Leptonic Interactions Research Articles

Charged-Particle Multiplicity Moments as Described by Shifted Gompertz Distribution in e+e−, pp¯, and pp Collisions at High Energies

In continuation of our earlier work, in which we analysed the charged particle multiplicities in leptonic and hadronic interactions at different center-of-mass energies in full phase space as well as in restricted phase space using the shifted Gompertz distribution, a detailed analysis of the normalized moments and normalized factorial moments is reported here. A two-component model in which a probability distribution function is obtained from the superposition of two shifted Gompertz distributions, as introduced in our earlier work, has also been used for the analysis. This is the first analysis of the moments with the shifted Gompertz distribution. Analysis has also been performed to predict the moments of multiplicity distribution for the e+e− collisions at s=500 GeV at a future collider.

B -physics anomalies: The bridge between R -parity violating supersymmetry and flavored dark matter

In recent years, significant experimental indications that point towards Lepton Flavour Universality violating effects in B-decays, involving $b \to c \tau \nu$ and $b \to s \ell^+ \ell^-$ have been accumulated. A possible New Physics explanation can be sought within the framework of R-parity violating Supersymmetry, which contains the necessary ingredients to explain the anomalies via both leptoquark, tree-level exchange and one-loop diagrams involving purely leptonic interactions. In addition, an approximate $U(2)^2$ flavour symmetry, that respects gauge coupling unification, successfully controls the strength of these interactions. Nevertheless strong constraints from leptonic processes and $Z$ boson decays exclude most of the relevant parameter space at the $2 \sigma$ level. Moreover, R-parity violation deprives Supersymmetry of its Dark Matter candidates. Motivated by these deficiencies, we introduce a new gauge singlet superfield, charged under the flavour symmetry and show that its third-generation, scalar component may participate in loop diagrams that alleviate the above-mentioned tensions, while at the same time reproduce the observed relic abundance. We obtain a solution to both anomalies that is also fully consistent with the rich Flavour and Dark Matter phenomenology. Finally, we assess the prospect to probe the model at future experiments.

Towards a Z3-graded approach to quarks’ symmetries

Colour SU(3) group is an exact symmetry of Quantum Chromodynamics, which describes strong interactions between quarks and gluons. Supplemented by two internal symmetries, SU(2) and U(1), it serves as the internal symmetry of the Standard Model, describing as well the electroweak interactions of quarks and leptons. The colour SU(3) symmetry is exact, while two other symmetries are broken by means of the Higgs-Kibble mechanism. The three colours and fractional quarks charges with values 1/3 and 2/3 suggest that the cyclic group Z3 may play a crucial role in quark field dynamics.In this paper we consequently apply the Z3 symmetry to field multiplets describing colour quark fields. Generalized Dirac equation for coloured 12-component spinors is introduced and its properties are discussed. Imposing Z3-graded Lorentz and Poincaré covariance leads to enlargement of quark fields multiplets and incorporates additional Z2 × Z3 symmetry which leads to the appearance of three generations (families) of distinct quark doublets.

Neutrino scattering and B anomalies from hidden sector portals

We examine current constraints on and the future sensitivity to the strength of couplings between quarks and neutrinos in the presence of a form factor generated from loop effects of hidden sector particles that interact with quarks via new interactions. We consider models associated with either vector or scalar interactions of quarks and leptons generated by hidden sector dynamics. We study constraints on these models using data from coherent elastic neutrino-nucleus scattering and solar neutrino experiments and demonstrate how these new interactions may be discovered by utilizing the recoil spectra. We show that our framework can be naturally extended to explain the lepton universality violating neutral current B decay anomalies, and that in a model framework the constraints from neutrino scattering can have implications for these anomalies.

Lepton non-universality in B decays and fermion mass structure

We consider the possibility that the neutral-current B anomalies are due to radiative corrections generated by Yukawa interactions of quarks and leptons with new vector-like quark and lepton electroweak doublets and new Standard Model singlet scalars. We show that the restricted interactions needed can result from an underlying Abelian family symmetry and that the same symmetry can give rise to an acceptable pattern of quark and charged lepton masses and mixings, providing a bridge between the non-universality observed in the B-sector and that of the fermion mass matrices. We construct two simple models, one with a single singlet scalar in which the flavour changing comes from quark and lepton mixing and one with an additional scalar in which the flavour changing can come from both fermion and scalar mixing. We show that for the case the new quarks are much heavier than the new leptons and scalars the B anomalies can be due to box diagrams with couplings in the perturbative regime consistent with the bounds coming from {B}_s-{overline{B}}_s , K-overline{K} and D-overline{D} mixing as well as other lepton family number violating processes. The new states can be dark matter candidates and, in the two scalar model with a light scalar of O(60) GeV and vector-like lepton of O(100) GeV, there can be a simultaneous explanation of the B-anomalies, the muon anomalous magnetic moment and the dark matter abundance.

A challenge to lepton universality in B meson decays

One of the key assumptions of the Standard Model of fundamental particles is that the interactions of the charged leptons, namely electrons, muons, and taus, differ only because of their different masses. While precision tests have not revealed any definite violation of this assumption, recent studies of B meson decays involving the higher-mass tau lepton have resulted in observations that challenge lepton universality at the level of four standard deviations. A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics.

Relativistic corrections to the Bethe logarithm for the 2S3 and 2P3 states of He

With this work we start a project of calculating the QED contribution of order $\alpha^7m$ to the $2^3P$-$2^3S$ transition energy in helium, aiming for an accurate determination of the nuclear charge radius $r_E$ from measurements of the corresponding transition frequency. Together with the complementary determination of $r_E$ from muonic helium, this project will provide a stringent test of universality of electromagnetic interactions of leptons in the Standard Model. We report a calculation of the relativistic corrections to the Bethe logarithm for the $2^3S$ and $2^3P$ states, which is the most numerically demanding part of the project.

Manifestation of Nonuniversality of Lepton Interactions in Spontaneously Violated Mirror Symmetry

LHCb data indicate a significant difference in widths between the B->K (or K^*) + e^+ e^- and B->K (or K^*) + {\mu}^+ {\mu}^- decays (April 2017). The width of the e^+ e^--channel is noticeably larger than that of the {\mu}^+ {\mu}^--channel. SM-calculations require equality here. The difference may mean that a new interaction exists which changes generations and distinguishes leptons, with coupling constants much larger than, and inverse in power to, the SM-coupling of fermions with the Higgs boson. In the spontaneously violated mirror symmetry, the coupling between SM particles and the heavy (in principle) second Higgs scalar, necessarily present here, is characterized by precisely such properties. The inverse strong power of such a coupling is not an additional hypothesis but also a necessary condition for the qualitative reproduction of the observed mass hierarchy of charged leptons and the structure of lepton weak mixing, the PMNS matrix. In the mirror model being discussed, all properties, including the inverse power of the new interaction, are dictated by the hierarchical character of mass spectra for quarks and charged leptons.

Probing Seesaw with Parity Restoration.

We present a novel way of testing the seesaw origin of neutrino mass in the context of the minimal left-right symmetric model. It is based on the connection between the leptonic interactions of the doubly charged scalars, whose presence is at the core of the seesaw mechanism, and the neutrino Dirac Yukawa couplings which govern, among other processes, the right-handed neutrino decays into left-handed charged leptons. We prove that any physical quantity depending on these couplings is a function of the Hermitian part only, which can significantly simplify their future experimental determination.

Testing fundamental interactions on the helium atom

We critically examine the current status of theoretical calculations of the energies, the fine structure, and the isotope shift of the lowest-lying states of helium, searching for unresolved discrepancies with experiments. Calculations are performed within the quantum electrodynamics expansion in powers of the fine structure constant $\alpha$ and the electron-to-nucleus mass ratio $m/M$. For energies, theoretical results are complete through orders $\alpha^6m$ and $\alpha^6m^2/M$, with the resulting accuracy ranging from $0.5$ to $2$~MHz for the $n=2$ states. The fine-structure splitting of the $2^3P$ state is predicted with a much better accuracy, 1.7~kHz, as a consequence of a calculation of the next-order $\alpha^7m$ effect. An excellent agreement of the theoretical predictions with the recent measurements of the fine structure provides one of the best tests of the bound-state QED in few-electron systems and determines the fine-structure constant $\alpha$ with an accuracy of 31~ppb. The isotope shift between $^3$He and $^4$He is treated theoretically with a sub-kHz accuracy, which allows for a high-precision determination of the differences of the nuclear charge radii $\delta r^2$. Several such determinations, however, yield results that are in a 4$\sigma$ disagreement with each other, what remains unexplained. Apart from this, we find no significant discrepancies between theory and experiment for the helium atom. In the future, a calculation of the next-order $\alpha^7m$ effect for energy levels will enable determinations of the nuclear charge radii from atomic transition frequencies with an accuracy better than 1\%. Combined with the complementary determinations from muonic atoms, this will provide a sensitive test of universality in electromagnetic interactions of leptons and contribute to the solution of the proton charge radius puzzle.

A challenge to lepton universality in B-meson decays.

One of the key assumptions of the standard model of particle physics is that the interactions of the charged leptons, namely electrons, muons and taus, differ only because of their different masses. Whereas precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, recent studies of B-meson decays involving the higher-mass tau lepton have resulted in observations that challenge lepton universality at the level of four standard deviations. A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics.

Beta spectrum of unique first-forbidden decays as a novel test for fundamental symmetries

Within the Standard Model, the weak interaction of quarks and leptons is characterized by certain symmetry properties, such as maximal breaking of parity and favored helicity. These are related to the V−A structure of the weak interaction. These characteristics were discovered by studying correlations in the directions of the outgoing leptons in nuclear beta decays. Presently, correlation measurements in nuclear beta decays are intensively studied to probe for signatures for deviations from these couplings, which are an indication of Beyond Standard Model physics. We show that the structure of the energy spectrum of emitted electrons in unique first-forbidden β-decays is sensitive to the symmetries of the weak interaction, and thus can be used as a novel probe of physics beyond the standard model. Furthermore, the energy spectrum gives constraints both in the case of right and left couplings of the new beyond standard model currents. We show that a measurement with modest energy resolution of ≈20 keV is expected to lead to new constraints on beyond the standard model interactions with tensor couplings.

Simulation study of the formation of a non-relativistic pair shock

We examine with a particle-in-cell (PIC) simulation the collision of two equally dense clouds of cold pair plasma. The clouds interpenetrate until instabilities set in, which heat up the plasma and trigger the formation of a pair of shocks. The fastest-growing waves at the collision speed $c/5$, where $c$ is the speed of light in vacuum, and low temperature are the electrostatic two-stream mode and the quasi-electrostatic oblique mode. Both waves grow and saturate via the formation of phase space vortices. The strong electric fields of these nonlinear plasma structures provide an efficient means of heating up and compressing the inflowing upstream leptons. The interaction of the hot leptons, which leak back into the upstream region, with the inflowing cool upstream leptons continuously drives electrostatic waves that mediate the shock. These waves heat up the inflowing upstream leptons primarily along the shock normal, which results in an anisotropic velocity distribution in the post-shock region. This distribution gives rise to the Weibel instability. Our simulation shows that even if the shock is mediated by quasi-electrostatic waves, strong magnetowaves will still develop in its downstream region.

Gluon dominance model

Study of multi-particle production has longer than the semi-centennial history. As it is known, with the growth of energy of accelerators, the new channels of reaction are being opened, the average number of secondary particles is increasing. Physicists are able to accelerate stable particles, such as electrons, positrons, protons, antiprotons, ions (both light and heavy). Rarely, they accelerate kaons and pions. The obtained experimental material stimulates the development of the different theoretical approaches. Since appearance of the modern theory of strong interactions, quantum chromodynamics (QCD), our understanding of multi-particle production is advanced significantly. The language of quarks and gluons is basic one at the explanation of observable phenomena. This review is devoted to the history of appearance and the following development of the gluon dominance model. This model is based on the pQCD and the phenomenological description of the hadronization stage. It permits to describe multiplicity distributions both for lepton and hadron interactions, especially in the high multiplicity region.

Probing models of Dirac neutrino masses via the flavor structure of the mass matrix

We classify models of the Dirac neutrino mass by concentrating on flavor structures of the mass matrix. The advantage of our classification is that we do not need to specify detail of models except for Yukawa interactions because flavor structures can be given only by products of Yukawa matrices. All possible Yukawa interactions between leptons (including the right-handed neutrino) are taken into account by introducing appropriate scalar fields. We also take into account the case with Yukawa interactions of leptons with the dark matter candidate. Then, we see that flavor structures can be classified into seven groups. The result is useful for the efficient test of models of the neutrino mass. One of seven groups can be tested by measuring the absolute neutrino mass. Other two can be tested by probing the violation of the lepton universality in $\ell \to \ell^\prime \nu \overline{\nu}$. In order to test the other four groups, we can rely on searches for new scalar particles at collider experiments.

Scalar doublet models confrontτandbanomalies

There are indications of a possible breakdown of the standard model, suggesting that $\tau$ lepton interactions violate flavor universality. BABAR, Belle and LHCb report high ratios of $B\to D^{(*)}\tau\nu$. There are long-standing excesses in $B\to\tau\nu$ and $W\to\tau\nu$ decays, and a deficit in inclusive $\tau$ to strange decays. We investigate whether two Higgs doublet models with the most general allowed couplings to quarks, and a large coupling to $\tau$ leptons, can explain these anomalies while respecting other flavor constraints and technical naturalness. Fits to $B\to D^{(*)}\tau\nu$ data require couplings of the new Higgs doublet to down-type quarks, opening the door to many highly constrained flavor-changing neutral current (FCNC) processes. We confront these challenges by introducing a novel ansatz that relates the new up- and down-type Yukawa couplings, and demonstrate viable values of the couplings that are free from fine tuning. LEP and LHC searches for new Higgs bosons decaying via $H^0\to \tau^+\tau^-$ and $H^\pm\to\tau^\pm\nu$ allow a window of masses $m_H = [100$-$125]\,$GeV and $m_\pm\sim 100\,$GeV that is consistent with the predictions of our model. Contamination of the $W^+\to\tau^+\nu$ signal by $H^+\to\tau^+\nu$ decays at LEP could explain the apparent $W\to\tau\nu$ excess. We predict that the branching ratio for $B_s\to \tau^+\tau^-$ is not far below its current limit of several percent. An alternative model with decays of $B\to D^{(*)}\tau\nu_s$ to a sterile neutrino is also argued to be viable.

Z′model forb→sℓℓ¯flavor anomalies

We study the implications of flavour-changing neutral currents (FCNC's) in a model with the $SU(2)_l\times SU(2)_h\times U(1)_Y$ electroweak gauge symmetry for several anomalies appearing in $b\to s \ell\bar \ell$ induced $B$ decays in LHCb data. In this model, $SU(2)_l$ and $SU(2)_h$ govern the left-handed fermions in the first two generations and the third generation, respectively. The physical $Z$ and $Z'$ generate the $b\to s$ transition at tree level, leading to additional contributions to the $b \to s$ semileptonic operators ${\cal O}_{9,10}$. We find that although $B_s$-$\bar B_s$ mixing constrains the parameters severely, the model can produce values of ${\cal C}^{\rm NP}_{9,10}$ in the range determined by Descotes-Genon {\it et. al.} in Ref.~\cite{Descotes-Genon:2015uva} for this scenario to improve the global fit of observables in decays induced by the $b\to s \mu \bar \mu$ transition. The $Z'$ boson in this model also generates tree-level FCNC's for the leptonic interactions that can accommodate the experimental central value of $R_K = {\cal B}(B\to K \mu \bar \mu)/{\cal B}(B\to K e\bar e)=0.75$. In this case, the model predicts sizeable branching ratios for $B\to K e \bar \tau$, $B\to K \tau \bar e$, and an enhancement of $B\to K \tau \bar \tau$ with respect to its SM value.

Quark flavour observables in the Littlest Higgs model with T-parity after LHC Run 1.

The Littlest Higgs model with T-parity (LHT) belongs to the simplest new physics scenarios with new sources of flavour and CP violation. The latter originate in the interactions of ordinary quarks and leptons with heavy mirror quarks and leptons that are mediated by new heavy gauge bosons. Also a heavy fermionic top partner is present in this model which communicates with the SM fermions by means of standard W^pm and Z^0 gauge bosons. We present a new analysis of quark flavour observables in the LHT model in view of the oncoming flavour precision era. We use all available information on the CKM parameters, lattice QCD input and experimental data on quark flavour observables and corresponding theoretical calculations, taking into account new lower bounds on the symmetry breaking scale and the mirror quark masses from the LHC. We investigate by how much the branching ratios for a number of rare K and B decays are still allowed to depart from their SM values. This includes K^+rightarrow pi ^+nu bar{nu }, K_{L}rightarrow pi ^0nu bar{nu }, K_Lrightarrow mu ^+mu ^-, Brightarrow X_sgamma , B_{s,d}rightarrow mu ^+mu ^-, Brightarrow K^{(*)}ell ^+ell ^-, Brightarrow K^{(*)}nu bar{nu }, and varepsilon '/varepsilon . Taking into account the constraints from Delta F=2 processes, significant departures from the SM predictions for K^+rightarrow pi ^+nu bar{nu } and K_{L}rightarrow pi ^0nu bar{nu } are possible, while the effects in B decays are much smaller. In particular, the LHT model favours mathcal {B}(B_{s}rightarrow mu ^+mu ^-) ge mathcal {B}(B_{s}rightarrow mu ^+mu ^-)_mathrm{SM}, which is not supported by the data, and the present anomalies in Brightarrow K^{(*)}ell ^+ell ^- decays cannot be explained in this model. With the recent lattice and large N input the imposition of the varepsilon '/varepsilon constraint implies a significant suppression of the branching ratio for K_{L}rightarrow pi ^0nu bar{nu } with respect to its SM value while allowing only for small modifications of K^+rightarrow pi ^+nu bar{nu }. Finally, we investigate how the LHT physics could be distinguished from other models by means of indirect measurements and discuss the consequences for quark flavour observables of not finding any LHT state in the coming years.

The denoised, deconvolved, and decomposedFermiγ-ray sky

We analyze the 6.5yr all-sky data from the Fermi LAT restricted to gamma-ray photons with energies between 0.6-307.2GeV. Raw count maps show a superposition of diffuse and point-like emission structures and are subject to shot noise and instrumental artifacts. Using the D3PO inference algorithm, we model the observed photon counts as the sum of a diffuse and a point-like photon flux, convolved with the instrumental beam and subject to Poissonian shot noise. D3PO performs a Bayesian inference in this setting without the use of spatial or spectral templates;i.e., it removes the shot noise, deconvolves the instrumental response, and yields estimates for the two flux components separately. The non-parametric reconstruction uncovers the morphology of the diffuse photon flux up to several hundred GeV. We present an all-sky spectral index map for the diffuse component. We show that the diffuse gamma-ray flux can be described phenomenologically by only two distinct components: a soft component, presumably dominated by hadronic processes, tracing the dense, cold interstellar medium and a hard component, presumably dominated by leptonic interactions, following the hot and dilute medium and outflows such as the Fermi bubbles. A comparison of the soft component with the Galactic dust emission indicates that the dust-to-soft-gamma ratio in the interstellar medium decreases with latitude. The spectrally hard component exists in a thick Galactic disk and tends to flow out of the Galaxy at some locations. Furthermore, we find the angular power spectrum of the diffuse flux to roughly follow a power law with an index of 2.47 on large scales, independent of energy. Our first catalog of source candidates includes 3106 candidates of which we associate 1381(1897) with known sources from the 2nd(3rd) Fermi catalog. We observe gamma-ray emission in the direction of a few galaxy clusters hosting radio halos.

Composite leptoquarks and anomalies in B-meson decays

We attempt to explain recent anomalies in semileptonic $B$ decays at LHCb via a composite Higgs model, in which both the Higgs and an $SU(2)_L$-triplet leptoquark arise as pseudo-Goldstone bosons of the strong dynamics. Fermion masses are assumed to be generated via the mechanism of partial compositeness, which largely determines the leptoquark couplings and implies non-universal lepton interactions. The latter are needed to accommodate tensions in the $b \to s \mu \mu$ dataset and to be consistent with a discrepancy measured at LHCb in the ratio of $B^+ \to K^+ \mu^+ \mu^-$ to $B^+ \to K^+ e^+ e^-$ branching ratios. The data imply that the leptoquark should have a mass of around a TeV. We find that the model is not in conflict with current flavour or direct production bounds, but we identify a few observables for which the new physics contributions are close to current limits and where the leptoquark is likely to show up in future measurements. The leptoquark will be pair-produced at the LHC and decay predominantly to third-generation quarks and leptons, and LHC13 searches will provide further strong bounds.