higgs bosons

First Glimpse of Higgs Bosons at Work Revealed.

Posted by

0


An extremely rare collision of massive subatomic particles could reveal the nuts and bolts of how the subatomic particles called Higgs bosons impart mass to other particles.

an abstract version of the higgs boson

The Higgs boson particle, which was detected for the first time in 2012, is essentially tossed around like a ball between two force-carrying particles known as W-bosons when they scatter, or bounce off of one another, a new data analysis revealed.

The data comes from the ATLAS experiment, the same proton-collision experiment that revealed the Higgs boson, at the Large Hadron Collider(LHC), a 17-mille-long (27 kilometers) underground atom smasher on the border of Switzerland and France.

By studying how much the Higgs sticks to the W-bosons during this scattering process, the team could learn new details about how strongly the elusive Higgs boson interacts with the field that gives all particles their mass.

“We are basically observing the Higgs boson at work to see whether it does its job the way we expect it to,” said study co-author Marc-André Pleier, a physicist with the ATLAS project, and a researcher at Brookhaven National Laboratory in Upton, New York. [Beyond Higgs: 5 Elusive Particles That May Lurk in the Universe]

Higgs Field

For decades, the Standard Model, the reigning physics theory that describes the menagerie of subatomic particles, was both astonishingly predictive and obviously incomplete.

The long-sought missing piece of the Standard Model was the Higgs boson, a particle proposed by English physicist Peter Higgs and others in 1964 to explain how certain particles get their mass. The theory held that particles like W-bosons pick up mass as they travel through a field, now known as the Higgs field. The more particles “drag” through the field, the more massive they are. If the Higgs field did exist, then by extension another particle, the now-famous Higgs boson (dubbed “the God Particle,” a nickname scientists dislike), should also exist as a vibration of that field when other subatomic particles interact with the field.

In 2012, scientists announced they had found the Higgs boson. In the years since, physicists have been busy analyzing data from collisions at the LHC to figure out exactly how the Higgs boson does its job of giving particles mass.

Impossible physics

Other parts of the Standard Model didn’t add up without the Higgs boson. For instance, in theory proton collisions could produce pairs ofW-bosons that would then scatter, or bounce off of, one another. (W-bosons mediate the weak nuclear force, which governs radioactive decay and fuels the chemical reactions at the hearts of stars, Pleier said.)

At high-enough collision energies, however, the theory predicted that W-boson scattering would occur more than 100 percent of the time, which is physically impossible, Pleier said.

So physicists proposed a subatomic game of catch, where a Higgs boson could bounce off one W-boson in a colliding pair, and be absorbed by the other member of the pair, Pleier said.

The extra Higgs, in essence, fixed the mathematical glitch in the theory.

But W-boson scattering was incredibly rare: It occurs only once in 100 trillion proton-proton collisions, so scientists never had a chance to test their theory, Pleier said.

“It’s even rarer to observe than the Higgs boson,” Pleier told Live Science.

Higgs at work

While poring over data from the ATLAS experiment, researchers saw, for the first time, glimpses of elusive W-boson scattering, Pleier said.

So far, the team has seen hints of just 34 W-boson scattering events, which showed that the Higgs boson does play some role in this scattering process.

But there is still too little data to say exactly how “sticky” the Higgs boson is to these W-bosons, which would reveal how sticky the Higgs field is. That, in turn, could help reveal more details about how the Higgs field gives other particles their mass, Pleier said.

If follow-up data reveals that the Higgs Boson doesn’t seem to be sticky enough, that’s an indication that other subatomic particles may be involved in W-boson scattering, he said.

When the LHC ramps up again in 2015 at higher energies, the team should be able to produce 150 times more data than they were collecting when the atom smasher shut down in 2013, which could help flesh out the now-shadowy picture of the Higgs boson in action.

 

Have Scientists Found 2 Different Higgs Bosons?

Posted by

0


gammagamma_run194108_evt564224000_ispy_3d-nologo-1024x656A month ago scientists at the Large Hadron Collider released the latest Higgs boson results. And although the data held few obvious surprises, most intriguing were the results that scientists didn’t share.

The original Higgs data from back in July had shown that the Higgs seemed to be decaying into two photons more often than it should—an enticing though faint hint of something new, some sort of physics beyond our understanding. In November, scientists at the Atlas and LHC CMS experiments updated just about everything except the two-photon data.* This week we learned why.

Yesterday researchers at the Atlas experiment finally updated the two-photon results. What they seem to have found is bizarre—so bizarre, in fact, that physicists assume something must be wrong with it. Instead of one clean peak in the data, they have found two an additional peak.* There seems to be a Higgs boson with a mass of 123.5 GeV (gigaelectron volts, the measuring unit that particle physicists most often use for mass), and another Higgs boson at 126.6 GeV—a statistically significant difference of nearly 3 GeV. Apparently, the Atlas scientists have spent the past month trying to figure out if they could be making a mistake in the data analysis, to little avail. Might there be two Higgs bosons?

Although certain extensions of the Standard Model of particle physics postulate the existence of multiple Higgs bosons, none of them would predict that two Higgs particles would have such similar masses. They also don’t predict why one should preferentially decay into two Z particles (the 123.5 GeV bump comes from decays of the Higgs into Zs), while the other would decay into photons.

The particle physicist Adam Falkowski (under the nom de plume Jester) writes that the results “most likely signal a systematic problem rather than some interesting physics.” (By “systematic problem” he means something like a poorly-calibrated detector.) The physicist Tommaso Dorigo bets that it’s a statistical fluke that will go away with more data. Indeed, he’s willing to bet $100 on it with up to five people, in case you’re the kind of person who likes to wager on the results of particle physics experiments with particle physicists. The Atlas physicists are well aware of both of these possibilities, of course, and have spent the past month trying to shake the data out to see if they can fix it. Still, the anomaly remains.

But let’s not let this intriguing blip distract us from the original scent of new physics. Back when the preliminary data seemed to show that the Higgs was decaying into two photons more often than it should, I wrote that it could be “a statistical blip that would wash away in the coming flood of data.” But more data has now arrived, and the blip hasn’t gone anywhere. The Higgs boson continues to appear to be decaying into two photons nearly twice as often as it should.

All the more reason to stay tuned for the next big data release, currently scheduled for March.

*Update 12/17/12: In November, scientists at the Atlas and CMS experiments (not the “LHC” experiment—apologies for the dumb typo) updated everything except for the two-photon data and, in the case of Atlas, the data regarding the decay of the Higgs into four leptons. I have added “just about” to indicate that the two-photon data wasn’t the only thing missing. I apologize for the imprecise language.

*Update 12/17/12: The sentence as originally written inadvertently implied that there are two peaks in the two-photon data. In fact the two photon data has one peak, but at a different mass than the peak found in other data sets. The Higgs to two-photon data shows a peak at 126.6 GeV, while the Higgs to four-lepton data (newly updated) shows a peak at 123.5 GeV. Apologies for the confusion.

Source: Scientific American

Tantalizing Hints of Elusive Higgs Particle Announced.

Posted by

0


The long-sought Higgs boson is tied to the leading theory of how quarks, electrons and other particles get their mass.

The two largest collaborations of physicists in history Tuesday presented intriguing but tentative clues to the existence of the Higgs boson, the elementary particle thought to endow ordinary matter with mass.

Representing the 6,000 physicists who work on two separate detectors at the Large Hadron Collider (LHC), called CMS and ATLAS, two spokespersons said that both experiments seemed to agree, as both their data sets suggested that the Higgs has a mass close to that of about 125 hydrogen atoms. The LHC is an international facility hosted by CERN, the European particle physics laboratory outside Geneva.

“We are talking of intriguing, tantalizing hints,” said CMS spokesperson Guido Tonelli, speaking to a room filled with dozens of journalists and TV crews. “It’s not evidence.”

The experiments, in which protons traveling at nearly the speed of light collide head-on, cannot directly detect the Higgs, because the boson would decay within a fraction of a nanosecond into other particles. Instead, physicists must search through the debris of many different types of particle decay to find precise combinations of by-products that the Higgs would produce—and different chains of particle decays may well have the same signatures. A particular combination that appears more often than expected from other, “background” processes may signal the presence of the Higgs. But if it does not appear often enough compared with the expected background, it could just be a statistical fluctuation. Today, neither CMS nor ATLAS could claim to have the “3-sigma” statistical significance needed to claim evidence for a new particle—let alone 5 sigma for the accepted standard to claim a discovery. (A 3-sigma result implies a fraction of a 1 percent chance of a statistical fluke.) Instead, so far each experiment could only claim a statistical significance of around 2 sigma.

Both the detectors and the LHC accelerator itself, however, have been performing better than expected; so all the ducks are now in a row for settling the question soon, according to the researchers. “The nice thing to know is that by the end of 2012—sooner if we are lucky—we should be able to say the final word,” Fabiola Gianotti, the ATLAS spokesperson, said at the press conference.

“I find it fantastic that we have the first results on the search for the Higgs, but keep in mind that these are preliminary results. And keep in mind that we have small numbers,” said CERN Director General Rolf-Dieter Heuer in summarizing presentations that both Tonelli and Gianotti gave during a CERN seminar earlier that day.

“I think the evidence is very encouraging, though it’s still too early to be sure,” comments Steven Weinberg, a leading theoretical physicist at the University of Texas at Austin and a winner of the Nobel Prize in Physics.

A generation of high-energy physicists came of age studying and testing the Standard Model of particle physics, a theory devised in the 1970s that has withstood all experimental challenges. One final piece is missing, though, and it is one without which the whole model could fall. Without the Higgs boson, physicists cannot explain how other particles have mass. The Higgs itself has mass, and going by exclusion, researchers from the LHC and from its predecessor particle colliders were able narrow down the range of its value to between 115 and 140 giga–electron volts, or GeV. (One GeV is roughly the mass of a hydrogen atom.)

Together, the LHC detectors have now reduced the allowed range further: Tonelli said that according to CMS data its mass cannot be greater than 127 GeV. That was not for lack of data—in fact, quite the opposite. “We were not able to exclude the range below 127 GeV because of excesses,” or more of certain particle by-products than would be expected in the absence of the Higgs, he remarked during his seminar talk—which was an understated way of saying that the CMS experiment had actually seen hints of a Higgs existing and having a mass of 124 GeV or so. ATLAS saw excesses in a similar range of energies, although the graphs did not quite line up—the ATLAS data favor a Higgs around 126 GeV. Not everyone is impressed with the new findings. The data are
“unconvincing,” says Matt Strassler, a theoretical physicist at Rutgers University who was visiting CERN for the occasion. “I was a little disappointed,” he adds, that the results did not live up to the expectations and the rumors—some called it a “Higgsteria”—that had circulated in the run-up to the announcement. On the other hand, he grants, no one expected to have a discovery at this stage—the experiments have not yet amassed enough data.

Vivek Sharma, Higgs search coordinator at the CMS collaboration, agrees that the two experiments have a small discrepancy on what the supposed Higgs mass would be, and that tantalizing hints of new physics from other experiments have often turned out to be statistical anomalies. “People should curb their enthusiasm,” he cautions.

Joe Lykken, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., who is a member of the CMS collaboration, is more optimistic about the discrepancy. “Even though we are only seeing hints of the Higgs boson, it is encouraging that the ATLAS and CMS hints seem to be consistent with each other,” he says.

A Higgs with a mass of 125 GeV would fit with a hypothesized extension of the Standard Model called supersymmetry, which posits that every known particle has a heavier, as-yet-undiscovered partner. “The low-mass Higgs is not so bad for supersymmetry, to say it diplomatically,” CERN’s Heuer said.

The LHC first fired up in September 2008, but within a week it was crippled by a serious accident that put it out of order for more than a year. “It was a big setback,” says Lyn Evans, a CERN accelerator physicist who oversaw the construction and commissioning of the LHC from 1994 until his retirement a year ago. After repairs, however, the machine restarted in 2009 and has delivered more collisions than predicted, enabling the ATLAS and CMS collaborations to amass data five times faster than expected.

As recently as a year ago, one would not have thought that the LHC would make so much progress in its Higgs search by the end of 2011, observes Dmitri Denisov, spokesperson for the DZero experiment, one of the detectors at Fermilab’s recently retired Tevatron collider. “It performed better than anyone expected,” Denisov says.

If the Higgs really exists, it will answer the long-standing question of how matter gets its mass. It will also reveal the nature of the connection between two fundamental forces, the weak nuclear force and the electromagnetic force—a relationship termed the electroweak interaction. The two forces were unified for the first instants of our universe, but now they behave differently. Weinberg says the new results suggest that “it should be possible to reach a definite decision about whether this is the particle associated with the breakdown of the symmetries of the electroweak theory. I’ll bet that it is.”

Source:Scientific American