Published: Wed, June 06, 2018
Sci-tech | By Patricia Wade

Direct Coupling Of Higgs Boson To Top Quark Observed

Direct Coupling Of Higgs Boson To Top Quark Observed

Because of the Higgs boson's reputation as a giver of mass and the top quark's status as physics' heaviest particle, scientists assumed the two were linked.

The discovery was announced at the Large Hadron Collider Physics (LHCP) 2018 conference in Bologna.

On 4 July 2012, two of the experiments at the CERN's Large Hadron Collider (LHC), ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact-Muon-Solenoid), reported independently the discovery of the Higgs boson.

While today's result is scientifically fascinating, it is also one that is personally exciting for me. The energy field permeates the universe.

After almost 50 years of searching, researchers at the LHC found the Higgs boson in 2012. In 2013, British physicist Peter Higgs and Belgian physicist Francois Englert shared the Nobel Prize in Physics for their work. These new studies focused on the rare process in which two gluons inside the colliding protons fuse and produce two virtual top quarks, which are quantum mechanical fluctuations and not yet fully formed discrete particles. There are six known quarks.

Other particles interact much less, like a barracuda, which can dart through water very quickly. A single top quark has a mass comparable to an atom of tungsten.

"Higgs boson production is rare - but Higgs production with top quarks is rarest of them all, amounting to only about 1 percent of the Higgs boson events produced at the LHC", Chris Neu, a physicist at the University of Virginia, said in a news release. This is the first time scientists have measured the Higgs boson's direct interaction with top quarks.

Fundamental particles gain mass through their interaction with the Higgs field, so it would make sense that the top quark-the most massive particle ever discovered-would have a strong coupling with the Higgs boson. Accordingly, interactions of this sort are an ideal laboratory in which to do detailed studies of the origins of mass.

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This measurement was particularly challenging. This leads to a wide variety of signatures in events with two top quarks and a Higgs boson.

The top quark played a key role in Higgs research even before scientists found the Higgs. This was a very hard accomplishment.

"Much of what we think we know about the Higgs boson hinges on its relationship with the top quark", says Rachel Hyneman, a graduate student at the University of MI who worked on the ATLAS analysis. It is therefore impossible for a Higgs boson to decay into a top quark/antiquark pair.

After sifting through around a quadrillion (10) collisions, we identified a handful of instances in which a Higgs boson was created at the same time as a top quark/antiquark pair. Each top quark decays into three particles, and the Higgs boson decays into two.

The Higgs boson was predicted in the 1960s and discovered by the CMS and ATLAS experiments in 2012 using particle collisions generated by the LHC. Among them: Why does the Higgs boson have the mass that it does?

First and foremost is the fact that the Higgs theory is not motivated by a deeper theoretical framework. It is simply added on. They have observed the most massive known fundamental subatomic particle directly interacting with an energy field that gives mass to the building blocks of the universe.

In addition, the numerical value for the mass of the Higgs boson is a bit of a mystery. The Higgs field gives mass to fundamental subatomic particles, including the Higgs boson itself. However, the story is more complex than that. The heaviest of the quarks is the "top" quark. A big question has been whether the Higgs boson can interact with quarks and, if so, what this relationship might look like.

"The Higgs boson was originally predicted because it helped explain why some force-carrying bosons had mass while others remained massless", says Anadi Canepa, the new head of the CMS Department at Fermilab. This is a pressing mystery for modern physics and, hopefully, better measurements of the interactions of Higgs bosons will shed light on this conundrum. Future measurements will be far more accurate. At the end of this year, scientists will have collected only about 3% of the data that is expected over the lifetime of the facility. At the end of 2018, the LHC will shut down for two years for refurbishment and upgrades and then return with a vengeance, running through 2030 and making who knows what kind of discoveries. "This is what we set out to find".

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