TWO teams working at the in Batavia, Illinois, have found hints of a new generation of fundamental particles – to add to the three generations we already know about. What’s so special about these new particles?
If they really do exist, they might explain a long-standing puzzle – how the universe avoided self-destruction in its earliest moments after the big bang.
First a rundown on what we know already. Each of the three known generations of matter contains two types of fundamental particle – quarks and leptons. First generation leptons include the familiar electron and neutrino (see images, right).
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The first generation of matter can explain everything we encounter in everyday life. Atomic nuclei are composed of protons and neutrons, which are in turn composed solely of “up” and “down” quarks.
The second and third generations were introduced to explain the dozens of varieties of short-lived, subatomic particles spotted in the debris of particle smashers. Each of these two generations contains a pair of quarks – much heavier than those of the first generation – as well as muons and taus, heavy versions of the electron. They also each have their own version of the neutrino.
New generations of matter have tended to show up every 30 or 40 years – the last time was in 1975, when the tau was discovered. “We’ve seen three generations, why not four?” says of Brookhaven National Laboratory in Upton, New York. A fourth generation would be “a very simple continuation of the trend we’ve seen”, he says.
Now hints of this fourth generation have turned up in data from the , which smashes together protons and antiprotons.
In March, researchers at the at the Tevatron finished combing through the collision debris created there between March 2002 and March 2009. They were looking for hints of a fourth-generation quark, which would be heavier than those in the other three generations. That would explain why it hadn’t been seen in past experiments – the heavier a particle is, the more energy is needed to forge it, and collisions in previous experiments involved too little energy to produce such a massive beast.
A heavy fourth-generation quark would unleash a lot of energy as it decayed, producing very energetic muons, among other things. The other three generations of matter also produce these decay products, and calculations suggest these three generations should account for two decay events at the highest energy measured in the experiment. But the CDF team eight – a surplus that hints at a fourth-generation quark.
“Hints of a new quark have turned up in the decay products of the Tevatron particle smasher”
The excess is small enough to be a statistical fluke, so the team is not claiming to have seen signs of a fourth generation. “Extraordinary claims require extraordinary evidence, and we definitely don’t have that,” admits of the University of California at Davis, one of the study’s authors.
No fluke
Not everyone is ready to dismiss the excess, however. of Northern Illinois University in DeKalb, who was not involved in the study, says, “It’s interesting enough that we’ll be paying attention to future analyses and hoping. It would be very exciting if there was a [fourth-generation] quark.”
Though the significance of the CDF excess is debated, fresh evidence from Tevatron’s other main detector, DZero, shows another possible hint of a fourth generation that is harder to dismiss.
A new analysis of proton-antiproton collisions in DZero found the decay products were unexpectedly skewed – slightly more muons were created than antimuons, their antimatter counterparts ().
“If it is confirmed, it’s an extremely important discovery,” says Soni. “It has very important repercussions for all of particle physics.”
This result is at odds with the of particle physics, the best theory we have so far to describe the subatomic world. The model predicts a much smaller difference between the number of matter and antimatter particles produced in collisions, about 1/40th of what DZero actually saw. A new and unknown influence seems to be at work.
Some physicists have previously pointed out that a fourth generation of particles could skew the matter-antimatter balance in the sort of process observed at DZero.
How might this happen? The weird rules of quantum mechanics permit virtual particles to briefly pop into existence, and if fourth-generation quarks were to arise this way in DZero, they could interfere with the normal sequence of events by which particles in the experiment decay. For example, pairs of quarks that include third-generation “bottoms” normally go through a series of reactions that produce muons and anti-muons. A fourth-generation quark could interfere with this process, upsetting the normal balance between matter and antimatter production and skewing the results in favour of matter.
If the anomaly at DZero is the result of fourth-generation particles, the implications would be profound. For decades, physicists have puzzled over the fact that the universe as we know it exists at all.
According to the standard model, matter and antimatter should have condensed in nearly equal amounts from the energy available in the early universe. Since matter and antimatter annihilate each other on contact, most of both “species” would have been quickly destroyed, leaving a barren sea of radiation almost completely devoid of the matter needed to make stars, galaxies and planets. Clearly that didn’t happen, so something must have boosted production rates for matter, leaving an excess to survive the orgy of annihilation and give rise to the universe.
“A fourth generation of particles could explain how matter survived to form stars and galaxies”
If fourth-generation quarks are responsible for upsetting this balance, then we would not exist without them. “To me, this is the single most important motivation for the existence of [the fourth generation],” says George Hou of the National Taiwan University in Taipei. By a mere extension from three to four generations, he adds, we may have enough asymmetry to explain how matter survived annihilation in the early universe.
Though the DZero asymmetry fits with the existence of a fourth generation, it does not prove it. It is also possible to generate matter-antimatter asymmetry in theories that attempt to explain particle physics by introducing hidden extra dimensions, as well as in supersymmetry – a theory in which each particle in the three known generations of matter, as well as those that carry forces, has a heavier partner.
Fourth-generation particles could also help explain the origin of the dark matter that seems to make up most of the universe’s mass. Key to this idea is a heavy neutrino. Like the neutrinos in all the other generations of particles, this one does not interact with the electromagnetic force, making it transparent to light and hence invisible.
While the other three known neutrinos are too lightweight to account for a significant fraction of dark matter, heavier fourth-generation neutrinos might be able to clump together and form the seeds of galaxies.
Exciting as the idea is, it is not watertight. For one, a heavy neutrino would ordinarily decay in a fraction of a second into a lighter version from another generation, so no heavy neutrinos from the early universe should have survived to form the dark matter we think exists today. Physicists would have to come up with a way to explain how a heavy neutrino stayed stable for billions of years since the big bang.
Luckily the Large Hadron Collider at CERN should be able to clarify things. It is now colliding particles with a combined energy of 7 teraelectronvolts, dwarfing the Tevatron’s 2 TeV collisions. Given the extra power, it should not take long for the LHC to spot a fourth-generation quark with a mass of around 450 GeV. “The LHC is going to be able to definitively test this,” says Martin.
An important milestone along the way will be spotting the heaviest particle that is already known – a third-generation quark called the top, which has a mass of 170 GeV. The LHC looks on track to spot the top within a few months, Conway says, and then it should not take much longer to see if the CDF excess is more than a fluke.
A positive discovery would be a win for the fourth-generation theory, says Hou. “It would truly turn the world upside down.”
