Bósons W e Z: diferenças entre revisões
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'''Bósons W e Z⁰''' são [[partículas elementares]] mediadoras da [[força nuclear fraca]]. Sua descoberta, no [[CERN]] em [[1983]], foi anunciada como um grande sucesso para o [[Modelo Padrão]] da [[física de partículas]].
O bóson '''W''' foi nomeado por causa do "W" de "''Weak nuclear force''". O bóson '''Z''' recebeu a última letra do alfabeto porque humoristicamente seria a última partícula a ser descoberta. Outra explicação é de que "Z" é a inicial de "zero", a carga elétrica que o bóson possui.
== Propriedades básicas ==
<--
Two kinds of W [[boson]]s exist with +1 and −1 elementary units of [[electric charge]]; the {{SubatomicParticle|W boson+}} is the [[antiparticle]] of the {{SubatomicParticle|W boson-}}. The Z boson (or Z) is electrically [[Neutral particle|neutral]] and is its own antiparticle. All three particles are very short-lived with a [[half-life|mean life]] of about {{val|3|e=-25|u=s}}.
These bosons are heavyweights among the elementary particles. With a mass of {{val|80.4|u=GeV/c2}} and {{val|91.2|u=GeV/c2}}, respectively, the W and Z particles are almost 100 times as massive as the [[proton]]—heavier than entire [[atom]]s of [[iron]]. The [[mass]]es of these bosons are significant because they act as [[force carriers]]; their masses thus limit the range of the weak interaction. The [[electromagnetism|electromagnetic force]], by contrast, has an infinite range because its force carrier (the [[photon]]) is massless.
All three types have a [[spin (physics)|spin]] of 1. The emission of a {{SubatomicParticle|W boson+}} or {{SubatomicParticle|W boson-}} boson either raises or lowers the electric charge of the emitting particle by 1 unit, and alters the spin by 1 unit. At the same time a W boson can change the generation of the particle, for example changing a [[strange quark]] to an [[up quark]]. The {{SubatomicParticle|Z boson}} boson cannot change either electric charge nor any other charges (like strangeness, charm, etc.), only spin and momentum, so it never changes the generation or flavour of the particle emitting it (see ''[[weak neutral current]]'').
== Weak nuclear force ==
[[Image:Beta Negative Decay.svg|thumb|right|280px|The [[Feynman diagram]] for beta decay of a [[neutron]] into a [[proton]], [[electron]], and [[electron antineutrino]] via an intermediate heavy [[W boson]] ]]
The W and Z bosons are carrier particles that mediate the weak nuclear force, much like the photon is the carrier particle for the electromagnetic force. The W bosons are best known for their role in [[nuclear decay]]. Consider, for example, the [[beta decay]] of [[cobalt-60]], an important process in [[supernova]] explosions.
: {{Nuclide|Link|Cobalt|60}} → {{Nuclide|Link|Nickel|60}} + {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Electron antineutrino}}
This reaction does not involve the whole cobalt-60 [[Atomic nucleus|nucleus]], but affects only one of its 33 [[neutron]]s. The neutron is converted into a [[proton]] while also emitting an [[electron]] (called a [[beta particle]] in this context) and an [[electron antineutrino]]:
: {{SubatomicParticle|link=yes|Neutron0}} → {{SubatomicParticle|link=yes|Proton+}} + {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Electron antineutrino}}
Again, the neutron is not an elementary particle but a composite of an [[up quark]] and two [[down quark]]s (udd). It is in fact one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (uud). At the most fundamental level, then, the weak force changes the [[flavour (particle physics)|flavour]] of a single quark:
: {{SubatomicParticle|link=yes|Down quark}} → {{SubatomicParticle|link=yes|Up quark}} + {{SubatomicParticle|link=yes|W boson-}}
which is immediately followed by decay of the {{SubatomicParticle|W boson-}} itself:
: {{SubatomicParticle|link=yes|W boson-}} → {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Electron antineutrino}}
Being its own antiparticle, all the [[flavour quantum numbers]] and all the [[charge (physics)|charges]] of the Z boson are zero. The exchange of a Z boson between particles, called a [[neutral current]] interaction, therefore leaves the interacting particles unaffected, except for a transfer of [[momentum]]. Unlike beta decay, the observation of neutral current interactions requires huge investments in [[particle accelerator]]s and [[detector]]s, such as are available in only a few [[high-energy physics]] laboratories in the world.
== Predicting the W and Z ==
[[Image:Kaon-box-diagram.svg|thumb|right|A [[Feynman diagram]] showing the exchange of a pair of W bosons. This is one of the leading terms contributing to neutral [[Kaon]] oscillation.]]
Following the spectacular success of [[quantum electrodynamics]] in the 1950s, attempts were undertaken to formulate a similar theory of the weak nuclear force. This culminated around 1968 in a unified theory of electromagnetism and weak interactions by [[Sheldon Glashow]], [[Steven Weinberg]], and [[Abdus Salam]], for which they shared the 1979 Nobel Prize in physics.<ref>[http://www.nobel.se/physics/laureates/1979/ 1979 Nobel Prize in physics]</ref> Their [[electroweak theory]] postulated not only the W bosons necessary to explain beta decay, but also a new Z boson that had never been observed.
The fact that the W and Z bosons have mass while photons are massless was a major obstacle in developing electroweak theory. These particles are accurately described by an SU(2) [[gauge theory]], but the bosons in a gauge theory must be massless. As a case in point, the [[photon]] is massless because electromagnetism is described by a U(1) gauge theory. Some mechanism is required to break the SU(2) symmetry, giving mass to the W and Z in the process. One explanation, the [[Higgs mechanism]], was forwarded by [[Peter Higgs]] and others in the mid 1960s. It predicts the existence of yet another new particle, the [[Higgs boson]].
The combination of the SU(2) gauge theory of the weak interaction, the electromagnetic interaction, and the Higgs mechanism is known as the [[Glashow-Weinberg-Salam model]]. These days it is widely accepted as one of the pillars of the Standard Model of particle physics. {{As of|2009}}, despite intensive search for the Higgs boson carried out at [[CERN]] and [[Fermilab]], its existence remains the main prediction of the Standard Model not to be confirmed experimentally.
== Discovery ==
[[Image:CERN-20060225-24.jpg|thumb|The Gargamelle bubble chamber, now exhibited at CERN]]
The discovery of the W and Z particles was considered a major success for CERN. First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The huge [[Gargamelle]] [[bubble chamber]] photographed the tracks of a few electrons suddenly starting to move, seemingly of their own accord. This is interpreted as a [[neutrino]] interacting with the electron by the exchange of an unseen Z boson. The neutrino is otherwise undetectable, so the only observable effect is the momentum imparted to the electron by the interaction.
The discovery of the W and Z particles themselves had to wait for the construction of a [[particle accelerator]] powerful enough to produce them. The first such machine that became available was the [[Super Proton Synchrotron]], where unambiguous signals of W particles were seen in January 1983 during a series of experiments conducted by [[Carlo Rubbia]] and [[Simon van der Meer]]. The actual experiments were called [[UA1]] (led by Rubbia) and [[UA2]] (led by [[Peter Jenni]])<ref>[http://library.web.cern.ch/library/Archives/isad/isaua2.html The UA2 Collaboration collection]</ref>, and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end ([[stochastic cooling]]). UA1 and UA2 found the Z a few months later, in May 1983. Rubbia and van der Meer were promptly awarded the 1984 Nobel Prize in Physics, a most unusual step for the conservative [[Nobel Prize|Nobel Foundation]].<ref>[http://www.nobel.se/physics/laureates/1984/ 1984 Nobel Prize in physics]</ref>
The {{SubatomicParticle|W boson+}}, {{SubatomicParticle|W boson-}}, and {{SubatomicParticle|Z boson0}} [[bosons]], together with the [[photon]] ({{SubatomicParticle|Photon}}), build up the four [[gauge boson]]s of the [[electroweak interaction]].
== Decay ==
The W and Z [[boson]]s decay to [[fermion]]-antifermion pairs but neither the W nor the Z boson can decay into the higher-mass [[top quark]]. Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the [[coupling constant]]s.
=== W bosons ===
'''W bosons''' can decay to a [[lepton]] and [[neutrino]] or to an [[quark#Electric charge|up-type quark and a down-type quark]]. The [[decay width]] of the W boson to a quark-antiquark pair is proportional to the corresponding squared [[CKM matrix]] element and the number of [[quark colour]]s, N<sub>C</sub> = 3. The decay widths for the W boson are then proportional to:
{| class="wikitable" style="text-align:center;"
!colspan="2" width="100"|Leptons
!colspan="2" width="100"|Up quarks
!colspan="2" width="100"|Charm quarks
|-
| {{SubatomicParticle|Electron+}}{{SubatomicParticle|Electron neutrino}}
| 1
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down antiquark}}
| 3<nowiki>|</nowiki>V<sub>ud</sub><nowiki>|</nowiki><sup>2</sup>
| {{SubatomicParticle|Charm quark}}{{SubatomicParticle|Down antiquark}}
| 3<nowiki>|</nowiki>V<sub>cd</sub><nowiki>|</nowiki><sup>2</sup>
|-
| {{SubatomicParticle|Muon+}}{{SubatomicParticle|Muon neutrino}}
| 1
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Strange antiquark}}
| 3<nowiki>|</nowiki>V<sub>us</sub><nowiki>|</nowiki><sup>2</sup>
| {{SubatomicParticle|Charm quark}}{{SubatomicParticle|Strange antiquark}}
| 3<nowiki>|</nowiki>V<sub>cs</sub><nowiki>|</nowiki><sup>2</sup>
|-
| {{SubatomicParticle|Tauon+}}{{SubatomicParticle|Tauon neutrino}}
| 1
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Bottom antiquark}}
| 3<nowiki>|</nowiki>V<sub>ub</sub><nowiki>|</nowiki><sup>2</sup>
| {{SubatomicParticle|Charm quark}}{{SubatomicParticle|Bottom antiquark}}
| 3<nowiki>|</nowiki>V<sub>cb</sub><nowiki>|</nowiki><sup>2</sup>
|-
|}
Here, {{SubatomicParticle|Electron+}}, {{SubatomicParticle|Muon+}}, {{SubatomicParticle|Tauon+}} denoted the three [[Flavour (particle physics)|flavour]]s of [[lepton]]s (more exactly, the positive charged [[anti matter|anti lepton]]s). {{SubatomicParticle|Electron neutrino}}, {{SubatomicParticle|Muon neutrino}}, {{SubatomicParticle|Tauon neutrino}} denote the three [[Flavour (particle physics)|flavour]]s of [[neutrino#Types of neutrinos|neutrino]]s. The other particles starting with {{SubatomicParticle|Up quark}} and {{SubatomicParticle|Down antiquark}} all denote [[quark]]s and anti-quarks (factor N<sub>C</sub> is applied). V is the CKM matrix with its coefficients.
[[Unitary matrix|Unitarity]] of the CKM matrix implies that
|V<sub>ud</sub>|<sup>2</sup> + |V<sub>us</sub>|<sup>2</sup> + |V<sub>ub</sub>|<sup>2</sup> =
|V<sub>cd</sub>|<sup>2</sup> + |V<sub>cs</sub>|<sup>2</sup> + |V<sub>cb</sub>|<sup>2</sup> = 1. Therefore the leptonic [[branching ratio]]s of the W boson are approximately B({{SubatomicParticle|Electron+}}{{SubatomicParticle|Electron neutrino}}) = B({{SubatomicParticle|Muon+}}{{SubatomicParticle|Muon neutrino}}) = B({{SubatomicParticle|Tauon+}}{{SubatomicParticle|Tauon neutrino}}) = {{frac|1|9}} (~11.11%). The hadronic branching ratio is dominated by the CKM favored {{SubatomicParticle|Up quark}}{{SubatomicParticle|down antiquark}} and {{SubatomicParticle|Charm quark}}{{SubatomicParticle|strange antiquark}} final states, and the sum of the [[hadron]]ic branching ratios is roughly {{frac|2|3}} (~66.67%). The branching ratios have been measured experimentally: B(l<sup>+</sup>ν<sub>l</sub>) = {{val|10.80|0.09|s=%}} and B(hadrons) ={{val|67.60|0.27|s=%}}.<ref>[http://pdg.lbl.gov/2007/listings/s043.pdf W.-M. Yao ''et al.'', J. Phys. '''G33''', 1 (2006) and 2007 partial update for the 2008 edition]</ref>
=== Z bosons ===
'''Z bosons''' decay into a fermion and its antiparticle. The [[decay width]] of a Z boson to a fermion-antifermion pair is proportional to the square of the weak charge ''T''<sub>3</sub> − ''Q''·''x'', where ''T''<sub>3</sub> is the third component of the [[weak isospin]] of the fermion, ''Q ''is the [[electric charge|charge]] of the fermion (in units of the [[elementary charge]]), and ''x'' = sin<sup>2</sup>''θ''<sub>W</sub>, where ''θ''<sub>W</sub> is the [[Weinberg angle|weak mixing angle]]. Because the weak isospin is different for fermions of different [[Chirality (physics)|chirality]], either [[Standard Model (mathematical formulation)#Right handed singlets, left handed doublets|left-handed or right-handed]]), the coupling is different as well. The decay width of the Z boson for quarks is also proportional to ''N''<sub>''C''</sub>.
{| class="wikitable" style="text-align:center;"
!colspan=2| Particles
!colspan=2| Weak charge
!Decay width of Z Boson
!colspan=2|Branching ratios BR(particle, antiparticle)
|-
! Name
! Symbols
! L
! R
! (proportional to)
! Predicted for ''x'' = 0.23
! Experimental measurements{{Citation needed|date=February 2010}}
|-
| Neutrinos
| {{SubatomicParticle|Electron neutrino}}, {{SubatomicParticle|Muon neutrino}}, {{SubatomicParticle|Tauon neutrino}}
| {{frac|1|2}}
|
| {{frac|1|2}}<sup>2</sup>
| {{val|20.5|s=%}}
| {{val|20.00|0.06|s=%}}
|-
| Leptons
| {{SubatomicParticle|Electron}}, {{SubatomicParticle|Muon}}, {{SubatomicParticle|Tauon}}
| −{{frac|1|2}} + x
| x
| (−{{frac|1|2}} + x)<sup>2</sup> + x<sup>2</sup>
| {{val|3.4|s=%}}
| {{val|3.3658|0.0023|s=%}}
|-
| Up-type Quarks
| {{SubatomicParticle|Up quark}}, {{SubatomicParticle|Charm quark}}
| {{frac|1|2}} − {{frac|2|3}}x
| −{{frac|2|3}}x
| 3({{frac|1|2}} − {{frac|2|3}}x)<sup>2</sup> + 3(−{{frac|2|3}}x)<sup>2</sup>
| {{val|11.8|s=%}}
| {{val|11.6|0.6|s=%}}
|-
| Down-type quarks
| {{SubatomicParticle|Down quark}}, {{SubatomicParticle|Strange quark}}, {{SubatomicParticle|Bottom quark}}
| −{{frac|1|2}} + {{frac|1|3}}x
| {{frac|1|3}}x
| 3(−{{frac|1|2}} + {{frac|1|3}}x)<sup>2</sup> + 3({{frac|1|3}}x)<sup>2</sup>
| {{val|15.2|s=%}}
| {{val|15.6|0.4|s=%}}
|-
| [[Hadron]]s
|
|
|
|
| {{val|69.2|s=%}}
| {{val|69.91|0.06|s=%}}
|}
Here, L and R denote the chirality of the fermions, i. e. left-handed and right-handed, respectively. The right-handed neutrinos do not exist in the standard model. However, in some extensions of the standard model they do.<ref>[[Standard Model (mathematical formulation)#Right handed singlets, left handed doublets|Right-handed neutrino and Standard Model]]</ref><ref>[[neutrino#Handedness|Chirality of neutrinos]]</ref>
== See also ==
* [[Standard model (mathematical formulation)]]
* [[List of particles]]
* [[X and Y bosons]]: analogous pair of bosons predicted by [[Grand unification theory|GUT]]
* [[W' and Z' bosons]]
==References==
{{Reflist}}
==External links==
* [http://pdg.lbl.gov/ The Review of Particle Physics], the ultimate source of information on particle properties.
*[http://intranet.cern.ch/Chronological/Announcements/CERNAnnouncements/2003/09-16WZSymposium/Courier/HeavyLight/Heavylight.html W and Z] page from [[CERN]]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html#c4 W and Z particles at Hyperphysics]
* [http://www.everything2.com/index.pl?node=Z%20particle Z particle at Everything2]
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