IT’S an odd thought that the banana on your kitchen counter, squished in your lunch bag or tucked away in your desk drawer is the embodiment of one of the universe’s great mysteries, just waiting to be unpeeled.
Whatever its state of ripeness, that banana is made of particles of matter, just like you: its intrinsic matteryness is why you can see, feel and taste it. What you don’t see is what a banana does 15 times a day or so. Blip! It produces a particle of something else, something that vanishes almost instantaneously in a flash of light.
That something else is antimatter.
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The prediction, and subsequent discovery, of antimatter counts as a great triumph of physics. It represents a whole mirror world of particles, identical in mass to those of normal matter, but with opposite electrical charge. But it seems rather an afterthought. In our neck of the woods, antimatter particles are only produced during interactions of high-energy cosmic rays in the atmosphere, or in radioactive decays – such as those from the tiny amount of radioactive potassium-40 every banana contains.
In one sense that’s unsurprising, given that antimatter and matter “annihilate” whenever they meet, giving out a puff of energy in the form of light. In our matter-dominated world, antimatter simply doesn’t stand a chance. But for physicists studying the fundamentals of the material world, the imbalance is deeply unsettling. According to their best-laid theories, the big bang should have created matter and antimatter in exactly equal amounts. In the orgy of annihilation that followed, the cosmos would have been left filled with nothing but a sea of light.
But it wasn’t. Some blip at the beginning of the universe caused some matter to survive – and make everything from bananas to black holes, seahorses to stars. It didn’t need much: calculations show a difference of just one part in a billion would have been enough. “It’s that one-part-in-a-billion more matter that forms the universe that we’re in now,” says physicist of the University of Liverpool, UK. But how did it happen?
At Geneva International Airport on the French-Swiss border, it’s easy to miss that such deep questions are being explored in an underground cavern just a kilometre away. Planes take off and land tantalisingly close to the building that houses the control room of the experiment Shears works on, . This detector at CERN’s Large Hadron Collider – the $10 billion particle smasher buried beneath Geneva’s outskirts – isn’t as famous as its two bigger cousins, ATLAS and CMS. They garnered glory back in 2012 for co-discovering the elusive Higgs boson particle, the signature of an all-pervasive field that gives the basic particles of matter mass.
But LHCb is after an even more fundamental prize.
The b stands for beauty – not as in salons, but as in quarks. Quarks are the fundamental particles found in the protons that the LHC smashes to smithereens. There are six types of quark, and they all have the odd quirk that they are never found by themselves, but always bundled into larger composite particles made up of two, three or possibly more (see “Tetraquirk?“). Protons, for example, are made up of three of the lightest – two up quarks and one down quark.
These days, the beauty quark is often more prosaically called the bottom quark, or just b. It is much more massive than the up and down quarks, but thanks to Einstein’s E = mc2, particles containing more massive quarks can be made from the energy liberated in the LHC’s proton collisions. One example would be particles called B mesons, which contain either a b quark or its antimatter equivalent, as well as one other quark.
B mesons could hold the key to how matter came to win out over antimatter. To understand why, we must rewind 50 years, to the discovery in the 1960s of a phenomenon known as charge-parity (CP) violation. James Cronin and Val Fitch later for showing that, under certain circumstances, antiparticles decay at different rates from their matter counterparts.
It was just the sort of thing that might explain matter’s dominance, but unfortunately the effect was far too small even to explain a one-in-a-billion imbalance. Subsequent experiments such as at SLAC National Accelerator Laboratory in California and at the KEK Laboratory in Japan have shown that CP violation also occurs when particles containing b quarks decay, and the imbalance is greater in this case (see diagram). “The behaviour of the matter version and the antimatter version of the beauty quark happen to be the most different of any fundamental particle that we’ve studied,” says Shears. “We don’t know why, but it means we can measure it with more accuracy.”
Even so, decays of known particles so far account for only about one part in a trillion of the CP violation needed to explain the universe. Meanwhile, the discovery of the Higgs boson has only deepened the mystery. Its mass of 125 gigaelectronvolts and observed decay rate fit predictions from the standard model of particle physics, a theoretical construct laboriously built up over decades, boringly well. If it had defied them, that might have pointed to the influence of particles as-yet unknown – which might have provided another source of CP violation.
No such luck. “The known type of CP violation we have in the equations so far is not enough. Now that we know more about the Higgs particle, it’s not enough to explain the matter-antimatter asymmetry in the universe today,” says , a theorist at Harvard University. “People are looking for something wrong with the equations, something wrong with the standard model, something beyond it.”
And perhaps unexpectedly, that throws the focus back on B mesons. The “beauty factories” BaBar and Belle collided electrons and their antimatter equivalents, positrons, with energies in the range of billions of electronvolts – enough to produce only the lightest B mesons in appreciable numbers. But to arrive at any meaningful answer as to why matter’s dominance arose, we need to recreate the more energetic conditions of the cosmic primordial soup.
“Just one part in a billion more matter created the universe we see today”
After a two-year shutdown, the LHC is now expected to produce collisions releasing 13 trillion electronvolts of energy, at a higher rate than ever before, bringing us much closer to doing that. The results could perhaps open up a crack between experiments and standard-model predictions. “The energy increase brings us increased B particle yields that will allow more sensitive studies of CP violation in areas we have not yet probed,” says LHCb physicist of Syracuse University in New York state.
And LHCb is custom-made to do that. It is a relatively small fish in a big pond – or perhaps more appropriately, given the LHC’s 27-kilometre circular structure, in a very large moat. ATLAS and CMS each cost seven times as much to build, and have three times as many researchers working on them. But unlike these general purpose detectors, which were built to observe all sorts of particles created in the energetic melee of the LHC’s collisions, LHCb is more choosy. Whereas ATLAS and CMS are cylindrical, fully encompassing the point at which the protons collide, LHCb’s detectors are in a cone or wedge shape, with the collisions taking place at the pointy end.
That makes the experiment much less sensitive to detecting Higgs bosons, which tend to emerge at high angles outside LHCb’s zone of detection. But it’s just the thing for measuring B mesons, which tend to be produced with their momentum directed along the line of the original proton beams. “For certain measurements, LHCb is the best game in town,” says Strassler.
Not that getting hold of a B meson is easy. They are produced travelling close to the speed of light and exist for just 10-12 seconds before decaying. That’s not long, but it is an eternity compared with some other particles: those containing the even heavier top quarks, for example, decay after just 10-20 seconds. Crucially, it means B mesons carve characteristic trails through the LHCb detectors that are a centimetre or so long – long enough for the LHCb’s analysis teams to begin to get a handle on them. “That’s what we do,” says Stone. “That’s what we live on.”
Recent results from data taken before the LHC’s shutdown, , suggest something interesting. They confirm earlier hints of deviations from standard-model predictions in the rare decay of a B meson to a K meson and two muons, heavier versions of the electron. The signal isn’t yet certain enough to tell if some unknown physics is responsible for the anomaly, or whether it can help to explain the matter mystery. Following up on that lead is a priority for LHCb’s physicists once the LHC is fully rebooted.
“The signal is not yet certain enough to tell whether unknown physics causes it”
Another focus is the decay of a different beauty-containing particle, the “B-sub-s”, or strange B meson. This agglomeration of a b-antiquark and a strange quark can transform into its antiparticle incredibly fast, so looking at whether the reverse process happens in the same way would provide another avenue to study CP violation. Also, three times in every billion, it decays into a muon and an antimuon. This decay may be rare, but its end state is very easy to see because muons leave a trail right through the detector all the way to the outermost layer. That makes it a “golden channel” to search for new physics, says Shears.
Here the interest may not just be in CP violation, but also in hints of phenomena predicted by theories such as supersymmetry. SUSY proposes the existence of a raft of exotic particles and might explain a lot of things the standard model can’t, such as why the fundamental forces have very different strengths, and the nature of the mysterious dark matter that seems to make up most of the stuff in the cosmos. But if these particles truly exist, their masses are so large that neither the LHC nor any other particle smasher has created them to date (7 March, p 30).
Rare B-meson decays should be especially susceptible to the influence of unseen massive particles, perhaps giving the LHCb experiment a sneaky way to prove their existence without detecting them directly. Any deviation from B-mesons’ expected rates of decay could mean hidden particles are participating in a “ghostly manner”, says of the University of Oxford, the leader of the LHCb collaboration.
The strange B meson decay is so unusual that experiments to date have seen very few examples, although both LHCb and CMS did spot it happening in 2013. More observations with the revamped LHC could make all the difference. “If you have certain varieties of supersymmetry as the real nature of the universe, then the way in which this particle behaves is modified, and it can produce these pairs of muons more often or less often,” says Shears. “It’s a very, very precise probe of what could be out there.”
Nonetheless, this particular decay hasn’t yielded any insights into why the fates of matter and antimatter were so very different. “Unfortunately, it all matches up with our predictions,” says Shears.
It sounds a little funny to hear experimental physicists say it’s unfortunate how well theory and experiment match, but it’s a refrain Stone echoes. “It’s not a big deal,” he says about the strange B meson findings so far, but then pauses and corrects himself: “The result is a big deal, but unfortunately it confirmed the standard model. That’s the problem.”
That yearning for something new will continue as long as that existential question of what caused the blip at the beginning of the universe remains unanswered.
Maybe, just maybe, LHCb will be the underdog experiment to satisfy that longing as the LHC kicks back into gear. “For the first time, we’re going to a regime in the universe where what we really need to see, and what we’re really looking to see, is the unexpected,” says Shears.
And if we do find a convincing explanation for the matter around us any time soon – well, that really would be reason to go bananas.
Has the LHC found a surprise massive particle? Read “Bigger than the Higgs, bigger even than gravitational waves…“
Tetraquirk?
There are six types of quark: up, down, strange, charm, bottom (or beauty) and top (or truth). Each has an antimatter doppelganger that is identical in mass, but has opposite electrical charge.
Quarks of any kind only ever turn up in larger composite particles. Heavier “baryons” contain three quarks, as the familiar proton and neutron do; they also have antimatter equivalents containing three antiquarks. Lighter mesons contain a quark and an antiquark.
That might not be the whole story, though. Before the LHC’s recent two-year shutdown, the LHCb experiment amassed the most significant evidence yet for the existence of tetraquarks – particles with four quarks.
The standard model doesn’t explicitly limit the number of quarks in a single particle, but if tetraquarks do exist they represent a whole new form of matter beyond the pairs and triplets previously confirmed, says of the University of Pittsburgh. “Now comes along another option for the first time.”
More observations of LHCb’s particular tetraquark find – the jazzily named Z(4430) – could give us a better understanding of how quarks are glued together. More broadly, it could tell us what exotic states of matter may have existed when the universe was just a baby – another insight into matter’s mysterious early history (see main story).
This article appeared in print under the headline “The big blip”
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