arXiv subjects

Ulrich Nierste

Publications and source records attributed to Ulrich Nierste.

At least 19 recordsLinked to original sources

arXiv2026

Next-to-next-to-leading QCD corrections to the $\mathbf{B^+}$-$\mathbf{B_d^0}$, $\mathbf{D^+}$-$\mathbf{D^0}$, and $\mathbf{D_s^+}$-$\mathbf{D^0}$ lifetime ratios

The total decay widths of heavy mesons can be systematically calculated in terms of an expansion in the two parameters $1/m_Q$ and $\alpha_s(m_Q)$, where $Q=c,b$ denotes the heavy quark. The dominant contributions to meson lifetime splittings stem from terms which are suppressed by $1/m_Q^3$ with respect to the leading universal contribution to the total decay width. We calculate three-loop contributions of order $\alpha_s^2/m_q^3$ to the lifetime ratios $\tau(B^+)/\tau(B_d^0)$, $\tau(D^+)/\tau(D^0)$, and $\tau(D_s^+)/\tau(D^0)$ in the limit of exact isospin and V-spin symmetry, respectively. Furthermore, we present new $\alpha_s/m_q^3$ corrections to the Cabibbo-suppressed terms in $\tau(B^+)/\tau(B_d^0)$. Combining our perturbative coefficients with hadronic matrix elements calculated from Heavy Quark Effective Theory sum rules, we find $\tau(B^+)/\tau(B_d^0)= {1.072 \pm 0.024 }$. Using hadronic matrix elements from a recent lattice QCD calculation we find $\tau(D^+)/\tau(D^0)={2.344 \pm 0.170} $ and $\tau(D_s^+)/\tau(D^0)={1.289 \pm 0.042}$. We find good agreement of our predictions with experimental data, which constitutes a successful probe of the calculations of hadronic matrix elements and permits estimates of the unknown $1/m_Q^4$ contributions as well as the V-spin breaking terms in $\tau(D_s^+)/\tau(D^0)$.

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arXiv2025

Complete next-to-next-to-leading order QCD corrections to the decay matrix in $\boldsymbol{B}$-meson mixing at leading power

We compute next-to-next-to-leading order corrections to the decay width difference of mass eigenstates and the charge-parity (CP) asymmetry $a_{\rm fs}$ in flavour-specific decays of neutral $B$ mesons. We include both current-current and penguin operators at three-loop order. All input integrals in the transition amplitude are reduced to a small set of master integrals which depend on the ratio of the charm and bottom quark masses. The latter are computed using semi-analytic methods which provide deep expansions around properly selected values of $m_c/m_b$. We provide numerical results for $\Delta\Gamma$ and $\Delta\Gamma/\Delta M$, both for the $B_d$ and $B_s$ system, including a detailed uncertainty analysis. Using the experimental value for the mass difference $\Delta M_s$ we predict $\Delta\Gamma_s=(0.078 \pm 0.015) ~\mbox{ps}^{-1}$. For the CP asymmetries we find $ a_{\rm fs}^s = (2.27 \pm 0.13 ) \times 10^{-5}$ and $a_{\rm fs}^d = -(5.19 \pm 0.30)~\times~10^{-4}$. Furthermore, we show that the ratios $(\Delta\Gamma_s/\Delta M_s) / (\Delta\Gamma_d/\Delta M_d)$ and $\Delta\Gamma_d / \Delta\Gamma_s$ can be predicted with high precision. The former quantity permits the prediction $\Delta\Gamma_d=(0.00215\pm 0.00013)~\mbox{ps}^{-1}$ from the measurements of $\Delta M_{d,s}$ and $\Delta\Gamma_s$. We further discuss the impact of $\Delta\Gamma_d/\Delta\Gamma_s$ on the CKM unitarity triangle and present ready-to-use formulae which permit improved predictions once updated results for the operator matrix elements are available.

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arXiv2025

QCD corrections to charged-current decays with Heavy Sterile Neutrinos in initial or final state and their impact on $\tau$ decays

Searches for a Heavy Sterile Neutrino $N$ profit from precise predictions of inclusive decay rates, entering predictions for branching fractions and lifetime. Once decay channels into semi-hadronic final states are open, a reliable calculation of inclusive decay rates is only possible if $N$ is heavy enough to permit a perturbative calculation. We adopt the scenario in which $N$ only interacts with SM particles through $N$-$\nu_\ell$ mixing, where $\ell=e,\mu,\tau$. Using literature results for $W$ boson correlators calculated to $\mathcal{O}(\alpha_s^4)$, we study the quality of the perturbation series for $N\to \ell +\mbox{hadrons}$ to determine mass ranges for which inclusive decay widths can be predicted robustly. We present novel analytic results for the decay rate $N\to \tau +\mbox{hadrons}$ in terms of $m_\tau/m_N$. Our expressions equally apply to $\tau \to N +\mbox{hadrons}$, perturbatively calculable for $m_N\lesssim 600\,$MeV. Applying our result to the $\tau$ lifetime, we determine the allowed parameter space for the $N$-$\nu_\tau$ mixing angle $\theta$ and $m_N$. We find $|\sin\theta| \leq 0.2 $ for $m_N=600\,$MeV and weaker bounds for a lighter $N$. For $m_N\geq m_{\tau}$ we find constraints from the dependence of $\tau$ decay rates on $\cos\theta$. Combining $\tau \to \pi^- \nu_\tau$ and $\tau \to K^- \nu_\tau$ data gives $|\sin\theta| = (9.1^{+3.7}_{-7.8}) \cdot 10^{-2}$ while $N$-$\nu_\tau$ mixing does not improve the agreement between theory and data for $\tau \to \ell \bar\nu_\ell \nu_\tau $. We find current data for the decay rate $\Gamma(\tau \to \ell+\mbox{nothing})$ about 1$\sigma$ above the SM prediction for $\Gamma(\tau \to \ell \bar\nu_\ell \nu_\tau)$, which leads to useful constraints on $\Gamma(\tau \to \ell X_{\mathrm{dark}})$ with dark-sector particles $X_{\mathrm{dark}}$ and might stimulate additional experimental effort on $\tau \to \ell+\mbox{nothing}$.

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arXiv2025

CP Violation in Charmed Meson Decays into Final States with $\eta'$

We derive Standard Model predictions for the CP asymmetries of singly-Cabibbo suppressed $D\rightarrow P\eta'$ decays, where $P=K,\pi,\eta$. Our predictions are based on the approximate SU(3)$_F$ symmetry of QCD and include first-order symmetry-breaking effects in a systematic way. The underlying symmetry leads to correlations between different decay modes. To this end we predict the correlations between $ a_{\rm CP}^{\rm dir}(D^+\to \pi^+ \eta')$ and $a_{\rm CP}^{\rm dir}(D_s^+ \to K^+ \eta')$ as well as $a_{\rm CP}^{\rm dir}(D^0\to \pi^0 \eta')$ and $ a_{\rm CP}^{\rm dir}(D^0\to \eta \eta')$. Our results can be used to probe the Standard Model with future measurements by LHCb, Belle II and BESIII. At the same time, such future measurements permit the extraction of the key theory parameter related to the ratio of color-suppressed to color-favoured contributions to the decay amplitudes. Future data on both branching ratios and CP asymmetries will automatically improve our predictions and thereby also their sensitivity to physics beyond the Standard Model.

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arXiv2025

Decay matrix of B-\bar{B} mixing: Mixing of dimension-seven operators into dimension-six operators under renormalization

The precise measurement of the width difference \Delta\Gamma_s among the mass eigenstates of the B_s-\bar{B}_s system requires the calculation of the corresponding decay matrix to order \alpha_s/m_b. QCD corrections to power-suppressed terms in the Heavy Quark Expansion involve the renormalization of dimension-7 four-quark operators for which no general methodology is available yet. In the \overline{MS} scheme one-loop corrections to matrix elements of dimension-7 operators violate the power counting, but we find the responsible terms to be infrared-finite and show that they can be absorbed into finite counterterms proportional to dimension-6 operators. We calculate all these counterterms and subsequently verify the consistency of our results with hadronic matrix elements calculated in the limit of a large number N_c of colours. The condition of correct power counting implies constraints on the possible definitions of evanescent operators.

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arXiv2025

Meson-antimeson mixing

Meson-antimeson transitions are flavor-changing neutral current processes in which the strangeness, charm, or beauty quantum number changes by two units. In the Standard Model (SM) these transitions originate from box diagrams with two W bosons. They permit the preparation of time-dependent, oscillating quantum states which are superpositions of a meson and its antimeson. By studying their decays one gains information on both the meson-antimeson mixing amplitude and the decay amplitude involved and one can measure complex phases quantifying the violation of charge-parity (CP) violation. I present a comprehensive overview on the topic, starting with phenomenological presentations of $K$-$\bar K$, $B_d$-$\bar B_d$, $B_s$-$\bar B_s$, and $D$-$\bar D$ mixing. Highlights are the discovery of the violation of CP and other discrete symmetries, the predictions of the charm quark and its mass and a heavy top quark, and the confirmation of the Kobayashi-Maskawa mechanism of CP violation. Further sections cover the theoretical formalism needed to describe meson-antimeson mixing and to calculate observables in terms of the fundamental parameters of the SM. I discuss the unitarity triangle of the Cabibbo-Kobayashi-Maskawa matrix, which is used to visualize how various CP-violating and CP-conserving quantities combine to probe the SM. I describe the emergence of precision flavor physics and the role of reliable theory calculations to link $K$-$\bar K$ mixing to $B_d$-$\bar B_d$ mixing, which was essential to confirm the Kobayashi-Maskawa mechanism, and present the current status of theory predictions. Today, the focus of the field is on physics beyond the SM, because meson-antimeson mixing amplitudes are sensitive to virtual effects of heavy particles with masses which are several orders of magnitude above the reach of current particle colliders.

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arXiv2025

TeV-scale scalar leptoquarks motivated by B anomalies improve Yukawa unification in SO(10) GUT

It is common practice to explain deviations between data and Standard-Model (SM) predictions by postulating new particles at the TeV scale ad-hoc. This approach becomes much more convincing, if one successfully embeds the postulated particles into a UV completion which addresses other conceptual or phenomenological shortcomings of the SM. We present a study of an SO(10) grand unified theory which contains scalar leptoquark fields employed to explain the ``flavour anomalies'' in $b\rightarrow s$ and $b\rightarrow c$ decays. We find that the additional degrees of freedom improve the renormalization-group (RG) evolution of the SM parameters. In particular, the light leptoquarks modify the RG evolution of the Yukawa couplings such that successful bottom-tau unification becomes possible in a minimal SO(10) GUT with only a $126$-plet coupling to fermions. If we amend the Yukawa interaction of the minimal one-generation model with a second fermion multiplet and small flavor-violating terms, we find the flavour violation in the leptoquark couplings growing with the RG evolution while it stays small in the Yukawa interaction of the SM Higgs boson. By employing mass splittings among the members of the $126$-plet one can increase the effect and obtain large flavor violation in leptoquark couplings from tiny perturbations at the GUT scale, because the flavour-conserving limit is an unstable initial condition for the RG equations.

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arXiv2025

$B\rightarrow K + \text{axion-like particles}$: effective versus UV-complete models and enhanced two-loop contributions

An axion-like particle $a$ (ALP) can explain the excess of $B\rightarrow K + \text{invisible}$ events at Belle-II. However, many analyses of ALP scenarios are over-simplified. We revisit the $B\rightarrow K a$ transition rate in a popular minimal and UV complete model with two Higgs doublets (2HDM) and a complex singlet (DFSZ model). To this end we compare our results with previous studies which derived the $\overline{b}sa$ vertex from the $\overline{b}sA$ vertex, where $A$ is the heavy pseudo-scalar of the 2HDM, in terms of an $a-A$ mixing angle. We find this approach to work only at the leading one-loop order, while it fails at the two-loop level. Furthermore, while an approximate $Z_2$ symmetry suppresses the leading-order amplitude by a factor of $1/\tan\beta$, which is the ratio of the two vacuum expectation values of the Higgs doublets, we find the two-loop contribution unsuppressed and phenomenologically relevant for $\tan\beta \gtrsim 5$. We determine the allowed parameter space and underline the importance of better searches for $\Upsilon\rightarrow \gamma+$invisible and for a possible excess in $B\rightarrow K\mu^+\mu^-$. We further study the low-energy axion effective theory which leads to a divergent and basis-dependent amplitude. As a conceptual result, we clarify the ambiguities and identify which low-energy framework is consistent with the DFSZ model.

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arXiv2025

Current-current operator contribution to the decay matrix in $B$-meson mixing at next-to-next-to-leading order of QCD

We compute next-to-next-to-leading order perturbative corrections to the decay width difference of mass eigenstates and the charge-parity asymmetry $a_{\rm fs}$ in flavour-specific decays of neutral $B$ mesons. In our calculation we take into account the full dependence on the charm and bottom quark masses for the current-current operator contributions up to three-loop order. Special emphasis is put on the proper construction of the so-called $|\Delta B|=2$ theory such that Fierz symmetry is preserved. We provide updated phenomenological predictions, for $\Delta\Gamma$, $\Delta\Gamma/\Delta M$ and $a_{\rm fs}$ for the $B_d$ and $B_s$ system, including a detailed analysis of the uncertainties of our predictions. The calculated NNLO correction reduce the perturbative uncertainty of the leading term of the $1/m_b$ expansion of the width difference $\dg_s$ in the $B_s$ system to the level of the current experimental error. The uncertainty of our prediction $\dg_s=({0.077}\pm 0.016)\,\mbox{ps}^{-1}$ is dominated by the sub-leading term of this expansion. We further illustrate how better future measurements of $\dg_d$ and $a_{\rm fs}^d$ will help to gain a better understanding of $B_d$-$\bar B_d$ mixing.

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arXiv2025

Branching fractions and CP asymmetries in charm meson decays

I present a consistent way to include $\eta$-$\eta^\prime$ mixing in global analyses of two-body decays of heavy hadrons employing the approximate flavour-SU(3) symmetry of QCD. The framework is applied to $D\to P \eta^\prime$ decays, where $P$ denotes a pseudoscalar meson. The result shows that flavour-SU(3) symmetry holds in the decay rates of these modes to better than 30%. With future data we expect the branching ratios of $D_s\to K^+ \eta^\prime$ and $D \to K^+\eta^\prime$ to move upward and downward by $\sim\!\! 1\sigma$, respectively. Subsequently I discuss the implications of the LHCb measurements of the CP asymmetries in $D\to K^+K^-$ and $D\to \pi^+\pi^-$ for generic scenarios of new physics. New-physics contributions should have imprints on other CP asymmetries as well and can be tested through sum rules. Promising decays are $D_s^+\to K^0\pi^+$, $D^+\to \bar K^0K^+$, $D^0\to K^0 \bar K^{*0}$, $D^0\to \bar K^0 K^{*0}$, $D_s^+\to K^{*0}\pi^+$, and $D^+\to \bar K^{*0}K^+$.

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arXiv2024

Model independent bounds on heavy sterile neutrinos from the angular distribution of $\mathbf{B\to D^*\ell\nu}$ decays

In this paper we study the bounds that can be inferred on New Physics couplings to heavy sterile neutrinos $N$ from the recent measurements performed by the Belle collaboration of the angular analysis of $B\to D^*\ell\bar\nu_\ell$ decays, with $\ell=e,\mu$. Indeed, a sterile neutrino $N$ may lead to competing $B\to D^*\ell\bar N$ decays and Belle might have measured an incoherent sum of these two independent channels. After reviewing the theoretical formalism required to describe this phenomenon in full generality, we first perform a bump hunt in the $M_{\rm miss}^2$ Belle distribution to search for evidences of an additional massive neutrino. We found in such a way a small hint at $M_{\rm miss}^2 \sim (350\ {\rm MeV})^2$. However, the Belle angular analysis is sensitive to $N$ masses up to $\mathcal{O}$(50 MeV), preventing us to further inspect this hint. Nevertheless, we study the potential impact of this additional channel in the allowed mass range on the measured angular distributions and extract model-independent bounds on the new-physics couplings which could mediate such an interaction. In particular, in the mass window here inspected, we obtain the most stringent bounds for vector and left-handed scalar operators to date.

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arXiv2024

Anatomy of Non-Leptonic Two-Body Decays of Charmed Mesons into Final States with $\eta'$

We show that $D\rightarrow P\eta^\prime$ decay amplitudes, where $P=K,\pi,\eta$, cannot be simply related to their $D\rightarrow P\eta$ counterparts with a single $\eta_0$--$\eta_8$ mixing angle. We propose a novel, consistent treatment of $\eta_0$--$\eta_8$ mixing for application in $D\to P\eta$ and $D\to P\eta^\prime$ decays. Using this framework, we perform a global analysis of $D\rightarrow P\eta^\prime$ decays employing SU(3)$_F$ symmetry including linear SU(3)$_F$ breaking. We find that the assumption of 30\% SU(3)$_F$ breaking is in slight tension ($2.5\sigma$) with the data when compared to a fit that allows for 50\% SU(3)$_F$ breaking, the latter giving a perfect description of the data. In order to allow for further scrutinization of SU(3)$_F$-breaking effects in the future, we give branching ratio predictions for all $D\rightarrow P\eta'$ modes. Our predictions deviate from the current data in case of the branching ratios $\mathcal{B}(D_s^+\rightarrow K^+\eta^\prime)$ and $\mathcal{B}(D^+\rightarrow K^+\eta')$. Future more precise measurements of these channels are therefore highly important in order to clarify the quality of the SU(3)$_F$ expansion in nonleptonic $D\rightarrow P\eta^\prime$ decays.

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arXiv2024

Radiative corrections relating leptoquark-fermion couplings probed at low and high energy

Scalar leptoquarks (LQ) with masses between 2 TeV and 50 TeV are prime candidates to explain deviations between measurements and Standard-Model predictions in decay observables of $b$-flavored hadrons (``flavor anomalies''). Explanations of low-energy data often involve order-one LQ-quark-lepton Yukawa couplings, especially when collider bounds enforce a large LQ mass. This calls for the calculation of radiative corrections involving these couplings. Studying such corrections to LQ-mediated $b\to c\tau \nu$ and $b\to s\ell^+\ell^-$ amplitudes, we find that they can be absorbed into finite renormalizations of the LQ Yukawa couplings. If one wants to use Yukawa couplings extracted from low-energy data for the prediction of on-shell LQ decay rates, one must convert the low-energy couplings to their high-energy counterparts, which subsume the corrections to the on-shell LQ-quark-lepton vertex. We present compact formulae for these correction factors and find that in scenarios with $S_1$, $R_2$, or $S_3$ LQ the high-energy coupling is always smaller than the low-energy one, which weakens the impact of collider data on the determination of the allowed parameter spaces. For the $R_2$ scenario addressing $b\to c\tau \nu$, in which one of the two involved Yukawa coupling must be significantly larger than 1, we find this coupling reduced by 15\% at high energy. If both $S_1$ and $R_2$ are present, the high-energy coupling can also be larger and the size of the correction is unbounded, because tree contribution and vertex corrections involve different couplings. We further present the conversion formula to the $\overline{\rm MS}$ scheme for the Yukawa couplings of the $S_3$ scenario.

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arXiv2024

$\rm{SU}(3)_F$ sum rules for CP asymmetry of $D_{(s)}$ decays

Charge-parity (CP) asymmetries in charm decays are extremely suppressed in the Standard Model and may well be dominated by new physics contributions. The LHCb collaboration reported the results of direct CP asymmetry measurements in $D^0\to K^+ K^-$ and $D^0\to \pi^+\pi^-$ decays with unprecedented accuracy: $a_{\rm{CP}}(K^+ K^-)=(7.7\pm5.7)\times 10^{-4}$ and $a_{\rm{CP}}(\pi^+\pi^-)=(23.2\pm6.1)\times 10^{-4}$, with the latter quantity inferred from the precise measurement of $\Delta a_{\rm{CP}} =\, a_{\rm{CP}}(K^+ K^-) -a_{\rm{CP}}(\pi^+\pi^-) =\, (-15.7\pm2.9)\times 10^{-4}$. When interpreted within the Standard Model, these values indicate a breakdown of the approximate $U$-spin symmetry of QCD. If, however, this symmetry holds and the data stem from new physics, other CP asymmetries should be enhanced as well. We derive CP asymmetry sum rules based on $\rm{SU}(3)$ flavor symmetry for $D$ meson decays into a pair of pseudoscalar mesons as well as a pair of a pseudoscalar and a vector meson for two generic scenarios, with $\Delta U=0$ and $|\Delta U|=1$ interactions, respectively. The correlations implied by the sum rules can be used to check the consistency between different measurements and to discriminate between these scenarios with future data. For instance, we find $a_{\mathrm{CP}}(\pi^{+}K^{* 0}) + a_{\mathrm{CP}}(K^{+}\overline{K}^{* 0}) = 0$ for $\Delta U=0$ new physics and the opposite relative sign for the $|\Delta U|=1$ case. One sum rule, connecting four decay modes, holds in both scenarios. We further extend our sum rules to certain differences of CP asymmetries from which the $D$ production asymmetries drop out.

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arXiv2024

NNLO QCD corrections to $\Delta \Gamma_s$ in the $B_s-\overline{B}_s$ system

This report summarises recent advances made in the calculation of the NNLO QCD corrections to the width difference $\Delta\Gamma_s$ in the $B_s-\overline{B}_s$ system. The inclusion of the effects due to current-current operators leads to an updated prediction of $\Delta\Gamma_s = (0.076\pm 0.017)\,\text{ps}^{-1}$, which narrows the gap between theory and experiment.

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arXiv2023

Renormalisation group analysis of scalar Leptoquark couplings addressing flavour anomalies: emergence of lepton-flavour universality

Leptoquarks with masses between 2 TeV and 50 TeV are commonly invoked to explain deviations between data and Standard-Model (SM) predictions of several observables in the decays $b\to c\tau \bar\nu$ and $b\to s \ell^+\ell^-$ with $\ell=e,\mu$. While Leptoquarks appear in theories unifying quarks and leptons, the corresponding unification scale $M_{QLU}$ is typically many orders of magnitude above this mass range. We study the case that the mass gap between the electroweak scale and $M_{QLU}$ is only populated by scalar Leptoquarks and SM particles, restricting ourselves to scenarios addressing the mentioned flavour anomalies, and determine the renormalisation-group evolution of Leptoquark couplings to fermions below $M_{QLU}$. In the most general case, we consider three SU(2) triplet Leptoquarks $S_3^\ell$, $\ell=e,\mu,\tau$, which couple quark doublets to the lepton doublet $(\nu_\ell,\ell^-)$ to address the $b\to s \ell^+\ell^-$ anomalies. In this case, we find a scenario in which the Leptoquark couplings to electrons and muons are driven to the same infrared fixed point, so that lepton flavour universality emerges dynamically. However, the corresponding fixed point for the couplings to taus is necessarily opposite in sign, leading to a unique signature in $b \to s\tau^+\tau^-$. For $b\to c\tau \bar\nu$ we complement these with either an SU(2) singlet $S_1^\tau$ or doublet $R_2^\tau$ and study further the cases that also these Leptoquarks come in three replicas. The fixed point solutions for the $S_3^\ell$ couplings explain the $b\to s \ell^+\ell^-$ data for $S_3^{e,\mu}$ masses around 10 TeV. $b\to c\tau \bar\nu$ data can only be fully explained by couplings exceeding their fixed-point values and evolving into Landau poles at high energies, so that one can place an upper bound on $M_{QLU}$ between $10^{8}$ and $10^{11}$ GeV.

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arXiv2023

Discriminating $B\to D^{*}\ell\nu$ form factors via polarization observables and asymmetries

Form factors are crucial theory input in order to extract $|V_{cb}|$ from $B \to D^{(*)}\ell\nu$ decays, to calculate the Standard Model prediction for ${\cal R}(D^{(*)})$ and to assess the impact of New Physics. In this context, the Dispersive Matrix approach, a first-principle calculation of the form factors, using no experimental data but rather only lattice QCD results as input, was recently applied to $B \to D^{(*)}\ell\nu$. It predicts (within the Standard Model) a much milder tension with the ${\cal R}(D^*)$ measurements than the other form factor approaches, while at the same time giving a value of $|V_{cb}|$ compatible with the inclusive value. However, this comes at the expense of creating tensions with differential $B\to D^*\ell\nu$ distributions (with light leptons). In this article, we explore the implications of using the Dispersive Matrix method form factors, in light of the recent Belle (II) measurements of the longitudinal polarization fraction of the $D^*$ in $B\to D^*\ell\nu$ with light leptons, $F_L^{\ell}$, and the forward-backward asymmetry, $A_{\rm FB}^{\ell}$. We find that the Dispersive Matrix approach predicts a Standard Model value of $F_L^{\ell}$ that is in significant tension with these measurements, while mild deviations in $A_{\rm FB}^{\ell}$ appear. Furthermore, $F_L^{\ell}$ is very insensitive to New Physics such that the latter cannot account for the tension between Dispersive Matrix predictions and its measurement. While this tension can be resolved by deforming the original Dispersive Matrix form factor shapes within a global fit, a tension in ${\cal R}(D^*)$ reemerges. As this tension is milder than for the other form factors, it can be explained by New Physics not only in the tau lepton channel but also in the light lepton modes.

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arXiv2022

$B_s \to \mu^+ \mu^-$ in a Two-Higgs-Doublet Model with flavour-changing up-type Yukawa couplings

We present a Two-Higgs-Doublet Model in which the structure of the quark Yukawa matrices is governed by three spurions breaking the flavour symmetries of the quark Yukawa sector. The model naturally suppresses flavour-changing neutral current (FCNC) amplitudes in the down-type sector, but permits sizable FCNC couplings in the up sector. We calculate the branching ratio of $B_s \to \mu^+ \mu^-$ to leading and next-to-leading order of QCD for the case with FCNC couplings of the heavy neutral Higgs bosons to up-type quarks and verify that all counterterms follow the pattern dictated by the spurion expansion of the Yukawa matrices. We find correlations between $B_s \to \mu^+ \mu^-$, $b\to s\gamma$, and the Higgs masses. The $B_s - \bar B_s$ mixing amplitude is naturally suppressed in the model but can probe a portion of the parameter space with very heavy Higgs bosons.

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