After Higgs is the Turn to Dark Matter and Gravity

For physicists like Bhaskar Dutta, a theoretical physicist at the Texas A&M University, the Higgs boson is more a gateway into the unknown than the final missing piece of a theory that describes the building blocks of the universe. 

An illustration of the Higgs-Boson particle.

A typical “candidate event” in the Higgs-hunting CMS experiment. Red lines represent high-energy proton beams while yellow lines show the tracks of particles produced in the collision. Credit: CERN

Dutta is hoping his pencil-and-paper research work featuring the Higgs boson will help resolve one of cosmology’s biggest mysteries: invisible dark matter.

Scientists at the European Organisation for Nuclear Research, or CERN, announced on July 4th their discovery of a candidate subatomic particle with signatures of the Higgs boson, the last piece of an elegant theory of physics called the Standard Model that explains almost all particles and forces in nature, except gravity.

But research teams at CERN are also seeking answers to deeper mysteries.

“The Standard Model with the Higgs boson explains only about 4% of the universe,” said Dutta, who had studied physics at Calcutta’s Presidency College and Calcutta University before moving to the US for a PhD.

Many physicists believe the Large Hadron Collider (LHC), a giant accelerator at CERN where protons collide with protons, will help them understand the so-called dark matter, an invisible form of matter that makes up 23% of the Universe.

Astronomers have for decades inferred the existence of dark matter only through its gravitational force on stars and galaxies. Ordinary matter and dark matter together make up 27% of the universe. The other 73% is dark energy, an even more mysterious entity discovered in the late 1990s that is causing the expansion of the universe to accelerate.

In a research paper published four years ago, Dutta had predicted that the debris of the collisions in the LHC might contain a set of signatures of a hypothetical particle called the neutralino, a candidate for dark matter.

“With these signatures and other measurements at the LHC, it’d be possible to explain why the universe has 23 per cent dark matter — no more, no less,” he said. “No one has found a signature of the neutralino yet but we have many years of experiments ahead.”

Physicists Bhaskar Dutta. Credit: Texas A&M University

Physicists expect the Large Hadron Collider (LHC) to work for another 20 years. Some are already planning changes in experiments so that the rate of the proton-proton collisions can be increased to five or ten times the rate today.

“This will make possible the investigation of several non-Higgs goals,” said Ashutosh Kotwal, a physicist at Duke University, who heads a Higgs search team at the LHC experiment called ATLAS and is now planning modifications to it for physics beyond the Standard Model. The modifications are planned to be completed by 2022, giving physicists an eight-year window for experiments at the higher rate of data.

Over the past several decades, theoretical physicists have extended the Standard Model through a set of theories collectively called Supersymmetry (SUSY), and many of them are hoping the LHC will throw up evidence for SUSY.

“There are many reasons to search for (signals of) SUSY — one is related to the Higgs boson,” said David Toback, professor of experimental physics at the Texas A&M University, and a colleague of Dutta.

The Higgs boson, predicted 48 years ago by Peter Higgsa British physicist, is the key to explaining how all other subatomic particles in the universe acquired mass. But physicists don’t really understand how the Higgs boson works.

“Our best guess is that there are undiscovered particles out there that will help us understand how the Higgs works,” Toback said. “If SUSY theories are correct, it could explain why the Higgs behaves the way it does.”

These theories predict a zoo of so-called SUSY particles, counterparts for each subatomic particle in the Standard Model. Physicist Manas Maity at Visva-Bharati, India, has predicted the possible signatures of the SUSY counterpart of a subatomic particle called the top quark that might be observed in the debris of the proton-proton collisions at the LHC.

The Large Hadron Collider. Image credit: CERN

“It’s clear from existing data that predicted SUSY particles are much heavier than the top quark, which is the heaviest among Standard Model particles,” Maity said. The top quark has a mass close to that of an atom of gold.

The heavier the subatomic particle, the more volume of data scientists need to find it, because heavier particles are rarer events in the collision debris, produced less frequently than lighter particles.

The search for SUSY particles hasn’t yielded any signal yet, but the absence of evidence hasn’t dampened the researchers’ enthusiasm. “There are many versions of SUSY and this absence of early evidence might only mean there are no low-hanging fruits,” said Toback.

Gravity also features among LHC’s non-Higgs goals

“The understanding of gravity is completely missing from the Standard Model,” Kotwal said. “We now believe that gravity could be understood better if, at very small distances, space contained extra dimensions beyond the three we are familiar with. The LHC is the most powerful microscope ever built and the best possible to explore this exciting idea of extra dimensions in space.”

At Calcutta’s Saha Institute of Nuclear Physics, Satyaki Bhattacharya is among researchers looking for a specific signature of extra dimensions — a packet of missing energy and a single photon emerging from a proton-proton collision.

“Just because humans cannot visualise extra dimensions, it doesn’t mean they cannot exist,” Bhattacharya said.

“Imagine a sheet of paper which is two-dimensional with length and breadth; if we fold the paper, we get a cylinder. If we curl it enough, the cylinder looks like a single dimensional line. The extra dimensions we’re looking for are also similarly curled up and we’re familiar with only three spatial dimensions.”

The LHC experiments haven’t yet revealed any signs of extra dimensions, but many believe the search represents a long-standing goal of physics. “It would complete Einstein’s dream of understanding gravity at the microscopic level,” Kotwal said.

Source: The Texas A&M University and The Telegraph of India

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