ATLAS joint polarization measurement

First ATLAS measurement offers new look at the Higgs

Luka Selem says he has always been a curious kid. Growing up in France, he received copies of Junior Science and Lifea science magazine for young people, by his parents.

“Since I was little, I’ve always been interested in a lot of things,” he says. “I was always asking, ‘Why? But why is that? Why that, and then why that? I wanted to go all the way. I have never been satisfied with the response.

Particle physics, the study of fundamental particles and the forces that make up everything around us, has proven to be a good way for Selem to search for answers. “In particle physics, there is no other ‘why’,” he says. “Nobody can tell me the rest of the story. I have to find it myself with my colleagues.

Selem recently found a new way to research the rest of the story while researching his doctoral dissertation. Along with other physicists from the ATLAS experiment at CERN’s European laboratory, Selem has successfully made a measurement of particles called W and Z bosons that will allow physicists to further probe the mechanism that gives particles mass.

How the boson got its polarization

Bosons are a class of subatomic particles associated with the four fundamental forces of the universe: gravity, electromagnetism, strong nuclear force and weak nuclear force. The best known boson is the photon, the force carrier of light, responsible for electromagnetism.

While photons are massless, massive bosons, such as W and Z bosons, have properties that set them apart from other particles. One concerns their polarization, the degree of alignment of the boson’s quantum spin in a given direction. A boson can be transversely or longitudinally polarized. (There are technically two types of transverse polarization – left and right – but they are not yet distinguishable in the data of joint polarizations of two bosons.) Massive bosons can be polarized longitudinally, which means that their spin can be oriented perpendicularly. to their direction of movement.

“Among physicists, polarization is something…you’re not sure you understand exactly what it is,” says Selem, who admits he wasn’t very familiar with polarization himself when he chose his doctoral project at the Annecy Laboratory of Particle Physics, or LAPP.

The W and Z bosons activate the weak force, the force responsible for radioactive decay, in the same way that photons activate the electromagnetic force. But unlike photons, W and Z bosons have mass, thanks to their interactions with a field generated by yet another boson, the Higgs. The interactions of fundamental particles with the Higgs field are what physicists believe generate mass.

This is what makes polarization so interesting. “It’s a very indirect way of probing the Higgs mechanism,” says Selem.

The W and Z bosons gain mass in a process called electroweak symmetry breaking. When the symmetry “breaks”, four particles are created, including the Higgs boson, explains Junjie Zhu, a professor at the University of Michigan and an ATLAS physicist.

The other three particles are incorporated into the W+, W- and Z bosons, giving them mass and giving them a new longitudinal polarization. “[It is] very important to study the longitudinal components of the W and Z bosons because the longitudinal components are the origin of the symmetry breaking,” explains Zhu.

This provides a unique window through which to search for new physics – things that cannot be explained by the current Standard Model of particle physics. Physicists want to measure longitudinal joint polarization more accurately because new physics is “often sensitive to longitudinal components,” says Zhu.

Since the era of the Large Electron-Positron Collider of the 1990s, the predecessor to the Large Hadron Collider at CERN, the polarizations of the W and Z bosons have been studied individually. But studies of single boson polarization measurements have revealed no hints of new physics.

To continue probing the Standard Model, the physicists then sought to measure joint polarization: two bosons polarized simultaneously. If the value they measured deviated from the Standard Model prediction, it would be an indicator of new physics, such as an unknown aspect of the Higgs mechanism.

A new measure

Selem and his colleagues used ATLAS data taken between 2015 and 2018 during Run 2 of the Large Hadron Collider. The researchers aligned this data with models based on theoretical predictions for the four different combinations of polarizations.

“We have different [templates] from the simulations that tell us what the shapes look like for longitudinal-longitudinal, transverse-transverse, longitudinal-transverse, and transverse-longitudinal,” says Zhu, who performs similar analyzes with ATLAS.

The physicists scaled their four models to match the histogram of the data. The proportion of each matrix at the end revealed the fraction of W and Z bosons with the respective joint polarizations. In the analysis, they found that the longitudinal longitudinal fraction was statistically significant.

The results confirm the model of the universe as we know it. “With the current sensitivity, no sign of new physics has been observed, pushing the Standard Model’s domain of validity even further,” explains Emmanuel Sauvan, ATLAS physicist at LAPP and Selem’s thesis supervisor. “That in itself is an interesting result, although I myself would have preferred to see new physical effects.

“A next step is to refine this measurement, with a view to reducing its uncertainties, with more data than we are recording at this time. [during LHC Run 3].”

An important step in the future will be to measure the scattering of two longitudinally polarized bosons. This interaction is even more sensitive to new physics, but researchers need 10 to 20 times more data than they currently have to hope to detect this event.

“Such polarization measurements in vector boson scattering will require much more luminosity, and we’ll have to wait for the high-luminosity LHC,” Sauvan says, referring to the Large Hadron Collider upgrade proposal scheduled for the end of the 2010s. 2020s. “This will allow us to fully validate the Higgs mechanism of the Standard Model.”

For now, the result of joint polarization is a significant development. The researchers overcame challenges that had limited attempts to measure previous experiments, and they developed new analysis techniques, including machine learning algorithms.

Prachi Atmasiddha, a University of Michigan graduate student working with Zhu, performed the event generation for the joint polarization result. “Once we had those events, it was one of the biggest [reasons] why this analysis was motivated,” she says. “We [determined that we] can produce these polarized events separately, so we can build models on that, and we can adapt to these models of different polarization states.

Selem says he can’t wait to find out what physicists can do next. “I would say that the most important part of this measure is the roadmap it provides to do other measures, because we have encountered many challenges that will occur in other similar measures, and we have given ideas on how to solve them.”

Selem recently accepted a postdoctoral research position in Grenoble, France, where he will work on detector development and exotic research. There, he and his fellow physicists will continue to search for their own answers to the question “Why?” »

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