Particle Physics

23
Dec

Did the LHC Really Discover a New Particle?

In today’s world of constant connectivity, rumors abound, even in science. Over the last few days, you may have seen in your social media feeds that scientists might have discovered a new particle that might be a sibling of the Higgs boson. The Higgs boson was supposed to be an only child. If another one is found, this will force us to rewrite our theories and give us a better idea of the rules that govern our universe. But note that I said “if.” Perhaps it’s time to pause for a moment and take a hard look at what the scientists involved in the measurement did and didn’t say.

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A 13 TeV proton collision recorded by CMS. The two green lines show two photons generated by the collision. (Image: CERN/CMS)

The Large Hadron Collider (LHC) is the scientific wonder of our times. Seventeen miles around, over twenty years in the making and costing around ten billion dollars, the LHC accelerates beams of protons to almost the speed of light and collides them. These collisions generate temperatures that last prevailed when the universe was only a tenth of a trillionth of a second old and probe distances as tiny as 5×10-20 meters: that’s as small compared to you as you are compared to the thickness of the entire Milky Way galaxy.

So, it is not surprising that the scientific world was waiting with barely concealed excitement for the seminar at CERN, the LHC’s host laboratory, announcing measurements generated during the accelerator’s 2015 run. The announcements this year were especially anticipated. After running from 2010 to 2012 at a collision energy of seven or eight trillion electron volts, the LHC shut down for a couple of years for retrofits, refurbishments and upgrades. In 2015, the accelerator resumed operations at a much higher energy: thirteen trillion electron volts, over 60% higher than when it generated the data that revealed the Higgs boson. And, because in particle physics more energy means more discovery potential, this year’s data might have held something unexpected.

On December 15, 2015, the new LHC physics results were unveiled to the scientific community. Keep in mind that the LHC is simply the accelerator and that the measurements are made by detectors. While there are four detectors arrayed around the LHC, two of them are large, multi-purpose, detectors, designed to be able to study anything nature throws at us. These two detectors are called ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid). The two detectors were built to study the same phenomena and so therefore have similar capabilities. However, the designs were different, and this is a good thing, as the measurements made by the two detectors can cross-check one another.

The data presented in the seminar represent a dazzling success. Between the two experiments, approximately sixty new physics results were announced. Most of the results were in good agreement with theoretical predictions, but those weren’t the ones that scientists were interested in. What the world wanted to know were about the discrepancies.

One thing stood out: When scientists studied the characteristics of events in which two highly energetic photons were made, there seemed to be too many of them at an energy in the range of 700-750 billion electron volts, just shy of five times heavier than the Higgs boson. So this is when the story gets interesting. One of the noteworthy ways in which the Higgs boson was observed was via its decay into two photons, so this slight excess could be the first indications of a heavier Higgs.

But anyone who has looked at real data knows that it doesn’t perfectly follow theoretical predictions. There are little statistical fluctuations, with the data sometimes being a little above the predictions and sometimes lower. It takes real expertise and good statistical techniques to determine whether an excess is the signature of something unexpected or just a fluke.

One way to test this is to verify that both experiments saw an excess at the same position, and both did. So that’s a reason to be more interested. However, when CMS separated its data into two categories, distinguished by where the photons hit the detector, the two data sets didn’t agree perfectly. That was a down vote.

There were other discrepancies. The ATLAS excess covered a broader mass range than the CMS one. And neither experiment’s measurement was, statistically speaking, very significant. The experimental physics community rated these small observed excesses as something to keep an eye on, but certainly nothing to get too excited about. Until more data is recorded, it is impossible to know if these measurements are really the first glimpses at a paradigm-shifting measurement.

Still, the theoretical community must ask, “Well, what if these excesses are real? What could they be?” By the very next day after the announcement, at least ten papers had been submitted to the arXiv server, interpreting the new results. (The arXiv server is an online repository of physics papers. It is not refereed and therefore the papers should be viewed with caution.)

What is the bottom line? It is really hard to say. The excesses could indeed be the signature of new physics and that would be incredibly exciting. Or they could be statistical will-o’-the-wisp, destined to disappear when more data is recorded. As a cautionary note, it is important to remember that in the data taken between 2010 and 2012, both ATLAS and CMS saw a bump in the mass spectrum of events in which two bosons were made. This excess was at a mass of about two trillion electron volts. However, in the data of 2015, there is no hint of a bump. Thus that particular example of insider buzz has quickly faded, replaced by the new “diphoton” bump.

Now, the LHC will shut down for the next few months before resuming operations, probably in the middle of April. The two experiments will then record more data, hoping to see something new. Only time will tell if the new bump is a discovery or will join the many earlier statistical flukes that have come and gone. Personally, I’m not taking any bets; but you can rest assured that I’m really looking forward to the next few months.

Go Deeper
Editor’s picks for further reading

Nature News: LHC sees hint of boson heavier than Higgs
Davide Castelvecchi reports for Nature on the “intriguing bump” in the LHC’s second-run data set, and why it should be interpreted with caution.

The Nature of Reality: Thanks, Mom! Finding the Quantum of Ubiquitous Resistance
Physicist Frank Wilczek explains the decay modes by which the Higgs boson was discovered.

Resonaances: A new boson at 750 GeV?
In this blog, physicist Adam Falkowski breaks down the ATLAS and CMS data that hint at the presence of a new boson.

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dlincoln

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.