Apr 26, 2011 14:59 GMT  ·  By
Antihelium-4 nuclei were discovered at RHIC thanks to a detector built at the Rice University
   Antihelium-4 nuclei were discovered at RHIC thanks to a detector built at the Rice University

A collaboration of physicists in the United States announces the discovery of antihelium-4, the heaviest antimatter nucleus ever discovered. The discovery, which was made last month, was possible only though the use of a detector built at the Rice University.

Together with their collaborators, researchers here were able to positively identify this antimatter nucleus out of a myriad of false results and leads. The sensitive instrument that made this a reality was developed at the Rice Bonner Lab.

Chinese investigators contributed some $2.5 million to the $7.5 million detector. The international collaboration was led by Rice physicists, and the device was then installed, and carried out studies, at the US Department of Energy (DOE) Brookhaven National Laboratory (BNL).

The detector was put inside the BNL Relativistic Heavy Ion Collider (RHIC), and attached to the STAR experiment. This position enabled it to carry out its observations from an amazing vantage point.

Additional details of the new work were published in the April 25 issue of the top journal Nature.

Experts explain that the anti-helium 4 particles, also called anti-alpha, do not exhibit radioactive decay. They are made up of 2 antiprotons and 2 antineutrons, and some 15 of them were discovered by the Rice time-of-flight particle detector.

Observations were carried out as the RHIC was colliding gold atoms head-on at nearly the speed of light. This also led to the formation of quark gluon plasma (QGP), a state of matter that astrophysicists believe permeated the Universe fractions of a second after the Big Bang.

Antihelium-4 particles were found in a cloud of hadron gas, that formed in the collision area right after the QGP cooled down. Despite trillions of particles being formed here, only 15 such particles were identified using the new detector.

“These are massive clumps of antimatter. The fact that two antineutrons and two antiprotons find each other, produce an anti-alpha, travel two to three meters of air and give us a measurable signal when they pass through the time-of-flight detector is astounding,” says scientist Frank Geurts.

The expert holds an appointment as a Rice assistant professor of physics and astronomy, and he is also one of the lead authors of the Nature paper.

Antihelium-3, the predecessor of the newly-found particles, was obtained in 1970, and it took more than 40 years to produce antihelium-4. RHIC scientists explain that it will take some time until the next antihelium is found. Current collider technology is insufficient to produce it, they add.

The BNL team is also happy to report that the production of this type of antimatter occurred exactly in the steps predicted by the standard theory of thermodynamics, and in accordance to the predictions made by the Standard Model of particle physics.

The work also indicates that antihelium-4 will never occur naturally, so there is no reason to waste resources looking for it. But there are also other implications that the study highlights.

“Any observation of antihelium or even heavier antinuclei in space would indicate the existence of a large amount of antimatter elsewhere in the Universe,” a STAR Collaboration press release says.