Editing Nuclear physics (section)

==History==
[[File:Paul Nadar - Henri Becquerel.jpg|thumb|[[Henri Becquerel]]]]
[[File:Cloud chambers played an important role of particle detectors.jpg|thumb|Since the 1920s, [[cloud chamber]]s played an important role of particle detectors and eventually lead to the discovery of [[positron]], [[muon]] and [[kaon]].]]

The history of nuclear physics as a discipline distinct from [[atomic physics]], starts with the discovery of [[radioactivity]] by [[Henri Becquerel]] in 1896,<ref name=brm>{{cite book
|title=Nuclear and Particle Physics
|author=B. R. Martin
|publisher=John Wiley & Sons, Ltd.
|year=2006
|isbn=978-0-470-01999-3
}}</ref> made while investigating [[phosphorescence]] in [[uranium]] salts.<ref>{{cite journal
|author=Henri Becquerel
|title=Sur les radiations émises par phosphorescence
|journal=Comptes Rendus
|volume=122
|pages=420–421
|year=1896
|url=http://gallica.bnf.fr/ark:/12148/bpt6k30780/f422.chemindefer
|access-date=2010-09-21
|archive-date=2017-09-04
|archive-url=https://web.archive.org/web/20170904040827/http://gallica.bnf.fr/ark:/12148/bpt6k30780/f422.chemindefer
|url-status=live
}}</ref> The discovery of the [[electron]] by [[J. J. Thomson]]<ref>{{cite journal|last1=Thomson|first1=Joseph John|author1-link=Joseph John Thomson|title=Cathode Rays|year=1897|journal=[[Royal Institution|Proceedings of the Royal Institution of Great Britain]]|volume=XV|pages=419–432|url=https://archive.org/stream/proceedings15roya#page/418/mode/2up}}</ref> a year later was an indication that the atom had internal structure. At the beginning of the 20th century the accepted model of the atom was J. J. Thomson's [[Plum pudding model|"plum pudding" model]] in which the atom was a positively charged ball with smaller negatively charged electrons embedded inside it.

In the years that followed, radioactivity was extensively investigated, notably by [[Marie Curie]], a Polish physicist whose maiden name was Sklodowska, [[Pierre Curie]], [[Ernest Rutherford]] and others. By the turn of the century, physicists had also discovered three types of [[radiation]] emanating from atoms, which they named [[alpha decay|alpha]], [[beta decay|beta]], and [[gamma decay|gamma]] radiation. Experiments by [[Otto Hahn]] in 1911 and by [[James Chadwick]] in 1914 discovered that the beta decay [[spectrum]] was continuous rather than discrete. That is, [[electron]]s were ejected from the atom with a continuous range of energies, rather than the discrete amounts of energy that were observed in gamma and alpha decays. This was a problem for nuclear physics at the time, because it seemed to indicate that [[conservation of energy|energy was not conserved]] in these decays.

The 1903 [[Nobel Prize]] in Physics was awarded jointly to Becquerel, for his discovery and to Marie and Pierre Curie for their subsequent research into radioactivity. Rutherford was awarded the Nobel Prize in Chemistry in 1908 for his "investigations into the disintegration of the elements and the chemistry of radioactive substances".

In 1905, [[Albert Einstein]] formulated the idea of [[mass–energy equivalence]]. While the work on radioactivity by [[Henri Becquerel|Becquerel]] and [[Marie Curie]] predates this, an explanation of the source of the energy of radioactivity would have to wait for the discovery that the nucleus itself was composed of smaller constituents, the [[nucleon]]s.

=== Rutherford discovers the nucleus ===
In 1906, [[Ernest Rutherford]] published "Retardation of the a Particle from Radium in passing through matter."<ref>{{cite journal|last=Rutherford|first=Ernest|author-link1=Ernest Rutherford|title=On the retardation of the α particle from radium in passing through matter|journal=[[Philosophical Magazine]]|year=1906|volume=12|number=68|pages=134–146|doi=10.1080/14786440609463525|url=https://zenodo.org/record/1430810|access-date=2019-07-01|archive-date=2022-03-31|archive-url=https://web.archive.org/web/20220331091115/https://zenodo.org/record/1430810|url-status=live}}</ref> [[Hans Geiger]] expanded on this work in a communication to the [[Royal Society]]<ref>{{cite journal|last=Geiger|first=Hans|author-link1=Hans Geiger|title=On the scattering of α-particles by matter|journal=[[Proceedings of the Royal Society A]]|year=1908|volume=81|number=546|pages=174–177|doi=10.1098/rspa.1908.0067 |bibcode=1908RSPSA..81..174G|doi-access=free}}</ref> with experiments he and Rutherford had done, passing alpha particles through air, aluminum foil and gold leaf. More work was published in 1909 by Geiger and [[Ernest Marsden]],<ref>{{cite journal | last1 =Geiger | first1 =Hans| author-link1=Hans Geiger|last2=Marsden|first2=Ernest|author-link2=Ernest Marsden| title=On the diffuse reflection of the α-particles | year =1909 | volume=82| pages =495|number=557|journal=[[Proceedings of the Royal Society A]] | doi=10.1098/rspa.1909.0054 |bibcode=1909RSPSA..82..495G| doi-access=free}}</ref> and [[Geiger–Marsden experiments|further greatly expanded work was published in 1910 by Geiger]].<ref>{{cite journal|last=Geiger|first=Hans|author-link1=Hans Geiger|title=The scattering of the α-particles by matter|year=1910|volume=83|number=565|pages=492–504|journal=[[Proceedings of the Royal Society A]]|doi=10.1098/rspa.1910.0038|bibcode=1910RSPSA..83..492G|doi-access=free}}</ref> In 1911–1912 Rutherford went before the Royal Society to explain the experiments and propound the new theory of the atomic nucleus as we now understand it.

Published in 1909,<ref>H. Geiger and E. Marsden, PM, 25, 604 1913, ''citing'', H. Geiger and E. Marsden, Roy. Soc. Proc. vol. LXXXII. p. 495 (1909), in,  [http://www.physics.utah.edu/~lebohec/P5110/Material/geiger_marsden_1913.pdf The Laws of Deflexion of α Particles Through Large Angles \\ H. Geiger and E. Marsden] {{Webarchive|url=https://web.archive.org/web/20190501022445/http://www.physics.utah.edu/~lebohec/P5110/Material/geiger_marsden_1913.pdf |date=2019-05-01 }} (1913), (published subsequently online by – physics.utah.edu ([[University of Utah]])) Retrieved June 13, 2021 (p.1):"..In an earlier paper, however, we pointed out that α particles are sometimes turned through very large angles..."(p.2):"..Professor Rutherford has recently developed a theory to account for the scattering of α particles through these large angles, the assumption being that the deflexions are the result of an intimate encounter of an α particle with a single atom of the matter traversed. In this theory an atom is supposed to consist of a strong positive or negative central charge concentrated within a sphere of less than about 3 × 10–12 cm. radius, and surrounded by electricity of the opposite sigh distributed throughout the remainder of the atom of about 10−8 cm. radius..."</ref> with the eventual classical analysis by Rutherford published May 1911,<ref name=Radvanyi>{{cite journal |last1=Radvanyi |first1=Pierre |date=January–February 2011 |title=Physics and Radioactivity after the Discovery of Polonium and Radium |url=http://publications.iupac.org/publications/ci/2011/3301/8_radvanyi.html |access-date=13 June 2021 |format=electronic |journal=Chemistry International |language=English |location=online |publisher=[[International Union of Pure and Applied Chemistry]] |publication-date=January–February 2011 |volume=33 |issue=1 |quote="..Geiger and an English-New Zealand student, E. Marsden, to study their scattering through thin metallic foils. In 1909, the two physicists observe that some alpha-particles are scattered backwards by thin platinum or gold foils (Geiger 1909)...It takes Rutherford one and a half years to understand this result. In 1911, he concludes that the atom contains a very small 'nucleus'..." |archive-date=9 July 2023 |archive-url=https://web.archive.org/web/20230709185302/http://publications.iupac.org/publications/ci/2011/3301/8_radvanyi.html |url-status=live }}</ref><ref>{{cite journal
| url = https://www.chemteam.info/Chem-History/Rutherford-1911/Rutherford-1911.html
| last1 = Rutherford F.R.S.
| first1 = E.
| title = The Scattering of α and β Particles by Matter and the Structure of the Atom
| date = May 1911
| journal = Philosophical Magazine
| series = 6
| volume = 21 May 1911
| pages = 669–688
| archive-url = https://web.archive.org/web/20200212052356/https://www.chemteam.info/Chem-History/Rutherford-1911/Rutherford-1911.html
| archive-date = 12 February 2020
| accessdate = 13 June 2021
| url-status = live
}}</ref><ref>{{cite journal |last1=Rutherford |first1=E. |title=LXXIX. The scattering of α and β particles by matter and the structure of the atom |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |date=May 1911 |volume=21 |issue=125 |pages=669–688 |doi=10.1080/14786440508637080}}</ref><ref>{{cite web
| url = https://www.fnal.gov/pub/inquiring/timeline/03.html
| title = 1911 John Ratcliffe and Ernest Rutherford (smoking) at the Cavendish Laboratory...
| publisher = [[Fermilab]]
| archive-url = https://web.archive.org/web/20210401022934/https://www.fnal.gov/pub/inquiring/timeline/03.html
| archive-date = 1 April 2021
| accessdate = 13 June 2021
| url-status = live
}}"..that would become a classic technique of particle physics..."</ref> the key preemptive experiment was performed during 1909,<ref name=Radvanyi/><ref>*{{cite web 
|url=https://micro.magnet.fsu.edu/electromag/java/rutherford/ 
|last1=Davidson 
|first1=Michael W 
|department=micro.magnet. 
|website=micro.magnet.fsu.edu 
|publisher=[[Florida State University]] 
|title=The Rutherford Experiment 
|location=[[Florida State]] 
|access-date=13 June 2021 
|archive-url=https://web.archive.org/web/20210613142921/https://micro.magnet.fsu.edu/electromag/java/rutherford/ 
|archive-date=13 June 2021 
|url-status=live 
}} "experiment was conducted 1911"
*{{cite web
 |url          = https://cerncourier.com/a/rutherford-transmutation-and-the-proton/
 |title        = <small>CULTURE AND HISTORY FEATURE</small> Rutherford, transmutation and the proton 8 May 2019 The events leading to Ernest Rutherford's discovery of the proton, published in 1919.
 |work         = [[CERN Courier]]
 |date         = 8 May 2019
 |publisher    = [[IOP Publishing]]
 |archive-url  = https://web.archive.org/web/20210418113242/https://cerncourier.com/a/rutherford-transmutation-and-the-proton/
 |archive-date = 18 April 2021
 |accessdate   = 13 June 2021
 |url-status   = live
}}"...1909...a couple of years later..."
*{{cite journal
 |date         = May 2006
 |url          = https://www.aps.org/publications/apsnews/200605/history.cfm
 |title        = This Month in Physics History: May, 1911: Rutherford and the Discovery of the Atomic Nucleus
 |volume       = 15
 |issue        = 5
 |journal      = [[APS News]]
 |archive-url  = https://web.archive.org/web/20210613142920/https://www.aps.org/publications/apsnews/200605/history.cfm
 |archive-date = 13 June 2021
 |accessdate   = 13 June 2021
 |url-status   = live
}}"..1909..published – 1911.."
*{{cite web 
| url =http://ffden-2.phys.uaf.edu/212_spring2005.web.dir/ashley_anderson/atomic_timeline.html 
| author =Anderson, Ashley 
| title =Timeline 
| publisher =University of Alaska-Fairbanks 
| archive-url =https://web.archive.org/web/20210613142921/http://ffden-2.phys.uaf.edu/212_spring2005.web.dir/ashley_anderson/atomic_timeline.html 
| archive-date =13 June 2021 
| accessdate =13 June 2021 
| url-status =live 
}}  "1911 performed "
*1911 discovers:
**Leonard, P.  and Gehrels, N. (November 28, 2009) [https://heasarc.gsfc.nasa.gov/docs/history/ A History of Gamma-Ray Astronomy Including Related Discoveries] {{Webarchive|url=https://web.archive.org/web/20210613142921/https://heasarc.gsfc.nasa.gov/docs/history/ |date=2021-06-13 }} [[National Aeronautics and Space Administration]] [[Goddard Space Flight Center]]:  [[High Energy Astrophysics Science Archive Research Center]] (HEASARC), Retrieved 13 June 2021
**Rizvi, Eram – [http://pprc.qmul.ac.uk/~rizvi/Talks/Lecture1.pdf Quantum Mechanics and Particle Scattering Lecture 1] {{Webarchive|url=https://web.archive.org/web/20210613142922/http://pprc.qmul.ac.uk/~rizvi/Talks/Lecture1.pdf |date=2021-06-13 }}, p.9, pprc.qmul.ac.uk [[Queen Mary University London]]: School of Physics and Astronomy – Particle Physics Research Centre, Retrieved 13 June 2021 "..by Rutherford.."
*[https://www.nobelprize.org/prizes/chemistry/1908/rutherford/biographical/ rutherford/biographical] {{Webarchive|url=https://web.archive.org/web/20230603075847/https://www.nobelprize.org/prizes/chemistry/1908/rutherford/biographical/ |date=2023-06-03 }}, [[Nobel Prize]],  "..In 1910, his investigations into the scattering of alpha rays and the nature of the inner structure of the atom which caused such scattering led to the postulation of his concept of the 'nucleus'..."
*{{cite web
| url = https://spark.iop.org/collections/case-studies-history-physics
| title = Case studies from the history of physics
| publisher = [[Institute of Physics]]
| archive-url = https://web.archive.org/web/20210422133723/https://spark.iop.org/collections/case-studies-history-physics
| archive-date = 22 April 2021
| quote = "..It is suggested that, in 1910, the 'plum pudding model' was suddenly overturned by Rutherford's experiment. In fact, Rutherford had already formulated the nuclear model of the atom before the experiment was carried out.."
| accessdate = 13 June 2021
| url-status = live
}}</ref><ref name=Jariskog>{{cite web
| url = https://cds.cern.ch/record/1734171/files/vol48-issue10-p019-e.pdf
| title = <small>ANNIVERSARY</small> The nucleus and more
| last1 = Jariskog
| first1 = Cecilia
| date = December 2008
| magazine = [[CERN Courrier]]
| volume = 48
| issue = 10
| page = 21
| archive-url = https://web.archive.org/web/20210613142922/https://cds.cern.ch/record/1734171/files/vol48-issue10-p019-e.pdf
| archive-date = 13 June 2021
| quote = ".. in 1911, Rutherford writes: "I have been working recently on scattering of alpha and beta particles and have devised a new atom to explain the results.."
| accessdate = 13 June 2021
| url-status = live
}}</ref><ref name=Godenko>{{cite book |last1=Godenko |first1=Lyudmila |title=The Making of the Atomic Bomb |url=https://cuny.manifoldapp.org/read/he-making-of-the-atomic-bomb/section/21c33a72-ee12-4198-a0cd-0b618a14b5ff#topic-4-the-nuclear-atom-1911-1920.-alpha-scattering-and-the-discovery-of-the-nucleus.-nuclear-sizes-vs-atomic-sizes.-niels-bohr-and-the-structure-of-the-nuclear-atom.-moseleys-work-with-x-rays-and-the-significance-of-the-atomic-number. |format=E-Book |publisher=cuny.manifoldapp.org CUNY's Manifold ([[City University of New York]]) |archive-url= |archive-date= |access-date=13 June 2021 |quote="The discovery for which Rutherford is most famous is that atoms have nuclei; ...had its beginnings in 1909...Geiger and Marsden published their anomalous result in July, 1909...The first public announcement of this new model of atomic structure seems to have been made on March 7, 1911, when Rutherford addressed the Manchester Literary and Philosophical Society;..." }}{{Dead link|date=December 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> at the [[University of Manchester]]. Ernest Rutherford's assistant, Professor <ref name=Godenko/> Johannes <ref name=Jariskog/> "Hans" Geiger, and an undergraduate, Marsden,<ref name=Godenko/> performed an [[Geiger–Marsden experiment|experiment in which Geiger and Marsden]] under Rutherford's supervision fired alpha particles ([[Helium-4 nucleus|helium 4 nuclei]]<ref>{{cite web |last=Watkins |first=Thayer |title=The Structure and Binding Energy of the Alpha Particle, the Helium 4 Nucleus |access-date=14 June 2021 |url=https://www.sjsu.edu/faculty/watkins/He4.htm |publisher=[[San Jose University]] |archive-url=https://web.archive.org/web/20200130165819/http://www.sjsu.edu/faculty/watkins/He4.htm |archive-date=30 January 2020 |url-status=dead }}</ref>) at a thin film of [[gold]] foil. The [[plum pudding model]] had predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent. But Rutherford instructed his team to look for something that shocked him to observe: a few particles were scattered through large angles, even completely backwards in some cases. He likened it to firing a [[bullet]] at tissue paper and having it bounce off. The discovery, with Rutherford's analysis of the data in 1911, led to the Rutherford model of the atom, in which the atom had a very small, very dense [[Atomic nucleus|nucleus]] containing most of its mass, and consisting of heavy positively charged particles with embedded electrons in order to balance out the charge (since the neutron was unknown). As an example, in this model (which is not the modern one) nitrogen-14 consisted of a nucleus with 14 protons and 7 electrons (21 total particles) and the nucleus was surrounded by 7 more orbiting electrons.

===Eddington and stellar nuclear fusion===
Around 1920, [[Arthur Eddington]] anticipated the discovery and mechanism of [[nuclear fusion]] processes in [[star]]s, in his paper ''The Internal Constitution of the Stars''.<ref name=eddington>{{cite journal |last=Eddington |first=A. S. |title=The Internal Constitution of the Stars |journal=The Scientific Monthly |volume=11 |issue=4 |year=1920 |pages=297–303 |jstor=6491|bibcode=1920SciMo..11..297E }}</ref><ref name=eddington2>{{cite journal|bibcode=1916MNRAS..77...16E|title=On the radiative equilibrium of the stars|journal=Monthly Notices of the Royal Astronomical Society|volume=77|pages=16–35|last1=Eddington|first1=A. S.|year=1916|doi=10.1093/mnras/77.1.16|doi-access=free}}</ref> At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source was [[nuclear fusion|fusion]] of hydrogen into helium, liberating enormous energy according to Einstein's equation ''[[Mass–energy equivalence|E = mc<sup>2</sup>]]''. This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of [[hydrogen]] (see [[metallicity]]), had not yet been discovered.

===Studies of nuclear spin===
The Rutherford model worked quite well until studies of [[Spin (physics)|nuclear spin]] were carried out by [[Franco Rasetti]] at the [[California Institute of Technology]] in 1929. By 1925 it was known that protons and electrons each had a spin of {{frac|±|1|2}}. In the Rutherford model of nitrogen-14, 20 of the total 21 nuclear particles should have paired up to cancel each other's spin, and the final odd particle should have left the nucleus with a net spin of {{frac|1|2}}. Rasetti discovered, however, that nitrogen-14 had a spin of 1.

=== James Chadwick discovers the neutron ===
{{Main|Discovery of the neutron}}
In 1932 Chadwick realized that radiation that had been observed by [[Walther Bothe]], [[Herbert Becker (physicist)|Herbert Becker]], [[Irène Joliot-Curie|Irène]] and [[Frédéric Joliot-Curie]] was actually due to a neutral particle of about the same mass as the proton, that he called the [[neutron]] (following a suggestion from Rutherford about the need for such a particle).<ref>{{cite journal|last=Chadwick|first=James|author-link1=James Chadwick|title=The existence of a neutron|journal=[[Proceedings of the Royal Society A]]|year=1932|volume=136|number=830|pages=692–708|doi=10.1098/rspa.1932.0112 |bibcode=1932RSPSA.136..692C|doi-access=free}}</ref> In the same year [[Dmitri Ivanenko]] suggested that there were no electrons in the nucleus — only protons and neutrons — and that neutrons were spin {{frac|1|2}} particles, which explained the mass not due to protons. The neutron spin immediately solved the problem of the spin of nitrogen-14, as the one unpaired proton and one unpaired neutron in this model each contributed a spin of {{frac|1|2}} in the same direction, giving a final total spin of 1.

With the discovery of the neutron, scientists could at last calculate what fraction of [[binding energy]] each nucleus had, by comparing the nuclear mass with that of the protons and neutrons which composed it. Differences between nuclear masses were calculated in this way. When nuclear reactions were measured, these were found to agree with Einstein's calculation of the equivalence of mass and energy to within 1% as of 1934.

=== Proca's equations of the massive vector boson field ===
[[Alexandru Proca]] was the first to develop and report the massive vector [[boson]] [[field equation]]s and a theory of the [[meson]]ic field of [[nuclear force]]s. Proca's equations were known to [[Wolfgang Pauli]]<ref>W. Pauli'', Nobel lecture'', December 13, 1946.</ref> who mentioned the equations in his Nobel address, and they were also known to Yukawa, Wentzel, Taketani, Sakata, Kemmer, Heitler, and Fröhlich who appreciated the content of Proca's equations for developing a theory of the atomic nuclei in Nuclear Physics.<ref>{{cite journal |doi=10.1051/epn:2006504  |title=Alexandru Proca (1897–1955) and his equation of the massive vector boson field |first1=Dorin N. |last1=Poenaru |first2=Alexandru |last2=Calboreanu |journal=[[Europhysics News]] |volume=37|issue=5|pages=25–27 |year=2006 |bibcode=2006ENews..37e..24P |s2cid=123558823 |doi-access=free }}</ref><ref>''G. A. Proca, Alexandre Proca.Oeuvre Scientifique Publiée'', S.I.A.G., Rome, 1988.</ref><ref>{{cite journal | last1 = Vuille | first1 = C. | last2 = Ipser | first2 = J. | last3 = Gallagher | first3 = J. | year = 2002 | title = Einstein–Proca model, micro black holes, and naked singularities | journal = General Relativity and Gravitation | volume = 34 | issue = 5| page = 689 | doi=10.1023/a:1015942229041| arxiv = 1406.0497 | bibcode = 2002GReGr..34..689V | s2cid = 118221997 }}</ref><ref>{{cite journal | last1 = Scipioni | first1 = R. | year = 1999 | title = Isomorphism between non-Riemannian gravity and Einstein–Proca–Weyl theories extended to a class of scalar gravity theories | journal = Class. Quantum Gravity | volume = 16 | issue = 7| pages = 2471–2478 | doi=10.1088/0264-9381/16/7/320|arxiv = gr-qc/9905022 |bibcode = 1999CQGra..16.2471S | s2cid = 6740644 }}</ref><ref>{{cite journal  | doi=10.1016/s0920-5632(97)00399-x |bibcode = 1997NuPhS..57..259T |title = An Einstein–Proca-fluid model for dark matter gravitational interactions |journal = Nuclear Physics B: Proceedings Supplements |volume = 57 |issue = 1–3 |pages = 259–262 |last1 = Tucker |first1 = R. W |last2 = Wang |first2 = C |year = 1997 }}</ref>

===Yukawa's meson postulated to bind nuclei===
In 1935 [[Hideki Yukawa]]<ref>{{cite journal |last1=Yukawa |first1=Hideki |title=On the Interaction of Elementary Particles. I |journal=Proceedings of the Physico-Mathematical Society of Japan |series=3rd Series |date=1935 |volume=17 |pages=48–57 |doi=10.11429/ppmsj1919.17.0_48 |doi-access=free |url=https://www.jstage.jst.go.jp/article/ppmsj1919/17/0/17_0_48/_pdf/-char/en |url-status=live |archive-url= https://web.archive.org/web/20231122190008/http://www.jstage.jst.go.jp/article/ppmsj1919/17/0/17_0_48/_pdf/-char/en |archive-date= Nov 22, 2023 }}</ref> proposed the first significant theory of the [[strong force]] to explain how the nucleus holds together. In the [[Yukawa interaction]] a [[virtual particle]], later called a [[meson]], mediated a force between all nucleons, including protons and neutrons. This force explained why nuclei did not disintegrate under the influence of proton repulsion, and it also gave an explanation of why the attractive [[strong force]] had a more limited range than the electromagnetic repulsion between protons. Later, the discovery of the [[pi meson]] showed it to have the properties of Yukawa's particle.

With Yukawa's papers, the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force, unless it is too large. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron (or [[positron]]). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high-energy photons (gamma decay).

The study of the strong and weak nuclear forces (the latter explained by [[Enrico Fermi]] via [[Fermi's interaction]] in 1934) led physicists to collide nuclei and electrons at ever higher energies. This research became the science of [[particle physics]], the crown jewel of which is the [[Standard Model|standard model of particle physics]], which describes the strong, weak, and [[Electromagnetism|electromagnetic forces]].