arXiv subjects

Claudio Andrea Manzari

Publications and source records attributed to Claudio Andrea Manzari.

35 records · Page 2Linked to original sources

arXiv2022

Towards excluding a light $Z^\prime$ explanation of $b\to s\ell^+\ell^-$

The discrepancies between $b\to s\ell^+\ell^-$ data and the corresponding Standard Model predictions constitute the most significant hints for new physics (at the TeV scale or below) currently available. In fact, many scenarios that can account for these anomalies have been proposed in the literature. However, only a single light new physics explanation, i.e. with a mass below the $B$ meson scale, is possible: a light $Z^\prime$ boson. In this article, we point out that improved limits on $B\to K^{(*)}νν$, including the experimental sensitivities required for a proper treatment of the necessarily sizable $Z^\prime$ width, together with the forthcoming Belle~II analyses of $e^+e^-\toμ^+μ^-+{\rm invisible}$, can rule out a $Z^\prime$ explanation of $b\to s\ell^+\ell^-$ data with a mass below $\approx4\,$GeV. Importantly, such a light $Z^\prime$ is the only viable single particle solution to the $b\to s\ell^+\ell^-$ anomalies predicting $R(K^{(*)})>0$ in high $q^2$ bins, therefore providing an essential consistency test of data.

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arXiv2021

Hunting for tetraquarks in ultra-pheripheral heavy ion collisions

Ultra-peripheral heavy ion collisions constitute an ideal setup to look for exotic hadrons because of their low event multiplicity and the possibility of an efficient background rejection. We propose to look for four-quark states produced by photon-photon fusion in these collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.5~{\rm TeV}$. In particular, we focus on those states that would represent a definite smoking gun for the compact tetraquark model. We show that the $X(6900)$, a likely $cc\bar{c}\bar{c}$ compact state, is a perfect candidate for this search, and estimate a production cross section ranging from around $250$ nb to $1150$ nb, depending on its quantum numbers. Furthermore, we discuss the importance of ultra-peripheral collisions to the search for the scalar and tensor partners of the $X(3872)$ predicted by the diquarkonium model, and not yet observed. The completion of such a flavor-spin multiplet would strongly speak in favor of the compact tetraquark model.

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arXiv2021

Explaining the Cabibbo Angle Anomaly

The Cabibbo-Cobayashi-Maskawa (CKM) matrix parametrizes the misalignement between the up- and down-quark mass basis in the Standard Model (SM). The observation of first row CKM unitarity violation has recently emerged as a new anomaly of the SM, known as the "Cabibbo Angle Anomaly" (CAA). With current measurements, comparing the elements $V_{ud}$ and $V_{us}$ extracted from beta and kaon decays respectively, the tension with the SM prediction amounts to $\sim$$3\,σ$. Recently, it has been pointed out that this anomaly can also be seen as a discrepancy in the determination of the Fermi constant from muon decay vs $β$ and K decays, once CKM unitarity is assumed. In fact, possible explanations in terms on New Physics fall under two broad classes: contributions to $β$ decay and/or to $μ$ decay. In this proceedings, we discuss these solutions in terms of gauge invariant dimension 6 operators in SMEFT and simplified extensions of the Standard Model. The latter could introduce correlations with other anomalies in the SM, pointing to new and interesting directions for model building.

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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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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

The Fermi constant from muon decay versus electroweak fits and CKM unitarity

The Fermi constant ($G_F$) is extremely well measured through the muon lifetime, defining one of the key fundamental parameters in the Standard Model (SM). Therefore, to search for physics beyond the SM (BSM) via $G_F$, the constraining power is determined by the precision of the second-best independent determination of $G_F$. The best alternative extractions of $G_F$ proceed either via the global electroweak (EW) fit or from superallowed $β$ decays in combination with the Cabibbo angle measured in kaon, $τ$, or $D$ decays. Both variants display some tension with $G_F$ from muon decay, albeit in opposite directions, reflecting the known tensions within the EW fit and hints for the apparent violation of CKM unitarity, respectively. We investigate how BSM physics could bring the three determinations of $G_F$ into agreement using SM effective field theory and comment on future perspectives.

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arXiv2021

Combined Explanation of the $Z\to b\bar b$ Forward-Backward Asymmetry, the Cabibbo Angle Anomaly, $τ\toμνν$ and $b\to s\ell^+\ell^-$ Data

In this article we propose a simple model which can provide a combined explanation of the $Z\to b\bar b$ forward-backward asymmetry, the Cabibbo Angle Anomaly (CAA), $τ\toμνν$ and $b\to s\ell^+\ell^-$ data. This model is obtained by extending the Standard Model (SM) by two heavy vector-like quarks (an $SU(2)_L$ doublet (singlet) with hypercharge $-5/6$ (-1/3)), two new scalars (a neutral and a singly charged one) and a gauged $L_μ-L_τ$ symmetry. The mixing of the new quarks with the SM ones, after electroweak symmetry breaking, does not only explain $Z\to b\bar b$ data but also generates a lepton flavour universal contribution to $b\to s\ell^+\ell^-$ transitions. Together with the lepton flavour universality violating effect, generated by loop-induced $Z^\prime$ penguins involving the charged scalar and the heavy quarks, it gives an excellent fit to data ($6.1\,σ$ better than the SM). Furthermore, the charged scalar (neutral vector) gives a necessarily constructive tree-level (loop) effect in $μ\to eνν$ ($τ\to μνν$), which can naturally account for the CAA (${\rm Br}[τ\toμνν]/{\rm Br}[τ\to eνν]$ and ${\rm Br}[τ\toμνν]/{\rm Br}[μ\to eνν]$).

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arXiv2021

Vector-like Leptons in Light of the Cabibbo-Angle Anomaly

The first row of the Cabibbo-Cobayashi-Maskawa (CKM) matrix shows a discrepancy of $\sim $$3\,σ$ with unitarity, known as the "Cabibbo Angle Anomaly" (CAA). This tension can be explained via modified $W$ couplings to leptons, but in order to consistently assess the validity of this solution, a global fit including the electroweak (EW) precision constraints and the low energy tests of lepton flavour universality (LFU) is necessary. Performing such a fit for gauge-invariant dimension-6 operators, we find that even when assuming LFU, including the CKM elements $V_{us}$ and $V_{ud}$ into the electroweak fit has a relevant impact, shifting the best fit point significantly. Vector-like leptons (VLLs) are prime candidates for a corresponding UV completion since they can affect the $W$ and $Z$ couplings to leptons already at tree-level. Studying each pattern of new physics (NP) given by the six possible representations of VLLs (under the SM gauge group) we find that any single representation describes experimental data at most slighlty better than the SM hypothesis. However, allowing for several representations of VLLs at the same time, we find a simple scenario consisting of a singlet, $N$, coupling to electrons and a triplet, $Σ_1$, coupling to muons which not only explains the CAA but also improves the electroweak fit in such a way that its best fit point is preferred by more than $4\,σ$ with respect to the SM.

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arXiv2021

Searching for Lepton Flavour (Universality) Violation and Collider Signals from a Singly-Charged Scalar Singlet

In recent years, evidence for lepton flavour universality violation beyond the Standard Model has been accumulated. In this context, a singly charged $SU(2)_L$ singlet scalar ($ϕ^\pm$) is very interesting, as it can only have flavour off-diagonal couplings to neutrinos and charged leptons, therefore necessarily violating lepton flavour (universality). In fact, it gives a (necessarily constructive) tree-level effect in $\ell\to\ell^\primeνν$ processes, while contributing to charged lepton flavour violating only at the loop-level. Therefore, it can provide a common explanation of the hints for new physics in $τ\toμνν/τ(μ)\to eνν$ and of the Cabibbo Angle Anomaly. Such an explanation predicts ${\rm Br }[τ\to eγ]$ to be of the order of a few times $10^{-11}$ while ${ \rm Br}[τ\to eμμ]$ can be of the order of $10^{-9}$ for order one couplings and therefore in the reach of forthcoming experiments. Furthermore, we derive a {novel} coupling-independent lower limit on the scalar mass of $\approx 200\,$GeV by recasting LHC slepton searches. In the scenario preferred by low energy precision data, the lower limit is even strengthened to $\approx300\,$GeV, showing the complementary between LHC searches and flavour observables. Furthermore, we point out that this model can be tested by reinterpreting DM mono-photon searches at future $e^+e^-$ colliders.

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arXiv2021

Global Analysis of Leptophilic Z' Bosons

New neutral heavy gauge bosons ($Z^\prime$) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic $Z^\prime$ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like $(g-2)_μ$) and generate flavour violating decays of charged leptons. In particular, $\ell\to\ell^\primeν\barν$ decays, anomalous magnetic moments of charged leptons, $\ell\to\ell^\primeγ$ and $\ell\to3\ell^\prime$ decays place stringent limits on leptophilic $Z^\prime$ bosons. Furthermore, in case of mixing $Z^\prime$ with the SM $Z$, $Z$ pole observables are affected. In light of these many observables we perform a global fit to leptophilic $Z^\prime$ models with the main goal of finding the bounds for the $Z^\prime$ couplings to leptons. To this end we consider a number of scenarios for these couplings. While in generic scenarios correlations are weak, this changes once additional constraints on the couplings are imposed. In particular, if one considers an $L_μ-L_τ$ symmetry broken only by left-handed rotations, or considers the case of $τ-μ$ couplings only. In the latter setup, on can explain the $(g-2)_μ$ anomaly and the hint for lepton flavour universality violation in $τ\toμν\barν/τ\to eν\barν$ without violating bounds from electroweak precision observables.

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arXiv2021

Correlating Non-Resonant Di-Electron Searches at the LHC to the Cabibbo-Angle Anomaly and Lepton Flavour Universality Violation

In addition to the existing strong indications for lepton flavour university violation (LFUV) in low energy precision experiments, CMS recently released an analysis of non-resonant di-lepton pairs which could constitute the first sign of LFUV in high-energy LHC searches. In this article we show that the Cabibbo angle anomaly, an (apparent) violation of first row and column CKM unitarity with $\approx3\,σ$ significance, and the CMS result can be correlated and commonly explained in a model independent way by the operator $[Q_{\ell q}^{(3)}]_{1111} = (\bar{\ell}_1γ^μσ^I\ell_1)(\bar{q}_1γ_μσ^Iq_1)$. This is possible without violating the bounds from the non-resonant di-lepton search of ATLAS (which interestingly also observed slightly more events than expected in the electron channel) nor from $R(π)=π\toμν/π\to e ν$. We find a combined preference for the new physics hypothesis of $4.5\,σ$ and predict $1.0004<R(π)<1.0009$ (95\%~CL) which can be tested in the near future with the forthcoming results of the PEN experiment.

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arXiv2021

Global Electroweak Fit and Vector-Like Leptons in Light of the Cabibbo Angle Anomaly

The "Cabibbo Angle Anomaly" (CAA) originates from the disagreement between the CKM elements $V_{ud}$ and $V_{us}$ extracted from superallowed beta and kaon decays, respectively, once compared via CKM unitarity. It points towards new physics with a significance of up to $4\,σ$, depending on the theoretical input used, and can be explained through modified $W$ couplings to leptons. In this context, vector-like leptons (VLLs) are prime candidates for a corresponding UV completion since they can affect $W\ellν$ couplings at tree-level, such that this modification can have the dominant phenomenological impact. In order to consistently asses the agreement with the data, a global fit is necessary which we perform for gauge-invariant dimension-6 operators and all patterns obtained for the six possible representations (under the SM gauge group) of VLLs. We find that even in the lepton flavour universal case, including the measurements of the CKM elements $V_{us}$ and $V_{ud}$ into the electroweak fit has a relevant impact, shifting the best fit point significantly. Concerning the VLLs we discuss the bounds from charged lepton flavour violating processes and observe that a single representation cannot describe experimental data significantly better than the SM hypothesis. However, allowing for several representations of VLLs at the same time, we find that the simple scenario in which $N$ couples to electrons via the Higgs and $Σ_1$ couples to muons not only explains the CAA but also improves the rest of the electroweak fit in such a way that its best fit point is preferred by more than $4\,σ$ with respect to the SM.

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arXiv2021

Explaining $b\to s\ell^+\ell^-$ and the Cabibbo Angle Anomaly with a Vector Triplet

The most statistically significant hints for new physics in the flavor sector are discrepancies between theory and experiment in $B$ decays to lepton pairs ($b\to s\ell^+\ell^-$) and a deficit in $1^{\rm st}$ row CKM unitarity (the Cabibbo Angle anomaly). We propose that these anomalies can be reconciled by a simplified model with massive gauge bosons transforming in the adjoint representation of $SU(2)_L$. After calculating the impact of this model on $B$ decays, observables testing charged current lepton flavor universality (LFU), electro-weak precision observables and LHC searches we perform a global fit to all available data. We find that our model can provide a consistent common explanation of both anomalies and that the fit to the data is more than $7\,σ$ better than the fit of the SM. The model also predicts interesting correlations between LFU violation in the charged current and $b\to s\ell^+\ell^-$ data which can be tested experimentally in the near future.

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arXiv2020

Global Fit to Modified Neutrino Couplings and the Cabibbo-Angle Anomaly

Recently, discrepancies of up to $4\,σ$ between the different determinations of the Cabibbo angle were observed. This "Cabibbo-angle anomaly" could 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 global analysis in a Bayesian approach and find that the tension in the Cabibbo angle is significantly reduced, while the agreement with other data is also mostly improved. In fact, non-zero modifications of electron and muon neutrino couplings are preferred at more than 99\% CL. Still, since constructive effects in the muon sector are necessary, simple models with right-handed neutrinos (whose global fit we update as a by-product) cannot fully explain data, pointing towards more sophisticated NP models.

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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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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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