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本文作者：小红猪小分队
What is dark matter? Or gravity? Why is the universe so smooth? There’s a particle for every conundrum
WHEN, in the late 1930s, the Nobel laureate Isidor Rabi learned of the discovery of a heavier version of the electron, he asked “Who ordered that?”. Three-quarters of a century on, he could repeat that question many times over. We now know that Rabi’s intruder, the muon, is one of a family of three electron-like particles that differ only in their mass.
It doesn’t end there. What is called the standard model of matter and its interactions relies on a panoply of particles, some familiar, some less familiar (see diagram). The Large Hadron Collider (LHC), which this month is preparing to smash protons together at CERN near Geneva, Switzerland, for its third full season, is looking for the definitive trace of the only particle predicted by the standard model still to be discovered - the Higgs boson, giver of mass.
And much more besides. The standard model leaves many questions unanswered. Why does matter dominate antimatter in our cosmos? What is the true nature of gravity? What is the “dark matter” that appears to hold galaxies together made of? Attempts to answer such questions lead physicists time and time again to the same expedient: invent a new particle.
Stringballs
String theory is a popular shot at bringing together two disparate scales – the tiny world of quantum particles, where the standard model holds sway, and the cosmic distances over which gravity acts. It holds that particles such electrons and quarks are really strings of energy a mere 10-35 metres long vibrating in different ways.
If such predictions are at all correct, some interesting things might turn up at the LHC. Miniature black holes are one notorious possibility. Stringballs are another. These are made when two strings slam into one another and, rather than combining to form a stretched string, make a tangled ball.
Energy is in plentiful supply at the LHC, so stringballs could show up there in large quantities. That would be a revolutionary event, says Savas Dimopoulos of Stanford University in California, one of the originators of the stringball concept. That’s not least because string theory goes hand-in-hand with the idea that there are extra dimensions of space in addition to the three we know about. “Finding an extra dimension would be more exciting than discovering a new continent,” says Dimopoulos.
So far, none has made its presence felt. But it is still early days for the particle smasher. “Theorists are good at predicting phenomena,” shrugs Dimopoulos. “They just can’t tell you where.”
Tetraquarks
Sometimes, experiments lead the way in suggesting new particles. A decade ago, particle physics was abuzz when more than 10 experiments worldwidereported hints of a “pentaquark” - an agglomeration of four quarks and one antiquark that weighed half as much again as a proton.
Protons and other composite particles known in the standard model are either made from three quarks bound together, or are a ménage à deux of a quark and an antimatter antiquark. Yet there is no fundamental reason to believe weightier particles with combinations of four, five, six or even seven quarks and antiquarks don’t exist, says particle theorist Frank Close of the University of Oxford. There are, however, very good reasons to believe we would be hard-pressed to spot them. A pentaquark, for example, would be expected to decay within less than 10-23 seconds – “about the time it takes for 0light to cross a particle”, says Close.
Our glimpse of pentaquarks certainly proved suitably fleeting. The presumed discoveries melted away in 2005 when a dedicated search for the particlesturned up nothing.
Yet even as pentaquarks have faded, there has been an uptick in sightings of “tetraquarks”. These composite particles of two quarks and two antiquarks might be produced when an electron and its antiparticle, the positron, annihilate. The problem here, says Close, is one of interpretation: are we actually seeing true closely bound particles, or something more like “molecules” of two conventional quark-antiquark pairings loosely, and fleetingly, linked together?
Such tales of particles found and lost gain resonance as we sieve through the LHC’s outpourings. Is that really the Higgs boson, or a sign of supersymmetry – or will it too dissolve into thin air?
Glueballs
The proton’s inner life is a complex affair. The three “valence” quarks that make up its charge live in a seething sea of shorter-lived quarks that pop into and out of existence from the quantum vacuum.
Pulling the strings is a melange of particles called gluons. Quarks carry both electrical charge and a property known as colour charge. Just as photons are exchanged between particles with electric charge to produce the electromagnetic force, gluons are exchanged between colour-charged quarks. This exchange produces the strong nuclear force that binds them together.
Except there is a difference. Photons are electrically neutral, but gluons themselves carry colour charge, and so feel their own force. That raises an interesting question: can we forget the quarks altogether, and make matter just of gluons stuck to each other?
The possibility of “glueballs” has tantalised physicists for three decades. In 1994, CERN’s Crystal Barrel experiment provided the first of a series of putative sightings. Yet two decades on, says particle theorist Frank Close, we are no closer to saying what truth there was in the claims. Any number of electrically neutral, strongly-interacting particles will in all practically conceivable situations mix with glueballs and muddy the waters. “There is nothing that goes against the idea of glueballs existing,” says Close. “But how to prove it is still bugging me.”
Inflatons
Why is space so smooth and the contents of the cosmos so evenly distributed? According to the plain-vanilla big bang model of the universe’s origins, space could be jaggedy or warped in all manner of curious ways.
The current standard explanation is that just after its birth the universe went through a period of breakneck expansion in which regions of space were pulled apart faster than the speed of light, ironing out all the wrinkles. The driving force behind this “inflation” was a hugely energetic field that briefly dominated the cosmos before dissolving into other matter and radiation.
Quantum theory says that every field has an associated particle – in this case the inflaton. Its existence would have some intriguing implications. Quantum fluctuations in the inflaton field make it very difficult to turn off completely, so parts of the original cosmos will still be inflating, making for a “multiverse” of independently developing universes.
Direct evidence for the inflaton won’t be coming any time soon, though. At a minimum, you would require an accelerator capable of producing a trillion times the LHC’s energy density, says Paul Steinhardt of Princeton University. “But then you have to figure out what to accelerate such that, when it collides, it would produce inflatons.”
Pomerons
Even if we never succeed in isolating a glueball (see left), there’s one place physicists are convinced they will turn up – at the LHC, as packets of energy exchanged when protons suffer only a glancing collision in the accelerator.
These “virtual” glueballs come in all shapes and sizes depending on the nature of the collision, creating a mathematical headache for theorists. But there is a ready remedy, says theorist Joe Polchinski at the University of California, Santa Barbara: simplify them all into an “effective” particle known as a pomeron.
Pomerons have a long history in models of proton interactions, pre-dating theories involving quarks, gluons and the strong force. They were even one of the inspirations for string theory (see “Stringballs”), which originally aimed to explain protons before it was radically repurposed. “String theory was supposed to be a theory of the strong interactions,” says Polchinski. “Instead it turned out to be a theory of gravity.”
Now things are moving back the other way. String theory with an extra fifth dimension turns out to look very much like the strong force in our conventional four dimensions - so it can now be used to understand pomerons and the goings-on in glancing reactions at the LHC.
That could help hook the big fish: such brushing encounters produce much less debris than a standard head-on LHC collision. Detectors are under development that can spot protons that have swapped pomerons, and they could provide a particularly clear sight of the vaunted Higgs boson, says theorist Jeff Forshaw at the University of Manchester in the UK.
Leptoquarks
In 1994, a team of physicists were colliding electrons head-on with protons at the DESY laboratory in Hamburg, Germany, when they saw an electron apparently turn into its heavier counterpart, a muon. Such a transformation is unheard of in the standard model. So what happened?
One possibility is that the collisions created a heavyweight crossbreed known as a leptoquark. In the standard model, electrons and protons are very different sorts of particles, set apart by the forces they feel. Protons and the like are composites made when quarks combine under the influence of the strong force (see “Tetraquark”). Particles such as electrons and muons are elementary particles, collectively known as leptons, that do not feel the strong force at all.
Grand unified theories aim to cut across such boundaries by rolling three of the four forces of nature into one. In some theories, when an electron hits a proton, as at DESY’s HERA accelerator, leptoquarks can form and decay to a muon and a quark. “HERA seemed like a good place to make a leptoquark,” says John Ellis, a theoretical physicist at King’s College London.
In the event, there were no further sightings, and the excitement faded. Yet the lure of grand unified theories remains – and the search for leptoquarks continues at the LHC today.
Winos
Particle physicists are generally a sober bunch. The same is not always true of their particles.
Winos pop up in supersymmetry, the grand theoretical construction that is favoured to subsume the standard model. Supersymmetry patches some of the standard model’s structural weaknesses by suggesting that each known particle has an as-yet-undiscovered, generally heavier partner.
Fermions, for example, are a class of standard-model particle that embrace the building blocks of matter – electrons and quarks – and their ghostly neutrino relatives (see diagram). In supersymmetry, they all have “sfermion” cousins: selectrons, sneutrinos and the parrot-like squarks. The other main group of standard-model particles, the force-transmitting bosons, have “-ino” partners: photinos for photons, gluinos for gluons, and so on. Hence the Winos: they are partners of the W bosons, particles that transmit the weak nuclear force.
According to supersymmetry, all sfermions are bosons and all “bosinos” are fermions. If this all sounds rather confusing, don’t worry: the LHC has yet to turn up the expected haul of supersymmetric particles
. For many particle physicists and cosmologists a lack of Winos and the like would be a serious headache, as supersymmetric particles provide a ready recipe for the obscure dark matter that binds galaxies together (see “Wimpzilla”).
Anyons
Forget the rules: with anyons, anything goes. These denizens of two-dimensional worlds do not obey the normal, clean division of particles into fermions and bosons (see “Winos”), but lie somewhere between the two – an ambiguous status that led Nobel prizewinning particle theorist Frank Wilczekof the Massachusetts Institute of Technology to give them their name.
Conventional particles such electrons and photons can be regarded as aberrations in the energy of free space, as point-like “excitations” of the quantum vacuum. Similarly, anyons crop up as energetic excitations, each apparently carrying just a fraction of an electron’s charge, in two-dimensional layers of some metals when exposed to a strong magnetic field.
In such a situation, the moving parts are actually the photons of the magnetic fields and the free electrons of the metal. So why invent a new particle? For the same reason we invent things like protons, says Wilczek: they work. Protons are made up of quarks, but no one has ever seen a quark on its own, so it often makes sense, for example when describing how atomic nuclei work, to deal in protons. “In principle you could do without identifying the excitations as separate entities,” says Wilczek. “But it would be awkward and perverse.”
The advent of 2D materials such as graphene – the single layers of carbon atoms that earned Andre Geim and Konstantin Novoselov the Nobel prize in physics in 2010 – means anyons could soon be anyone’s. Their unique characteristics also make them hot favourites to power a future generation of superfast quantum computers.
Galileons
The discovery that the universe’s expansion is accelerating came from observations of far-off supernovae in the 1990s, and was deemed worthy of the latest Nobel prize in physics.
It has left theorists scratching their heads as to what causes it. The leading candidate is a “dark energy” that emanates from the quantum vacuum, and somehow manages to trump the steadying force of gravity. Other suggestions are that the effect is an illusion born of where we are sitting in the cosmos - or simply that gravity itself is weakened on large cosmic scales.
This last explanation has a big hurdle to overcome. Our current theory of gravity, Einstein’s general theory of relativity, says that the force works in the same way everywhere. Its predictions have been confirmed on scales up to that of the solar system, which is as far as we have roamed to test it.
Galileons provide a neat workaround. They are the quantum particles associated with a field that is hypothesised to weaken gravity. Like the related “chameleon” particles, their influence is screened by the presence of matter. In a region of relatively high density, such as our solar system, their weakening effect is imperceptible, kicking in only over vast, emptier swathes of the cosmos – thus explaining the supernova observations.
It’s a nice idea, but is it true? The answer depends on finding testable effects that we might use to prove the existence of the particles, says theorist Mark Trodden of the University of Pennsylvania in Philadelphia. “We are trying very hard to work out what they might be.”
Majorana particles
When Italian theoretical physicist Ettore Majorana disappeared en route from Palermo to Naples in 1938, he left behind many riddles. Among them was under what circumstances a particle can be its own antiparticle.
Particles and antiparticles are identical except for their opposite electrical charges. Unlike the partner particles of supersymmetry (see “Winos”), antimatter is real – although it only emerged from the realms of conjecture in 1932, when a positively charged anti-electron, or positron, was seen in cosmic rays.
Majorana suggested that a chargeless particle belonging to the same group as the electron, the fermions, might have an antiparticle with identical, zero charge. That seems absurd: surely that would just be the same particle twice over? But Majorana’s particles are a fixture in a supersymmetric world. There, the chargeless photon has a fermionic superpartner, the photino, which is its own antiparticle. The same goes for the Higgsino, the superpartner of the Higgs boson.
Something answering Majorana’s description also popped up in a lab-bound nanoscale semiconducting wire just last month, confirming a long-standing theoretical prediction. They are quite possibly also passing through our heads every second: neutrinos and antineutrinos seem to interact differently, but might be the same chargeless particle in different states of motion.
The experimental proof might be spotting a nuclear process called neutrinoless double beta decay. Conventional beta decay comes with the emission of an antineutrino or neutrino. In rare cases where a nucleus can undergo two such decays, two of those particles should be emitted. If the neutrino were its own antiparticle, the two would annihilate, and no neutrino emission would be observed. Such processes might in turn shed light on one of the biggest riddles of them all: why it is that matter, rather than antimatter, came to dominate the cosmos.
Wimpzilla
Physicist Rocky Kolb was doing his grocery shopping in Warrenville, Illinois, one day, and wondering what he should call the dark-matter particle he and his colleagues had just invented. A movie poster on a passing bus provided the answer. It was 1998 and the Godzilla remake had just been released. The wimpzilla was born.
No one knows what dark matter is made of: we just know that 80 per cent of universe’s matter is invisible to our telescopes. Weakly-interacting massive particles, or WIMPs, are a popular idea. Between 10 and 100 times as weighty as protons, they would have been produced in the universe’s hot primordial soup and corralled by gravity to seed today’s galaxies.
But that is not the only possibility. Within the universe’s first second, during the period of inflation (see “Inflatons”), the expansion of space itself ripped particles out of the vacuum. Kolb and his colleagues calculated that among them could have been dark particles weighing a billion times more than a WIMP.
Their monstrous mass means wimpzillas would be exceedingly rare. They can’t be made in particle accelerators and are unlikely to amble into one of the myriad underground detectors looking for WIMPs. “They are possibly the most elusive dark-matter particles ever proposed,” concedes Kolb.
They might still leave subtle features in the cosmic microwave background radiation, the big bang’s afterglow that suffuses the sky. If we find any trace, for example in the detailed maps of the cosmic background expected from the European Space Agency’s Planck satellite, “then after 10,000 years of thinking about it, we’ll know what the universe is made of”, says Kolb. Such are the grand prizes that can come from inventing new particles.
Richard Webb and Valerie Jamieson are feature editors at New Scientist
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