Scientists Discover Heaviest Antimatter Ever

Scientists at the Relativistic Heavy Ion Collider (RHIC) have discovered the anti-matter partner of the helium nucleus: antihelium-4.

AsianScientist (Apr. 30, 2011) – Scientists working as part of the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), a particle accelerator used to recreate and study conditions of the early universe at the Brookhaven National Laboratory, have discovered the anti-matter partner of the helium nucleus: antihelium-4.

This new particle, also known as anti-alpha, is the heaviest antimatter nucleus ever detected, breaking the record held by an antimatter particle discovered by the same collaboration just a year ago. The international STAR collaboration, composed of scientists and engineers from 54 institutions in 12 countries, including China, India, and Korea, describe the discovery in a paper in Nature, published online on April 24, 2011.

At the RHIC, gold ions are accelerated to nearly the speed of light and directed into head-on collisions to shatter particles like protons into their constituent quarks and gluons. In these smashups, quarks and anti-quarks have a roughly equal chance of emerging and this is thought to simulate conditions at the Big Bang when matter and anti-matter are understood to have been created in equal amounts. In the instant after the collision, thousands more particles form as the area cools off, and these are the particles that physicists are most interested in.

The STAR detector, which specializes in tracking the thousands of particles produced by each ion collision at the RHIC, gained measurement capabilities vital to antihelium-4 identification with the installation of a tracking device funded by both US and Chinese sources and constructed jointly by US and Chinese institutions in 2009. This detector captured traces of the antihelium-4 particles which existed for only about 10 trillionths of a trillionth of a second before being annihilated when they came into contact with ordinary matter particles.

The probability of two anti-protons and two anti-neutrons coming together to form an antihelium-4 nucleus after an ion collision is so exceedingly small that the STAR scientists had to sift through an enormous amount of data (traces for half a trillion charged particles emitted from almost a billion collisions) before finding 18 examples of the unique signature they were looking for.

This graph plots particle counts by mass, showing ordinary helium nuclei (He-3 and He-4) in orange, and their antimatter counterparts (antihelium-3 and antihelium-4) in blue. The plot illustrates that the newly discovered antimatter nuclei, antihelium-4, are very cleanly separated from the lighter isotopes, and are at the correct mass (Source: Brookhaven National Laboratory).

Physicists do not expect to find anything heavier than antihelium-4 in the near future as the odds of finding the next heavier anti-matter nucleus is predicted to be so vanishingly small that it would be beyond today’s technology.

Steven Vigdor, Brookhaven’s Associate Lab Director for Nuclear and Particle Physics, who leads the RHIC program, said:

“Barring a new breakthrough in accelerator technology, or the discovery of a completely new production mechanism, it is likely that antihelium-4 will remain the heaviest stable antimatter nucleus observed for the foreseeable future.”

The article can be found at: Agakishiev, H et al. (2011) Observation of the antimatter helium-4 nucleus.


Source: Chinese Academy of Sciences, DOE/Brookhaven National Laboratory.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Yew Chung is a postdoctoral research fellow at the Duke-NUS Graduate Medical School, Singapore.

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