Scenarios Of New Physics Research Articles

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 qoverline{q} → 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.

Impact of B→Kνν¯ measurements on beyond the Standard Model theories

Semileptonic flavor changing neutral current transitions with a pair of neutrinos in the final state are very accurately determined in the standard model (SM) and thus provide an accurate and sensitive probe for physics beyond the SM. Until recently, the poor tagging efficiency for the $B\to K^{(*)}\nu \bar{\nu}$ modes made them less advantageous as a probe of new physics (NP) compared to the charged lepton counterparts. The most recent Belle II result on $B\to K \nu \bar{\nu}$ uses an innovative inclusive tagging technique resulting in a higher tagging efficiency; this together with previous BaBar and Belle results indicates a possible enhancement in the branching fraction of $B^+\to K^+ \nu \bar{\nu}$. A reanalysis of the full Belle dataset together with upcoming Belle II dataset is expected to result in a much more precise measurement of this mode. If the branching ratio is indeed found to be enhanced with improved measurements, this would provide an unambiguous signal of NP without uncertainties due to long-distance non-factorizable effects or power corrections (in contrast to $B\to K^{(*)} \ell \ell$). We have explored the possibilities of such an enhancement as a signal of NP within several scenarios, which can also explain some of the other tensions observed in neutral as well as charged current $B$-decays. In an effective field theory approach, with the most general dimension-six Hamiltonian including light right-handed neutrinos, we explore the viability of all scalar and vector leptoquarks as well as the parameter space possible with a generic vector gauge boson $Z^\prime$ model assuming minimal new particle content. While being consistent with all data, correlations between the observed intriguing discrepancies in $B$-decays are also obtained, which will discriminate between the various NP scenarios.

New physics constraints from atomic parity violation in Cs133

Our improved calculation of the nuclear spin-independent parity violating electric dipole transition amplitude ($E1_{PV}$) for $6s ~ ^2S_{1/2} - 7s ~ ^2S_{1/2}$ in $^{133}$Cs in combination with the most accurate (0.3\%) measurement of this quantity yields a new value for the nuclear weak charge $Q_W=-73.71(26)_{ex} (23)_{th}$ against the Standard Model (SM) prediction $Q_W^{\text{SM}}=-73.23(1)$. The advances in our calculation of $E1_{PV}$ have been achieved by using a variant of the perturbed relativistic coupled-cluster theory which treats the contributions of the core, valence and excited states to $E1_{PV}$ on the same footing unlike the previous high precision calculations. Furthermore, this approach resolves the controversy regarding the sign of the core correlation effects. We discuss the implications of the deviation of our result for $Q_W$ from the SM value by considering different scenarios of new physics.

New physics and the tau polarization vector in b \u2192 mathrm{c}tau {overline{nu}}_{tau } decays

For a general Hb → {H}_ctau {overline{nu}}_{tau } decay we analyze the role of the τ polarization vector {mathcal{P}}^{mu } in the context of lepton flavor universality violation studies. We use a general phenomenological approach that includes, in addition to the Standard Model (SM) contribution, vector, axial, scalar, pseudoscalar and tensor new physics (NP) terms which strength is governed by, complex in general, Wilson coefficients. We show that both in the laboratory frame, where the initial hadron is at rest, and in the center of mass of the two final leptons, a overrightarrow{mathcal{P}} component perpendicular to the plane defined by the three-momenta of the final hadron and the τ lepton is only possible for complex Wilson coefficients, being a clear signal for physics beyond the SM as well as time reversal (or CP-symmetry) violation. We make specific evaluations of the different polarization vector components for the Λb → Λc, {overline{B}}_c → ηc, J/ψ and overline{B} → D(*) semileptonic decays, and describe NP effects in the complete two-dimensional space associated with the independent kinematic variables on which the polarization vector depends. We find that the detailed study of {mathcal{P}}^{mu } has great potential to discriminate between different NP scenarios for 0− → 0− decays, but also for Λb → Λc transitions. For this latter reaction, we pay special attention to corrections to the SM predictions derived from complex Wilson coefficients contributions.

Flavour dynamics and violations of the CP symmetry

An overview of flavour physics and CP-violating phenomena is presented. The Standard Model quark-mixing mechanism is discussed in detail and its many successful experimental tests are summarized. Flavour-changing transitions put very stringent constraints on new-physics scenarios beyond the Standard Model framework. Special attention is given to the empirical evidences of CP violation and their important role in our understanding of flavour dynamics.The current status of the so-called flavour anomalies is also reviewed.

Testing leptoquark/EFT in {bar{B}} rightarrow {D^{(*)}}l{bar{nu }} at the LHC

We investigate the current LHC bounds on New Physics (NP) that contributes to {bar{B}} rightarrow {D^{(*)}}l{bar{nu }} for l = (e,mu ,tau ) by considering both leptoquark (LQ) models and an effective-field-theory (EFT) Hamiltonian. Experimental analyses from l+text {missing} searches with high p_T are applied to evaluate the NP constraints with respect to the Wilson coefficients. A novel point of this work is to show difference between LQs and EFT for the applicable LHC bound. In particular, we find that the EFT description is not valid to search for LQs with the mass less than lesssim 10,text {TeV} at the LHC and leads to overestimated bounds. We also discuss future prospects of high luminosity LHC searches including the charge asymmetry of background and signal events. Finally, a combined summary for the flavor and LHC bounds is given, and then we see that in several NP scenarios the LHC constraints are comparable with the flavor ones.

New physics at nuSTORM

In this work we investigate the usefulness of nuSTORM as a probe of two new-physics scenarios which are sterile neutrinos and non-unitarity of the neutrino mixing matrix. For the sterile neutrino we show the importance of the neutral current events when combined with the charged current events to constrain the effective mixing angle, $\theta_{\mu \mu}$, and the sterile mixing angles $\theta_{14}$ and $\theta_{24}$. We also study the role nuSTORM will play in the study of neutrino oscillation physics if the three generation neutrino mixing matrix is non-unitary. In this context we elucidate the role of nuSTORM, considering both charged current and neutral current events, in constraining the various non-unitarity parameters such as $\alpha_{11}$, $|\alpha_{21}|$ and $\alpha_{22}$.

The measurable angular distribution of {Lambda}_b^0to {Lambda}_c^{+}left(to {Lambda}^0{pi}^{+}right){tau}^{-}left(to {pi}^{-}{v}_{tau}right){overline{v}}_{tau } decay

In {Lambda}_b^0to {Lambda}_c^{+}left(to {Lambda}^0{pi}^{+}right){tau}^{-}{overline{v}}_{tau } decay, the solid angle of the final-state particle τ− cannot be determined precisely since the decay products of the τ− include an undetected ντ. Therefore, the angular distribution of this decay cannot be measured. In this work, we construct a measurable angular distribution by considering the subsequent decay τ−→ π−ντ. The full cascade decay is {Lambda}_b^0to {Lambda}_c^{+}left(to {Lambda}^0{pi}^{+}right){tau}^{-}left(to {pi}^{-}{v}_{tau}right){overline{v}}_{tau } . The three-momenta of the final-state particles Λ0, π+, and π− can be measured. Considering all Lorentz structures of the new physics (NP) effective operators and an unpolarized initial Λb state, the five-fold differential angular distribution can be expressed in terms of ten angular observables {mathcal{K}}_ileft({q}^2,{E}_{pi}right) . By integrating over some of the five kinematic parameters, we define a number of observables, such as the Λc spin polarization {P}_{Lambda_c}left({q}^2right) and the forward-backward asymmetry of π− meson AFB(q2), both of which can be represented by the angular observables {hat{mathcal{K}}}_ileft({q}^2right) . We provide numerical results for the entire set of the angular observables {hat{mathcal{K}}}_ileft({q}^2right) and {hat{mathcal{K}}}_i both within the Standard Model and in some NP scenarios, which are a variety of best-fit solutions in seven different NP hypotheses. We find that the NP which can resolve the anomalies in overline{B}to {D}^{left(ast right)}{tau}^{-}{overline{v}}_{tau } decays has obvious effects on the angular observables {hat{mathcal{K}}}_ileft({q}^2right) , except {hat{mathcal{K}}}_{1 ss}left({q}^2right) and {hat{mathcal{K}}}_{1 cc}left({q}^2right) .

The present and future of four top operators

We study the phenomenology of a strongly-interacting top quark at future hadron and lepton colliders, showing that the characteristic four-top contact operators give rise to the most significant effects. We demonstrate the extraordinary potential of a 100 TeV proton-proton collider to directly test such non-standard interactions in four-top production, a process that we thoroughly analyze in the same-sign dilepton and trilepton channels, and explore in the fully hadronic channel. Furthermore, high-energy electron-positron colliders, such as CLIC or the ILC, are shown to exhibit an indirect yet remarkable sensitivity to four-top operators, since these constitute, via renormalization group evolution, the leading new-physics deformations in top-quark pair production. We investigate the impact of our results on the parameter space of composite Higgs models with a strongly-coupled (right-handed) top quark, finding that four-top probes provide the best sensitivity on the compositeness scale at the future energy frontier. In addition, we investigate mild yet persisting LHC excesses in multilepton plus jets final states, showing that they can be consistently described in the effective field theory of such a new-physics scenario.

B¯c→ηc , B¯c→J/ψ and B¯→D(*) semileptonic decays including new physics

We apply the general formalism derived in N. Penalva et al. [Phys. Rev. D 101, 113004 (2020)] to the semileptonic decay of pseudoscalar mesons containing a $b$ quark. While present $B\to D^{(*)}$ data give the strongest evidence in favor of lepton flavor universality violation, the observables that are normally considered are not able to distinguish between different new physics (NP) scenarios. In the above reference we discussed the relevant role that the various contributions to the double differential decay widths $d^2\Gamma/(d\omega d\cos\theta_\ell)$ and $d^2\Gamma/(d\omega dE_\ell)$ could play to this end. Here $\omega$ is the product of the two hadron four-velocities, $\theta_\ell$ is the angle made by the final lepton and final hadron three-momenta in the center of mass of the final two-lepton system, and $E_\ell$ is the final charged lepton energy in the laboratory system. The formalism was applied there to the analysis of the $\Lambda_b\to\Lambda_c$ semileptonic decay showing the new observables were able to tell apart different NP scenarios. Here we analyze the $\bar B_c\to \eta_c \tau\bar\nu_\tau$, $\bar B_c\to J/\psi\tau\bar\nu_\tau$, $\bar B\to D\tau\bar\nu_\tau$ and $\bar B\to D^*\tau\bar\nu_\tau$ semileptonic decays. We show that, as a general rule, the $\bar B_c \to J/\psi$ observables, even including $\tau$ polarization, are less optimal for distinguishing between NP scenarios than those obtained from $\bar B_c \to \eta_c$ decays, or those presented in N. Penalva et al. for the related $\Lambda_b \to \Lambda_c$ semileptonic decay. Finally, we show that $\bar B\to D$ and $\bar B_c\to \eta_c$, and $\bar B\to D^*$ and $\bar B_c\to J/\psi$ decay observables exhibit similar behaviors.

Enhanced CP asymmetries in B rightarrow K mu ^+ mu ^-

We show that the current values of R_K^mathrm {exp} and R_{K^*}^mathrm {exp} can be accommodated by allowing a nonzero New Physics coupling delta C_9^{mu mu } to be complex, both in the scenario in which only delta C_9^{mu mu } is affected, and in the scenario with complex delta C_{9,10}^{mu mu } satisfying delta C_{9}^{mu mu }=-delta C_{10}^{mu mu }. A presence of the weak CP-violating phase can then be tested by measuring the CP-asymmetry, mathcal {A}_mathrm {CP}. We show that this asymmetry is enhanced around the peak of each cbar{c}-resonance, and in fact more pronounced in the close vicinity of J/psi and psi (2S). Therefore, measuring mathcal {A}_mathrm {CP} before and after the resonances’ peak could be revelatory of the CP-violation that originates from beyond the Standard Model, or to be a significant constrain when building a realistic scenario of New Physics.

Novel flavour-changing neutral currents in the top quark sector

We demonstrate that flavour-changing neutral currents in the top sector, mediated by leptophilic scalars at the electroweak scale, can easily arise in scenarios of new physics, and in particular in composite Higgs models. We moreover show that such interactions are poorly constrained by current experiments, while they can be searched for at the LHC in rare top decays and, more generally, in the channels $pp\to tS(S)+j$, with $S\to\ell^+\ell^-$. We provide dedicated analyses in this respect, obtaining that cut-off scales as large as $\Lambda\sim$ 90 TeV can be probed with an integrated luminosity of $\mathcal{L} = 150$ fb$^{-1}$.

Das ist der HAMMER: consistent new physics interpretations of semileptonic decays

Precise measurements of brightarrow ctau bar{nu } decays require large resource-intensive Monte Carlo (MC) samples, which incorporate detailed simulations of detector responses and physics backgrounds. Extracted parameters may be highly sensitive to the underlying theoretical models used in the MC generation. Because new physics (NP) can alter decay distributions and acceptances, the standard practice of fitting NP Wilson coefficients to SM-based measurements of the R(D^{(*)}) ratios can be biased. The newly developed Hammer software tool enables efficient reweighting of MC samples to arbitrary NP scenarios or to any hadronic matrix elements. We demonstrate how Hammer allows avoidance of biases through self-consistent fits directly to the NP Wilson coefficients. We also present example analyses that demonstrate the sizeable biases that can otherwise occur from naive NP interpretations of SM-based measurements. The Hammer library is presently interfaced with several existing experimental analysis frameworks and we provide an overview of its structure.

Bayesian fit analysis to full distribution data of overline{mathrm{B}}to {mathrm{D}}^{left(ast right)}mathrm{ell}overline{nu }:left|{mathrm{V}}_{mathrm{cb}}right| determination and new physics constraints

We investigate the semi-leptonic decays of Bto {D}^{left(ast right)}mathrm{ell}overline{nu } in terms of the Heavy-Quark-Effective-Theory (HQET) parameterization for the form factors, which is described with the heavy quark expansion up to mathcal{O}left(1/{m}_c^2right) beyond the simple approximation considered in the original CLN parameterization. An analysis with this setup was first given in the literature, and then we extend it to the comprehensive analyses including (i) simultaneous fit of |Vcb| and the HQET parameters to available experimental full distribution data and theory constraints, and (ii) New Physics (NP) contributions of the V2 and T types, such as left(overline{c}{upgamma}^{mu }{P}_Rbright)left(overline{mathrm{ell}}{gamma}_{mu }{P}_Lnu mathrm{ell}right)kern0.5em mathrm{and}kern0.5em left(overline{c}{sigma}^{mu nu}{P}_Lbright)left(overline{mathrm{ell}}{sigma}_{mu nu}{P}_L{nu}_{mathrm{ell}}right) , to the decay distributions and rates. For this purpose, we perform Bayesian fit analyses by using Stan program, a state-of-the-art public platform for statistical computation. Then, we show that our |Vcb| fit results for the SM scenarios are close to the PDG combined average from the exclusive mode, and indicate significance of the angular distribution data. In turn, for the SM + NP scenarios, our fit analyses find that non-zero NP contribution is favored at the best fit point for both SM + V2 and SM + T depending on the HQET parameterization model. A key feature is then realized in the overline{B}to {D}^{left(ast right)}tau overline{nu} observables. Our fit result of the HQET parameters in the SM(+T) produces a consistent value for RD while smaller for {R}_{D^{ast }} , compared with the previous SM prediction in the HFLAV report. On the other hand, SM + V2 points to smaller and larger values for RD and {R}_{D^{ast }} than the SM predictions. In particular, the {R}_{D^{ast }} deviation from the experimental measurement becomes smaller, which could be interesting for future improvement on measurements at the Belle II experiment.

Evading the Grossman-Nir bound with \u2206I = 3/2 new physics

Rare kaon decays with missing energy, K → π+Emiss, have received considerable attention because their rates can be calculated quite precisely within the standard model (SM), where the missing energy is carried away by an undetected neutrino- antineutrino pair. Beyond the SM, clean theoretical predictions can also be made regarding these processes. One such prediction is the so-called Grossman-Nir (GN) bound, which states that the branching fractions of the KL and K+ modes must satisfy the relation mathrm{mathcal{B}}left({K}_Lto {pi}^0+{E}_{mathrm{miss}}right)underset{sim }{<}4.3mathrm{mathcal{B}}left({K}^{+}to {pi}^{+}+{E}_{mathrm{miss}}right) and applies within and beyond the SM, as long as the hadronic transitions change isospin by ∆I = 1/2. In this paper we extend the study of these modes to include new-physics scenarios where the missing energy is due to unobserved lepton-number-violating neutrino pairs, invisible light new scalars, or pairs of such scalars. The new interactions are assumed to arise above the electroweak scale and described by an effective field theory. We explore the possibility of violating the GN bound through ∆I = 3/2 contributions to the K → π transitions within these scenarios and find that large violations are only possible in the case where the missing energy is due to an invisible light new scalar.

Symmetries in B \u2192 D\u2217\u2113\u03bd angular observables

We apply the formalism of amplitude symmetries to the angular distribution of the decays B → D∗ℓν for ℓ = e, μ, τ . We show that the angular observables used to describe the distribution of this class of decays are not independent in absence of New Physics contributing to tensor operators. We derive sets of relations among the angular coefficients of the decay distribution for the massless and massive lepton cases which can be used to probe in a very general way the consistency among the angular observables and the underlying New Physics at work. We use these relations to access the longitudinal polarisation fraction of the D∗ using different angular coefficients from the ones used by Belle experiment. This in the near future can provide an alternative strategy to measure {F}_L^{Dast } in B → D∗τν and to understand the relatively high value measured by the Belle experiment. Using the same symmetries, we identify three observables which may exhibit a tension if the experimental value of {F}_L^{Dast } remains high. We discuss how these relations can be exploited for binned measurements. We also propose a new observable that could test for specific scenarios of New Physics generated by light right-handed neutrinos. Finally we study the prospects of testing these relations based on the projected experimental sensitivity of new experiments.

High-energy constraints from low-energy neutrino nonstandard interactions

Many scenarios of new physics predict the existence of neutrino Non-Standard Interactions, new vector contact interactions between neutrinos and first generation fermions beyond the Standard Model. We obtain model-independent constraints on the Standard Model Effective Field Theory at high energies from bounds on neutrino non-standard interactions derived at low energies. Our analysis explores a large set of new physics scenarios and includes full one-loop running effects below and above the electroweak scale. Our results show that neutrino non-standard interactions already push the scale of new physics beyond the TeV. We also conclude that bounds derived by other experimental probes, in particular by low-energy precision measurements and by charged lepton flavor violation searches, are generally more stringent. Our study constitutes a first step towards the systematization of phenomenological analyses to evaluate the impact of neutrino Non-Standard Interactions for new physics scenarios at high energies.

MUonE sensitivity to new physics explanations of the muon anomalous magnetic moment

The MUonE experiment aims at a precision measurement of the hadronic vacuum polarization contribution to the muon g − 2, via elastic muon-electron scattering. Since the current muon g − 2 anomaly hints at the potential existence of new physics (NP) related to the muon, the question then arises as to whether the measurement of hadronic vacuum polarization in MUonE could be affected by the same NP as well. In this work, we address this question by investigating a variety of NP explanations of the muon g − 2 anomaly via either vector or scalar mediators with either flavor-universal, non-universal or even flavor-violating couplings to electrons and muons. We derive the corresponding MUonE sensitivity in each case and find that the measurement of hadronic vacuum polarization at the MUonE is not vulnerable to any of these NP scenarios.

D * Polarization as an Additional Constraint on New Physics in the b → cτ ν ¯ τ Transition

Measurements of the branching fractions of the semileptonic decays B → D ( * ) τ ν ¯ τ and B c → J / ψ τ ν ¯ τ systematically exceed the Standard Model predictions, pointing to possible signals of new physics that can violate lepton flavor universality. The unknown origin of new physics realized in these channels can be probed using a general effective Hamiltonian constructed from four-fermion operators and the corresponding Wilson coefficients. Previously, constraints on these Wilson coefficients were obtained mainly from the experimental data for the branching fractions. Meanwhile, polarization observables were only theoretically studied. The situation has changed with more experimental data having become available, particularly those regarding the polarization of the tau and the D * meson. In this study, we discuss the implications of the new data on the overall picture. We then include them in an updated fit of the Wilson coefficients using all hadronic form factors from our covariant constituent quark model. The use of our form factors provides an analysis independent of those in the literature. Several new-physics scenarios are studied with the corresponding theoretical predictions provided, which are useful for future experimental studies. In particular, we find that under the one-dominant-operator assumption, no operator survives at 1 σ . Moreover, the scalar operators O S L and O S R are ruled out at 2 σ if one uses the constraint B ( B c → τ ν τ ) ≤ 10 % , while the more relaxed constraint B ( B c → τ ν τ ) ≤ 30 % still allows these operators at 2 σ , but only minimally. The inclusion of the new data for the D * polarization fraction F L D * reduces the likelihood of the right-handed vector operator O V R and significantly constrains the tensor operator O T L . Specifically, the F L D * alone rules out O T L at 1 σ . Finally, we show that the longitudinal polarization P L τ of the tau in the decays B → D * τ ν ¯ τ and B c → J / ψ τ ν ¯ τ is extremely sensitive to the tensor operator. Within the 2 σ allowed region, the best-fit value T L = 0.04 + i 0.17 predicts P L τ ( D * ) = − 0.33 and P L τ ( J / ψ ) = − 0.34 , which are at about 33% larger than the Standard Model (SM) prediction P L τ ( D * ) = − 0.50 and P L τ ( J / ψ ) = − 0.51 .

Searching for dark photons in hyperon decays

That massless dark photons could exist and have flavor-changing magnetic-dipole interactions with down-type light quarks is an attractive possibility which may be realized in various new-physics scenarios. It is potentially testable not only in kaon processes but also via two-body hyperon decays involving missing energy carried away by the massless dark photon. We explore the latter within a simplified model approach and take into account constraints from the kaon sector. We find that the branching fractions of some of these hyperon modes are allowed to be as high as a few times $10^{-4}$. Such numbers are likely to be within the sensitivity reaches of ongoing experiments like BESIII and future ones at super charm-tau factories.