Heavy Neutral Particles Research Articles

Charged resonances and MDM bound states at a multi-TeV muon collider

A multi-TeV muon collider proves to be very efficient not only for the search for new heavy neutral particles, but also for the discovery of charged bosons of the W′ type. We find that, by analyzing the associated production with a Standard Model W, charged resonances can be probed directly up to multi-TeV mass values close to the collision energy, and for very small couplings with the SM fermions, of the order of 10−3 − 10−4 times the SM weak coupling. Additionally, charged bound states of WIMP Minimal Dark Matter, specifically a Majorana fermionic 5-plet, can be discovered with low statistics by running above the kinematic threshold, at a center-of-mass energy just slightly above the mass of the MDM bound state. This opens up a very interesting possibility for the discovery of WIMPs, complementary to the search for the resonant production of the neutral MDM bound state component, which relies on an on-peak search. For 5-plet MDM, indeed, the proposed search strategy is more efficient than the WIMP searches based on mono-X, missing-mass and disappearing tracks signatures.

Dynamical symmetry breaking and fermion mass hierarchy in the scale-invariant 3-3-1 model

We propose an extension of the Standard Model (SM) based on the $SU(3)_C\otimes SU(3)_L\otimes U(1)_X$ (3-3-1) gauge symmetry and scale invariance. Maintaining the main features of the so-called 3-3-1 models, such as the cancellation of gauge anomalies related to the number of chiral fermion generations, this model exhibits a very compact scalar sector. Only two scalar triplets and one singlet are necessary and sufficient to break the symmetries dynamically via the Coleman-Weinberg mechanism. With the introduction of an Abelian discrete symmetry and assuming a natural hierarchy among the vacuum expectation values of the neutral scalar fields, we show that all particles in the model can get phenomenologically consistent masses. In particular, most of the standard fermion masses are generated via a seesaw mechanism involving some extra heavy fermions introduced for consistency. This mechanism provides a partial solution for the fermion mass hierarchy problem in the SM. Furthermore, the simplicity of the scalar sector allows us to analytically find the conditions for the potential stability up to one-loop level and show how they can be easily satisfied. Some of the new particles, such as the scalars $H$, $H^\pm$ and all the non-SM vector bosons, are predicted to get masses around the TeV scale and, therefore, could be produced at the high-luminosity LHC. Finally, we show that the model features a residual symmetry which leads to the stability of a heavy neutral particle; the latter is expected to show up in experiments as missing energy.

Freeze-in and freeze-out of dark matter with charged long-lived partners

We present a novel framework capable of addressing the dark matter problem through freeze-in and freeze-out mechanisms, separately or together, depending on the region of the parameter space considered. In the dark matter dynamics, the model features an interplay of thermal production along with sizeable contribution through feeble decay of associated dark fermionic partner, which finally freezes out to the right relic density for a wide range of masses and couplings. Apart from the fermionic dark matter candidate, the model introduces two charged partners, one fermionic and another scalar, which often have delayed decays leading to distinct characteristics of such long-lived particles (LLP) in the colliders like the LHC . Our analysis shows that within the present scenario, LLP of decay length that could be probed at the LHC experiments are compatible with dark matter masses ranging from a few GeV to close to a TeV, as opposed to the requirement of keV-MeV dark matter in simple FIMP scenarios with LLP. In addition, the model presents hitherto unexplored interesting possibilities in the LLP searches, like (i) LLP to LLP to SM cascade decays, which could be searched for within the LHC detectors and (ii) heavy neutral particle decaying within MATHUSLA with two jets and large missing energy. A supplementary aspect of the model is the presence of a heavy neutrino facilitating Type-I seesaw mechanism without disturbing the dark matter side.

Prospects for heavy scalar searches at the LHeC

In this paper, we study the prospects of the proposed Large Hadron electron Collider (LHeC) in the search for heavy neutral scalar particles. We consider a minimal model with one additional complex scalar singlet that interacts with the Standard Model (SM) via mixing with the Higgs doublet, giving rise to an SM-like Higgs boson [Formula: see text] and a heavy scalar particle [Formula: see text]. Both scalar particles are produced via vector boson fusion and can be tested via their decays into pairs of SM particles, analogously to the SM Higgs boson. Using multivariate techniques, we show that the LHeC is sensitive to [Formula: see text] with masses between 200 and 800 GeV down to scalar mixing of [Formula: see text].

Effect of the magnetized medium on the decay of neutral scalar bosons

The decay of a heavy neutral scalar particle into fermions and into charged scalars are analyzed when in the presence of an external magnetic field and finite temperature. Working in the one-loop approximation for the study of these decay channels, it is shown that the magnetic field leads in general to a suppression of the decay width whenever the kinematic constrain depends explicitly on the magnetic field. Our results are also compared with common approximations found in the literature, e.g., when the magnitude of the external magnetic field is smaller than the decaying product particle masses, i.e., in the weak field approximation, and in the opposite case, i.e., in the strong field approximation. Possible applications of our results are discussed.

Semialigned two Higgs doublet model

In the left-right symmetric model based on $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ gauge symmetry, there appear heavy neutral scalar particles mediating quark flavor changing neutral currents (FCNCs) at tree level. We consider a situation where such FCNCs give the only sign of the left-right model while $W_R$ gauge boson is decoupled, and name it "semi-aligned two Higgs doublet model" because the model resembles a two Higgs doublet model with mildly-aligned Yukawa couplings to quarks. We predict a correlation among processes induced by quark FCNCs in the model, and argue that future precise calculation of meson-antimeson mixings and CP violation therein may hint at the semi-aligned two Higgs doublet model and the left-right model behind it.

Search for a Heavy Neutral Particle Decaying to eμ, eτ, or μτ in pp Collisions at sqrt[s]=8 TeV with the ATLAS Detector.

This Letter presents a search for a heavy neutral particle decaying into an opposite-sign different-flavor dilepton pair, e^{±}μ^{∓}, e^{±}τ^{∓}, or μ^{±}τ^{∓} using 20.3 fb^{-1} of pp collision data at sqrt[s]=8 TeV collected by the ATLAS detector at the LHC. The numbers of observed candidate events are compatible with the standard model expectations. Limits are set on the cross section of new phenomena in two scenarios: the production of ν[over ˜]_{τ} in R-parity-violating supersymmetric models and the production of a lepton-flavor-violating Z^{'} vector boson.

Search for long-lived neutral particles decaying to quark-antiquark pairs in proton-proton collisions ats=8 TeV

A search is performed for long-lived massive neutral particles decaying to quark-antiquark pairs. The experimental signature is a distinctive topology of a pair of jets, originating at a secondary vertex. Events were collected with the CMS detector at the CERN LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV. The data analyzed correspond to an integrated luminosity of 18.5 inverse femtobarns. No significant excess is observed above standard model expectations. Upper limits at 95% confidence level are set on the production cross section of a heavy neutral scalar particle, H, in the mass range of 200 to 1000 GeV, decaying promptly into a pair of long-lived neutral X particles in the mass range of 50 to 350 GeV, each in turn decaying into a quark-antiquark pair. For X with mean proper decay lengths of 0.4 to 200 cm, the upper limits are typically 0.5-200 fb. The results are also interpreted in the context of an R-parity-violating supersymmetric model with long-lived neutralinos decaying into a quark-antiquark pair and a muon. For pair production of squarks that promptly decay to neutralinos with mean proper decay lengths of 2-40 cm, the upper limits on the cross section are typically 0.5-3 fb. The above limits are the most stringent on these channels to date.

ΔF = 2 observables and B → X q γ decays in the left-right model: Higgs particles striking back

We present a complete study of ΔS = 2 and ΔB = 2 processes in the left-right model (LRM) based on the weak gauge group SU(2) L × SU(2) R × U(1) B−L . This includes ε K , ΔM K , ΔMq, $ A_{\text{SL}}^q $ , ΔΓ q with q = d, s and the mixing induced CP asymmetries $ {S_{\psi {K_S}}} $ and S ψϕ . Compared to the Standard Model (SM) these observables are affected by tree level contributions from heavy neutral Higgs particles (H 0) as well as new box diagrams with W R gauge boson and charged Higgs (H ±) exchanges. We also analyse the B → X s,d γ decays that receive important new contributions from the W L − W R mixing and H ± exchanges. Compared to the existing literature the novel feature of our analysis is the search for correlations between various observables that could help us to distinguish this model from other extensions of the SM and to obtain an insight into the structure of the mixing matrix V R that governs right-handed currents. Moreover, we perform the full phenomenology including both gauge boson and Higgs boson contributions. We find that even for $ {M_{{H^0}}} \approx {M_{{H^\pm }}} \sim \mathcal{O}\left( {20} \right) $ TeV, the tree level H 0 contributions to ΔF = 2 observables are by far dominant and the H ± contributions to B → X q γ can be very important, even dominant for certain parameters of the model. While in a large fraction of the parameter space this model has to struggle with the experimental constraint from ε K , we demonstrate that there exist regions in parameter space which satisfy all existing ΔF = 2, B → X s,d γ, tree level decays and electroweak precision constraints for scales $ {M_{{W_R}}} $ ≃ 2 − 3 TeV in the reach of the LHC. We also show that the $ {S_{\psi {K_S}}} $ - ε K tension present in the SM can be removed in the LRM. Simultaneously Br(B → X s γ) can be brought closer to the data. However, we point out that with the increased lower bound on $ {M_{{W_R}}} $ , the LRM cannot help in explaining the difference between the inclusive and exclusive determinations of |V ub |, when all constraints are taken into account, unless allowing for large fine-tuning. Finally we present a rather complete list of Feynman rules involving quarks, gauge bosons and Higgs particles.

Search for a heavy neutral particle decaying into an electron and a muon using 1 fb−1 of ATLAS data

AbstractA search is presented for a high mass neutral particle that decays directly to the e±μ∓ final state. The data sample was recorded by the ATLAS detector in $\sqrt{s}=7\mbox{~TeV}$pp collisions at the LHC from March to June 2011 and corresponds to an integrated luminosity of 1.07 fb−1. The data are found to be consistent with the Standard Model background. The high e±μ∓ mass region is used to set 95% confidence level upper limits on the production of two possible new physics processes: tau sneutrinos in an R-parity violating supersymmetric model and Z′-like vector bosons in a lepton flavor violating model.

The Neutron – The Curie Family's Legacy

This article is concerned with the scientific developments that led to the discovery of the neutron by Sir James Chadwick at the Cavendish Laboratory Cambridge in 1932. The Rutherford atom with a heavy nucleus and the problem of the ‘intra-nuclear’ electrons (needed to reconcile nuclear mass and charge) coupled with Marie Curie's discovery of radium as a prime example of natural radioactivity coming from the nucleus were key milestones. Frédéric Joliot and Irène Curie–Joliot almost discovered the neutron in 1931. But the predisposition of the thinking in Chadwick's laboratory allowed conclusive identification of the emission of a heavy neutral particle to be published about a month after the Curie–Joliot experiment. Their Nobel Prize came a few years later with the discovery of artificial radioactivity.

Possible astrophysical signatures of heavy stable neutral relics in supergravity models

We consider heavy stable neutral particles in the context of supergravity and show that a gravitationally suppressed inflaton decay can produce such particles in cosmologically interesting abundances within a wide mass range $10^3 {\rm GeV} \leq m_X \leq 10^{11} {\rm GeV}$. In gravity-mediated supersymmetry breaking models, a heavy particle can decay into its superpartner and a photon-photino pair or a gravitino. Such decays only change the identity of a possible dark matter candidate. However, for $10^3 {\rm GeV} \leq m_X \leq 10^7 {\rm GeV}$, astrophysical bounds from gamma-ray background and photodissociation of light elements can be more stringent than the overclosure bound, thus ruling out the particle as a dark matter candidate.

WIMPs at Last? Or More Wimpy Sightings?

ASTROPHYSICSResearchers from the Dark Matter experiment at Gran Sasso National Laboratory in Italy reported on 25 February that the number of heavy neutral particles entering their underground detector varies slightly with the seasons. The result, they say, proves that the Milky Way galaxy twirls in the midst of a gigantic cloud of Weakly Interacting Massive Particles (WIMPs), the leading candidate for the dark matter thought to account for 90% of the universe's mass. But physicists conducting the Cold Dark Matter Search at Stanford say they see no evidence of the particles.

On Possible Sources of Heavy Neutral Particles in Cosmic Rays**The project supported in part by National Natural Science Foundation of China

We explore some possible mechanisms by which the heavy neutral cosmic particles are accelerated to gain sufficiently high kinetic energies and cause observable effects at detectors on the earth. Our results indicate that all the known sources which are based on the established theories and observations fail to give rise to very high energy with sizable flux intensity. If the heavy neutral particles of kinetic energies of GeV order indeed exist in cosmic rays, there must be certain mechanisms beyond our present knowledge of dark matter and universe evolution.

Towards automatic analytic evaluation of diagrams with masses

A method to calculate two-loop self-energy diagrams of the Standard Model is demonstrated. A direct physical application is the calculation of the two-loop electroweak contribution to the anomalous magnetic moment of the muon 1 2 (g−2) μ . Presently, we confine ourselves to a “toy” model with only μ, γ and a heavy neutral scalar particle (Higgs). The algorithm is implemented as a FORM-based program package. For generating and automatically evaluating any number of two-loop self-energy diagrams, a special C program has been written. This program creates the initial FORM-expression for every diagram generated by QGRAF, executes the corresponding subroutines and sums up the final results.

Heavy Majorana neutrinos in e + e - → W + W -

In this paper we investigate the effects of heavy Majorana neutrinos in the reaction e+e− → W+W−. We consider neutrino masses in the 1–10 TeV region. We show that at LEP II and NLC energies it is possible to use this processes to verify indirect evidence of heavy neutral particles with mixing angles of the order sin2 α = 0.01. We discuss the unitarity restrictions that can be obtained for vector singlet and fermion-mirror-fermion models.

Search for a neutral particle of mass 33.9 MeV in pion decay

We have measured the muon momentum distribution in charged pion decay in flight in order to search for a small branching fraction η of pion decays π + → μ + + X, in which a heavy neutral particle X with a mass of 33.9 MeV would be emitted. The upper limit of η at a confidence level of 95% is found to be 2.6×10 −8.

Bounds on compositeness from neutrinoless double beta decay.

Assuming the existence of a heavy Majorana neutral particle arising from a composite model scenario we discuss the constraints imposed by present experimental limits of half-life neutrinoless double beta decay $(0\nu\beta\beta ) $ measurements on the coupling of the heavy composite neutrinos to the gauge bosons. For neutrino masses $M_N = 1 $ TeV we obtain a rather weak lower bound on the compositeness scale: $\Lambda \ge 0.23 $ TeV.

The behaviour of heavy particles in the expanding plasma jet in argon

This paper concerns the behaviour of heavy particles in an argon plasma expanding from a cascaded arc. The plasma is characterized using high-accuracy spatially resolved Thomson-Rayleigh measurements. It is shown that the expansion of the heavy particles, neutral particles and ions, is close to adiabatic and that three-particle recombination has a small effect on the ion density. The measurements are compared with an adiabatic model and are found to be in good agreement. The behaviour of the neutral particles in the shock front is independent from that of the ions. The neutral particle shock front is identical to the shock front found in neutral gases and is accordingly characterized by the neutral particle Mach number. It is experimentally confirmed that the shape of the shock front is of the Mott-Smith type. The motion of the ions in the shock front is influenced by the presence of the electrons in such a way that the electrons are compressed by the ions. The shape of the ion shock front is of the Mott-Smith type, however, with a different Mach number which includes the electron temperature.

Heavy triplet neutrinos as a new dark-matter option

We propose a new scheme where dark matter is made of heavy stable neutral particles N, possibly even lighter than 1 2 m Z , but uncoupled to the Z 0. Such particles should be given an efficient way to annihilate, otherwise their fossil density would overclose the universe. In this scheme, the annihilation results from a delicate interplay between N and its heavier charged electroweak partner E ±. A closure density may be naturally reached if the mass splitting is ∼10 GeV, which suggests that the E ± may be discovered at LEP 200. The species N results from the mixing between a singlet and the neutral member of a triplet, here induced by the vacuum expectation value of a Higgs triplet. The latter is kept naturally small with respect to the electroweak scale, as a result of a new approximate discrete symmetry, the triplet-parity T p. We discuss various implications of this model for astrophysics and particle physics.