scenarios.

hep-ph↗

arXiv2020

LHC constraints on $W^\prime,~Z^\prime$ that couple mainly to third generation fermions

We use the results of CMS and ATLAS searches for resonances that decay to $τν$ or $tb$ and $τ^+τ^-$ or $t\bar t$ final states to constrain the parameters of non-universal $W^\prime$ and $Z^\prime$ gauge bosons that couple preferentially to the third generation. For the former we consider production from $c\bar{b}$ annihilation and find very weak constraints on the strength of the interaction and only for the mass range between 800 and 1100 GeV from the $pp \to τ_h p_T^{\rm miss}$ channel. The constraints on the latter are much stronger and arise from both $t\bar t$ and $τ^+τ^-$ production. Treated separately, we find that the weak constraints on the $W^\prime$ still permit an explanation of the $R(D^{(\star)})$ anomalies with a light sterile neutrino whereas the stronger constraints on the $Z^\prime$ exclude significant light sterile neutrino contributions to the $K \to πν\barν$ rates. Within specific models the masses of $W^\prime$ and $Z^\prime$ are of course related and we briefly discuss the consequences.

hep-ph↗

arXiv2020

Evading the Grossman-Nir bound with $ΔI=3/2$ new physics

Rare kaon decays with missing energy, $K\toπ$+$E_{\rm miss}$, have received considerable attention because their rates can be calculated quite precisely within the standard model (SM), where the missing energy is carried away by an undetected neutrino-antineutrino pair. Beyond the SM, clean theoretical predictions can also be made regarding these processes. One such prediction is the so-called Grossman-Nir (GN) bound, which states that the branching fractions of the $K_L$ and $K^+$ modes must satisfy the relation $\mathcal{B}(K_L\toπ^0$+$E_{\rm miss})\lesssim4.3\,\mathcal{B}(K^+\toπ^+$+$E_{\rm miss})$ and applies within and beyond the SM, as long as the hadronic transitions change isospin by $ΔI=1/2$. In this paper we extend the study of these modes to include new-physics scenarios where the missing energy is due to unobserved lepton-number-violating neutrino pairs, invisible light new scalars, or pairs of such scalars. The new interactions are assumed to arise above the electroweak scale and described by an effective field theory. We explore the possibility of violating the GN bound through $ΔI=3/2$ contributions to the $K\toπ$ transitions within these scenarios and find that large violations are only possible in the case where the missing energy is due to an invisible light new scalar.

hep-ph↗

arXiv2020

Constraints on new physics from $K \to πν\barν$

We study generic effects of new physics on the rare decay modes $K_L \to π^0 ν\barν$ and $K^+ \to π^+ ν\barν$. We discuss several cases: left-handed neutrino couplings; right handed neutrino couplings; neutrino lepton flavour violating (LFV) interactions; and $ΔI =3/2$ interactions. The first of these cases has been studied before as it covers many new physics extensions of the standard model; the second one requires the existence of a new light (sterile) right-handed neutrino and its contribution to both branching ratios is always additive to the SM. The case of neutrino LFV couplings introduces a CP conserving contribution to $K_L \to π^0 ν\barν$ which affects the rates in a similar manner as a right handed neutrino as neither one of these interferes with the standard model amplitudes. Finally, we consider new physics with $ΔI =3/2$ interactions to go beyond the Grossman-Nir bound. We find that the rare kaon rates are only sensitive to new physics scales up to a few GeV for this scenario.

hep-ph↗

arXiv2020

Breaking the Grossman-Nir Bound in Kaon Decays

The ratio $\mathcal{B}(K_L\toπ^0ν\barν)/\mathcal{B}(K^+\toπ^+ν\barν)$ of the branching fractions of kaon decays $K_L\toπ^0ν\barν$ and $K^+\toπ^+ν\barν$ has a maximum of about 4.3 under the assumption that the underlying interactions change isospin by $ΔI=1/2$. This is referred to as the Grossman-Nir (GN) bound, which is respected by the standard model (SM) and by many scenarios beyond it. Recent preliminary results of the KOTO and NA62 Collaborations searching for these kaon modes seem to imply a violation of this bound. The KOTO findings also suggest that $\mathcal{B}(K_L\toπ^0ν\barν)$ could be much larger, by nearly two orders of magnitude, than that predicted in the SM. In this work we study the possibility of violating the GN bound in an effective field theory approach with only SM fields. We show that the bound holds, in addition to the original GN scenarios, whether or not the kaon decays conserve lepton number. We demonstrate that the inclusion of $ΔI=3/2$ operators can lead to a violation of the GN bound and illustrate with an example of how the KOTO numbers may be reached with a new physics scale of order tens of GeV.

hep-ph↗

arXiv2019

A slice tour for finding hollowness in high-dimensional data

Taking projections of high-dimensional data is a common analytical and visualisation technique in statistics for working with high-dimensional problems. Sectioning, or slicing, through high dimensions is less common, but can be useful for visualising data with concavities, or non-linear structure. It is associated with conditional distributions in statistics, and also linked brushing between plots in interactive data visualisation. This short technical note describes a simple approach for slicing in the orthogonal space of projections obtained when running a tour, thus presenting the viewer with an interpolated sequence of sliced projections. The method has been implemented in R as an extension to the tourr package, and can be used to explore for concave and non-linear structures in multivariate distributions.

stat.CO↗

arXiv2019

Lepton-flavor-violating semileptonic $τ$ decay and $K\toπν\barν$

We consider lepton-flavor violation in strangeness changing ($|ΔS|=1$) semileptonic $τ$-lepton decays arising from new physics encoded in a standard model effective Lagrangian. Its invariance under the standard model gauge group entails the relevance of other processes which can serve as complementary probes of the new physics operators. We show in particular that for some of them the bounds implied by current data on the rare kaon decays involving a neutrino pair, $K\to πν\barν$, are stronger than the existing limits from direct searches for lepton-flavor-violating semileptonic $τ$ decays. We discuss additional processes affected by the same operators and find that certain leptonic charged-meson decays also provide stricter constraints on a few more of them. Upcoming results of ongoing experiments such as Belle II and NA62 will further test the new physics parameter space.

hep-ph↗

arXiv2019

Fitting in or odd one out? Pulls vs residual responses in $b\to s \ell^+\ell^-$

New results in processes with an underlying quark transition $b\to s \ell^+\ell^-$ have been recently reported by the LHCb and Belle II collaborations. In this note we show how the main implications of a handful of new measurements can be understood with the tools introduced in our recent paper, arXiv:1811.10793, without the need to redo the global fits. We find that the main impact of the new results, due to $R_K^{[1.1,6]}$ from LHCb, is a decrease in $C_{10μ}^{NP}$ with a reduced uncertainty. We validate this conclusion by presenting the result of a new global fit.

hep-ph↗

arXiv2019

Charged-lepton-flavor violation in $|ΔS|=1$ hyperon decays

Studies of lepton-flavor violation in strangeness-changing ($|ΔS|=1$) transitions have a~long tradition in the kaon sector where they provide some of the strongest limits on physics beyond the standard model. Recent hints of violation of lepton-flavor universality in $B$-meson decays have revived interest in lepton-flavor violation as the two phenomena appear simultaneously in many extensions of the standard model. At the same time, the LHCb experiment has produced new results for the hyperon process $Σ^+\to pμ^+μ^-$ and may be in a position to study other rare hyperon decay modes. With this in mind, we investigate $|ΔS|=1$ hyperon decays into different-flavor lepton pairs $e^\pmμ^\mp$ in a model-independent manner and contrast the coverage of parameter space that can be achieved with what is known from kaon modes. We include a comparison with selected two-body leptonic decays of charged mesons, with $K\to πν\barν$ modes, with $μ\to e$ conversion, and with lepton-flavor violating decays of other neutral mesons, all of which constrain the same parameter space in a complementary~way.

hep-ph↗

arXiv2019

Anatomy of a six-parameter fit to the $b\to s \ell^+\ell^-$ anomalies

Discrepancies between measurements of decay modes with an underlying quark level transition $b\to s \ell^+\ell^-$ and standard model (SM) predictions have persisted for several years, particularly for the muon channels. The inadequacy of the SM becomes more compelling in a global fit. For example, Ref. [1] described 175 observables by six parameters encoding new physics and quantified the disagreement with the SM at about the $5σ$ level. While certain one and two parameter fits have previously been considered in detail, we establish a framework for the detailed discussion of the full 6d fit. We visualize and quantify the 6d $1σ$ region around the best fit point and define fit uncertainties for both current and future observables. We then define metrics quantifying the deviations between measurements and both SM and best fit predictions. These metrics relate observables to directions in parameter space, revealing their precise role in the fit, thus providing guidance for future theoretical and experimental work. Some metrics further quantify the role of correlated uncertainties, which turns out to be significant. For example the relevance of angular observables such as $P_5^\prime$ is reduced in this context. Finally, studying the space of observables allows us to discuss the internal tensions in the fit.

hep-ph↗

arXiv2018

Decay rate and asymmetries of $Σ^+\to pμ^+μ^-$

The LHCb Collaboration has recently reported evidence for the rare hyperon decay $Σ^+\to pμ^+μ^-$ that is consistent with the standard model expectation. Motivated by this new result we revisit the calculation of this mode including both long and short distance contributions. In the standard model this mode is completely dominated by long distance physics and thus subject to large uncertainties. We present a range of predictions for the rate, the dimuon invariant mass spectrum, and a muon polarization asymmetry that covers these uncertainties as well as known constraints. We study the interplay between short and long distance contributions which could result in additional asymmetries, but we find that they are negligible within the standard model. We propose a parameterization of these asymmetries in terms of a couple of constants that can arise from new physics.

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