antiproton - Occurrence in Nature, Uses

The antiparticle partner of the proton; symbol ?. Spin and mass are as for the proton, but the charge is ?1. Discovered in 1955, antiprotons are created in particle accelerators by the collisions of protons with nuclei, and can in turn be accelerated and used in particle physics experiments.

Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy.

In mid-June 2006, CERN succeeded in determining the mass of antiproton, which they measured at 1836.153674 times more massive than an electron, with uncertainty of +/- 5 at the sixth decimal digit.

Occurrence in Nature

Antiprotons have been detected in cosmic rays for over 25 years, first by balloon-borne experiments and more recently by satellite-based detectors. The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction:

The secondary antiprotons () then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium, and antiprotons can also be lost by "leaking out" of the galaxy.

The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions. Since the galactic storage time of antiprotons is about 10 million years, an intrinsic decay lifetime would modify the galactic residence time and distort the spectrum of cosmic ray antiprotons. This is significantly more stringent than the best laboratory measurements of the antiproton lifetime:

LEAR collaboration at CERN: 0.08 year Antihydrogen Penning trap of Gabrielse et al: 0.28 year APEX collaboration at Fermilab: 50,000 years for and 300,000 years for

The properties of the antiproton are predicted by CPT symmetry to be exactly related to those of the proton. In particular, CPT symmetry predicts the mass and lifetime of the antiproton to be the same as those of the proton, and the electric charge and magnetic moment of the antiproton to be opposite in sign and equal in magnitude to those of the proton.

List of recent antiproton cosmic ray detection experiments

BESS: balloon-borne experiment, flown in 1993, 1995, and 1997.

Uses

Antiprotons are routinely produced at Fermilab for collider physics operations in the Tevatron, where they are collided with protons. The use of antiprotons allows for a higher average energy of collisions between quarks and antiquarks than would be possible in proton-proton collisions. This is because the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the largest fraction of the proton or antiproton's momentum.