Antiproton
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The '''antiproton''' ({{SubatomicParticle|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 [[annihilation|annihilated]] in a burst of energy. It was discovered in [[1955]] by [[University of California, Berkeley]] [[physicist]]s [[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 ({{SubatomicParticle|link=yes|Up antiquark}}{{SubatomicParticle|link=yes|Up antiquark}}{{SubatomicParticle|link=yes|Down antiquark}}).
{{antimatter}}
{{Infobox Particle
| bgcolour =
| classification = [[Baryon]]
| name = AntiProton
| image = [[Image:Quark structure proton.svg|250px]]
| caption = The quark structure of the proton.
}}
Their formation requires energy equivalent to a temperature of 10 trillion [[Kelvin|K]] (10<sup>13</sup> K), and [[Big Bang]]s aside, this does not tend to happen naturally. However, at [[CERN]], protons are accelerated in the Proton [[Synchrotron]] (PS) to an energy of 26 [[giga|G]][[electron volt|eV]], and then smashed into an [[iridium]] rod. The protons bounce off the iridium nuclei with [[mass-energy equivalence|enough energy for matter to be created]]. A range of particles and [[antiparticle]]s 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 {{val|1836.153674|(5)}} times more massive than an [[electron]]. 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.{{Fact|date=February 2007}}
==Occurrence in nature==
Antiprotons have been detected in [[cosmic ray]]s 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 [[proton]]s with nuclei in the [[interstellar medium]], via the reaction:
{{SubatomicParticle|Proton}} A → {{SubatomicParticle|Proton}} {{SubatomicParticle|Antiproton}} {{SubatomicParticle|Proton}} A <!-- Who wrote this? What is "A"? Should there not be plus signs, as in p + A -> etc...? -->
The secondary antiprotons ({{SubatomicParticle|Antiproton}}) then propagate through the [[galaxy]], confined by the galactic [[magnetic field]]s. 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.<ref>{{cite journal |last=Kennedy |first=Dallas C. |authorlink= |coauthors= |year=2000 |month= |title=Cosmic Ray Antiprotons |journal= |volume= 2806|issue= |pages= 113|id= |url=http://arxiv.org/abs/astro-ph/0003485v2 |accessdate= |quote=|doi=10.1117/12.253971 }}</ref> This sets upper limits on the number of antiprotons that could be produced in exotic ways, such as from annihilation of [[Supersymmetry|supersymmetric]] [[dark matter]] particles in the galaxy or from the [[Hawking radiation|evaporation]] of [[primordial black hole]]s. 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]]: {{val|0.08|u=year}}
* [[Antihydrogen]] [[Penning trap]] of Gabrielse et al: {{val|0.28|u=year}} <ref>{{cite journal |last=Caso |first=C. |authorlink= |coauthors=''et al.'' |year=1998 |month= |title=[[Particle Data Group]] |journal=Eur. Phys. J. |volume=C3 |issue= |pages=613 |id= |url=http://pdg.ihep.su/1999/s041.pdf |accessdate= |quote= }}</ref>
* APEX collaboration at [[Fermilab]]: {{val|50000|u=years}} for {{SubatomicParticle|Antiproton}} → {{SubatomicParticle|link=yes|Muon}} + X<!-- What is "X"?? --> and {{val|300000|u=years}} for {{SubatomicParticle|Antiproton}} → {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Gamma}}
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.
===List of recent antiproton cosmic ray detection experiments===
* [[BESS]]: balloon-borne experiment, flown in 1993, 1995, and 1997.
* CAPRICE: balloon-borne experiment, flown in 1994.<ref>[http://ida1.physik.uni-siegen.de/caprice.html Caprice Experiment<!-- Bot generated title -->]</ref>
* HEAT: balloon-borne experiment, flown in 2000.
* [[Alpha Magnetic Spectrometer|AMS]]: space-based experiment, prototype flown on the [[space shuttle]] in 1998, intended for the [[International Space Station]] but not yet launched.
* [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]]: satellite experiment to detect cosmic rays and antimatter from space, launched June 2006.
==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 [[quark]]s and [[antiquark]]s than would be possible in proton-proton collisions. This is because the [[valence quark]]s in the proton, and the valence antiquarks in the antiproton, [[Parton (particle physics)|tend to carry the largest fraction of the proton or antiproton's momentum]].
== References ==
{{Reflist}}
==See also==
*[[Antimatter]]
*[[Antineutron]]
*[[Positron]]
*[[List of particles]]
{{Particles}}
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[[Category:Nucleons]]
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