arXiv subjects

Andreas Crivellin

Publications and source records attributed to Andreas Crivellin.

At least 109 records · Page 6Linked to original sources

arXiv2020

Beta decays as sensitive probes of lepton flavor universality

Nuclear $β$ decays as well as the decay of the neutron are well-established low-energy probes of physics beyond the Standard Model (SM). In particular, with the axial-vector coupling of the nucleon $g_A$ determined from lattice QCD, the comparison between experiment and SM prediction is commonly used to derive constraints on right-handed currents. Further, in addition to the CKM element $V_{us}$ from kaon decays, $V_{ud}$ from $β$ decays is a critical input for the test of CKM unitarity. Here, we point out that the available information on $β$ decays can be re-interpreted as a stringent test of lepton flavor universality (LFU). In fact, we find that the ratio of $V_{us}$ from kaon decays over $V_{us}$ from $β$ decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in $W$-$μ$-$ν$ couplings thanks to a CKM enhancement by $(V_{ud}/V_{us})^2\sim 20$. From this perspective, recent hints for the violation of CKM unitarity can be viewed as further evidence for LFUV, fitting into the existing picture exhibited by semi-leptonic $B$ decays and the anomalous magnetic moments of muon and electron. Finally, we comment on the future sensitivity that can be reached with this LFU violating observable and discuss complementary probes of LFU that may reach a similar level of precision, such as $Γ(π\toμν)/Γ(π\to eν)$ at the PEN and PiENu experiments or even direct measurements of $W\toμν$ at an FCC-ee.

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arXiv2020

Modified lepton couplings and the Cabibbo-angle anomaly

Significant discrepancies between the different determinations of the Cabibbo angle have been observed. Here, we point out that this "Cabibbo-angle anomaly" can be explained by lepton flavour universality (LFU) violating New Physics (NP) in the neutrino sector. However, modified neutrino couplings to Standard Model gauge bosons also affect many other observables sensitive to LFU violation, which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent Bayesian global analysis and find that non-zero modifications of electron and muon neutrino couplings are preferred at more than $99.99\%\,{\rm C.L.}$ (corresponding to more than $4\,σ$). Our results show that constructive effects in the muon sector are necessary, meaning simple models with right-handed neutrinos are discarded and more sophisticated NP models required.

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arXiv2020

Hadronic vacuum polarization: $(g-2)_μ$ versus global electroweak fits

Hadronic vacuum polarization (HVP) is not only a critical part of the Standard Model (SM) prediction for the anomalous magnetic moment of the muon $(g-2)_μ$, but also a crucial ingredient for global fits to electroweak (EW) precision observables due to its contribution to the running of the fine-structure constant encoded in $Δα^{(5)}_\text{had}$. We find that with modern EW precision data, including the measurement of the Higgs mass, the global fit alone provides a competitive, independent determination of $Δα^{(5)}_\text{had}\big|_\text{EW}=270.2(3.0)\times 10^{-4}$. This value actually lies below the range derived from $e^+e^-\to\text{hadrons}$ cross-section data, and thus goes into the opposite direction as would be required if a change in HVP were to bring the SM prediction for $(g-2)_μ$ into agreement with the Brookhaven measurement. Depending on the energy where the bulk of the changes in the cross section occurs, reconciling experiment and SM prediction for $(g-2)_μ$ by adjusting HVP would thus not necessarily weaken the case for physics beyond the SM (BSM), but to some extent shift it from $(g-2)_μ$ to the EW fit. We briefly explore some options of BSM scenarios that could conceivably explain the ensuing tension.

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arXiv2020

Emerging patterns of New Physics with and without Lepton Flavour Universal contributions

We perform a model-independent global fit to $b\to s\ell^+\ell^-$ observables to confirm existing New Physics (NP) patterns (or scenarios) and to identify new ones emerging from the inclusion of the updated LHCb and Belle measurements of $R_K$ and $R_{K^*}$, respectively. Our analysis, updating Refs. [1,2] and including these new data, suggests the presence of right-handed couplings encoded in the Wilson coefficients ${\cal C}_{9'μ}$ and ${\cal C}_{10'μ}$. It also strengthens our earlier observation that a lepton flavour universality violating (LFUV) left-handed lepton coupling (${\cal C}_{9μ}^{\rm V}=-{\cal C}_{10μ}^{\rm V}$), often preferred from the model building point of view, accommodates the data better if lepton-flavour universal (LFU) NP is allowed, in particular in ${\cal C}_{9}^{\rm U}$. Furthermore, this scenario with LFU NP provides a simple and model-independent connection to the $b\to cτν$ anomalies, showing a preference of $\approx 7\,σ$ with respect to the SM. It may also explain why fits to the whole set of $b\to s\ell^+\ell^-$ data or to the subset of LFUV data exhibit stronger preferences for different NP scenarios. Finally, motivated by $Z^\prime$ models with vector-like quarks, we propose four new scenarios with LFU and LFUV NP contributions that give a very good fit to data. We provide also an addendum collecting our updated results after including the data for the $B\to K^*μμ$ angular distribution released in 2020 by the LHCb collaboration.

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arXiv2020

Effective Field Theory with Two Higgs Doublets

In this article we extend the effective field theory framework describing new physics effects to the case where the underlying low-energy theory is a Two-Higgs-Doublet model. We derive a complete set of independent operators up to dimension six assuming a $Z_2$-invariant CP-conserving Higgs potential. The effects on Higgs and gauge boson masses, mixing angles in the Higgs sector as well as couplings to fermions and gauge bosons are computed. At variance with the case of a single Higgs doublet, we find that pair production of SM-like Higgses, arising through dimension-six operators, is not fixed by fermion-fermion-Higgs couplings and can therefore be sizable.

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arXiv2019

Correlating Tauonic B Decays to the Neutron EDM via a Scalar Leptoquark

In this article we investigate the correlations between tauonic $B$ meson decays (e.g. $B\to τν$, $B\to D^{(*)}τν$, $B\to πτν$) and electric dipole moments (EDMs), in particular the one of the neutron, in the context of the $S_1$ scalar leptoquark (LQ). This LQ naturally arises in the R-parity violating MSSM as the right-handed down-squark. We perform the matching of this model on the effective field theory taking into account the leading renormalization group effect for the relevant observables. We find that one can explain the hints for new physics in $b\to cτν$ transitions without violating bounds from other observables. Even more interesting, it can also give sizable effects in $B\toτν$, to be tested at BELLE II, which are correlated to (chromo) electric dipole operators receiving $m_τ/m_u$ enhanced contributions. Therefore, given a deviation from the Standard Model (SM) expectations in $B\toτν$, this model predicts a sizable neutron EDM. In fact, even if new physics has CP conserving real couplings, the CKM matrix induces a complex phase and already a 10\% change of the $B\toτν$ branching ratio (with respect to the SM) will lead to an effect observable with the n2EDM experiment at PSI.

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arXiv2019

$Z^\prime$ models with less-minimal flavour violation

We study the phenomenology of simplified $Z^\prime$ models with a global $U(2)^3$ flavour symmetry in the quark sector, broken solely by the Standard Model Yukawa couplings. This flavour symmetry, known as less-minimal flavour violation, protects $ΔF=2$ processes from dangerously large new physics (NP) effects, and at the same time provides a free complex phase in $b\to s$ transitions, allowing for an explanation of the hints for additional direct CP violation in kaon decays ($ε^\prime/ε$) and in hadronic $B$-decays ($B\to Kπ$ puzzle). Furthermore, once the couplings of the $Z^\prime$ boson to the leptons are included, it is possible to address the intriguing hints for NP (above the 5$\,σ$ level) in $b\to s \ell^+\ell^-$ transitions. Taking into account all flavour observables in a global fit, we find that $ε^\prime/ε$, the $B\to Kπ$ puzzle and $b\to s \ell^+\ell^-$ data can be explained simultaneously. Sizeable CP violation in $b\to s \ell^+\ell^-$ observables, in particular $A_8$, is predicted, which can be tested in the near future, and an explanation of the $B\to Kπ$ and $ε^\prime/ε$ puzzles leads to effects in di-jet tails at the LHC, that are not far below the current limits. Once $b\to s \ell^+\ell^-$ is included, cancellations in di-muon tails, possibly by a second $Z^\prime$, are required by LHC data.

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arXiv2019

Correlating $ε^\prime/ε$ to hadronic $B$ decays via $U(2)^3$ flavour symmetry

There are strong similarities between charge-parity (CP) violating observables in hadronic $B$ decays (in particular $ΔA^-_{\rm CP}$ in $B\to Kπ$) and direct CP violation in Kaon decays ($ε^\prime$): All these observables are very sensitive to new physics (NP) which is at the same time CP and isospin violating (i.e. NP with complex couplings which are different for up quarks and down quarks). Intriguingly, both the measurements of $ε^\prime$ and $ΔA^-_{\rm CP}$ show deviations from their Standard Model predictions, calling for a common explanation (the latter is known as the $B\to Kπ$ puzzle). For addressing this point, we parametrize NP using a gauge invariant effective field theory approach combined with a global $U(2)^3$ flavor symmetry in the quark sector (also known as less-minimal flavour violation). We first determine the operators which can provide a common explanation of $ε^\prime$ and $ΔA^-_{\rm CP}$ and then perform a global fit of their Wilson coefficients to the data from hadronic $B$ decays. Here we also include e.g. the recently measured CP asymmetry in $B_s\to KK$ as well as the purely isospin violating decay $B_s\toϕρ^0$, finding a consistent NP pattern providing a very good fit to data. Furthermore, we can at the same time explain $ε^\prime/ε$ for natural values of the free parameters within our $U(2)^3$ flavour approach, and this symmetry gives interesting predictions for hadronic decays involving $b\to d$ transitions.

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arXiv2019

Addendum to "Impact of polarization observables and $B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly"

In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${\cal R}(D)$ and ${\cal R}(D^*)$ by the Belle collaboration: ${\cal R}(D)_{\rm Belle} = 0.307\pm0.037\pm0.016$ and ${\cal R}(D^*)_{\rm Belle}=0.283\pm0.018\pm0.014$, resulting in the new HFLAV fit result ${\cal R}(D) = {0.340\pm0.027 \pm 0.013}$, ${\cal R}(D^*) = {0.295\pm0.011 \pm 0.008 }$, exhibiting a $3.1\,σ$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${\cal R}(Λ_c)$ from our sum rule reads ${\cal R}(Λ_c)= \mathcal{R}_{\rm SM}(Λ_c) \left( 1.15 \pm 0.04 \right) = 0.38 \pm 0.01 \pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $b\to B_c$ and their implication on the constraint from $B_{u/c}\toτν$ data.

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arXiv2019

$\boldsymbol{C\!P}\!$ violation in Higgs-gauge interactions: from tabletop experiments to the LHC

We investigate the interplay between the high- and low-energy phenomenology of $C\!P$-violating interactions of the Higgs boson with gauge bosons. For this purpose we use an effective field theory approach and consider all dimension-6 operators arising in so-called universal theories. We compute their loop-induced contributions to electric dipole moments and the $C\!P$ asymmetry in $B\to X_sγ$, and compare the resulting current and prospective constraints to the projected sensitivity of the LHC. Low-energy measurements are shown to generally have a far stronger constraining power, which results in highly correlated allowed regions in coupling space, a distinctive pattern that could be probed at the high-luminosity LHC.

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arXiv2019

Explaining the Flavor Anomalies with a Vector Leptoquark (Moriond 2019 update)

Several experiments revealed intriguing hints for lepton flavor universality (LFU) violating new physics (NP) in semi-leptonic $B$ meson decays, mainly in $b \to c τν$ and $b \to s \ell^+ \ell^-$ transitions at the $3-5\,σ$ level. Leptoquarks (LQ) are prime candidates to address these anomalies as they contribute to semi-leptonic decays already at tree level while effects in other flavor observables, agreeing with the standard model (SM), are loop suppressed. In these proceedings we review the vector leptoquark $SU(2)_L$ singlet, contained in the famous Pati-Salam model, which is able to address both $b \to cτν$ and $b \to s μ^+ μ^-$ data simultaneously. Due to the large couplings to tau leptons needed to account for the $b \to c τν$ data, sizable loop effects arise which we include in our phenomenological analysis. Updating our previous result with the recent measurements of LHCb and BELLE we find an even better fit to data than before.

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arXiv2019

Impact of polarization observables and $ B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly

The combined analysis of the BaBar, Belle, and LHCb data on $B\to Dτν$, $B\to D^*τν$ and $B_c\to J/Ψτν$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $τ$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\toτν)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\toτν)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\toτν)$ constraint has a significant impact: depending on whether one uses a limit of $60\%$, $30\%$ or $10\%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60\%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10\%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $B\to Dτν$, $B\to D^*τν$ and $Λ_b\to Λ_cτν$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(Λ_b\to Λ_cτν)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data.

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arXiv2019

Combined explanations of $(g-2)_μ$, $(g-2)_e$ and implications for a large muon EDM

We consider possible beyond-the-Standard-Model (BSM) effects that can accommodate both the long-standing tension in the anomalous magnetic moment of the muon, $a_μ=(g-2)_μ/2$, as well as the emerging $2.5σ$ deviation in its electron counterpart, $a_e=(g-2)_e/2$. After performing an EFT analysis, we consider BSM physics realized above the electroweak scale and find that a simultaneous explanation becomes possible in models with chiral enhancement. However, this requires a decoupling of the muon and electron BSM sectors to avoid the strong constraints from $μ\to eγ$. In particular, this decoupling implies that there is no reason to expect the muon electric dipole moment (EDM) $d_μ$ to be correlated with the electron EDM $d_e$, avoiding the very stringent limits for the latter. While some of the parameter space for $d_μ$ favored by $a_μ$ could be tested at the $(g-2)_μ$ experiments at Fermilab and J-PARC, a dedicated muon EDM experiment at PSI would be able to probe most of this region.

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arXiv2019

$b\to s\ell^+\ell^-$ Transitions in Two-Higgs-Doublet Models

In this article we study $b\to sμ^+μ^-$ transitions and possible correlations with the anomalous magnetic moment of the muon ($a_μ$) within two-Higgs-doublet models with generic Yukawa couplings, including the possibility of right-handed neutrinos. We perform the matching on the relevant effective Hamiltonian and calculate the leading one-loop effects for $b\to s\ell\ell^{(\prime)}$, $b\to sγ$, $ΔB=ΔS=2$, $b\to sν\barν$ and $\ell\to\ell^\primeγ$ transitions in a general $R_ξ$ gauge. Concerning the phenomenology, we find that an explanation of the hints for new physics in $b\to sμ^+μ^-$ data is possible once right-handed neutrinos are included. If lepton flavour violating couplings are allowed, one can account for the discrepancy in $a_μ$ as well. However, only a small portion of parameter space gives a good fit to $b\to sμ^+μ^-$ data and the current bound on $h\toτμ$ requires the mixing between the neutral Higgses to be very small if one aims at an explanation of $a_μ$.

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arXiv2019

Low- and high-energy phenomenology of a doubly charged scalar

We explore the phenomenology of an $SU(2)$-singlet doubly charged scalar at the high and low energy frontier. Such a particle is predicted in different new physics models, like left-right symmetric models or the Zee-Babu model. Nonetheless, since its interactions with Standard Model (SM) leptons are gauge invariant, it can be consistently studied as a UV complete SM extension. Its signatures range from same-sign di-lepton pairs to flavour changing decays of charged leptons to muonium-antimuonium oscillations. In this article, we use a systematic effective-field-theory approach for studying the low-energy observables and comparing them consistently to collider bounds. For this purpose, experimental searches for doubly charged scalars at the Large Hadron Collider are reinterpreted, including large width effects, and projections for exclusion and discovery reaches in the high-luminosity phase are provided. The sensitivities of the future International Linear Collider and Compact Linear Collider for the doubly charged scalar are presented with focus on di-lepton final states and resonant production. Theoretically and phenomenologically motivated benchmark scenarios are considered showing the different impact of low- and high-energy observables. We find that future low- and high-energy experiments display strong complementarity in studying the parameter space of the model.

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arXiv2019

Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark

In recent years experiments revealed intriguing hints for new physics (NP) in $B$ decays involving \bctaunu and $b\to s\ell^+\ell^-$ transitions at the $4\,σ$ and $5\,σ$ level, respectively. In addition, there are slight disagreements in $b\to u τν$ and $b\to dμ^+μ^-$ observables. While not significant on their own, they point in the same direction. Furthermore, $V_{us}$ extracted from $τ$ decays shows a slight tension ($\approx2.5\,σ$) with its value determined from CKM unitarity. Additionally, BELLE found hints for an excess in $B_d\toτ^+τ^-$. Concerning NP explanations, the vector leptoquark $SU(2)$ singlet is of special interest since it is the only single particle extension of the SM which can (in principle) address all the anomalies described above. For this purpose, large couplings to $τ$ leptons are necessary and loop effects, which we calculate in this article, become important. Including them in our phenomenological analysis, we find that neither the tension in $V_{us}$ nor the excess in $B_d\toτ^+τ^-$ can be fully explained without violating bounds from $K\toπν\barν$. However, one can account for $b\to cτν$ and $b\to uτν$ data finding intriguing correlations with $B_{q}\toτ^+τ^-$ and $K\to πν\barν$. Furthermore, the explanation of $b\to cτν$ predicts a positive shift in $C_7$ and a negative one in $C_9$, being nicely in agreement with the global fit to $b\to s\ell^+\ell^-$ data. Finally, we point out that one can fully account for \bctaunu and $b\to s\ell^+\ell^-$ without violating bounds from $τ\to ϕμ$, $Υ\toτμ$ or $b\to sτμ$ processes.

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arXiv2018

Combined explanations of $(g-2)_{μ,e}$ and implications for a large muon EDM

With the long-standing tension between experiment and Standard-Model (SM) prediction in the anomalous magnetic moment of the muon, $a_μ=(g-2)_μ/2$, at the level of 3-4$σ$, it is natural to ask if there could be a sizable effect in the electric dipole moment (EDM) $d_μ$ as well. In this context it has often been argued that in UV complete models the electron EDM, which is very precisely measured, excludes a large effect in $d_μ$. However, the recently observed 2.5$σ$ tension in $a_e=(g-2)_e/2$, if confirmed, requires that the muon and electron sectors effectively decouple to avoid constraints from $μ\to eγ$. We briefly discuss UV complete models that possess such a decoupling, which can be enforced by an Abelian flavor symmetry $L_μ-L_τ$. We show that, in such scenarios, there is no reason to expect a correlation between the electron and muon EDM, so that the latter can be sizable. New limits on $d_μ$ improved by up to two orders of magnitude are expected from the upcoming $(g-2)_μ$ experiments at Fermilab and J-PARC. Beyond, a proposed dedicated muon EDM experiment at PSI could further advance the limit. In this way, future improved measurements of $a_e$, $a_μ$, as well as the fine-structure constant $α$ are not only set to provide exciting precision tests of the SM, but, in combination with EDMs, to reveal crucial insights into the flavor structure of physics beyond the SM.

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arXiv2018

A model of vector leptoquarks in view of the $B$-physics anomalies

Lepton number as a fourth color is an intriguing theoretical idea which is combined with a possible left-right symmetry within the famous Pati-Salam (PS) model. In the conventional PS model, a spontaneous breaking of the PS gauge group down to the SM one can only take place at very high scales (above the PeV scale) due to the stringent bounds from $K_L\toμe$ and $K\toπμe$ induced by the resulting vector leptoquarks. In this paper, we show that these constraints can be avoided once additional vector-like fermions are introduced and thus a breaking at the TeV scale is possible. We consider the flavor phenomenology of this model in the context of the intriguing hints for new physics in semileptonic $B$ decays. The necessary violation of lepton flavor universality is induced by mixing SM and vector-like fermions. Concerning $R(D)$ and $R(D^*)$ we find that sizable effects are possible while respecting the bounds from other flavor observables but predicting a large enhancement of $B_s\toτ^+τ^-$. Furthermore, also in $b\to s\ell^+\ell^-$ transitions the observed deviations from the SM predictions (including $R(K)$ and $R(K^*)$) can be explained with natural values for the free parameters of the model without any fine-tuning, predicting sizable decay rates for $b\to sτμ$. Finally, the anomaly in anomalous magnetic moment of the muon can be accounted for by a loop-contribution involving the vector leptoquark and vector-like leptons.

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