CP violation
Encyclopedia
In particle physics
, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry
(charge
conjugation symmetry) and P-symmetry
(parity symmetry). CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle (C symmetry), and left and right were swapped (P symmetry). The discovery of CP violation in 1964 in the decays of neutral kaon
s resulted in the Nobel Prize in Physics
in 1980 for its discoverers James Cronin
and Val Fitch.
It plays an important role both in the attempts of cosmology
to explain the dominance of matter
over antimatter
in the present Universe
, and in the study of weak interaction
s in particle physics.
: C for charge conjugation, which transforms a particle into its antiparticle
, and P for parity, which creates the mirror image of a physical system. The strong interaction
and electromagnetic interaction seem to be invariant under the combined CP transformation operation, but this symmetry is slightly violated during certain types of weak decay. Historically, CP-symmetry was proposed to restore order after the discovery of parity violation in the 1950s.
The idea behind parity symmetry is that the equations of particle physics are invariant under mirror inversion. This leads to the prediction that the mirror image of a reaction (such as a chemical reaction
or radioactive decay
) occurs at the same rate as the original reaction. Parity symmetry appears to be valid for all reactions involving electromagnetism
and strong interaction
s. Until 1956, parity conservation was believed to be one of the fundamental geometric conservation laws (along with conservation of energy
and conservation of momentum). However, in 1956 a careful critical review of the existing experimental data by theoretical physicists Tsung-Dao Lee
and Chen Ning Yang revealed that while parity conservation had been verified in decays by the strong or electromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. The first test based on beta decay
of Cobalt-60
nuclei was carried out in 1956 by a group led by Chien-Shiung Wu
, and demonstrated conclusively that weak interactions violate the P symmetry or, as the analogy goes, some reactions did not occur as often as their mirror image.
Overall, the symmetry of a quantum mechanical
system can be restored if another symmetry S can be found such that the combined symmetry PS remains unbroken. This rather subtle point about the structure of Hilbert space
was realized shortly after the discovery of P violation, and it was proposed that charge conjugation was the desired symmetry to restore order.
Simply speaking, charge conjugation is a simple symmetry between particles and antiparticles, and so CP-symmetry was proposed in 1957 by Lev Landau
as the true symmetry between matter and antimatter.
In other words a process in which all particles are exchanged with their antiparticle
s was assumed to be equivalent to the mirror image of the original process.
, Val Fitch with coworkers provided clear evidence (which was first announced at the 12th ICHEP
conference in Dubna
) that CP-symmetry could be broken, too, winning them the 1980 Nobel Prize
. This discovery showed that weak interactions violate not only the charge-conjugation symmetry C between particles and antiparticles and the P or parity, but also their combination. The discovery shocked particle physics and opened the door to questions still at the core of particle physics and of cosmology today. The lack of an exact CP-symmetry, but also the fact that it is so nearly a symmetry, created a great puzzle.
Only a weaker version of the symmetry could be preserved by physical phenomena, which was CPT symmetry
. Besides C and P, there is a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time reversal implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. The combination of CPT is thought to constitute an exact symmetry of all types of fundamental interactions. Because of the CPT symmetry, a violation of the CP-symmetry is equivalent to a violation of the T symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of quantum field theory
, charge conjugation, parity, and time reversal are applied together.
The kind of CP violation discovered in 1964 was linked to the fact that neutral kaon
s can transform into their antiparticle
s (in which each quark
is replaced with the other's antiquark) and vice versa, but such transformation does not occur with exactly the same probability in both directions; this is called indirect CP violation.
Despite many searches, no other manifestation of CP violation was discovered until the 1990s, when the NA31 experiment at CERN
suggested evidence for CP violation in the decay process of the very same neutral kaons (direct CP violation). The observation was somewhat controversial, and final proof for it came in 1999 from the KTeV experiment at Fermilab
and the NA48 experiment at CERN
.
In 2001, a new generation of experiments, including the BaBar Experiment
at the Stanford Linear Accelerator Center (SLAC) and the Belle Experiment
at the High Energy Accelerator Research Organisation (KEK
) in Japan
, observed direct CP violation in a different sector of particle physics, namely in decays of the B meson
s. By now a large number of CP violation processes in B meson
decays have been discovered. Before these "B factory" experiments, it was a logical possibility that all CP violation was confined to kaon physics. However, this raised the question of why it's not extended to the strong force, and furthermore, why this is not predicted in the unextended Standard Model
, despite the model being undeniably accurate with "normal" phenomena.
The CP violation is incorporated in the Standard Model by including a complex phase in the CKM matrix describing quark
mixing. In such scheme a necessary condition for the appearance of the complex phase, and thus for CP violation, is the presence of at least three generations of quarks.
. This presents the strong CP problem (see below).
, the strong CP problem is the puzzling question of why quantum chromodynamics
(QCD) does not seem to break the CP-symmetry.
QCD does not violate the CP-symmetry as easily as the electroweak theory; unlike the electroweak theory in which the gauge fields couple to chiral
currents constructed from the fermion
ic fields, the gluons couple to vector currents. Experiments do not indicate any CP violation in the QCD sector. For example, a generic CP violation in the strongly interacting sector would create the electric dipole moment
of the neutron
which would be comparable to 10−18 e
·m
while the experimental upper bound is roughly a trillion times smaller.
This is a problem because at the end, there are natural terms in the QCD Lagrangian
that are able to break the CP-symmetry.
For a nonzero choice of the θ angle and the chiral quark mass phase θ′ one expects the CP-symmetry to be violated. One usually assumes that the chiral quark mass phase can be converted to a contribution to the total effective
angle, but it remains to be explained why this angle is extremely small instead of being of order one; the particular value of the θ angle that must be very close to zero (in this case) is an example of a fine-tuning problem
in physics, and is typically solved by physics beyond the Standard Model.
There are several proposed solutions to solve the strong CP problem. The most well-known is Peccei-Quinn theory
, involving new scalar particles called axion
s. A newer, more radical approach not requiring the axion is a theory involving two time dimensions
first proposed in 1998 by Bars, Deliduman, and Andreev.
The strong CP problem may also be solved within a theory of quantum gravity
.
. It refers to an issue related to the enhanced new physics contributions to the neutron EDM in flavor anarchic models.
, rather than consisting of equal parts of matter and antimatter
. It can be demonstrated that, to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang
. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter–antimatter imbalance was present at the beginning, or on other admittedly exotic assumptions.
The Big Bang should have produced equal amounts of matter and antimatter if CP-symmetry was preserved; as such, there should have been total cancellation of both—protons should have cancelled with antiproton
s, electrons with antielectrons
, neutrons with antineutron
s, and so on. This would have resulted in a sea of radiation in the universe with no matter. Since this is not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP-symmetry.
The Standard Model
contains only two ways to break CP-symmetry. The first of these, discussed above, is in the QCD Lagrangian, and has not been found experimentally; but one would expect this to lead to either no CP violation or a CP violation that is many, many orders of magnitude too large. The second of these, involving the weak force, has been experimentally verified, but can account for only a small portion of CP violation. It is predicted to be sufficient for a net mass of normal matter equivalent to only a single galaxy in the known universe.
Since the Standard Model does not accurately predict this discrepancy, it would seem that the current Standard Model has gaps (other than the obvious one of gravity and related matters) or physics is otherwise in error. Moreover, experiments to probe these CP-related gaps may require the practically impossible-to-obtain energies that may be necessary to probe the gravity-related gaps (see Planck mass).
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry
C-symmetry
In physics, C-symmetry means the symmetry of physical laws under a charge-conjugation transformation. Electromagnetism, gravity and the strong interaction all obey C-symmetry, but weak interactions violate C-symmetry.-Charge reversal in electromagnetism:...
(charge
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
conjugation symmetry) and P-symmetry
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...
(parity symmetry). CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle (C symmetry), and left and right were swapped (P symmetry). The discovery of CP violation in 1964 in the decays of neutral kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
s resulted in the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
in 1980 for its discoverers James Cronin
James Cronin
James Watson Cronin is an American nuclear physicist.Cronin was born in Chicago, Illinois and attended Southern Methodist University in Dallas, Texas. Cronin and co-researcher Val Logsdon Fitch were awarded the 1980 Nobel Prize in Physics for a 1964 experiment that proved that certain subatomic...
and Val Fitch.
It plays an important role both in the attempts of cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
to explain the dominance of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
over antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
in the present Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
, and in the study of weak interaction
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...
s in particle physics.
What is CP?
CP is the product of two symmetriesSymmetry in physics
In physics, symmetry includes all features of a physical system that exhibit the property of symmetry—that is, under certain transformations, aspects of these systems are "unchanged", according to a particular observation...
: C for charge conjugation, which transforms a particle into its antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
, and P for parity, which creates the mirror image of a physical system. The strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
and electromagnetic interaction seem to be invariant under the combined CP transformation operation, but this symmetry is slightly violated during certain types of weak decay. Historically, CP-symmetry was proposed to restore order after the discovery of parity violation in the 1950s.
The idea behind parity symmetry is that the equations of particle physics are invariant under mirror inversion. This leads to the prediction that the mirror image of a reaction (such as a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
or radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
) occurs at the same rate as the original reaction. Parity symmetry appears to be valid for all reactions involving electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
and strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
s. Until 1956, parity conservation was believed to be one of the fundamental geometric conservation laws (along with conservation of energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...
and conservation of momentum). However, in 1956 a careful critical review of the existing experimental data by theoretical physicists Tsung-Dao Lee
Tsung-Dao Lee
Tsung-Dao Lee is a Chinese born-American physicist, well known for his work on parity violation, the Lee Model, particle physics, relativistic heavy ion physics, nontopological solitons and soliton stars....
and Chen Ning Yang revealed that while parity conservation had been verified in decays by the strong or electromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. The first test based on beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
of Cobalt-60
Cobalt-60
Cobalt-60, , is a synthetic radioactive isotope of cobalt. Due to its half-life of 5.27 years, is not found in nature. It is produced artificially by neutron activation of . decays by beta decay to the stable isotope nickel-60...
nuclei was carried out in 1956 by a group led by Chien-Shiung Wu
Chien-Shiung Wu
Chien-Shiung Wu was a Chinese-American physicist with expertise in the techniques of experimental physics and radioactivity. Wu worked on the Manhattan Project...
, and demonstrated conclusively that weak interactions violate the P symmetry or, as the analogy goes, some reactions did not occur as often as their mirror image.
Overall, the symmetry of a quantum mechanical
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
system can be restored if another symmetry S can be found such that the combined symmetry PS remains unbroken. This rather subtle point about the structure of Hilbert space
Hilbert space
The mathematical concept of a Hilbert space, named after David Hilbert, generalizes the notion of Euclidean space. It extends the methods of vector algebra and calculus from the two-dimensional Euclidean plane and three-dimensional space to spaces with any finite or infinite number of dimensions...
was realized shortly after the discovery of P violation, and it was proposed that charge conjugation was the desired symmetry to restore order.
Simply speaking, charge conjugation is a simple symmetry between particles and antiparticles, and so CP-symmetry was proposed in 1957 by Lev Landau
Lev Landau
Lev Davidovich Landau was a prominent Soviet physicist who made fundamental contributions to many areas of theoretical physics...
as the true symmetry between matter and antimatter.
In other words a process in which all particles are exchanged with their antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
s was assumed to be equivalent to the mirror image of the original process.
Indirect CP violation
In 1964, James CroninJames Cronin
James Watson Cronin is an American nuclear physicist.Cronin was born in Chicago, Illinois and attended Southern Methodist University in Dallas, Texas. Cronin and co-researcher Val Logsdon Fitch were awarded the 1980 Nobel Prize in Physics for a 1964 experiment that proved that certain subatomic...
, Val Fitch with coworkers provided clear evidence (which was first announced at the 12th ICHEP
ICHEP
ICHEP or International Conference on High Energy Physics is one of the most prestigious international scientific conferences in the field of particle physics, bringing together leading theorists and experimentalists of the world. It was first held in 1950, and is biennial since 1960...
conference in Dubna
Dubna
Dubna is a town in Moscow Oblast, Russia. It has a status of naukograd , being home to the Joint Institute for Nuclear Research, an international nuclear physics research centre and one of the largest scientific foundations in the country. It is also home to MKB Raduga, a defence aerospace company...
) that CP-symmetry could be broken, too, winning them the 1980 Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
. This discovery showed that weak interactions violate not only the charge-conjugation symmetry C between particles and antiparticles and the P or parity, but also their combination. The discovery shocked particle physics and opened the door to questions still at the core of particle physics and of cosmology today. The lack of an exact CP-symmetry, but also the fact that it is so nearly a symmetry, created a great puzzle.
Only a weaker version of the symmetry could be preserved by physical phenomena, which was CPT symmetry
CPT symmetry
CPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity, and time simultaneously.-History:...
. Besides C and P, there is a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time reversal implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. The combination of CPT is thought to constitute an exact symmetry of all types of fundamental interactions. Because of the CPT symmetry, a violation of the CP-symmetry is equivalent to a violation of the T symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...
, charge conjugation, parity, and time reversal are applied together.
Direct CP violation
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
s can transform into their antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
s (in which each quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
is replaced with the other's antiquark) and vice versa, but such transformation does not occur with exactly the same probability in both directions; this is called indirect CP violation.
Despite many searches, no other manifestation of CP violation was discovered until the 1990s, when the NA31 experiment at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
suggested evidence for CP violation in the decay process of the very same neutral kaons (direct CP violation). The observation was somewhat controversial, and final proof for it came in 1999 from the KTeV experiment at Fermilab
Fermilab
Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...
and the NA48 experiment at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
.
In 2001, a new generation of experiments, including the BaBar Experiment
BaBar experiment
The BaBar experiment, or simply BaBar, is an international collaboration of more than 500 physicists and engineers studying the subatomic world at energies of approximately ten times the rest mass of a proton . Its design was motivated by the investigation of CP violation...
at the Stanford Linear Accelerator Center (SLAC) and the Belle Experiment
Belle experiment
The Belle experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation in Tsukuba, Ibaraki Prefecture, Japan.The...
at the High Energy Accelerator Research Organisation (KEK
KEK
, known as KEK, is a national organization whose purpose is to operate the largest particle physics laboratory in Japan, which is situated in Tsukuba of Ibaraki prefecture. Established in 1997. The term "KEK" is also used to refer to the laboratory itself, which employs approximately 900 employees...
) in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, observed direct CP violation in a different sector of particle physics, namely in decays of the B meson
B meson
B mesons are mesons composed of a bottom quark or bottom antiquark and either an up , down , strange or charm quark . The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's short lifetime...
s. By now a large number of CP violation processes in B meson
B meson
B mesons are mesons composed of a bottom quark or bottom antiquark and either an up , down , strange or charm quark . The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's short lifetime...
decays have been discovered. Before these "B factory" experiments, it was a logical possibility that all CP violation was confined to kaon physics. However, this raised the question of why it's not extended to the strong force, and furthermore, why this is not predicted in the unextended Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
, despite the model being undeniably accurate with "normal" phenomena.
The CP violation is incorporated in the Standard Model by including a complex phase in the CKM matrix describing quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
mixing. In such scheme a necessary condition for the appearance of the complex phase, and thus for CP violation, is the presence of at least three generations of quarks.
CP violation in strong sector
There is no experimentally known violation of the CP-symmetry in quantum chromodynamicsQuantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
. This presents the strong CP problem (see below).
Strong CP problem
In particle physicsParticle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, the strong CP problem is the puzzling question of why quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(QCD) does not seem to break the CP-symmetry.
QCD does not violate the CP-symmetry as easily as the electroweak theory; unlike the electroweak theory in which the gauge fields couple to chiral
Chirality (physics)
A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...
currents constructed from the fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
ic fields, the gluons couple to vector currents. Experiments do not indicate any CP violation in the QCD sector. For example, a generic CP violation in the strongly interacting sector would create the electric dipole moment
Electric dipole moment
In physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system's overall polarity with SI units of Coulomb-meter...
of the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
which would be comparable to 10−18 e
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
·m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
while the experimental upper bound is roughly a trillion times smaller.
This is a problem because at the end, there are natural terms in the QCD Lagrangian
Lagrangian
The Lagrangian, L, of a dynamical system is a function that summarizes the dynamics of the system. It is named after Joseph Louis Lagrange. The concept of a Lagrangian was originally introduced in a reformulation of classical mechanics by Irish mathematician William Rowan Hamilton known as...
that are able to break the CP-symmetry.
For a nonzero choice of the θ angle and the chiral quark mass phase θ′ one expects the CP-symmetry to be violated. One usually assumes that the chiral quark mass phase can be converted to a contribution to the total effective
angle, but it remains to be explained why this angle is extremely small instead of being of order one; the particular value of the θ angle that must be very close to zero (in this case) is an example of a fine-tuning problemFine-tuning
In theoretical physics, fine-tuning refers to circumstances when the parameters of a model must be adjusted very precisely in order to agree with observations. Theories requiring fine-tuning are regarded as problematic in the absence of a known mechanism to explain why the parameters happen to...
in physics, and is typically solved by physics beyond the Standard Model.
There are several proposed solutions to solve the strong CP problem. The most well-known is Peccei-Quinn theory
Peccei-Quinn theory
In particle physics, the Peccei–Quinn theory is the best known proposal for the resolution of the strong CP problem. The theory proposes that the QCD Lagrangian be extended with a CP-violating term known as the θ parameter...
, involving new scalar particles called axion
Axion
The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics...
s. A newer, more radical approach not requiring the axion is a theory involving two time dimensions
Multiple time dimensions
The possibility that there might be more than one dimension of time has occasionally been discussed in physics and philosophy.- Physics :Special relativity describes spacetime as a manifold whose metric tensor has a negative eigenvalue. This corresponds to the existence of a "time-like" direction...
first proposed in 1998 by Bars, Deliduman, and Andreev.
The strong CP problem may also be solved within a theory of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
.
Little CP problem
The little CP problem is a term coined by Lisa RandallLisa Randall
Lisa Randall is an American theoretical physicist and a leading expert on particle physics and cosmology. She works on several of the competing models of string theory in the quest to explain the fabric of the universe. Her most well known contribution to the field is the Randall-Sundrum model,...
. It refers to an issue related to the enhanced new physics contributions to the neutron EDM in flavor anarchic models.
CP violation and the matter–antimatter imbalance
One of the unsolved theoretical questions in physics is why the universe is made chiefly of matterMatter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
, rather than consisting of equal parts of matter and antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
. It can be demonstrated that, to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter–antimatter imbalance was present at the beginning, or on other admittedly exotic assumptions.
The Big Bang should have produced equal amounts of matter and antimatter if CP-symmetry was preserved; as such, there should have been total cancellation of both—protons should have cancelled with antiproton
Antiproton
The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy....
s, electrons with antielectrons
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
, neutrons with antineutron
Antineutron
The antineutron is the antiparticle of the neutron with symbol . It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, but has opposite baryon number...
s, and so on. This would have resulted in a sea of radiation in the universe with no matter. Since this is not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP-symmetry.
The Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
contains only two ways to break CP-symmetry. The first of these, discussed above, is in the QCD Lagrangian, and has not been found experimentally; but one would expect this to lead to either no CP violation or a CP violation that is many, many orders of magnitude too large. The second of these, involving the weak force, has been experimentally verified, but can account for only a small portion of CP violation. It is predicted to be sufficient for a net mass of normal matter equivalent to only a single galaxy in the known universe.
Since the Standard Model does not accurately predict this discrepancy, it would seem that the current Standard Model has gaps (other than the obvious one of gravity and related matters) or physics is otherwise in error. Moreover, experiments to probe these CP-related gaps may require the practically impossible-to-obtain energies that may be necessary to probe the gravity-related gaps (see Planck mass).
See also
- B-factoryB-FactoryIn particle physics, a B-factory, or sometimes a beauty factory, is a collider-based scientific machine designed to produce a large number of B mesons and analyze their properties. The tauons and D mesons are also copiously produced at B-factories, which allows precise studies of their...
- LHCbLHCbLHCb is one of six particle physics detector experiments collecting data at the Large Hadron Collider accelerator at CERN. LHCb is a specialized b-physics experiment, that is measuring the parameters of CP violation in the interactions of b-hadrons...
- BTeVBTeVThe BTeV experiment — for B meson TeV — was an experiment in high-energy particle physics designed to challenge the Standard Model explanation of CP violation, mixing and rare decays of bottom and charm quark states. The Standard Model has been the baseline particle physics theory for several...
- Cabibbo–Kobayashi–Maskawa matrix
- Penguin diagramPenguin diagramIn quantum field theory, penguin diagrams are a class of Feynman diagrams which are important for understanding CP violating processes in the standard model. They refer to one-loop processes in which a quark temporarily changes flavor , and the flavor-changed quark engages in some tree interaction,...