When researchers operating the Large Hadron Collider, or LHC, outside Geneva discovered the Higgs boson, it ended a decades-long effort to fill the final gap in physicists’ catalog of matter’s particles and forces. Now comes the hard part. Physicists know that their catalog is only a start; there’s far more to the universe that needs to be explained. But the LHC’s first round of results offered no clues, no roadmap for delving beyond.
“We don’t know how to link what we know to what we don’t,” says Tara Shears, an experimental physicist at the University of Liverpool in England.
As an upgraded LHC begins collecting data from high-speed proton collisions on June 3 after a two-plus-year hiatus, physicists are anxiously wondering whether the machine’s second act will lead to discoveries of new particles and forces that add pages to the catalog. “I’m cautiously optimistic,” says Nathaniel Craig, a theoretical physicist at the University of California, Santa Barbara. “There’s a significant likelihood of something happening.”
Researchers say hints of new physics could come in the form of strange behavior of particles, a slightly peculiar Higgs or mysteriously disappearing energy. Those developments could show up soon after the machine restarts, or they may require a decade or more of painstaking analysis.
In the 1970s, physicists devised the standard model of particle physics, which described a set of puzzle pieces — particles and forces — and showed how they could fit together to form a coherent picture that describes nature. By the time the LHC started its first experimental run in the spring of 2010, physicists had discovered all the puzzle pieces except one: the Higgs boson, which was needed to explain how some other particles acquire mass. Within a little more than two years, LHC physicists found the Higgs and completed the standard model puzzle (SN: 7/28/12, p. 5).
The standard model beautifully describes ordinary matter, but it has no explanation for the vast majority of the universe’s composition, which includes mysterious stuff like the dark matter that dominates the mass of galaxies. Any strange finding at the LHC, such as an unexpected property of the Higgs or an oddly behaving set of particles, could have provided clues to particles and forces beyond the standard model. Yet the Higgs discovery was arguably the only highlight of the LHC’s inaugural three-year run, disappointing physicists who expected more surprising discoveries. “The fact that the Higgs was the only exciting thing is a little disturbing,” says Boston University theoretical physicist Kenneth Lane.
The lack of non-Higgs developments after more than a million billion proton collisions may seem to portend the same outcome once the LHC returns to action. Yet the revamped LHC is not a retread of the old one. During a May 20 test to prepare for the restart, LHC physicists collided protons at a record-setting energy of 13 trillion electron volts, or 13 TeV, nearly doubling the energy of the machine’s first collisions in 2010. Higher energy translates into an improved ability to produce and detect more massive particles. For example, the gluino, a proposed particle beyond the standard model that could weigh in at between 1 and 2 TeV, should stick out like a sore thumb if it exists.
In addition to the energy upgrade, protons will also collide about a billion times a second, versus about 600 million collisions per second five years ago. The increased collision rate will allow experimentalists to more rapidly separate the signatures of new particles from the inevitable noise that comes when tracking a maelstrom of subatomic shrapnel.
Thanks to the dual improvements, over the next three years the LHC should produce about 10 times as many Higgs bosons as the roughly 500,000 churned out by the first-generation machine (only a fraction of those Higgs were actually detected). The boosted efficiency will enable physicists to meticulously probe every property of the particle and compare the measurements with standard model predictions. A discrepancy in, say, the rate at which the Higgs decays into photons would suggest that a particle or force new to physics is interfering with the process.
The Higgs could reveal new physics in other ways, too. Perhaps Higgs bosons are produced by yet-to-be-discovered heavier particles, says Matt Strassler, a Harvard University theoretical physicist. Or the Higgs might decay into lighter particles never seen before. Based on the limited number of Higgs bosons observed so far, Strassler says, “as many as 1 in 10 decays could be to something the standard model does not predict.”
Physicists aren’t just praying for hints of new physics, Strassler stresses. He says there is very good reason to believe that the LHC should find new particles. For one, the mass of the Higgs boson, about 125.09 billion electron volts, seems precariously low if the census of particles is truly complete. Various calculations based on theory dictate that the Higgs mass should be comparable to a figure called the Planck mass, which is about 17 orders of magnitude higher than the boson’s measured heft (SN Online: 10/22/13).
As a remedy to this problem, many physicists have proposed new particles that essentially cancel out the influences that would otherwise cause the Higgs’ mass to skyrocket. Those theories come in many forms and go by many names, including twin Higgs and supersymmetry (SN Online: 10/17/13). Despite the plethora of options, all these proposals hinge on the existence of particles with masses not much higher than that of the Higgs — a mass range that the supercharged LHC is optimized to explore.
Other physicists have high hopes that new physics is hiding in seemingly minor blips in measurements from the LHC’s inaugural run. Lane and other theorists are eyeing decays of particles called B mesons. Some decays produce, among other things, electrons and positrons; other decays produce muons, which are about 200 times as massive as electrons but otherwise identical, and antimuons. According to the standard model, the B decays should produce muons just as often as electrons. Yet Lane says that multiple measurements suggest that only three muon/antimuon pairs are produced for every four pairs of electrons and positrons.
No single measurement meets physicists’ strict statistical criteria to qualify as a discovery, Lane warns. But he says it’s worth pursuing whether a new particle or force interacts preferentially with muons over electrons. Along with Nobel laureate Sheldon Glashow and another colleague, Lane also proposed that a B meson could decay into combinations such as one electron and one antimuon that are banned by the standard model. “That would be exciting as hell,” Lane says.
Finally, the LHC could present a pivotal piece for the puzzle of dark matter. Physicists know dark matter exists because of its gravitational influence on galaxies (SN: 5/16/15, p. 10), but nobody knows what this mysterious matter is made of because it rarely, if ever, interacts with ordinary matter. Some theories of supersymmetry predict that the LHC should produce dark matter particles. While no detector would sense those particles, they could leave their mark if energy seems to mysteriously disappear following a collision. Strassler says that dark matter or related particles could decay into detectable matter. It’s probably a long shot, but such a detection would offer that invaluable, elusive link between what physicists know and what they don’t.