RIVERSIDE, Calif. — Physicists on experiments at the Large Hadron Collider (LHC) at CERN, the particle physics laboratory on the border of Switzerland and France, announced yesterday (July 4) that they have observed a new particle. Whether the particle has the properties of the predicted Higgs boson, the world's most sought-after particle, remains to be seen.
The University of California, Riverside is a founding member of the Compact Muon Solenoid (CMS) experiment, a large particle-capturing detector at CERN, and is one of only five U.S. institutes to be a founding member. UC Riverside was a key contributor to the design, prototype testing, and construction of the endcap muon chambers, one of the principal detector components used in the search for the Higgs boson.
Other central UCR contributions include participation in the design, construction, commissioning, and operation of the silicon-based tracker, and the commissioning and operation of the hadron calorimeter, both of which are fundamental to most CMS physics studies including the Higgs boson search.
The observed new particle is in the mass region around 125-126 GeV. Publication of the analyses is expected around the end of this month. If the particle is confirmed to be the Higgs boson, it would represent a keystone in our knowledge of the elementary forces and particles that exist in our universe.
The next step for scientists will be to determine the precise nature of the particle and its significance for our understanding of the universe. Are its properties as expected for the Higgs boson, the final missing ingredient in the Standard Model of particle physics? Or is it something more exotic?
"The UCR group on CMS will take much more data this year and into 2013 — the 2012 run has just been extended by three months," said Gail Hanson, a distinguished professor of physics and astronomy, who recently met with UCR Chancellor Timothy P. White at CERN. "We will study the couplings of the new particle using this data. In supersymmetry, for example, there are multiple Higgs bosons, and a 125 GeV lightest Higgs boson is compatible with supersymmetric models. We will also continue to search for physics beyond the Standard Model."
The Standard Model of particle physics, the best explanation to date for how the universe works, has proven to explain correctly the elementary particles and forces of nature through more than four decades of experimental tests. But it cannot, without the Higgs boson, explain how most of these particles acquire their mass, a key ingredient in the formation of our universe.
"The question is whether the new particle discovered is the Standard Model Higgs or not," said J. William Gary, a professor of physics and astronomy at UCR. "We hope it is not the Standard Model Higgs, but rather a Higgs of a different variety, because we need a guide on how to extend the Standard Model to a deeper, more complete theory. We need more data to determine the new particle's properties, before the LHC begins a two-year shutdown."
In December 2011, the CMS and ATLAS experiments at the LHC announced seeing tantalizing hints of a new particle in their hunt for the Higgs. Since resuming data-taking in March 2012, the CMS and ATLAS experiments have more than doubled their collected data.
UCR currently has three postgraduate researchers and eight graduate students stationed at CERN, who are participating in CMS detector maintenance, operation, and upgrades.
UCR physics studies encompass a wide range of fundamental topics, including searches for the Higgs boson in the photon-photon channel, searches for physics beyond the standard model in signatures associated with supersymmetry and new heavy neutrinos, and studies of the top quark.
Besides Hanson and Gary, the UCR faculty who participate in CMS are Robert Clare, John Ellison, Owen Long, and Stephen Wimpenny. The UCR postgraduate researchers are Mauro Dinardo, Sudan Paramesvaran, and Suharyo Sumowidagdo. The UCR graduate students are John Babb, Kira Burt, Ferdinando Giordano, Jesse Heilman, Pawandeep Jandir, Elizabeth Kennedy, Arun Luthra, Harold Nguyen, Amithabh Shrinivas, Jared Sturdy, Rachel Wilken, and Xu Xu. Visiting scientist Hongliang Liu, a former UCR graduate student, is also part of the team.
Hundreds of scientists and graduate students from American institutions have played important roles in the search for the Higgs at the LHC. More than 1,700 people from U.S. institutions helped design, build and operate the LHC accelerator and its particle detectors.
The results presented yesterday at CERN are labeled preliminary. They are based on data collected in 2011 and 2012, with the 2012 data still under analysis. A more complete picture of the observations will emerge later this year after the LHC provides the experiments with more data.
"This is fundamental science at its best," said Clare, a professor of physics. "This observation of what could be the Higgs boson and potential future discoveries at the LHC will shed light on how the universe works."
Scientists proposed in 1964 the existence of the Higgs boson, whose coupling with other particles would determine their mass. Experiments at CERN and Fermilab have searched for the particle, but it has eluded discovery. Only now, after decades of developments in accelerator and detector technology and computing — not to mention advancements in the understanding of the rest of the Standard Model — are scientists approaching the moment of knowing whether the Higgs was the right solution to this problem.
When protons collide in the Large Hadron Collider, their energy can convert into mass, often creating short-lived particles. These particles quickly decay into pairs of lighter, more stable particles that scientists can record with their detectors.
Theoretical physicists have predicted the rate at which the Higgs boson will be produced in high-energy proton-proton collisions at the LHC and also how it decays into certain combinations of observable particles. Discovery of the Higgs — or another new particle — would represent only the first step into a new realm of understanding of the world around us.