Neutral Current Effects Research Articles

QCD analysis of ΔS = 0 hadronic parity violation

We present a QCD analysis of the effective weak Hamiltonian at hadronic energy scales for strangeness-nonchanging (ΔS=0) hadronic processes. Performing a leading-order renormalization group analysis in QCD from the W to the O(2GeV) energy scale, we derive the pertinent effective Hamiltonian for hadronic parity violation, including the effects of both neutral and charged weak currents. We compute the complete renormalization group evolution of all isosectors and the evolution through heavy-flavor thresholds for the first time. We show that the additional four-quark operators that enter below the W mass scale from QCD operator mixing effects form a closed set, and they result in a 12×12 anomalous dimension matrix. Computing the resulting effective Hamiltonian and comparing to earlier results, we affirm the importance of operator mixing effects and find, as an example, that the parity-violating pion-nucleon coupling constant, using the factorization Ansatz and an assessment of the pertinent quark charge of the nucleon in lattice QCD at the 2 GeV scale, is in better agreement with recent experiments.

A model-independent analysis of b{rightarrow }smu ^{+}mu ^{-}transitions with GAMBIT \u2019s FlavBit

The search for flavour-changing neutral current effects in B-meson decays is a powerful probe of physics beyond the Standard Model. Deviations from SM behaviour are often quantified by extracting the preferred values of the Wilson coefficients of an operator product expansion. We use the FlavBit module of the GAMBIT package to perform a simultaneous global fit of the Wilson coefficients C_7, C_9, and C_{10} using a combination of all current data on b{rightarrow }smu ^{+}mu ^{-} transitions. We further extend previous analyses by accounting for the correlated theoretical uncertainties at each point in the Wilson coefficient parameter space, rather than deriving the uncertainties from a Standard Model calculation. We find that the best fit deviates from the SM value with a significance of 6.6sigma . The largest deviation is associated with a vector coupling of muons to b and s quarks.

Neutrino masses and mixing in models with large extra dimensions and localized fermions

Using a low-energy effective field theory approach, we study some properties of models with large extra dimensions, in which quarks and leptons have localized wave functions in the extra dimensions. We consider models with two types of gauge groups: (i) the Standard-Model gauge group, and (ii) the left-right symmetric (LRS) gauge group. Our main focus is on the lepton sector of models with $n=2$ extra dimensions, in particular, neutrino masses and mixing. We analyze the requisite conditions that the models must satisfy to be in accord with data and present a solution for lepton wave functions in the extra dimensions that fulfills these conditions. As part of our work, we also present a new solution for quark wave function centers. Issues with flavor-changing neutral current effects are assessed. Finally, we remark on baryogenesis and dark matter in these models.

Affleck-Dine inflation

The Affleck-Dine mechanism in its simplest form provides baryogenesis from the out-of-equilibrium evolution of a complex scalar field with a simple renormalizable potential. We show that such a model, supplemented by nonminimal coupling to gravity, can also provide inflation, consistent with Planck constraints, simultaneously with the generation of the baryon asymmetry. The predictions of the model include significant tensor-to-scalar ratio and possibly baryon isocurvature fluctuations. The reheating temperature is calculable, making the model fully predictive. We require color triplet scalars for reheating and transfering the primordial baryon asymmetry to quarks; these could be observable at colliders. They can also be probed at higher scales by searches for quark compositeness in dijet angular distributions, and flavor-changing neutral current effects.

Matching for FCNC effects in the flavour-symmetric SMEFT

We calculate the complete tree and one-loop matching of the dimension-six Standard Model Effective Field Theory (SMEFT) with unbroken U(3)5 flavour symmetry to the operators of the Weak Effective Theory (WET) which are responsible for flavour changing neutral current effects among down-type quarks. We also explicitly calculate the effects of SMEFT corrections to input observables on the WET Wilson coefficients, a necessary step on the way to a well-defined, complete prediction. These results will enable high-precision flavour data to be incorporated into global fits of the SMEFT at high energies, where the flavour symmetry assumption is widespread.

Study of Bs0 → ℓ+ℓ−γ decays in a nonuniversal Z′ model

We investigate [Formula: see text] decays in a nonuniversal [Formula: see text] model derived from the extension of the Standard Model (SM). Considering the [Formula: see text]-mediated flavor-changing neutral current (FCNC) effects, we calculate the branching ratio and forward–backward asymmetry [Formula: see text] for [Formula: see text] decay processes. We compare the obtained results with predictions of the SM and discuss the sensitivity of the observables to [Formula: see text] boson coupling parameters. We find that the branching ratios are enhanced by one order from SM predictions in [Formula: see text] model scenario. We also observe that the variation of forward–backward asymmetry with the [Formula: see text] boson coupling parameters portrays discrimination between NP effects and SM results.

Low-frequency dielectric response of a periodic array of charged spheres in an electrolyte solution: The simple cubic lattice.

We study the low-frequency dielectric response of highly charged spheres arranged in a cubic lattice and immersed in an electrolyte solution. We focus on the influence of the out-of-phase current in the regime where the ionic charge is neutral. We consider the case where the charged spheres have no surface conductance and no frequency-dependent surface capacitance. Hence, the frequency dispersion of the dielectric constant is dominated by the effect of neutral currents outside the electric double layer. In the thin double-layer limit, we use Fixman's boundary condition at the outer surface of the double layer to capture the interaction between the electric field and the flow of the ions. For periodic conditions, we combine the methods developed by Lord Rayleigh for understanding the electric conduction across rectangularly arranged obstacles and by Korringa, Kohn, and Rostoker for the electronic band structure computation. When the charged spheres occupy a very small volume fraction, smaller than 1%, our solution becomes consistent with the Maxwell Garnett mixing formula together with the single-particle polarization response, as expected, because interparticle interactions become less prominent in the dilute limit. By contrast, the interparticle interaction greatly alters the dielectric response even when charged spheres occupy only 2% of the volume. We found that the characteristic frequency shifts to a higher value compared to that derived from the single-particle polarization response. At the same time, the low-frequency dielectric enhancement, a signature of charged spheres immersed in an electrolyte, becomes less prominent for the periodic array of charged spheres. Our results imply that the signature of the dielectric response of a system consisting of densely packed charged spheres immersed in an electrolyte can differ drastically from a dilute suspension.

Searching for flavor changing neutral currents int→Hc,H→ττdecays at the LHC

The prospects of searching for the flavor changing neutral current effect in the decay of $t\to H c, H\to \tau\tau$ are investigated with the simulated $p-p$ collision data for the ATLAS detector at the LHC, where the Higgs mass is assumed to be 125~GeV. A fit based on the constraints from the Higgs mass and the tau decay kinematics is performed for each event, which improves significantly the Higgs and top mass reconstruction and helps the signal-background separation. Boosted Decision Trees discriminants are developed to achieve an optimal sensitivity of searching for the FCNC signal. An expected upper limit of the branching ratio $\mathcal{B}(t\to H c)$ at $95\%$ confidence level of 0.25\% is obtained with a data set of 100~fb$^{-1}$ at $\sqrt{s}=13$ TeV during the LHC Run-2 period.

Search for doubly charged Higgs bosons through vector boson fusion at the LHC and beyond

Production and decays of doubly charged Higgs bosons at the LHC and future hadron colliders triggered by a vector boson fusion mechanism are discussed in the context of the minimal left-right symmetric model. Our analysis is based on the Higgs boson mass spectrum compatible with available constraints which include flavor changing neutral current (FCNC) effects and vacuum stability of the scalar potential. Though the parity breaking scale ${v}_{R}$ is large ($\ensuremath{\sim}$ few TeV) and scalar masses which contribute to FCNC effects are even larger, a consistent Higgs boson mass spectrum still allows us to keep doubly charged scalar masses below 1 TeV which is an interesting situation for LHC and future circular collider (FCC). We have shown that the allowed Higgs boson mass spectrum constrains the splittings (${M}_{{H}_{1}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}}\ensuremath{-}{M}_{{H}_{1}^{\ifmmode\pm\else\textpm\fi{}}}$), closing the possibility of ${H}_{1}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{W}_{1}^{\ifmmode\pm\else\textpm\fi{}}{H}_{1}^{\ifmmode\pm\else\textpm\fi{}}$ decays. Assuming that doubly charged Higgs bosons decay predominantly into a pair of same-sign charged leptons through the process $pp\ensuremath{\rightarrow}{H}_{1/2}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}{H}_{1/2}^{\ensuremath{\mp}\ensuremath{\mp}}jj\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{\ensuremath{\mp}}{\ensuremath{\ell}}^{\ensuremath{\mp}}jj$, we find that for the LHC operating at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ with an integrated luminosity at the level of $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ (HL-LHC), there is practically no chance to detect such particles at the reasonable significance level through this channel. However, at 33 TeV HE-LHC and (or) 100 TeV FCC-hh, a wide region opens up for exploring the doubly charged Higgs boson mass spectrum. In FCC-hh, the doubly charged Higgs bosons mass up to 1 TeV can be easily probed.

Lowest limits on the doubly charged Higgs boson masses in the minimal left-right symmetric model

The doubly charged Higgs bosons ${H}_{1,2}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$ would undoubtedly be clear messengers of the new physics. We discuss their mass spectrum and show how experimental data and relations between scalar masses put limits on it. In particular, both the masses of the particles ${H}_{1}^{0}$, ${A}_{1}^{0}$ that play a crucial role in flavor changing neutral current effects and the masses of the additional gauge bosons ${W}_{2}$, ${Z}_{2}$ are notably important. For instance, if ${M}_{{H}_{1}^{0},{A}_{1}^{0}}\ensuremath{\simeq}15\text{ }\text{ }\mathrm{TeV}$ and ${M}_{{W}_{2}}\ensuremath{\simeq}3.76\text{ }\text{ }\mathrm{TeV}$, then the lowest mass of ${H}_{1}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$ is 465 GeV. In contrast, because of the freedom in the parameter space of the full scalar potential, there is no lowest limit on the mass of ${H}_{2}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$. It is shown to which signals at hadron colliders such relatively light doubly charged scalars might correspond. LHC working at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ will enter into the region where the existence of such particles with minimal masses can be thoroughly explored for a much wider parameter space of the minimal and manifest version of the left-right symmetric model (MLRSM). Taking into account our considerations and present ATLAS and CMS exclusion limits on ${M}_{{H}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}}$, there exist already first partial bounds on some of the MLRSM scalar potential parameters.

Excited neutrino production by ultrahigh energy neutrinos traversing the Earth

We focus on the production of excited neutrinos by ultra high energy (UHE, ${E}_{\ensuremath{\nu}}>{10}^{4}\text{ }\text{ }\mathrm{GeV}$) neutrinos traversing the Earth. The surviving neutrino fluxes are calculated using the complete system of transport equations for ordinary neutrinos and their excited states, and we compare these results with the obtained using only Standard Model (SM) interactions. We extend the analysis by including the neutral current (NC) and decay regeneration effects, and computing the surviving flux for different values of $f/\ensuremath{\Lambda}$, where $\ensuremath{\Lambda}$ is the compositeness scale and $f$ a coupling factor representing non-perturbative physics. Finally, we analyze the possibilities of detecting such fluxes in a neutrino telescope such as IceCube showing the allowed regions in the $({m}^{*},f/\ensuremath{\Lambda})$ plane for two possible initial fluxes. We have considered the IC80 configuration of IceCube for an operation time of ten years.

Light charged Higgs boson in two-Higgs doublet model for CDFWjjanomaly

Motivated by recent anomalous CDF data on $Wjj$ events, we study a possible explanation within the framework of the two-Higgs doublet model. We find that a charged Higgs boson of mass $\sim$ 140 GeV with appropriate couplings can account for the observed excess. In addition, we consider the flavor-changing neutral current effects induced at loop level by the charged Higgs boson on the $B$ meson system to further constrain the model. Our study shows that the like-sign charge asymmetry $A_{s\ell}^b$ can be of ${\cal O}(10^{-3})$ in this scenario.

Flavour constraints on multi-Higgs-doublet models: Yukawa alignment

In multi-Higgs-doublet models, the alignment in flavour space of all Yukawa matrices coupling to a given right-handed fermion guarantees the absence of tree-level flavour-changing neutral couplings, while introducing new sources of CP violation. With N Higgs doublets (and no right-handed neutrinos) the Yukawa Lagrangian is characterized by the fermion masses, the CKM quark mixing matrix and 3(N-1) complex couplings. Quantum corrections break the alignment, generating a minimal-flavour-violation structure with flavour-blind phases. The aligned multi-Higgs-doublet models lead to a rich and viable phenomenology with an interesting hierarchy of flavour-changing neutral current effects, suppressing them in light-quark systems while allowing potentially relevant signals in heavy-quark transitions.

Large mixing of light and heavy neutrinos in seesaw models and the LHC

In the type-I seesaw model the size of mixing between light and heavy neutrinos, nu and N, respectively, is of order the square root of their mass ratio, (m_nu/m_N)^(1/2), with only one generation of the neutrinos. Since the light-neutrino mass must be less than an eV or so, the mixing would be very small, even for a heavy-neutrino mass of order a few hundred GeV. This would make it unlikely to test the model directly at the LHC, as the amplitude for producing the heavy neutrino is proportional to the mixing size. However, it has been realized for some time that, with more than one generation of light and heavy neutrinos, the mixing can be significantly larger in certain situations. In this paper we explore this possibility further and consider specific examples in detail in the context of type-I seesaw. We study its implications for the single production of the heavy neutrinos at the LHC via the main channel q qbar' -> W^* -> l N involving an ordinary charged lepton l. We then extend the discussion to the type-III seesaw model, which has richer phenomenology due to presence of the charged partners of the heavy neutrinos, and examine the implications for the single production of these heavy leptons at the LHC. In the latter model the new kinds of solutions that we find also make it possible to have sizable flavor-changing neutral-current effects in processes involving ordinary charged leptons.

The physics of heavyZ′gauge bosons

The U(1)' symmetry associated with a possible heavy Z' would have profound implications for particle physics and cosmology. The motivations for such particles in various extensions of the standard model, possible ranges for their masses and couplings, and classes of anomaly-free models are discussed. Present limits from electroweak and collider experiments are briefly surveyed, as are prospects for discovery and diagnostic study at future colliders. Implications of a Z' are discussed, including an extended Higgs sector, extended neutralino sector, and solution to the mu problem in supersymmetry; exotic fermions needed for anomaly cancellation; possible flavor changing neutral current effects; neutrino mass; possible Z' mediation of supersymmetry breaking; and cosmological implications for cold dark matter and electroweak baryogenesis.

Same-sign top pair production in an extra-dimension model of flavor at the CERN Large Hadron Collider

We study the same-sign top pair production mediated by the first Kaluza-Klein (KK) excitation of the gluon in the Randall-Sundrum model with flavor violation at the LHC, in which the nonuniversal couplings between fermions and KK gauge bosons will lead to observable tree-level flavor changing neutral current effects. We find that the same-sign top quarks produced in our case have property of high energy and high transverse momentum, and lead to an observable signal in the same-sign dilepton channel even when the mass of the KK gluon reach up to 3 TeV. We further investigate the potential of the LHC to probe the flavor violating parameters and find that the LHC can probe their values down to 0.06.

Constraints on economical 331 models from mixing ofK,Bd, andBsneutral mesons

We analyze the effect of flavor changing neutral currents within 331 models. In particular, we concentrate on the so-called economical models, which have a minimal scalar sector. Taking into account the experimental measurements of observables related to neutral $K$ and $B$ meson mixing, we study the resulting bounds for angles and phases in the mixing matrix for the down quark sector, and the mass and mixing parameters related to the new ${Z}^{\ensuremath{'}}$ gauge boson.

Quark masses in supersymmetricSU(3)color⊗SU(3)W⊗U(1)Xmodel with discreteT′flavor symmetry

We propose a supersymmetric model of quark flavor symmetry based on $\mathrm{SU}(3{)}_{\mathrm{color}}\ensuremath{\bigotimes}\mathrm{SU}(3{)}_{W}\ensuremath{\bigotimes}\mathrm{U}(1{)}_{X}\ensuremath{\bigotimes}{T}^{\ensuremath{'}}$ where ${T}^{\ensuremath{'}}$ (binary tetrahedral group) is a discrete flavor symmetry group. The third (heavy) generation of quarks are assigned to singlet representation of ${T}^{\ensuremath{'}}$ while light quark generations are treated differently and assigned to doublet representation of ${T}^{\ensuremath{'}}$. The model generates masses for heavy quarks as in 3-3-1 case. The quark mass terms for the second generation are obtained when ${T}^{\ensuremath{'}}$ symmetry is broken at leading order and for first generation with small higher order corrections. The flavor-changing neutral current effects are suppressed at leading order for $3\ensuremath{\rightarrow}2$ quark transitions due to ${T}^{\ensuremath{'}}$ breaking which is an attractive feature of the 3-3-1 $\ensuremath{\bigotimes}{T}^{\ensuremath{'}}$ model.

Molecular homochirality and the parity‐violating energy difference. A critique with new proposals

Previous proposals for the origin of molecular homochirality, based on the effect of the weak neutral current (WNC) on enantiomers, and the amplification of the resultant parity-violating energy difference (PVED), are possibly flawed. The additive amplification of PVED in crystals and polymers ("Yamagata hypothesis") cannot lead to detectable levels of optical activity, the original theory apparently overestimating PVED by a factor equal to Avogadro's number. An alternative theory based on the irreversible and spontaneous evolution of a dynamically fluctuating system is apparently impractical. However, the nonlinear amplification of PVED via autocatalytic polymerization may be possible as indicated by a simplified physico-chemical approach. This may also occur during crystallization and melting, and form the basis of the second order asymmetric transformation. (Thus, reported differences in the melting points of enantiomers in several cases may well be real). Also, the preponderance of racemic compounds over conglomerates may be based on the destabilization of the conglomerate by the action of the WNC on the crystalline lattice. The WNC may also be involved in the anomalous scattering of X-rays, which possibly arises from their circular polarization: the current theory would need to be revised accordingly.

Top hypercharge

We propose a top hypercharge model with gauge symmetry SU(3)C × SU(2)L × U(1)1 × U(1)2 where the first two families of the Standard Model (SM) fermions are charged under U(1)1 while the third family is charged under U(1)2. The U(1)1 × U(1)2 gauge symmetry is broken down to the U(1)Ygauge symmetry, when a SM singlet Higgs field acquires a vacuum expectation value. We consider the electroweak constraints, and compare the fit to experimental observables to that of the SM. We study the quark CKM mixing between the first two families and the third family, the neutrino masses and mixing, the flavour changing neutral current effects in meson mixing and decays, the Z' discovery potential at the Large Hadron Collider, the dark matter with a gauged Z2 symmetry, and the Higgs boson masses.