Right-handed Charged Currents Research Articles

Anomalies in global SMEFT analyses. A case study of first-row CKM unitarity

Recent developments in the Standard Model analysis of semileptonic charged-current processes involving light quarks have revealed ~ 3σ tensions in Cabibbo universality tests involving meson, neutron, and nuclear beta decays. In this paper, we explore beyond the Standard Model explanations of this so-called Cabibbo Angle Anomaly in the framework of the Standard Model Effective Field Theory (SMEFT), including not only low-energy charged current processes (‘L’), but also electroweak precision observables (‘EW’) and Drell-Yan collider processes (‘C’) that probe the same underlying physics across a broad range of energy scales. The resulting ‘CLEW’ framework not only allows one to test explanations of the Cabibbo Angle Anomaly, but is set up to provide near model-independent analyses with minimal assumptions on the flavor structure of the SMEFT operators. Besides the global analysis, we consider a large number of simpler scenarios, each with a subset of SMEFT operators, and investigate how much they improve upon the Standard Model fit. We find that the most favored scenarios, as judged by the Akaike Information Criterion, are those that involve right-handed charged currents. Additional interactions, namely oblique operators, terms modifying the Fermi constant, and operators involving right-handed neutral currents, play a role if the CDF determination of the W mass is included in the analysis.

Right-handed charged currents in the era of the Large Hadron Collider

We discuss the phenomenology of right-handed charged currents in the frame-work of the Standard Model Effective Field Theory, in which they arise due to a single gauge-invariant dimension-six operator. We study the manifestations of the nine complex couplings of the W to right-handed quarks in collider physics, flavor physics, and low-energy precision measurements. We first obtain constraints on the couplings under the assumption that the right-handed operator is the dominant correction to the Standard Model at observable energies. We subsequently study the impact of degeneracies with other Beyond-the-Standard-Model effective interactions and identify observables, both at colliders and low-energy experiments, that would uniquely point to right-handed charged currents.

An ϵ′ improvement from right-handed currents

Recent lattice QCD calculations of direct CP violation in $K_L \to \pi \pi$ decays indicate tension with the experimental results. Assuming this tension to be real, we investigate a possible beyond-the-Standard Model explanation via right-handed charged currents. By using chiral perturbation theory in combination with lattice QCD results, we accurately calculate the modification of $\epsilon'/\epsilon$ induced by right-handed charged currents and extract values of the couplings that are necessary to explain the discrepancy, pointing to a scale around $10^2$ TeV. We find that couplings of this size are not in conflict with constraints from other precision experiments, but next-generation hadronic electric dipole moment searches (such as neutron and ${}^{225}$Ra) can falsify this scenario. We work out in detail a direct link, based on chiral perturbation theory, between CP violation in the kaon sector and electric dipole moments induced by right-handed currents which can be used in future analyses of left-right symmetric models.

Neutrinoless double beta decay in the left-right symmetric models for linear seesaw

In a class of left-right symmetric models for linear seesaw, a neutrinoless double beta decay induced by the left- and right-handed charged currents together will only depend on the breaking details of left-right and electroweak symmetries. This neutrinoless double beta decay can reach the experimental sensitivities if the right-handed charged gauge boson is below the 100 TeV scale.

Heavy neutrinos and the [formula omitted] CMS data

We show that the excess in the pp→eejj CMS data can be naturally interpreted within the Minimal Left–Right Symmetric model (MLRSM), keeping gL=gR, if CP phases and non-degenerate masses of heavy neutrinos are taken into account. As an additional benefit, a natural interpretation of the reported ratio (14:1) of the opposite-sign (OS) pp→l±l∓jj to the same-sign (SS) pp→l±l±jj lepton signals is possible. Finally, a suppression of muon pairs with respect to electron pairs in the pp→lljj data is obtained, in accordance with experimental data. If the excess in the CMS data survives in the future, it would be a first clear hint towards presence of heavy neutrinos in right-handed charged currents with specific CP phases, mixing angles and masses, which will have far reaching consequences for particle physics directions.

B→Xdγand constraints on new physics

We combine recent progress in measuring the branching ratio of the decay B->X_d gamma$ with the discovery that hadronic uncertainties in the CP-averaged branching ratio drop out to a large extent. Implications of these improvements on the size of possible new physics effects are investigated. We find the updated SM prediction for the CP-averaged branching ratio to be Br[B->X_d gamma]^SM_E_gamma>1.6 GeV = 1.54^+0.26-0.31*10^-5, which should be compared with the experimental value of Br[B->X_d gamma]^exp_E_gamma>1.6GeV = (1.41+-0.57) 10^-5. After performing a model independent analysis, we consider different new physics models: the MSSM with generic sources of flavor violation, the two Higgs doublet model of type III and a model with right-handed charged currents. It is found that the constraints on the SUSY parameters delta^d_13 have improved and that the absolute value of the right-handed quark mixing matrix element |V^R_td| must be smaller than 1.5*10^-4.

Effects of right-handed charged currents on the determinations of|Vub|and|Vcb|

We study the effect of a right-handed coupling of quarks to the W-boson on the measurements of |V_ub| and |V_cb|. It is shown that such a coupling can remove the discrepancies between the determinations of |V_ub| from B\to\pi\ell\nu, B^+\to\tau^+ \nu and B\to X_u\ell\nu. Further the measurements of |V_cb| from B\to D^*\ell\nu, B\to D\ell\nu and inclusive B\to X_c\ell\nu decays can be brought into better agreement. We demonstrate that a right-handed coupling can be generated within the MSSM by a finite gluino-squark loop. The effect involves the parameters \delta^{u RL}_{23} and \delta^{u RL}_{13} of the squark mass matrices, which are poorly constrained from other processes. On the other hand, all gluino-squark corrections to the regular left-handed coupling of the W-boson are found to be too small to be relevant.

Has a coupling of right-handed quarks to W been observed?

Stringent tests of non-standard electroweak (EW) couplings of (light) quarks are rare because it is not easy to disentangle QCD and EW quantities. We will discuss here a recently proposed test of right-handed charged quark currents, based on Kl3 decay data and the prediction of the Callan-Treiman low energy theorem. Non-standard couplings of fermions to W and Z are suggested by the minimal low-energy effective electroweak theory (LEET) to be the dominant new physics effects. It is shown that coherent values for these couplings emerge from an analysis of Z-pole observables, too, curing in particular the long-standing AbFB puzzle.

Can we probe right-handed charged quark currents?

Different scenarios of new physics beyond the standard model give rise to a direct coupling of right-handed quarks to W bosons. We will discuss how the prediction of the Callan-Treiman low energy theorem for the scalar K π form factor in combination with recent data on K μ 3 L decays can serve as a stringent test of the standard model (SM) and possible extensions, in particular right-handed charged quark currents (RHCs). In addition, we will comment on the impact of hadronic tau decay data on RHCs.

Measurement of high-Q2 deep inelastic scattering cross sections with a longitudinally polarised positron beam at HERA

The cross sections for charged and neutral current deep inelastic scattering in e^+p collisions with a longitudinally polarised positron beam have been measured using the ZEUS detector at HERA. The results, based on data corresponding to an integrated luminosity of 23.8 pb^-1 at sqrt(s) = 318 GeV, are given for both e^+p charged current and neutral current deep inelastic scattering for both positive and negative values of the longitudinal polarisation of the positron beam. Single differential cross sections are presented for the kinematic region Q^2 > 200 GeV^2 . The measured cross sections are compared to the predictions of the Standard Model. A fit to the data yields sigma^CC (P_e = -1) = 7.4 +/- 3.9 (stat.) +/- 1.2 (syst.) pb, which is consistent within two standard deviations with the absence of right-handed charged currents in the Standard Model.

Enhanced contribution to quark and neutron electric dipole moments with small mixing of right-handed currents and CKMCPviolation

We study the light quark and the neutron electric dipole moments (EDMs) under the assumptions that the CP source is still in the usual CKM matrix and that there is a small mixing of right-handed charged currents in the quark sector. We find that the EDMs arise already at two loop order that are much larger than the standard model (SM) result even for a small mixing.

Erratum: ‘‘Limits on right-handed charged weak currents from a polarization-asymmetry correlation experiment withIn107[Phys. Rev. Lett.70, 4047 (1993)

Erratum: ‘‘Limits on right-handed charged weak currents from a polarization-asymmetry correlation experiment with<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">In</mml:mi></mml:mrow><mml:mprescripts /><mml:mrow /><mml:mrow><mml:mn>107</mml:mn></mml:mrow><mml:mrow /><mml:mrow /></mml:mmultiscripts></mml:mrow></mml:math>[Phys. Rev. Lett.<b>70</b>, 4047 (1993)

Limits on right-handed charged weak currents from a polarization-asymmetry correlation experiment with 107In.

A new technique, based on relative measurements of the longitudinal polarization (P) of positrons emitted in two opoosite directions with respect to the nuclear polarization (R=${\mathit{P}}^{\mathrm{\ensuremath{-}}}$/${\mathit{P}}^{+}$), is used to provide constraints on extensions of the standard model (${\mathit{R}}_{0}$). The first result, i.e., R/${\mathit{R}}_{0}$=0.926\ifmmode\pm\else\textpm\fi{}0.041, was obtained with $^{107}\mathrm{In}$. Interpreted in the framework of left-right symmetric models and assuming no mixing, it provides a 90% C.L. lower limit of 225 GeV/${\mathit{c}}^{2}$ for the mass of a possible right-handed charged W gauge boson, reducing the mass range allowed by other semileptonic experiments.

Radiative background to the search for right-handed charged currents

The probability for an electron to emit a collinear, hard photon while undergoing helicity flip is finite in the limit that the electron mass tends to zero. We point out that this provides a background of order α to tests of the Standard Model, which may affect the search for right-handed, charged currents in deeply inelastic scattering, especially at HERA. It would appear to be exceedingly difficult to discriminate against this background in practice and may make it difficult to improve substantially the mass limits on a right-handed charged vector boson.

Physical consequences of mass singularities

We argue that the perturbation expansion of massless field theories are uniquely and unambiguously defined for properly formed observables. In so doing, we generalize the Kinoshita-Lee-Nauenberg theorem to show that there are no observable “mass-shell anomalies”. Paradoxically, in realistic circumstances, massive electrons give rise to non-intuitive, enhanced effects, for example, an unexpected background to the search for right-handed charged currents at HERA.

The neutrinoless double beta decay of 76Ge, 82Se, 86Kr, 114Cd, 128, 130Te and 134, 136Xe in the framework of a relativistic quark confinement model

The half-life of the 0 + → 0 + neutrinoless double beta decay is calculated for 76Ge, 82Se, 86Kr, 114Cd, 128, 130Te and 134, 136Xe and the upper limit for the effective neutrino mass of 3.0 eV is deduced from available experimental data. In addition, the contribution of the right-handed charged weak currents to the effective weak hamiltonian is estimated. The relevant parameters attain the values |〈 Λ〉| < 4.1 × 10 −6 and |〈 ν〉| < 6.6 × 10 −8. The nucleonic weak current is treated starting from the current quark level and evaluating the quark current using relativistic quark wave functions obtained from a Dirac equation with a harmonic confinement potential. The nuclear matrix elements of the thus obtained effective weak operators are evaluated using the proton-neutron quasi-particle random phase approximation approach avoiding the use of closure and the accompanying singular neutrino potentials. The dependence of the decay half-life on the recoil effects and the p-wave contribution is analyzed. Detailed presentation of the theory is pursued.

Mixing between ordinary and exotic fermions.

The results of a comprehensive analysis of the limits on mixings between ordinary fermions and possible heavy fermions with exotic SU(2)\ifmmode\times\else\texttimes\fi{}U(1) assignments (e.g., left-handed singlets and/or right-handed doublets) are presented. A general formalism for describing such mixings is given. It is shown that a variety of constraints, including the relation between the W and Z masses and the Fermi constant, charged-current universality, limits on induced right-handed charged currents, and flavor-diagonal neutral currents suffice to limit all directions in parameter space that are not excluded by the absence of flavor-changing neutral currents. Limits on ${s}^{2}$, the square of the mixing between ordinary and exotic fermions, are quite stringent for the ${\ensuremath{\nu}}_{\ensuremath{\mu}L}$, ${\ensuremath{\mu}}_{L}^{\mathrm{\ensuremath{-}}}$, ${u}_{L}$, and ${d}_{L}$ (${s}^{2}$\ensuremath{\le}0.002--0.005) if only one particle is allowed to mix at a time, but are weaker by an order of magnitude if fine-tuned cancellations between different mixings are allowed. Similar statements apply to quark mixings with heavy sequential doublets. Limits on ${s}^{2}$ for the other light fermions (${\ensuremath{\nu}}_{\mathrm{eL}}$, ${\mathit{e}}_{\mathit{L}}^{\mathrm{\ensuremath{-}}}$,${\mathit{e}}_{\mathit{R}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\mu}}}_{\mathit{R}}^{\mathrm{\ensuremath{-}}}$, ${\mathit{u}}_{\mathit{R}}$,${\mathit{d}}_{\mathit{R}}$) are in the range 0.02--0.06, while those for the s, c, b, ${\ensuremath{\nu}}_{\ensuremath{\tau}}$, and ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}}$ are considerably weaker. Slightly stronger limits are found in specific models (e.g., ${\mathrm{E}}_{6}$). Implications for the masses of the heavy exotic fermions are discussed.

Right-handed charged currents induced by a strongly interacting Higgs sector

A strongly interacting Higgs sector could give rise to right-handed charged currents in the quark sector. An analysis of nonleptonic weak K decays strongly restricts the possible size of these right-handed couplings. The experimental value of ε can be reproduced even for vanishing R = Γ( b→ ueν)/Γ( b→ ceν) . ε'/ε may have arbitrarily small values of either sign. The neutron electric dipole moment could be near to its present experimental bound.

“Virtual effects of excited quarks as probes of a possible new hardonic mass scale”

Abstract The effects of spin - 3 2 excited quarks are considered as probes of a possible new hadronic mass-scale Λ. A specific model is developed which describes virtual spin - 3 2 quarks of mass of O(Λ). Induced effects, which are corrections to the standard SU(2) × U(1) electroweak model, include right-handed charged currents and flavor changing neutral currents. A model-independent classification of all SU(3) × SU(2) × U(1) invariant quark operators of dimension six or less is also presented. Ambiguities in converting this analysis and existing experiments to a definitive lower-bound for Λ are discussed. It is found, depending on the strength of certain Higgs couplings and the underlying global flavor symmetries in the absence of Higgs couplings, that a lower bound as small as Λ > 500 GeV or as large as Λ > 100 TeV is possible.

PandCPnonconservation through Higgs exchange in gauge models with right-handed charged currents

The recent proposal of Weinberg that CP violation occurs through the exchange of charged Higgs bosons is examined in detail for gauge models with right-handed charged currents. We find that parity must be violated as well through the exchange of neutral Higgs bosons. In a five-quark model, this effect could be considerably bigger than that of the usual weak interaction, and should be looked for in psi' ..-->.. psi..pi pi.. and psi' ..-->.. psi..pi../sup 0/ as a test of this whole theoretical framework. Other interesting phenomena are the enhanced production of certain Higgs bosons and a value for the neutron electric dipole moment which is within experimental limits.