View source for Antiproton

{{Short description|Subatomic particle}}
{{Infobox particle
| bgcolour =
| name = Antiproton
| image = [[Image:Quark structure antiproton.svg|200px]]
| caption = The [[quark]] content of the antiproton.
| num_types = 
| classification = [[Antibaryon]]
| composition = 2 [[up quark|up antiquarks]], 1 [[down quark|down antiquark]]
| statistics = [[Fermionic]]
| group = Hadron
| generation = 
| interaction = [[Strong interaction|Strong]], [[Weak interaction|weak]], [[Electromagnetic interaction|electromagnetic]], [[gravity]]
| particle = 
| antiparticle = [[Proton]]
| status =
| theorized = 
| discovered = [[Emilio Segrè]] & [[Owen Chamberlain]] (1955)
| symbol = {{SubatomicParticle|Antiproton}}
| mass =  {{physconst|mp}}<br /> {{physconst|mpc2_MeV|unit={{val|ul=MeV/c2}}}}
| mean_lifetime = the same as that of the proton
| decay_particle = 
| electric_charge = {{val|-1|ul=e}}
| charge_radius = 
| electric_dipole_moment = 
| electric_polarizability = 
| magnetic_moment = {{val|−2.7928473441|(42)|u=[[Nuclear magneton|''μ''<sub>N</sub>]]}} <ref>{{cite journal|title=A parts-per-billion measurement of the antiproton magnetic moment|journal=Nature|volume=550|issue=7676|pages=371–374|doi=10.1038/nature24048|pmid=29052625|bibcode=2017Natur.550..371S|year=2017|last1=Smorra|first1=C.|last2=Sellner|first2=S.|last3=Borchert|first3=M. J.|last4=Harrington|first4=J. A.|last5=Higuchi|first5=T.|last6=Nagahama|first6=H.|last7=Tanaka|first7=T.|last8=Mooser|first8=A.|last9=Schneider|first9=G.|last10=Bohman|first10=M.|last11=Blaum|first11=K.|last12=Matsuda|first12=Y.|last13=Ospelkaus|first13=C.|last14=Quint|first14=W.|last15=Walz|first15=J.|last16=Yamazaki|first16=Y.|last17=Ulmer|first17=S.|s2cid=205260736|url=https://cds.cern.ch/record/2291601/files/nature24048.pdf|doi-access=free}}</ref>
| magnetic_polarizability = 
| color_charge = 
| spin = {{frac|1|2}}
| num_spin_states = 
| lepton_number = 
| baryon_number = 
| strangeness = 
| charm = 
| bottomness = 
| topness = 
| isospin = −{{frac|1|2}}
| weak_isospin = 
| hypercharge = 
| weak_hypercharge = 
| parity = 
| g_parity = 
| c_parity = 
| r_parity = 
| condensed_symmetries =
|theorised=[[Paul Dirac]] (1933)}}
{{antimatter}}
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.

The existence of the antiproton with electric charge of {{val|-1|u=e}}, opposite to the electric charge of {{val|+1|u=e}} of the proton, was predicted by [[Paul Dirac]] in his 1933 Nobel Prize lecture.<ref>
{{Cite book |last=Dirac |first=Paul A. M. |orig-date=1933 |chapter=Theory of electrons and positrons |chapter-url=https://www.nobelprize.org/prizes/physics/1933/dirac/lecture/ |title=Nobel Lectures, Physics 1922-1941 |publisher=Elsevier Publishing Company |location=Amsterdam |year=1965}}
</ref> Dirac received the Nobel Prize for his 1928 publication of his [[Dirac equation]] that predicted the existence of positive and negative solutions to [[Albert Einstein|Einstein]]'s energy equation (<math>E = mc^2</math>) and the existence of the [[positron]], the [[antimatter]] analog of the [[electron]], with opposite [[electric charge|charge]] and [[quantum spin|spin]].

The antiproton was first experimentally confirmed in 1955 at the [[Bevatron]] particle accelerator by [[University of California, Berkeley]] [[physicist]]s [[Emilio Segrè]] and [[Owen Chamberlain]], for which they were awarded the 1959 [[Nobel Prize in Physics]].

In terms of [[valence quark]]s, an antiproton consists of two [[up quark|up]] antiquarks and one [[down quark|down]] antiquark ({{SubatomicParticle|link=no|Up antiquark}}{{SubatomicParticle|link=no|Up antiquark}}{{SubatomicParticle|link=no|Down antiquark}}). 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 the [[Big Bang]], remain open problems, in part, due to the relative scarcity of antimatter in today's universe.

==Occurrence in nature==
Antiprotons have been detected in [[cosmic ray]]s 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 with [[Atomic nucleus|atomic nuclei]] in the [[interstellar medium]], via the reaction, where A represents a nucleus:

{{SubatomicParticle|Proton}} + A → {{SubatomicParticle|Proton}} + {{SubatomicParticle|Antiproton}} + {{SubatomicParticle|Proton}} + A

The secondary antiprotons ({{SubatomicParticle|Antiproton}}) then propagate through the Milky Way 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.<ref name="Kennedy2000" />

A belt of antiprotons surrounding Earth was reported in 2011 by the team behind the [[PAMELA detector|Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics]] detector.<ref>{{Citation |last=Adriani |first=O. |title=The discovery of geomagnetically trapped cosmic ray antiprotons |date=2011-07-25 |url=http://arxiv.org/abs/1107.4882 |access-date=2025-10-27 |publisher=arXiv |doi=10.48550/arXiv.1107.4882 |id=arXiv:1107.4882 |last2=Barbarino |first2=G. C. |last3=Bazilevskaya |first3=G. A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E. A. |last7=Bongi |first7=M. |last8=Bonvicini |first8=V. |last9=Borisov |first9=S.}}</ref> The antiprotons are trapped by Earth's magnetic field, and similar antiproton belts may exist around Jupiter, Saturn, Neptune, and Uranus. Interactions between cosmic rays and Saturn's rings may produce the largest number of antiprotons in the solar system.<ref>{{Cite journal |last=Spjeldvik |first=Walther |last2=Bickford |first2=James |last3=Schmitt |first3=William |last4=Spjeldvik |first4=Walther |last5=Martin |first5=Inácio M. |last6=Pugacheva |first6=Galina |last7=Gusev |first7=Anatoly |date=2008 |title=Antiproton distributions around the Earth, Saturn and Jupiter from cosmic ray driven albedo antineutron decay |url=https://ui.adsabs.harvard.edu/abs/2008cosp...37.3002S/abstract |journal=37th COSPAR Scientific Assembly |language=en |volume=37 |pages=3002}}</ref><ref>{{Cite web |last=Cowen |first=Ron |date=2011-08-09 |title=Antimatter Belt Found Circling Earth |url=https://www.science.org/content/article/antimatter-belt-found-circling-earth |access-date=2025-10-27 |website=www.science.org |language=en}}</ref>

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 name="Kennedy2000">{{cite book |last=Kennedy |first=Dallas C. |chapter=High-energy Antimatter Telescope (HEAT): Basic design and performance |editor-first1=Brian D. |editor-first2=Thomas A. |editor-last1=Ramsey |editor-last2=Parnell |date=2000 |title=Gamma-Ray and Cosmic-Ray Detectors, Techniques, and Missions |series=[[Proceedings of SPIE]] |volume= 2806 |pages= 113–120 |arxiv=astro-ph/0003485 |doi=10.1117/12.253971 |s2cid=16664737 }}</ref> These experimental measurements set 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]] caused by the evaporation of [[primordial black hole]]s. This also provides a lower limit on the antiproton lifetime of about 1.7 million years.<ref name=":0">{{Citation |last=Geer |first=Stephen H. |title=The Cosmic Ray Antiproton Spectrum and a Limit on the Antiproton Lifetime |date=1998-09-08 |url=http://arxiv.org/abs/astro-ph/9809101 |access-date=2025-10-27 |publisher=arXiv |doi=10.48550/arXiv.astro-ph/9809101 |id=arXiv:astro-ph/9809101 |last2=Kennedy |first2=Dallas C.}}</ref> Since the galactic storage time of antiprotons is about 10 million years,<ref name=":0" /> 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=years}}
* [[Antihydrogen]] [[Penning trap]] of [[Gerald Gabrielse|Gabrielse]] et al.: {{val|0.28|u=years}}<ref>{{cite journal |last=Caso |first=C. |date=1998 |title=Particle Data Group |journal=European Physical Journal C |volume=3 |issue=1–4 |pages=1–783 |url=http://pdg.ihep.su/1999/s041.pdf |doi=10.1007/s10052-998-0104-x |bibcode=1998EPJC....3....1P |citeseerx=10.1.1.1017.4419 |s2cid=195314526 |display-authors=etal |access-date=2008-03-16 |archive-date=2011-07-16 |archive-url=https://web.archive.org/web/20110716074638/http://pdg.ihep.su/1999/s041.pdf |url-status=dead }}</ref>
* BASE experiment at CERN: {{val|10.2|u=years}}<ref>{{cite journal |last=Sellner |first=S. |date=2017 |title=Improved limit on the directly measured antiproton lifetime |journal=New Journal of Physics  |volume=19 |issue=8 |pages=083023 |url=http://stacks.iop.org/1367-2630/19/i=8/a=083023 |doi=10.1088/1367-2630/aa7e73 |bibcode=2017NJPh...19h3023S |display-authors=etal|doi-access=free }}</ref>
* APEX collaboration at [[Fermilab]]: {{val|50000|u=years}} for {{SubatomicParticle|Antiproton}} → {{SubatomicParticle|link=yes|Muon}} + anything
* APEX collaboration at Fermilab: {{val|300000|u=years}} for {{SubatomicParticle|Antiproton}} → {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Gamma}}

The magnitude of 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 cosmic ray detection experiments===

* [[BESS (experiment)|BESS]]: balloon-borne experiment, flown in 1993, 1995, 1997, 2000, 2002, 2004 (Polar-I) and 2007 (Polar-II).
* CAPRICE: balloon-borne experiment, flown in 1994<ref>{{cite web |title=Cosmic AntiParticle Ring Imaging Cherenkov Experiment (CAPRICE) |url=http://ida1.physik.uni-siegen.de/caprice.html |publisher=Universität Siegen |access-date=14 April 2022 |archive-date=3 March 2016 |archive-url=https://web.archive.org/web/20160303203131/http://ida1.physik.uni-siegen.de/caprice.html |url-status=dead }}</ref> and 1998.
* 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]], launched May 2011.
* [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]]: satellite experiment to detect cosmic rays and antimatter from space, launched June 2006. Recent report discovered 28 antiprotons in the [[South Atlantic Anomaly]].<ref>{{cite journal | doi = 10.1088/2041-8205/737/2/L29 | title = The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons | date = 2011 | last1 = Adriani | first1 = O. | last2 = Barbarino | first2 = G. C. | last3 = Bazilevskaya | first3 = G. A. | last4 = Bellotti | first4 = R. | last5 = Boezio | first5 = M. | last6 = Bogomolov | first6 = E. A. | last7 = Bongi | first7 = M. | last8 = Bonvicini | first8 = V. | last9 = Borisov | first9 = S. | journal = The Astrophysical Journal Letters | volume = 737 | issue = 2 | pages = L29 | bibcode = 2011ApJ...737L..29A | arxiv=1107.4882 | last10 = Bottai | first10 = S. | last11 = Bruno | first11 = A. | last12 = Cafagna | first12 = F. | last13 = Campana | first13 = D. | last14 = Carbone | first14 = R. | last15 = Carlson | first15 = P. | last16 = Casolino | first16 = M. | last17 = Castellini | first17 = G. | last18 = Consiglio | first18 = L. | last19 = De Pascale | first19 = M. P. | last20 = De Santis | first20 = C. | last21 = De Simone | first21 = N. | last22 = Di Felice | first22 = V. | last23 = Galper | first23 = A. M. | last24 = Gillard | first24 = W. | last25 = Grishantseva | first25 = L. | last26 = Jerse | first26 = G. | last27 = Karelin | first27 = A. V. | last28 = Kheymits | first28 = M. D. | last29 = Koldashov | first29 = S. V. | last30 = Krutkov | first30 = S. Y. | display-authors = 29 }}</ref>

==Modern experiments and applications==
[[File:Photocopy of photograph (original print located in LBNL Photo Lab Collection). Photographer unknown. October 6, 1955. BEV-938. ANTI-PROTON SET-UP WITH WORK GROUP; E. SEGRE, C. HAER CAL,1-BERK,4-34.tif|thumb|BEV-938. Antiproton set-up with work group: [[Emilio Segre]], [[Clyde Wiegand]], [[Edward J. Lofgren]], [[Owen Chamberlain]], [[Thomas Ypsilantis]], 1955]]

=== Production ===
Antiprotons were routinely produced at Fermilab for collider physics operations in the [[Tevatron]], 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 largest [[Parton (particle physics)|fraction of the proton or antiproton's momentum]].

Formation of antiprotons requires energy equivalent to a temperature of 10 trillion [[Kelvin|K]] (10<sup>13</sup>&nbsp;K), and this does not tend to happen naturally. However, at CERN, protons are accelerated in the [[Proton Synchrotron]] 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”]] (transforming kinetic energy into new matter particles). A range of particles and antiparticles are formed, and the antiprotons are separated off using magnets in [[vacuum]].

=== Measurements ===
In July 2011, the [[ASACUSA]] experiment at CERN determined the mass of the antiproton to be {{val|1836.1526736|(23)}} times that of the electron.<ref name=Mhori>{{cite journal | journal=Nature | volume=475 |issue=7357 | pages=484–8 | date=2011 |last1=Hori |first1=M. |last2=Sótér | title=Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio |doi=10.1038/nature10260 | first2=Anna | last3=Barna | first3=Daniel | last4=Dax | first4=Andreas | last5=Hayano | first5=Ryugo | last6=Friedreich | first6=Susanne | last7=Juhász | first7=Bertalan | last8=Pask | first8=Thomas | last9=Widmann | first9=Eberhard | display-authors=8| pmid=21796208| arxiv=1304.4330 | s2cid=4376768 }}</ref> This is the same as the mass of a proton, within the level of certainty of the experiment.

In October 2017, scientists working on the [[BASE experiment]] at CERN reported a measurement of the antiproton [[magnetic moment]] to a precision of 1.5 parts per billion.<ref name="TT-20171025">{{cite web |last=Adamson |first=Allan |title=Universe Should Not Actually Exist: Big Bang Produced Equal Amounts Of Matter And Antimatter |url=http://www.techtimes.com/articles/214821/20171025/universe-should-not-actually-exist-big-bang-produced-equal-amounts-of-matter-and-antimatter.htm |date=19 October 2017 |work=TechTimes.com |access-date=26 October 2017 }}</ref><ref name="NAT-20171020">{{cite journal |author=Smorra C.|display-authors=et al |title=A parts-per-billion measurement of the antiproton magnetic moment |date=20 October 2017 |journal=[[Nature (journal)|Nature]] |volume=550 |issue=7676 |pages=371–374 |doi=10.1038/nature24048 |pmid=29052625 |bibcode=2017Natur.550..371S |s2cid=205260736 |url=https://cds.cern.ch/record/2291601/files/nature24048.pdf |doi-access=free }}</ref> 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.<ref>{{Cite web|title=BASE breaks new ground in matter–antimatter comparisons|url=http://home.cern/news/news/physics/base-breaks-new-ground-matter-antimatter-comparisons|access-date=2022-01-05|website=CERN|language=en}}</ref><ref>{{Cite journal|last1=Borchert|first1=M. J.|last2=Devlin|first2=J. A.|last3=Erlewein|first3=S. R.|last4=Fleck|first4=M.|last5=Harrington|first5=J. A.|last6=Higuchi|first6=T.|last7=Latacz|first7=B. M.|last8=Voelksen|first8=F.|last9=Wursten|first9=E. J.|last10=Abbass|first10=F.|last11=Bohman|first11=M. A.|date=2022-01-05|title=A 16-parts-per-trillion measurement of the antiproton-to-proton charge–mass ratio|url=https://www.nature.com/articles/s41586-021-04203-w|journal=Nature|volume=601 |issue=7891 |language=en|pages=53–57|doi=10.1038/s41586-021-04203-w|pmid=34987217 |bibcode=2022Natur.601...53B |s2cid=245709321 |issn=1476-4687|url-access=subscription}}</ref>

=== Possible applications ===
Antiprotons have been shown within laboratory experiments to have the potential to treat certain cancers, in a similar method currently used for [[Particle therapy|ion (proton) therapy]].<ref>{{cite journal|url=http://www.engr.psu.edu/antimatter/Papers/pbar_med.pdf |title=Antiproton portable traps and medical applications |date=1997 |url-status=dead |archive-url=https://web.archive.org/web/20110822150631/http://www.engr.psu.edu/antimatter/Papers/pbar_med.pdf |archive-date=2011-08-22 |doi=10.1023/A:1012653416870 |last1=Lewis |first1=R.A. |last2=Smith |first2=G.A. |last3=Howe |first3=S.D. |journal=Hyperfine Interactions |volume=109 |issue=1–4 |pages=155–164 |bibcode=1997HyInt.109..155L }}</ref> 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.

==See also==
*[[Antineutron]]
*{{section link|Deuterium|Antideuterium}}
*[[Antiprotonic helium]]
*[[List of particles]]
*[[Recycling antimatter]]
*[[Positron]]

==References==
{{Reflist}}

{{Particles}}

{{Authority control}}

[[Category:Antimatter]]
[[Category:Baryons]]
[[Category:Nucleons]]
[[Category:Proton]]