Posts Tagged ‘W boson

World’s best measurement of W boson mass …

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… points to Higgs mass and tests Standard Model

The world’s most precise measurement of the mass of the W boson, one of nature’s elementary particles, has been achieved by scientists from the CDF and DZero collaborations at the Department of Energy’s Fermi National Accelerator Laboratory. The new measurement is an important, independent constraint of the mass of the theorized Higgs boson. It also provides a rigorous test of the Standard Model that serves as the blueprint for our world, detailing the properties of the building blocks of matter and how they interact.

The Higgs boson is the last undiscovered component of the Standard Model and theorized to give all other particles their masses. Scientists employ two techniques to find the hiding place of the Higgs particle: the direct production of Higgs particles and precision measurements of other particles and forces that could be influenced by the existence of a Higgs particles. The new measurement of the W boson mass falls into the precision category.

The CDF collaboration measured the W boson mass to be 80387 +/- 19 MeV/c2. The DZero collaboration measured the particle’s mass to be 80375 +-23 MeV/c2. The two measurements combined along with the addition of previous data from the earliest operation of the Tevatron produces a measurement of 80387 +- 17 MeV/c2, which has a precision of 0.02 percent.

These ultra-precise, rigorous measurements took up to five years for the collaborations to complete independently. The collaborations measured the particle’s mass in six different ways, which all match and combine for a result that is twice as precise as the previous measurement. The results were presented at seminars at Fermilab over the past two weeks by physicists Ashutosh Kotwal from Duke University and Jan Stark from the Laboratoire de Physique Subatomique et de Cosmologie in Grenoble, France.

“This measurement illustrates the great contributions that the Tevatron has made and continues to make with further analysis of its accumulated data,” said Fermilab Director Pier Oddone. “The precision of the measurement is unprecedented and allows rigorous tests of our underlying theory of how the universe works.”

The new W mass measurement and the latest precision determination of the mass of the top quark from Fermilab triangulate the location of the Higgs particle and restrict its mass to less than 152 GeV/c2 .This is in agreement with the latest direct searches at the LHC, which constrain the Higgs mass to less than 127 GeV/c2, and direct-search limits from the Tevatron, which point to a Higgs mass of less than 156 GeV/c2, before the update of their results expected for next week.

“The Tevatron has expanded the way we view particle physics,” said CDF co-spokesperson and Fermilab physicist Rob Roser. “Tevatron experiments discovered the top quark, made precision measurements of the W boson mass, observed B_s mixing and set many limits on potential new physics theories.”

The new measurement comes at a pivotal time, just days before physicists from the Tevatron and the Large Hadron Collider at CERN will present their latest direct-search results in the hunt for the Higgs at the annual conference on Electroweak Interactions and Unified Theories known as Rencontres de Moriond in Italy. The CDF and DZero experiments plan to present their latest results on Wednesday, March 7.

“It is a very exciting time to analyze data at particle colliders,” said Gregorio Bernardi, DZero co-spokesperson and physicist at the Laboratoire de Physique Nucléaire et de Hautes Energies in Paris. “The next few months will confirm if the Standard Model is correct, or if there are other particles and forces yet to be discovered.”

The existence of the world we live in depends on the W boson mass being heavy rather than massless as the Standard Model predicts. The W boson is a carrier of the electroweak nuclear force that is responsible for such fundamental process as the production of energy in the sun.

“The W mass is a very distinctive feature of the universe we live in, and requires an explanation,” said Giovanni Punzi, CDF co-spokesperson and physicist from the University of Pisa. “Its precise value is perhaps the most striking evidence for something “out there” still to be found, be it the Higgs or some variation of it.”

“The measurement of the W boson mass will be one of the great scientific legacies of the Tevatron particle collider,” added DZero co-spokesperson and Fermilab scientist Dmitri Denisov.

Notes for Editors:

Funding for the CDF and DZero experiments comes from DOE’s Office of Science, the U.S. National Science Foundation, and numerous international funding agencies.

CDF collaborating institutions are at

DZero collaborating institutions are at

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Written by physicsgg

March 2, 2012 at 5:21 pm

Posted in High Energy Physics

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W and double W

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The W boson carries the weak nuclear force. At the Large Hadron Collider pairs of them may be showing the first signs of the Higgs boson. But they are a decidedly mixed blessing.
The current data from the LHC show an effect which might, or might not, be the first indication of the presence of a Higgs boson. Most of this effect is due to the number of pairs of W bosons which are being produced in proton-proton collisions.

Diagram showing a Higgs boson decaying to a W+ and a W-, which then decay to leptons. This might be happening.

The W boson (along with the Z boson) is responsible for carrying the weak nuclear force, just as the photon carries the electromagnetic force. It is intimately connected with the Higgs boson. In the standard theory, the Higgs gives mass to the W and Z, and so breaks a symmetry which otherwise would mean the weak force was pretty much as strong as the electromagnetic one.

If the Higgs boson has enough mass (that is, enough energy if it isn’t moving, since E = mc2), it will decay very quickly into a pair of W bosons. So if the Higgs is there, extra pairs of W bosons are one way we might first see it.

Generally this is a common way of seeing new particles. It’s called resonant production. If you can measure the energy and momentum of the decay products (two W bosons in this case), you can reconstruct the mass of the original (in this case the Higgs boson). W  bosons can be produced in lots of different ways, but if one way is via the Higgs, you would expect a bump in the WW mass distribution at the mass of the Higgs….. Read the rest of this entry »

Written by physicsgg

August 8, 2011 at 1:34 pm

Posted in High Energy Physics

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