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My story of the Higgs discovery

30 June 2022

Particle physicist Sau Lan Wu chronicles the weeks leading up to the historic 4 July 2012 announcement at CERN.

Higgs anniversary illustration.
Credit: Abigail Malate for Physics Today

The discovery of the Higgs boson, announced on 4 July 2012 at CERN, was the result of more than two decades of work by the ingenious Large Hadron Collider (LHC) machine physicists and by many thousands of ATLAS and CMS experimental physicists. Those physicists built and now operate the detectors, designed and now manage a computer system that distributes data around the world, created novel hardware and software to identify the most interesting collisions, and wrote the algorithms that dig out the most pertinent events from the great amounts of data being recorded. They all worked feverishly, contributing greatly to the Higgs discovery.

What follows is the story of my own personal experience of the discovery. Other research groups within the ATLAS and CMS collaborations observed the same result, with the same excitement. They also have success stories to tell.

In 1974, as a postdoc in the MIT group of Samuel Ting, I took part in the discovery of the J/ψ particle, which provided the first experimental evidence of charm quarks. Three years later I was appointed an assistant professor at the University of Wisconsin–Madison and joined the TASSO collaboration at the German Electron Synchrotron (DESY) in Hamburg. In the spring of 1979, I led the effort to discover the gluon through three-jet events. This was the observation of the first Yang–Mills non-abelian gauge boson.

Interestingly, there were only 14 authors for the 1974 publication of the J particle discovery and 88 authors for the TASSO group’s 1979 gluon discovery. In contrast, there were 3172 authors in the ATLAS collaboration’s 2012 Higgs discovery paper. The working dynamics, the decision-making process, and the credit allocation were grossly different and proportionally more complicated and difficult.

In 1985 my Wisconsin experimental group and I made the important decision to move from DESY to CERN in Geneva, where I was already working with the ALEPH experiment using the Large Electron–Positron Collider. The move would allow us to concentrate on the experimental search for the Higgs particle, which was theoretically proposed but not yet observed. I must admit, I did not realize at the time that it would take another 27 years to find this Higgs particle. In 1993 my group became the first one from the US to join the ATLAS collaboration at the LHC.

The joy of discovery

Let me begin my Higgs discovery story by quoting from “Chasing the Higgs,” an article by science writer Dennis Overbye in the 5 March 2013 edition of the New York Times:

“The job of ultimately confirming the boson’s discovery had been entrusted to another pair of graduate students, Haoshuang Ji, a Wisconsin student, and Aaron Armbruster of the University of Michigan—who had sent the plot that Dr. Gross had woken up to in November. They were each working to combine all the Higgs data from all the myriad ways it could fall apart and leave a trace in the detectors. This calculation would make or break the Higgs, because the boson had to behave properly in all its guises.

On the afternoon of June 25, Mr. Ji announced he had gotten a result of 5.08 sigma, causing cheers to go ringing down the corridor outside Dr. Wu’s office; everybody ran to sign the printout. The next day, Mr. Armbruster arrived at the same result.

Atlas was at 5-sigma.”

This date of 25 June 2012 was indeed an exciting one. At 3:00pm that day, there was a commotion in the Wisconsin corridor on the ground floor of CERN Building 32. My graduate student Haichen Wang was saying loudly, “Haoshuang is going to announce the Higgs discovery!” Our first reaction was that it was a joke; thus when we entered Haoshuang’s office, we all had smiles on our faces. Those smiles suddenly became much bigger when we got to look at the result of Haoshuang’s combination: It showed the 5.08σ close to the Higgs mass of 125 GeV/c2. Pretty soon, cheers were ringing down the Wisconsin corridor.

ATLAS had a discovery!

ATLAS and CMS results presented on 4 July 2012.
Figure 1. On 4 July 2012 the ATLAS (left) and CMS (right) collaborations presented preliminary results on the background fluctuation probability p0 in the Higgs search using the full 2011 and partial 2012 data sets. Credit: ATLAS and CMS collaborations

Somebody said, “Oh my god, this is real!” Haichen videotaped the excitement. We made a large copy of the Higgs signal plot, and all my group members signed it. This signed document is now displayed on the wall of the Wisconsin corridor at CERN.

Amid the cheering, science historian Michael Riordan was visiting me and my Wisconsin group. (See the accompanying article by Riordan.) He was napping in the office of my postdoc Luis Flores Castillo in the Wisconsin corridor at CERN and was awakened by the shouting. He wrote later to me, “I’ll never forget awakening in your CERN offices from my jetlag-induced stupor to shouts of glee in the next room!”

A great day for physics

Rolf Heuer, then the director general of CERN, was constantly being informed separately by the ATLAS and CMS collaborations of their progress. The planning of the announcement came together within a week. The original idea was to make the discovery of the Higgs particle public in Melbourne, Australia, on 4 July, the first day of the 2012 International Conference on High Energy Physics (ICHEP). However, since the experiments were sited at CERN, the decision was made to announce the result from Geneva instead. Thus a special symposium was arranged in the CERN auditorium on the morning of 4 July, US Independence Day.

On the day of the announcement, the auditorium at CERN was locked until 9:00am. Physicists and physics students slept just outside the auditorium the night before to ensure themselves a seat inside. To encourage all the students and postdocs of my group to witness the scientific event of the century, I promised a reward of $100 to anyone who would line up outside the auditorium overnight. They all got in. One of my graduate students held a spot for me and escorted me to the auditorium. The doorman recognized me, and we both got in. I ended up in a seat not far from Peter Higgs.

Celebrating the discovery.
Figure 2. Scenes of jubilation in the CERN auditorium on 4 July 2012. Standing, from left to right, are Christopher Llewellyn Smith (CERN director general, 1994–98), Lyn Evans (leader of the LHC project), Herwig Schopper (CERN director general, 1981–88), Luciano Maiani (CERN director general, 1999–2003), and Robert Aymar (CERN director general, 2004–8). Credit: CERN

Higgs, François Englert, Gerald Guralnik, and Carl Hagen, whose theoretical work had led to the discovery, walked into the auditorium to a standing ovation. Another important contributor to the theory, Robert Brout, had unfortunately passed away.

Figure 1 shows the preliminary plots that the ATLAS and CMS collaborations presented at the CERN special symposium. The same figures were shown at the ICHEP conference in Melbourne. Both ATLAS and CMS showed their final significance, obtained by combining all their decay channels, with ATLAS obtaining 5σ and CMS 4.9σ.

Right after the ATLAS and CMS presentations, Heuer declared, “I think we have it. . . . We have now found the missing cornerstone of particle physics. We have a discovery. We have observed a new particle that is consistent with a Higgs boson.”

Sau Lan Wu meets Peter Higgs.
Figure 3. The author (center) met Peter Higgs (foreground) at the end of the 4 July 2012 CERN seminar. Credit: Jana Schaarschmidt

A scene in the auditorium is shown in figure 2. At the end of the symposium, I went to shake hands with Higgs (figure 3). “I have been looking for you for over 20 years,” I told him. In fact, it had taken me 32 years, from 1980 to 2012. And I will always cherish his reply: “Now you have found me.”

Later the ATLAS Higgs working group had a celebratory drink (figure 4). Everyone was watching the corks of the champagne bottles flying up to the ceiling.

The Higgs at last

On 31 July 2012, both the ATLAS collaboration and the CMS collaboration updated their preliminary analyses and submitted their first publications on the discovery of the Higgs particle to Physics Letters B. Both the published result of the ATLAS collaboration of 5.9σ and the published result of the CMS collaboration of 5.0σ can be seen in figure 5.

The ATLAS team celebrates.
Figure 4. Following the CERN seminar, members of the ATLAS Higgs working group had a celebratory drink. The physicists watched the corks of champagne bottles fly to the ceiling. Credit: Natalia Panikashvili

The Higgs discovery caught the imagination of the press around the world. Peter Higgs was prominently featured on the front page of the 5 March 2013 New York Times together with four ATLAS and CMS physicists. I was honored to be one of them. Later that year Englert and Higgs were jointly awarded the Nobel Prize in Physics.

With Riordan and former CMS spokesperson Guido Tonelli, I authored the article “The Higgs at last,” which was published both in the October 2012 issue of Scientific American and in a special edition of the magazine the following year. “After decades in the doldrums, the discipline is energized once again by the heady intercourse of theory and experiment,” we wrote. “Questions abound that may find answers from further research on this fascinating particle or its potential partners.”

First published results of ATLAS and CMS.
Figure 5. Final results of the background fluctuation probability p0 in the Higgs search using the full 2011 and partial 2012 data sets collected by ATLAS (left) and CMS (right). The results were obtained by combining all available standard-model decay channels. The Gaussian significances corresponding to the peak in p0 is 5.9σ for ATLAS and 5.0σ for CMS. Credit: ATLAS collaboration, Phys. Lett. B 716, 1 (2012); CMS collaboration, Phys. Lett. B 716, 30 (2012)

As is typical of any major finding, the experimental discovery of the Higgs particle will have important consequences for our understanding of fundamental physics. (See the accompanying article by Haider Abidi, Heather Gray, and Martina Ojeda.) All of us have our own ideas of what the most important consequences may be, and we are now diligently pursuing our own ideas.

One big question that immediately comes to my mind is the mass problem. The breakthrough generated by the Higgs discovery was that elementary particles acquire their masses through the Higgs particle. A deeper question that needs to be answered is how to explain the values of the individual masses of the elementary particles. In my mind, this mass problem remains a big topic to be explored in the years to come.

Sau Lan Wu is the Enrico Fermi Distinguished Professor of Physics at the University of Wisconsin–Madison and a member of the ATLAS collaboration at CERN.

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