Mar 21 2013
The new particle discovered at experiments at the CERN’s Large Hadron Collider (LHC) last summer is looking more like a Higgs boson than ever before. The CERN (European Organization for Nuclear Research) scientists, including a number of BU physicists, released new findings about the Higgs boson last week at CERN’s Moriond Conference in La Thuile, Italy.
Event recorded with the CMS detector in 2012 at a proton-proton centre of mass energy of 8 TeV. The event shows characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers). The event could also be due to known standard model background processes. Source: CMS, CERN
On July 4, 2012, physicists on the CMS and ATLAS experiments announced the discovery of a particle with a close resemblance to a Higgs, a particle thought to give mass to other elementary particles. The discovery of such a particle could finish a job almost five decades in the making: It could confirm the last remaining piece of the Standard Model of particle physics, a menu of the smallest particles and forces that make up the universe and how they interact.
Boston University physicists been involved in the two general purpose collider detectors at CERN almost 20 years and have made important contributions to the CMS and ATLAS experiments. BU physicists helped design the CMS Hadron Calorimeter, an integral part of the experiment used to measure the energy of particles that emerge from the proton-proton collisions and have been instrumental in the design and commissioning of the CMS ``trigger'' system—an online selection system designed to select, in real time, potentially interesting physics events from Higgs and other decays. BU team members also play leading roles in the analysis of the collision data and are searching for exotic particles (including exotic versions of the Higgs Boson) predicted by several new physics theories.
The ATLAS and CMS collaborations have analyzed two and a half times more data than was available for the discovery announcement in July, and, in their preliminary results, they find that the new particle is looking more and more like a Higgs boson. Scientists have specific predictions for how often a Standard Model Higgs boson of a certain mass will decay into different patterns of particles. The latest results indicate that the new particle is sticking to the Standard Model’s script.
“The search for the Higgs Boson has been the Holy Grail of particle physics, and the latest results indicate that we have indeed found a Higgs boson,” said Tulika Bose, assistant professor of physics at Boston University. “Whether it is the Higgs Boson predicted by the Standard Model of particle physics or something more exotic, only new data and time will tell.” Bose adds that the LHC will restart in 2014 at close to double its current energy. “We expect this new energy regime to answer this question as well as discover new physics, which will help solve the other unsolved mysteries in the universe, such as the dominance of matter over antimatter in our universe, dark matter and dark energy,” said Bose.
Leading BU’s CMS group are Bose, James Rohlf and Larry Sulak, both professors of physics, and Arno Heister, research assistant professor.
BU faculty members of the ATLAS experiment are Stephen Ahlen and John Butler, both professors of physics, and Kevin Black, assistant professor of physics. The BU ATLAS team also includes James Shank, research professor, and Saul Youssef, research associate professor. These BU scientists have been involved in the design, development, maintenance, and software for the outer most part of the ATLAS detector, which is specially designed to identify and measure the properties of a subatomic particles called the muons, which, because of their unique characteristics, can penetrate a large amount of material and exit the detector.
The BU ATLAS researchers specifically were involved in the search for evidence of Higgs particle decay into two tau particles (tau particles are essentially heavier partners to the electron). While this type of decay is relatively rare for the Higgs boson, it is important to confirm and measure its occurrence in order to compare the decay to the Standard Model prediction, thereby confirming the existence of a Standard Model Higgs particle.
BU’s ATLAS team also played an active role in the 'trigger' of the experiment for the LHC, which decides which hundred of the million events per second will be recorded for further analysis. In addition, the BU team hosts a large computing center, which simulates and reconstructs events from the detector.
Although scientists will need to analyze substantially more data before they can conclusively declare the new particle is the Standard Model Higgs boson, results announced at the Rencontres de Moriond conference bolster scientists’ confidence that the particle they discovered is the Standard Model Higgs.
“Clear evidence that the new particle is the Standard Model Higgs boson still would not complete our understanding of the universe,” said Patty McBride, head of the CMS Center at Fermilab. “We still wouldn’t understand why gravity is so weak and we would have the mysteries of dark matter to confront. But it is satisfying to come a step closer to validating a 48-year-old theory.”
“These models aim to resolve some of the other great mysteries of particle physics, such as the nature of the dark matter making up about 20 percent of the mass of the Universe and the explanation for the dominance of matter over antimatter in the Universe—these are the very reason we are here,” said Bose. “We are now eagerly looking forward to the new run in 2014, which will provide unique opportunities to search for new physics.”
“When we discovered the particle, we knew we found something significant,” ATLAS scientist and New York University professor Kyle Cranmer said. “Now, we’re just trying to establish the properties.”
“It is important to keep in mind that this is really just the beginning of the exploration of the Higgs sector of particle physics,” said Black. “While many articles in the popular press seem to indicate that since this would be the final piece of the Standard Model, this somehow means particle physics is over. Nothing could be farther from the truth.” Black adds that researchers are just starting to understand the basic properties of this particle, and even if it turns out to be a Standard Model Higgs, many questions remain to be answered.
Researchers look for the Higgs boson at the LHC by accelerating protons to high energies and crashing them into one another. The energy of those colliding protons can briefly convert into mass, bringing into being heavier particles such as the Higgs bosons. The heavy particles are unstable and decay almost immediately into pairs of less massive particles.
One of those properties is the spin, an intrinsic identifier of the boson. As predicted by the Standard Model, the Higgs boson should have a spin of zero. If thought of as a color, it’s as if scientists know the new boson is blue, but they can’t tell if it’s a light or dark shade of blue yet. It’s a subtle point, but the difference is night and day for a detailed understanding of the particle’s properties.
The analysis included the data from about 500 trillion proton-proton collisions collected in 2011 and from about 1,500 trillion collisions in 2012. The LHC stopped operation on Feb. 16, for two years of maintenance and upgrades, but researchers will continue to study the data collected before the shutdown.
Hundreds of scientists and students from American institutions have played important roles in the search for the Higgs at the LHC. Fermi National Accelerator Laboratory and Brookhaven National Laboratory host the U.S. contingents of the CMS and ATLAS experiments, respectively. More than 1,700 people from U.S. institutions–including 89 American universities and seven U.S. Department of Energy (DOE) national laboratories–helped design, build and operate the LHC accelerator and its four particle detectors. The United States, through DOE’s Office of Science and the National Science Foundation, provides support for research, detector operations, and upgrades at the LHC, as well as supplies computing for the ATLAS and CMS experiments.
The vast majority of U.S. scientists participate in the LHC experiments from their home institutions, remotely accessing and analyzing the data through high-capacity networks and grid computing. Working collaboratively, these international organizations are able to analyze an incredible amount of data.
After further analysis, scientists will be able to say whether this new particle is the Standard Model Higgs boson or something more surprising.
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