Charm Quark
Composition :
Statistics :
Generation :
Family :
Interaction forces :
Symbol :
Antiparticle :
Mass :
Decays into :
Electric charge :
Color charge
Spin :
Weak isospin :
Weak hypercharge :
Elementary particle
Fermionic
Second
Quark
strong, weak,
electromagnetic force
gravity
c
Charm Antiquark ( c )
1,275 MeV/c²
strange quark (~95%)
down quark (~5%)
+²∕₃ e
Yes
¹/₂
LH : - ¹/₂, RH : 0
LH : + ¹/₃, RH : +⁴/₃
_
+ 0,025
- 0,035
CHARM QUARK
Definition
A charm quark, often denoted as c, is one of the six types of quarks in the Standard Model of particle physics. Quarks are fundamental particles that are considered the building blocks of protons, neutrons, and other hadrons, which are the particles that make up atomic nuclei. The charm quark is one of the heavier quarks and is an essential component of our understanding of the strong and weak nuclear forces. Here's a detailed description of the charm quark:
Characteristics of the Charm Quark
Electric Charge: The charm quark has an electric charge of +2/3e, where "e" represents the elementary charge. This positive charge makes it one of the positively charged quarks, along with the up quark (u) and the top quark (t).
Mass: The charm quark is relatively massive compared to the up and down quarks, with a mass of approximately 1.27 GeV/c² (gigaelectronvolts divided by the speed of light squared). This mass gives it the name "charm," as it was initially discovered as a particle with unusual charm compared to lighter quarks.
Spin: The charm quark has a half-integer spin of 1/2, which is a fundamental property of all quarks and is associated with their intrinsic angular momentum.
Color Charge: Like all quarks, the charm quark carries a "color charge," which is a property related to the strong nuclear force (quantum chromodynamics or QCD). Quarks come in three "colors" (red, green, and blue) and can combine in ways that result in color-neutral hadrons.
Lifetime: The charm quark has a relatively short lifetime, on the order of picoseconds (10⁻¹² seconds), before it decays into other particles through the weak interaction.
Context in Quantum Physics and Quantum Field Theory
Strong Nuclear Force (Quantum Chromodynamics, QCD): Quarks, including the charm quark, interact through the strong nuclear force described by QCD. This force is mediated by particles called gluons, which carry the color charge. QCD is a crucial component of the Standard Model and governs the behavior of quarks and gluons within hadrons, such as mesons (quark-antiquark pairs) and baryons (three-quark combinations).
Weak Interaction: The charm quark, like other quarks, can participate in weak interactions, which are responsible for processes such as quark flavor-changing decays. Weak interactions involve the exchange of W and Z bosons and are described by electroweak theory, a component of the Standard Model.
Quantum Field Theory (QFT): The behavior of quarks, including the charm quark, is described within the framework of quantum field theory, specifically in the context of QCD. In QFT, particles are viewed as excitations of underlying quantum fields. Quarks are the excitations of quark fields, and their interactions with one another and with gluons are described mathematically using field equations.
The charm quark is of particular interest in particle physics because of its relatively large mass and its role in certain types of decays and reactions, which allow physicists to test the predictions of the Standard Model and explore the dynamics of the strong and weak nuclear forces. Studies involving charm quarks are conducted at particle colliders like the Large Hadron Collider (LHC) to gain insights into the fundamental forces and particles that make up the universe.
Combinations with Charm Quarks
Quarks combine in various ways to form hadrons, such as mesons and baryons. Here's an overview of some of the known combinations involving charm quarks:
Mesons:
D Mesons (charmed mesons): Charm quarks can combine with antiquarks of different flavors to form D mesons.
Baryons:
Charmed Baryons: Charm quarks can also combine with other quarks to form charmed baryons.
Lambda Baryon
These are some of the known combinations involving charm quarks within the framework of the Standard Model. Particle physics research continues to explore and discover new particles and interactions beyond the Standard Model, which may involve exotic combinations of quarks and other fundamental particles.
Figure 88 - Λ⁺c baryon
Figure 89 - Σ⁰c baryon
Figure 90 - Ξ⁺⁺c baryon
Figure 91 - Ω⁺⁺c baryon
Figure 85 - D°/ D⁺ Meson
Figure 86 - D⁺s meson
Figure 87 - J/ψ meson
(charmonium state)
Creation of Charm Quarks
Charm quarks are typically created in high-energy particle collisions, such as those that occur in particle accelerators or during high-energy cosmic ray interactions. Several processes can lead to the creation of charm quarks, and these processes are crucial for studying the properties of charm quarks and exploring the fundamental forces of the universe. Here are some of the primary processes where charm quarks are created:
Hard Scattering in Hadron Colliders:
Charm quarks can be produced in collisions between high-energy hadrons, such as protons and antiprotons, in particle accelerators like the Large Hadron Collider (LHC) at CERN. In these hard scattering processes, two quarks or gluons from the colliding hadrons collide with high momentum, creating new particles, including charm quarks.
Quark-Antiquark Annihilation:
Charm quarks can be produced through quark-antiquark annihilation processes. In high-energy collisions, a quark from one hadron and an antiquark from another hadron can come close enough to annihilate each other, leading to the creation of a pair of new quarks, which may include charm quarks. The same can happen with an electron-positron annihilation.
Gluon Fusion:
Gluons, which are the carriers of the strong nuclear force, can also participate in the creation of charm quarks. In processes like gluon-gluon fusion, high-energy gluons collide and produce quark-antiquark pairs, potentially including charm quarks.
Decays of Heavier Quarks
Charm quarks can be created indirectly through the decays of heavier quarks. For example, top quarks (t quarks) can decay into a W boson and a charm quark, among other possibilities.
Parton Shower and Fragmentation:
In the initial stages of high-energy collisions, partons (quarks and gluons) inside the colliding hadrons undergo parton shower and fragmentation processes. These involve the emission of additional quarks and gluons before hadronization occurs. Charm quarks can be produced in this complex process and subsequently combine with other quarks and gluons to form hadrons.
Charmonium Production:
Charmonium states, such as J/ψ mesons (which consist of a charm quark and an anti-charm quark), can be produced in high-energy collisions. These states are essential for studying the properties of charm quarks and understanding the strong force.
High-Energy Cosmic Rays:
In extremely high-energy cosmic ray interactions with Earth's atmosphere, charm quarks can also be created as part of the cascade of particle production. These events are detected by ground-based observatories and provide insights into the properties of cosmic ray particles.
Figure 92 - Lambda Λb decay in Λc4
Figure 93 - gluon-gluon fusion creating charm quarks
Figure 94 - top quark decay into charm quark
Figure 95 - Feynman diagrams for B0 → J/ψ K + K − , and B0 → J/ψ φ
Decay of Charm Quarks
Semileptonic Decay
In semileptonic charm quark decay, a charm quark (c) transitions into a strange quark (s) while emitting a charged lepton (l) and an associated neutrino (ν). The charged lepton can be an electron (e), muon (μ), or tau (τ), depending on the specific decay mode. The neutrino accompanying the charged lepton is usually an electron neutrino (νe), muon neutrino (νμ), or tau neutrino (ντ).
Nonleptonic Decay
In nonleptonic charm quark decay, the charm quark transforms into a strange quark (s) by emitting two or more light quarks (q), such as up (u) and down (d) quarks. This type of decay involves strong interactions mediated by gluons, and the specific combination of quarks emitted can vary.
Radiative Decay (c → s + γ):
Radiative charm quark decay involves the transition of a charm quark (c) into a strange quark (s) while emitting a photon (γ). This decay process occurs through electromagnetic interactions.
Charm Mixing (c ↔ c'):
Charm quarks can also undergo mixing with their antiparticles (c ↔ c') through weak interactions.
Rare Decays:
In addition to the common decay modes mentioned above, charm quarks can also participate in rare decay processes, which are less frequent but of great interest to particle physicists. Examples of rare charm quark decays include those involving the violation of certain conservation laws, such as (FCNCs).
Figure 96 -Feynman diagram for Semileptonic charm quark decay
Figure 97 – Feynman diagram for Nonleptonic decay (cds)
Figure 99 – Feynman diagram for Charm mixing (c )
Figure 98 -Feynman diagram for Radiative Decay (cdu)
Figure 100 - Standard Model box diagrams of flavor-changing neutral currentscontributing to D0 − 0 mixing at the quark level
Annihilation of Charm Quarks
Charm quarks can be produced through quark-antiquark annihilation processes. In high-energy collisions, a quark from one hadron and an antiquark from another hadron can come close enough to annihilate each other, leading to the creation of a pair of new quarks, which may include charm quarks.The same can happen with an electron-positron annihilation.
Figure 101 - electron-positron annihilation into charm-anticharm
The ALICE collaboration
The ALICE Collaboration delves into the concealed intricacies of quark-gluon plasma. The ALICE research team has unveiled distinctive alterations in various composite forms of charm quarks and their antimatter counterparts within the quark-gluon plasma. This discovery paves the way for novel avenues of research into this unique state of matter and its consequential impacts.
Quark-gluon plasma represents an incredibly high-temperature and dense phase of matter where the basic building blocks, quarks, and gluons, exist without confinement within composite particles like protons and neutrons found in atomic nuclei. It is believed to have been present in the early stages of the universe and can be recreated in experiments at the Large Hadron Collider (LHC) through collisions involving lead nuclei.
A recent investigation conducted by the international ALICE collaboration at the LHC delves into the impact of quark-gluon plasma on diverse bound states of a charm quark and its corresponding antimatter counterpart, which are also produced during these collisions. These findings open up novel avenues for the study of the strong interaction.
The bound states in question, known as charmonia or hidden-charm particles, consist of a charm quark and a charm antiquark (fig. 37) held together by the strong interaction. They serve as excellent probes for exploring quark-gluon plasma. Within the plasma, their production is hindered due to "screening" brought about by the abundance of quarks and gluons present in this unique form of matter. The degree of screening, and consequently the suppression of charmonia production, escalates with the temperature of the plasma. For instance, the production of the ψ(2S) state, which is less tightly bound and approximately 20% more massive than the J/ψ state, is anticipated to be more significantly suppressed than that of the J/ψ state.
Nevertheless, hierarchical suppression isn't the sole fate of charmonia within quark-gluon plasma. The large quantity of charm quarks and antiquarks in the plasma, sometimes numbering around a hundred in head-on collisions, gives rise to a mechanism referred to as recombination. This process leads to the formation of new charmonia and counteracts the suppression to a certain extent. Recombination is expected to be influenced by the type and momentum of the charmonia, with those that are less tightly bound potentially being produced through recombination later in the plasma's evolution, and charmonia possessing the lowest transverse momentum exhibiting the highest recombination rate.
Previous studies, utilizing data from CERN's Super Proton Synchrotron and subsequently from the LHC, have indicated that the production of the ψ(2S) state is indeed more suppressed than that of the J/ψ state. ALICE has also previously provided evidence of the recombination mechanism in J/ψ production. However, until now, no precise studies of ψ(2S) production at low particle momentum had been conducted, preventing a comprehensive understanding of ψ(2S) production from emerging.
The ALICE collaboration has now presented the initial measurements of ψ(2S) production, extending to zero transverse momentum, based on data from lead-lead collisions at the LHC collected in 2015 and 2018.
The measurements reveal that irrespective of particle momentum, the ψ(2S) state experiences approximately twice the level of suppression compared to the J/ψ state. This represents the first instance of a clear hierarchy in suppression observed for the overall production of charmonia at the LHC. A similar observation had been made by the LHC collaborations concerning bound states of a bottom quark and its corresponding antiquark.
Upon further examination, as a function of particle momentum, the suppression of ψ(2S) is observed to decrease at lower momentum levels. This characteristic, previously noted by ALICE in the case of the J/ψ state, serves as a signature of the recombination process.
Future studies with even greater precision, utilizing data from LHC Run 3, which commenced in July, hold the potential to provide a definitive comprehension of the modification of hidden-charm particles and, consequently, the strong interaction that binds them together, within the extreme environment of quark-gluon plasma. [1]
The above illustration shows the effect of quark–gluon plasma on the formation of charmonia in lead-nuclei collisions. When the plasma temperature increases, the more weakly bound ψ(2S) state is more likely to be “screened”, and thus not form, due to the larger number of quarks and gluons in the plasma (the coloured circles). The increase in the number of charm quarks and antiquarks (c and c̄) can lead to the formation of additional charmonia by quark recombination. [2]
Literature : https://alice-publications.web.cern.ch/publications
Figure 102 - Overall view of the ALICE detector [4]
Figure 103 - Illustration of the effect of quark–gluon plasma on the formation of charmonia in lead-nuclei collisions [3]
Online Library
References
[1] CERN accelerating science. 20 october 2020. ALICE collaboration
[2] Image ALICE collaboration
[3] The ALICE collaboration
[4] © CERN – ALICE collaboration. alice.cern