Sky-high observatory sheds light on origin of excess anti-matter

[Judi Lynn]

New study excludes nearby pulsars, points to dark matter as possible culprit
Chris Cesare, Miguel Mostafá, Gail McCormick
November 16, 2017

UNIVERSITY PARK, Pa. — The High-Altitude Water Cherenkov (HAWC) Observatory in Mexico, built and operated by an international team that includes Penn State scientists, has captured the first wide-angle view of very-high-energy light emanating from two rapidly spinning stars. The fresh perspective on these stellar neighbors — which are both close to Earth in cosmic terms — casts serious doubt on one possible origin for a mysterious excess of particles near Earth whose origin has tantalized scientists in recent years.

“Scientists have debated the cause of an unexpectedly high number of positrons — the anti-matter cousins of electrons — near the Earth since a space-borne detector measured the anomaly in 2008,” said Miguel Mostafá, professor of physics and of astronomy and astrophysics and principal investigator of the HAWC group at Penn State. “Some have speculated that the extra positrons have an exotic source, perhaps originating from as-yet undetected processes involving dark matter — the invisible but pervasive substance seen only through its gravitational pull. Others have suggested something more pedestrian: The extra particles might originate from nearby collapsed stars, called pulsars, that spin around several times a second and throw off electrons, positrons and other matter with violent force.”

Now, using new data from the HAWC Gamma-Ray Observatory, an international team of researchers has made the first detailed measurements of two pulsars, Geminga and its unnamed sibling, that had been identified as possible sources of the mysterious excess of positrons. By catching and counting particles of light streaming forth from these nearby stars, the HAWC collaboration has shown that the two pulsars are very unlikely to be the origin of the positron excess despite being the right age and the right distance from Earth to contribute. Positrons from these sources simply have not traveled far enough to reach the Earth in great numbers. The results appear in the Nov. 17 issue of the journal Science.

“After excluding two of the main source candidates, we are closer to understanding the origin of the positron excess,” said Francisco Salesa Greus, lead author of the new paper. A postdoctoral researcher in the HAWC group at Penn State at the time of the research, Salesa Greus is now at the Polish Academy of Sciences Institute of Nuclear Physics in Krakow, Poland.

More:
http://news.psu.edu/story/493719/2017/11/16/research/sky-high-observatory-sheds-light-origin-excess-anti-matter




High-Altitude Water Cherenkov (HAWC) Observatory in Mexico







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[Judi Lynn]

1. Nearby pulsars shed light on the antimatter puzzle

PUBLIC RELEASE: 16-NOV-2017

THE HENRYK NIEWODNICZANSKI INSTITUTE OF NUCLEAR PHYSICS POLISH ACADEMY OF SCIENCES

There are too many high-energy positrons in the cosmic rays reaching the Earth. These positrons (particles that are antimatter equivalents of electrons) could be being produced by pulsars in our vicinity. The most recent measurements from the HAWC Observatory in Mexico have practically excluded this possibility, strengthening the competing and much more exotic hypothesis concerning the origin of the excess positrons.

Our planet is immersed in cosmic rays. The particles reaching the Earth from the depths of the Universe include positrons - antimatter equivalents of electrons. Astrophysicists have long been intrigued by why there are far more high-energy positrons in the cosmic rays than could be expected by current theoretical models. The latest attempt at an answer is the observations made by a team of several dozen researchers from the United States, Mexico, Germany and Poland, conducted using the recently activated High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) detector. The analysis of the measurements of the cosmic ray particles, which has just been published in the prestigious scientific journal Science, included the participation of a research group from the Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN) in Cracow, financed by the Polish National Science Centre OPUS grant.

"We know that high-energy particles of cosmic rays, travelling through our galaxy, quickly dissipate their energy by interacting with other radiation and magnetic fields. This is how particles of primary cosmic rays behave. Positrons are secondary, they come from interactions in which primary radiation is involved. We would therefore expect a similar dependence: a marked decrease in the number of high-energy positrons," explains Prof. Sabrina Casanova (IFJ PAN) and adds: "The reality is different. Satellite and terrestrial observatories record many more high-energy positrons than they should. Our aim was to check whether the source of the positron excess was astronomical objects in our vicinity, such as pulsars and their surrounding nebulae."

The HAWC Observatory is located on the slopes of the Mexican volcano Sierra Negra at an altitude of over 4100 metres above sea level. It houses 300 water tanks, surrounded by detectors sensitive to fleeting flashes of light, known as Cherenkov radiation. This radiation appears in the tank when a particle moving at a speed greater than the speed of light in water falls into it. Each day at HAWC, in this manner, the presence of cosmic gamma photons with energies from 100 gigaelectronvolts (GeV) to 100 teraelectronvolts (TeV) is recorded. These are energies even trillions of times greater than the energy of visible light photons and are over a dozen times greater than the energy of protons in the LHC accelerator. (It is worth noting that throughout the history of cosmic ray measurements, particles with energies of even up to 300 000 000 TeV have been recorded.)

More:
https://www.eurekalert.org/pub_releases/2017-11/thni-nps111617.php

[Judi Lynn]

2. Antimatter Particles Are Crashing into Earth, And Scientists Don't Know Why

By Harrison Tasoff, Space.com Staff Writer | November 18, 2017 07:40am ET

More antimatter particles stream toward Earth than scientists can explain — and new research from a mountaintop observatory in central Mexico deepens the mystery by crossing off one possible source.

The Earth is constantly showered by high-energy particles from a variety of cosmic sources. Physicist Victor Hess used a balloon to provide the first evidence of the extraterrestrial nature of cosmic rays in 1912. Since then, scientists have identified and accounted for a variety of different types, but the origin of some of these particles continues to elude experts.

The recent finding, detailed in the journal Science today (Nov. 17), concerns positrons, the antimatter complements of electrons. High-energy particles, usually protons, traveling across the galaxy can create pairs of positrons and electrons when they interact with dust and gas in space, study co-author Hao Zhou, at Los Alamos National Lab, told Space.com. In 2008, the space-based PAMELA detector measured unexpectedly high numbers of earthbound positrons. This was about 10 times what they were expecting to see, according to Zhou. [Supernova Face-Off May Solve 40-Year-Old Antimatter Mystery]

After years of work, camps coalesced around two distinct explanations, according to a statementby Michigan Technological University, which was involved in the new study. One hypothesis suggests the particles come from nearby pulsars, rapidly spinning cores of burnt-out stars, which can whip particles like electrons and positrons to incredible speeds. The other group posits a more exotic origin for the excess positrons, perhaps involving dark matter, an unknown yet pervasive entity that accounts for 80 percent of the universe's mass.

More:
https://www.livescience.com/60973-antimatter-particles-crashing-into-earth.html?utm_source=notification