Anti-Helium Nucleus: Heaviest Anti-Matter Particle Detected at RHIC, Brookhaven

Exciting news has been tumbling out of the Relativistic Heavy Ion Collider (RHIC), since today morning. Scientists have found the distinct signature of an anti-helium nucleus, the heaviest anti-matter particle detected till date. They can also figure out the production rates and compare them to theoretical values, verifying known calculations. This is big news!

Anti-helium tracks
Note the red line. This is the track made by the anti-helium nucleus.

Smashing Particles

The STAR collaboration at the RHIC, Brookhaven National Laboratories, smashed together extremely fast moving gold nuclei,  producing conditions similar to that of the hot, early Universe. Out of these billions of collisions, trillions of charged particles and anti-particles are produced. The huge data sets are sifted through to identify the details of the particles and anti-particles produced. Generally, anti-matter are stable for long enough to be detected. They eventually collide with matter on the outer margins of the detector and get annihilated.

The Relativistic Heavy Ion Collider

The Data

Sifting through this particular dataset, the STAR team found at least 18-20 distinct signatures’ of anti-helium(IV) nuclei. This is a bound state of two anti-protons and two anti-neutrons, having an overall double negative charge (just opposite to the helium(IV) nucleus, which is made up of two protons and two neutrons with an overall double positive charge). The data clearly shows the anti-He-3 (bound state 2 anti-protons and one anti-neutron) and anti-He-4 (bound state of two anti-protons and two anti-neutrons) peaks.

Anti-Helium Graph
The graph obtained from the dataset. Note the anti-matter peaks (purple) overlapping the matter peaks (orange). Note also the slight difference in the energy at which the matter and anti-matter peaks are located. This slight shift in energy is due to CP violation and is vital to our very existence.

The exciting part

The exciting part is that the rate can also be measured. This rate is then compared with theoretical values. Scientists are ecstatic with the present data as the rates match theoretical predictions extremely well. This also augers well for an experimental project called Alpha Magnetic Spectrometer (AMS), which will be sent to the International Space Station by early May this year. AMS is designed to search for anti-matter in space. This experiment by the STAR team will set the expected rates and provide a good calibration rate for comparisons for AMS. If there is anti-matter concentrated somewhere in the Universe, AMS will catch it. This will go a long way in explaining the asymmetry in the matter-antimatter production rates. (If anti-matter is produced at the same rate as matter, as predicted in theory and observed in the laboratories, why are we surrounded by only matter and not anti-matter?)

Anti Helium Nucleus
The Anti-Helium Nucleus. (Regard the red balls as anti-protons, carrying a unit negative charge and the blue balls being neutral anti-neutrons.)

The STAR collaboration is jubilant about the discovery and reckons that this will be the heaviest anti-particle detected for quite some time. The next heavy nucleus of anti-lithium is 2.25 times heavier and a trillion times rarer, at least theoretically. Finding such a particle is beyond today’s technology.

Published by

Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.