Two-photon physics, also called gamma–gamma
physics, is a branch of particle physics that
describes the interactions between two photons.
Normally, beams of light pass through each
other unperturbed.
Inside an optical material, and if the intensity
of the beams is high enough, the beams may
affect each other through a variety of non-linear
effects.
In pure vacuum, some weak scattering of light
by light exists as well.
Also, above some threshold of this center-of-mass
energy of the system of the two photons, matter
can be created.
== Astronomy ==
Photon–photon scattering limits the spectrum
of observed gammas to a photon energy below
80 TeV, that is, a wavelength of more than
~ 1.5×10−20 m.
The other photon is one of the many photons
of the cosmic microwave background.
In the frame of reference where the invariant
mass of the two photons is at rest, both photons
are gammas with just enough energy to pair-produce
an electron–positron pair.
== 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 Collider
(LEP) at CERN.
If the transverse momentum transfer and thus
the deflection is large, one or both electrons
can 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.
Frequently, photon-photon interactions will
be studied via ultraperipheral collisions
(UPCs) of heavy ions, such as gold or lead.
These are collisions in which the colliding
nuclei do not touch each other; i.e., the
impact parameter
b
{\displaystyle b}
is larger than the sum of the radii of the
nuclei.
The strong interaction between the quarks
composing the nuclei is thus greatly suppressed,
making the weaker electromagnetic
γ
γ
{\displaystyle \gamma \gamma }
interaction much more visible.
In UPCs, because the ions are heavily charged,
it is possible to have two independent interactions
between a single ion pair, such as production
of two electron-positron pairs.
UPCs are studied with the STARlight simulation
code.
Light-by-light scattering can be studied using
the strong electromagnetic fields of the hadrons
collided at the LHC, it has first been seen
in 2016 by the ATLAS collaboration at the
LHC.
The best previous constraint on the elastic
photon–photon scattering cross section was
set by PVLAS, which reported an upper limit
far above the level predicted by the Standard
Model.
Observation of a cross section larger than
that predicted by the Standard Model could
signify new physics such as axions, the search
of which is the primary goal of PVLAS and
several similar experiments.
== Processes ==
From quantum electrodynamics 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 virtual 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, somewhat metaphorically, what is "inside"
the photon.
There are three interaction processes:
Direct or pointlike: The photon couples directly
to a quark inside the target photon.
If a lepton–antilepton pair is created,
this process involves only quantum electrodynamics
(QED), but if a quark–antiquark pair is
created, it involves both QED and perturbative
quantum chromodynamics (QCD).
The intrinsic quark content of the photon
is described by the photon structure function,
experimentally analyzed in deep-inelastic
electron–photon scattering.
Single resolved: The quark pair of the target
photon form a vector meson.
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 ==
Channelling radiation has been considered
as a method to generate polarized high energy
photon beams for gamma–gamma colliders.
Matter creation
Pair production
Delbrück scattering
