On 4 July, CERN, the European particle physics laboratory near Geneva, Switzerland, grabbed worldwide attention when it announced that it had found a new particle that looked very much like the long-sought Higgs boson. Two teams of scientists working with the Large Hadron Collider (LHC) at CERN—each using one of the two huge particle detectors at the LHC: ATLAS and CMS—reported that all the high-energy atom smashing that they had been doing over the last couple of years had paid off, providing a glimpse of what might well be the missing piece of the standard model of particle physics. “We have reached a milestone in our understanding of nature,” CERN Director General Rolf-Dieter Heuer stated during a seminar held at CERN.
Because the Higgs boson is extremely short-lived, scientists must look for evidence of its existence by recording its decay into various combinations of lighter particles predicted by the standard model. The efforts of the ATLAS and CMS teams—which each included some 3000 physicists—were organized around a complex chain of data analysis and crosschecks. Among those playing a key role in the discovery were many early-career scientists. Science Careers talked to three of them about what it took to get involved in the project and what it was like to be part of such a large and high-profile scientific endeavor.
“It was very exciting to see that, as then you know that it’s actually something physical in nature that you’re observing and not just some ghost that you’re chasing.” —Aaron Armbruster
Christos Anastopoulos, ATLAS
Breakthrough of the Year
Read more about the discovery of the Higgs boson and other important breakthroughs that occurred this year, in this week’s issue of Science.
Christos Anastopoulos was first exposed to the ATLAS experiment in 2004 when, as a B.Sc. student at the Aristotle University of Thessaloniki in Greece, he contributed to the construction of the ATLAS muon spectrometer, a part of the machinery that detects electrons' heavier cousins. While working on his master’s degree at Thessaloniki and the first year of his Ph.D. at the University of Sheffield in the United Kingdom, he became involved in the computing side of the ATLAS project, writing and validating algorithms. The LHC was still under construction, so the goal was to put in place the key parts of the software needed to analyze ATLAS data. According to Anastopoulos, the effort worked very well: “Most of the things we did for the discovery were already part of that exercise. We figured them out back then,” he says.
Anastopoulos moved on to developing algorithms to improve the identification and analysis of electrons, which, together with muons, make up a class of particles known as leptons. When the first LHC collisions occurred, at the end of November 2009, Anastopoulos was finally able to work with real particles. When that happened, “the game changed completely,” he says.
Inside the collaboration, those heady early days when real data started rolling in generated as much excitement as the Higgs discovery did 3 years later, Anastopoulos says. As the LHC started ramping up to ever-higher energies, the team's efforts focused on studying the decay of already known particles. Still, “at that time, any data, any peak, any small study … was a huge excitement.”
On 30 March 2010, the LHC reached high enough collision energies for the search for the Higgs to really begin. In the final year of his Ph.D., Anastopoulos joined the search, analyzing electron signatures in the channel corresponding to the predicted decay of the Higgs into four leptons. He also joined the effort to estimate the background in the results coming from the four-lepton decay channel.
In 2011, Anastopoulos became a CERN fellow, essentially a 2-year postdoc. He took on increasingly important roles. He became an electron expert, ensuring that the four-lepton group electron observations were made with high efficiency and were properly taken into account in the data analysis. One year before the discovery, he took on responsibility for background-signal calculations. When the ATLAS discovery paper came out on arXiv at the end of July, he was a corresponding editor.
To avoid bias when tweaking algorithms, the team adopted a strategy of looking at the region where they expected the Higgs to appear at only long, predetermined intervals. When they looked in late 2011, they saw something that looked like the Higgs, but they couldn't be sure because the statistics weren't strong enough yet. When they looked again several months later, confirmation came almost overnight. Anastopoulos was one of a handful of people who did the analysis that revealed the particle's signature in the four-lepton channel about 2 weeks before the July announcement. He was the one who presented the results to the whole Higgs search group. It was an intimidating challenge. “The critical part is not only convincing them that the particle is there, it’s convincing them that everything … was done right and all the plots are right.”
The month that followed was spent “working around the clock to … make every crosscheck, make sure that there was nothing wrong or we didn’t fool ourselves,” Anastopoulos says. The pressure came not just from the time crunch but also from the scrutiny of peers. It wasn’t until the 4 July announcement that he felt relaxed enough to experience the excitement of the discovery.
Now that the discovery is in the bag, Anastopoulos is studying the properties of the new particle in the four-lepton channel to find out whether this is the Higgs boson predicted by the standard model or something more exotic. In October, he was put in charge of coordinating the 102 people involved in this study. The greatest challenge, he says, is collaborating directly with so many people.
When Anastopoulos finishes next summer—his fellowship was extended by 6 months—he plans to apply for a 5-year position at CERN or a tenure-track job in another institution that would allow him to continue working with ATLAS.
His contribution to the discovery of the Higgs boson bodes well for his career. “My generation, we were the lucky ones. We just looked for it for 1 to 2 years and we found it. … We get more visibility and it makes it a little easier for us to find jobs.”
Cristina Botta, CMS
CREDIT: Cristina Botta
Cristina Botta joined the CMS experiment in 2007, when the detector was still being assembled and many scientists were working on software to prepare for the data acquisition phase. While she was still a master’s degree student at the University of Torino in Italy—and then, after graduating, during a 1-year stint supported by a grant from Italy's Piedmont region—Botta was involved in planning a strategy for analyzing the decay of the Higgs boson into four leptons.
After starting her Ph.D. at Torino, she worked on the detector itself, optimizing it for the detection of muons. In 2010, when CMS started collecting real data from collisions of sufficient energy to start the search, Botta returned to the analysis strategy that she had worked on before, developing it further and using it to search for the Higgs.
Botta finished her Ph.D. in December 2011 and became a CERN postdoctoral fellow this past March. “These were the most interesting months, from March to July,” she says. CMS stopped taking data on 18 June, and on the run up to the 4 July announcement, “it went so fast and it went so well; it means that really 3000 people were each of them doing their little pieces and everything was working; this was incredible,” she says. Botta’s contribution was especially important because the four-lepton channel was one of the channels with the greatest sensitivity for spotting the Higgs. The day that all of the channels presented their final results to the whole CMS Higgs group, Botta gave the talk for the four-lepton channel team.
Being on the frontlines during the search for the Higgs boson required a great deal of work and resilience, Botta says. “If the data have a problem in the night, you have to be there and to solve it,” she says. She also had to adapt to a very competitive environment. There were about a hundred people in the CMS four-lepton group. In contrast to ATLAS, CMS was further divided into subgroups that competed to present the best analysis strategy. Even within her own subgroup, with the data completely open and no specific tasks assigned, all 30 members of the team competed to produce the best ideas and results. “It is competitive at different levels, but then at the end you have always a moment in which you feel like a group,” she says. When the announcement was made on 4 July, the Higgs group at CMS was “there all together to say to the world, ‘These were our best results,’ and to compare them with ATLAS.”
The analysis strategy that Botta started working on as a master's degree student turned out to have the highest sensitivity in the days leading up to the discovery, she says. “It’s not that you are the only one doing the major work, of course; it is a collaboration of 3000 people working on one experiment,” she says. “But still, you feel that you were there when they discovered it.”
Botta is now studying the properties of the Higgs, but she is looking forward to seeing the LHC upgraded to higher energies, which is expected to be completed in 2015. “We will move from analyzing data to again doing simulation and preparing ourselves for the new era,” she says.
Aaron Armbruster, ATLAS
CREDIT: Aaron Armbruster
Aaron Armbruster joined ATLAS in 2007, about 2 years before the LHC started generating data. He was in the final year of his bachelor's degree in physics at the University of Michigan, Ann Arbor, when he was initiated into the analysis of two possible Higgs decay routes. After graduating, he spent a year at CERN contributing to the construction of the ATLAS muon spectrometer.
In 2008, Armbruster came back to Ann Arbor to begin a Ph.D. in physics. In April 2010, just as the LHC was starting to produce high-energy collisions and real data to fuel the search for the Higgs, Armbruster moved back to CERN to pursue his research. He contributed to the analysis of the decay of the Higgs into two particles called W bosons.
In Geneva, he became a statistics expert, contributing to the statistical interpretation of the results and building a common statistics framework for analyses in the two–W boson decay channel. When scientists across all of the different channels pulled together their 2011 and 2012 results, Armbruster was put in charge of determining how statistically significant the signal was. His results showed that the chance that random backgrounds from run-of-the-mill particles would produce a spurious signal as big as the one they were seeing was about 1 in 3 million.
Armbruster says that the size of the collaboration presented some problems for the younger scientists. “The biggest challenges are really when you’re first starting out, because there are so many people and actually it’s hard to find your place in such a huge collaboration.” Young scientists, he says, can go unnoticed. But things got easier as time went on, with “a move from everybody trying to compete to do the same thing to everyone having specialized tasks,” he says.
As for the project itself, it has been “a roller coaster,” Armbruster says. “The ups usually happened when you saw something exciting in the data, something that could potentially be a signal” for the Higgs, he says. But, several times, promising signals disappeared as scientists did the crosschecks or the LHC produced more collisions. “If you collect more and more data you can see more clearly whether you have a signal or if this is just a statistical fluctuation above the background.”
In December 2011, CERN announced that both ATLAS and CMS had seen “[t]antalising hints” of a Higgs signal. But it wasn’t until late June—just a couple of weeks before the 2012 announcement—that one more data set convinced everyone at ATLAS that the signal was real, Armbruster says. “It was very exciting to see that, as then you know that it’s actually something physical in nature that you’re observing and not just some ghost that you’re chasing,” he says. But for him, the most rewarding time was when ATLAS and CMS shared their results with each other on the same day that they announced those results to the rest of the world. Armbruster continues,“We both saw basically the exact same thing. This is a huge, huge confirmation … that we actually did a good job and didn’t make any mistakes.”
Armbruster will finish his Ph.D. within a month. He plans to continue working on ATLAS as a postdoc, but he would like to explore new topics such as supersymmetry and dark matter. After the LHC upgrade, higher-energy collisions should greatly improve the study of the properties of the Higgs boson and may even allow researchers to “create heavier particles that might be just around the corner, that we just barely can’t create right now.”
What’s most fascinating is “the fact that we can make such statements that we do about the nature of the universe … at such small sizes and such high energies,” Armbruster says. “It’s very exciting to be part of this.”