The standard model of particle physics is based on a core set of fundamental particles. But, there are hints that we may have found a new one that no one was really looking for in experiments that collide at high speeds short lived exotic particles that have already exhibited strange properties when they decay.
Standard Model Particle Background
Quarks, which are best known for making protons and neutrons, come in two basic types; these in turn come in "standard weight" and two heavier and short lived versions of each of the basic types. Electrons and neutrinos, in standard and two heavier and short lived version also exist. Each of these particles come in particle and anti-particle versions. These two basic types of quarks, electrons and neutrinos make up what we think of at a non-quantum level as "matter."
There are also three known kinds of "force carrier" particles for each of the fundamental forces. Photons mediate electromagnetism. Gluons mediate the "strong force" that holds atoms together. W particles (and their kin, the Z particle) mediate the weak nuclear force that causes the radioactive decay of large atoms. Gluons are never observed free standing and must be inferred. W/Z particles are heavy and short lived. Unlike "matter" particles, force carrier particles have not been observed to repeat themselves in heavier versions of themselves.
There are also two hypothesized but undiscovered particles in the standard model, the graviton to mediate gravity, and the Higgs boson to create inertia.
All currently observed particles have a spin of one-half (particles in the same category as quarks, neutrinos and electrons), or one (photons, gluons, Ws and Zs). The proposed Higgs particle would have a spin of zero; the proposed graviton would have a spin of two.
All current particles have a charge of zero (photons, gluons, neutrinos and Z particles and their anti-particles and gravitons), plus or minus one-third (down, strange and bottom quarks and their anti-particles), plus or minus two-thirds (up, charm and top quarks and their anti-particles), or plus or minus one (electron variants, W particles and their anti-particles) relative to an electron's charge. There are both charged and neutral versions of the proposed Higgs particle.
There are many types of compound particles as well. Two quark particles are called mesons. Most matter is made up of two of the many possible types of three quark particles, protons and neutrons. All compound particles other than protons and neutrons are highly unstable and quickly decay into something else.
Standard Model Symmetry Background
One of the important rules of quantum physics is that certain symmetries are observed. "Parity" is a property of particles that is conservated in all situations except weak force interactions. The first party violations were discovered in 1956.
"Charge parity" symmetry violations, which holds that if parity is not conserved in a set of particles, that electrical charge is reversed in that set of particles, moreover, is true in all but two known kinds of weak force interactions, neutral kaon decay (a type of meson), and B meson decay. The first CP symmetry violations were discovered in 1964 in netural kaons. CP violations in B mesons were discovered in the 1990s and definively confirmed in 1999.
All 140 or so of the two quark combinations (called mesons) are unstable with mean lifetimes on the order of a hundred millionth of a second or less. W and Z particles are likewise very short-lived with a mean life of about 3 × 10^−25 second. CP violations have been observed in only a handful of the 140 or so of the two quark combinations (there are four kinds of neutral kaons and a similarly small number of kinds of B mesons).
Current theories predict that CP violations would exist in strong nuclear force interactions, but they are not observed. A hypothetical particle called an axion could explain why this is the case and is also a candidate to explain the dark matter that is indirectly observed in cosmological observations.
These two rare types of CP symmetry violating weak force mediated decays still preserve "charge partity time reversal" symmetry (CPT). The combination of three properties of particles is preserved in all known circumstances.
The Experimental Results
New particle accelerator experiments have produced data that suggest that a new particle may have to be added to the roster.
The evidence involves 230 instances where a rare type of weak force particle mediated decay of a B meson were observed. The type of B meson decays studied are already of great interest to physicists because they are one of the few situations where charge parity symmetry is broken.
The data were out of line with what would have been predicted with precisely understood quantum mechanical laws and all known particles that were expected in this type of B meson decay.
The B meson has a mass of about 5.279 giga-electron volts (an electron volt is a unit of mass based upon Einstein's E=mc^2 relationship commonly used for fundamental physical particles). The experimental results point to a particle that would be somewhat heavier than a B meson. Particles that heavy are also always very short lived, lasting only a tiny fraction of a second before decaying.
Not all particle properties of the hypothetical new particle have been determined from the experimental results so far. So, we don't know the spin or charge of this hypothetical particle, at this point. If further experiments or data analysis could pin down these properties of the hypothetical new particle, we would be much closer to determining where it fits in the particle zoo.
What is it and what does it mean?
Basically, this is one of those "who ordered that?" moments. Scientists were expecting to fine tune their understanding CP violations in these experiments. They weren't expecting a new particle to be discovered, and scientists are still exercising great caution in the conclusions that they draw from these experiments until they know more.
By comparison, the heaviest weight quarks (top and bottom) have masses of 171.2 GeV and 4.2 GeV respectively, the heaviest neutrino weighs less than 15.5 MeV, the heaviest version of the electron (the Tau) weighs 1.777 GeV, the W particle weighs 80.4 GeV, and the Z particle weighs 91.2 GeV. Gluons and photons are massless.
The data seem to rule out the possibility that some sort of Higgs particle has been observed, and is not consistent with the hypothetical axion particle. There is no reason to believe that it would be a graviton. It would also be a poor fit for any other proposed dark matter particle, as it is unstable. The data suggest a particle that is too light to be a fourth generation quark or electron. And, the particle would weigh less than a W or Z particle, so they can't be heavier versions of these particles. The properties of almost all possible combinations of known fundamental particles are already well documented, and don't seem like likely fits.
Hypothetical particles that could fit the bill include a fourth generation neutrino, a preon or rare form of compound preon (preons are subcomponents of standard model fundamental particles), a supersymmetric particle of some kind, or something entirely different. The results could also be a result of some new quantum mechanical law. Then again, the results could also be a result of a subtle problem with the data analysis or application of existing quantum mechanical law, a problem with the experimental set up, or a statistical fluke.
A short lived, heavy particle that is observed only during a rare form of weak force mediated exotic B meson decay would probably be more or less devoid of any direct practical implications. The CP violations that we observe in experiments can't even adequately explain more than a small percentage of the matter-antimatter imbalance that we observe in the real world, and have no current practical implications.
But, because a new particle would expand the standard model of particle physics, this experiment could rule out theoretical all physics theories that don't predict its existence, and these experimental findings could open up new lines of theoretical physics inquiry.
The fact that this potential new particle was observed in one of the few little corners of the world of physics where CP violations are observed is probably not coincidental. These particle decays act in a weird way for some reason that we don't yet fully understand, so, it is a natural place to look for new physics. These observations could point to a new fundamental force of physics, new quantum physical law, or a more elaborate understanding of the weak force that better explains why CP violations happen.