Goodman and colleagues in their recent paper studied the cosmology and detection of heavy composite dark matter (DM) that internally accelerates nuclei, resulting in copious collisional radiation and nuclear fusion.
The presence of dark matter has become manifest through galactic dynamics, the lensing of light, and temperature fluctuations in the cosmic microwave background. But setting aside these gravitational signifiers, little is known about dark matter despite extensive laboratory and astrophysical efforts. It is a high priority of modern science to uncover dark matter, identify its mass and couplings, and determine what influence it may have on other particles that compose the known universe.
In the past decade theorists have enunciated how a certain variety of dark matter could bear a striking resemblance to known matter. Atoms, nuclei, and nucleons, which comprise the bulk of known particles, are all built from fundamental fermions – electrons, protons, quarks – bound together by photons and gluons into composite states. Similarly, dark matter could also be comprised of many particles bound together in a composite state. One simple composite dark matter model consists of fermions (X) bound by a new attractive force provided by a massive scalar field (ϕ). If this force is strong enough, then in the early universe large dark matter states would be built from successive fusion of X particles into increasingly massive states, in a process similar to big bang nucleosynthesis (BBN). In the absence of the repulsive Coulomb force between protons in Standard Model nuclei, these dark composites can become extremely massive after accumulating oodles of X particles. Now, Goodman and colleagues, performing IceCube and liquid scintillator experiments, showed that, if X has a TeV-EeV mass, this can imply dark matter composite masses ranging from a few micrograms to thousands of tons.
They have found that such large dark matter (DM) composites imply novel dynamical interactions with Standard Model nuclei. In their paper, they presented these newly identified dynamics. Large composite dark matter can cause Standard Model (SM) nuclei to accelerate, radiate, and fuse in the composite interior, as shown in Fig. 1.
These dynamics occur because the scalar field binding X particles together can have an extremely high potential (ϕ) inside the dark matter composite. Under the influence of this potential, SM nuclei are accelerated to energies ∆E ∼ gn (ϕ) ∼ MeV, sufficient to initiate nuclear fusion and radiation from high energy collisions, even for a miniscule Yukawa coupling gn between ϕ and nucleons. This implies new signatures and even potential uses for composite dark matter, including nuclear fusion and bremsstrahlung radiation as unique signatures in particle detectors, nuclear reactions in stars and planets, and speculatively the use of composites as compact fusion reactors. In addition, they found that the largest fusion capable composites would make white dwarfs explode.
“… it would be interesting to study whether fusion-capable composites could detectably alter isotopic abundances in the Earth over geological time periods. We leave these and other inquests into accelerative dark matter to future work.”, concluded authors of the study.
Reference: Javier F. Acevedo, Joseph Bramante, Alan Goodman, “Nuclear Fusion Inside Dark Matter”, ArXiv, pp. 1-8, 2020. https://arxiv.org/abs/2012.10998v1
Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us.
Usually, each cell has exactly one nucleus. But the cells of our skeletal muscles are different: These long, fibrous cells have a comparatively large cytoplasm that contains hundreds of nuclei. But up to now, we have known very little about the extent to which the nuclei of a single muscle fiber differ from each other in terms of their gene activity, and what effect this has on the function of the muscle.
In this single muscle fiber, a multitude of nuclei can be clearly seen. The researchers used DAPI for staining, it stains the DNA in the nuclei blue. Credit: C. Birchmeier Lab, MDC
A team led by Professor Carmen Birchmeier, head of the research group on Developmental Biology/Signal Transduction at the Max Delbrueck Center for Molecular Medicine in the Helmholtz Association (MDC), has now unlocked some of the secrets contained in these muscle cell nuclei. As the researchers report in the journal Nature Communications, the team investigated the gene expression of cell nuclei using a still quite novel technique called single-nucleus RNA sequencing—and in the process, they came across an unexpectedly high variety of genetic activity.
Muscle fibers resemble entire tissues
“Due to the heterogeneity of its nuclei, a single muscle cell can act almost like a tissue, which consists of a variety of very different cell types,” explains Dr. Minchul Kim, a postdoctoral researcher in Birchmeier’s team and one of the two lead authors of the study. “This enables the cell to fulfill its numerous tasks, like communicating with neurons or producing certain muscle proteins.”
Kim undertook the majority of the experimental work in the study, and his data was also evaluated at the MDC. The bioinformatics analyses were performed by Dr. Altuna Akalin, head of the Bioinformatics and Omics Data Science Platform at the MDC’s Berlin Institute of Medical Systems Biology (BIMSB), and Dr. Vedran Franke, a postdoctoral fellow in Akalin’s team and the study’s co-lead author. “It was only thanks to the constant dialogue between the experiment-based and theory-based teams that we were we able to arrive at our results, which offer important insight for research into muscle diseases,” emphasizes Birchmeier. “New techniques in molecular biology such as single cell sequencing create large amounts of data. It is essential that computational labs are part of the process early on as analysis is as important as data generation,” adds Akalin.
The researchers began by studying the gene expression of several thousand nuclei from ordinary muscle fibers of mice, as well as nuclei from muscle fibers that were regenerating after an injury. The team genetically labeled the nuclei and isolated them from the cells. “We wanted to find out whether a difference in gene activity could be observed between the resting and the growing muscle,” says Birchmeier.
And they did indeed find such differences. For example, the researchers observed that the regenerating muscle contained more active genes responsible for triggering muscle growth. “What really astonished us, however, was the fact that, in both muscle fiber types, we found a huge variety of different types of nuclei, each with different patterns of gene activity,” explains Birchmeier.
In this part of a muscle fiber, Rian was stained as well. Rian is a long, non-coding RNA (lncRNA) that is highly expressed in a cluster. This indicates that the nuclei have a function in metabolism of the cell. Credit: C. Birchmeier Lab, MDC
Stumbling across unknown nuclei types
Before the study, it was already known that different genes are active in nuclei located in the vicinity of a site of neuronal innervation than in the other nuclei. “However, we have now discovered many new types of specialized nuclei, all of which have very specific gene expression patterns,” says Kim. Some of these nuclei are located in clusters close to other cells adjacent to the muscle fiber: for example, cells of the tendon or perimysium—a connective tissue sheath that surrounds a bundle of muscle fibers.
“Other specialized nuclei seem to control local metabolism or protein synthesis and are distributed throughout the muscle fiber,” Kim explains. However, it is not yet clear what exactly the active genes in the nuclei do: “We have come across hundreds of genes in previously unknown small groups of nuclei in the muscle fiber that appear to be activated,” reports Birchmeier.
Muscle dystrophy seemingly causes many nuclei types to be lost
In a next step, the team studied the muscle fiber nuclei of mice with Duchenne muscular dystrophy. This disease is the most common form of hereditary muscular dystrophy (muscle wasting) in humans. It is caused by a mutation on the X chromosome, which is why it mainly affects boys. Patients with this disease lack the protein dystrophin, which stabilizes the muscle fibers. This results in the cells gradually dying off.
“In this mouse model, we observed the loss of many types of cell nuclei in the muscle fibers,” reports Birchmeier. Other types were no longer organized into clusters, as the team had previously observed, but scattered throughout the cell. “I couldn’t believe this when I first saw it,” she recounts. “I asked my team to repeat the single-nucleus sequencing immediately before we investigated the finding any further.” But the results remained the same.
The mouse nuclei resemble those of human patients
“We also found some disease-specific nuclear subtypes,” reports Birchmeier. Some of these are nuclei that only transcribe genes to a small extent and are in the process of dying off. Others are nuclei that contain genes that actively repair damaged myofibers. “Interestingly, we also observed this increase in gene activity in muscle biopsies of patients with muscle diseases provided by Professor Simone Spuler’s Myology Lab at the MDC,” says Birchmeier. “It seems this is how the muscle tries to counteract the disease-related damage.”
“With our study, we are presenting a powerful method for investigating pathological mechanisms in the muscle and for testing the success of new therapeutic approaches,” concludes Birchmeier. As muscular malfunction is also observed in a variety of other diseases, such as diabetes and age- or cancer-related muscle atrophy, the approach can be used to better research these changes too. “We are already planning further studies with other disease models,” Kim confirms.
Reference: Minchul Kim et al, Single-nucleus transcriptomics reveals functional compartmentalization in syncytial skeletal muscle cells, Nature Communications (2020). DOI: 10.1038/s41467-020-20064-9
Gaining a better understanding of the limiting factors for the existence of stable, superheavy elements is a decade-old quest of chemistry and physics. Superheavy elements, as are called the chemical elements with atomic numbers greater than 103, do not occur in nature and are produced artificially with particle accelerators. They vanish within seconds. A team of scientists from GSI Helmholtzzentrum fuer Schwerionenforschung Darmstadt, Johannes Gutenberg University Mainz (JGU), Helmholtz Institute Mainz (HIM) and the University of Jyvaeskylae, Finland, led by Dr. Jadambaa Khuyagbaatar from GSI and HIM, has provided new insights into the fission processes in those exotic nuclei and for this, has produced the hitherto unknown nucleus mendelevium-244. The experiments were part of “FAIR Phase 0”, the first stage of the FAIR experimental program. The results have now been published in the journal Physical Review Letters.
Heavy and superheavy nuclei are increasingly unstable against the fission process, in which the nucleus splits into two lighter fragments. This is due to the ever-stronger Coulomb repulsion between the large number of positively charged protons in such nuclei, and is one of the main limitations for the existence of stable superheavy nuclei.
The nuclear fission process was discovered more than 80 years ago and is being studied intensely to this day. Most experimental data on the spontaneous fission are for nuclei with even numbers of protons and neutrons – called “even-even nuclei”. Even-even nuclei consist entirely of proton and neutron pairs and their fission properties are rather well describable by theoretical models. In nuclei with an odd number of either neutrons or protons, a hindrance of the fission process when compared to the properties of even-even nuclei has been observed and traced back to the influence of such a single, unpaired constituent in the nucleus.
However, the fission hindrance in “odd-odd nuclei”, containing both, an odd number of protons and an odd number of neutrons, is less well known. Available experimental data indicate that the spontaneous fission process in such nuclei is greatly hindered, even more so than in nuclei with only one odd-numbered type of constituents.
Once the fission probability is most reduced, other radioactive decay modes like alpha decay or beta decay become probable. In beta decay, one proton transforms into a neutron (or vice versa) and, accordingly, odd-odd nuclei turn into even-even nuclei, which typically have a high fission probability. Accordingly, if a fission activity is observed in experiments on the production of an odd-odd nucleus, it is often difficult to identify whether fission occurred in the odd-odd nucleus, or not rather started from the even-even beta-decay daughter, which can then undergo beta-delayed fission. Recently, Dr. Jadambaa Khuyagbaatar from GSI and HIM predicted that this beta-delayed fission process may be very relevant for the heaviest nuclei and – in fact – may be one of the main decay modes of beta-decaying superheavy nuclei.
In superheavy nuclei, which are exceedingly difficult to be produced experimentally, beta-decay has not yet been observed conclusively. For instance, in the case of the heaviest element produced at GSI Darmstadt, tennessine (element 117), only two atoms of the odd-odd nucleus tennessine-294 were observed in an experiment that lasted about one month. This small production rates limit the verification and detailed study of the beta-decay delayed fission process. Still, new experimental data to shed light on this process are best gained in exotic nuclei, like those which have an extremely unbalanced ratio of protons to neutrons. For this, the team from GSI, JGU, HIM and University of Jyväskylä has produced the hitherto unknown nucleus mendelevium-244, an odd-odd nucleus consisting of 101 protons and 143 neutrons.
The theoretical estimate suggests that beta decay of this nucleus will be followed by fission in about one out of five cases. Due to the large energy release of the fission process, this can be detected with high sensitivity, whereas beta decays are more difficult to measure. The researchers used an intense beam of titanium-50 available at GSI’s UNILAC accelerator to irradiate a gold target. The reaction products of titanium and gold nuclei were separated in the Transactinide Separator and Chemistry TASCA, which guided mendelevium nuclei into a silicon detector suitable to register the implantation of the nuclei as well as their subsequent decay.
A first part of the studies, performed in 2018, led to the observation of seven atoms of mendelevium-244. In 2020, the researchers used a lower titanium-50 beam energy, which is insufficient to lead to mendelevium-244 production. Indeed, signals like those assigned to mendelevium-244 in the 2018 study were absent in this part of the data set, corroborating the proper assignment of the 2018 data and confirming the discovery of the new isotope.
All of the seven registered atomic nuclei underwent alpha decay, i.e., the emission of a helium-4 nucleus, which led to the daughter isotope einsteinium-240, discovered four years ago by a preceding experiment carried out at the University of Jyväskylä. Beta decay was not observed, which allows establishing an upper limit on this decay mode of 14 percent. If the 20 percent fission probability of all beta-decaying nuclei were correct, the total probability for beta delayed fission would be at most 2.8 percent and its observation would necessitate the production of substantially more mendelevium-244 atoms than in this discovery experiment.
In addition to the alpha-decaying mendelevium-244, the researchers found signals of short-lived fission events with unexpected characteristics concerning their number, production probability, and half-life. Their origin cannot currently be pinpointed exactly, and is in fact not readily explicable with current knowledge of the production and decay of isotopes in the region of mendelevium-244. This motivates follow-up studies to get more detailed data, which will help shed further light on the fission process in odd-odd nuclei.
References: J. Khuyagbaatar, H. M. Albers, M. Block, H. Brand, R. A. Cantemir, A. Di Nitto, Ch. E. Düllmann, M. Götz, S. Götz, F. P. Heßberger, E. Jäger, B. Kindler, J. V. Kratz, J. Krier, N. Kurz, B. Lommel, L. Lens, A. Mistry, B. Schausten, J. Uusitalo, and A. Yakushev, “Search for Electron-Capture Delayed Fission in the New Isotope 244Md”, Phys. Rev. Lett. 125, 142504 – Published 1 October 2020. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.142504
Researchers from MIPT have developed a prototype detector of solar particles. The device is capable of picking up protons at kinetic energies between 10 and 100 megaelectronvolts, and electrons at 1-10 MeV. This covers most of the high-energy particle flux coming from the sun. The new detector can improve radiation protection for astronauts and spaceships, as well as advancing our understanding of solar flares. The research findings are reported in the Journal of Instrumentation.
As energy gets converted from one form to another in the active regions of the solar atmosphere, streams of particles — or cosmic rays — are born with energies roughly between 0.01-1,000 MeV. Most of these particles are electrons and protons, but nuclei from helium to iron are also observed, albeit in far smaller numbers.
The current consensus is that the particle flux has two principal components. First, there are the narrow streams of electrons in brief flares lasting from tens of minutes to several hours. And then there are the flares with broad shockwaves, which last up to several days and mostly contain protons, with some occasional heavier nuclei.
Despite the vast arrays of data supplied by solar orbiters, some fundamental questions remain unresolved. Scientists do not yet understand the specific mechanisms behind particle acceleration in the shorter- and longer-duration solar flares. It is also unclear what the role of magnetic reconnection is for particles as they accelerate and leave the solar corona, or how and where the initial particle populations originate before accelerating on impact waves. To answer these questions, researchers require particle detectors of a novel type, which would also underlie new spaceship security protocols that would recognize the initial wave of electrons as an early warning of the impending proton radiation hazard.
A recent study by a team of physicists from MIPT and elsewhere reports the creation of a prototype detector of high-energy particles. The device consists of multiple polystyrene disks, connected to photodetectors. As a particle passes through polystyrene, it loses some of its kinetic energy and emits light, which is registered by a silicon photodetector as a signal for subsequent computer analysis.
The project’s principal investigator Alexander Nozik from the Nuclear Physics Methods Laboratory at MIPT said: “The concept of plastic scintillation detectors is not new, and such devices are ubiquitous in Earth-based experiments. What enabled the notable results we achieved is using a segmented detector along with our own mathematical reconstruction methods.”
Part of the paper in the Journal of Instrumentation deals with optimizing the detector segment geometry. The dilemma is that while larger disks mean more particles analyzed at any given time, this comes at the cost of instrument weight, making its delivery into orbit more expensive. Disk resolution also drops as the diameter increases. As for the thickness, thinner disks determine proton and electron energies with more precision, yet a large number of thin disks also necessitates more photodetectors and bulkier electronics.
The team relied on computer modeling to optimize the parameters of the device, eventually assembling a prototype that is small enough to be delivered into space. The cylinder-shaped device has a diameter of 3 centimeters and is 8 centimeters tall. The detector consists of 20 separate polystyrene disks, enabling an acceptable accuracy of over 5%. The sensor has two modes of operation: It registers single particles in a flux that does not exceed 100,000 particles per second, switching to an integrated mode under more intense radiation. The second mode makes use of a special technique for analyzing particle distribution data, which was developed by the authors of the study and does not require much computing power.
“Our device has performed really well in lab tests,” said study co-author Egor Stadnichuk of the MIPT Nuclear Physics Methods Laboratory. “The next step is developing new electronics that would be suitable for detector operation in space. We are also going to adapt the detector’s configuration to the constraints imposed by the spaceship. That means making the device smaller and lighter, and incorporating lateral shielding. There are also plans to introduce a finer segmentation of the detector. This would enable precise measurements of electron spectra at about 1 MeV.”
UR 