LHC’s new particle looking ever more Higgs-like

Interactions with the Higgs field provide everything but photons and gluons with mass.

The teams behind the two general-purpose detectors of the Large Hadron Collider, ATLAS and CMS, tend to go through their results when facing a deadline, usually provided by one of the large physics conferences. Last week, the Moriond conference took place in Italy and, as expected, there were a number of updates on the Higgs based on the full data collected over the past year. So far, Peter Higgs and the others that first added the Higgs mechanism to physics' Standard Model are looking pretty good.

When the LHC teams announced that they'd found a new particle this summer, they were very careful to note a caveat. While it showed up at an energy where you might expect to see a Higgs (125GeV or so), they weren't certain it was a Higgs. With the additional data, the uncertainty is beginning to fall by the wayside. As a variety of releases (here's a couple of examples) this week have indicated, the Higgs particle was predicted to have a spin of 0 and be even parity. Further studies of the 125GeV particle produced in the LHC have indicated that it has both of these.

Meanwhile, the numbers of Higgs like particles detected by different decay pathways (two photons, four leptons, etc.) are all within the range of predictions. The ATLAS detector has posted a number of animations that show how the signal at 125GeV appeared in various decay channels as the data piled up, rising above the background of other Standard Model events. This one, which shows the Higgs-ZZ decay pathway, is especially clear. There were also some hints that the Higgs was decaying into two photons more often than expected, but physicist Matt Strassler notes the additional data has pretty much eliminated that prospect.

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Finding the Higgs? Good news. Finding its mass? Not so good.

A collision in the LHC's CMS detector.

Research.gov

Ohio State's Christopher Hill joked he was showing scenes of an impending i-Product launch, and it was easy to believe him: young people were setting up mats in a hallway, ready to spend the night to secure a space in line for the big reveal. Except the date was July 3 and the location was CERN—where the discovery of the Higgs boson would be announced the next day.

It's clear the LHC worked as intended and has definitively identified a Higgs-like particle. Hill put the chance of the ATLAS detector having registered a statistical fluke at less than 10^-11, and he noted that wasn't even considering the data generated by its partner, the CMS detector. But is it really the one-and-only Higgs and, if so, what does that mean? Hill was part of a panel that discussed those questions at the meeting of the American Association for the Advancement of Science.

As theorist Joe Lykken of Fermilab pointed out, the answers matter. If current results hold up, they indicate the Universe is currently inhabiting what's called a false quantum vacuum. If it were ever to reach the real one, its existing structures (including us), would go away in what Lykken called "fireballs of doom."

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Large Hadron Collider shuts down for two years of upgrades

No, finding the Higgs boson doesn't mean the end of physics. But as of today, no atoms will be smashed at the LHC (the Large Hadron Collider at CERN) for approximately two years. During that time, every piece of hardware around the accelerator's full circumference will get some attention, as will the detectors that track collisions.

The LHC was designed to collide protons with a total energy of 14TeV (Tera-electron Volts), but a catastrophic failure early in its history revealed some of the superconducting connectors within the hardware wasn't up to the task. As a result, the LHC hasn't run collisions at energies above 8TeV. Each of these connectors, which link segments of the pipe that the beam travels within, will be replaced over the next two years. While the machine is shut down, the detectors used to track particles will receive maintenance and upgrades.

We're at the annual meeting of the American Association for the Advancement of Science this week, and there will be updates on the properties of the Higgs, as well as the search for dark matter particles. Hopefully we'll hear more about the work that went on during the physics runs of the past several years.

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New Large Hadron Collider data may thin out theories in particle physics

Photograph by Fermilab

Although the Large Hadron Collider is often viewed as a Higgs discovery machine—a task for which it turned out to be admirably suited—the collider isn't a one-trick pony. Its general purpose detectors, ATLAS and CMS, should be able to spot any other unusual particles out there, while the ALICE detector is specialized for heavy ion collisions. But this week, attention fell on LHCb, the Large Hadron Collider beauty experiment.

Beauty is the alternate name for the bottom quark, which was discovered back in the 1980s and is the second heaviest of the quarks. Bottom quarks are often found in particles, called B mesons, in which they're paired with another quark (or an antimatter equivalent). LHCb is designed specifically to track how these B mesons decay, since their pattern of decays provides a sensitive test of the Standard Model of particle physics. Now, the LHCb team has announced they've spotted a number of rare decays—not one-in-a-billion, but close—and they've found that the rate at which decays occur agrees remarkably well with that predicted by the Standard Model. This in turn puts some limits on alternative theories.

With bottom quarks being 30 years old, you might think there would be little left to learn from them. But the fact is that they were heavy enough that they weren't produced in vast numbers by earlier particle colliders. That means that very rare events involving a bottom quark either weren't detected at all or were detected in such small numbers that it was impossible to say anything about these events with any statistical certainty.

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Hottest temps ever at LHC, and more hints about early Universe

Brookhaven National Laboratory

Washington, DC — This week is the Quark Matter 2012 (QM2012) conference—probably the preeminent meeting for those studying high-energy collisions between heavy ions. I attended a number of talks on Monday, August 13, during which researchers announced the major new results from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN. The conference offered fresh insights on the transition between ordinary matter and the soup of quarks that existed in the early Universe—including a tantalizing hint that might tell us about why the modern cosmos has more matter than antimatter.

We recently ran a detailed review of heavy-ion physics; here's an executive summary. Heavy nuclei (lead at LHC, gold, copper, and uranium at RHIC) are completely stripped of electrons, leaving massive, positively charged ions. These are accelerated to well over 99 percent of the speed of light and smashed into each other. If the energy is sufficiently high, the protons and neutrons in the nuclei "melt" into their constituent quarks and gluons. The result is a substance known as the quark-gluon plasma (QGP), which theory predicts existed during the first 10 microseconds after the Big Bang.

While the hunt for the Higgs boson has dominated press coverage of the LHC, the collider also performs heavy ion experiments using lead (Pb+Pb). In addition to the ATLAS and CMS detectors, which are used both for proton-proton and heavy ion collisions, LHC has a dedicated heavy ion detector named ALICE (A Large Ion Collider Experiment, pronounced "ahLEES"). The two active detectors at RHIC are PHENIX (Pioneering High-Energy Nuclear Interacting Experiment) and STAR (Solenoidal Tracker at RHIC). These study the products of collisions between gold ions (Au+Au); in the most recent experiments, researchers have added gold and copper asymmetric collisions (Au+Cu) and uranium (U+U). The two major colliders are complimentary in many aspects: the LHC has a larger temperature range and can reach lower density, while RHIC is able to explore much higher baryon densities.

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Heavy ion collisions reveal the earliest instants of our Universe

Lead ion collisions give the detectors lots of particles to track.

Duke.edu

The LHC's proton collisions, which have now successfully nailed down the existence of the Higgs boson, get most of the attention, both in the media and at CERN itself. But, for a few weeks each year, the collider is switched over to smashing lead ions. Heavy ion collisions, in fact, are considered to provide such distinct information that the US has kept open the Relativistic Heavy Ion Collider, which is dedicated to smashing heavy ions, even as it shut down the Tevatron, its dedicated proton/antiproton collider.

Right on the heels of the Higgs announcement, Science is running a review of heavy ion collisions, which nicely explains why they tell us something completely different from what's revealed by proton colliders. Plus it provides a nice picture of how the LHC will provide new data, and upgrades that have taken place at the RHIC to help keep it relevant.

The matter we see around us is comprised mainly of protons and neutrons. These, in turn, are composed of quarks and gluons, which mediates the strong force that binds them together. Because the potency of the strong force increases with distance, breaking up a nucleon (proton or neutron) typically requires high levels of energy that basically blasts the nucleon in part. That's precisely the sort of thing that happens during the proton collisions that take place at the LHC.

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A Higgs boson Q&A

From "The Higgs Boson (God Particle) Explained"

TheScienceHD

Last week's announcement of the discovery of a new particle seemed to answer one of the great outstanding questions in physics. But for those who haven't been immersed in all things LHC, the results were likely to raise all sorts of new questions (along with "what was all the fuss about again?"). So, to help navigate the post-Higgs world, we put together a short Q&A, based on questions that some of the Ars staff had.

I know we detected it in the Large Hadron Collider, but how did they actually make Higgs bosons?

There are two ways to answer that question. The first is that we're simply converting energy into matter. The protons in the collider carry a tremendous amount of energy, and it has to go somewhere. Given Einstein's E = mc², we know that some of that energy can be converted into matter. That's why things that are much heavier than two protons at rest can pop out of the collisions.

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CERN celebrates as Higgs signal reaches significance

A four-lepton decay, a possible sign of the Higgs, seen by the ATLAS detector.

CERN

Today, in two seminars held at CERN, the European center for physics, announced evidence that the elusive Higgs particle has finally been discovered.

Physics' Standard Model describes the fundamental particles that make up all matter, like quarks and electrons, as well as the particles that mediate their interactions through forces like electromagnetism and the weak force. Back in the 1960s, theorists extended the model to incorporate what has become known as the Higgs mechanism, which provides many of the particles with mass. One consequence of the Standard Model's version of the Higgs is that there should be a force-carrying particle, called a boson, associated with the Higgs field.

For decades, physicists have been sifting through the output of colliders like the Tevatron and LEP, looking for an indication that the Higgs was present in the spray of exotic particles they detected. The closest they got was a hint of a signal that didn't rise far enough above the background. Now, in less than two years of operation, the Large Hadron Collider's detectors have found clear evidence of a particle that looks a lot like the Higgs.

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"We’ve observed a new particle": leaked video reveals Higgs confirmation

Two superconducting dipole magnets used in the LHC

spadger

"We've observed a new particle." That's the opening statement in a video featuring Joe Incandela, the spokesman for the Large Hadron Collider's CMS detector. The video, first spotted by ScienceNews, was publicly accessible on the CERN website earlier today, but has now apparently been pulled. It appears to preempt the big announcement scheduled for early tomorrow morning, and implies that this year's data was enough to push the evidence for the Higgs past the five standard deviations needed to declare discovery.

"When we say we've observed the particle, it means we've just got enough data to say it's definitely there, and it's very unlikely to go away," Incandela says in the video. In addition, we know it's a boson because it decays into two photons. Its mass is roughly 130 times that of the proton, making it the heaviest boson we've discovered so far—and the heaviest particle other than the top quark.

According to the video, the two-photon decay is one of the strongest bits of evidence, and provides a narrow peak that helps define its mass. Strong evidence is also available through decay into two Z particles; the signal is there in other decay channels as well, which is reassuring, but those are less definitive because of the background noise present.

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Higgs week starts with Fermilab updates, strong evidence expected

A four-lepton decay, one option that the Standard Model provides for getting rid of a Higgs.

Quantum Diaries

This week, the European physics lab CERN will be hosting an announcement (scheduled for early on July 4th). It's expected that strong evidence for the Higgs boson will be presented. Rumors about exactly what will be announced are swirling, but most informed expectations indicate a very strong signal that falls just short of the standard for discovery. To set the stage for that announcement, Fermilab hosted two seminars today that gave an update on the search for the Higgs performed at the Tevatron, its now-defunct particle collider.

The Higgs is the last undiscovered particle predicted by the Standard Model; it mediates the interactions that give particles mass. Although its existence was proposed decades ago, direct evidence for the particle has been hard to come by. This is because it is very heavy, and because other processes produce very similar signals. (See this for an explanation about how we find a signal in the background noise). When we last checked in on the search back in March, the Tevatron had left the door open in a broad region around 115-135GeV, while the LHC's detectors had seen hints of a signal around 125GeV.

With the Tevatron having been shut down, there was no new data for the folks from Fermilab to discuss. What they have done in the mean time, however, is improve their analysis of the data they do have. Many of the decay processes that look like the Higgs can be partly distinguished from background events based on the specific details of the spray of particles produced. Scientists at Fermi have been developing neural networks that are better at separating out the different types of collisions. For the D0 detector alone, the improved analysis got them a 20 to 30 percent boost in sensitivity using the same data.

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