parton (particle physics)

The parton model was proposed by Richard Feynman in 1969, used originally for analysis of high-energy hadron collisions.{{cite conference |last=Feynman |first=R. P. |year=1969 |title=The Behavior of Hadron Collisions at Extreme Energies |book-title=High Energy Collisions: Third International Conference at Stony Brook, N.Y. |pages=237–249 |publisher=Gordon & Breach |isbn=978-0-677-13950-0 }} It was applied to electron-proton deep inelastic scattering by James Bjorken and Emmanuel Anthony Paschos.{{cite journal |title=Inelastic Electron-Proton and γ-Proton Scattering and the Structure of the Nucleon |year=1969 |last1=Bjorken |first1=J. |last2=Paschos |first2=E. |journal=Physical Review |volume=185 |issue=5 |pages=1975–1982 |bibcode=1969PhRv..185.1975B |doi=10.1103/PhysRev.185.1975 }} Later, with the experimental observation of Bjorken scaling, the validation of the quark model, and the confirmation of asymptotic freedom in quantum chromodynamics, partons were matched to quarks and gluons. The parton model remains a justifiable approximation at high energies, and others{{who|date=August 2018}} have extended the theory{{how|date=December 2016}} over the years.

Murray Gell-Mann preferred to use the term "put-ons" to refer to partons.{{Cite web |date=2019-05-30 |title=Remembering Murray Gell-Mann (1929–2019), Inventor of Quarks—Stephen Wolfram Writings |url=https://writings.stephenwolfram.com/2019/05/remembering-murray-gell-mann-1929-2019-inventor-of-quarks/ |access-date=2024-02-02 |website=writings.stephenwolfram.com |language=en}}

In 1994, partons were used by Leonard Susskind to model holography.{{Cite journal|last=Susskind|first=Leonard|year=1995|title=The world as a hologram|journal=Journal of Mathematical Physics|volume=36 |issue=11 |pages=6377–6396 |doi=10.1063/1.531249 |arxiv=hep-th/9409089 |bibcode=1995JMP....36.6377S |s2cid=17316840 }}

File:Parton scattering.PNG

Any hadron (for example, a proton) can be considered as a composition of a number of point-like constituents, termed "partons".

Just as accelerated electric charges emit QED radiation (photons), the accelerated coloured partons will emit QCD radiation in the form of gluons. Unlike the uncharged photons, the gluons themselves carry colour charges and can therefore emit further radiation, leading to parton showers.Bryan Webber (2011). [http://www.scholarpedia.org/article/Parton_shower_Monte_Carlo_event_generators Parton shower Monte Carlo event generators.] Scholarpedia, 6(12):10662., revision #128236.

• {{webarchive|url=https://web.archive.org/web/20130402235841/http://www.scholarpedia.org/article/Parton_shower_Monte_Carlo_event_generators |date=2013-04-02 }}[http://indico.cern.ch/getFile.py/access?contribId=0&resId=0&materialId=slides&confId=49675 Parton Shower Monte Carlo Event Generators.] Mike Seymour, MC4LHC EU Networks’ Training Event

May 4th – 8th 2009.[http://www.stfc.ac.uk/PPD/resources/pdf/Krauss_08_Pheno_5.pdf Phenomenology at collider experiments. Part 5: MC generators] {{webarchive|url=https://web.archive.org/web/20120703153443/http://www.stfc.ac.uk/PPD/resources/pdf/Krauss_08_Pheno_5.pdf |date=2012-07-03 }}, Frank Krauss. HEP Summer School 31.8.-12.9.2008, RAL.

{{see also|DGLAP}}

The hadron is defined in a reference frame where it has infinite momentum – a valid approximation at high energies. Thus, parton motion is slowed by time dilation, and the hadron charge distribution is Lorentz-contracted, so incoming particles will be scattered "instantaneously and incoherently".{{citation needed|date=August 2018}}

Partons are defined with respect to a physical scale (as probed by the inverse of the momentum transfer).{{clarify|date=August 2018}} For instance, a quark parton at one length scale can turn out to be a superposition of a quark parton state with a quark parton and a gluon parton state together with other states with more partons at a smaller length scale. Similarly, a gluon parton at one scale can resolve into a superposition of a gluon parton state, a gluon parton and quark-antiquark partons state and other multiparton states. Because of this, the number of partons in a hadron actually goes up with momentum transfer.{{cite journal | author = G. Altarelli and G. Parisi | title = Asymptotic Freedom in Parton Language | journal = Nuclear Physics | volume = B126 | issue = 2 | pages = 298–318 | year = 1977 | doi= 10.1016/0550-3213(77)90384-4| bibcode = 1977NuPhB.126..298A }} At low energies (i.e. large length scales), a baryon contains three valence partons (quarks) and a meson contains two valence partons (a quark and an antiquark parton). At higher energies, however, observations show sea partons (nonvalence partons) in addition to valence partons.

{{cite journal |last1=Drell |first1=S.D. |last2=Yan |first2=T.-M. |s2cid=16827178 |year=1970 |title=Massive Lepton-Pair Production in Hadron-Hadron Collisions at High Energies |journal=Physical Review Letters |volume=25 |issue=5 |pages=316–320 |bibcode=1970PhRvL..25..316D |doi=10.1103/PhysRevLett.25.316 |osti=1444835 }}

::And erratum in {{cite journal |last1=Drell |first1=S. D. |last2=Yan |first2=T.-M. |year=1970 |title=none |journal=Physical Review Letters |volume=25 |issue=13 |page=902 |bibcode=1970PhRvL..25..902D |doi=10.1103/PhysRevLett.25.902.2 |osti=1444835 |doi-access=free }}

{{See also|Transverse momentum distributions}}

File:CTEQ6 parton distribution functions.png

A parton distribution function (PDF) within so called collinear factorization is defined as the probability density for finding a particle with a certain longitudinal momentum fraction x at resolution scale Q². Because of the inherent non-perturbative nature of partons which cannot be observed as free particles, parton densities cannot be calculated using perturbative QCD. Within QCD one can, however, study variation of parton density with resolution scale provided by external probe. Such a scale is for instance provided by a virtual photon with virtuality Q² or by a jet. The scale can be calculated from the energy and the momentum of the virtual photon or jet; the larger the momentum and energy, the smaller the resolution scale—this is a consequence of Heisenberg's uncertainty principle. The variation of parton density with resolution scale has been found to agree well with experiment;PDG: Aschenauer, Thorne, and Yoshida, (2019). "Structure Functions", [http://pdg.lbl.gov/2019/reviews/rpp2019-rev-structure-functions.pdf online]. this is an important test of QCD.

Parton distribution functions are obtained by fitting observables to experimental data; they cannot be calculated using perturbative QCD. Recently, it has been found that they can be calculated directly in lattice QCD using large-momentum effective field theory.{{Cite journal|last=Ji|first=Xiangdong|date=2013-06-26|title=Parton Physics on a Euclidean Lattice|journal=Physical Review Letters|volume=110|issue=26|pages=262002|doi=10.1103/PhysRevLett.110.262002|pmid=23848864|arxiv=1305.1539|bibcode=2013PhRvL.110z2002J|s2cid=27248761}}{{Cite journal|last=Ji|first=Xiangdong|date=2014-05-07|title=Parton physics from large-momentum effective field theory|journal=Science China Physics, Mechanics & Astronomy|language=en|volume=57|issue=7|pages=1407–1412|doi=10.1007/s11433-014-5492-3|issn=1674-7348|arxiv=1404.6680|bibcode=2014SCPMA..57.1407J|s2cid=119208297}}

Experimentally determined parton distribution functions are available from various groups worldwide. The major unpolarized data sets are:

• [http://mail.ihep.ru/~alekhin/pdfs.html ABM] {{Webarchive|url=https://web.archive.org/web/20220119175433/https://mail.ihep.ru/~alekhin/pdfs.html |date=2022-01-19 }} by S. Alekhin, J. Bluemlein, S. Moch
• [http://www.cteq.org/#PDFs CTEQ], from the CTEQ Collaboration
• [https://archive.today/20160604082205/http://doom.physik.tu-dortmund.de/pdfserver/index.html GRV/GJR], from M. Glück, P. Jimenez-Delgado, E. Reya, and A. Vogt
• [https://www.desy.de/h1zeus/combined_results/index.php?do=proton_structure HERA] PDFs, by H1 and ZEUS collaborations from the Deutsches Elektronen-Synchrotron center (DESY) in Germany
• [http://www.hep.ucl.ac.uk/mmht/ MSHT/MRST/MSTW/MMHT], from A. D. Martin, R. G. Roberts, W. J. Stirling, R. S. Thorne, and collaborators
• [http://nnpdf.hepforge.org/ NNPDF], from the NNPDF Collaboration

The [http://lhapdf.hepforge.org/ LHAPDF] {{cite arXiv |last1=Whalley |first1=M. R. |last2=Bourilkov |first2=D |last3=Group |first3=R. C. |year=2005 |title=The Les Houches accord PDFs (LHAPDF) and LHAGLUE |eprint=hep-ph/0508110 }} library provides a unified and easy-to-use Fortran/C++ interface to all major PDF sets.

Generalized parton distributions (GPDs) are a more recent approach to better understand hadron structure by representing the parton distributions as functions of more variables, such as the transverse momentum and spin of the parton.{{cite journal | journal = Phys. Rev. D| volume = 29 | issue = 3 | pages= 567–569 | year= 1984 | title= Spin structure of the nucleon | doi= 10.1103/PhysRevD.29.567 | author1 = DJE Callaway | author2 = SD Ellis | s2cid = 15798912 | bibcode = 1984PhRvD..29..567C}} They can be used to study the spin structure of the proton, in particular, the Ji sum rule relates the integral of GPDs to angular momentum carried by quarks and gluons.{{Cite journal|last=Ji|first=Xiangdong|date=1997-01-27|title=Gauge-Invariant Decomposition of Nucleon Spin|journal=Physical Review Letters|volume=78|issue=4|pages=610–613|doi=10.1103/PhysRevLett.78.610|arxiv=hep-ph/9603249|bibcode=1997PhRvL..78..610J|s2cid=15573151}} Early names included "non-forward", "non-diagonal" or "skewed" parton distributions. They are accessed through a new class of exclusive processes for which all particles are detected in the final state, such as the deeply virtual Compton scattering.{{Cite journal|last=Ji|first=Xiangdong|date=1997-06-01|title=Deeply virtual Compton scattering|journal=Physical Review D|volume=55|issue=11|pages=7114–7125|doi=10.1103/PhysRevD.55.7114|bibcode=1997PhRvD..55.7114J|arxiv=hep-ph/9609381|s2cid=1975588}} Ordinary parton distribution functions are recovered by setting to zero (forward limit) the extra variables in the generalized parton distributions. Other rules show that the electric form factor, the magnetic form factor, or even the form factors associated to the energy-momentum tensor are also included in the GPDs. A full 3-dimensional image of partons inside hadrons can also be obtained from GPDs.{{cite journal |last1=Belitsky |first1=A. V. |last2=Radyushkin |first2=A. V. |author2-link=Anatoly Radyushkin |year=2005 |title=Unraveling hadron structure with generalized parton distributions |journal=Physics Reports |volume=418 |issue=1–6 |pages=1–387 |arxiv=hep-ph/0504030 |bibcode=2005PhR...418....1B |doi=10.1016/j.physrep.2005.06.002 |s2cid=119469719 }}

Parton showers simulations are of use in computational particle physics either in automatic calculation of particle interaction or decay or event generators, in order to calibrate and interpret (and thus understand) processes in collider experiments.{{ cite web | url=https://www.phys.psu.edu/~cteq/schools/summer09/talks/Soper1.pdf | title=The physics of parton showers | first=Davison E. | last=Soper | date=June 2009 | website=Pennsylvania State University | archive-url=https://web.archive.org/web/20110524061119/http://www.phys.psu.edu/~cteq/schools/summer09/talks/Soper1.pdf | url-status=dead | archive-date=24 May 2011 | access-date=17 November 2013 }} They are particularly important in large hadron collider (LHC) phenomenology, where they are usually explored using Monte Carlo simulation.

The scale at which partons are given to hadronization is fixed by the Shower Monte Carlo program. Common choices of Shower Monte Carlo are PYTHIA and HERWIG.Johan Alwall, [https://web.archive.org/web/20140114023830/http://phys.cts.ntu.edu.tw/workshop/2012/1010525LHC/PDF/LEC2.pdf Complete simulation of collider events], pg 33. NTU MadGraph school, May 25–27, 2012.M Moretti. [http://www1b.physik.rwth-aachen.de/~kolleg/fileadmin/data/gk/de/veranstaltungen/moretti.pdf Understanding events at the LHC: Parton Showers and Matrix Element tools for physics simulation at the hadronic colliders],{{Dead link|date=June 2024}} p. 19. 28/11/2006.

{{div col|colwidth=20em}}

• Hadronization
• Jet (particle physics)
• Particle shower
• Proton structure function
• Photon structure function
• SLAC bag model

{{div col end}}

{{reflist|25em}}

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{{external links|date=July 2020}}

• {{Cite journal|title=Introduction to Parton Distribution Functions |journal=Scholarpedia |year=2010 |language=en |doi=10.4249/scholarpedia.10160 |issn=1941-6016|doi-access=free |last1=Feltesse |first1=Joël |volume=5 |issue=11 |page=10160 |bibcode=2010SchpJ...510160F }}
• Event Generator Physics (http://www.hep.phy.cam.ac.uk/theory/webber/MCnet/MClecture2.pdf)
• {{Cite web |title=Introduction to QCD |url=https://people.phys.ethz.ch/~pheno/QCDcourse/ |access-date=2022-08-04 |website=people.phys.ethz.ch}}
• http://www.kceta.kit.edu/grk1694/img/2013_10_01_Hangst.pdf
• http://d-nb.info/1008230227/34
• {{Cite thesis |title=Applying SCET to parton showers |url=https://dspace.mit.edu/handle/1721.1/62649 |publisher=Massachusetts Institute of Technology |date=2010 |degree= |first=Claudio |last=Marcantonini|hdl=1721.1/62649 }}

{{Richard Feynman}}

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Category:Quantum chromodynamics

Category:Richard Feynman