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Unpuzzling the Higgs Particle

The laws of physics have served human knowledge very well so far. But as physicists follow them further and further backward in time, things start to get hazy.

Physics and astronomy professor Mayda Velasco investigates the earliest moments of the universe: the first fractions of the second after the Big Bang. Although she studies the tiniest constituents of the universe, the question her work seeks to answer is a big one: Why is there something rather than nothing?

Velasco, who leads a team of researchers in Northwestern’s High Energy Physics Group, says the answer is the Higgs boson — the once-elusive particle that gives mass to the familiar elementary particles of the Standard Model. Although British physicist Peter Higgs proposed the particle in 1964, evidence of its existence first surfaced in 2012 at the Large Hadron Collider in Geneva, Switzerland.

When particles collide with enough force, they break apart. The LHC’s detectors record the evidence left behind as the fragments release their energy, or “decay,” and researchers like Velasco use the data to trace each energy signature back to its source. Specifically, they look for energy that couldn’t have come from any of the particles that collided and check it against Higgs’ equations.

There is something beautiful about the way the data match the mathematical models. You can look at the records of energy lost, plot the points and watch the telltale curves pop out, just as the equations predicted they would. But even for a trained professional, spotting a Higgs interaction amid the chaotic jumble of proton debris inside the LHC is no small task. For every interaction that suggests the existence of the Higgs boson, there are 10 billion other interactions to sift through, many of which look similar to Higgs interactions at first glance.

Kristian Hahn, an assistant professor of physics and astronomy, says this task will only get harder in 2015, when the LHC will begin slamming particles together with twice as much energy as it has so far. Hahn — whose work on decay models also played an important role in the discovery of the presumed Higgs — and his research partners are designing devices that will be able to sort through huge quantities of data, highlight traces of the new particle and discard everything else.

Even as they work backward, putting sub-atomic pieces of the universe back together to learn more about its past, Velasco and Hahn and their colleagues are thinking ahead. Velasco is focused on an unusual Higgs decay mode that may reveal particles no one has yet discovered. Hahn says a greater understanding of the Higgs could shed some light on dark matter and dark energy, which are perhaps the two greatest mysteries of modern physics.

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