For decades, particle physicists have yearned for physics beyond their tried-and-true standard model. Now, they are finding signs of something unexpected at the Large Hadron Collider (LHC), the world’s biggest atom smasher at CERN, the European particle physics laboratory near Geneva, Switzerland. The hints come not from the LHC’s two large detectors, which have yielded no new particles since they bagged the last missing piece of the standard model, the Higgs boson, in 2012, but from a smaller detector, called LHCb, that precisely measures the decays of familiar particles.
The latest signal involves deviations in the decays of particles called B mesons—weak evidence on its own. But together with other hints, it could point to new particles lying on the high-energy horizon. “This has never happened before, to observe a set of coherent deviations that could be explained in a very economical way with one single new physics contribution,” says Joaquim Matias, a theorist at the Autonomous University of Barcelona in Spain. Matias says the evidence is strong enough for a discovery claim, but others urge caution.
The LHC smashes protons together at unprecedented energy to try to blast into existence massive new particles, which its two big detectors, ATLAS and CMS, would spot. LHCb focuses on familiar particles, in particular B mesons, using an exquisitely sensitive tracking detector to sniff out the tiny explosive decays.
B mesons are made of fundamental particles called quarks. Familiar protons and neutrons are made of two flavors of quarks, up and down, bound in trios. Heavier quark flavors—charm, strange, top, and bottom—can be created, along with their antimatter counterparts, in high-energy particle collisions; they pair with antiquarks to form mesons.
Lasting only a thousandth of a nanosecond, B mesons potentially provide a window onto new physics. Thanks to quantum uncertainty, their interiors roil with particles that flit in and out of existence and can affect how they decay. Any new particles tickling the innards of B mesons—even ones too massive for the LHC to create—could cause the rates and details of those decays to deviate from predictions in the standard model. It’s an indirect method of hunting new particles with a proven track record. In the 1970s, when only the up, down, and strange quarks were known, physicists predicted the existence of the charm quark by discovering oddities in the decays of K mesons (a family of mesons all containing a strange quark bound to an antiquark).
In their latest result, reported today in a talk at CERN, LHCb physicists find that when one type of B meson decays into a K meson, its byproducts are skewed: The decay produces a muon (a cousin of the electron) and an antimuon less often than it makes an electron and a positron. In the standard model, those rates should be equal, says Guy Wilkinson, a physicist at the University of Oxford in the United Kingdom and spokesperson for the 770-member LHCb team. “This measurement is of particular interest because theoretically it’s very, very clean,” he says.
The result is just one of half a dozen faint clues LHCb physicists have found that all seem to jibe. For example, in 2013, they examined the angles at which particles emerge in such B meson decays and found that they didn’t quite agree with predictions.
What all those anomalies point to is less certain. Within the standard model, a B meson decays to a K meson only through a complicated “loop” process in which the bottom quark briefly turns into a top quark before becoming a strange quark. To do that, it has to emit and reabsorb a W boson, a “force particle” that conveys the weak force (see graphic, previous page).
The new data suggest the bottom quark might morph directly into a strange quark—a change the standard model forbids—by spitting out a new particle called a Z′ boson. That hypothetical cousin of the Z boson would be the first particle beyond the standard model and would add a new force to theory. The extra decay process would lower production of muons, explaining the anomaly. “It sort of an ad hoc construct, but it fits the data beautifully,” says Wolfgang Altmannshofer, a theorist at the University of Cincinnati in Ohio. Others have proposed that a quark–electron hybrid called a leptoquark might briefly materialize in the loop process and provide another way to explain the discrepancies.
Of course, the case for new physics could be a mirage of statistical fluctuations. Physicists with ATLAS and CMS 18 months ago reported hints of a hugely massive new particle only to see them fade away with more data. The current signs are about as strong as those were, Altmannshofer says.
The fact that physicists are using LHCb to search in the weeds for signs of something new underscores the fact that the LHC hasn’t yet lived up to its promise. “ATLAS and CMS were the detectors that were going to discover new things, and LHCb was going to be more complementary,” Matias says. “But things go as they go.”
If the Z′ or leptoquarks exist, then the LHC might have a chance to blast them into bona fide, albeit fleeting, existence, Matias says. The LHC is now revving up after its winter shutdown. Next month, the particle hunters will return to their quest.