The journey of particle rediscovery is starting again (Image: Enrico Sacchetti)
THEY call it “the desert” – a vast, empty landscape separating us from a promised land that shimmers like a mirage on the horizon. A land full of answers, where we finally achieve a complete understanding of material reality.
Stop dreaming: we can’t get to this nirvana. The way across the desert is too long and hot, and we have no vehicle to take us there. But if physicists’ hopes are realised, a machine just waking from a two-year slumber could bring us a decisive step closer – and might even reveal answers closer to home.
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The machine in question is the Large Hadron Collider, or LHC. This most muscular of particle smashers, situated at CERN near Geneva, Switzerland, garnered fame for discovering the Higgs boson in 2012. That was the last uncharted feature of a landscape that’s now pretty well explored: the landscape of the standard model, our current best theory of matter and its workings.
Since February 2013, the LHC has been undergoing a comprehensive overhaul. Now it is gearing up again, more powerful than ever before, for a journey towards the desert – and the complete unknown. The excitement is palpable. “We are living in a once-in-a-lifetime experience, opening the curtains on a totally new energy scale,” says Jim Olsen of the LHC’s CMS experiment.
Particle physicists measure out territory using energy as their yardstick. Through Einstein’s famous equation E = mc2, smashing particles together at high speed converts their mass into pinpricks of enormous energy that can in turn form other, more massive particles. Our universe started out in a hot, dense big bang, so the higher the energies we reach by ramping up the speed of the colliding particles, the further back we reach into the origins of the cosmos and matter itself.
In this way the LHC and its predecessors have allowed us to map out the standard model. This theory describes the fundamental particles of matter, known as leptons and quarks, and the particles responsible for transmitting three of the four forces operating on them: electromagnetism and the weak and strong nuclear forces.
The crowning glory of this effort was the Higgs boson. This particle represents a disturbance in an all-pervasive, invisible entity, the Higgs field, with which all other fundamental particles interact to acquire their mass. The Higgs is essential to explain a pivotal moment in the early cosmos. Originally, electromagnetism and the weak nuclear force were part of the same unified electroweak force, until the Higgs field switched on around 10-12 seconds after the big bang. This “broke the symmetry” of the force, giving the particles that transmitted it different masses. Since that point electromagnetism, carried by the massless photon, has had an effectively infinite range, whereas the weak force, carried by the meaty W and Z bosons, has been confined to the subatomic scale.
Complete yet incomplete
The Higgs’s existence was first predicted in 1964, but finding it required the energy of the LHC: 8 teraelectronvolts (TeV) produced in head-on collisions between protons. Even then, theorists were not certain about exactly what mass the Higgs should have, assuming it did exist. But it was a no-lose situation: even if it did turn out to be a mathematical mirage, they would have had the impetus to tear up the standard model and start afresh.
As it was, the elusive particle was found with a mass of 125 gigaelectronvolts (GeV), well within the LHC’s energy range. Theorists were vindicated and two of the Higgs theory’s pioneers, François Englert and Peter Higgs, shared the crowning glory of physics, , in October 2013.
But here’s the thing: although the standard model is now complete, it is incomplete. The theory includes no description of dark matter – a sizeable omission, given that this stuff apparently makes up 85 per cent of all matter in the universe, judging by observations such as how galaxies move as if whirled around by an invisible gravitational hand. It gives no indication of how a tiny imbalance could have arisen between normal matter and antimatter to ensure a matter-filled cosmos. It is also mute on gravity, the fourth of the four fundamental forces.
More technically, the standard model is dependent on a bevy of arbitrary numbers to make it work. The Higgs is just one example: its mass is one of many quantities the standard model cannot predict, but which have to be measured instead. Left to its own devices, the model does come out with a number of sorts: it says the Higgs interacts so strongly with heavy particles that it acquires a nonsensically large mass of at least 1013 TeV.
For most physicists, the conclusion is that the standard model is part of a bigger theory – one that brings us closer to unifying all forces and understanding matter at all energy scales. The problem is, although precise predictions vary, our best guess is that further bouts of force unification only lie at scales of trillions of TeV and above, that were last attained in the first trice of the universe – within 10-36 seconds of the big bang (see diagram).FIG-mg30110501.jpg
No accelerator on Earth could conceivably achieve such energies. In this picture, what lies between us and the unattainable promised land is a desert devoid of interest. It makes the LHC’s upgraded collision energy of 13 TeV seem a rather forlorn gesture.
“No accelerator on Earth could achieve the energies needed to unify the forces”
Not so, says theorist of the University of Cambridge. If the favourite candidate for a next-generation theory is right, the sliver of new territory we are about to enter could contain particles and phenomena that will take us a decisive step closer to an ultimate answer.
The theory in question is supersymmetry, or SUSY to its friends. First dreamed up in the 1970s, SUSY provides the first whiff of a “grand unified theory” that combines the electroweak and the strong forces. It pulls off this trick by introducing a forest of “sparticles”, heavier supersymmetric doppelgängers for each particle already known, that might appear at scales the LHC can probe. “If they find signals of a new particle at the LHC you will see a shock go through the theoretical community,” says Allanach. “We’ll be dancing in the streets.”
Not least because the lightest sparticle provides an excellent candidate for dark matter. In concert, the sparticles also naturally cancel out troublesome quantum fluctuations that cause the Higgs mass to balloon out of control. A dark-matter-like SUSY particle is likely to slip unseen out of the detectors, leaving a hole when adding up the energy and momentum produced. But, in general, SUSY particles are expected to make themselves known by the way they decay into lighter standard model particles in telltale patterns.
So beloved of theorists is SUSY, and so earnest is the desire to catch a glimpse of it, that several safeguards must be built into the analysis procedure to guard against overenthusiasm. Once the crew is confident the machine and the analysis are running smoothly, the data will be “blinded” – computers doing the analysis will not read out the results of certain calculations until sufficient data has been accumulated, to prevent standard model processes being misinterpreted as something new (see “Faster, higher, stronger“).
To confirm an anomaly is the result of fresh physics, it must be present in data from both of the two big LHC experiments, ATLAS and CMS, as was the case for the Higgs discovery. “The moment where fluctuations deepen and are mutually confirmed is magical, and it is hard to predict how it will occur,” says Andreas Hoecker of ATLAS.
Or if it will occur at all. In a perfectly supersymmetric world, sparticles would have identical masses to their partner particles and would have been seen long ago. Theorists suppose that, rather like the electroweak force, SUSY is “broken” to make the sparticles much heavier. Make them too heavy, however, and they cease to be useful in addressing problems such as the Higgs mass, the nature of dark matter or force unification.
The fact that no sparticles were discovered in the LHC’s first run has already limited the breathing space for the simplest, most aesthetically attractive variants of the theory. Like many theorists, Allanach says the next few years of LHC data will decide whether or not he waves goodbye to SUSY. If nothing’s found, “it will show that there is something really fundamental at high energy scales that we don’t understand,” he says.
While physicists keep a keen eye out for a satisfactory solution to the SUSY conundrum, there remains the vexed question of gravity. The conventional view is that there is no chance of seeing even a hint of unification with this force at the energy scales the LHC can probe. Gravity is so much weaker than the other three forces – by some 40 orders of magnitude – that it can only hope to be unified with them at blistering energies of 1016 TeV and above, that correspond almost to the heat of the big bang itself.
But in 1998 theorists proposed a mind-bending alternative. What if our world existed on a “brane” of three spatial dimensions floating in a higher-dimensional space? Then the true strength of gravity might be spread over all these dimensions, leaving it looking strangely diluted from our perspective. If so, gravity’s true strength might be such that the promised land of unification lies at energies much closer to where we are now, perhaps even within the LHC’s reach. Then the desert would be no desert, but full of a host of strange objects such as miniature black holes, which the LHC might be able to squeeze into existence by warping and pinching space-time in its collisions. These entities would decay into showers of more familiar particles in highly distinctive patterns.
Even if such things turn out to be too energetic for direct observation, there is a chance that new particles, SUSY or otherwise, might influence the behaviour of things at lower energy scales in ways that are measurable. Some whispers of unexplained effects do lurk in data from the LHC and previous colliders, and these will grow or fade as further collisions amass after the restart. If such a blip grew large enough, that in itself would be a “huge thing”, says Allanach. But it wouldn’t necessarily tell you what sort of new particle is at play. The trouble with theorists like him, he says, is that they will come up with 20 or more different hypotheses to explain the same blip – all of which work.
For an experimentalist like Hoecker, though, the excitement lies less in proving any “this or that” scenario, but precisely in not knowing what’s to come. “There certainly is more room for surprise in the vast space of possible new physics effects than was the case for the Higgs discovery,” he says.
So the hope is that the territory about to be explored is a lush forest bristling with particles that give us clues to the nature of the desert – and beyond. If not, then we’re stuck, with no indication of what comes next, and no “natural” explanation for why aspects of the standard model landscape look as they do. Faced with such a prospect, physicists are left postulating that only some spooky fine-tuning, some strange coincidence of factors, is responsible. Perhaps our standard model is just one of countless others, in a space in which all possible universes exist. Or perhaps we can move the goalposts and say the answers can be found only with a machine that can reach higher energy scales.
But in the meantime let’s saddle up for the upgraded LHC ride, wherever it takes us – so as not to be left back staring longingly at a unreachable kingdom, with a bag containing just the Higgs and a load of unanswered questions.
Leader: “Double or nothing: the stakes are high for the new LHC“
Faster, higher, stronger
The souped-up LHC will not only be delivering collisions at higher energy, but will also double their rate, amassing a gigabyte of data a second. Further upgrades are planned, meaning that all the data the particle smasher has collected so far would represent just 1 per cent of the total the machine should collect in its full 20-year expedition.
The first job after the restart will be to see if the LHC’s two big multipurpose detectors, ATLAS and CMS, are dealing with this huge flow of data properly, by calibrating and testing them on known physics. “Rediscovering” the Higgs boson and making precise measurements of its properties, especially how it interacts with other particles such as the top quark and the W and Z bosons, are high priorities, says Jim Olsen of CMS.
In the heady days immediately following the Higgs discovery, there were hints that it decayed into pairs of photons at twice the rate expected in the standard model, but the discrepancy melted away once more of the particles had been produced. The consensus now is that the Higgs has a disappointing “plain vanilla” flavour, conforming to every prediction made of it by the standard model. But perhaps more data will once again hint at something more interesting, such as the Higgs being not one particle, but a composite bundle.
In any case, it is important to investigate how the familiar landscape of the standard model looks from the perspective of the upgraded machine. The particle processes in this landscape will form the expected “background” against which any blips – the signatures of fresh physics – will manifest themselves.
“We need both to confirm that the standard model still holds at higher energy, and to prove that we are able to predict and model the main background to new physics searches,” says Olsen. Only then can the hunt for next-generation theories such as supersymmetry begin (see main story).
This article appeared in print under the headline “It’s back”
