Particle Physics


It’s Back! The LHC Prepares for Round Two

Beneath the ground, just west of Geneva, Switzerland, a leviathan is stirring, slowly rousing from a two-year-long nap. The surrounding area has hosted giants before; the nearby Jura Mountains lend their name to the Jurassic period. However, this colossus isn’t the familiar Diplodocus or Stegosaurus, but rather a wonder of technology. The Large Hadron Collider is back.

A worker helps prepare the LHC for its next phase of operations. Image credit: Anna Pantelia/CERN. Copyright CERN

The LHC was a quarter-century in making when it began operations in early 2010. From 2010 to 2012, the world’s largest particle accelerator smashed together beams of protons traveling nearly at the speed of light. The temperatures in those collisions were a hundred thousand times hotter than the center of the sun and ten times hotter than the center of a raging supernova; the last time the universe was that hot was just a tenth of a trillionth of a second after the Big Bang. This first working phase of the LHC experiment led to the discovery of the Higgs boson, the final missing piece of the Standard Model of particle physics.

At the end of 2012, the LHC was ready for a well-deserved rest and was shut down for refurbishments, upgrades, and retrofits. That process is now complete, and this spring, the LHC will power up with substantially enhanced capabilities. The beam energy has been raised from eight to 13 trillion electron volts and the collision rate is expected to nearly triple. The increase in collision energy will raise the temperature inside the collisions by 60%. All of the detectors surrounding the ring have had their capabilities enhanced, with faster electronics and more modern detector technologies. The four experiments are ready to receive the onslaught of data that will soon arrive. With the increased beam intensity and anticipated multi-year running period, the data rate will triple and the total recorded data will be more than ten times what the instruments had gathered previously.

The new LHC won’t just do more of what it did before. In accelerators, energy means discovery: Higher-energy collisions make more, and potentially more exotic, particles. For instance, depending on exactly how the Higgs boson is produced, the LHC’s energy boost will make three to five times as many Higgs bosons per second from beams of the same intensity. However, given that the beam intensity is expected to triple, we should see between nine and 15 times as many Higgs bosons as we did back in 2012. In fact, while Higgs bosons haven’t quite yet become pedestrian, they will soon turn into either a pesky background that must be understood and suppressed so that we can study even rarer physical phenomena, or we will turn to events in which the Higgs boson is produced in conjunction with another particle so that we can investigate more mysterious corners of the Standard Model.

However, before that happens, scientists will first “rediscover” the Higgs boson in the new accelerator and detector environment and then make precision measurements of its properties. This is extremely important, as the Higgs boson and the associated Higgs field offer a promising window into new physics beyond the standard model. Indeed, one of the most distressing outcomes of the first measurements from the LHC is just how well they agree with the predictions of the Standard Model. This is comforting, but it doesn’t point us in the right direction—or any direction at all, really—to deepen our understanding of the universe. If the data agrees with your model, you’re kind of stuck. When you finally discover a measurement that disagrees with your prediction, you have a crucial thread at which to tug. This could lead to the entire sweater of the Standard Model being unraveled before we knit a new model that does a better job of describing the data.

The Higgs discovery raises one glaring question that completely vexes scientists. The mass of the Higgs boson is much smaller than seems “natural” from the theory. In the most naïve reading of the Standard Model, the mass of the Higgs boson should be in the vicinity of the Planck energy (~1019 GeV), rather than the 125 GeV that was measured. There are many proposed explanations of this mystery, ranging from supersymmetry to extra spatial dimensions to a Higgs boson consisting of even smaller objects. Hopefully the data taken over the next few years will shed light on and perhaps answer that nagging conundrum.

While it’s hard to make predictions, especially about the future, we hope that the first circulating beams will occur in early March, with first collisions occurring in May and the full data-taking program commencing in June. While these dates might change a little due to the teething pains that accompany the commissioning of such a huge instrument, there is one thing we know: The LHC is about to plow ahead into the unknown and maybe, just maybe, reveal of mystery of the universe that causes us to rewrite the textbooks.

Go Deeper
Our picks for further reading

CERN: LHC Season 2: A stronger machine
This backgrounder from CERN’s press office details the LHC’s equipment upgrades.

Fermilab: #Hashtag3 #RestartLHC
In this video parody, two US LHC scientists give a Twitter-style preview of the LHC restart.

NOVA: Big Bang Machine
Watch NOVA’s in-depth look at the discovery of the Higgs.

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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.