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The antiproton (, pronounced p-bar) is the antiparticle of the proton. 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. It was discovered in 1955 by University of California, Berkeley physicists Emilio Segrè and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics. An antiproton consists of two anti-up quarks and one anti-down quark ().

Their formation requires energy equivalent to a temperature of 10 trillion K (10¹³ K), and Big Bangs aside, this does not tend to happen naturally. However, at CERN, protons are accelerated in the Proton Synchrotron (PS) to an energy of 26 GeV, and then smashed into an iridium rod. The protons bounce off the iridium nuclei with enough energy for matter to be created. A range of particles and antiparticles are formed, and the antiprotons are separated off using magnets in vacuum.

In mid-June 2006, CERN succeeded in determining the mass of the antiproton, which they measured at 1836.153674 times more massive than an electron, with uncertainty of +/- 5 at the sixth decimal digit. This is exactly the same as the mass of a "regular" proton, necessitating further research into the nature of difference between matter and anti-matter, in order to explain how our universe survived the Big Bang and why so little remains of antimatter today in our solar system.^{[citation needed]}

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:

p A p p A

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.^[1] This sets upper limits on the number of antiprotons that could be produced in exotic ways, such as from annihilation of supersymmetric dark matter particles in the galaxy or from the evaporation of primordial black holes. This also provides a lower limit on the antiproton lifetime of about 1-10 million years. 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 ^[2]
• 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. CPT symmetry is a basic consequence of quantum field theory and no violations of it have ever been detected.

• BESS: balloon-borne experiment, flown in 1993, 1995, and 1997.
• CAPRICE: balloon-borne experiment, flown in 1994.[1]
• HEAT: balloon-borne experiment, flown in 2000.
• AMS: space-based experiment, prototype flown on the space shuttle in 1998, intended for the International Space Station but not yet launched.
• PAMELA: satellite experiment to detect cosmic rays and antimatter from space, launched June 2006.

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.

• Antimatter
• Antineutron
• Positron
• List of particles