Antiproton
Written by Jean Tate
This proton-antiproton collision, recorded by the DZero collaboration, is among the single top quark candidate events. The top quark decayed and produced a bottom quark jet (b jet), a muon and a neutrino. Credit: DZero collaboration.
The nucleus of an atom consists of protons and neutrons (except if it's ordinary hydrogen, that has just one proton); the nucleus of an anti-atom consists of antiprotons and antineutrons (except if it's 'ordinary' anti-hydrogen). Yep, that's right, the antiproton is the antimatter (or anti-matter) counterpart of the proton.
The proton has a positive charge, so the antiproton has a negative one; the proton is composed of two up quarks and one down quark, so the antiproton is composed of two anti-up quarks and one anti-down one; BUT the antiproton and proton have the same spin and isospin (1/2), and the same mass (938 MeV/c2).
Who got the Nobel Prize for Physics in 1959? Emilio Gino Segrè and Owen Chamberlain. And what did they get it for? Discovering the antiproton! In 1955.
The mass of the antiproton has been measured to ten significant figures, and it's the same as the mass of the proton. This is important, because it puts constraints on theories of why the universe we observe is made of matter (and not equal parts of matter and antimatter).
Antiprotons have been used in colliders (they collide with protons) to produce single top quarks (rather than top-antitop pairs), and many other things too (they may turn out to be more effective in treating some cancers than protons).
Antiprotons are one kind of cosmic ray particle. There's a lot of interest in studying them, because they might tell us about the nature of dark matter (some kinds of hypothetical dark matter particles might decay into antiprotons, and when primordial black holes evaporate – if they exist – they might produce antiprotons). So far, antiproton cosmic ray research points to all the antiproton cosmic rays we observe being produced by collisions of high energy cosmic ray protons with the nuclei of atoms (or ions) in the interstellar medium. AMS (Alpha Magnetic Spectrometer experiment) would continue this research, but it has yet to fly (even though it's been ready for quite a while).
Searching for Antimatter in Antarctica, Antimatter, and Repaired too Late? Tevatron May Beat LHC in Hunt for Higgs Boson; just three Universe Today articles with more on antiprotons.
More from Astronomy Cast: Antimatter.
Filed under: Astronomy
Tags: antimatter, cosmic rays, Physics
