Particle physics and the discovery of scattering of W and Z bosons

Particle physics and the discovery of scattering of W and Z bosons
ATLAS © Maximilien Brice CERN CC BY-SA 4.0 via Wikimedia Commons

Particle physicists from Technische Universität Dresden, Germany, together with international research colleagues, have discovered an extremely rare process that can be compared to tiny lightsabers.

At the ATLAS experiment at CERN in Geneva, the particle physics group is conducting research on messenger particles of the so-called weak interaction. They have shown that these particles interact with each other in a scattering process.

Science Fiction writers have a passion for lightsabers. But this idea will probably remain a vision forever, because light consists of photons – in physics they are also called messenger particles of the electromagnetic interaction.

As the laws of physics explains, photons can only interact with objects that carry an electrical charge. Since they are electrically neutral themselves, they simply penetrate each other completely unaffected. That is why light beams can be crossed but will never bend each other.

Particle physics at ATLAS

The groups analysed ATLAS data from 2015 and 2016. The ATLAS detector records the collision products of protons that travel near the speed of light in the particle accelerator LHC and finally collide. W and Z particles can be radiated from quarks.

However, according to the university, due to their short lifetime, these quanta of ‘weak light’, emitted by the quarks, are only able to fly 0.1 femtometre, that is, about 1/10 of a proton radius, before transforming into other particles.

To be able to interact with each other, these ultra-short ‘lightsabers’ have to approach each other by 0.002 femtometre (about 1/500 of a proton´s radius). Such an extremely improbable coincidence happens only about once in 20,000 billion proton-proton interactions.

Two PhD students from the Institute of Nuclear and Particle Physics were able to extract the signal from the background noise. They found 60 candidates for scattering events WW → WW of like-sign charged W particles. This is more than three times what was seen in data from 2012, where ATLAS had obtained the first evidence for this process.

Further investigation of the scattering process with more data could provide clues to new particles or substructures of known particles.

Michael Kobel head of the Dresden group said: “Exactly six years after the discovery of the Higgs particle, we now have two processes at hand that will help us to better understand the properties of the new species of Higgs particles.”


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