Physicists at the University of New Mexico are among several thousand scientists around the world who’ve been collaborating for 17 years and have played an instrumental part in the search for the Higgs boson at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. Recently, a new particle was observed in the search, but whether the particle has the properties of the predicted Higgs boson is now a matter of intense scrutiny.
The Higgs boson or Higgs particle is a proposed elementary particle in the Standard Model of particle physics. The Standard Model of particle physics 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.
UNM Researchers Professor Sally Seidel, Research Professor Igor Gorelov, Research Engineer Martin Hoeferkamp, Graduate Research Assistant Rui Wang, and Post-doctoral fellow Konstantin Toms have all played important roles through their involvement in the development, construction, and operation of the pixel detector, which was of critical importance in the discovery of the new particle.
“The design and construction of the pixel detector were instrumental in finding this particle,” said Seidel, the group's leader at UNM. “We participated in the design and testing of the silicon detectors of the pixel detector which is at the very heart of the ATLAS Detector. The pixel detector tracks the most short-lived and rare particles, providing information that is essential in the discovery of the Higgs boson. By contributing to the design of the instrument, we helped to enable our collaboration to make this discovery.”
Construction of the Atlas Detector
Seidel’s research group has been collaborating and working with the ATLAS Detector for nearly two decades. “We not only worked on building the detector, we also take shifts in the main control room at CERN monitoring data,” said Seidel. “Our group has been developing silicon detectors since 1991. UNM has lots of experience in this area so it was natural for us to contribute to that aspect. We previously developed silicon detectors for the CDF Experiment at Fermilab, and those were critical to the discovery of the top quark. We’ve been working on it for decades and have established expertise in that field.”
In 1995, Seidel’s group started brainstorming with collaborators in Europe. Prototypes were then fabricated with silicon pixel sensors designed and characterized by UNM students and scientific personnel. Once the design was finalized, different parts of the detector were constructed at many universities and laboratories around the world. Some members of the UNM team participated in commissioning it at CERN. The ATLAS Detector began taking data as soon as the first LHC collisions occurred in fall 2008.
The ATLAS Detector is featured in the opening scenes of "Angels and Demons." A link to the clip of the movie showing the experimental hall and access shaft is available on the ATLAS Collaboration web site.
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 – 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 and detector operations at the LHC and also supplies computing for the ATLAS and CMS experiments.
Shaft Pixel Detector
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.
“Our graduate students do part of their research at UNM and then go to CERN with us to take data and work side-by-side with scientists from other parts of the world analyzing it. This is an excellent educational experience for students. Our undergrads, who carry out their research here at UNM, have the satisfaction of knowing that their work is incorporated in the commissioned experiment. It makes science real for them and helps them appreciate the subtle details of an enormous, complex project.”
Some of the work at UNM made use of facilities at Los Alamos National Lab to study radiation hardness of detector materials. “There are some unique facilities for science in New Mexico,” said Seidel. “It’s a great opportunity for our students to work with experts there.”
Working with so many scientists from around the world, Seidel compared the construction of the detector to a recipe.
“The ATLAS Experiment has hundreds of millions of detector elements and electronic components. Each part of the hardware has its own special role to play in the discovery of a new particle. No single group could design, build, and operate everything. Think of it like a recipe with all the various ingredients such as eggs and milk,” said Seidel. “You could say that we contributed the eggs while other researchers and scientists from around the world contributed the milk and other ingredients. But we had to invent the eggs before we could contribute them! The process involving all the scientists is quite democratic. Certain scientists are interested and experienced in building particular types of detectors. We would visit and talk, and after a while the collaboration emerged organically.”
Scientists proposed in 1964 the existence of a new particle, now known as the Higgs boson, whose coupling with other particles would determine their mass. Experiments at the LEP collider at CERN and the Tevatron collider at the Department of Energy's Fermilab have searched for the Higgs boson, 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.
The new particle is in the mass region around 125-126 GeV. When protons collide in the Large Hadron Collider, their energy can convert into mass, often creating short-lived particles. These particles quickly decay into 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. Experimental physicists at the ATLAS and CMS experiments have been studying the collisions and have observed a new particle. Still, more data will need to be collected and further analysis run to determine its properties.
In December the CMS and ATLAS experiments announced seeing tantalizing hints of a new particle in their hunt for the Higgs, the missing piece in the Standard Model of particle physics. Since resuming data-taking in March 2012, the CMS and ATLAS experiments have more than doubled their collected data. The statistical significance of the earlier hints has grown.
“We understand that all of nature is built from elementary particles – for example, atoms are made up of protons and electrons and other particles that form bound states,” said Seidel. “Atoms and molecules comprise our planetary systems and galaxies. Having mass is essential to forming a gravitationally bound state. The Higgs boson answers the question of why do particles have mass. The Higgs boson helps us understand the structures of the universe.”
Seidel’s group has an ongoing research program investigating many open questions in particle physics that may involve the Higgs boson and as well as other questions in different directions.
“This is new data in an energy regime never explored before. We’ll try to look into many corners of it over the coming years,” she added.
The results announced recently are 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. Publication of the analyses shown recently is expected in August.