Z-boson
Composition :
Statistics :
Family :
Interaction forces :
Symbol :
Mass :
Electric charge :
Color charge
Spin :
Weak isospin
Weak hypercharge :
Elementary particle
Bosonic
Gauge boson
weak
Z
91.1876 ± 0.0021 GeV/c2 [1]
0 e
none
1
0
0
General
These bosons, classified as elementary particles, carry considerable mass. The W and Z bosons weigh 80.4 GeV/c² and 91.2 GeV/c², respectively. This makes them nearly 80 times heavier than a proton and even surpasses the weight of entire iron atoms.
Their substantial masses significantly constrain the range of the weak interaction. In contrast, the photon, which mediates the electromagnetic force, boasts a zero mass, aligning with the infinite range of electromagnetism. Similarly, the graviton, though hypothetical, is also theorized to possess zero mass. (While gluons are believed to have zero mass as well, the color force they convey is restricted due to different factors, as outlined in color confinement theory.)
All three bosons possess a particle spin of s = 1. When a W+ or W- boson is emitted, it either elevates or diminishes the electric charge of the emitting particle by one unit, concurrently altering the spin by one unit. Additionally, the emission or absorption of a W± boson can lead to a change in the particle's type, such as transforming a strange quark into an up quark. On the other hand, the neutral Z boson, with its mass of 91.2 GeV/c², is incapable of altering the electric charge of any particle or modifying any other "charges" (like strangeness, baryon number, charm, etc.). Its emission or absorption solely affects the spin, momentum, and energy of the involved particle.
Specific characteristics of the Z°-boson
The Z0 boson, unlike its counterparts, is its own antiparticle. As a result, all of its flavor quantum numbers and charges are zero. During the exchange of a Z boson between particles, known as a neutral current interaction, the interacting particles remain unaffected, except for a transfer of spin and/or momentum.
Interactions involving the Z boson and neutrinos exhibit distinct characteristics. They represent the sole mechanism known for the elastic scattering of neutrinos in matter. Neutrinos are nearly as likely to scatter elastically (via Z boson exchange) as they are to scatter inelastically (via W boson exchange). The confirmation of weak neutral currents via Z boson exchange occurred shortly thereafter, in 1973, during a neutrino experiment conducted in the Gargamelle bubble chamber at CERN.
Creation of the Z-boson
The production of Z bosons in proton-proton collisions at the LHC serves as a standard candle in the initial phase of data-taking for the ATLAS experiment. The decay of Z bosons into an electron-positron pair provides a distinct signature in the detector, enabling performance studies and calibration. With a cross-section of approximately 1 nb, this allows for initial LHC measurements of parton density distributions. This study focuses on simulations of 10 TeV collisions and discusses the challenges of experimentally measuring the cross-section with an integrated luminosity of 100 pb^(-1). The efficiencies of single electrons are determined both through simulation-based methods and testing a data-based approach, showing excellent agreement within approximately 3%. Various components of an inclusive and differential Z production cross-section measurement at ATLAS are discussed, along with their potential contributions to systematic uncertainties. For a selection of signal and background events, the expected uncertainty on the inclusive cross-section for an integrated luminosity of 100 pb^(-1) is determined to be Δσ_pp→γ* / Z+X→e+e-+X = 1.5%stat ± 4.2%syst ± 10%lumi. The possibilities for simple-differential cross-section measurements in rapidity and transverse momentum of the Z boson are outlined, crucial for understanding parton density distributions and studying non-perturbative effects within pQCD. The challenges of efficiency correction based on electron efficiencies, which are functions of electron transverse momentum and pseudorapidity, are studied. An alternative approach, involving an additional dimension - namely, the invariant mass of the lepton pair - is suggested. [1]
Figure 313 - Proton-Proton collision creating Z-boson (Drell-Yan) [2]
Decay of the Z-boson
Z bosons undergo decay into a fermion and its corresponding antiparticle. The Z0 boson, a combination of the pre-symmetry-breaking W0 and B0 bosons (refer to weak mixing angle), involves a vertex factor with a coefficient of T3 − Q sin2 θW, where T3 represents the third component of the fermion's weak isospin (indicating the "charge" for the weak force), Q denotes the electric charge of the fermion (measured in elementary charge units), and θW symbolizes the weak mixing angle. Since the weak isospin (T3) differs for fermions of distinct chirality, be it left-handed or right-handed, the coupling also varies accordingly.
Estimations of the relative strengths of each coupling involve considering that the decay rates encompass the square of these coefficients, factoring in all feasible diagrams (e.g., summation over quark families, left and right contributions). The tabulated outcomes provided below are approximations, as they solely account for tree-level interaction diagrams within the framework of the Fermi theory.
Figure 314 - Z-Boson decay - Table of Relative strengths of the couplings
To keep the notation compact, the table uses x = sin2 θW [3]
In 2018, the CMS collaboration observed the first exclusive decay of the
Z- boson to a ψ meson and a lepton–antilepton pair.
In-depth reading
-
Production of Z bosons and neutrinos in early universe. Cosmin Crucean. the European Physical Journal. (2019)
References
[1] Production of Z Bosons in Proton-Proton Collisions at √ s = 10 TeV: Expectations for Early Measurements at the ATLAS Experiment. Markus Bendel
[2] Quantum Diaries. Richard Ruiz.
[3] Wikipedia W and Z bosons