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Tag Archives: particle physics

An article in ScienceNews reports that data from the IceCube experiment under Antarctic ice have shown that the highest energy neutrinos they detect come from all directions, indicating that they are probably at cosmological distances (see Section 12.7c, pp. 324-325). The results were first announced at the American Physical Society’s meeting in April 2014.

Neutrinos open a window into the very distant and high-energy Universe that is extremely difficult to access by any other means. This is because neutrinos, unlike every other subatomic particle, are electrically neutral and rarely interact with matter. By detecting these particles and charting the directions they come from, scientists aim to identify the sources of neutrinos: star-forming galaxies, supermassive black holes or perhaps some as-yet unknown violent objects. These sources can accelerate neutrinos and other subatomic particles to energies far greater than any human-made machine could achieve.

Credit: Sven Lidstrom, IceCube/NSF

IceCube was specifically built to aid in this quest. For three years, strings of sensors stretching as deep as 2.5 kilometers below the surface of an Antarctic glacier have detected subtle flashes of light created when neutrinos and other particles collide with atoms. Last year, IceCube researchers identified 28 high-energy neutrinos from all directions that are almost certainly from outside the Solar System. The researchers have since found nine more, including the highest energy neutrino ever detected.

To complement this painstaking search for the highest energy neutrinos, Christopher Weaver, an IceCube physicist at the University of Wisconsin-Madison, decided to cast a wider net for the larger population of slightly lower-energy astronomical neutrinos. His approach relied on selecting particles that fell from the skies of the Northern Hemisphere, whizzed through Earth’s interior and arrived at IceCube from below. Only neutrinos, and not other particles that often trigger IceCube’s sensors from above, can make it through Earth’s dense crust and core. He also limited his search to detections at a specific energy – about 100 trillion electron volts – so that the number of neutrinos from space wouldn’t be dwarfed by the amount of neutrinos produced in the atmosphere. (IceCube’s sensors can’t distinguish between the two.)

That left Weaver with about 35,000 neutrinos, at least some of which began their journeys beyond the Solar System. He tracked the directions they came from and found no evidence of clustering in any particular parts of the sky – a finding that confirmed previous analyses and suggests that no local source is primarily responsible for the population of neutrinos whizzing by Earth. As IceCube continues to collect more data, scientists hope these two independent neutrino search methods will converge on trends in the neutrinos’ direction of arrival. It’s an exciting time in neutrino astrophysics.

Links: ScienceNews article

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Using data from NASA’s Van Allen Probes, scientists have discovered a massive particle accelerator in the heart of one of the harshest regions of near-Earth space, the super-energetic, charged particles surrounding the globe known as the Van Allen radiation belts.

Credit: NASA/Van Allen Probes/Goddard Space Flight Center

Local bumps of energy kick particles inside the belts to ever-faster speeds, much like a well-timed push on a moving swing. Knowing the location of the acceleration within the radiation belts will help scientists improve predictions of space weather, which can be hazardous to satellites near Earth. The results were published earlier this year in the journal Science.

The twin Van Allen Probes fly straight through this intense area of space. By taking simultaneous measurements with their instruments, the satellites were able to distinguish between two broad possibilities of what accelerates the particles to such amazing speeds, deducing that the particles are undergoing local acceleration, rather than radial acceleration.

The data showed an increase in energy that started right in the middle of the radiation belts and gradually spread both inward and outward, which implies a local acceleration source. The research shows this local energy comes from electromagnetic waves coursing through the belts, tapping energy from other particles residing in the same region of space.

The challenge for scientists now is to determine which waves are at work. The Van Allen Probes, which are designed to measure and distinguish between many types of electromagnetic waves, will tackle this task, too.

Links: NASA press release and Van Allen Probes mission page.

François Englert and Peter W. Higgs have been awarded the 2013 Nobel Prize in Physics “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, at CERN’s Large Hadron Collider.” The announcement by the ATLAS and CMS experiments took place on July 4 last year. (See Figure 19-15, p. 523.)

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Credit: Vince Higgs

The Brout-Englert-Higgs (BEH) mechanism was first proposed in 1964 in two papers published independently, the first by Belgian physicists Robert Brout (now deceased) and François Englert, and the second by British physicist Peter Higgs. Among other things, it explains the mechanism that endows fundamental particles with mass. A third paper by Americans Gerald Guralnik and Carl Hagen with their British colleague Tom Kibble contributed to the development of the new idea, which now forms an essential part of the Standard Model of particle physics. As was pointed out by Higgs, a key prediction of the idea is the existence of a massive particle of a new type, dubbed the Higgs boson, which was discovered by the ATLAS and CMS experiments at CERN in 2012.

The Standard Model describes the fundamental particles from which we, and all the visible matter in the Universe, are made, along with the interactions that govern their behavior. It’s a remarkably successful theory that has been thoroughly tested by experiment over many years. Until last year, the BEH mechanism was the last remaining piece of the model to be experimentally verified. Now that the Higgs has been found, experiments at CERN are eagerly looking for physics “beyond the Standard Model”.

Links: the CERN press release, a Higgs boson poster courtesy of the Institute of Physics; an introductory cartoon explaining the Higgs field, courtesy of the New York Times; and Sean Carroll’s op-ed article, also in the New York Times.