Particle trio exceeds expectations at Large Hadron Collider

In the wake of proton-proton collisions within the Large Hadron Collider, the ATLAS experiment has verified that a trio of particles—a top-antitop quark pair and a W boson—occurs more frequently than predicted (LHC).

Just one out of every 50,000 collisions at the LHC produces the trio, known as ttW, which indicates how uncommon the mechanism is that produces these three particles post-impact. Top quarks and W bosons have a limited lifetime and disintegrate nearly instantly, thus the scientists detected ttW events by looking at the electrons and muons they decay into.

In order to increase the precision and depth of the analysis of the measurement, members of the ATLAS group at the Department of Energy's SLAC National Accelerator Laboratory have spent the last three years finishing a complex analysis to measure the process. This work included creating novel methods to estimate and remove background and detector effects. The findings will aid experimentalists looking at various particle physics processes as well as researchers testing elementary particle physics ideas more effectively.

Brendon Bullard, research associate at SLAC National Accelerator Laboratory and coordinator of this data analysis, noted that the LHC is the only collider that can create these kinds of events at a rate great enough to be observed.

a mysterious excess

Data from the LHC's Run 1, which took place between 2010 and 2012, were used by ATLAS to make the first observation of the ttW process in 2015. A portion of the data acquired during Run 2 (2015–2018) was used in subsequent measurements to imply that ttW was occurring more frequently than expected by the Standard Model of particle physics, which describes the behavior of subatomic particles.

The most recent measurement, which used the whole dataset gathered by ATLAS during Run 2, allowed for a more accurate measurement of ttW, which revealed that the total production rate was nearly 20% greater than predicted by theory. Recent CMS experiment findings support this excess.

Although the precise cause of the disparity is still unknown, Bullard stated that the data "truly do appear to imply that there's something going on that we're not taking into consideration."

It's plausible that new, Standard Model-defying physics is to blame.

Alternately, it's conceivable that the current models don't include the components needed to anticipate ttW production properly. Theorists use piecewise approximations of increasing difficulty to construct predictions from the Standard Model, and the disparity may be due to subtle effects that have not yet been taken into account by these approximations.

In any case, theorists must now attempt to determine the truth by approximating ttW while taking into consideration these yet-to-be-calculated subtle effects.

"Because of its difficulty, this has never been attempted before. Yet, in light of our finding, there are already theories eager to put out the effort "said Bullard. This measurement will be highly helpful for furthering our understanding of the Standard Model and, if we're lucky, for identifying any impacts that go beyond it.

Even too much can be beneficial.

Studying various aspects of ttW events offers new opportunities for scientists to investigate the fundamental forces at work between the two quarks and the W boson, including the strong interaction, which holds quarks together, and the electroweak interaction, which controls electromagnetism and radioactive decay.

The study of even more uncommon events happening during proton collisions will benefit from improved observations. To identify the signal they were looking for, researchers previously had to estimate ttW production and delete it from data since ttW is a significant background of two other processes seen at the LHC. They can now identify these unusual signals with more accuracy because to this more exact measurement of ttW.

The creation of two top quarks and a Higgs boson, the particle that gives some particles, such quarks and W bosons, mass, is one of these processes. By hunting for the electrons and muons that this event, known as ttH, decays into, it is found to be 10 times more uncommon than ttW. Improved measurements of ttH will aid scientists in determining the strength of the Higgs-Top quark coupling, a crucial test of the Standard Model that potentially reveal the source of mass.

The generation of four top quarks, which is 50 times rarer and was just seen for the first time by ATLAS and CMS, is the second process that ttW muddles. The most powerful particle in the Standard Model, top quarks, may play a role in novel physics that may be explored further.

Zhi Zheng, a research associate at SLAC who led the four top quark study at ATLAS, stated, "Improved understanding of the ttW process, especially with this discovery, can further enhance the four top measurements and precision, allowing us to examine additional aspects of this process." She further supported Bullard's ttW analysis. Cross-checking these related metrics was made easier by the pair's collaboration at SLAC.

Being at SLAC together has improved communication and cooperation between these two metrics, according to Bullard.