W⁺ Boson
Composition :
Statistics :
Family :
Interaction forces :
Symbol :
Mass :
Electric charge :
Color charge
Spin :
Weak isospin
Weak hypercharge :
W⁻ Boson
Composition :
Statistics :
Family :
Interaction forces :
Symbol :
Mass :
Electric charge :
Color charge
Spin :
Weak isospin
Weak hypercharge :
Elementary particle
Bosonic
Gauge boson
weak
W
80.379 ± 0.012 GeV/c² [1] [2] [3]
- 1 e
none
1
-1
0
General
The W-boson (W⁺ and W⁻) is one of the elementary particles in the Standard Model of particle physics. It belongs to the category of gauge bosons, which are force carrier particles responsible for mediating fundamental forces between particles. Specifically, the W-boson mediates the weak nuclear force, one of the four fundamental forces in nature, along with gravity, electromagnetism, and the strong nuclear force.
The W-bosons are named after the weak force. The physicist Steven Weinberg named the additional particle the “Z-particle” and later gave the explanation that it was the last additional particle needed by the model.The W- bosons had already been named, and the Z- bosons were named for having zero electric charge
Particle Characteristics
The W-boson is an elementary particle with a nonzero rest mass.There are two types of W-bosons: the W⁺ and the W⁻. They are antiparticles of each other. The W⁺ has a positive electric charge, while the W⁻ has a negative electric charge. The electric charges of the W-bosons make them capable of participating in weak interactions, which involve the change of electric charge and flavor of particles.
Quantum Numbers
The W-boson has a spin of 1, which means it is a vector particle. It carries a unit of the weak isospin, which is related to the symmetry properties of the weak force. Each type of W-boson has a corresponding weak isospin quantum number: +1/2 for W⁺ and -1/2 for W⁻.
Interaction
The W-boson mediates the weak nuclear force, which is responsible for processes like beta decay, neutrino interactions, and some forms of radioactive decay. Weak interactions involve the transformation of one type of elementary particle into another, such as the conversion of a neutron into a proton, an electron, and an antineutrino in beta decay.
The W-bosons are best known for their role in nuclear decay, for example the decay of a neutron into a proton (intermediated via the W⁻-boson) or the decay of a proton into a neutron (intermediated via the W⁺-boson)
Figure 315 - Neutron and Proton decay via W-bosons
The interaction strength of the weak force is significantly weaker than that of electromagnetism and the strong nuclear force.
The W⁺ and W⁻ bosons mediate different types of weak interactions, depending on the specific process involved.
Production and Decay
W-bosons are produced in high-energy particle collisions, such as those occurring in particle accelerators like the Large Hadron Collider (LHC). They have a very short lifetime, on the order of 10^(-25) seconds, which means they decay almost immediately after being produced.
The primary decay modes of W⁺ and W⁻ bosons involve the transformation of a quark into another quark plus either a positron (for W⁺) or an electron neutrino (for W⁻), or their antiparticles in the corresponding decays.
Some examples of W-boson production/decay
Figure 316 - First observation of Z-boson production via weak-boson fusion | ATLAS Experiment at CERN [1]
Figure 318 - W boson pair production [4]
W- bosons can decay to a lepton and neutrino (one of them charged and another neutral) or to a quark and antiquark of complementary types (with opposite electric charges ±1/3 and ±2/3). 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 colours, NC = 3 . The decay widths for the W+ boson are then proportional to:
Figure 319 – W+-boson decay widths
Here, e+, μ+, τ+ denote the three flavours of leptons (more exactly, the positive charged antileptons). νe,νμ,ντ τ denote the three flavours of neutrinos. The other particles, starting with u and , all denote quarks and antiquarks (factor NC is applied). The various Vij denote the corresponding CKM matrix coefficients.
Unitarity of the CKM matrix implies that
z.png){kind=small}
(26)
thus each of two quark rows sums to 3. Therefore, the leptonic branching ratios of the W- boson are approximately
z.jpg){kind=small}
(27)
The hadronic branching ratio is dominated by the CKM-favored ud- and cs- final states. The sum of the hadronic branching ratios has been measured experimentally to be 67.60±0.27%, with B(ℓ+Vℓ) = 10.80±0.09%
Discussions about the mass of the W-bosons
W bosons mediate the weak interaction, one of the fundamental forces in physics. Because the Standard Model (SM) of particle physics places tight constraints on the mass of the W boson, measuring the mass puts the SM to the test. The Collider Detector at Fermilab (CDF) Collaboration now reports a precise measurement of the W boson mass extracted from data taken at the Tevatron particle accelerator (see the Perspective by Campagnari and Mulders). Surprisingly, the researchers found that the mass of the boson was significantly higher than the SM predicts, with a discrepancy of 7 standard deviations
The mass of the W boson, a mediator of the weak force between elementary particles, is tightly constrained by the symmetries of the standard model of particle physics. The Higgs boson was the last missing component of the model. After observation of the Higgs boson, a measurement of the W boson mass provides a stringent test of the model. The CDF collaburation measured the W boson mass, MW, using data corresponding to 8.8 inverse femtobarns of integrated luminosity collected in proton-antiproton collisions at a 1.96 tera–electron volt center-of-mass energy with the CDF II detector at the Fermilab Tevatron collider. A sample of approximately 4 million W boson candidates was used to obtain
MW = 80,433.5±6.4stat±6.9syst = 80,433.5±9.4 MeV/c²,
the precision of which exceeds that of all previous measurements combined (stat, statistical uncertainty; syst, systematic uncertainty; MeV, mega–electron volts; c, speed of light in a vacuum). This measurement is in significant tension with the standard model expectation.
Litarature
References
[1] First observation of Z-boson production via weak-boson fusion. CERN Courrier 2014
[2] Observation of photon-induced W+W− production in pp collisions at TeV using the ATLAS detector. Physics Letters B Volume 816, 10 May 2021, 136190
[3] High-precision measurement of the W boson mass with the CDF II detector Science VOL. 376, NO. 6589. DOI: 10.1126/science.abk1781
[4] A simulation study of SUGRA and GMSB signatures in the ATLAS detector at LHC, CERN. Halvor Kippe 2001