 Thequark content of the antiproton. | |
| Classification | Antibaryon |
|---|---|
| Composition | 2up antiquarks, 1down antiquark |
| Statistics | Fermionic |
| Family | Hadron |
| Interactions | Strong,weak,electromagnetic,gravity |
| Symbol | p |
| Antiparticle | Proton |
| Theorised | Paul Dirac (1933) |
| Discovered | Emilio Segrè &Owen Chamberlain (1955) |
| Mass | 1.67262192595(52)×10−27 kg[1] 938.27208943(29) MeV/c2[2] |
| Mean lifetime | the same as that of the proton |
| Electric charge | −1 e |
| Magnetic moment | −2.7928473441(42) μN[3] |
| Spin | 1⁄2 |
| Isospin | −1⁄2 |
| Antimatter |
|---|
 |
Theantiproton,p, (pronouncedp-bar) is theantiparticle of theproton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to beannihilated in a burst of energy.
The existence of the antiproton with electric charge of−1 e, opposite to the electric charge of+1 e of the proton, was predicted byPaul Dirac in his 1933 Nobel Prize lecture.[4] Dirac received the Nobel Prize for his 1928 publication of hisDirac equation that predicted the existence of positive and negative solutions toEinstein's energy equation () and the existence of thepositron, theantimatter analog of theelectron, with oppositecharge andspin.
The antiproton was first experimentally confirmed in 1955 at theBevatron particle accelerator byUniversity of California, BerkeleyphysicistsEmilio Segrè andOwen Chamberlain, for which they were awarded the 1959Nobel Prize in Physics.
In terms ofvalence quarks, an antiproton consists of twoup antiquarks and onedown antiquark (uud). The properties of the antiproton that have been measured all match the corresponding properties of the proton, with the exception that the antiproton has electric charge and magnetic moment that are the opposites of those in the proton, which is to be expected from the antimatter equivalent of a proton. The questions of how matter is different from antimatter, and the relevance of antimatter in explaining how our universe survived theBig Bang, remain open problems, in part, due to the relative scarcity of antimatter in today's universe.
Antiprotons have been detected incosmic rays beginning in 1979, 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 withatomic nuclei in theinterstellar medium, via the reaction, where A represents a nucleus:
p + A →p +p +p + A
The secondary antiprotons (p) then propagate through thegalaxy, confined by the galacticmagnetic 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.[5]
The antiproton cosmic rayenergy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[5] These experimental measurements set upper limits on the number of antiprotons that could be produced in exotic ways, such as from annihilation ofsupersymmetricdark matter particles in the galaxy or from theHawking radiation caused by the evaporation ofprimordial 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:
The magnitude of properties of the antiproton are predicted byCPT 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 ofquantum field theory and no violations of it have ever been detected.
Antiprotons were routinely produced at Fermilab for collider physics operations in theTevatron, where they were 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 largestfraction of the proton or antiproton's momentum.
Formation of antiprotons requires energy equivalent to a temperature of 10 trillionK (1013 K), and this does not tend to happen naturally. However, at CERN, protons are accelerated in theProton Synchrotron to an energy of 26GeV and then smashed into aniridium rod. The protons bounce off the iridium nuclei withenough energy for matter to be created. A range of particles and antiparticles are formed, and the antiprotons are separated off using magnets invacuum.
In July 2011, theASACUSA experiment at CERN determined the mass of the antiproton to be1836.1526736(23) times that of the electron.[10] This is the same as the mass of a proton, within the level of certainty of the experiment.
In October 2017, scientists working on theBASE experiment at CERN reported a measurement of the antiprotonmagnetic moment to a precision of 1.5 parts per billion.[11][12] It is consistent with the most precise measurement of the proton magnetic moment (also made by BASE in 2014), which supports the hypothesis of CPT symmetry. This measurement represents the first time that a property of antimatter is known more precisely than the equivalent property in matter.
In January 2022, by comparing the charge-to-mass ratios between antiproton and negatively charged hydrogen ion, the BASE experiment has determined the antiproton's charge-to-mass ratio is identical to the proton's, down to 16 parts per trillion.[13][14]
Antiprotons have been shown within laboratory experiments to have the potential to treat certain cancers, in a similar method currently used forion (proton) therapy.[15] The primary difference between antiproton therapy and proton therapy is that following ion energy deposition the antiproton annihilates, depositing additional energy in the cancerous region.
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