From physicsworld.com: “Optical tomography brings exploding stars into view”

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From physicsworld.com

02 Aug 2019

1
The new optical tomography technique has been used to produce optical emission images of the remnants of supernovae in the Large Magellanic Cloud (Courtesy: PRL/I R Seitenzahl et al.)

Large Magellanic Cloud. Adrian Pingstone December 2003

New updates to the Very Large Telescope (VLT) in Chile have allowed a team of astronomers to detect elusive optical emissions in the remnants of three type Ia supernovae.

ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).
elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,

The international team, led by Ivo Seitenzahl at the University of New South Wales in Canberra, used the improvements to observe Doppler shifts in the spectral lines emitted by highly ionized states of iron and sulphur in the gases.

Formed when white dwarf stars collapse in colossal thermonuclear explosions, type Ia supernovae are known to influence processes including star formation and galaxy evolution. Extensive optical surveys have provided astronomers with huge amounts of data about the events, allowing for reliable theoretical models to explain their formation and evolution.

Among the predictions of these models are that type Ia supernovae must be triggered by white dwarf stars above the Chandrasekhar mass limit, which can be reached by accreting material from companion stars. In addition, the models predict that different elements will be more abundant in different layers of the explosion due to the onion-like structure of the white dwarf’s progenitor star, in which heavier elements reside in layers closer to the core.

Yet despite their successes so far, these models remain plagued with uncertainties due to limitations in previous observations of the events. In the first year of a supernova, for example, its remnants are optically thick, which means that astronomers can only measure the composition of its outermost layers. While X-rays emitted by the superheated fronts of shockwaves in the remnants can reveal their composition to an extent, the limited spectral resolutions of current instruments makes them difficult to detect.

In their study, Seitenzahl’s team exploited a new spectrometer that has recently been added to the VLT to study the remnants of a supernova at visible rather than X-ray wavelengths. The MUSE spectrometer combines high spectral resolution with a wide field-of-view, which makes it possible to acquire spectra at thousands of positions at the same time.

ESO MUSE on the VLT on Yepun (UT4)

The team used the spectrometer to search for visible wavelengths emitted by highly ionized states of iron and sulphur in the slocked, nonradiative remnants of three supernovae in the Large Magellanic Cloud. Though the light is extremely faint due to these transitions being optically forbidden, the VLT’s new setup had high enough spectral resolution to detect the characteristic transmission lines of several different ions.

Seitenzahl and colleagues then used a new technique, dubbed “supernova remnant tomography”, to relate the Doppler shifts of the spectral lines to the velocities of supernova remnants at different positions. This allowed them to test previous models of supernova explosions, and their subsequent evolution, more rigorously than ever before.

Their analysis revealed a clearly layered structure in one of the supernova remnants, with sulphur emission occurring in a region outside of one dominated by iron emissions. The team also found that one supernova appeared to have originated from a white dwarf with a lower mass than the Chandrasekhar limit. Though the dynamics of this event appeared consistent with current models, the observed spectral lines were less Doppler shifted than predicted.

Seitenzahl’s team believe that this new technique represents an important advance in supernova analysis. They now aim to use observed their observed shifts in spectral lines to update current models of supernova formation and evolution.

See the full article here .


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From physicsworld.com: Cosmic gamma-ray energy record shattered by high-altitude observatory

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From physicsworld.com

06 Jul 2019
Hamish Johnston

Cosmic gamma rays with energies as high as 450 TeV (1012 eV) have been observed by the ASgamma observatory in Tibet – which is run jointly by China and Japan. This shatters the previous record of 75 TeV, which was set by the High-Energy-Gamma-Ray Astronomy observatory on the Canary Islands.

ASgamma Observatory in Tibet-China and Japan

ASgamma detected 24 gamma rays with energies in the 100-450 TeV range. The particles appear to originate in the Crab Nebula, which is a supernova remnant about 6000 light-years away. It is home to a pulsar – a rapidly rotating neutron star that broadcasts a bright beam of electromagnetic radiation.

Astronomers believe that gamma rays in the 100-450 TeV range are created when much higher energy electrons in the petaelectronvolt (1015 eV) range interact with the cosmic microwave background – radiation released just after the Big Bang that permeates the universe. These electrons are believed to be accelerated to such high energies by the swirling magnetic fields generated by the pulsar. Indeed, the ASgamma researchers describe the Crab Nebula pulsar as “the most powerful natural electron accelerator known so far in our galaxy”.

Cosmic shower

ASgamma looks for high-energy gamma rays by detecting the shower of secondary particles that rain down on Earth when a cosmic ray interacts with the atmosphere. By analysing the components of a shower, physicists can work out what type of cosmic ray caused the shower (gamma ray or charged particle) and the energy of the cosmic ray. Located at 4300 m above sea level, ASgamma does this using two types of detectors – plastic scintillators on the surface and Cherenkov detectors buried underground.

Now that cosmic gamma rays with energies above 100 TeV have been detected, ASgamma scientists are keen to find other regions where electrons are accelerated to petaelectronvolt energies. This could help solve one of the most important mysteries of astrophysics – what are the origins of the very highest energy cosmic rays?

A paper describing the observations has been accepted for publication in Physical Review Letters.

See the full article here .


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From physicsworld.com: “Our universe has antimatter partner on the other side of the Big Bang, say physicists”

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From physicsworld.com

03 Jan 2019

1
(Courtesy: shutterstock/tomertu)

Our universe could be the mirror image of an antimatter universe extending backwards in time before the Big Bang. So claim physicists in Canada, who have devised a new cosmological model positing the existence of an “antiuniverse” [Physical Review Letters] which, paired to our own, preserves a fundamental rule of physics called CPT symmetry. The researchers still need to work out many details of their theory, but they say it naturally explains the existence of dark matter.

Standard cosmological models tell us that the universe – space, time and mass/energy – exploded into existence some 14 billion years ago and has since expanded and cooled, leading to the progressive formation of subatomic particles, atoms, stars and planets.

However, Neil Turok of the Perimeter Institute for Theoretical Physics in Ontario reckons that these models’ reliance on ad-hoc parameters means they increasingly resemble Ptolemy’s description of the solar system. One such parameter, he says, is the brief period of rapid expansion known as inflation that can account for the universe’s large-scale uniformity. “There is this frame of mind that you explain a new phenomenon by inventing a new particle or field,” he says. “I think that may turn out to be misguided.”

Instead, Turok and his Perimeter Institute colleague Latham Boyle set out to develop a model of the universe that can explain all observable phenomena based only on the known particles and fields. They asked themselves whether there is a natural way to extend the universe beyond the Big Bang – a singularity where general relativity breaks down – and then out the other side. “We found that there was,” he says.

The answer was to assume that the universe as a whole obeys CPT symmetry. This fundamental principle requires that any physical process remains the same if time is reversed, space inverted and particles replaced by antiparticles. Turok says that this is not the case for the universe that we see around us, where time runs forward as space expands, and there’s more matter than antimatter.

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In a CPT-symmetric universe, time would run backwards from the Big Bang and antimatter would dominate (L Boyle/Perimeter Institute of Theoretical Physics)

Instead, says Turok, the entity that respects the symmetry is a universe–antiuniverse pair. The antiuniverse would stretch back in time from the Big Bang, getting bigger as it does so, and would be dominated by antimatter as well as having its spatial properties inverted compared to those in our universe – a situation analogous to the creation of electron–positron pairs in a vacuum, says Turok.

Turok, who also collaborated with Kieran Finn of Manchester University in the UK, acknowledges that the model still needs plenty of work and is likely to have many detractors. Indeed, he says that he and his colleagues “had a protracted discussion” with the referees reviewing the paper for Physical Review Letters [link is above] – where it was eventually published – over the temperature fluctuations in the cosmic microwave background. “They said you have to explain the fluctuations and we said that is a work in progress. Eventually they gave in,” he says.

In very broad terms, Turok says, the fluctuations are due to the quantum-mechanical nature of space–time near the Big Bang singularity. While the far future of our universe and the distant past of the antiuniverse would provide fixed (classical) points, all possible quantum-based permutations would exist in the middle. He and his colleagues counted the instances of each possible configuration of the CPT pair, and from that worked out which is most likely to exist. “It turns out that the most likely universe is one that looks similar to ours,” he says.

Turok adds that quantum uncertainty means that universe and antiuniverse are not exact mirror images of one another – which sidesteps thorny problems such as free will.

But problems aside, Turok says that the new model provides a natural candidate for dark matter. This candidate is an ultra-elusive, very massive particle called a “sterile” neutrino hypothesized to account for the finite (very small) mass of more common left-handed neutrinos. According to Turok, CPT symmetry can be used to work out the abundance of right-handed neutrinos in our universe from first principles. By factoring in the observed density of dark matter, he says that quantity yields a mass for the right-handed neutrino of about 5×108 GeV – some 500 million times the mass of the proton.

Turok describes that mass as “tantalizingly” similar to the one derived from a couple of anomalous radio signals spotted by the Antarctic Impulsive Transient Antenna (ANITA). The balloon-borne experiment, which flies high over Antarctica, generally observes cosmic rays travelling down through the atmosphere. However, on two occasions ANITA appears to have detected particles travelling up through the Earth with masses between 2 and 10×108 GeV. Given that ordinary neutrinos would almost certainly interact before getting that far, Thomas Weiler of Vanderbilt University and colleagues recently proposed that the culprits were instead decaying right-handed neutrinos [Letters in High Energy Physics].

Turok, however, points out a fly in the ointment – which is that the CPT symmetric model requires these neutrinos to be completely stable. But he remains cautiously optimistic. “It is possible to make these particles decay over the age of the universe but that takes a little adjustment of our model,” he says. “So we are still intrigued but I certainly wouldn’t say we are convinced at this stage.”

See the full article here .


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From physicsworld.com: “Cosmic expansion rate remains a mystery despite new measurement”

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From physicsworld.com

21 Nov 2018

1
Galaxy far away: an image taken by the Dark Energy Camera. (Courtesy: Fermilab)

Dark Energy Survey


Dark Energy Camera [DECam], built at FNAL


NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

A new value for the Hubble constant – the expansion rate of the universe — has been calculated by an international group of astrophysicists. The team used primordial distance scales to study more than 200 supernovae observed by telescopes in Chile and Australia. The new result agrees well with previous values of the constant obtained using a specific model of cosmic expansion, while disagreeing with more direct observations from the nearby universe – so exacerbating a long-running disagreement between cosmologists and astronomers.

The Hubble constant is calculated by looking at distant celestial objects and determining how fast they are moving away from Earth. A plot of the speeds of the objects versus their distance from Earth falls on a straight line, the slope of which is the Hubble constant.

Obtaining an object’s speed is straightforward and involves measuring the redshift of the light it emits, but quantifying its distance is much more complicated. Historically, this has been done using a “distance-ladder”, whereby progressively greater length scales are measured by using one type of “standard candle” to calibrate the output of another standard candle. The distance to stars known as Cepheid variables (one type of standard candle) is first established via parallax, and that information is used to calibrate the output of type Ia supernovae (another type of standard candle) located in galaxies containing Cepheids. The apparent brightness of other supernovae can then be used to work out distances to galaxies further away.

Large discrepancy

This approach has been refined over the years and has most recently yielded a Hubble constant of 73.5 ± 1.7 kilometres per second per magaparsec (one megaparsec being 3.25 million light-years). That number, however – obtained by starting close to Earth and moving outwards – is at odds with calculations of the Hubble constant that take the opposite approach — moving inwards from the dawn of time. The baseline in that latter case comes from length scales of temperature fluctuations in the radiation dating back to just after the Big Bang, known as the cosmic microwave background. The cosmic expansion rate at that time is extrapolated to the present day by assuming that the universe’s growth has accelerated under the influence of a particular kind of dark energy. Using the final results from the European Space Agency’s Planck satellite, a very different Hubble constant of 67.4 ± 0.5 is obtained.

ESA/Planck 2009 to 2013

To try to resolve the problem by using an alternative approach, scientists have in recent years created what is known as an “inverse distance ladder”. This also uses the cosmic microwave background as a starting point, but it calculates the expansion rate at a later time – about 10 billion years after the Big Bang – when the density fluctuations imprinted on the background radiation had grown to create clusters of galaxies distributed within “baryon acoustic oscillations”. The oscillations are used to calibrate the distance to supernovae – present in the galaxies – thanks to the fact that the oscillations lead to a characteristic separation between galaxies of 147 megaparsecs.

In the latest work, the Dark Energy Survey collaboration draws on galaxy data from the Sloan Digital Sky Survey as well as 207 newly-studied supernovae captured by the Dark Energy Camera mounted on the 4-metre Víctor M Blanco telescope in Chile. Using spectra obtained mainly at the similarly-sized Anglo-Australian Telescope in New South Wales, the collaboration calculates a value for the Hubble constant of 67.8 ± 1.3 – so agreeing with the Planck value while completely at odds with the conventional distance ladder.


AAO Anglo Australian Telescope near Siding Spring, New South Wales, Australia, Altitude 1,100 m (3,600 ft)

Siding Spring Mountain with Anglo-Australian Telescope dome visible near centre of image at an altitude of 1,165 m (3,822 ft)

Fewer assumptions

“The key thing with these results,“ says team member Ed Macaulay of the University of Portsmouth in the UK, “is that the only physics you need to assume is plasma physics in the early universe. You don’t need to assume anything about dark energy.”

Adam Riess, an astrophysicist at the Space Telescope Science Institute in Baltimore, US who studies the distance-ladder, says that the new work “adds more weight” to the disparity in values of the Hubble constant obtained from the present and early universe.

Cosmic Distance Ladder, skynetblogs


Dark Energy Camera Enables Astronomers a Glimpse at the Cosmic Dawn. CREDIT National Astronomical Observatory of Japan

(Indeed, the distance-ladder itself has gained independent support from expansion rates calculated using gravitational lensing.) He reckons that the similarity between the Planck and Dark Energy Survey results means that redshifts out to z=1 (going back about 8 billion years) are “probably not where the tension develops” and that the physics of the early universe might be responsible instead.

Chuck Bennett of Johns Hopkins University, who led the team on Planck’s predecessor WMAP, agrees. He points to a new model put forward by his Johns Hopkins colleagues Marc Kamionkowski, Vivian Poulin and others that adds extra dark energy to the universe very early on (before rapidly decaying). This model, says Bennett, “proves that it is theoretically possible to find cosmological solutions to the Hubble constant tension”.

Macaulay is more cautious. He acknowledges the difficulty of trying to find an error, reckoning that potential systematic effects in any of the measurements “are about ten times smaller” than the disparity. But he argues that more data are needed before any serious theoretical explanations can be put forward. To that end, he and his colleagues are attempting to analyse a further 2000 supernovae observed by the Dark Energy Camera, although they are doing so without the aid of (costly) spectroscopic analysis. Picking out the right kind of supernovae and then working out their redshift “will be very difficult,” he says, “and not something that has been done with this many supernovae before”.

A preprint describing the research is available on arXiv.

See the full article here .


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From Columbia University via physicsworld.com: “Not Even Wrong”

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From Columbia University

via

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1

August 13, 2018
woit

In recent weeks string theory has been again getting a lot of press attention, because of claims that new progress is being made in the study of the relation of string theory and the real world, via the study of the “swampland”. This is a very old story, and I’ve often written about it here. I just added a new category, so anyone who wants to can go follow it by clicking on the Swampland category of posts.

Recent press coverage of this includes an article by Clara Moskowitz at Scientific American, entitled String Theory May Create Far Fewer Universes Than Thought. This motivated Avi Loeb to write his own Scientific American piece highlighting the dangers of string theory speculation unmoored to any possible experimental test, which appeared as Theoretical Physics is Pointless without Experimental Tests. Loeb reports:

“…There is a funny anecdote related to the content of this commentary. In my concluding remarks at the BHI conference we held at Harvard in May 2018, I recommended boarding a futuristic spacecraft directed at the nearest black hole to experimentally test the validity of string theory near the singularity. Nima Arkani-Hamed commented that he suspects I have an ulterior motive for sending string theorists into a black hole. For the video of this exchange, see

https://www.youtube.com/watch?v=WdFkbsPFQi0 ”

Last week Natalie Wolchover reported on this controversy, with an article that appeared at Quanta magazine as Dark Energy May Be Incompatible With String Theory and at The Atlantic as The Universe as We Understand It May Be Impossible (The Atlantic headline writer misidentifies “we” as “string theorists”).

Wolchover accurately explains part of this story as a conflict between string theorists over whether certain solutions (such as the KKLT solution and the rest of the so-called “string theory landscape”) to string theory really exist. Vafa argues they may not exist, since the proposed solutions are complicated and “Usually in physics, we have simple examples of general phenomena.” In response Eva Silverstein argues:

“…They [Vafa and others] essentially just speculate that those things don’t exist, citing very limited and in some cases highly dubious analyses….”

On Twitter, Jim Baggott explains the problem

“Let’s be clear. This is not a ‘test’ of string theory. There is no ‘evidence’ here. This is yet another conjecture that ‘might be true’, on which there is no consensus in the string theory community.”

and points to an earlier tweet thread of his about this. Sabine Hossenfelder replies with the comment that

“…The landscape itself is already a conjecture build on a conjecture, the latter being strings to begin with. So: conjecture strings, then conjecture the landscape (so you don’t have to admit the theory isn’t unique), then conjecture the swampland because it’s still not working….”

The Simons Center summer workshop this year has been devoted to Recent Developments in the Swampland, videos are here (this was also the case in 2006, see here). Next month in Madrid a conference will be devoted to Vistas over the Swampland, and I’m sure many more such gatherings are planned.

Unfortunately I think the fundamental problem here somehow never gets clearly explained: String theorists don’t actually have a theory, what they have is an approximation to an unknown theory supposed to be valid in certain limits, and a list of properties they would like the unknown theory to have. If this is all you have, there’s no way to distinguish when you’re on dry land (a solution to string theory) from when you’re in the swamp (a non-solution to string theory). Different string theorists can generate different opinions, conjectures and speculations about whether some location is swamp or dry land, but in the absence of an actual theory, no one can tell who is right and who is wrong. I don’t know why Vafa back in 2005 chose “Swampland” as the metaphor for this subject, but it’s an unfortunately apt one: string theorists are stuck in a swamp, with no way of getting out since they can’t tell what’s dry land and what isn’t.

[I do not normally “poach” a blog post, especially wordpress material, but there was no other way to get this out]

See the full article here .

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From physicsworld.com: “Did dark matter have a chilling effect on the early universe?”

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From physicsworld.com

10 Jul 2018
Edwin Cartlidge

1
Early days: artist’s impression of stars forming from primordial hydrogen gas. (Courtesy: E R Fuller/National Science Foundation)

New research lends further support to the idea that a detection of surprisingly strong absorption by primordial hydrogen gas, reported earlier this year, could be evidence of dark matter. The new results, described in three papers in Physical Review Letters, are theoretical and do not settle the issue. Indeed, one group is sceptical of the dark-matter interpretation. But the work heightens interest in ongoing observations of the “cosmic dawn”, with new results from radio telescopes expected within the next year.

According to cosmologists, the hydrogen gas that existed in the very early universe was in thermal equilibrium with the cosmic microwave background (CMB), which meant that the gas would not have been visible either through absorption of the microwave photons or through emission. But at the start of the cosmic dawn about 100 million years after the Big Bang, ultraviolet light from the first stars would have excited the hydrogen atoms and shifted the distribution of electrons within the lower and upper levels of the hyperfine transition. As such, the hydrogen would have started to absorb much more radiation at the transition wavelength (21 cm), which would be seen today as a dip at longer, re-shifted wavelengths in the CMB spectrum.

Dark Energy Camera Enables Astronomers a Glimpse at the Cosmic Dawn

In February, researchers working on the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) telescope reported in Nature that they had seen just such a dip at a wavelength of 380 cm in data from their small ground-based antenna system in Western Australia.

EDGES telescope in a radio quiet zone at the Murchison Radio-astronomy Observatory in Western Australia.

The observation was exciting news, but nevertheless in line with standard cosmological theory. However, the dip was actually twice as deep as expected – immediately leading theorists to speculate that the hydrogen was in fact interacting with particles of dark matter.

“The stakes are high because if the signal is real, this experiment is worth two Nobel prizes,” says Abraham Loeb of Harvard University. “One for being first to detect the 21 cm signal from the cosmic dawn and the second for finding an unexpected level of hydrogen absorption that may be indicative of new physics.”

New or old force?

The idea is that the dark matter would have been colder than the hydrogen atoms and so interactions between the two would have transferred energy from the gas to the dark matter – so cooling the gas and boosting absorption. The possibility of this mechanism being tied to the switching on of the first stars was proposed by Rennan Barkana of Tel Aviv University in Israel, but Barkana suggested that the interaction could involve a new fundamental force between dark and ordinary matter.

However, Loeb and Harvard colleague Julián Muñoz argued that there could be no such force as it would have led to stars cooling more quickly than is observed. Instead, they reckon that the interaction could be that of familiar electromagnetism – requiring that a small fraction of dark matter particles have little mass and carry about a millionth of the charge of the electron.

That view has now won cautious backing from other researchers in the US. By imposing constraints from a wider range of cosmological and astrophysical observations, Asher Berlin of the SLAC National Accelerator Laboratory in California and colleagues have shown in a new paper [Physical Review Letters] that dark matter interactions could indeed explain the EDGES results if up to 2% of dark matter weighs in at less than a tenth the mass of the proton and has a charge less than 0.01% of the electron’s. Berlin and colleagues do, however, add that this scenario would require “additional forces” to subsequently deplete the dark matter so its abundance is in line with observations of the present universe. “Although it’s possible that dark matter could produce the EDGES result, it is not easy or simple to do so,” says Berlin’s colleague Dan Hooper of Fermilab near Chicago.

Extraordinary claims

Loeb acknowledges that “extraordinary claims require extraordinary evidence,” adding that the apparent 21 cm signal from EDGES could be nothing more than instrumental noise or absorption by dust grains in our galaxy. He looks forward to new results from other experiments operating at different sites – including SARAS-2, LEDA, and PRIzM – and expects new data to be available within the next year.

Even if the signal is confirmed, however, dark matter is not necessarily the culprit. Guido D’Amico and colleagues at CERN in Geneva argue in the second new paper [Physical Review Letters] that proponents of the dark-matter interpretation have carried out an “incomplete analysis” by neglecting the heating effect of dark-matter annihilation. In particular, they say that annihilations could inject electrons and low-energy photons into the hydrogen gas, thereby potentially heating the gas more than it is cooled. As such, they conclude, dark-matter annihilations are “strongly constrained” by a 21 cm signal.

In a third new paper [Physical Review Letters], on the other hand, Anastasia Fialkov of the Harvard-Smithsonian Center for Astrophysics in the US and colleagues (including Barkana) show that the dark-matter hypothesis yields an additional prediction that can be tested using different kinds of radio telescope. They have found that the 21 cm signal should vary across the sky by up to 30 times as much as it would do if there were no charged interactions between ordinary and dark matter – and pointing out that this prediction can be tested using low-frequency interferometers.

Muñoz is enthusiastic about these spatial measurements, explaining that they are far more immune to foreground noise and other potential systematic errors than the data collected by EDGES, and are therefore, he says, “more reliable”. He reckons that a couple of interferometers – LOFAR in the Netherlands and HERA in South Africa – might have gathered sufficient data within the next five to ten years to establish definitively whether or not the dip at 21 cm really is due to charged dark matter.

ASTRON LOFAR Radio Antenna Bank, Nethrlands

UC Berkeley Hydrogen Epoch of Reionization Array (HERA), South Africa

See the full article here .


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From physicsworld.com: “Muon antineutrino oscillation spotted by NOvA”

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From physicsworld.com

07 June 2018
Hamish Johnston

FNAL NOvA detector in northern Minnesota

NOvA Far Detector Block

The best evidence yet that muon antineutrinos can change into electron antineutrinos has been found by the NOvA experiment in the US. The measurement involved sending a beam of muon antineutrinos more than 800 km through the Earth from Fermilab near Chicago to a detector in northern Minnesota. After running for about 14 months, NOvA found that at least 13 of the muon antineutrinos had changed type, or “flavour”, during their journey.

The results were presented at the Neutrino 2018 conference, which is being held in Heidelberg, Germany, this week. Although the measurement is still below the threshold required to claim a “discovery”, the result means that fundamental properties of neutrinos and antineutrinos can be compared in detail. This could shed light on important mysteries of physics, such as why there is very little antimatter in the universe.

Neutrinos and antineutrinos come in three flavours: electron, muon and tau. The subatomic particles also exist in three mass states, which means that neutrinos (and antineutrinos) will continuously change flavour (or oscillate). Neutrino oscillation came as a surprise to physicists, who had originally thought that neutrinos have no mass. Indeed, the origins of neutrino mass are not well-understood and a better understanding of neutrino oscillation could point to new physics beyond the Standard Model.
Pion focusing

NOvA has been running for more than three years and comprises two detectors – one located at Fermilab and the other in Minnesota near the border with Canada.

FNAL Near Detector

The muon antineutrinos in the beam are produced at Fermilab’s NuMI facility by firing a beam of protons at a carbon target. This produces pions, which then decay to produce either muon neutrinos or muon antineutrinos – depending upon the charge of the pion. By focusing pions of one charge into a beam, researchers can create a beam of either neutrinos or antineutrinos.

The beam is aimed on a slight downward trajectory so it can travel through the Earth to the detector in Minnesota, which weighs in at 14,000 ton. Electron neutrinos and antineutrinos are detected when they very occasionally collide with an atom in a liquid scintillator, which produces a tiny flash of light. This light is converted into electrical signals by photomultipler tubes and the type of neutrino (or antineutrino) can be worked-out by studying the pattern of signal produced.

The experiment’s first run with antineutrino began in February 2017 and ended in April 2018. The first results were presented this week in Heidelberg by collaboration member Mayly Sanchez of Iowa State University, who reported that a total of 18 electron antineutrinos had been seen by the Minnesota detector. If muon antineutrinos did not oscillate to electron antineutrinos, then only five detections should have been made.
“Strong evidence”

“The result is above 4σ level, which is strong evidence for electron antineutrino appearance,” Sanchez told Physics World, adding that this is the first time that the appearance of electron antineutrinos has been seen in a beam of muon antineutrinos. While this is below the 5σ level normally accepted as a discovery in particle physics, it is much stronger evidence than found by physicists working on the T2K detector in Japan – which last year reported seeing hints of the oscillation.

In 2014-2017 NOvA detected 58 electron neutrinos that have appeared in a muon neutrino beam. This has allowed NOvA physicists to compare the rates at which muon neutrinos and antineutrinos oscillate to their respective electron counterparts. According to Sanchez, the team has seen a small discrepancy that has a statistical significance of just 1.8σ. While this difference is well within the expected measurement uncertainty, if it persists as more data are collected it could point towards new physics.

Sanchez says that NOvA is still running in antineutrino mode and the amount of data taken will double by 2019.

See the full article here .


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