The ATLAS Collaboration announces the first observation of two W bosons produced from the scattering of two photons — particles of light — at the International Conference on High-Energy Physics (ICHEP 2020).
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 LHC collisions, 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 (see Figure 2). Although the ATLAS and CMS Collaborations saw first evidence of this process in data recorded during Run 1 of the LHC (2011–2012), its observation required the substantially larger dataset taken during Run 2 (2015–2018).
This rare 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 Collaboration has observed the rare interaction of two W bosons with two photons with statistical significance of 8.4 standard deviations.
ATLAS physicists had to overcome several unique challenges to observe this process, starting with separating the signal from background. LHC protons can break up into their constituents and fragment into several detectable particles at very low energy. In particular, W-boson pairs can be produced from the proton’s constituents. This background process is hundreds of times more likely to occur than the photon–photon production of W-boson pairs, and can mimic its signature. To enhance the signal over such a background, physicists only selected collisions where no other charged particles are measured in the vicinity of the electron and the muon, as reconstructed in the ATLAS Inner Detector.
Further, a typical collision event contains particles from 20 to 60 additional proton–proton interactions occurring simultaneously as bunches of proton cross in ATLAS. These additional particles can prevent the identification of signal events if they are produced in close proximity to the photon–photon interaction (see Figure 1).
Physicists developed novel experimental techniques to precisely determine the contributions of these effects. Simulated events can be used to estimate the expected backgrounds, but detailed tuning on the data is needed to ensure that they provide a faithful description. Physicists performed auxiliary measurements using data consistent with resonant Z boson production, a well-understood process produced with high frequency and purity at the LHC. This dataset was used to count particles from both the additional proton-proton interactions and the proton fragmentation, and the findings allowed ATLAS physicists to tune the simulation of such events. The accurate description of these background processes made the observation of this rare phenomenon possible (see Figure 3, where the photon–photon signal interactions accumulate at low particle multiplicity).
A total of 307 events matching the selection requirements were found in the analysed dataset, 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. The cross section is measured to be 3.13 ± 0.42 fb. This means that only one or two such interactions occurred in the 30 trillion proton–proton interactions of a typical day of data-taking in 2018.
The four force-carrier interaction is an integral part of electroweak theory and, at the same time, can be sensitive to modifications of the Standard Model by unaccounted-for new physics. The experimental techniques presented by the ATLAS Collaboration will enable future measurements that can probe such modifications and test the electroweak theory in a novel way.
- Observation of photon-induced W+ W– production in proton–proton collisions at 13 TeV using the ATLAS detector (ATLAS-CONF-2020-038)
- Rare phenomenon observed by ATLAS features the LHC as a high-energy photon collider, CERN Press Update, 5 August 2020
- Measurement of exclusive γγ→W+W− production and search for exclusive Higgs boson production in proton–proton collisions at 8 TeV using the ATLAS detector (arXiv: 1607.03745)
- CMS Collaboration: Evidence for exclusive gamma gamma to W+W− production and constraints on anomalous quartic gauge couplings in proton–proton collisions at 7 and 8 TeV (arXiv: 1604.04464)
- CMS Collaboration: Study of exclusive two-photon production of W+W− in proton–proton collisions at 7 TeV and constraints on anomalous quartic gauge couplings (arXiv: 1305.5596)
- ATLAS observes light scattering off light, Physics Briefing, March 2019
- ATLAS finds evidence of three massive vector boson production, Physics Briefing, March 2019
- New milestone reached in the study of electroweak symmetry breaking, Physics Briefing, July 2019
- Quarks observed to interact via minuscule “weak lightsabers”, Physics Briefing, June 2018
- ATLAS sees first direct evidence of light-by-light scattering at high energy, Press Statement, April 2017
- ATLAS finds evidence for the rare electroweak W±W± production, Physics Briefing, September 2014
- See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.