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DOE OSTI2021

An experimental program with high duty-cycle polarized and unpolarized positron beams at Jefferson Lab

Positron beams, both polarized and unpolarized, are identified as important ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors of the energy-momentum tensor. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Measurements of the $\mathrm {p}\mathrm {p}\rightarrow \mathrm{Z}\mathrm{Z}$ production cross section and the $\mathrm{Z}\rightarrow 4\ell $ branching fraction, and constraints on anomalous triple gauge couplings at $\sqrt{s} = 13\,\text {TeV} $

Four-lepton production in proton-proton collisions, $\mathrm {p}\mathrm {p}\rightarrow (\mathrm{Z}/ \gamma ^*)(\mathrm{Z}/\gamma ^*) \rightarrow 4\ell $ , where $\ell = \mathrm {e}$ or $\mu $ , is studied at a center-of-mass energy of 13 $\,\text {TeV}$ with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 35.9 $\,\text {fb}^{-1}$ . The ZZ production cross section, $\sigma (\mathrm {p}\mathrm {p}\rightarrow \mathrm{Z}\mathrm{Z}) = 17.2 \pm 0.5\,\text {(stat)} \pm 0.7\,\text {(syst)} \pm 0.4\,\text {(theo)} \pm 0.4\,\text {(lumi)} \text { pb} $ , measured using events with two opposite-sign, same-flavor lepton pairs produced in the mass region $60< m_{\ell ^+\ell ^-} < 120\,\text {GeV} $ , is consistent with standard model predictions. Differential cross sections are measured and are well described by the theoretical predictions. The Z boson branching fraction to four leptons is measured to be $\mathcal {B}(\mathrm{Z}\rightarrow 4\ell ) = 4.8 \pm 0.2\,\text {(stat)} \pm 0.2\,\text {(syst)} \pm 0.1\,\text {(theo)} \pm 0.1\,\text {(lumi)} \times 10^{-6}$ for events with a four-lepton invariant mass in the range $80< m_{4\ell } < 100\,\text {GeV} $ and a dilepton mass $m_{\ell \ell } > 4\,\text {GeV} $ for all opposite-sign, same-flavor lepton pairs. The results agree with standard model predictions. The invariant mass distribution of the four-lepton system is used to set limits on anomalous ZZZ and ZZ $\gamma $ couplings at 95% confidence level: $-0.0012

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Forward-Backward Asymmetry of Top Quark Pair Productionn at the Fermilab Tevatron

This dissertation presents the final measurements of the forward-backward asymmetry (A FB ) of top quark-antiquark pair events (t t - ) at the Collider Detector at Fermilab (CDF) experiment. The t t - events are produced in proton{anti-proton collisions with a center of mass energy of 1:96 TeV during the Run II of the Fermilab Tevatron. The measurements are performed with the full CDF Run II data (9.1 fb -1 ) in the final state that contain two charged leptons (electrons or muons, the dilepton final state), and are designed to con rm or deny the evidence-level excess in the AFB measurements in the final state with a single lepton and hadronic jets (lepton+jets final state) as well as the excess in the preliminary measurements in the dilepton final state with the first half of the CDF Run II data. New measurements include the leptonic AFB (A l FB ), the lepton-pair A FB (A ll FB ) and the reconstructed top AFB (At t FB ). Each are combined with the previous results from the lepton+jets final state measured at the CDF experiment. The inclusive A l FB , A ll FB , and At t FB measured in the dilepton final state are 0.072 ± 0.060, 0.076 ± 0.081, and 0.12 ± 0.13, to be compared with the Standard Model (SM) predictions of 0.038 ± 0.003, 0.048 ± 0.004, and 0.010 ± 0.006, respectively. The CDF combination of A l FB and At t FB are 0.090 +0:028 -0.026 , and 0.160 ± 0.045, respectively. The overall results are consistent with the SM predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Research in Theoretical Particle Physics: Neutrino Masses and The Origin of CP-Violation

The main goal of this project was to investigate the theories of neutrino masses at the low scale and the origin of CP-violation in physics beyond the Standard Model. The origin of neutrino masses is one of the most pressing issues in particle physics. In this proposal, we investigated the theories for neutrino masses based on local total lepton number. In these theories anomaly cancellation predicts the existence of extra fermions with lepton number, and one of them is a good dark matter (DM) candidate. The cosmological constraints on the DM relic density implies that the lepton number symmetry breaking scale must be below the multi-TeV scale. Therefore, one can hope to test the origin of neutrino masses in current or future experiments. We investigated in great detail the anomaly cancellation in these gauge theories, study the different mechanisms for neutrino masses, study the predictions for direct and indirect dark matter experiments. Since these theories predict new sources of CP violation, we investigated the predictions for the electric dipole moments (EDM). The Higgs decays and the different signatures at the Large Hadron Collider were investigated in great detail. Finally, we studied the possible baryogenesis mechanisms in these theories in agreement with the EDM, DM and collider constraints. The origin of CP violation in the Standard Model is unknown. In this proposal, we investigated the simplest mechanisms for spontaneous CP-violation to explain the CP-violation in the CKM matrix and the value of the QCD vacuum angle. We discussed the Nelson-Barr mechanism in gauge theories predicting vector-like quarks from anomaly cancellation such as theories for local baryon number. We discussed the Bento-Branco-Parada mechanism in gauge theories to explain the CP-Violation in the CKM matrix. We will investigate models with two Higgs doublets in the context of the minimal theory for quark-lepton unification. We will show the non-decoupling effects in the Higgs sector, the main constraints coming from flavour violating processes when the Yukawa couplings are related by the gauge symmetry in theories for quark-lepton unification. We will investigate the predictions for electric dipole moments in these theories. The possibility to have successful baryogenesis was investigated. Finally, we investigated the relation between CP-violation in the quark and leptonic sectors. The future results from these studies could help us to understand two main issues in physics beyond the Standard Model: The Origin of Neutrino Masses and CP-violation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Observation of the doubly strange b-Baryon Ω b -

This thesis reports the first experimental evidence of the doubly strange b-baryon Ω b - (ssb) following the decay channel Ω b - → J/Ψ(1S) μ + μ - Ω - Λ K - p π - in p$\bar{p}$ collisions at √s = 1.96 Tev. Using approximately 1.3 fb -1 of data collected with the D0 detector at the Fermilab Tevatron Collider, they observe 17.8 ± 4.9(stat) ± 0.8(syst) Ω b - signal events at 6.165 ± 0.010(stat) ± 0.013(syst) GeV/c 2 with a corresponding significance of 5.4 σ, meaning that the probability of the signal coming from a fluctuation in the background is 6.7 x 10 -8 . The theoretical model we have to describe what we believe are the building blocks of nature and the interactions between them, is known as Standard Model. The Standard Model is the combination of Electroweak Theory and Quantum Chromodynamics into a single core in the attempt to include all interactions of subatomic particles except those due to gravity in a simple framework. This model has proved highly accurate in predicting certain interactions, but it does not explain all aspects of subatomic particles. For example, it cannot say how many particles there should be or what their masses are. The search goes on for a more complete theory, and in particular an unified field theory describing the strong, weak, and electromagnetic forces. Twelve elementary particles are known in the Standard Model: the Fermions. They have spin -1/2 and obey the Pauli Exclusion Principle. Fermions are divided into six Quarks: up u, down d, charm c, strange s, top t and, bottom b; and six Leptons: electron e, muon μ, ττ, electron neutrino v e , muon neutrino v μ and, τ neutrino v τ . Quarks interact via the strong force because they carry color charge, electromagnetically because of their electric charge and via the weak nuclear interaction because of the weak isospin. Quarks form color-neutral composite particles known as Hadrons which are divided in Mesons, containing a quark and an antiquark and Baryons, made up three quarks. Leptons have no color charge and can not interact via the strong force. Only three of them have electric charge, hence interact electromagnetically. The motion of non-electrically charged leptons, the neutrinos, is influenced only by the weak nuclear interaction. Every fermion have an associated antiparticle. For quarks, the antiparticle carry opposite electric charge, color charge and baryon number. For leptons, the antiparticle carry opposite electric charge and lepton number. Fermions are suitably grouped together considering their properties and three generations of them are defined. A higher generation fermion have greater mass than those in lower generations. Charged members of the first generation do not decay and form the ultimate building blocks for all the baryonic matter we know about. Charged members of higher generations have very short half lives and are found normally in high-energy environments. Non-electrically charged fermions do not decay and rarely interact with baryonic matter. The way particles interact and influence each other in the Standard Model is result from matter particles exchanging other particles, known as Force Mediating Particles. They are believed to be the reason of the existence of the forces and interactions between particles observed in the laboratory and the universe. Force mediating particles have spin 1, i.e., they are Bosons, and do not follow the Pauli Exclusion Principle. The types of force mediating particles are: the photon γ, three gauge bosons W ± and Z and, eight gluons g. Photons have no mass, the theory of Quantum Electrodynamics describe them very well and are responsible for mediation of the electromagnetic force between electrically charged particles. Gauge bosons are massive, being Z heavier than W ± . They are responsible for the mediation of the weak interactions between particles of different flavors but W ± act only on left-handed particles and right-handed antiparticles while Z with both left-handed particles and antiparticles. Due to the electric charge of W ± , they couple also to electromagnetic interactions. Photons and the three gauge bosons are grouped together and collectively mediate the electroweak interactions. Finally, gluons have no mass, the theory of Quantum Chromodynamics describe them and are responsible for the mediation of the strong interactions between particles with color charge. Having an effective color charge, gluons can interact among themselves. The Higgs Boson is the only particle in the SM without direct experimental evidence. Its detection would help in the explanation of the difference between massive bosons mediating the weak force and the massless photon mediating the electromagnetism.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Measuring the 2{gamma} Exchange Effect in Electron-Proton Scattering at CLAS

Recent results from experiments at Jefferson Laboratory, Newport News VA, which measure the ratio of the electric to magnetic form factors of the proton, G{sub E}/G{sub M}, have forced us to reexamine the single photon exchange approximation in lepton-proton elastic scattering. Discrepancies between the ratio obtained via the time-tested Rosenbluth separation method and new polarization transfer measurements, which differ by as much as a factor of three, may be resolved by considering the effect of two photon exchange (TPE). Theoretical corrections for the TPE effect are difficult due to the large number of excited nucleon states that can contribute to the process. The TPE effect can, however, be determined directly by measuring the ratio of the positron-proton to electron-proton elastic scattering cross sections, R = {sigma}(e{sup +})/{sigma}(e{sup -}), since the TPE effect changes sign with the charge of the incident particle. The CLAS TPE experiment at Jefferson Laboratory, will determine the effect of two-photon exchange in elastic lepton-proton scattering by precisely measuring the ratio of positron-proton to electron-proton elastic cross sections over a large kinematic range (0.1<{epsilon}<0.96, 0.02{<=}Q{sup 2}{<=}2.0). We will accomplish this by directing the 5.5 GeV primary electron beam, provided by the Continuous Electron Beam Accelerator Facility (CEBAF), onto a set of radiators and converters to produce simultaneous and identical beams of electron and positrons. Acceptance and efficiency concerns are minimized by only considering the ratios of the elastic cross sections and switching polarity of magnets in the beam line and the spectrometer. Guided by the results of a short 2006 test run and extensive GEANT based modeling, new shielding and beam line components were designed to maximize luminosity. The unique experimental design and challenges of the TPE experiment and the projected luminosities will be presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Quantum leptogenesis I

Research Highlights: > The paper presents a full quantum mechanical description of leptogenesis. > The lepton asymmetry is calculated directly in terms of Green's functions. > A key element is the nonequilibrium statistical propagator of the heavy neutrino. > The full quantum mechanical description is compared with results from Boltzmann equations. - Abstract: Thermal leptogenesis explains the observed matter-antimatter asymmetry of the universe in terms of neutrino masses, consistent with neutrino oscillation experiments. We present a full quantum mechanical calculation of the generated lepton asymmetry based on Kadanoff-Baym equations. Origin of the asymmetry is the departure from equilibrium of the statistical propagator of the heavy Majorana neutrino, together with CP violating couplings. The lepton asymmetry is calculated directly in terms of Green's functions without referring to 'number densities'. Compared to Boltzmann and quantum Boltzmann equations, the crucial difference are memory effects, rapid oscillations much faster than the heavy neutrino equilibration time. These oscillations strongly suppress the generated lepton asymmetry, unless the standard model gauge interactions, which cause thermal damping, are properly taken into account. We find that these damping effects essentially compensate the enhancement due to quantum statistical factors, so that finally the conventional Boltzmann equations again provide rather accurate predictions for the lepton asymmetry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Hypermagnetic helicity evolution in early universe: leptogenesis and hypermagnetic diffusion

We study hypermagnetic helicity and lepton asymmetry evolution in plasma of the early Universe before the electroweak phase transition (EWPT) accounting for chirality flip processes via inverse Higgs decays and sphaleron transitions which violate the left lepton number and wash out the baryon asymmetry of the Universe (BAU). In the scenario where the right electron asymmetry supports the BAU alone through the conservation law B/3−L{sub eR} = const at temperatures T > T{sub RL} ≅ 10 TeV the following universe cooling leads to the production of a non-zero left lepton (electrons and neutrinos) asymmetry. This is due to the Higgs decays becoming more faster when entering the equilibrium at T = T{sub RL} with the universe expansion, Γ{sub RL} ∼ T > H ∼ T{sup 2}, resulting in the parallel evolution of both the right and the left electron asymmetries at T < T{sub RL} through the corresponding Abelian anomalies in SM in the presence of a seed hypermagnetic field. The hypermagnetic helicity evolution proceeds in a self-consistent way with the lepton asymmetry growth. The role of sphaleron transitions decreasing the left lepton number turns out to be negligible in given scenario. The hypermagnetic helicity can be a supply for the magnetic one in Higgs phase assuming a strong seed hypermagnetic field in symmetric phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Scalar triplet flavored leptogenesis: a systematic approach

Type-II seesaw is a simple scenario in which Majorana neutrino masses are generated by the exchange of a heavy scalar electroweak triplet. When endowed with additional heavy fields, such as right-handed neutrinos or extra triplets, it also provides a compelling framework for baryogenesis via leptogenesis. We derive in this context the full network of Boltzmann equations for studying leptogenesis in the flavored regime. To this end we determine the relations which hold among the chemical potentials of the various particle species in the thermal bath. This takes into account the standard model Yukawa interactions of both leptons and quarks as well as sphaleron processes which, depending on the temperature, may be classified as faster or slower than the Universe Hubble expansion. We find that when leptogenesis is enabled by the presence of an extra triplet, lepton flavor effects allow the production of the B-L asymmetry through lepton number conserving CP asymmetries. This scenario becomes dominant as soon as the triplets couple more to leptons than to standard model scalar doublets. In this case, the way the B-L asymmetry is created through flavor effects is novel: instead of invoking the effect of L-violating inverse decays faster than the Hubble rate, it involves the effect of L-violating decays slower than the Hubble rate. We also analyze the more general situation where lepton number violating CP asymmetries are present and actively participate in the generation of the B-L asymmetry, pointing out that as long as L-violating triplet decays are still in thermal equilibrium when the triplet gauge scattering processes decouple, flavor effects can be striking, allowing to avoid all washout suppression effects from seesaw interactions. In this case the amount of B-L asymmetry produced is limited only by a universal gauge suppression effect, which nevertheless goes away for large triplet decay rates.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

WIMPless dark matter and the excess gamma rays from the Galactic center

In this paper we discuss the excess gamma rays from the Galactic center, the WMAP haze and the CoGeNT and DAMA results in WIMPless models. At the same time we also investigate the low energy constraints from the anomalous magnetic moment of leptons and from some lepton flavor violating decays. It is found that, for scalar or vector WIMPless dark matter, neither the WMAP haze nor the CoGeNT and DAMA observations could be explained simultaneously with the excess gamma rays from the Galactic center. As to fermion WIMPless dark matter, it is only marginally possible to accommodate the CoGeNT and DAMA results with the excess gamma rays from the Galactic center with vector connector fields. On the other hand, only scalar connector fields could interpret the WMAP haze concerning the constraints of anomalous magnetic moment of leptons. Furthermore, if there is only one connector field for all the charged leptons, some lepton flavor violating decays could happen with too large branching ratios severely violating the experimental bounds.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Higgs quadruplet for the type III seesaw model and implications for {mu}{yields}e{gamma} and {mu}-e conversion

In the type III seesaw model, the heavy neutrinos are contained in leptonic triplet representations. The Yukawa couplings of the triplet fermion and the left-handed neutrinos with the doublet Higgs field produce the Dirac mass terms. Together with the Majorana masses for the leptonic triplets, the light neutrinos obtain nonzero seesaw masses. We point out that it is also possible to have a quadruplet Higgs field to produce the Dirac mass terms to facilitate the seesaw mechanism. The vacuum expectation value of the quadruplet Higgs is constrained to be small by electroweak precision data. Therefore, the Yukawa couplings of a quadruplet can be much larger than those for a doublet. We also find that unlike the usual type III seesaw model where at least two copies of leptonic triplets are needed, with both doublet and quadruplet Higgs representations, just one leptonic triplet is possible to have a phenomenologically acceptable model because light neutrino masses can receive sizable contributions at both tree and one-loop levels. Large Yukawa couplings of the quadruplet can induce observable effects for lepton flavor violating processes {mu}{yields}e{gamma} and {mu}-e conversion. Implications of the recent {mu}{yields}e{gamma} limit from MEG and the limit on {mu}-e conversion on Au are also given. Some interesting collider signatures for the doubly charged Higgs boson in the quadruplet are discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Tau Trigger at the ATLAS Experiment

Many theoretical models, like the Standard Model or SUSY at large tan({beta}), predict Higgs bosons or new particles which decay more abundantly to final states including tau leptons than to other leptons. At the energy scale of the LHC, the identification of tau leptons, in particular in the hadronic decay mode, will be a challenging task due to an overwhelming QCD background which gives rise to jets of particles that can be hard to distinguish from hadronic tau decays. Equipped with excellent tracking and calorimetry, the ATLAS experiment has developed tau identification tools capable of working at the trigger level. This contribution presents tau trigger algorithms which exploit the main features of hadronic tau decays and describes the current tau trigger commissioning activities. Many of the SM processes being investigated at ATLAS, as well as numerous BSM searches, contain tau leptons in their final states. Being able to trigger effectively on the tau leptons in these events will contribute to the success of the ATLAS experiment. The tau trigger algorithms and monitoring infrastructure are ready for the first data, and are being tested with the data collected with cosmic muons. The development of efficiency measurements methods using QCD and Z {yields} {tau}{tau} events is well advanced.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Nuclear Corrections in Neutrino-Nucleus Deep Inelastic Scattering and their Compatibility with Global Nuclear Parton-Distribution-Function Analyses

We perform a global {chi}{sup 2} analysis of nuclear parton distribution functions using data from charged current neutrino-nucleus ({nu}A) deep-inelastic scattering (DIS), charged-lepton-nucleus (l{sup {+-}A}) DIS, and the Drell-Yan (DY) process. We show that the nuclear corrections in {nu}A DIS are not compatible with the predictions derived from l{sup {+-}A} DIS and DY data. We quantify this result using a hypothesis-testing criterion based on the {chi}{sup 2} distribution which we apply to the total {chi}{sup 2} as well as to the {chi}{sup 2} of the individual data sets. We find that it is not possible to accommodate the data from {nu}A and l{sup {+-}A} DIS by an acceptable combined fit. Our result has strong implications for the extraction of both nuclear and proton parton distribution functions using combined neutrino and charged-lepton data sets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Possibility of observing an MSSM charged Higgs boson in association with a W boson at the LHC

The possibility of observing associated production of the charged Higgs and W boson in the framework of the minimal supersymmetric standard model (MSSM) at LHC is studied. Both leptonic and hadronic decays of the W boson are studied while the charged Higgs boson is considered to decay to a {tau} lepton and a neutrino. Therefore two search categories are defined based on the leptonic and hadronic final states, i.e., l{tau}+E{sub T}{sup miss} and jj{tau}+E{sub T}{sup miss} where l=e or {mu} and j is a light jet from W decay. The discovery chance of the two categories is evaluated at an integrated luminosity of 300 fb{sup -1} at LHC. It is shown that both leptonic and hadronic final states have the chance of discovery at high tan{beta}. Finally 5{sigma} and 3{sigma} contours are provided for both search categories.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Top-Quark Mass Measurement Using Events with Missing Transverse Energy and Jets at CDF

We present a measurement of the top-quark mass using a sample of tt events in 5.7 fb{sup -1} of integrated luminosity from pp collisions at the Fermilab Tevatron with {radical}(s)=1.96 TeV and collected by the CDF II Detector. We select events having large missing transverse energy, and four, five, or six jets with at least one jet tagged as coming from a b quark, and reject events with identified charged leptons. This analysis considers events from the semileptonic tt decay channel, including events that contain tau leptons. The measurement is based on a multidimensional template method. We fit the data to signal templates of varying top-quark masses and background templates, and measure a top-quark mass of M{sub top}=172.32{+-}2.4(stat){+-}1.0(syst) GeV/c{sup 2}.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Differential top-antitop cross-section measurements as a function of observables constructed from final-state particles using pp collisions at $\sqrt{s}=7 $ TeV in the ATLAS detector

Various differential cross-sections are measured in top-quark pair (tt) events produced in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV at the LHC with the ATLAS detect or. These differential cross-sections are presented in a data set corresponding to an integrated luminosity of 4.6 fb -1 . The differential cross-sections are presented in terms of kinematic variables, such as momentum, rapidity and invariant mass, of a top-quark proxy referred to as the pseudo-top-quark as well as the pseudo-top-quark pair system. The dependence of the measurement on theoretical models is minimal. The measurements are performed on tt events in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of them tagged as originating from a b-quark. The hadronic and leptonic pseudo-top-quarks are defined via the leptonic or hadronic decay mode of the W boson produced by the top-quark decay in events with a single charged lepton. Differential cross-section measurements of the pseudo-top-quark variables are compared with several Monte Carlo models that implement next-to-leading order or leading-order multi-leg matrix-element calculations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Cosmic ray electron and positron excesses from a fourth generation heavy Majorana neutrino

Unexpected features in the energy spectra of cosmic rays electrons and positrons have been recently observed by PAMELA and Fermi-LAT satellite experiments, opening to the exciting possibility of an indirect manifestation of new physics. A TeV-scale fourth lepton family is a natural extension of the Standard Model leptonic sector (also linked to the hierarchy problem in Minimal Walking Technicolor models). The heavy Majorana neutrino of this setup mixes with Standard Model charged leptons through a weak charged current interaction. Here, we first study analytically the energy spectrum of the electrons and positrons originated in the heavy Majorana neutrino decay modes, also including polarization effects. We then compare the prediction of this model with the experimental data, exploiting both the standard direct method and our recently proposed Sum Rules method. We find that the decay modes involving the tau and/or the muon charged leptons as primary decay products fit well the PAMELA and Fermi-LAT lepton excesses while there is tension with respect to the antiproton to proton fraction constrained by PAMELA.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗