Singlet Particles Research Articles

Next-to-soft radiation from a different angle

Soft and collinear radiation in collider processes can be described in a universal way, that is independent of the underlying process. Recent years have seen a number of approaches for probing whether radiation beyond the leading soft approximation can also be systematically classified. In this paper, we study a formula that captures the leading next-to-soft QCD radiation affecting processes with both final- and initial-state partons, by shifting the momenta in the nonradiative squared amplitude. We first examine W+jet production, and show that a previously derived formula of this type indeed holds in the case in which massive color singlet particles are present in the final state. Next, we develop a physical understanding of the momentum shifts, showing precisely how they disrupt the well-known angular ordering property of leading soft radiation. Published by the American Physical Society 2024

Primordial black holes and mirror dark matter

If mirror matter exists but cannot comprise all of the dark matter (DM) in the universe, we can expect that the additional DM component may only interact with the other sectors gravitationally. One of the natural candidates of a gravitationally interacting component is a primordial black hole (PBH). Therefore, if mirror matter exists but cannot comprise all of the DM in the universe, the existence of PBH may be expected as a candidate of the additional DM component. In this case, the remaining DM components may be PBHs or [Formula: see text] singlet particles from PBH. We show constraints on PBH with the mirror DM. Particularly, the initial PBH mass is estimated to be [Formula: see text], if the DM comprises mirror baryons and PBHs.

Low scale Dirac leptogenesis and dark matter with observable Delta N_{mathrm{eff}}

We propose a gauged B-L extension of the standard model (SM) where light neutrinos are of Dirac type by virtue of tiny Yukawa couplings. To achieve leptogenesis, we include additional heavy Majorana fermions without introducing any B-L violation by two units. An additional scalar doublet with appropriate B-L charge can allow such heavy fermion coupling with SM leptons so that out-of-equilibrium decay of the former can lead to generation of lepton asymmetry. Due to the B-L gauge interactions of the decaying fermion, the criteria of successful Dirac leptogenesis can also constrain the gauge sector couplings. The same B-L gauge sector parameter space can also be constrained from dark matter requirements if the latter is assumed to consist of SM singlet particles with non-zero B-L charges, which also keep the model anomaly free. The same B-L gauge interactions also lead to additional thermalised relativistic degrees of freedom Delta N_{mathrm{eff}} from light Dirac neutrinos which are tightly constrained by Planck 2018 data. While there exists parameter space from the criteria of successful leptogenesis, dark matter and Delta N_{mathrm{eff}} even after incorporating the latest collider bounds, the currently allowed parameter space can be probed by future measurements of Delta N_{mathrm{eff}}.

Systematically testing singlet models for (g − 2)μ

We comprehensively study all viable new-physics scenarios that resolve the muon (g − 2)μ anomaly with only Standard Model singlet particles coupled to muons via renormalizable interactions. Since such models are only viable in the MeV–TeV mass range and require sizable muon couplings, they predict abundant accelerator production through the same interaction that resolves the anomaly. We find that a combination of fixed-target (NA64μ, M3), B-factory (BABAR, Belle II), and collider (LHC, muon collider) searches can cover nearly all viable singlets scenarios, independently of their decay modes. In particular, future muon collider searches offer the only certain test of singlets above the GeV scale, covering all higher masses up to the TeV-scale unitarity limit for these models. Intriguingly, we find that mathcal{O}left(100 mathrm{GeV}right) muon colliders may yield better coverage for GeV-scale singlets compared to TeV-scale concepts, which has important implications for the starting center-of-mass energy of a staged muon collider program.

Scalar singlet dark matter in non-standard cosmologies

We study production of dark matter (DM) in models with a non-standard expansion history. We consider both freeze-out and freeze-in mechanisms for producing the observed DM abundance in a model where the DM consists of scalar singlet particles coupled to the Standard Model sector via the Higgs portal. We show that a non-standard expansion phase can lead to a significant change in the DM abundance and therefore to observational ramifications. For example, for DM freeze-in the required portal coupling can be much larger, whereas for DM freeze-out much smaller values become allowed. We evaluate the relevant constraints and discuss prospects for direct detection of such DM.

LHC signals for singlet neutrinos from a natural warped seesaw mechanism. I

Recently, it was shown in arXiv:1512.06742 that a straightforward implementation of the type I seesaw mechanism in a warped extra dimensional framework is in reality a {\em natural} realization of "inverse" seesaw, i.e., the Standard Model (SM) neutrino mass is dominantly generated by exchange of pseudo-Dirac {\em TeV}-mass SM singlet neutrinos. By the AdS/CFT correspondence, this scenario is {\em dual} to these singlet particles being composites of some new strong dynamics, along with the SM Higgs boson, with the rest of the SM particles being mostly elementary. We study signals from production of these heavy neutrinos at the Large Hadron Collider (LHC). We focus on the scenario where the strong sector has a global $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry; such a left-right (LR) structure being motivated by consistency with the electroweak (EW) precision tests. The singlet neutrinos are charged under $SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry, thus can be produced from $W^{ \pm }_R$ exchange, as in four-dimensional (4D) LR symmetric models. However, the direct coupling of light quarks to $W^{ \pm }_R$ is negligible, due to $W^{ \pm }_R$ also being composite; nonetheless, a sizable coupling can be induced by mixings among the various types of $W^{ \pm }$ bosons. Furthermore, $W^{ \pm }_R$ decays dominantly into the singlet and {\em composite} partner of charged lepton. This heavy charged lepton, in turn, decays into SM lepton, {\em plus} $Z$/Higgs, thus the latter can be used for extra identification of the signal. For a benchmark scenario with $W^{ \pm }_R$ of mass 2 TeV and singlet neutrino of mass 750 GeV, we find that, in both the di-lepton + di-jet + Higgs and tri-lepton + Higgs channels, significant evidence can be seen at the LHC14 for an integrated luminosity of 300/fb and that even discovery is possible with slightly more luminosity.

Formation and evolution of carbon particles in coflow diffusion air flames of vaporized biodiesel, diesel and biodiesel-diesel blends

Evolution profiles on the formation of the carbon particulate (soot) in vaporized coflow diffusion flames of biodiesel (BD), No. 2 diesel and blended fuel mixtures were obtained. The evolution profiles contain the different stages of soot formation, including soot inception, particle growth and agglomeration, and oxidation. Carbon samples were collected directly from inside the flame’s yellow luminous zone along the axial direction at various heights above the burner (HAB) using a well-accepted approach. The studied BD flames were formed using canola methyl ester (CME), cotton methyl ester (COME) and soy methyl ester (SME) in their neat form (B100). The blends consisted of B80 (80% CME/20% No. 2 diesel), B50 (50% CME/50% No. 2 diesel) and B20 (20% CME/80% No. 2 diesel). The evolution profile of the No. 2 diesel flame was compared to the evolution profiles of the neat (B100) BD and blended fuel flames. Soot evolution profiles in the studied vaporized BD and diesel flames are consistent with the general trends of particle formation present during the combustion of diesel and gaseous fuels. That is, particle inception (singlet particles) present in a region at the lower part of the flame followed by particle growth and agglomeration, and the subsequent soot carbonization and oxidation in the upper regions of the flame. However, the soot evolution profiles also show that significant differences exist in the soot morphological properties of the tested BD and blended flames. The presence of “irregular-shaped” fragments such as the “liquid-like” droplets or “globules” at the different stages of soot formation is evident in these oxygenated flames. The “irregular-shaped” fragments resemble short chain-like aggregates that appear to be formed of fused particles having undefined shapes and boundaries. The “irregular-shaped” fragments resemble eutectic (solid/liquid) phases manifesting their viscous liquid nature. Some of these “irregular-shaped” structures contain embedded carbonized inclusions. By a small axial variation of the flame, the fragments are transformed into fully carbonized aggregates that are significantly larger and of complex fractal morphology (multi-branched). From the transmission electron microscopy analysis it can be suggested that the “liquid-like” droplets or “globule” structures serve as possible growth pathways for the formation of the fully carbonized aggregates composed of spherical particles in the upper part of the flame. The size and number density of the “irregular-shaped” fragments in the vaporized BD are much larger than those present in the flame formed from the vaporized diesel. It is also observed that as the percentage of BD is increased in the blended flames, these “irregular-shaped” structures become more pronounced. The evolution profiles also present other morphological properties of the carbon particulates (particle size and nanostructure) along the flame’s axial direction.

Separations of spherical and disc-shaped polystyrene particles and blood components (red blood cells and platelets) using pinched flow fractionation device with a tilted sidewall and vertical focusing channels (t-PFF-v)

Shape-based separation capabilities of a novel pinched flow fractionation device with a tilted sidewall and vertical focusing channels (t-PFF-v) were demonstrated for (1) spherical and disc-shaped polystyrene (PS) particles; (2) platelets (PLT) and red blood cells (RBC); and (3) singlet, doublet, and triplet clusters of disc-shaped PS particles. The tilted sidewalls and vertical focusing channels of the t-PFF-v device allowed us to perform enhanced separation of non-spherical particles. Using this t-PFF-v device with a Wp (pinched segment width) of 20μm and 15μm, not only spherical (diameter of 2μm) and disc-shaped (thickness of ∼2μm, diameter of ∼5.0μm) PS particles but also PLTs and RBCs from diluted blood were well separated with good separation resolutions (Rdisc,sphere=1.40 and RPLT,RBC=1.28). Additionally, time-lapse images of flowing particles indicated that both disc-shaped PS particles and RBCs are probably having type I orientation in the pinched segment (i.e., leaning against the tilted sidewalls at the pinched segment). Moreover, we observed streamlines of singlet, doublet, and triplet clustered disc-shaped PS particles, which suggested that these clustered particles could be well separated according to their aspect ratio. The doublet or triplet clustered PS particles are probably lay on the floor and rotated at the pinched segment (type II orientation). We believe that this shape-based separation capability achieved by the t-PFF-v device can be utilized for many application areas, such as medical, biological, material and colloidal sciences.

Singlet particles as cold dark matter in θ-exact non-commutative space-time

Singlet particles as cold dark matter in θ-exact non-commutative space-time

Observational constraints on decoupled hidden sectors

We consider an extension of the Standard Model with a singlet sector consisting of a real (pseudo)scalar and a Dirac fermion coupled with the Standard Model only via the scalar portal. We assume that the portal coupling is weak enough for the singlet sector not to thermalize with the Standard Model allowing the production of singlet particles via the freeze-in mechanism. If the singlet sector interacts with itself sufficiently strongly, it may thermalize within itself, resulting in dark matter abundance determined by the freeze-out mechanism operating within the singlet sector. We investigate this scenario in detail. In particular, we show that requiring the absence of inflationary isocurvature fluctuations provides lower bounds on the magnitude of the dark sector self-interactions and in parts of the parameter space favors sufficiently large self-couplings, supported also by the features observed in the small-scale structure formation.

Regeneration of neurite-like cells from induced pluripotent stem cells in self-assembled hyaluronic acid-gelatin microhydrogel

Hybrid microhydrogel comprising hyaluronic acid (HA) and gelatin (Gel) was self-assembled with repeating units and used to culture induced pluripotent stem (iPS) cells for neuronal production. HA and Gel were conjugated with methacrylic anhydride (MA), photocrosslinked to fabricate singlet particles, organized for cell-laden HAMA-GelMA constructs, and employed to differentiate iPS cells. We found that an increase in the microhydrogel concentration from 6% to 10% (w/v) reduced the swelling ratio of self-assembled HAMA-GelMA scaffolds and enhanced the encapsulation efficiency of iPS cells. In addition, an increase in the weight percentage of HAMA promoted the swelling ratio of self-assembled HAMA-GelMA scaffolds and decreased the encapsulation efficiency of iPS cells. Immunochemical staining and flow cytometry demonstrated that the order in the ability to differentiate iPS cells toward neural lineage was HAMA-GelMA construct < construct with CDPGYIGSR < construct with CSRARKQAASIKVAVSADR < construct with the two defined peptides. The current self-assembled HAMA-GelMA constructs with neurite-inductive factor can be of potential to duplicate an engineered tissue from iPS cells to reframe nervous systems.

Particle production with left-right neutrino oscillations

When the Higgs field starts oscillation after Higgs inflation, gauge bosons are produced non-perturbatively near the Enhanced Symmetry Point (ESP). Just after the particle production, when the Higgs field is going away from the ESP, these gauge bosons gain mass and decay or annihilate into Standard Model (SM) fermions. Left-handed neutrinos can be generated in that way. If one assumes the see-saw mechanism, the mass matrix of a pair of left and right-handed neutrinos is non-diagonal. Although their mixing in the mass eigenstates is negligible in the true vacuum, it could be significant near the edge of the Higgs oscillation, where the off-diagonal component is large. Therefore, the left-handed neutrinos generated from the gauge bosons can start neutrino oscillation between the right-handed neutrinos. We study the particle production when such L-R neutrino oscillation is significant. For a working example, the non-thermal leptogenesis scenario after Higgs inflation is examined, which cannot be realized without the L-R neutrino oscillation. The same mechanism could be applied to other singlet particles whose abundance has been neglected.

General Approach to the Evolution of Singlet Nanoparticles from a Rapidly Quenched Point Source

Among the numerous point vapor sources, microsecond-pulsed spark ablation at atmospheric pressure is a versatile and environmentally friendly method for producing ultrapure inorganic nanoparticles ranging from singlets having sizes smaller than 1 nm to larger agglomerated structures. Due to its fast quenching and extremely high supersaturation, coagulational growth already begins at the atomic scale at room temperature. On the basis of this knowledge, we develop a simple semiempirical yet versatile model for predicting the size distribution of singlet particles as a function of the process conditions. The model assumes that a plume of a turbulent aerosol flow flares out from a concentrated point source, eventually reaching the walls of the confinement where a fraction of the particles is deposited. Despite the complexity of the entire process, the concentration and size evolution of particles can be adequately described by a first-order differential equation accounting for coagulation, turbulent dilution,...

Cosmic inflation constrains scalar dark matter

In a theory containing scalar fields, a generic consequence is a formation of scalar condensates during cosmic inflation. The displacement of scalar fields out from their vacuum values sets specific initial conditions for post-inflationary dynamics and may lead to significant observational ramifications. In this work, we investigate how these initial conditions affect the generation of dark matter in the class of portal scenarios where the standard model fields feel new physics only through Higgs-mediated couplings. As a representative example, we will consider a Z2 symmetric scalar singlet s coupled to Higgs via λΦ†Φs2. This simple extension has interesting consequences as the singlet constitutes a dark matter candidate originating from non-thermal production of singlet particles out from a singlet condensate, leading to a novel interplay between inflationary dynamics and dark matter properties.

Non-unitarity of the leptonic mixing matrix: present bounds and future sensitivities

The non-unitarity of the effective leptonic mixing matrix at low energies is a generic signal of extensions of the Standard Model (SM) with extra fermionic singlet particles, i.e. sterile or right-handed neutrinos, to account for the observed neutrino masses. The low energy effects of such extensions can be described in a model-independent way by the Minimal Unitarity Violation (MUV) scheme, an effective field theory extension of the SM. We perform a global fit of the MUV scheme parameters to the present experimental data, which yields the up-to-date constraints on leptonic non-unitarity. Furthermore, we investigate the sensitivities and discovery prospects of future experiments. In particular, FCC-ee/TLEP would be a powerful probe of flavour-conserving non-unitarity for singlet masses up to ~ 60 TeV. Regarding flavour-violating non-unitarity, future experiments on muon-to-electron conversion in nuclei could even probe extensions with singlet masses up to ~ 0.3 PeV.

Finite Packing of Shape-Anisotropic Colloidal Particles

Small aggregates (or clusters) of silica doublet or dumbbell particles were produced inside oil-in-water or water-in-oil emulsions. First, forced aggregation of silica microsphere suspension was induced under a high concentration of ammonia, followed by the addition of tetraethylorthosilicate for bonding and fixation of the particle aggregates. Then, density gradient centrifugation was applied for the fractionation of silica doublets or dumbbell particles. The resultant shape-anisotropic particles, which were re-dispersed in oil phase such as hexane or toluene after surface modification with octadecyltrmethoxysilane, were emulsified and small clusters of silica doublets or dumbbells were obtained by evaporation-driven self-assembly. Most configurations of the doublet clusters are similar to minimal second-moment clusters from singlet particles. However, for a few particular cases of n = 6, 8, and 12, we observed some isomeric structures. In addition, the pear-shaped microparticles as asymmetric dimers were also assembled by self-organization of the shape-anisotropic particles inside aerosol droplets by electrospray-assisted atomization, and the resultant configurations were quite similar to the clusters of simple microspheres.

Two-photon annihilation of singlet cold dark matter due to noncommutative space-time

Detecting the cosmic rays, in particular gamma-ray, coming from the dark matter annihilation or decay is an indirect way to survey the nature of the dark matter. In the commutative space-time, the annihilation of the dark matter candidates (WIMPs) to photons proceeds through loop corrections. However, it is possible for WIMPs as well as the other standard model singlet particles to couple with photons directly in the noncommutative space-time. In this paper, we study two-photon annihilation of singlet WIMPs in the noncommutative space-time. If the noncommutative interactions are relevant to the relic abundance, one can exclude some dark matter masses using Fermi-Lat data.

Large mixing angles from many right-handed neutrinos

A beautiful understanding of the smallness of the neutrino masses may be obtained via the seesaw mechanism, whereby one takes advantage of the key qualitative distinction between the neutrinos and the other fermions: right-handed neutrinos are gauge singlets, and may therefore have large Majorana masses. The standard seesaw mechanism, however, does not address the apparent lack of hierarchy in the neutrino masses compared to the quarks and charged leptons, nor the large leptonic mixing angles compared to the small angles of the CKM matrix. In this paper, we will show that the singlet nature of the right-handed neutrinos may be taken advantage of in one further way in order to solve these remaining problems: Unlike particles with gauge interactions, whose numbers are constrained by anomaly cancellation, the number of gauge singlet particles is essentially undetermined. If large numbers of gauge singlet fermions are present at high energies - as is suggested, for example, by various string constructions - then the effective low energy neutrino mass matrix may be determined as a sum over many distinct Yukawa couplings, with the largest ones being the most important. This can reduce hierarchy, and lead to large mixing angles. Assuming a statistical distribution of fundamental parameters, we will show that this scenario leads to a good fit to low energy phenomenology, with only a few qualitative assumptions guided by the known quark and lepton masses. The scenario leads to predictions of a normal hierarchy for the neutrino masses, and a value for the |m_ee| mass matrix element of about 1-6 meV.

Chromomagnetism in Nuclear Matter

Quarks are color charged particles. Due to their motion there is a strong possibility of generation of color magnetic field. It is shown that however hadrons are color singlet particles they may have non-zero color magnetic moment. Due to this color magnetic moment hadrons can show color interaction. In this paper we have studied the chromomagnetic properties of nuclear matter.

Nanoparticles and the Brain: Cause for Concern?

Engineered nanoparticles (NPs) are in the same size category as atmospheric ultrafine particles, < 100 nm. Per given volume, both have high numbers and surface areas compared to larger particles. The high proportion of surface atoms/molecules can give rise to a greater chemical as well as biological activity, for example the induction of reactive oxygen species in cell-free medium as well as in cells. When inhaled as singlet particles, NPs of different sizes deposit efficiently in all regions of the respiratory tract by diffusion. A major difference to larger size particles is the propensity of NPs to translocate across cell barriers from the portal of entry (e.g., the respiratory tract) to secondary organs and to enter cells by various mechanisms and associate with subcellular structures. This makes NPs uniquely suitable for therapeutic and diagnostic uses, but it also leaves target organs such as the central nervous system (CNS) vulnerable to potential adverse effects (e.g., oxidative stress). Neuronal transport of NPs has been described, involving retrograde and anterograde movement in axons and dendrites as well as perineural translocation. This is of importance for access of inhaled NPs to the CNS via sensory nerves existing in the nasopharyngeal and tracheobronchial regions of the respiratory tract. The neuronal pathway circumvents the very tight blood brain barrier. In general, translocation rates of NP from the portal of entry into the blood compartment or the CNS are very low. Important modifiers of translocation are the physicochemical characteristics of NPs, most notably their size and surface properties, particularly surface chemistry. Primary surface coating (when NPs are manufactured) and secondary surface coating (adsorption of lipids/proteins occurring at the portal of entry and during subsequent translocation) can significantly alter NP biokinetics and their effects. Implications of species differences in respiratory tract anatomy, breathing pattern and brain anatomy for extrapolation to humans of NP effects observed in rodents need to be considered. Although there are anecdotal data indicating a causal relationship between long-term ultrafine particle exposures in ambient air (e.g., traffic related) or at the workplace (e.g., metal fumes) and resultant neurotoxic effects in humans, more studies are needed to test the hypothesis that inhaled nanoparticles cause neurodegenerative effects. Some but probably not the majority of NPs will have a significant toxicity (hazard) potential, and this will pose a significant risk if there is a sufficient exposure. The challenge is to identify such hazardous NPs and take appropriate measures to prevent exposure.