Tevatron collider’s mighty boost for Higgs hunt

In its day, the circular Tevatron produced about 10 million proton-antiproton collisions per second - about 200 collisions per second were recorded at each detector for further analysis (Image: Fermilab)

by Lisa Grossman
The Tevatron may now be defunct, but it is still detangling the nature of matter from beyond the grave. The late particle-smasher’s two main experiments, CDF and DZero, have released the most precise measurement yet of the mass of the W boson, one of the fundamental particles in the standard model of particle physics.

The new measurements, combined with earlier data from other detectors, places the W boson’s mass at 80.385 gigaelectronvolts (GeV), plus or minus 0.015 GeV. The measurement puts constraints on the mass of the Higgs boson – the long-sought missing piece that would complete the standard model and explain why all other particles have mass – placing it right where experimentalists want it.

The standard model is our current best picture of the menagerie of particles and forces that make up the universe and explains how they interact. But it cannot predict exactly how much each of those particles weighs; it can only describe how their masses are related to each other.
Prediction game

The Tevatron, which was housed at Fermilab in Batavia, Illinois, and turned off in September 2011, and the still-running Large Hadron Collider at CERN, near Geneva, Switzerland, have hunted the Higgs boson directly. Last December, physicists at the LHC announced tentative hopes that the Higgs had finally turned up, with a mass of about 125 gigaelectronvolts (GeV).

But thanks to the relationships laid out in the standard model, physicists can also predict at which mass the Higgs should show itself based on the masses of other particles, like the W boson and the top quark.

“The mass of the W boson is correlated with the mass of the top quark, which we here at Fermilab can measure very precisely,” says Dmitri Denisov, spokesman for the DZero experiment. And if the Higgs exists, the mass of the W boson is also correlated with the Higgs mass, he says.

Weight and see

Previous to the latest result, DZero and CDF – which use different and independent techniques to make similar particle measurements – had placed the W boson’s mass at 80.4 gigaelectronvolts, give or take about 0.045 GeV. That was the most precise measurement of the W boson’s mass at the time, but it was still not good enough, Denisov says. Taken together with the top quark, the earlier W boson measurement pointed to a Higgs that weighed about 90 GeV, a mass that had already been searched through and excluded by previous experiments.

That opened the door to possible physics beyond the standard model. But now, armed with about half the data the Tevatron produced over the course of its 28-year lifetime, the standard model is looking healthy again.

“Before this measurement, we thought maybe it’s not the standard model,” Denisov says. “But now it’s all fitting together nicely.” He recalls one comment when the team saw their latest result: “Let’s drink to the standard model, it wins!”

Bottoms up

The result is consistent with a Higgs weighing between 115 and 127 GeV, which fits with the standard model’s predictions, and with CERN’s tentative findings. But it also allows the Higgs to be as massive as 152 GeV, which would require some extra particles or other deviations from the standard model.

Denisov says the Fermilab teams still have half of the Tevatron’s data to comb through, so their precision will probably improve. The LHC collaborations are also refining their searches, and expect to have enough data to rule the Higgs boson in or out by the end of this year.

“In the next six to 12 months at most, it will be clarified,” Denisov says. “But if [the Higgs] will be found here, then I will drink a second time. Because then everything will really fit.”………..
Read more: newscientist.com

World’s best measurement of W boson mass …

… 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 http://www-cdf.fnal.gov/collaboration/index.html

DZero collaborating institutions are at http://www-d0.fnal.gov/ib/Institutions.html

Read more: http://fnal.gov