Thursday, March 26, 2009

How Massive is the Higgs?

Probing for the Higgs Boson may be the greatest quest in high energy physics. But two Duke-led efforts are narrowing the search. As a result, the Higgs may be discovered sooner at a particle smasher outside Chicago rather than later at its replacement, the CERN Large Hadron Collider spanning the Swiss and French borders.

Scientists at the Fermi National Accelerator Laboratory in Batavia, Ill. are hoping that the energy of smashing protons together at near light speed will summon up these Higgs particles, which can't exist in today's low-energy universe. But those researchers want to shrink the range of possibilities, because each analysis involves interpreting a bewildering array of subatomic fragments from two million collisions a second.

On March 13, Fermilab announced that work-to-date suggests the Higgs' will likely show itself at a mass less than 160 billion electron volts but more than 114. (Using an energy measurement to define massiveness is a rule-of-thumb conversion under Einstein's famous E=MC2 equation.)

Energy ranges higher than 160 and less than 170 were excluded from the Fermilab search after an intensive analysis led by Duke physics professor Mark Kruse and his graduate student Dean Hidas. Under the Standard Model that defines the theoretical underpinnings of matter, a Higgs boson that heavy should usually immediately decay to two other fundamental particles known as W bosons. But "we see no indication of the Higgs," Kruse says.

Meanwhile, other work by Duke physics professor Ashutosh Kotwal and his students -- also involving W bosons -- is moving the search toward the lower end of likely masses. Ws are "force" particles mainly known as agents of radioactivity. But, under the Standard Model, knowing the mass of the W also predicts the mass of the Higgs.

And painstaking measurements by Kotwal's group have pegged the W's mass at about 80 times heavier than a proton. As a result, a Standard Model mass ratio suggests the Higgs should be comparatively light.

Kotwal initially made those measurements in 2007 by developing special software and novel techniques to analyze W decays within a barrel-shaped Fermilab fragment tracking chamber. Followup efforts could result in measurements three times more precise, Kotwal says.

A low-mass Higgs might weigh-in at about 120 billion electron volts, adds Kruse, who is also helping overseeing all Higgs searches at Fermilab. A 120 billion EV Higgs would be hard to detect there because its breakdown products are less than obvious, Kruse said. But detection would be harder still at the more powerful CERN collider because there would be many more breakdown products.

So Kotwal's graduate student Ravi Shekhar is now looking for the most characteristic fragments from a Higgs of that lower mass range. Those would include two bottom quarks from each Higgs breakdown, plus two electrons or two muons from the associated devolution of a Z boson. Z particles, like the W, are ordinarily involved in radioactive decay.

Stay tuned.

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