+ 0,5
- 0,3
Down Quark
Composition :
Statistics : Generation :
Family :
Interaction forces :
Symbol :
Antiparticle :
Mass :
Decays into :
Electric charge :
Color charge
Spin :
Weak isospin :
Weak hypercharge :
_
Elementary particle
Fermionic
First
Quark
strong, weak,
electromagnetic force
gravity
d
Down Antiquark ( d )
4,7 MeV/c²
stable or
u + e + νₑ
-¹∕₃ e
Yes
¹/₂
LH : - ¹/₂, RH : 0
LH : + ¹/₃, RH : -²∕₃
DOWN QUARK
General
The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up quarks, one down quark) of atomic nuclei. It is part of the first generation of matter, has an electric charge of −1/3 e and a bare mass of 4.7 (+0.5/−0.3) MeV/c2.
Like all quarks, the down quark is an elementary fermion with spin 1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the down quark is the down antiquark (sometimes called antidown quark or simply antidown), which differs from it only in that some of its properties have equal magnitude but opposite sign.
Quantum Numbers of the Down-Quark:
The down-quark carries a negative electric charge of -1/3e, making it the only one of the six quark flavors with a negative charge.
Quarks also possess color charge, which is unrelated to visual colors but is a quantum property. Quarks come in three "colors" (red, green, blue), while antiquarks come in anticolors (anticolor charges: antired, antigreen, antiblue).
The strong nuclear force, described by Quantum Chromodynamics (QCD), governs the interactions between quarks and is mediated by gluons, which carry color charge.
Role in Particle Structure:
Down-quarks, along with up-quarks, are fundamental components of hadrons, such as protons and neutrons. A proton consists of two up-quarks and one down-quark, while a neutron consists of one up-quark and two down-quarks.
The combination of quarks within these hadrons, held together by the strong nuclear force, is essential for the stability of atomic nuclei.
Quantum Field Theory (QFT) and the Strong Force:
In the framework of quantum field theory, quarks are described as excitations of the quark field, which permeates all of spacetime.
Quantum Chromodynamics (QCD) is the quantum field theory that describes the strong force interactions between quarks and gluons. It is an integral part of the Standard Model of particle physics.
QCD explains the behavior of quarks within hadrons and the phenomenon of confinement, where quarks are never observed as free particles because the strong force becomes stronger as quarks are separated.
Experimental Observations:
Experimental evidence for the down-quark, along with other quark flavors, has been obtained through high-energy particle physics experiments conducted in accelerators like the Large Hadron Collider (LHC).
The properties and behavior of down-quarks, as predicted by the Standard Model, have been confirmed in numerous experiments, supporting the current understanding of particle physics.
Combinations with Down Quarks
Down quarks, as one of the six types of quarks in particle physics, can combine with other quarks to form various types of hadrons (composite particles made up of quarks). Here are some of the known combinations with down quarks:
Nucleons (Baryons):
Proton (uud): A combination of two up quarks (u) and one down quark (d). It is the nucleus of a hydrogen atom.
Neutron (udd): Comprising one up quark (u) and two down quarks (d), it is found in the nuclei of most atoms.
Hyperons (Baryons with Strange Quarks):
Lambda baryon (Λ0, uds): Consists of an up quark (u), a down quark (d), and a strange quark (s).
Mesons (Quark-Antiquark Pairs):
Pion (π±, π0): A combination of an up quark (u) and a down quark (d), or their antiquarks (ū, d̄).
Kaon (K±, K0): Comprising a strange quark (s) and an up quark (u) or down quark (d), or their respective antiquarks.
These are some of the common combinations involving down quarks, which play a crucial role in the formation of atomic nuclei and the interactions between particles in the standard model of particle physics.
Figure 68 - Proton (uud)
Figure 69 - Neutron (udd)
Figure 70 - Lambda baryon Λ0 (uds)
Figure 71 - Pion mesons (π±, π0) d d̄ and d ū)
Figure 72 - Kaon mesons (K±, K0)
Creation of Down quarks
The creation of down quarks, like other quarks, is a fundamental process in the universe's history, and our understanding of it is based on the framework of particle physics and the Big Bang theory, along with extensive experimental evidence. Here's what we know about the creation of down quarks:
Early Universe: In the very early universe, shortly after the Big Bang, the conditions were extremely hot and energetic. During this period, the universe was dominated by a state of matter known as quark-gluon plasma (QGP). In this state, quarks and gluons, which are the fundamental building blocks of matter and carriers of the strong nuclear force, existed independently and freely. Down quarks, along with up quarks and other quark flavors, were present in this primordial plasma.
Hadronization: As the universe cooled and expanded, the quark-gluon plasma underwent a phase transition known as hadronization. During this process, quarks combined to form hadrons, which are composite particles made up of quarks. Down quarks combined with up quarks to create hadrons like protons and neutrons. Protons are composed of two up quarks and one down quark (uud), while neutrons are made up of one up quark and two down quarks (udd).
Nucleosynthesis: After hadronization, the universe continued to evolve. During the first few minutes after the Big Bang, a process known as Big Bang nucleosynthesis occurred. This process involved the formation of atomic nuclei from protons and neutrons, which, in turn, are made up of down and up quarks. Down quarks played a crucial role in the creation of helium-4 nuclei, which are the most abundant nuclei in the universe after hydrogen.
Subsequent Evolution: As the universe continued to expand and cool, matter aggregated into structures, leading to the formation of galaxies, stars, and planets. Within stars, nuclear fusion reactions involving down quarks (as part of protons and neutrons) played a central role in powering stars and producing heavier elements through stellar nucleosynthesis.
Decay Processes
Down quarks can participate in various decay processes within the framework of the Standard Model of particle physics. Here are some of the known decay processes involving down quarks:
Beta Decay (Neutron Decay): One of the most well-known decay processes involving down quarks is the beta decay of neutrons. A neutron, which consists of two down quarks and one up quark (udd), can spontaneously decay into a proton (uud), an electron (e⁻), and an electron antineutrino (ν̄e):
This process is mediated by the exchange of a virtual W-boson (W⁻) and occurs inside atomic nuclei, contributing to the stability of matter in the universe.
Kaon Decays: Kaons (K± and K0) are mesons that contain down quarks. They can undergo various decay modes. For example, a neutral long kaon (K0) can decay into a neutral pion, a neutrino and an antineutrino.
Lambda Baryon Decays: Lambda baryons (Λ0) containing a down quark can decay into a proton (uud) or a neutron (udd) by emitting various mesons. For example:
Hyperon Decays: Hyperons, such as Sigma (Σ) and Xi (Ξ) baryons, may contain down quarks and can undergo a variety of decay modes, depending on their quark composition.
These are some of the primary decay processes involving down quarks. These decays play a crucial role in understanding the behavior of subatomic particles and the interactions governed by the weak force within the framework of the Standard Model. They are also important for studying fundamental symmetries and particle properties.
Figure 26 - Lambda Baryon
Figure 24 - β⁻decay of a neutron in a proton
Figure 73 – Neutral Long Kaon (K0) decay
Figure 28 - Sigma decay
Fig. 77
Figure 74 - Down quark pair annihilation creating electrons
Annihilation
The down quark pair annihilation can occur through various processes, including:
Pair annihilation into photons: When a down quark and an anti-down quark annihilate, they can produce two photons. This process can be observed in phenomena such as neutral pion decay, where a pion composed of a down and anti-down quark decays into two photons.
Pair annihilation into gluons: Since quarks are governed by the strong interaction, the annihilation of a down quark pair more often results in the production of two gluons. However, due to quark confinement, which means that quarks cannot be observed outside the particles they compose, this process cannot be directly observed through normal scattering experiments.
Pair annihilation mediated by weak force: Quark and antiquark pairs of different types can also interact through the weak force, mediated by W- and Z-bosons. These interactions can have various outcomes, such as the production of two gauge bosons or the creation of another quark and anti-quark pair. The specific outcomes depend on the original particles involved in the annihilation process.
Figure 65 - e-e+ pair production leads to ūu
Figure 75 - Down quark pair annihilation creating gluons
Literature
In-depth reading
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On the generation of the quarks through spontaneous symmetry breaking. Voicu Dolocan. Faculty of Physics, University of Bucharest, Bucharest, Romania
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The theory of quark confinement. V. N. Gribov (Landau Institute for Theoretical Physics, Moscow) (1999)
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Up and Down-Quark Masses. P. R. Silva (Departamento de Física – ICEx – Universidade Federal de Minas Gerais) (2016)
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Precise Calculation of the Up and Down Quark Mass Using an Adjusted Compton Wavelength Common Factor Analysis. Brian DN (University of Wisconsin-Platteville 1 University Plaza, Platteville WI 53818, USA) (2015)