arXiv subjects

Andreas Crivellin

Publications and source records attributed to Andreas Crivellin.

At least 91 records · Page 5Linked to original sources

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

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

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

Implications of $\textit{SU}(2)_L$ gauge invariance for constraints on Lorentz violation

Lorentz invariance is one of the basic ingredients of quantum field theories and violations of it are stringently constrained experimentally. Therefore, the possibility of Lorentz violation (LV) is usually realized at very high energy scales, resulting in a strong suppression of it (by the new scale) in experiments. The Standard-Model Extension (SME) parameterizes LV in a model-independent way, respecting $SU(2)_L$ gauge invariance. This means, e.g., that the neutrino and charged-lepton sectors are linked to each other. Hence, on the one hand, any modification of neutrino properties simultaneously gives rise to effects for charged leptons, which is why the tight limits on flavour-off-diagonal LV for neutrinos imply new bounds on modifications of charged leptons. On the other hand, LV for left-handed charged leptons implies LV for neutrinos. Since LV modifications of the charged-lepton sector are, in general, even more constraining than effects in the flavour-diagonal neutrino sector, we obtain novel tight bounds on LV in the latter. Subsequently, we apply the same approach to an analysis of time-of-flight data for neutrinos (detected by IceCube) and photons from gamma ray bursts where discrepancies have been observed. Our finding is that an explanation of the arrival time difference between neutrino and photon events by dim-5 operators in the neutrino sector would lead to unacceptably large LV effects in the charged-lepton sector.

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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

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

Addendum to: Combined Constraints on First Generation Leptoquarks

In this addendum to arXiv:2101.07811 we discuss the implications of the recent CMS analysis of lepton flavour universality violation in non-resonant di-lepton pairs for first generation leptoquarks. As CMS finds more electron events than expected from background, this analysis prefers the LQ representations $\tilde{S}_1, S_2, S_3, \tilde{V}_1, V_2\,(κ_2^{RL}\ne 0)$ and $V_3$ which lead to constructive interference with the SM. In principle the excess could also be (partially) explained by the representations $\tilde{S}_2, V_1\,(κ_1^R\ne 0), V_2\,(κ_2^{LR}\ne 0), \tilde{V}_2$ which are interfering destructively, as this would still lead to the right effect in bins with high invariant mass where the new physics contribution dominates. However, in these cases large couplings would be required which are excluded by other observables. The representations $S_1, V_1\, (κ_1^{L} \ne 0)$ cannot improve the fit to the CMS data compared to the SM.

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arXiv2021

Generic Loop Effects of New Scalars and Fermions in $b\to s\ell^+\ell^-$, $(g-2)_μ$ and a Vector-like $4^{\rm th}$ Generation

In this article we investigate in detail the possibility of accounting for the $b\to s\ell^+\ell^-$ and $(g-2)_μ$ anomalies via loop contributions involving with new scalars and fermions. For this purpose, we first write down the most general Lagrangian which can generate the desired effects and then calculate the generic expressions for all relevant $b\to s$ Wilson coefficients. Here we extend previous analysis by allowing that the new particles can also couple to right-handed Standard Model (SM) fermions as preferred by recent $b\to s\ell^+\ell^-$ data and the anomalous magnetic moment of the muon. In the second part of this article we illustrate this generic approach for a UV complete model in which we supplement the Standard Model by a $4^{\rm th}$ generation of vector-like fermions and a real scalar field. This model allows one to coherently address the observed anomalies in $b\to s\ell^+\ell^-$ transitions and in $a_μ$ without violating the bounds from other observables (in particular $B_s -\bar B_s$ mixing) or LHC searches. In fact, we find that our global fit to this model, after the recent experimental updates, is very good and prefers couplings to right-handed SM fermions, showing the importance of our generic setup and calculation performed in the first part of the article.

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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

Combined Constraints on First Generation Leptoquarks

In this article we perform a combined analysis of low energy precision constraints and LHC searches for leptoquarks which couple to first generation fermions. Considering all ten leptoquark representations, five scalar and five vector ones, we study at the precision frontier the constraints from $K\toπνν$, $K\toπe^+e^-$, $K^0-\bar K^0$ and $D^0-\bar D^0$ mixing, as well as from experiments searching for parity violation (APV and QWEAK). We include LHC searches for $s$-channel single resonant production, pair production and Drell-Yan-like signatures of leptoquarks. Interestingly, we find that the recent non-resonant di-lepton analysis of ATLAS provides stronger bounds than the resonant searches recasted so far to constrain $t$-channel production of leptoquarks. Taking into account all these bounds, we observe that none of the leptoquark representations can address the so-called "Cabibbo angle anomaly" via a direct contribution to super-allowed beta decays.

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arXiv2021

A new $B$-flavour anomaly in $B_{d,s}\to K^{0}\bar{K}^{0}$: anatomy and interpretation

In the context of the recently measured non-leptonic decays $B_{d}\to K^{*0}\bar{K}^{*0}$ and $B_{s}\to K^{*0}\bar{K}^{*0}$ we analyse the anatomy of the $L_{VV}$ observable that compares the longitudinal components of $B_s \to VV$ and $B_d \to VV$ decays. This observable is cleaner than the longitudinal polarisation fraction as it is afflicted only at subleading order in a $1/m_b$ expansion by the theoretical uncertainties arising in the transverse components entering the polarisation fraction. Focusing on the particular case of $B_{d}\to K^{*0}\bar{K}^{*0}$ and $B_{s}\to K^{*0}\bar{K}^{*0}$, we discuss the main sources of hadronic uncertainty in the SM. We find for the SM prediction $L_{K^*\bar{K}^*}=19.5^{+9.3}_{-6.8}$, which implies a $2.6σ$ tension with respect to the most recent data, pointing to a deficit in the $b \to s$ transition of the non-leptonic decay versus the corresponding $b \to d$ transition. We discuss possible New Physics explanations for this deviation, first at the level of the Weak Effective Theory and we identify that the two Wilson coefficients ${\cal C}_{4}$ and ${\cal C}_{8g}$ can play a central role in explaining this anomaly. Finally, we briefly explore two different simplified New Physics models which can explain the anomaly through a contribution either in ${\cal C}_4$ (Kaluza-Klein gluon) or in ${\cal C}_{8g}$, with a significant amount of fine tuning, but possible connections to the $b \to s \ell \ell$ anomalies.

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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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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

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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