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| [[Image:Drell-Yan.svg|300px|thumb|Drell–Yan process: a quark from one hadron and an antiquark from another hadron annihilate to create a pair of leptons through the exchange of a virtual photon.]]
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| The '''Drell–Yan process''' occurs in high energy hadron–hadron scattering. It takes place when a [[quark]] of one [[hadron]] and an [[antiquark]] of another hadron annihilate, creating a [[virtual particle|virtual]] [[photon]] or [[Z boson]] which then decays into a pair of oppositely-charged [[leptons]]. This process was first suggested by [[Sidney Drell]] and [[Tung-Mow Yan]] in 1970<ref name="DrellYan">
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| {{cite journal
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| |last1=Drell |first1=S.D.
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| |last2=Yan |first2=T.-M.
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| |year=1970
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| |title=Massive Lepton-Pair Production in Hadron-Hadron Collisions at High Energies
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| |journal=[[Physical Review Letters]]
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| |volume=25 |issue=5 |pages=316–320
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| |bibcode=1970PhRvL..25..316D
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| |doi=10.1103/PhysRevLett.25.316
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| }}
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| ::And ''erratum'' in {{cite journal
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| |last1=Drell |first1=S. D.
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| |last2=Yan |first2=T.-M.
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| |year=1970
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| |title=
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| |journal=[[Physical Review Letters]]
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| |volume=25 |issue=13 |page=902
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| |bibcode=1970PhRvL..25..902D
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| |doi=10.1103/PhysRevLett.25.902.2
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| }}</ref> to describe the production of [[lepton]]–[[antilepton]] pairs in high-energy hadron collisions. Experimentally, this process was first observed by J.H. Christenson ''et al.''<ref name="Christenson">
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| {{cite journal
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| |last1=Christenton |first1=J. H.
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| |coauthors=''et al.''
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| |year=1970
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| |title=Observation of Massive Muon Pairs in Hadron Collisions
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| |journal=[[Physical Review Letters]]
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| |volume=25 |issue=21 |pages=1523–1526
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| |bibcode=1970PhRvL..25.1523C
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| |doi=10.1103/PhysRevLett.25.1523
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| }}</ref> in proton–uranium collisions at the [[Alternating Gradient Synchrotron]].
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| ==Overview==
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| The Drell–Yan process is studied both in fixed-target and collider experiments. It provides valuable information about the [[Parton (particle physics)|parton distribution functions]] (PDFs) which describe the way the momentum of an incoming high-energy nucleon is partitioned among its constituent partons. These PDFs are basic ingredients for calculating essentially all processes at hadron colliders. Although PDFs should be derivable in principle, current ignorance of some aspects of the [[strong force]] prevents this. Instead, the forms of the PDFs are deduced from experimental data.
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| ==Sensitivity to Light Quark Flavor Asymmetry in the Proton==
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| One recent measurement using Drell-Yan scattering probed the flavor asymmetry of the proton.<ref>
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| {{cite journal
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| |last=Hawker |first=E. A.
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| |coauthors=''et al.''
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| |year=1998
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| |title=Measurement of the light anti-quark flavor asymmetry in the nucleon sea
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| |journal=[[Physical Review Letters]]
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| |volume=80 |issue=17 |pages=3715–3718
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| |arxiv = hep-ex/9803011
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| |bibcode=1998PhRvL..80.3715H
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| |doi=10.1103/PhysRevLett.80.3715
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| }}</ref><ref>
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| {{cite journal
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| |last=Towell |first=R. S.
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| |coauthors=''et al.''
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| |year=2001
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| |title=Improved measurement of the {{overline|d}}/{{overline|u}} asymmetry in the nucleon sea
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| |journal=[[Physical Review D]]
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| |volume=64 |issue=5 |pages=052002
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| |arxiv = hep-ex/0103030
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| |bibcode = 2001PhRvD..64e2002T
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| |doi=10.1103/PhysRevD.64.052002
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| }}</ref><ref>
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| {{cite journal
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| |last=Baldit |first=A.
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| |coauthors=''et al.''
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| |year=1994
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| |title=Study of the isospin symmetry breaking in the light quark sea of the nucleon from the Drell-Yan process
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| |journal=[[Physics Letters B]]
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| |volume=332 |issue= |pages=244–250
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| |arxiv=
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| |bibcode= 1994PhLB..332..244B
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| |doi= 10.1016/0370-2693(94)90884-2
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| }}</ref> It had been naively believed that the quark sea in the proton was formed by [[quantum chromodynamics]] (QCD) processes that did not discriminate between up and down quarks. To [[leading-order|leading order]] in the strong interaction coupling constant, α<sub>s</sub>, the ratio of the Drell-Yan cross section from a proton beam on a deuterium target to a proton beam on a proton target is given by
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| <math>
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| \frac{\sigma^{pd}}{2\sigma^{pp}} = \frac{1}{2}\left[1+ \frac{\bar{d}(x)}{\bar{u}(x)}\right]
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| </math>
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| ==Z boson production ==
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| The production of Z bosons through the Drell–Yan process affords the opportunity to study the [[coupling]]s of the Z boson to [[quarks]]. The main observable is the ''[[forward–backward asymmetry]]'' in the angular distribution of the two leptons in their [[center of mass|center-of-mass]] [[frame of reference|frame]].
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| If heavier neutral gauge bosons exist (see [[Z' boson]]), they might be discovered as a peak in the [[dilepton]] [[invariant mass]] spectrum in much the same way that the standard Z boson appears by virtue of the Drell–Yan process.
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| ==See also==
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| *[[NA51 experiment]]
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| *[[Fermilab E-866/NuSea]]
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| *[[Fermilab E-906/SeaQuest]]
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| ==References==
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| {{Reflist}}
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| {{DEFAULTSORT:Drell-Yan Process}}
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| [[Category:Quantum mechanics]]
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