Particle Physics


Higgs Fireworks on July 4

This week, we’ve come one step closer to understanding the rules that govern the universe. Two days ago, my colleagues at Fermilab announced our final results in a search for the answer to a mystery nearly 50 years old. In an intellectual tour de force, the CDF and my own DZero experiments analyzed a decade of data, combining dozens of hints that together tell an interesting tale. This announcement was an aperitif for an even more dramatic statement made today.

CMS detector

The construction of the CMS detector at the LHC. CMS is one of the detectors involved in the hunt for the Higgs. Credit: Mark Thiessen/National Geographic Society/Corbis

As physicists gathered in Melbourne, Australia, for the International Conference on High Energy Physics, one of the most anticipated conferences of the year, the two large collaborations at CERN made an extraordinary announcement. In back-to-back seminars held at CERN and simulcast to the conference (and the world), the leaders of two different experiments, CMS and ATLAS, gave strong evidence that we found something that can’t be explained by well-understood physics—something which could (and it’s worth emphasizing the “could”) be the Higgs boson.

The Higgs boson is the missing piece in the current best model of the universe, the Standard Model. In the Standard Model, building blocks called quarks and leptons are held together by the four known forces: gravity, electromagnetism and the strong and weak nuclear forces. Using these basic ideas, physicists can explain most of the measurements we have made. But one thing we have not been able to explain is one of the most fundamental and vexing questions in physics: Why do those building blocks have mass?

In 1964, Peter Higgs took some ideas that were floating around at the time, added a few of his own, and proposed a solution to this conundrum, which included a new particle that we now call the Higgs boson. The search for the Higgs boson is an energetic activity, directly involving as many as six thousand physicists—myself included—and the most powerful particle collider on Earth, the Large Hadron Collider (LHC) at CERN.

One of the fantastic benefits of being a physicist doing research at CERN and Fermilab is that I have been privileged to see this discovery evolve with an insider’s perspective in more than one world-class experiment and in collaboration with some of the finest minds on the planet. Over the past few years, we have searched through the data at both laboratories. Our measurements so far have shown where the Higgs boson isn’t. The results released today may finally show where it is.

The first tantalizing suggestions of the Higgs came in December of 2011, when scientists working with CMS and ATLAS announced that their data contained hints that the Higgs boson might be starting to show its face, and that it could have a mass about 125 times heavier than a proton. However, neither experiment had enough data to claim a discovery—or even to be certain that they were seeing anything at all.

In March, the search picked up again. This time, though, the LHC’s energy level and beam intensity were dialed up. If the LHC had been making Higgs bosons before, it would be making even more of them now—about 25% more, depending on the boson’s mass. The CERN management made their plans for 2012 so that both CMS and ATLAS would have enough “beam time” to independently discover the Higgs boson—if, that is, our hypotheses about its mass and other properties were correct. However, given the intellect and work ethic of the scientists involved, nobody really thought it would take the whole year to see a signal that “looked like” a Higgs boson, although proving anything we found was the actual Higgs boson predicted by the Standard Model could well take the entire years’ worth of data.

By June of this year, both LHC experiments had already recorded as much data in 2012 as in all of 2011. The accelerator and its detectors were performing superbly. Now the race was on to be the first to finish the job and find—or rule out—the Higgs boson.

ATLAS and CMS won’t find the Higgs itself, though; it disappears too quickly, decaying into other subatomic particles. It’s those particles that we’re looking for in the ATLAS and CMS data. Depending on the true mass of the Higgs boson, it could decay in several different ways. Seeing an excess of these decay products is an indication that we might have discovered the Higgs.

And that’s what we found! In the shrapnel of the LHC’s powerful collisions, the CMS experiment detected more pairs of photons and Z bosons than we can explain without some new kind of physics appearing. CMS also looked for supporting evidence in predicted decays to bottom quarks, W bosons and tau leptons. The ATLAS experiment also found an excess of events decaying into two photons and two Z bosons, but the ATLAS did not announce the results of their investigations into other decay modes.

To be certain that we didn’t adjust our analysis techniques to produce a preconceived result, we did the searches “blind,” meaning that we designed the analysis before we looked at the relevant data. This was especially important given that we saw hints in December 2011. We didn’t want that information to bias our searches in any way. That way, if the 2012 data told the same story as that of 2011, it would tell us something about the universe and not ourselves.

When all of our results are combined, CMS claims to have found a new boson with a mass of 125 GeV (or about 133 times heavier than a proton) and a statistical significance of about five sigma (which means that this result could happen 1 time in 3.5 million by accident), while ATLAS’ measurement indicates the existence of a particle with about the same mass (126 GeV) and the same statistical significance. While both experiments’ results are significant individually, the fact that both experiments are announcing similar observations and the 2011 and 2012 measurements are compatible lends tremendous credence to today’s announcement.

It is very important to stress that neither experiment team has claimed to have observed the Higgs boson. They have observed something without a doubt, but the Standard Model Higgs boson is a very specific thing. To be sure we’re seeing the Higgs boson and not a lookalike, we need to see it in all of the predicted decay modes.

For instance, the Higgs theory makes specific predictions about the relative probabilities of the Higgs decaying into pairs of bottom quarks, tau leptons and a whole myriad of possibilities. If all of the predicted possibilities aren’t seen, or aren’t seen in the right ratio, it might be that what we’re observing isn’t the Higgs boson after all. Furthermore, the Higgs boson is predicted to have exactly zero quantum mechanical spin. Until those and other properties are confirmed, it is possible that the experiments might be picking up traces of something entirely different. So, although what has been observed is consistent with being a Higgs boson, these measurements cannot rule out some other possibilities. In fact, this announcement is not the end of the story but rather the very beginning.

Tell us what you think on Twitter, Facebook, or email.

Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Yatin Dhareshwar

      Brilliant post – brilliant because it is SIMPLE and lucid.

      Also, since you are part of the team, Congratulations to you and the rest of the team!

    • Vivian Nelson Melle

      What a wonderful post. Thank you for posting and congratulations. An amazing beginning, for sure.

      • That’s exactly right….”an amazing beginning.”

    • Wasn’t the current Higgs discovered at much lower energies 125? Instead of >150? aren’t FermiLab Tevatron folks looking at their earilier data results for Higgs?

      • The Tevatron folks >>did<< look at their data. (I happen to be on a Tevatron experiment in addition to being on the LHC.) However, the extra collision energy at the LHC increases the probability that the Higgs boson will be created at the LHC.

        In fact, on July 2, the Tevatron experiments announced our quasi-final results regarding searches for the Higgs boson. The results, while suggestive, fell short of the quality of the measurements announced by the LHC.

        • Andrew

          I was wondering about this. Having recently read your book Understanding the Universe, I figured that at 125 GeV, the Tevatron should have picked picked it up. Thanks for clearing this up.

    • Zaheer Zaheeruddin

      The discovery will raise serious questions, like the one faced by Gallileo. WPBS in its usual manner does the community a tutorial service next to none. Thanks

    • Jean Fox

      i am no physicist but i really like how they made this article easy to understand, nice job. by the way, i have been closely following this one and i am excited when they finally announce that the higgs is indeed real.

    • Botsynis

      So cool. How is it going to affect our daily lives?

      • It won’t. It’s basic and fundamental research. It adds to our understanding of the universe and helps us answer better questions that have plagued the curious for 2,500 years (at least). There have been many technical things developed to help scientists study this kind of physics, for instance improvements in worldwide Grid computing, the WWW and magnetism. These are not central to the physics being studied….just technical advances that have helped advance the physics and happen to have considerable independent commercial value.

    • Botsynis

      Can further experiments create a big bang leading to creation of another Universe (and destroying this one)?

      • No. We can be confident of this because our planet is constantly bombarded by cosmic rays which involve collisions at much higher energy than the LHC can create. We’re still here, ergo it is totally safe.

      • Anonymous

        Science Rule #1: Don’t believe anything a journalist says.

    • Ignace Saenen

      Small amendment for an otherwise very clear article:

      In 1964, Peter Higgs took some ideas that were floating around at the time, added a few of his own, and proposed a solution to this conundrum, which included a new particle that we now call the Higgs boson. ”

      What actually happened is that 3 teams described the theory and posted them to scientific magazines in the same year barely 2 months apart. Higgs’ name stuck because the most visible journalist on that field at the time did not know of earlier work that was published a few months earlier. Higgs developed the model in an autonomous way, but because of similarities in his findings, he added a reference to the paper by Englert and Brout after he was finished, before publishing his work. A 3rd team again published the same theory, oblivious of the earlier publications.

      So it wasn’t just “ideas floating around”, the theory was already there and later acknowledged by the scientific community.

      • There is a lot of history here. The mathematics predates any of these guys and originates in some superconductivity theory. Guralnek, Brout, etc. certainly did some work on this. This is all described in Ian Sample’s “Massive,” which is an excellent history of the history of what we call the Higgs mechanism.

        We call it the Higgs boson mostly because Higgs predicted a boson as a consequence of the theory. In fact, his original paper was rejected until he added that.

        I remain happy with my word selection, even if it doesn’t tell the entire, nuanced, tale.

        • Jayxjayx

          fantastic all of it
          what an amazing leap of human learning and to think that there was global cooperation for all of this should give us all hope for our combined future as a species!

      • Anonymous

        Stigler’s Law strikes again.

    • Very easy way to explained the topic. Thanks!

      Bella Rossi

    • ProfessorNow

      Reality is founded on Paradox
      Life is riddled with Paradox
      Mind is seized by Paradox
      Right justified is a Paradox

    • Seiji Yawata

      Very specific and understandable. The higgs boson is an step closer to understanding the nature of particles and their differing masses