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Two new particles made of exotic types of quarks have appeared inside the Large Hadron Collider (LHC) near Geneva, Switzerland. The particles are never-before-seen species of baryons—a category of particles that also includes the familiar protons and neutrons inside atoms. The new baryons had been long predicted to exist, but their specific characteristics, such as their mass, were unknown until they were discovered in the flesh. The new measurements serve to confirm and refine the existing theory of subatomic particles and help pave the way for a deeper theory that could include even more exotic particles.
Scientists at the collider’s Large Hadron Collider beauty (LHCb) experiment reported the discovery of the baryons, called Xib'- and Xib*- (pronounced “zi-b-prime” and “zi-b-star”), February 10 in Physical Review Letters. (They posted a preprint of their paper in November on the arXiv server.) “These were two things that very much should have existed,” says Matthew Charles of Paris 6 University Pierre and Marie Curie, a co-author of the study. “Of course, you still have to check because every now and then you get a surprise.” Both particles contain one beauty, or b, quark, one strange quark and one down quark. What differentiates these particles from one another, and from one other conglomeration of the same three types of quarks that was previously found at the LHC, is the arrangement of the quarks' spins.
Quantum spin
Spin is one of the basic quantum characteristics intrinsic to any particle, and comes in unitless, discrete amounts. All quarks have a spin of one half. When two quarks inside the same particle are spinning in the same direction, their spins add together; when they rotate in opposite directions their spins cancel out. Spins are like magnets in that like repels like, so quarks prefer to spin in opposite directions. Extra energy is needed to align two quarks to spin in the same direction. The lowest-energy configuration of a Xib particle is for the two lightest quarks (the down and strange) to be antialigned, with their spins canceling out to zero, and the heavy b quark spinning in either direction, adding another one-half spin for a total spin of one half. That ground state, called Xib*0, was found at the LHC in 2012.
The two newfound baryons are higher-energy configurations. Both have the lightest two quarks spinning in parallel, adding to a combined spin of 1. Xib'- has its b quark spinning opposite those two, giving the particle a total spin of one half (from 1 minus one-half). In Xib* the spin of all three quarks is aligned, giving it a total spin of 1 and a half. This triple alignment requires the most energy of any configuration, causing Xib* to be the heaviest of the three states.
Before the particles were discovered, physicists had estimated their masses based on a theory called quantum chromodynamics (QCD), which describes the strong force—one of the four fundamental forces of nature—that is responsible for binding quarks together. The strong force is carried by particles called gluons, so inside any particle held together by the strong force there will also be gluons. And in addition to the main quarks and gluons “virtual” pairs of quarks and antiquarks (the antimatter counterpart of quarks) continuously pop into and out of existence. This particle zoo makes calculations based on QCD incredibly difficult, to the point that mass estimates can only be accomplished using powerful supercomputers running complex simulations that aim to take all of the constituents of the particle into account. “We supposedly have a theory that tells us how these particles are supposed to behave and in principle it should open new doors. But in practice, our ability to calculate is quite limited,” says Frank Wilczek, a theoretical physicist at the Massachusetts Institute of Technology who won the Nobel Prize for helping to formulate QCD.
The new LHCb measurements agree with the best QCD predictions of the Xib masses. “This is a validation that the theoretical approach is the correct one and that we have the calculation under control,” says theorist Richard Woloshyn of the Canadian particle physics laboratory TRIUMF, who published a prediction of the Xib masses in 2009. The measurements will serve as new data points to anchor down the theory. “We need more examples to test out computational methods and explore what the different methods can teach us,” Wilczek says. “This system will help us to refine those techniques.”
Testing the standard model
So far, the newfound baryons behave according to QCD and to the larger “standard model” of physics, which describes all the known particles in the universe. Yet scientists know that the standard model cannot be the final word, because it does not account for dark matter—the invisible material that seems to dwarf normal matter in the cosmos. By making increasingly precise measurements of all the predictions of the standard model, researchers hope eventually to find cracks that lead the way to a larger theory to supersede it. “These two particles themselves are perfectly standard-model and expected,” Charles says, “but we’re hoping that we will be able to build on these in the long run to move beyond the standard model.”
The Xib particles, like all new species discovered at the LHC (including the famed Higgs boson), arose in the aftermath of collisions between speeding protons inside the accelerator’s 27-kilometer underground ring. When the protons disintegrate, their mass and energy is converted into new particles. The higher a collision's energy, the more massive newly appearing particles can be. This spring the LHC will rev up again at higher energies than ever before, following a two-year hiatus for upgrades. Those higher energies should allow more and heavier particles to arise than earlier runs saw, potentially revealing exotic particles that finally push the bounds of the standard model.