arXiv subjects

Matthew Kirk

Publications and source records attributed to Matthew Kirk.

At least 19 recordsLinked to original sources

arXiv2026

New insights into the $b\rightarrow c \bar{u}q$ puzzle through Top-Bottom synergies

Anomalies in the non-leptonic $\bar{B}^0\rightarrow D^{(*)+}K^{(*)-}$ and $\bar{B}^0_s\rightarrow D^{(*)+}_s\pi^-$ decays may be an indication of physics beyond the Standard Model, but the large deviations require strongly coupled new physics that should be visible at colliders. We explore three new directions that could lead to viable new physics models, performing a detailed collider study to examine the possible weakening of previously known constraints on additional $SU(2)_L$ doublets. Our results show that, despite the difficulty of probing $t\bar{t}$ final states, increasing the branching ratio to this decay mode does not significantly weaken the bounds on weak doublet scalars, as additionally existing charged Higgs searches are equally strong. Beyond this, we analyse a potentially large breakdown of QCD factorisation by including large-power corrections to $B$ decays, and the effect of diluting collider searches with multi-scalar extensions. We find that these typical model-building routes for constructing a viable scenario remain constrained by collider measurements, indicating that these non-leptonic anomalies remain among the most puzzling discrepancies from the SM.

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arXiv2025

Global Determination of $|V_{us}|$

In light of ongoing issues with the first-row unitarity test of the CKM matrix, we showcase a global fit to measurements of $K\to \ell \nu$, $K\to \pi\ell \nu$, $\tau \to K\nu$, and $\tau\to K\pi \nu$ decays for the first time. Fitting the semileptonic and 3-body $\tau$ decay data simultaneously becomes computationally feasible because we employ a simple form factor parametrisation for the $K\pi$ form factor that manifestly connects the semileptonic and pair-production regions. We find good agreement with the data and use our analyses to infer $|V_{us}|$ and the parameters for the $K\pi$ form factors. Our result for $|V_{us}|$ sits close to the results extracted from exclusive $K_{\ell 2}$ and somewhat above the $K_{\ell 3}$ decay and inclusive $\tau$ decay result. Moreover, our results for the $K\to \pi$ vector form factor at zero momentum transfer are compatible with lattice QCD determinations, despite not using lattice QCD inputs for this quantity in our fit of the hadronic matrix elements. We are further able to determine some of the pole parameters of the individual scalar and vector $K\pi$ resonances with masses below the $\tau$ mass. We caution that our results account only for short-distance electromagnetic corrections but not long-distance contributions; this is due to the lack of a consistent description of long-distance corrections to the $K\pi$ matrix elements both above and below threshold for an arbitrary model of the form factors.

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arXiv2024

Collider-Flavour Complementarity from the bottom to the top

Motivated by recently observed anomalies in the flavour sector, we analyse the potential of measurements of top quarks at the Large Hadron Collider (LHC) to provide complementary constraints on interactions that shape low-energy precision investigations in the $B$ sector. The measurement of top quark properties, such as the top width and the abundant top pair production channels, are already reaching the percent level at this relatively early stage of the LHC phenomenology program. A focused analysis of four-fermion interactions, employing effective field theory without flavour structure assumptions and incorporating renormalization group evolution effects, bridges $B$ meson scale phenomena with key top quark measurements. We demonstrate that the LHC is increasingly competitive with, and complementary to, flavour physics constraints. Our results, which include a first comprehensive analysis of non-leptonic B decays in this context, suggest that the LHC's top physics program could serve as a valuable, complementary tool in the search for physics beyond the Standard Model within the flavour sector.

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arXiv2024

A Simple Parametrisation of the Pion Form Factor

We discuss a novel and simple parametrisation of the pion vector form factor that transparently connects spacelike and timelike regions of the momentum transfer $q^2$. Our parametrisation employs the framework of conformal mapping and respects the known analyticity properties of the form factor, accounting explicitly for the $\rho(770)$-meson pole. The parametrisation manifestly fulfils the normalisation condition at $q^2 = 0$ as well as further restrictions at the pion production threshold and in the limit $|q^2| \to \infty$. In contrast to the widely used Omn\`es parametrisation, our approach does not use the pion-pion scattering phase shift as input. We confront the parametrisation with experimental data from $\pi H$ scattering and $\tau^- \to \pi^-\pi^0\nu$ decay. We already find a good description of the data with only five free parameters, which include the pole mass and decay width of the $\rho(770)$.

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arXiv2023

Diquark Explanation of $b\to s\ell^+\ell^-$

The discrepancies between $b\to s\ell^+\ell^-$ data and the corresponding Standard Model predictions point to the existence of new physics with a significance at the $5\sigma$ level. While previously a lepton flavour universality violating effect was preferred, the new $R(K^{(*)})$ and $B_s\to\mu^+\mu^-$ measurements are now compatible with the Standard Model, favouring a lepton flavour universal beyond the Standard Model contribution to $C_9$. Since heavy new physics is generally chiral, and because of the stringent constraints from charged lepton flavour violation, this poses a challenge for model building. In this article, we point out a novel possibility: a diquark, i.e. a coloured scalar, induces the Wilson coefficient of the $(\bar s \gamma^\mu P_L b) (\bar c \gamma_\mu P_L c)$ operator at tree-level, which then mixes into $O_9$ via an off-shell photon penguin. This setup allows for a lepton flavour universal effect of $C_9\approx-0.5$, without violating bounds from $\Delta M_s$, $\Delta\Gamma$, $B\to X_s\gamma$ and $D^0-\bar D^0$ mixing. This scenario predicts a small and negative $C_9^{\prime}$ and a light diquark, preferably with a mass around $500\,$GeV, as compatible with the CMS di-di-jet analysis, and a deficit in the inclusive $b\to c\bar c s$ rate.

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arXiv2023

Cabibbo angle anomalies and a global fit to vector-like quarks

The most recent determinations of $V_{ud}$ from superallowed beta decays lead to a discrepancy when compared to the value implied by mesonic CKM measurements combined with CKM unitarity. On top of this, improved precision in lattice QCD calculations have revealed another discrepancy between the $V_{us}$ determinations from kaon and pion semi-leptonic decays. The combination of these can be referred to as the Cabibbo angle anomaly, which we find has a significance of around $3\,\sigma$. After summarising the current state of these issues, I will talk about new physics models that modify semi-leptonic decays as potential explanations, and why vector-like quarks in particular appear the most promising candidates. I will then discuss the results of a global fit to various vector-like quark models, and how other constraints are important in determining the most likely explanation. Finally I will touch on future experiments that could shed further light on the situation.

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arXiv2023

A $\nu$ window onto leptoquarks?

Upcoming neutrino telescopes promise a new window onto the interactions of neutrinos with matter at ultrahigh energies ($E_\nu = 10^7$-$10^{10}$ GeV), and the possibility to detect deviations from the Standard Model predictions. In this paper, we update previous predictions for the enhancement of the neutrino-nucleon cross-section for motivated leptoquark models and show the latest neutrino physics bound, as well as analyse the latest LHC pair production and Drell-Yan data, and flavour constraints (some of which were previously missed). We find that, despite the next generation of neutrino experiments probing the highest energies, they will not be enough to be competitive with collider searches.

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arXiv2023

Minimal model for the $W$-boson mass, $(g-2)_\mu$, $h\to\mu^+\mu^-$ and quark-mixing-matrix unitarity

The $SU(2)_L$ triplet scalar with hypercharge $Y=0$ predicts a positive definite shift in the $W$ mass, w.r.t.~the Standard Model prediction, if it acquires a vacuum expectation value. As this new field cannot couple directly to SM fermions (on its own), it has no significant impact on other low-energy precision observables and is weakly constrained by collider searches. In fact, the multi-lepton anomalies at the LHC even point towards new scalars that decay dominantly to $W$ bosons, as the neutral component of the triplet naturally does. In this article, we show that with a minimal extension of the scalar triplet model by a heavy vector-like lepton, being either I) an $SU(2)_L$ doublet with $Y=-1/2$ or II) an $SU(2)_L$ triplet with $Y=-1$, couplings of the triplet to Standard Model leptons are possible. This minimal extension can then provide, in addition to the desired positive shift in the $W$ mass, a chirally enhanced contribution to $(g-2)_\mu$. In addition version I) and II) can improve on $Z\to\mu^+\mu^-$ and alleviate the tension in first-row CKM unitarity (known as the Cabibbo angle anomaly), respectively. Finally, both options, in general, predict sizable changes of $h\to\mu^+\mu^-$, i.e.,~much larger than most other $(g-2)_\mu$ explanations where only $O(\%)$ effects are expected, making this channel a smoking gun signature of our model.

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arXiv2022

Global Fit of Modified Quark Couplings to EW Gauge Bosons and Vector-Like Quarks in Light of the Cabibbo Angle Anomaly

There are two tensions related to the Cabibbo angle of the CKM matrix. First, the determinations of $V_{us}$ from $K_{\mu 2}$, $K_{\ell3}$, and $\tau$ decays disagree at the $3\sigma$ level. Second, using the average of these results in combination with $\beta$ decays (including super-allowed $\beta$ decays and neutron decay), a deficit in first-row CKM unitarity with a significance of again about $3\sigma$ is found. These discrepancies, known as the Cabibbo Angle anomaly, can in principle be solved by modifications of $W$ boson couplings to quarks. However, due to $SU(2)_L$ invariance, $Z$ couplings to quarks are also modified and flavour changing neutral currents can occur. In order to consistently assess the agreement of a new physics hypothesis with data, we perform a combined analysis for all dimension-six Standard Model Effective Field Theory operators that generate modified $W$ couplings to first and second generation quarks. We then study models with vector-like quarks, which are prime candidates for a corresponding UV completion as they can affect $W$-quark couplings at tree level, and we perform a global fit including flavour observables (in particular loop effects in $\Delta F=2$ processes). We find that the best fit can be obtained for the $SU(2)_L$ doublet vector-like quark $Q$ as it can generate right-handed $W$-$u$-$d$ and $W$-$u$-$s$ couplings as preferred by data.

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arXiv2022

Large $t\to cZ$ as a Sign of Vector-Like Quarks in Light of the $W$ Mass

The rare flavour changing top quark decay $t\to cZ$ is a clear sign of new physics and experimentally very interesting due to the huge number of top quarks produced at the LHC. However, there are few (viable) models which can generate a sizable branching ratio for $t\to cZ$ -- in fact vector-like quarks seem to be the only realistic option. In this paper, we investigate all three representations (under the Standard Model gauge group) of vector-like quarks ($U$, $Q_1$ and $Q_7$) that can generate a sizable branching ratio for $t\to cZ$ without violating bounds from $B$ physics. Importantly, these are exactly the three vector-like quarks which can lead to a sizable positive shift in the prediction for $W$ mass, via the couplings to the top quark also needed for a sizable Br($t\to cZ$). Calculating and using the one-loop matching of vector-like quarks on the Standard Model Effective Field Theory, we find that Br($t\to cZ$) can be of the order of $10^{-6}$, $10^{-5}$ and $10^{-4}$ for $U$, $Q_1$ and $Q_7$, respectively and that in all three cases the large $W$ mass measurement can be accommodated.

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arXiv2021

Unveiling Hidden Physics at the LHC

The field of particle physics is at the crossroads. The discovery of a Higgs-like boson completed the Standard Model (SM), but the lacking observation of convincing resonances Beyond the SM (BSM) offers no guidance for the future of particle physics. On the other hand, the motivation for New Physics has not diminished and is, in fact, reinforced by several striking anomalous results in many experiments. Here we summarise the status of the most significant anomalies, including the most recent results for the flavour anomalies, the multi-lepton anomalies at the LHC, the Higgs-like excess at around 96 GeV, and anomalies in neutrino physics, astrophysics, cosmology, and cosmic rays. While the LHC promises up to 4/ab of integrated luminosity and far-reaching physics programmes to unveil BSM physics, we consider the possibility that the latter could be tested with present data, but that systemic shortcomings of the experiments and their search strategies may preclude their discovery for several reasons, including: final states consisting in soft particles only, associated production processes, QCD-like final states, close-by SM resonances, and SUSY scenarios where no missing energy is produced. New search strategies could help to unveil the hidden BSM signatures, devised by making use of the CERN open data as a new testing ground. We discuss the CERN open data with its policies, challenges, and potential usefulness for the community. We showcase the example of the CMS collaboration, which is the only collaboration regularly releasing some of its data. We find it important to stress that individuals using public data for their own research does not imply competition with experimental efforts, but rather provides unique opportunities to give guidance for further BSM searches by the collaborations. Wide access to open data is paramount to fully exploit the LHCs potential.

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arXiv2021

First-Generation New Physics in Simplified Models: From Low-Energy Parity Violation to the LHC

New-physics (NP) constraints on first-generation quark-lepton interactions are particularly interesting given the large number of complementary processes and observables that have been measured. Recently, first hints for such NP effects have been observed as an apparent deficit in first-row CKM unitarity, known as the Cabibbo angle anomaly, and the CMS excess in $q\bar q\to e^+e^-$. Since the same NP would inevitably enter in searches for low-energy parity violation, such as atomic parity violation, parity-violating electron scattering, and coherent neutrino-nucleus scattering, as well as electroweak precision observables, a combined analysis is required to assess the viability of potential NP interpretations. In this article we investigate the interplay between LHC searches, the Cabibbo angle anomaly, electroweak precision observables, and low-energy parity violation by studying all simplified models that give rise to tree-level effects related to interactions between first-generation quarks and leptons. Matching these models onto Standard Model effective field theory, we derive master formulae in terms of the respective Wilson coefficients, perform a complete phenomenological analysis of all available constraints, point out how parity violation can in the future be used to disentangle different NP scenarios, and project the constraints achievable with forthcoming experiments.

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arXiv2020

Cabibbo anomaly versus electroweak precision tests: An exploration of extensions of the Standard Model

There is a newly emerging tension between determinations of $V_{us}$ from different sources (known as the Cabibbo anomaly), which is clearly demonstrated by the new $R(V_{us})$ observable. We explore this observable from the perspective of the Standard Model Effective Field Theory and show there is a discrepancy between $R(V_{us})$ and existing electroweak precision observables (EWPO) in a simple single operator dominated scenario. We explore all possible single particle extensions of the Standard Model that can generate the Cabibbo anomaly effect and show how they cannot reconcile the current data. We further examine the future of EWPO at the ILC or FCC-ee experiments and discuss the effect on the tension of a change in specific EW observables.

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arXiv2020

Anomalies and accidental symmetries: charging the scalar leptoquark under $L_\mu$-$L_\tau$

While the $S_3$ scalar leptoquark presents a possible tree-level explanation of the $b \to s \ell \ell$ flavour anomalies, it suffers from two conceptual problems which are often disregarded by model-builders. Firstly, the quantum numbers of the $S_3$ allow for a renormalisable diquark operator that would trigger rapid proton decay unless its coupling were tuned away. Secondly, one expects the leptoquark to have generic couplings to leptons, which require tuning to avoid stringent experimental bounds on lepton flavour violation. By gauging a $U(1)$ current that acts as $L_\mu - L_\tau$ on the Standard Model (SM) fermions, and under which the leptoquark has charge $-1$, one can remedy both these problems. The additional $U(1)$, which is spontaneously broken at some high scale, is associated with a massive $Z^\prime$ gauge boson and a scalar SM singlet $\Phi$, which play no direct role in mediating the anomalous $B$ meson decays. By computing one- and two-loop mass corrections, we show that this pair of particles can be hidden away at much higher mass scales without destabilising either the Higgs or the leptoquark masses. The only low-energy relic of gauging $L_\mu - L_\tau$ is thus the accidental global symmetry structure of the lagrangian. On the other hand, we find quite generally that an $S_3$ leptoquark that mediates the $b \to s \ell \ell$ anomalies cannot be much heavier than a few TeV without itself inducing large Higgs mass corrections.

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arXiv2019

$|V_{cb}|$ and $\gamma$ from $B$-mixing -- Addendum to "$B_s$ mixing observables and $|V_{td}/V_{ts}|$ from sum rules"

In this addendum to "$B_s$ mixing observables and $|V_{td}/V_{ts}|$ from sum rules" \cite{King:2019lal} we study the impact of the recent improvements in the theoretical precision of $B$ meson mixing onto CKM unitarity fits. Our key results are the most precise determination of the angle $\gamma = \left(63.4\pm0.9\right)^\circ$ in the unitarity triangle and a new value for the CKM element $|V_{cb}|=(41.6\pm0.7)\cdot10^{-3}$.

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arXiv2019

$\Delta M_s$ theory precision confronts flavour anomalies

Based on recent HQET sum rule and lattice calculations we present updated Standard Model predictions for the mass differences of neutral $B$ mesons: $\Delta M_s^{\rm SM} = \left(18.4^{+0.7}_{-1.2} \right) \mbox{ps}^{-1}$ and $\Delta M_d^{\rm SM} = \left(0.533^{+0.022}_{-0.036} \right) \mbox{ps}^{-1}$ and study their impact on new physics models that address the present hints of anomalous data in $b \to s \ell \ell$ transitions. We also examine future prospects of further reducing the theory uncertainties and discuss the implications of a 2025 scenario with $\Delta M_s^\text{SM 2025} = \left(18.4 \pm 0.5\right) \mbox{ps}^{-1}$. In particular, the latter yields upper bounds $M_{Z'}\lesssim 9~\text{TeV}$ and $M_{S_3}\lesssim 30~\text{TeV}$ for the minimal $Z'$ and $S_3$ lepto-quark explanations of the $b \to s \ell \ell$ anomalies, respectively.

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arXiv2018

$B_s$-$\bar B_s$ mixing interplay with $B$ anomalies

After reviewing the theoretical uncertainties entering the Standard Model determination of the mass difference of the neutral $B_s$-$\bar B_s$ meson system, $\Delta M_s^{\rm SM}$, we discuss the implications of its updated value for new physics models addressing the experimental anomalies in semi-leptonic $B$ decays. Using the most recent FLAG average of lattice results for the non-perturbative matrix elements and the CKM-fitter determination of $V_{cb}$ points to a $1.8 \, \sigma$ discrepancy in $\Delta M_s^{\rm SM} > \Delta M_s^{\rm exp}$. Extending the analysis in Ref. [1] we show that the latter tension cannot be easily accommodated within single mediator models, whenever the same mediator is also responsible for the $b \to s \ell \ell$ anomalies.

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arXiv2017

One constraint to kill them all?

Many new physics models that explain the intriguing anomalies in the $b$-quark flavour sector are severely constrained by $B_s$-mixing, for which the Standard Model prediction and experiment agreed well until recently. The most recent FLAG average of lattice results for the non-perturbative matrix elements points, however, in the direction of a small discrepancy in this observable. Using up-to-date inputs from standard sources such as PDG, FLAG and one of the two leading CKM fitting groups to determine $\Delta M_s^{\rm SM}$, we find a severe reduction of the allowed parameter space of $Z'$ and leptoquark models explaining the $B$-anomalies. Remarkably, in the former case the upper bound on the $Z'$ mass approaches dangerously close to the energy scales already probed by the LHC. We finally identify some model building directions in order to alleviate the tension with $B_s$-mixing.

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