In
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
, flavor-changing neutral currents or flavour-changing neutral currents (FCNCs) are hypothetical interactions that change the
flavor of a
fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks and ...
without altering its
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons res ...
.
Details
If they occur in nature (as reflected by
Lagrangian
Lagrangian may refer to:
Mathematics
* Lagrangian function, used to solve constrained minimization problems in optimization theory; see Lagrange multiplier
** Lagrangian relaxation, the method of approximating a difficult constrained problem with ...
interaction terms), these processes may induce phenomena that have not yet been observed in experiment. Flavor-changing neutral currents may occur in the
Standard Model
The Standard Model of particle physics is the theory describing three of the four known fundamental forces ( electromagnetic, weak and strong interactions - excluding gravity) in the universe and classifying all known elementary particles. It ...
beyond the
tree level, but they are highly suppressed by the
GIM mechanism In particle physics, the GIM mechanism (or Glashow–Iliopoulos–Maiani mechanism) is the mechanism through which flavour-changing neutral currents (FCNCs) are suppressed in loop diagrams. It also explains why weak interactions that change strang ...
. Several collaborations have searched for FCNC. The
Tevatron
The Tevatron was a circular particle accelerator (active until 2011) in the United States, at the Fermi National Accelerator Laboratory (also known as ''Fermilab''), east of Batavia, Illinois, and is the second highest energy particle collider ...
CDF experiment observed evidence of FCNC in the decay of the strange B-meson to phi mesons in 2005.
FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of
supersymmetry
In a supersymmetric theory the equations for force and the equations for matter are identical. In theoretical and mathematical physics, any theory with this property has the principle of supersymmetry (SUSY). Dozens of supersymmetric theories ...
or
technicolor
Technicolor is a series of Color motion picture film, color motion picture processes, the first version dating back to 1916, and followed by improved versions over several decades.
Definitive Technicolor movies using three black and white films ...
. Their suppression is necessary for an agreement with observations, making FCNCs important constraints on model-building.
Example
Consider a
toy model
In the modeling of physics, a toy model is a deliberately simplistic model with many details removed so that it can be used to explain a mechanism concisely. It is also useful in a description of the fuller model.
* In "toy" mathematical models ...
in which an undiscovered
boson
In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer spi ...
''S'' may couple both to the
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
as well as the
tau () via the term
:
Since the electron and the tau have equal charges, the electric charge of ''S'' clearly must vanish to respect the conservation of electric charge. A
Feynman diagram
In theoretical physics, a Feynman diagram is a pictorial representation of the mathematical expressions describing the behavior and interaction of subatomic particles. The scheme is named after American physicist Richard Feynman, who introduc ...
with ''S'' as the intermediate particle is able to convert a tau into an electron (plus some neutral decay products of the ''S'').
The
MEG experiment at the
Paul Scherrer Institute
The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the River ...
near
Zurich will search for a similar process, in which an
antimuon decays to a
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
and an antielectron (a
positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collide ...
). In the Standard Model, such a process proceeds only by emission and re-absorption of a charged
, which changes the into a
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
on emission and then a
positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collide ...
on re-absorption, and finally emits a
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
that carries away any difference in
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
, spin, and
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...
.
In most cases of interest, the boson involved is not a new boson ''S'' but the conventional
boson itself.
[FCNCs involving the ]photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
cannot occur at zero momentum transfer
In particle physics, wave mechanics and optics, momentum transfer is the amount of momentum that one particle gives to another particle. It is also called the scattering vector as it describes the transfer of wavevector in wave mechanics.
In the s ...
s, because of the unbroken electromagnetic gauge symmetry
In physics, a gauge theory is a type of field theory in which the Lagrangian (and hence the dynamics of the system itself) does not change (is invariant) under local transformations according to certain smooth families of operations (Lie group ...
; as such, FCNCs involving the photon at a non-zero momentum transfer are relatively suppressed compared to FCNCs involving the boson. This can occur if the coupling to weak neutral currents is (slightly) non-universal. The dominant universal coupling to the Z boson does not change flavor, but sub-dominant non-universal contributions can.
FCNCs involving the boson for the down-type
quark
A quark () is a type of 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. All commonly ...
s at zero momentum transfer are usually parameterized by the
effective action
In quantum field theory, the quantum effective action is a modified expression for the classical action taking into account quantum corrections while ensuring that the principle of least action applies, meaning that extremizing the effective ac ...
term
:
This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay of
mesons in
Belle and
BaBar
Babar ( ur, ), also variously spelled as Baber, Babur, and Babor is a male given name of Pashto, and Persian origin, and a popular male given name in Pakistan. It is generally taken in reference to the Persian ''babr'' (Persian: ببر), meanin ...
. The off-diagonal entries of ''U'' parameterizes the FCNCs and current constraints restrict them to be less than one part in a thousand for , ''U''
bs, . The contribution coming from the one-loop Standard Model corrections are actually dominant, but the experiments are precise enough to measure slight deviations from the Standard Model prediction.
Experiments tend to focus on flavor-changing neutral currents as opposed to
charged current
Charged current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the and bosons.
In simple terms
Charged current interactions are the most easily det ...
s, because the
weak neutral current ( boson) does not change flavor in the Standard Model proper at the tree level whereas the weak charged currents (
bosons) do. New physics in charged current events would be swamped by more numerous
boson interactions; new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics.
See also
*
Two-Higgs-doublet model The two-Higgs-doublet model (2HDM) is an extension of the Standard Model of particle physics. 2HDM models are one of the natural choices for beyond-SM models containing two Higgs doublets instead of just one. There are also models with more than tw ...
References
{{reflist, 25em
Standard Model
Physics beyond the Standard Model
Hypothetical processes