arXiv subjects

Francesco Saturnino

Publications and source records attributed to Francesco Saturnino.

8 recordsLinked to original sources

arXiv2023

Specific three-loop contributions to $b\to sγ$ associated with the current-current operators

We work out a specific class of three-loop diagrams (of order $α_s^2$) contributing to the decay amplitude for $b \to s γ$ associated with the current-current operators $O_1$ and $O_2$ at the physical value of the charm-quark mass $m_c$. For many of the considered diagrams we were able to solve the master integrals using differential equations in the canonical form. For some diagrams we did not find a transformation to canonical form and therefore calculated the corresponding master integral directly as an expansion around $z=m_c^2/m_b^2=0$, retaining power terms up to $z^{10}$ and keeping the accompanying $\log(z)$ terms to all powers. The results for the sum of all considered diagrams are given in tabular form, while contributions of individual diagrams (or combinations thereof) are given in electronic form.

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arXiv2021

Correlating $h\toμ^+μ^-$ to the Anomalous Magnetic Moment of the Muon via Leptoquarks

Recently, both ATLAS and CMS measured the decay $h\toμ^+μ^-$, finding a signal strength with respect to the Standard Model expectation of $1.2\pm0.6$ and $1.19\substack{+0.41+0.17\\-0.39 -0.16}$, respectively. This provides, for the first time, evidence that the Standard Model Higgs couples to second generation fermions. This measurement is particularly interesting in the context of the intriguing hints for lepton flavor universality violation, accumulated within recent years, as new physics explanations could also be tested in the $h\toμ^{+}μ^{-}$ decay mode. Leptoquarks are prime candidates to account for the flavor anomalies. In particular, they can provide the necessary chiral enhancement (by a factor $m_t/m_μ$) to address $a_μ$ with TeV scale new physics. In this letter we point out that such explanations of $a_μ$ also lead to enhanced effects in $h\toμ^+μ^-$ and we examine the correlations between $h\toμ^+μ^-$ and $a_μ$ within leptoquark models. We find that the effect in the branching ratio of $h\toμ^+μ^-$ ranges from several percent up to a factor three, if one aims at accounting for $a_μ$ at the $2\,σ$ level. Hence, the new ATLAS and CMS measurements already provide important constraints on the parameter space, rule out specific $a_μ$ explanations and will be very important to test the flavor anomalies in the future.

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arXiv2021

Leptoquarks in Oblique Corrections and Higgs Signal Strength: Status and Prospects

Leptoquarks (LQs) are predicted within Grand Unified Theories and are well motivated by the current flavor anomalies. In this article we investigate the impact of scalar LQs on Higgs decays and oblique corrections as complementary observables in the search for them. Taking into account all five LQ representations under the Standard Model gauge group and including the most general mixing among them, we calculate the effects in $h\toγγ$, $h\to gg$, $h \to Z γ$ and the Peskin-Takeuchi parameters $S$, $T$ and $U$. We find that these observables depend on the same Lagrangian parameters, leading to interesting correlations among them. While the current experimental bounds only yield weak constraints on the model, these correlations can be used to distinguish different LQ representations at future colliders (ILC, CLIC, FCC-ee and FCC-hh), whose discovery potential we are going to discuss.

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arXiv2021

Scalar Leptoquarks in Leptonic Processes

Leptoquarks are hypothetical new particles, which couple quarks directly to leptons. They experienced a renaissance in recent years as they are prime candidates to explain the so-called \textit{flavor anomalies}, i.e. the deviations between the Standard Model predictions and measurements in $b\to s\ell^{+}\ell^{-}$ and $b\to cτν$ processes and in the anomalous magnetic moment of the muon. At the one-loop level these particles unavoidably generate effects in the purely leptonic processes like $Z\to\ell^{+}\ell^{-}$, $Z\toν\barν$, $W\to\ellν$ and $h\to\ell^{+}\ell^{-}$ and can even generate non-zero rates for lepton flavor violating processes such as $\ell\to \ell^{\prime}γ$, $Z\to\ell^+\ell^{\prime -}$, $h\to\ell^+\ell^{\prime -}$ and $\ell\to 3\ell^\prime$. In this article we calculate these processes for all five representations of scalar Leptoquarks. We include their most general interaction terms with the Standard Model Higgs boson, which leads to Leptoquark mixing after the former acquires a vacuum expectation value. In our phenomenological analysis we investigate the effects in modified lepton couplings to electroweak gauge bosons, we study the correlations of the anomalous magnetic moment of the muon with $h\toμ^{+}μ^{-}$ and $Z\toμ^{+}μ^{-}$ as well as the interplay between different lepton flavor violating decays.

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arXiv2020

Flavor Phenomenology of the Leptoquark Singlet-Triplet Model

In recent years, experiments revealed intriguing hints for new physics (NP) in semi-leptonic $B$ decays. Both in charged current processes, involving $b \to c τν$ transitions, and in the neutral currents $b\to s\ell^+\ell^-$, a preference for NP compared to the standard model (SM) of more that $3σ$ and $5σ$ was found, respectively. In addition, there is the long-standing tension between the theory prediction and the measurement of the anomalous magnetic moment (AMM) of the muon ($a_μ$) of more than $3σ$. Since all these observables are related to the violation of lepton flavor universality (LFU), a common NP explanation seems not only plausible but is even desirable. In this context, leptoquarks (LQs) are especially promising since they give tree-level effects in semi-leptonic $B$ decays, but only loop-suppressed effects in other flavor observables that agree well with their SM predictions. Furthermore, LQs can lead to a $m_t/m_μ$ enhanced effect in $a_μ$, allowing for an explanation even with (multi) TeV particles. However, a single scalar LQ representation cannot provide a common solution to all three anomalies. In this article we therefore consider a model in which we combine two scalar LQs: the $SU(2)_L$ singlet and the $SU(2)_L$ triplet. Within this model we compute all relevant one-loop effects and perform a comprehensive phenomenological analysis, pointing out various interesting correlations among the observables. Furthermore, we identify benchmark points which are in fact able to explain all three anomalies ($b \to c τν$, $b\to s\ell^+\ell^-$ and $a_μ$), without violating bounds from other observables, and study their predictions for future measurements.

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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

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

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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