Form-factor Uncertainties Research Articles

Unleashing the power of EFT in neutrino-nucleus scattering

Neutrino physics is advancing into a precision era with the construction of new experiments, particularly in the few GeV energy range. Within this energy range, neutrinos exhibit diverse interactions with nucleons and nuclei. This study delves in particular into neutrino-nucleus quasi-elastic cross sections, taking into account both standard and, for the first time, non-standard interactions, all within the framework of effective field theory (EFT). The main uncertainties in these cross sections stem from uncertainties in the nucleon-level form factors, and from the approximations necessary to solve the nuclear many-body problem. We explore how these uncertainties influence the potential of neutrino experiments to probe new physics introduced by left-handed, right-handed, scalar, pseudoscalar, and tensor interactions. For some of these interactions the cross section is enhanced, making long-baseline experiments an excellent place to search for them. Our results, including tabulated cross sections for all interaction types and all neutrino flavors, can serve as the foundation for such searches.

The neutrino floor: a data-driven analysis

We reconsider the discovery limit of multi-ton direct detection dark matter experiments in the light of recent measurements of the coherent elastic neutrino-nucleus scattering process. Assuming the cross section to be a parameter entirely determined by data, rather than using its Standard Model prediction, we use the COHERENT CsI and LAr data sets to determine WIMP discovery limits. Being based on a data-driven approach, the results are thus free from theoretical assumptions and fall within the WIMP mass regions where XENONnT and DARWIN have best expected sensitivities. We further determine the impact of subleading nuclear form factor and weak mixing angle uncertainties effects on WIMP discovery limits. We point out that these effects, albeit small, should be taken into account. Moreover, to quantify the impact of new physics effects in the neutrino background, we revisit WIMP discovery limits assuming light vector and scalar mediators as well as neutrino magnetic moments/transitions. We stress that the presence of new interactions in the neutrino sector, in general, tend to worsen the WIMP discovery limit.

Implications for the ΔAFB anomaly in B¯0→D*+ℓ−ν¯ using a new Monte Carlo event generator

Recent experimental results in $B$ physics from Belle, BABAR, and LHCb suggest new physics (NP) in the weak $b\ensuremath{\rightarrow}c$ charged-current processes. Here we focus specifically on the decay modes ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\ell}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ with $\ensuremath{\ell}=e$ and $\ensuremath{\mu}$. The world averages of the ratios ${R}_{D}$ and ${R}_{D}^{*}$ currently differ from the Standard Model (SM) predictions by $3.4\ensuremath{\sigma}$ while recently a new anomaly has been observed in the forward-backward asymmetry measurement, ${A}_{\mathrm{FB}}$, in ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\mu}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ decay. It is found that $\mathrm{\ensuremath{\Delta}}{A}_{\mathrm{FB}}={A}_{\mathrm{FB}}(B\ensuremath{\rightarrow}{D}^{*}\ensuremath{\mu}\ensuremath{\nu})\ensuremath{-}{A}_{\mathrm{FB}}(B\ensuremath{\rightarrow}{D}^{*}e\ensuremath{\nu})$ is around $4.1\ensuremath{\sigma}$ away from the SM prediction in an analysis of 2019 Belle data. In this work we explore possible solutions to the $\mathrm{\ensuremath{\Delta}}{A}_{\mathrm{FB}}$ anomaly and point out correlated NP signals in other angular observables. These correlations between angular observables must be present in the case of beyond the Standard Model physics. We stress the importance of $\mathrm{\ensuremath{\Delta}}$ type observables that are obtained by taking the difference of the observable for the muon and the electron mode. These quantities cancel form-factor uncertainties in the SM and allow for clean tests of NP. These intriguing results also suggest an urgent need for improved simulation and analysis techniques in ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\ell}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ decays. Here we also describe a new Monte Carlo event generator tool based on EVTGEN that we developed to allow simulation of the NP signatures in ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\ell}}^{\ensuremath{-}}\ensuremath{\nu}$, which arise due to the interference between the SM and NP amplitudes. We then discuss prospects for improved observables sensitive to NP couplings with 1, 5, 50, and $250\text{ }\text{ }{\mathrm{ab}}^{\ensuremath{-}1}$ of Belle II data, which seem to be ideally suited for this class of measurements.

Dispersion-theoretical analysis of the electromagnetic form factors of the Lambda hyperon

The electromagnetic form factors of the Lambda hyperon in the time-like region are determined precisely through a dispersion-theoretical analysis of the world data for the cross section of the annihilation process e^+e^-rightarrow {bar{Lambda }}{Lambda }. The spectral function is represented by a superposition of narrow and broad vector meson poles. We test different scenarios for the spectral function and obtain a good description of the world data in the time-like region. The uncertainties in the extracted form factors are estimated by means of the bootstrap sampling method. The analytical continuation of the form factors to the space-like region introduces large errors due to the lack of data. When the electric Lambda radius from chiral perturbation theory is taken as a constraint, the magnetic radius is predicted as r_M = 0.681 pm 0.002 fm. We also extract various vector meson to baryon coupling constants.

Impact of COHERENT measurements, cross section uncertainties and new interactions on the neutrino floor

We reconsider the discovery limit of multi-ton direct detection dark matter experiments in the light of recent measurements of the coherent elastic neutrino-nucleus scattering process. Assuming the cross section to be a parameter entirely determined by data, rather than using its Standard Model prediction, we use the COHERENT CsI and LAr data sets to determine WIMP discovery limits. Being based on a data-driven approach, the results are thus free from theoretical assumptions and fall within the WIMP mass regions where XENONnT and DARWIN have best expected sensitivities. We further determine the impact of subleading nuclear form factor and weak mixing angle uncertainties effects on WIMP discovery limits. We point out that these effects, albeit small, should be taken into account. Moreover, to quantify the impact of new physics effects in the neutrino background, we revisit WIMP discovery limits assuming light vector and scalar mediators as well as neutrino magnetic moments/transitions. We stress that the presence of new interactions in the neutrino sector, in general, tend to worsen the WIMP discovery limit.

On the effective lifetime of Bs\u2192 \u03bc\u03bc\u03b3

We consider the {B}_s^0 → μ+μ−γ effective lifetime, and the related CP-phase sensitive quantity {A}_{{Delta Gamma}_s}^{mu mu gamma} , as a way to obtain qualitatively new insights on the current B-decay discrepancies. Through a fit comparing pre- to post-Moriond-2021 data we identify a few theory benchmark scenarios addressing these discrepancies, and featuring large CP violation in addition. We then explore the possibility of telling apart these scenarios with {A}_{{Delta Gamma}_s}^{mu mu gamma} , once resonance-modeling and form-factor uncertainties are taken into account. We do so in both regions of low and high invariant di-lepton mass-squared q2. For low q2, we show how to shape the integration range in order to reduce the impact of the ϕ-resonance modelling on the {A}_{{Delta Gamma}_s}^{mu mu gamma} prediction. For high q2, we find that the corresponding pollution from broad-charmonium resonances has a surprisingly small effect on {A}_{{Delta Gamma}_s}^{mu mu gamma} . This is due to a number of cancellations, that can be traced back to the complete dominance of semi-leptonic operator contributions for high q2 — at variance with low q2 — and to {A}_{{Delta Gamma}_s}^{mu mu gamma} behaving like a ratio-of-amplitudes observable. Our study suggests that {A}_{{Delta Gamma}_s}^{mu mu gamma} is — especially at high q2 — a potentially valuable probe of short-distance CP-violating effects in the very same Wilson coefficients that are associated to current b → s discrepancies. Its discriminating power, however, relies on progress in form-factor uncertainties. Interestingly, high q2 is the region where {B}_s^0 → μ+μ−γ is already being accessed experimentally, and the region where form factors are more accessible through non-perturbative QCD methods.

Optimized lepton universality tests in Brightarrow V ell {bar{nu }} decays

We propose improved lepton flavor universality (LFU) ratios in semileptonic Prightarrow V ell {bar{nu }} decays, when comparing mu and tau modes, that minimize the theoretical form-factor uncertainties. These optimized ratios are obtained with simple cuts or reweighting of the dilepton mass distributions, which imply a minimum loss of signal on the rare tauonic modes while maximizing the cancellation of theoretical uncertainties among the two modes. We illustrate the usefulness of these observables in B_crightarrow J/psi , B_crightarrow psi (2S), Brightarrow D^* and B_srightarrow D_s^* transitions, showing that in all cases we can reach {mathcal {O}}(1%) uncertainties on the SM predictions of the improved LFU ratios employing conservative form-factor uncertainties.

A comprehensive analysis of weak transition form factors for doubly heavy baryons in the light front approach

The transition form factors for doubly heavy baryons into a spin-1/2 or spin-3/2 ground-state baryon induced by both the charged current and the flavor changing neutral current are systematically studied within the light-front quark model. In the transition the two spectator quarks have two spin configurations and both are considered in this calculation. We use an updated vertex functions, and inspired by the flavor SU(3) symmetry, we also provide a new approach to derive the flavor-spin factors. With the obtained transition form factors, we perform a phenomenological study of the corresponding semi-leptonic decays of doubly heavy baryons induced by the crightarrow d/s ell ^+nu , brightarrow c/uell ^-bar{nu } and brightarrow d/sell ^+ell ^-. Results for partial decay widths, branching ratios and the polarization ratios Gamma _{L}/Gamma _{T}s are given. We find that most branching ratios for the semi-leptonic decays induced by the crightarrow d,s transitions are at the order of 10^{-3}sim 10^{-2}, which might be useful for the search of other doubly-heavy baryons. Uncertainties in form factors, the flavor SU(3) symmetry and sources of symmetry breaking effects are discussed. We find that the SU(3) symmetry breaking effects could be sizable in charmed baryon decays while in the bottomed case, the SU(3) symmetry breaking effects are less significant. Our results can be examined at the experimental facilities in the future.

Extraction of the Neutron Charge Radius from a Precision Calculation of the Deuteron Structure Radius

We present a high-accuracy calculation of the deuteron structure radius in chiral effective field theory. Our analysis employs the state-of-the-art semilocal two-nucleon potentials and takes into account two-body contributions to the charge density operators up to fifth order in the chiral expansion. The strength of the fifth-order short-range two-body contribution to the charge density operator is adjusted to the experimental data on the deuteron charge form factor. A detailed error analysis is performed by propagating the statistical uncertainties of the low-energy constants entering the two-nucleon potentials and by estimating errors from the truncation of the chiral expansion as well as from uncertainties in the nucleon form factors. Using the predicted value for the deuteron structure radius together with the very accurate atomic data for the difference of the deuteron and proton charge radii we, for the first time, extract the charge radius of the neutron from light nuclei. The extracted value reads r_{n}^{2}=-0.106_{-0.005}^{+0.007} fm^{2} and its magnitude is about 1.7σ smaller than the current value given by the Particle Data Group. In addition, given the high accuracy of the calculated deuteron charge form factor and its careful and systematic error analysis, our results open the way for an accurate determination of the nucleon form factors from elastic electron-deuteron scattering data measured at the Mainz Microtron and other experimental facilities.

Impact of form factor uncertainties on interpretations of coherent elastic neutrino-nucleus scattering data

The standard model coherent elastic neutrino-nucleus scattering (CEνNS) cross section is subject to nuclear form factor uncertainties, mainly driven by the root-mean-square radius of the neutron density distribution. Motivated by COHERENT phases I-III and future multi-ton direct detection dark matter searches, we evaluate these uncertainties in cesium iodide, germanium, xenon and argon detectors. We find that the uncertainties become relevant for momentum transfers q ≳ 20 MeV and are essentially independent of the form factor parameterization. Consequently, form factor uncertainties are not important for CEνNS induced by reactor or solar neutrinos. Taking into account these uncertainties, we then evaluate their impact on measurements of CEνNS at COHERENT, the diffuse supernova background (DSNB) neutrinos and sub-GeV atmospheric neutrinos. We also calculate the relative uncertainties in the number of COHERENT events for different nuclei as a function of recoil energy. For DSNB and atmospheric neutrinos, event rates at a liquid argon detector can be uncertain to more than 5%. Finally, we consider the impact of form factor uncertainties on searches for nonstandard neutrino interactions, sterile neutrinos and neutrino generalized interactions. We point out that studies of new physics using CEνNS data are affected by neutron form factor uncertainties, which if not properly taken into account may lead to the misidentification of new physics signals. The uncertainties quantified here are also relevant for dark matter direct detection searches.

Recent results from the NA48 experiment at CERN

The NA48/2 experiment presents a final result of the charged kaon semileptonic decays form factors measurement based on 4.28 million and 2.91 million selected decays collected in 2004. The result is competetive with other measurements in mode and has a smallest uncertainty for , that leads to the most precise combined result and allows to reduce the form factor uncertainty of |VUS|. The NA48/2 experiment at CERN collected a very large sample of charged kaon decays into multiple final states. From this data sample we have reconstructed about 1663 events of the very rare decay K± → μ±νe+e− over almost negligible background in the region with m(e+e−) above 140 MeV, which is of great interest in Chiral Perturbation Theory. We present the mee spectrum and a model-independent measurement of the decay rate for this region.

Recent measurements of form factors at NA48

The NA48/2 experiment presents a final result of the charged kaon semileptonic decays form factors measurement based on 4.28 million Ke3± and 2.91 million Kμ3± selected decays collected in 2004. The result is competetive with other measurements in Kμ3± mode and has a smallest uncertainty for Ke3± that leads to the most precise combined Kl3± result and allows to reduce the form factor uncertainty of | V US |.

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

Background: The spatial distribution of charge and magnetization within the proton is encoded in the elastic form factors. These have been precisely measured in elastic electron scattering, and the combination of proton and neutron form factors allows for the separation of the up- and down-quark contributions.Purpose: In this work, we extract the proton and neutron form factors from worldwide data with an emphasis on precise new data covering the low-momentum region, which is sensitive to the large-scale structure of the nucleon. From these, we separate the up- and down-quark contributions to the proton form factors.Method: We combine cross section and polarization measurements of elastic electron-proton scattering to separate the proton form factors and two-photon exchange (TPE) contributions. We combine the proton form factors with parametrization of the neutron form factor data and uncertainties to separate the up- and down-quark contributions to the proton's charge and magnetic form factors.Results: The extracted TPE corrections are compared to previous phenomenological extractions, TPE calculations, and direct measurements from the comparison of electron and positron scattering. The flavor-separated form factors are extracted and compared to models of the nucleon structure.Conclusions: With the inclusion of the precise new data, the extracted TPE contributions show a clear change of sign at low ${Q}^{2}$, which is necessary to explain the high-${Q}^{2}$ form factor discrepancy while being consistent with the known ${Q}^{2}\ensuremath{\rightarrow}0$ limit. We find that the new Mainz data yield a significantly different result for the proton magnetic form factor and its flavor-separated contributions. We also observe that the rms radius of both the up- and down-quark distributions are smaller than the rms charge radius of the proton.

What next for the CMSSM and the NUHM: improved prospects for superpartner and dark matter detection

We present an updated analysis of the CMSSM and the NUHM using the latest experimental data and numerical tools. We map out favored regions of Bayesian posterior probability in light of data from the LHC, flavor observables, the relic density and dark matter searches. We present some updated features with respect to our previous analyses: we include the effects of corrections to the light Higgs mass beyond the 2-loop order using FeynHiggs v2.10.0; we include in the likelihood the latest limits from direct searches for squarks and gluinos at ATLAS with ~20/fb; the latest constraints on the spin-independent scattering cross section of the neutralino from LUX are applied taking into account uncertainties in the nuclear form factors. We find that in the CMSSM the posterior distribution now tends to favor smaller values of Msusy than in the previous analyses. As a consequence, the statistical weight of the A-resonance region increases to about 30% of the total probability, with interesting new prospects for the 14 TeV run at the LHC. The most favored region, on the other hand, still features multi-TeV squarks and gluinos, and ~1TeV higgsino dark matter whose detection prospects by current and one-tonne detectors look very promising. The same region is predominant in the NUHM, although the A-resonance region is also present there as well as a new solution, of neutralino-stau coannihilation through the channel stau stau -> hh at very large \mu. We derive the expected sensitivity of the future CTA experiment to ~1 TeV higgsino dark matter for both models and show that the prospects for probing both models are realistically good. We comment on the complementarity of this search to planned direct detection one-tonne experiments.

On B → Vℓℓ at small dilepton invariant mass, power corrections, and new physics

We investigate rare semileptonic \bar B -> \bar K^* l^+ l^- decays, providing a comprehensive treatment of theoretical uncertainties in the low-q^2 region as needed for interpreting current and future LHCb and B-factory data in terms of the new physics search. We go beyond the usual focus on form-factor uncertainties, paying proper attention to non-factorizable terms. A central point is the systematic exploitation of the V-A structure of SM weak interactions, which leads to the suppression of two helicity amplitudes and some of the angular coefficients. We review how this works at the level of (helicity) form factors, and show that the hierarchies extend to non-factorizable terms. For virtual charm effects, we give an argument for it in terms of light-cone QCD sum rules that continues to hold at the level of "long-distance" Lambda_QCD^2/m_c^2 power corrections, reducing an important source of theoretical uncertainty in any \bar B, \bar B_s -> V l^+ l^- (or \bar B -> V gamma) decay. The contributions of the remaining hadronic weak Hamiltonian respect a similar hierarchy. We employ a resonance model to preclude (in the \bar B -> \bar K^* case) large long-distance corrections to this. A phenomenological part pays particular attention to the region of lowest dilepton mass, 4 m_l^2 <= q^2 <= 2 GeV^2. Two observables remain theoretically clean, implying a (theoretical) sensitivity to the real (imaginary) part of the "right-handed" Wilson coefficient C_7' to 10% (1%) of C_7^SM, both in the muonic and the electronic mode. We also show that there are two near-exact relations between angular coefficients, even in the presence of new physics and when lepton masses are not neglected.

Optimizing the basis of B → K * ℓ + ℓ − observables in the full kinematic range

AbstractWe discuss the observables for theB→K*(→Kπ)ℓ+ℓ−decay, focusing on both CP-averaged and CP-violating observables at large and low hadronic recoil with special emphasis on their low sensitivity to form-factor uncertainties. We identify an optimal basis of observables that balances theoretical and experimental advantages, which will guide the New Physics searches in the short term. We discuss some advantages of the observables in the basis, and in particular their improved sensitivity to New Physics compared to other observables. We present predictions within the Standard Model for the observables of interest, integrated over the appropriate bins including lepton mass corrections. Finally, we present bounds on the S-wave contribution to the distribution coming from the$ B\to K_0^{*}{\ell^{+}}{\ell^{-}} $decay, which will help to establish the systematic error associated to this pollution.

The [formula omitted] decay

In this paper the potential for the discovery of new physics in the exclusive decay $\bar{B}_d \to \bar{K}^{*0} \mu^+ \mu^-$ is discussed. Attention is paid to constructing observables which are protected from uncertainties in QCD form factors and at the same time observe the symmetries of the angular distribution. We discuss the sensitivity to new physics in the observables including the effect of CP-violating phases.

Precision flavour physics with [formula omitted] and [formula omitted

We discuss how the combined analysis of $B\to K\nu\bar\nu$ and $B\to Kl^+l^-$ can provide us with new physics tests practically free of form factor uncertainties. Residual theory errors are at the level of several percent. This study underlines the excellent motivation for measuring these modes at a Super Flavour Factory, or, in the case of $B\to Kl^+l^-$, also at a hadron collider.

Precision flavour physics withB→Kνν̄ andB→Kl+l−

We show that a combined analysis of B → Kν and B → Kl+l− allows for new physics tests practically free of form factor uncertainties. Residual theory errors are at the level of several percent. Our study underlines the excellent motivation for measuring these modes at a Super Flavour Factory.

Breit type equation for mesonic atoms

The finite size effects and relativistic corrections in pionic and kaonic hydrogen are evaluated by generalizing the Breit equation for a spin-0–spin-1/2 amplitude with the inclusion of the hadron electromagnetic form factors. The agreement of the relativistic corrections to the energies of the mesonic atoms with other methods used to evaluate them is not exact, but reasonably good. The precision values of the energy shifts due to the strong interaction, extracted from data, are however subject to the hadronic form factor uncertainties.