Physicists from the ATLAS (A Toroidal LHC ApparatuS) Collaboration at CERN’s Large Hadron Collider have observed production of W bosons, elementary particles that carry the weak force, from photons colliding with photons. This new result confirms one of the main predictions of electroweak theory — that force carriers can interact with themselves — and provides new ways to probe it.
In everyday life, two crossing light beams follow the rules of classical electrodynamics and do not deflect, absorb or disrupt one another.
However, at the high energies seen in collisions at the Large Hadron Collider (LHC), effects of quantum electrodynamics become important.
For a short moment, photons radiated off the incoming proton beams can scatter and transform into a particle-antiparticle pair which appears as light-by-light interactions in the detector. This process was first observed by the ATLAS Collaboration in 2019.
Indeed, the Standard Model describes quantum electrodynamics as part of electroweak theory, which not only predicts that force-carrying particles — the W bosons, Z boson and photon — interact with ordinary matter, but also among themselves.
The newly-observed process proceeds via a very rare type of phenomenon where two photons collide to directly produce two W bosons of opposite electric charge via a four force-carrier interaction, among others.
The process occurs as bunches of high-energy protons skim past each other in ‘ultra-peripheral collisions,’ if only their surrounding electromagnetic fields interact.
Quasi-real photons from these fields scatter off one another to produce a pair of W bosons and leave a distinct signature in the ATLAS experiment.
As the skimming protons stay intact, the only detectable particles produced in the interaction are the visible decay products of the W bosons — namely, for this measurement, an electron and a muon with opposite electric charge.
The ATLAS physicists found a total of 307 candidate events in the data recorded by the experiment between 2015 and 2018, of which 174 were attributed to be from the photon-photon production of W-boson pairs and the remaining events to various background processes.
Such a yield corresponds to a statistical significance of 8.4 standard deviations, which is well above the established 5 standard deviations criterion for the unambiguous observation of a process.
“This observation opens up a new facet of experimental exploration at the LHC using photons in the initial state,” said Dr. Karl Jakobs, spokesperson of the ATLAS Collaboration.
“It is unique as it only involves couplings among electroweak force carriers in the strong-interaction-dominated environment of the LHC.”
“With larger future datasets it can be used to probe in a clean way the electroweak gauge structure and possible contributions of new physics.”
ATLAS Collaboration. 2020. Observation of photon-induced W+W- production in pp collisions at ps = 13 TeV using the ATLAS detector. CERN, report # ATLAS-CONF-2020-038
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