Is CERN Closer to Finding the “Higgs”?–This Week’s 8 TeV Collision Most Powerful in Human History

 

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Just days after restarting from a winter shutdown, researchers at European Organization for Nuclear Research (CERN) using the Large Hadron Collider (LHC), report that stable beams of protons were smashed at four observation positions, with a combined energy of 8 trillion electron volts (TeV), earning a new world record, blowing away the previous record of 7 TeV — a record set by the LHC in 2010.

"The experience of two good years of running at 3.5 TeV per beam gave us the confidence to increase the energy for this year without any significant risk to the machine" according to Steve Myers, CERN's particle accelerator director. "Now it's over to the experiments to make the best of the increased discovery potential we're delivering them!"

Aside from setting a record for the highest energy collisions ever made by man, the LHC had some impressive early accomplishments, discovering a new kind of subatomic particle and detecting evidence of dark matter (image above is the most remote, known "dark matter" galaxy).

The accelerator's successes come thanks to its extraordinary design, with a track 17 miles long cooled to temperatures colder than outer space, and is capable of sending protons hurling at speeds in excess of 99.999999 percent the speed of light.  

 

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Evidence appears to be closing in on proving the existence of the Higgs boson: While sifting through the data of last year's collisions, researchers found evidence of a 125 gigaelectronvolt particle — 130 times as massive as the relatively large proton.  FermiLab's Tevatron — America's last major particle accelerator — has been shut down, but even in death it continues to yield useful discoveries and evidence.  Digging through its archives, CERN researchers found a similar bump in readings in this same mass region.
These results all point to a possible Higgs boson spotting, but more tests are needed to provide confirmation.

With the winter off, the LHC is back and recalibrated to try to more accurately spot the elusive subatomic particle.  The LHC's higher energy should go a long ways towards helping, as higher energy collisions tend to produce larger, more exotic types of particles.

CERN's search for the Higgs, provides a rigorous test of the Standard Model which serves as the blueprint for our Universe, detailing the properties of the building blocks of matter and how they interact. Standard model is a physics theory that explains every particle and force of nature. It remains one of the cornerstones of modern physics. While almost all particles described in the theory have been discovered by the scientists, the Higgs boson, which imparts mass to all other particles, remains elusive.

On March 7, Fermilab Tevatron physicists reported hints of Higgs boson sighting consistent with those from LHC. New measurements announced by scientists from the CDF and DZero collaborations at the Department of Energy’s Fermi National Accelerator Laboratory indicate that the elusive Higgs boson may nearly be cornered.

After analyzing the full data set from the Tevatron accelerator, which completed its last run in September 2011, the two independent experiments see hints of a Higgs boson.Physicists from the CDF and DZero collaborations found excesses in their data that might be interpreted as coming from a Higgs boson with a mass in the region of 115 to 135 GeV. In this range, the new result has a probability of being due to a statistical fluctuation at level of significance known among scientists as 2.2 sigma.

This new result also excludes the possibility of the Higgs having a mass in the range from 147 to 179 GeV.Physicists claim evidence of a new particle only if the probability that the data could be due to a statistical fluctuation is less than 1 in 740, or three sigmas.

A discovery is claimed only if that probability is less than 1 in 3.5 million, or five sigmas.This result sits well within the stringent constraints established by earlier direct and indirect measurements made by CERN’s Large Hadron Collider, the Tevatron, and other accelerators, which place the mass of the Higgs boson within the range of 115 to 127 GeV.

These findings are also consistent with the December 2011 announcement of excesses seen in that range by LHC experiments, which searched for the Higgs in different decay patterns. None of the hints announced so far from the Tevatron or LHC experiments, however, are strong enough to claim evidence for the Higgs boson.

"The end game is approaching in the hunt for the Higgs boson," said Jim Siegrist, DOE Associate Director of Science for High Energy Physics. "This is an important milestone for the Tevatron experiments, and demonstrates the continuing importance of independent measurements in the quest to understand the building blocks of nature."

Physicists from the CDF and DZero experiments made the announcement at the annual conference on Electroweak Interactions and Unified Theories known as Rencontres de Moriond in Italy. This is the latest result in a decade-long search by teams of physicists at the Tevatron.

Higgs bosons, if they exist, are short-lived and can decay in many different ways. Just as a vending machine might return the same amount of change using different combinations of coins, the Higgs can decay into different combinations of particles. Discovering the Higgs boson relies on observing a statistically significant excess of the particles into which the Higgs decays and those particles must have corresponding kinematic properties that allow for the mass of the Higgs to be reconstructed.

"There is still much work ahead before the scientific community can say for sure whether the Higgs boson exists," said Dmitri Denisov, DZero co-spokesperson and physicist at Fermilab. "Based on these exciting hints, we are working as quickly as possible to further improve our analysis methods and squeeze the last ounce out of Tevatron data."

Only high-energy particle colliders such as the Tevatron and LHC can recreate the energy conditions found in the universe shortly after the Big Bang. 

"Without something like the Higgs boson giving fundamental particles mass, the whole world around us would be very different from what we see today," said Giovanni Punzi, CDF co-spokesperson and physicist at the National Institute of Nuclear Physics, or INFN, in Pisa, Italy. "Physicists have known for a long time that the Higgs or something like it must exist, and we are eager to finally pin this phenomenon down and start learning more about it."

If a Higgs boson is created in a high-energy particle collision, it immediately decays into lighter more stable particles before even the world’s best detectors and fastest computers can snap a picture of it. To find the Higgs boson, physicists retraced the path of these secondary particles and ruled out processes that mimic its signal.

The experiments at the Tevatron and the LHC offer a complementary search strategy for the Higgs boson. The Tevatron was a proton/anti-proton collider, with a maximum center of mass energy of 2 TeV, whereas the LHC is a proton/proton collider that will ultimately reach 14 TeV.

Because the two accelerators collide different pairs of particles at different energies and produce different types of backgrounds, the search strategies are different. At the Tevatron, for example, the most powerful method is to search the CDF and DZero datasets to look for a Higgs boson that decays into a pair of bottom quarks if the Higgs boson mass is approximately 115-130 GeV.  

It is crucial to observe the Higgs boson in several types of decay modes because the Standard Model predicts different branching ratios for different decay modes. If these ratios are observed, then this is experimental confirmation of both the Standard Model and the Higgs.

"The search for the Higgs boson by the Tevatron and LHC experiments is like two people taking a picture of a park from different vantage points," said Gregorio Bernardi, DZero co-spokesperson at the Nuclear Physics Laboratory of the High Energies, or LPNHE, in Paris . "One picture may show a child that is blocked from the other’ s view by a tree. Both pictures may show the child but only one can resolve the child’s features. You need to combine both viewpoints to get a true picture of who is in the park. At this point both pictures are fuzzy and we think maybe they show someone in the park. Eventually the LHC with future data samples will be able to give us a sharp picture of what is there. The Tevatron by further improving its analyses will also sharpen the picture which is emerging today."

This new updated analysis uses 10 inverse femtobarns of data from both CDF and DZero, the full data set collected from 10 years of the Tevatron’s collider program. Ten inverse femtobarns of data represents about 500 trillion proton-antiproton collisions. Data analysis will continue at both experiments.

"This result represents years of work from hundreds of scientists around the world," said Rob Roser, CDF co-spokesperson and physicist at Fermilab. "But we are not done yet – together with our LHC colleagues, we expect 2012 to be the year we know whether the Higgs exists or not, and assuming it is discovered, we will have first indications that it behaves as predicted by the Standard Model."

 

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