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CDF precision measurement of W-boson mass suggests a lighter Higgs particle

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The Fermilab accelerator complex accelerates protons and antiprotons close to the speed of light. The Tevatron, four miles in circumference, is the world's most powerful proton-antiproton accelerator, producing collisions at the energy of 2 tera electron volts (TeV). Two experiments, CDF and DZero, record the particles emerging from billions of collisions per second. Each collision produces hundreds of particles.


The CDF detector, about the size of a 3-story house, weighs about 6,000 tons. Its subsystems record the "debris" emerging from high-energy proton-antiproton collisions. The detector surrounds the collision point and records the path, energy and charge of the particles emerging from the collisions. This information can be used to determine, for example, the mass of the W boson, the carrier of the weak nuclear force and a key parameter of the Standard Model of particles and forces.


According to the Standard Model of particles and forces, the Higgs mechanism gives mass to particles. Measuring the mass of the top quark and the mass of the W boson, scientists can restrict the allowable mass range of the not-yet-observed Higgs boson.


CDF scientists used a blind analysis technique to determine the mass of the W boson in an unbiased fashion. CDF physicist Ashutosh Kotwal, Duke University, unveiled the result to his CDF colleagues at an internal meeting on December 14, 2006. The CDF collaboration made the result public with a press release on January 8, 2007.


In the Standard Model of particles and forces, the masses of the W boson, the top quark and the Higgs boson are connected. If one knows the mass of any two of the three particles, then the mass of the third particle can be calculated. This plot illustrates that relationship. It depicts the mass of the Higgs boson as a function of top quark and W-boson mass. Each diagonal line represents a single Higgs boson mass; examples chosen are MH = 114, 300 and 1000 GeV/c2. Based on theoretical constraints and direct experimental searches, scientists expect the mass of the Higgs boson to lie somewhere in the green-banded region. The new CDF measurement of the W-boson mass (see this press release) indicates that the W-boson mass is heavier than previously measured (worldwide average). Since the top quark mass did not change, a heavier W-boson mass indicates a lighter Higgs Boson. The blue ellipse shows the most likely values for the top quark and W-boson masses, based on all available experimental information, including the CDF result, at the 68 percent confidence level. The intersection of this ellipse with the green band indicates the most likely Higgs boson mass. This result can be compared to an older result (red ellipse), which did little to constrain the Higgs boson mass.






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Last modified: 12/02/2009 |