Two-photon physics
Encyclopedia
Two-photon physics, also called gamma-gamma physics, is a branch of particle physics
for the interaction
s between two photons. If the energy
in the center of mass
system of the two photons is large enough, matter
can be created.
(LEP) at CERN
. If the transverse
momentum
transfer
is large, one or both electrons can be deflect
ed enough to be detected; this is called tagging. The other particles that are created in the interaction are tracked by large detectors
to reconstruct the physics of the interaction.
it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes.
A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion
-antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon
structure
, or what is "inside" the photon.
We distinguish three interaction processes:
For the latter two cases, the scale of the interaction is such as the strong coupling constant is large. This is called Vector Meson Dominance (VMD) and has to be modelled in non-perturbative QCD.
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...
for the interaction
Interaction
Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect...
s between two photons. If the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
in the center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
system of the two photons is large enough, 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...
can be created.
Experiments
Two-photon physics can be studied with high-energy particle accelerators, where the accelerated particles are not the photons themselves but charged particles that will radiate photons. The most significant studies so far were performed at the Large Electron-Positron ColliderLarge Electron-Positron Collider
The Large Electron–Positron Collider was one of the largest particle accelerators ever constructed.It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland. LEP was a circular collider with a circumference of 27 kilometres built in a...
(LEP) 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...
. If the transverse
Transversality
In mathematics, transversality is a notion that describes how spaces can intersect; transversality can be seen as the "opposite" of tangency, and plays a role in general position. It formalizes the idea of a generic intersection in differential topology...
momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
transfer
Transfer
Transfer may refer to:* Transfer * Transfer * Transfer DNA, the transferred DNA of the tumor-inducing plasmid of some species of bacteria such as Agrobacterium tumefaciens* Transfer...
is large, one or both electrons can be deflect
Deflection (physics)
In physics deflection is the event where an object collides and bounces against a plane surface.In such collisions involving a sphere and a plane, the collision angle formed with the surface normal must equal the bounce angle , \alpha = \beta.Magnetic deflection refers to Lorentz forces acting...
ed enough to be detected; this is called tagging. The other particles that are created in the interaction are tracked by large detectors
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...
to reconstruct the physics of the interaction.
Processes
From quantum electrodynamicsQuantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes.
A photon can, within the bounds of the uncertainty principle, fluctuate into a charged 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....
-antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon
structure
Structure
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...
, or what is "inside" the photon.
- Direct or pointlike: The photon couples directly to a quark inside the target photon. If a leptonLeptonA lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
-antilepton pair is created, this process involves only quantum electrodynamics (QED), but if a quarkQuarkA 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...
-antiquark pair is created, it involves both QED and perturbative quantum chromodynamicsQuantum chromodynamicsIn 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). - Single resolved: The quark pair of the target photon form a vector mesonVector mesonIn high energy physics, a vector meson is a meson with total spin 1 and odd parity . Compare to a pseudovector meson, which has a total spin 1 and even parity....
. The probing photon couples to a constituent of this meson. - Double resolved: Both target and probe photon have formed a vector meson. This results in an interaction between two hadrons.
For the latter two cases, the scale of the interaction is such as the strong coupling constant is large. This is called Vector Meson Dominance (VMD) and has to be modelled in non-perturbative QCD.
See also
- ChannellingChannelling (physics)Channelling is the process that constrains the path of a charged particle in a crystalline solid.Many physical phenomena can occur when a charged particle is incident upon a solid target, e.g., elastic scattering, inelastic energy-loss processes, secondary-electron emission, electromagnetic...
radiation has been considered as a method to generate polarized high energy photon beams for gamma-gamma colliders. - Matter creationMatter creationMatter creation is the process inverse to particle annihilation. It is the conversion of massless particles into one or more massive particles. This process is the time reversal of annihilation. Since all known massless particles are bosons and the most familiar massive particles are fermions,...
- Pair productionPair productionPair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...