high energy physics (9)

In Medias Res...

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Image source: Link below

Topics: Applied Physics, Astrophysics, Computer Modeling, Einstein, High Energy Physics, Particle Physics, Theoretical Physics

In the search for new physics, a new kind of scientist is bridging the gap between theory and experiment.

Traditionally, many physicists have divided themselves into two tussling camps: the theorists and the experimentalists. Albert Einstein theorized general relativity, and Arthur Eddington observed it in action as “bending” starlight; Murray Gell-Mann and George Zweig thought up the idea of quarks, and Henry Kendall, Richard Taylor, Jerome Freidman and their teams detected them.

In particle physics especially, the divide is stark. Consider the Higgs boson, proposed in 1964 and discovered in 2012. Since then, physicists have sought to scrutinize its properties, but theorists and experimentalists don’t share Higgs data directly, and they’ve spent years arguing over what to share and how to format it. (There’s now some consensus, although the going was rough.)

But there’s a missing player in this dichotomy. Who, exactly, is facilitating the flow of data between theory and experiment?

Traditionally, the experimentalists filled this role, running the machines and looking at the data — but in high-energy physics and many other subfields, there’s too much data for this to be feasible. Researchers can’t just eyeball a few events in the accelerator and come to conclusions; at the Large Hadron Collider, for instance, about a billion particle collisions happen per second, which sensors detect, process, and store in vast computing systems. And it’s not just quantity. All this data is outrageously complex, made more so by simulation.

In other words, these experiments produce more data than anyone could possibly analyze with traditional tools. And those tools are imperfect anyway, requiring researchers to boil down many complex events into just a handful of attributes — say, the number of photons at a given energy. A lot of science gets left out.

In response to this conundrum, a growing movement in high-energy physics and other subfields, like nuclear physics and astrophysics, seeks to analyze data in its full complexity — to let the data speak for itself. Experts in this area are using cutting-edge data science tools to decide which data to keep and which to discard and to sniff out subtle patterns.


Opinion: The Rise of the Data Physicist, Benjamin Nachman, APS News

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Grappling With Waste...

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The underground Onkalo repository in Finland is designed to safely and permanently store hazardous, radioactive waste. Credit: Posiva

Topics: Environment, High Energy Physics, Nuclear Power

Finland and the former Yugoslavia adopted nuclear energy only four years apart. In 1971 Finland began construction of its first nuclear plant, Loviisa, and the first of two planned reactors went into commercial operation in 1977. Yugoslavia started building the Krško plant in 1975. In the 1980s, both countries acknowledged the need for a long-term nuclear waste management strategy and started making plans for permanent disposal repositories.

Fast-forward four decades, and Finland is on the verge of becoming the world’s first country to achieve permanent deep geological disposal for spent nuclear fuel, the highly radioactive waste that contains uranium, plutonium, fission products, and other heavy elements. Meanwhile, the fate of the spent fuel generated at Krško, which is jointly owned by former Yugoslavian republics Croatia and Slovenia, is still very much unknown. Both countries have yet to get a handle on even low-level radioactive waste, including contaminated clothes and water filters, which is slowly overwhelming storage facilities and threatening to halt plant operations.

The US has long struggled to find a final resting place for its nuclear waste, to the point that it is now spending billions of dollars to reimburse plant operators for the costs of storing spent fuel. The dramatically different outcomes of Finland and Croatia’s lengthy searches for permanent nuclear waste solutions are reflections of the varied ways in which this long-standing worldwide problem is being tackled by the nations of the European Union. Whereas Finland, Sweden, and France are expected to open permanent underground spent-fuel repositories by the early 2030s, 12 other nuclear EU countries are far behind, planning to open deep geological disposal facilities sometime between the 2040s and the 2100s. According to a 2019 European Commission report on the implementation of its nuclear waste directive, only a few of those nations have made progress in selecting a site.

European Union nations grapple with nuclear waste storage, Vedrana Simičević, Physics Today.

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An X-ray flash illuminates a molecule. Credit: Raphael Jay

Topics: Chemistry, Climate Change, Green Tech, High Energy Physics, Research, X-rays

The use of short flashes of X-ray light brings scientists one big step closer to developing better catalysts to transform the greenhouse gas methane into a less harmful chemical. The result, published in the journal Science, reveals for the first time how carbon-hydrogen bonds of alkanes break and how the catalyst works in this reaction.

Methane, one of the most potent greenhouse gases, is being released into the atmosphere at an increasing rate by livestock farming and the unfreezing of permafrost. Transforming methane and longer-chain alkanes into less harmful and, in fact, useful chemicals would remove the associated threats and, in turn, make a huge feedstock for the chemical industry available. However, transforming methane necessitates, as a first step, the breaking of a C-H bond, one of the strongest chemical linkages in nature.

Forty years ago, molecular metal catalysts that can easily split C-H bonds were discovered. The only thing found to be necessary was a short flash of visible light to "switch on" the catalyst, and, as by magic, the strong C-H bonds of alkanes passing nearby are easily broken almost without using any energy. Despite the importance of this so-called C-H activation reaction, it remained unknown over the decades how that catalyst performs this function.

The research was led by scientists from Uppsala University in collaboration with the Paul Scherrer Institute in Switzerland, Stockholm University, Hamburg University, and the European XFEL in Germany. For the first time, the scientists were able to directly watch the catalyst at work and reveal how it breaks those C-H bonds.

In two experiments conducted at the Paul Scherrer Institute in Switzerland, the researchers were able to follow the delicate exchange of electrons between a rhodium catalyst and an octane C-H group as it gets broken. Using two of the most powerful sources of X-ray flashes in the world, the X-ray laser SwissFEL and the X-ray synchrotron Swiss Light Source, the reaction could be followed all the way from the beginning to the end. The measurements revealed the initial light-induced activation of the catalyst within 400 femtoseconds (0.0000000000004 seconds) to the final C-H bond breaking after 14 nanoseconds (0.000000014 seconds).

X-rays visualize how one of nature's strongest bonds breaks, Uppsala University, Phys.org.

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Rule Breakers...

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Credit: Matt Harrison Clough (original image at link)

Topics: Entanglement, High Energy Physics, Particle Physics, Quantum Mechanics

Breaking the rules is exciting, especially if they have been held for a long time. This is true not just in life but also in particle physics. Here the rule I’m thinking of is called “lepton flavor universality,” and it is one of the predictions of our Standard Model of particle physics, which describes all the known fundamental particles and their interactions (except for gravity). For several decades after the invention of the Standard Model, particles seemed to obey this rule.

Things started to change in 2004 when the E821 experiment at Brookhaven National Laboratory on Long Island announced its measurement of a property of the muon—a heavy version of the electron—known as its g-factor. The measurement wasn’t what the Standard Model predicted. Muons and electrons are both parts of a class of particles called leptons (along with a third particle, the tau, as well as the three generations of neutrinos). The rule of lepton flavor universality says that because electrons and muons are charged leptons, they should all interact with other particles in the same way (barring small differences related to the Higgs particle). If they don’t, then they violate lepton flavor universality—and the unexpected g-factor measurement suggested that’s just what was happening.

If particles really were breaking this rule, that would be exciting in its own right and also because physicists believe that the Standard Model can’t be the ultimate theory of nature. The theory doesn’t explain why neutrinos have mass, what makes up the invisible dark matter that seems to dominate the cosmos, or why matter won out over antimatter in the early universe. Therefore, the Standard Model must be merely an approximate description that we will need to supplement by adding new particles and interactions. Physicists have proposed a huge number of such extensions, but at most one of these theories can be correct, and so far none of them has received any direct confirmation. A measured violation of the Standard Model would be a flashlight pointing the way toward this higher theory we seek.

Rule-Breaking Particles Pop Up in Experiments around the World, Andreas Crivellin, Scientific American

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Weighty W...

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Living on: data taken by the now-defunct CDF experiment has revealed a surprising mass for the W boson. (Courtesy: Fermilab)

Topics: Fermilab, High Energy Physics, Modern Physics, Particle Physics, Steven Weinberg

The most precise measurement to date of the mass of the W boson has yielded a result seven standard deviations away from that predicted by the Standard Model of particle physics. The stunning result was obtained by a painstaking analysis of data taken at the Fermilab Tevatron collider in the US before it closed in 2011. The particle physics community must now study the results carefully to work out whether it is an incredible statistical fluke, an unknown experimental error, a flaw in the Standard Model, or a genuine indication of physics beyond the Standard Model.

The W boson is one of the most intriguing particles described by the Standard Model. Together with the neutral Z boson, the charged W boson mediates the weak interaction, which causes beta decay and several other important processes in particle physics. The weak interaction has long intrigued scientists searching for physics beyond the Standard Model, partly because it is the only force known to violate charge-parity symmetry. If particles in a process are exchanged for their antiparticles and the spatial coordinates are inverted, the weak interaction in this mirror image process is not always identical. This puzzle is not explained in the Standard Model.

W boson mass measurement surprises physicists, Tim Wogan, Physics World

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DUNE...

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Image Source: Fermilab, and link below

Topics: Fermilab, High Energy Physics, Modern Physics, Neutrinos, Particle Physics

Solving big mysteries

The Deep Underground Neutrino Experiment is an international flagship experiment to unlock the mysteries of neutrinos. DUNE will be installed in the Long-Baseline Neutrino Facility, under construction in the United States. DUNE scientists will paint a clearer picture of the universe and how it works. Their research may even give us the key to understanding why we live in a matter-dominated universe — in other words, why we are here at all.

DUNE will pursue three major science goals: find out whether neutrinos could be the reason the universe is made of matter; look for subatomic phenomena that could help realize Einstein’s dream of the unification of forces; and watch for neutrinos emerging from an exploding star, perhaps witnessing the birth of a neutron star or a black hole.

DUNE at LBNF, Fermilab

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Collider Neutrinos...

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New territory Two candidate collider-neutrino events from the FASERν pilot detector in the plane longitudinal to (top) and transverse to (bottom) the beam direction. The different lines in each event show charged-particle tracks originating from the neutrino interaction point. Credit: FASER Collaboration.

Topics: CERN, High Energy Physics, Particle Physics, Research

Think “neutrino detector” and images of giant installations come to mind, necessary to compensate for the vanishingly small interaction probability of neutrinos with matter. The extreme luminosity of proton-proton collisions at the LHC, however, produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a much more compact apparatus.

In March, the CERN research board approved the Scattering and Neutrino Detector (SND@LHC) for installation in an unused tunnel that links the LHC to the SPS, 480 m downstream from the ATLAS experiment. Designed to detect neutrinos produced in a hitherto unexplored pseudo-rapidity range (7.2 < 𝜂 < 8.6), the experiment will complement and extend the physics reach of the other LHC experiments — in particular FASERν, which was approved last year. Construction of FASERν, which is located in an unused service tunnel on the opposite side of ATLAS along the LHC beamline (covering |𝜂|>9.1), was completed in March, while installation of SND@LHC is about to begin.

Both experiments will be able to detect neutrinos of all types, with SND@LHC positioned off the beamline to detect neutrinos produced at slightly larger angles. Expected to commence data-taking during LHC Run 3 in spring 2022, these latest additions to the LHC experiment family are poised to make the first observations of collider neutrinos while opening new searches for feebly interacting particles and other new physics.

Collider neutrinos on the horizon, Matthew Chalmers, CERN Courier

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Illustration of the FASER experiment. Image Credit: FASER/CERN.

Topics: CERN, Dark Matter, High Energy Physics, Neutrinos, Particle Physics

Neutrinos are ubiquitous and notorious. Billions are passing through you at this moment. Occasionally described as a “ghost of a particle,” neutrinos are nearly massless, thereby making them extremely difficult to detect experimentally (“Neutrino,” meaning “little neutral one” in Italian, was first used by Enrico Fermi in the early 1930s). Neutrinos were first confirmed in 1956 (thanks to a nearby nuclear reactor), and they’ve since been detected from different sources, including the Sun and cosmic rays, but not yet in a particle collider. Their elusiveness has been the source of much intrigue (and, of course, research funding) within the particle physics community since.

What else makes them so curious? Neutrinos come in three flavors — electron neutrino, muon neutrino, and tau neutrino — and may switch between them through the process of oscillation. Neutrino oscillations have been experimentally confirmed only in the past decade at the Super-K Detector in Japan (physicists Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for it). This discovery signified an important direction in the search for physics beyond the Standard Model because the longstanding theory does not explain neutrino oscillations and describes them as completely massless particles. Something isn’t quite adding up.

Enter: FASER. Initially proposed in 2018, the ForwArd Search ExpeRiment (FASER) is CERN’s newest experiment poised to detect neutrinos, potentially up to 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos. Constructed in an unused service tunnel located about 500 meters from an Atlas experiment interaction point, FASER and its corresponding sub-detector, FASERν, have been designed to probe interactions of high-energy neutrinos (predicted to be between 600 GeV and 1 TeV).

FASER Poised to Further Our Understanding of Neutrinos, Dark Matter, Hannah Pell, Physics Central Buzz Blog

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Antistars...

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Astronomers searched for candidate antimatter stars among nearly 6000 gamma-ray sources. After eliminating known objects and sources that lacked the spectral signature of an antistar, 14 possibles remained. (Courtesy: Simon Dupourqué/IRAP)

Topics: Astronomy, Astrophysics, Cosmology, High Energy Physics

Fourteen possible antimatter stars (“antistars”) have been flagged up by astronomers searching for the origin of puzzling amounts of antihelium nuclei detected coming from deep space by the Alpha Magnetic Spectrometer (AMS-02) on the International Space Station.

Three astronomers at the University of Toulouse – Simon Dupourqué, Luigi Tibaldo, and Peter von Ballmoos – found the possible antistars in archive gamma-ray data from NASA’s Fermi Gamma-ray Space Telescope. While antistars are highly speculative, if they are real, then they may be revealed by their production of weak gamma-ray emission peaking at 70 MeV, when particles of normal matter from the interstellar medium fall onto them and are annihilated.

Antihelium-4 was created for the first time in 2011, in particle collisions at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory. At the time, scientists stated that if antihelium-4 were detected coming from space, then it would definitely have to come from the fusion process inside an antistar.

However, when it was announced in 2018 that AMS-02 had tentatively detected eight antihelium nuclei in cosmic rays – six of antihelium-3 and two of antihelium-4 – those unconfirmed detections were initially attributed to cosmic rays colliding with molecules in the interstellar medium and producing the antimatter in the process.

Subsequent analysis by scientists including Vivian Poulin, now at the University of Montpellier, cast doubt on the cosmic-ray origin since the greater the number of nucleons (protons and neutrons) that an antimatter nucleus has, the more difficult it is to form from cosmic ray collisions. Poulin’s group calculated that antihelium-3 is created by cosmic rays at a rate 50 times less than that detected by the AMS, while antihelium-4 is formed at a rate 105 times less.

The mystery of matter and antimatter

The focus has therefore turned back to what at first may seem an improbable explanation – stars made purely from antimatter. According to theory, matter and antimatter should have been created in equal amounts in the Big Bang, and subsequently, all annihilated leaving a universe full of radiation and no matter. Yet since we live in a matter-dominated universe, more matter than antimatter must have been created in the Big Bang – a mystery that physicists have grappled with for decades.

“Most scientists have been persuaded for decades now that the universe is essentially free of antimatter apart from small traces produced in collisions of normal matter,” says Tibaldo.

The possible existence of antistars threatens to turn this on its head. “The definitive discovery of antihelium would be absolutely fundamental,” says Dupourqué.

Are antimatter stars firing bullets of antihelium at Earth? Physics World, published in Physical Review D

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