Tag Archives: particle physics

BigBang

Tour de France and the Higgs Particle

Two exciting races tracked through Grenoble, France this passed week. First, the Tour de France held one of the definitive stages of the 2011 race in Grenoble, the individual time trial. Second, Grenoble hosted the European Physical Society conference on High-Energy Physics. Fans of professional cycling and high energy physics would not be disappointed.

In cycling, Cadel Evans set a blistering pace in his solo effort on stage 20 to ensure the Yellow Jersey and an overall win in this year’s Tour.

In the world of high energy physics, physicists from Fermilab and CERN presented updates on their competing searches to discover (or not) the Higgs boson. The two main experiments at Fermilab, CDF and DZero, are looking for traces of the Higgs particle in the debris of Tevatron collider’s proton-antiproton collisions. At CERN’s Large Hadron Collider scientists working at the two massive detectors, Atlas and CMS, are sifting through vast mountains of data accumulated from proton-proton collisions.

Both colliders have been smashing particles together in their ongoing quest to refine our understanding of the building blocks of matter, and to determine the existence of the Higgs particle. The Higgs is believed to convey mass to other particles, and remains one of the remaining undiscovered components of the Standard Model of physics.

The latest results presented in Grenoble show excess particle events, above a chance distribution, across the search range where the Higgs particle is predicted to be found. There is a surplus of unusual events at a mass of 140-145 GeV (gigaelectronvolts), which is at the low end of the range allowed for the particle. Tantalizingly, physicists’ theories predict that this is the most likely region where the Higgs is to be found.

[div class=attrib]Further details from Symmetry Breaking:[end-div]

Physicists could be on their way to discovering the Higgs boson, if it exists, by next year. Scientists in two experiments at the Large Hadron Collider pleasantly surprised attendees at the European Physical Society conference this afternoon by both showing small hints of what could be the prized particle in the same area.

“This is what we expect to find on the road to the Higgs,” said Gigi Rolandi, physics coordinator for the CMS experiment.

Both experiments found excesses in the 130-150 GeV mass region. But the excesses did not have enough statistical significance to count as evidence of the Higgs.

If the Higgs really is lurking in this region, it is still in reach of experiments at Fermilab’s Tevatron. Although the accelerator will shut down for good at the end of September, Fermilab’s CDF and DZero experiments will continue to collect data up until that point and to improve their analyses.

“This should give us the sensitivity to make a new statement about the 114-180 mass range,” said Rob Roser, CDF spokesperson. Read more about the differences between Higgs searches at the Tevatron and at the LHC here.

The CDF and DZero experiments announced expanded exclusions in the search for their specialty, the low-mass Higgs, this morning. On Wednesday, the two experiments will announce their combined Higgs results.

Scientists measure statistical significance in units called sigma, written as the Greek letter ?. These high-energy experiments usually require 3?  level of confidence, about 99.7 percent certainty, to claim they’ve seen evidence of something. They need 5? to claim a discovery. The ATLAS experiment reported excesses at confidence levels between 2 and 2.8?, and the CMS experiment found similar excesses at close to 3?.

After the two experiments combine their results — a mathematical process much more arduous than simple addition — they could find themselves on new ground. They hope to do this in the next few months, at the latest by the winter conferences, said Kyle Cranmer, an assistant professor at New York University who presented the results for the ATLAS collaboration.

“The fact that these two experiments with different issues, different approaches and different modeling found similar results leads you to believe it might not be just a fluke,” Cranmer said. “This is what it would look like if it were real.”

[div class=attrib]More from theSource here.[end-div]

[div class=attrib]CERN photograph courtesy Fabrice Coffrini/AFP/Getty Images. Tour de France image courtesy of NBCSports.[end-div]

BigBang

New Tevatron collider result may help explain the matter-antimatter asymmetry in the universe

[div class=attrib]From Symmetry Breaking:[end-div]

About a year ago, the DZero collaboration at Fermilab published  a tantalizing result in which the universe unexpectedly showed a preference for matter over antimatter. Now the collaboration has more data, and the evidence for this effect has grown stronger.

The result is extremely exciting: The question of why our universe should exist solely of matter is one of the burning scientific questions of our time. Theory predicts that matter and antimatter was made in equal quantities. If something hadn’t slightly favored matter over antimatter, our universe would consist of a bath of photons and little else. Matter wouldn’t exist.

The Standard Model predicts a value near zero for one of the parameters that is associated with the difference between the production of muons and antimuons in B meson decays. The DZero results from 2010 and 2011 differ from zero and are consistent with each other. The vertical bars of the measurements indicate their uncertainty. 

The 2010 measurement looked at muons and antimuons emerging from the decays of neutral mesons containing bottom quarks, which is a source that scientists have long expected to be a fruitful place to study the behavior of matter and antimatter under high-energy conditions. DZero scientists found a 1 percent difference between the production of pairs of muons and pairs of antimuons in B meson decays at Fermilab’s Tevatron collider. Like all measurements, that measurement had an uncertainty associated with it. Specifically, there was about a 0.07 percent chance that the measurement could come from a random fluctuation of the data recorded. That’s a tiny probability, but since DZero makes thousands of measurements, scientists expect to see the occasional rare fluctuation that turns out to be nothing.

During the last year, the DZero collaboration has taken more data and refined its analysis techniques. In addition, other scientists have raised questions and requested additional cross-checks. One concern was whether the muons and antimuons are actually coming from the decay of B mesons, rather than some other source.

Now, after incorporating almost 50 percent more data and dozens of cross-checks, DZero scientists are even more confident in the strength of their result. The probability that the observed effect is from a random fluctuation has dropped quite a bit and now is only 0.005 percent. DZero scientists will present the details of their analysis in a seminar geared toward particle physicists later today.

Scientists are a cautious bunch and require a high level of certainty to claim a discovery. For a measurement of the level of certainty achieved in the summer of 2010, particle physicists claim that they have evidence for an unexpected phenomenon. A claim of discovery requires a higher level of certainty.

If the earlier measurement were a fluctuation, scientists would expect the uncertainty of the new result to grow, not get smaller. Instead, the improvement is exactly what scientists expect if the effect is real. But the uncertainty associated with the new result is still too high to claim a discovery. For a discovery, particle physicists require an uncertainty of less than 0.00005 percent.

The new result suggests that DZero is hot on the trail of a crucial clue in one of the defining questions of all time: Why are we here at all?

[div class=attrib]More from theSource here.[end-div]

BigBang

More subatomic spot changing

[div class=attrib]From the Economist:[end-div]

IN THIS week’s print edition we report a recent result from the T2K collaboration in Japan which has found strong hints that neutrinos, the elusive particles theorists believe to be as abundant in the universe as photons, but which almost never interact with anything, are as fickle as they are coy.

It has been known for some time that neutrinos switch between three types, or flavours, as they zip through space at a smidgen below the speed of light. The flavours are distinguished by the particles which emerge on the rare occasion a neutrino does bump into something. And so, an electron-neutrino conjures up an electron, a muon-neutrino, a muon, and a tau-neutrino, a tau particle (muons and tau are a lot like electrons, but heavier and less stable). Researchers at T2K observed, for the first time, muon-neutrinos transmuting into the electron variety—the one sort of spot-changing that had not been seen before. But their results, with a 0.7% chance of being a fluke, was, by the elevated standards of particle physics, tenuous.

Now, T2K’s rival across the Pacific has made it less so. MINOS beams muon-neutrinos from Fermilab, America’s biggest particle-physics lab located near Chicago, to a 5,000-tonne detector sitting in the Soudan mine in Minnesota, 735km (450 miles) to the north-west. On June 24th its researchers annouced that they, too, had witnessed some of muon-neutrinos change to the electron variety along the way. To be precise, the experiment recorded 62 events which could have been caused by electron-neutrinos. If the proposed transmutation does not occur in nature, it ought to have seen no more than 49 (the result of electron-neutrinos streaming in from space or radioactive rocks on Earth). Were the T2K figures spot on, as it were, it should have seen 71.

As such, the result from MINOS, which uses different methods to study the same phenomenon, puts the transmutation hypothesis on a firmer footing. This advances the search for a number known as delta (?). This is one of the parameters of the formula which physicists think describes neutrinos spot-changing antics. Physicists are keen to pin it down, since it also governs the description of the putative asymmetry between matter and antimatter that left matter as the dominant feature of the universe after the Big Bang.

In light of the latest result, it remains unclear whether either the American or the Japanese experiment is precise enough to measure delta. In 2013, however, MINOS will be supplanted by NOvA, a fancier device located in another Minnesota mine 810km from Fermilab’s muon-neutrino cannon. That ought to do the trick. Then again, nature has the habit of springing surprises.

And in more ways than one. Days after T2K’s run was cut short by the earthquake that shook Japan in March, devastating the muon-neutrino source at J-PARC, the country’s main particle-accelerator complex, MINOS had its own share of woe when the Soudan mine sustained significant flooding. Fortunately, the experiment itself escaped relatively unscathed. But the eerie coincidence spurred some boffins, not a particularly superstitious bunch, to speak of a neutrino curse. Fingers crossed that isn’t the case.

[div class=attrib]More from theSource here.[end-div]

[div]Image courtesy of Fermilab.[end-div]

BigBang

The LHC Begins Its Search for the “God Particle

[div class=attrib]From Discover:[end-div]

The most astonishing thing about the Large Hadron Collider (LHC), the ring-shaped particle accelerator that revved up for the first time on September 10 in a tunnel near Geneva, is that it ever got built. Twenty-six nations pitched in more than $8 billion to fund the project. Then CERN—the European Organization for Nuclear Research—enlisted the help of 5,000 scientists and engineers to construct a machine of unprecedented size, complexity, and ambition.

Measuring almost 17 miles in circumference, the LHC uses 9,300 superconducting magnets, cooled by liquid helium to 1.9 degrees Kelvin above absolute zero (–271.3º C.), to accelerate two streams of protons in opposite directions. It has detectors as big as apartment buildings to find out what happens when these protons cross paths and collide at 99.999999 percent of the speed of light. Yet roughly the same percentage of the human race has no idea what the LHC’s purpose is. Might it destroy the earth by spawning tiny, ravenous black holes? (Not a chance, physicists say. Collisions more energetic than the ones at the LHC happen naturally all the time, and we are still here.)

In fact, the goal of the LHC is at once simple and grandiose: It was created to discover new particles. One of the most sought of these is the Higgs boson, also known as the God particle because, according to current theory, it endowed all other particles with mass. Or perhaps the LHC will find “supersymmetric” particles, exotic partners to known particles like electrons and quarks. Such a discovery would be a big step toward developing a unified description of the four fundamental forces—the “theory of everything” that would explain all the basic interactions in the universe. As a bonus, some of those supersymmetric particles might turn out to be dark matter, the unseen stuff that seems to hold galaxies together.

[div class=attrib]More from theSource here.[end-div]

[div class=attrib] Image courtesy of Maximillien Brice/CERN.[end-div]

BigBang

The First Few Microseconds

[div class=attrib]From Scientific American:[end-div]

In recent experiments, physicists have replicated conditions of the infant universe–with startling results.

For the past five years, hundreds of scientists have been using a powerful new atom smasher at Brookhaven National Laboratory on Long Island to mimic conditions that existed at the birth of the universe. Called the Relativistic Heavy Ion Collider (RHIC, pronounced “rick”), it clashes two opposing beams of gold nuclei traveling at nearly the speed of light. The resulting collisions between pairs of these atomic nuclei generate exceedingly hot, dense bursts of matter and energy to simulate what happened during the first few microseconds of the big bang. These brief “mini bangs” give physicists a ringside seat on some of the earliest moments of creation.

During those early moments, matter was an ultrahot, superdense brew of particles called quarks and gluons rushing hither and thither and crashing willy-nilly into one another. A sprinkling of electrons, photons and other light elementary particles seasoned the soup. This mixture had a temperature in the trillions of degrees, more than 100,000 times hotter than the sun’s core.

[div class=attrib]More from theSource here.[end-div]