The identity of dark matter is a question of central importance in both astrophysics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically ...predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief summary of the standard model of particle physics and its outstanding problems. I then discuss several dark matter candidates motivated by these problems, including weakly interacting massive particles (WIMPs), superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, I critically examine the particle physics motivations and present their expected production mechanisms, basic properties, and implications for direct and indirect detection, particle colliders, and astrophysical observations. Upcoming experiments will discover or exclude many of these candidates, and progress may open up an era of unprecedented synergy between studies of the largest and smallest observable length scales. PUBLICATION ABSTRACT
In quantum field theory, Lorentz invariance leads to three types of fermion-Dirac, Weyl and Majorana. Although the existence of Weyl and Majorana fermions as elementary particles in high-energy ...physics is debated, all three types of fermion have been proposed to exist as low-energy, long-wavelength quasiparticle excitations in condensed-matter systems. The existence of Dirac and Weyl fermions in condensed-matter systems has been confirmed experimentally, and that of Majorana fermions is supported by various experiments. However, in condensed-matter systems, fermions in crystals are constrained by the symmetries of the 230 crystal space groups rather than by Lorentz invariance, giving rise to the possibility of finding other types of fermionic excitation that have no counterparts in high-energy physics. Here we use angle-resolved photoemission spectroscopy to demonstrate the existence of a triply degenerate point in the electronic structure of crystalline molybdenum phosphide. Quasiparticle excitations near a triply degenerate point are three-component fermions, beyond the conventional Dirac-Weyl-Majorana classification, which attributes Dirac and Weyl fermions to four- and two-fold degenerate points, respectively. We also observe pairs of Weyl points in the bulk electronic structure of the crystal that coexist with the three-component fermions. This material thus represents a platform for studying the interplay between different types of fermions. Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in condensed-matter systems.
Summary
Little is known about the risk factors associated with hepatitis B virus (HBV) intrauterine transmission among HBsAg‐positive mothers. We conducted a study in Taiyuan, China, including 1133 ...HBsAg‐positive mothers and their babies. A total of 101 neonates had HBsAg and/or HBV DNA positive with an intrauterine transmission rate of 8.9%. Maternal menstrual irregularity (OR = 4.95, 95% CI: 1.71, 14.33) and severe nausea during the first trimester (OR = 1.86, 95% CI: 1.11, 3.09) were associated with an increased risk of intrauterine transmission, while caesarean delivery (OR = 0.32, 95% CI: 0.20, 0.51) was associated with a decreased risk after adjusting for potential confounders. Maternal HBeAg positive was a strong independent predictor for intrauterine transmission (OR = 2.56, 95% CI: 1.54, 4.27). A positive association between maternal HBV DNA levels and intrauterine transmission was suggested. Maternal HBIG administration during pregnancy, family history of HBV infection and premature rupture of membranes was not associated with the risk of intrauterine transmission. The study confirmed that maternal HBeAg positive was a risk factor and caesarean delivery was a protective factor for intrauterine transmission. The new findings associated with menstrual irregularity and severe nausea during the first trimester warrant further investigation.
Polycystic ovary syndrome (PCOS) is a common metabolic dysfunction and heterogeneous endocrine disorder in women of reproductive age. Although patients with PCOS are typically characterized by ...increased numbers of oocytes retrieved during IVF, they are often of poor quality, leading to lower fertilization, cleavage and implantation rates, and a higher miscarriage rate.
For this review, we searched the database MEDLINE (1950 to January 2010) and Google for all full texts and/or abstract articles published in English with content related to oocyte maturation and embryo developmental competence.
The search showed that alteration of many factors may directly or indirectly impair the competence of maturating oocytes through endocrine and local paracrine/autocrine actions, resulting in a lower pregnancy rate in patients with PCOS. The extra-ovarian factors identified included gonadotrophins, hyperandrogenemia and hyperinsulinemia, although intra-ovarian factors included members of the epidermal, fibroblast, insulin-like and neurotrophin families of growth factors, as well as the cytokines.
Any abnormality in the extra- and/or intra-ovarian factors may negatively affect the granulosa cell-oocyte interaction, oocyte maturation and potential embryonic developmental competence, contributing to unsuccessful outcomes for patients with PCOS who are undergoing assisted reproduction.
Dark photons in the MeV to GeV mass range are important targets for experimental searches. We consider the case where dark photons A′ decay invisibly to hidden dark matter X through A′→XX. For ...generic masses, proposed accelerator searches are projected to probe the thermal target region of parameter space, where the X particles annihilate through XX→A′→SM in the early universe and freeze out with the correct relic density. However, if mA′≈2mX, dark matter annihilation is resonantly enhanced, shifting the thermal target region to weaker couplings. For ∼10% degeneracies, we find that the annihilation cross section is generically enhanced by 4 (2) orders of magnitude for scalar (pseudo-Dirac) dark matter. For such moderate degeneracies, the thermal target region drops to weak couplings beyond the reach of all proposed accelerator experiments in the scalar case and becomes extremely challenging in the pseudo-Dirac case. Proposed direct detection experiments can probe moderate degeneracies in the scalar case. For greater degeneracies, the effect of the resonance can be even more significant, and both scalar and pseudo-Dirac cases are beyond the reach of all proposed accelerator and direct detection experiments. For scalar dark matter, we find an absolute minimum that sets the ultimate experimental sensitivity required to probe the entire thermal target parameter space, but for pseudo-Dirac fermions, we find no such thermal target floor.
The goal of ForwArd Search ExpeRiment (FASER) at the LHC is to discover light, weakly interacting particles with a small and inexpensive detector placed in the far-forward region of ATLAS or CMS. A ...promising location in an unused service tunnel 480 m downstream of the ATLAS interaction point (IP) has been identified. Previous studies have found that FASER has significant discovery potential for new particles produced at the IP, including dark photons, dark Higgs bosons, and heavy neutral leptons. In this study, we explore a qualitatively different, “beam dump” capability of FASER, in which the new particles are produced not at the IP, but through collisions in detector elements further downstream. In particular, we consider the discovery prospects for axionlike particles (ALPs) that couple to the standard model through the aγγ interaction. TeV-scale photons produced at the IP collide with the TAXN neutral particle absorber 130 m downstream, producing ALPs through the Primakoff process, and the ALPs then decay to two photons in FASER. We show that FASER can discover ALPs with masses ma∼30–400 MeV and couplings gaγγ∼10−6−10−3 GeV−1, and we discuss the ALP signal characteristics and detector requirements.
The unique negative thermal expansion coefficient and remarkable thermal stability of graphene make it an ideal candidate for nanoelectromechanical systems (NEMS) with electrothermal tuning. We ...report on the first experimental demonstration of electrothermally tuned single- and few-layer graphene NEMS resonators operating in the high frequency (HF) and very high frequency (VHF) bands. In single-, bi-, and trilayer (1L, 2L, and 3L) graphene resonators with carefully controlled Joule heating, we have demonstrated remarkably broad frequency tuning up to Δf/f 0 ≈ 310%. Simultaneously, device temperature variations imposed by Joule heating are monitored using Raman spectroscopy; we find that the device temperature increases from 300 K up to 1200 K, which is the highest operating temperature known to date for electromechanical resonators. Using the measured frequency and temperature variations, we further extract both thermal expansion coefficients and thermal conductivities of these devices. Comparison with graphene electrostatic gate tuning indicates that electrothermal tuning is more efficient. The results clearly suggest that the unique negative thermal expansion coefficient of graphene and its excellent tolerance to very high temperature can be exploited for engineering highly tunable and robust graphene transducers for harsh and extreme environments.
The ambient environmental instability and degradation mechanism of single‐ and few‐layer WTe2 are investigated. Oxidation of W and Te atoms appears to be a main reason for degradation. Single‐layer ...samples' Raman signals disappear within 20 min in air. Few‐layer WTe2 exhibits saturating degradation behavior: only the top layer WTe2 is oxidized; the degraded layer can protect inner layers from further degradation.
We propose that dark matter is composed of particles that naturally have the correct thermal relic density, but have neither weak-scale masses nor weak interactions. These models emerge naturally ...from gauge-mediated supersymmetry breaking, where they elegantly solve the dark-matter problem. The framework accommodates single or multiple component dark matter, dark-matter masses from 10 MeV to 10 TeV, and interaction strengths from gravitational to strong. These candidates enhance many direct and indirect signals relative to weakly interacting massive particles and have qualitatively new implications for dark-matter searches and cosmological implications for colliders.
Observation of a prethermal discrete time crystal Kyprianidis, A.; Machado, F.; Morong, W. ...
Science (American Association for the Advancement of Science),
06/2021, Letnik:
372, Številka:
6547
Journal Article
Recenzirano
Prethermal time crystal
Characterizing and understanding different phases of matter in equilibrium is usually associated with the process of thermalization, where the system equilibrates. Recent ...efforts probing nonequilibrium systems have revealed that periodic driving of the system can suppress the natural tendency for equilibration yet still form new, nonequilibrium phases. Kyprianidis
et al.
used a quantum simulator composed of 25 trapped ion qubits and spins to observe such a nonequilibrium phase of matter: the disorder-free prethermal discrete time crystal. The flexibility and tunability of their quantum simulator provide a powerful platform with which to study the exotic phases of matter.
Science
, abg8102, this issue p.
1192
An ion trap quantum simulator was used to observe signatures of a prethermal discrete time crystal.
Extending the framework of statistical physics to the nonequilibrium setting has led to the discovery of previously unidentified phases of matter, often catalyzed by periodic driving. However, preventing the runaway heating that is associated with driving a strongly interacting quantum system remains a challenge in the investigation of these newly discovered phases. In this work, we utilize a trapped-ion quantum simulator to observe the signatures of a nonequilibrium driven phase without disorder—the prethermal discrete time crystal. Here, the heating problem is circumvented not by disorder-induced many-body localization, but rather by high-frequency driving, which leads to an expansive time window where nonequilibrium phases can emerge. Floquet prethermalization is thus presented as a general strategy for creating, stabilizing, and studying intrinsically out-of-equilibrium phases of matter.
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