IISc Scientists Contribute To Key Finding On Higgs Boson Self-Interaction

One of the landmark scientific discoveries last decade was the discovery of the Higgs boson. The particle, named after British theoretical physicist Peter Higgs, existed in theory for five decades until evidence of its existence emerged in 2012 from experiments conducted at the European Organization for Nuclear Research or CERN.

The Higgs boson is a manifestation of a universal field, called the Higgs field, that gives matter its mass. Its discovery was also important in the context of the Standard Model of particle physics, which captures the workings of fundamental particles under the sway of fundamental forces, as it provides a means to test out the long-standing theory.

But the particle’s discovery was more a milestone than an end goal. The ATLAS and CMS (compact muon solenoid) experiments using the Large Hadron Collider (LHC) at CERN have been at work over the years producing data from the detector regularly that can be used to improve our understanding of fundamental particles and forces and how they operate.

Recently, using data from experimental runs extending from 2016 through 2018, the CMS collaboration at CERN arrived at an important result – they derived the most stringent constraint on the value of the Higgs self-coupling, denoted mathematically by the Greek letter ‘lambda’ (λ).

Saying it in simple words, this new result brings scientists closer than ever to measure the Higgs field, which manifests the Higgs boson, and to test out predictions from the Standard Model. An exciting byproduct is the possibility of new physics beyond the Standard Model.

This important result, as is nearly any work that is carried out at CERN, has been possible because of collaboration with international scientists and research groups.

For the lambda determination, two physicists from the Centre for High Energy Physics at the Indian Institute of Science, Bengaluru, assistant professor Dr Jyothsna Rani Komaragiri and her Doctor of Philosophy (PhD) student Lata Panwar, have made important contributions.

Speaking to Swarajya, Dr Komaragiri says, “For the first time, we have a stringent limit on the parameter lambda.”

Lambda is the Higgs self-coupling parameter and pertains to how the Higgs boson interacts with itself.

Studying the Higgs boson and its interactions with other particles – couplings – is one of the ways to vindicate the success of the Standard Model or, if deviations are observed, go in search of new physics beyond the model.

Measurements of the Higgs boson’s couplings to W and Z bosons, among other fundamental particles, have already strongly correlated with the theoretical framework proposed by Higgs along with collaborators Brout and Englert.

But this experiment observes and measures the self-interaction of the Higgs boson.

For the analysis, Dr Komaragiri and Panwar applied their expertise in computation. They helped with building the algorithm and, later, analysing the vast amount of data coming in from the detector.

“We also needed to make sure that the data matched the simulations. So, we did the initial exercise of all the matching parts,” Dr Komaragiri says.

The Higgs boson pair production is feeble and rare. The rate of production is a thousand times lesser than that for one Higgs boson. Yet, they pulled it off.

“With limited data and feeble production, we still put a constraint on the parameter,” she says.

The sensitivity of this analysis, said the report in CERN Courier, was better by about a factor of four compared to the previous analysis based on 2016 particle detector data. Innovative analysis techniques, including the application of advanced machine-learning methods, played a part in arriving at a more refined result.

The result is the best constraint to date set on the ratio of the measured lambda parameter to the Standard Model prediction, at 0.6 (+6.3, –1.8).

The CMS collaboration extends far and wide – about 40 countries, 200 research institutes, and 5,000 people participate in this grand particle physics project. India is a major contributor to CERN’s research activities and IISc is among India’s participating institutes.

The IISc CHEP sub-group participating in the CMS collaboration was formed by Dr Komaragiri herself in 2016.

“CHEP didn’t have a collaboration with the CMS group before 2016. Then, in that year, an experimental group comprising two members (Dr Komaragiri included) was formed and collaboration established,” she says.

Panwar, who is a collaborator on the CMS project, was Dr Komaragiri’s first PhD student. Since then, Dr Komaragiri has supervised the work of four physics PhD students.

On 15 February, she completed five years of working at the IISc.

Over the course of her career, she has co-authored over 850 experimental particle-physics publications in peer-reviewed journals.

The CMS collaboration has stated that the measurement of the Higgs boson self-coupling is one of the most important goals of the LHC physics programme.

The recent analysis helps the international particle physics collaboration take another step in the direction of measuring the Higgs field and verify the Standard Model.