In addition to grand unified theories of the Georgi–Glashow type, another approach to unification emerged in the early 1970s from theorists in the Soviet Union and was independently rediscovered in 1973 by CERN physicists Julius Wess and Bruno Zumino. This is called supersymmetry, often reduced to the acronym SUSY. There are many varieties of supersymmetric theories but one of the simplest – first proposed in 1981 and called the Minimal Supersymmetric Standard Model (MSSM) – features ‘super-multiplets’ which connect matter particles (fermions) with the bosons that carry forces between them.
In supersymmetric theories, the equations are invariant to the exchange of fermions for bosons, and vice versa. The very different properties and behaviours of fermions and bosons in the physics we observe today must then be the result of breaking or hiding this supersymmetry.
One consequence of this higher supersymmetry is the proliferation of more particles. For every fermion, the theory predicts a corresponding supersymmetric fermion (called a sfermion), which is actually a boson. This means that for every particle in the Standard Model, the theory requires a massive supersymmetric partner with a spin different by ½. The partner of the electron is called the selectron (a shortening of scalar-electron). Each quark is partnered by a corresponding squark.
Likewise, for every boson in the Standard Model, there is a corresponding supersymmetric boson, called a bosino, which is actually a fermion. Supersymmetric partners of the photon, W, and Z particles are the photino, wino, and zino.
One of the advantages of the MSSM is that it resolves the problem of the mass of the Higgs boson. In the MSSM, the loop corrections that lead the Higgs mass to inflate are cancelled by negative corrections resulting from interactions involving virtual super-symmetric particles. For example, the contribution to the Higgs mass arising from interactions with a virtual top quark is cancelled by interactions involving a virtual stop squark. This cancellation stabilizes the Higgs mass and hence the strength of the weak force. To make this mechanism work, the MSSM actually needs five Higgs particles, each with a different mass. Three of these particles are neutral and two carry electric charge.
The MSSM also irons out another wrinkle in the Standard Model. As Weinberg, Georgi, and Quinn had shown in 1974, the strengths of the Standard Model strong, weak, and electromagnetic forces become near-equal at high energies. But they do not become precisely equal, as might be expected in a field theory of a fully unified electro-nuclear force. In the MSSM, the strengths of the three particle forces are predicted to converge on a single point (see Figure 23).
Supersymmetry may also resolve a long-standing problem in cosmology. In 1934, the Swiss astronomer Fritz Zwicky discovered that the average mass of galaxies in the Coma Cluster, inferred from their gravitational effects, is not consistent with the average mass inferred from the galaxies’ luminosity in the night sky. As much as 90 per cent of the mass required to explain gravitational effects appeared to be ‘missing’, or invisible. This missing mass was called ‘dark matter’.
This problem was not confined to a single cluster of galaxies. Dark matter is a central component of the current Standard Model of big bang cosmology, the lambda-CDM model. Successive observations of the cosmic microwave background radiation by the COBE and, more recently, WMAP, satellites suggest that dark matter constitutes about 22 per cent of the mass-energy of the universe. About 73 per cent is ‘dark energy’, associated with an all-pervasive vacuum energy field, leaving the ‘visible’ matter of the universe: stars, neutrinos, and heavy elements – everything we are and everything we can see – to account for less than five per cent.
Supersymmetry predicts super-particles that are not affected by either the strong or electromagnetic forces. Super-particles, such as neutralinos, are therefore candidates for so-called weakly interacting massive particles, or WIMPs, which are thought to constitute a significant proportion of dark matter.*
FIGURE 23 (a) Extrapolating the strengths of the forces in the Standard Model implies an energy (and a time after the big bang) at which the forces have the same strength and are unified. However, the forces do not quite converge on a single point. (b) In the Minimum Supersymmetric Standard Model (MSSM) the additional quantum fields change this extrapolation, and the forces more nearly converge.
The existence of a host of supersymmetric particles may seem fantastic, but the history of particle physics is littered with fantastic discoveries based on theoretical predictions that many dismissed as absurd when they were made. If they do exist, some of the supersymmetric particles are anticipated to make their appearance at the TeV energy scale.
As the LHC began to take shape over 500 feet beneath French and Swiss soil at the beginning of a new millennium, it was obvious that its purpose was much more than finding the electro-weak Higgs boson, or indeed several Higgs bosons or supersymmetric particles as predicted by the MSSM. It was about pushing beyond the Standard Model; it was about our ability to understand what things are made of and how these things have shaped our universe.
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Work began on dismantling the LEP in December 2000. Forty thousand tonnes of material had to be removed. The tunnel was completely emptied by November 2001, as surveyors began to mark the first of seven thousand locations for the components of the LHC.
There were inevitable delays. Maiani identified substantial cost over-runs in October 2001 and subsequent budget constraints pushed completion of the project back a further year, from 2006 to 2007. Just as the Americans had discovered during their abortive project to build the SSC, the novel technology of superconducting magnets tended to chew up rather more budget than had been anticipated.
Construction of the world’s largest refrigeration system, capable of cooling the superconducting magnets to –271.4 °C, was completed in October 2006. The last of the LHC’s 1746 superconducting magnets was installed in May 2007.
Although the LHC would be housed in the 27-kilometre tunnel that had been used for the LEP, further excavation was necessary to make room for new detector facilities. In the original planning for the LHC, four detector facilities were envisaged. These were A Toroidal LHC Apparatus (ATLAS), the Compact Muon Solenoid (CMS), A Large Ion Collider Experiment (ALICE), designed for the study of heavy ion (lead nuclei) collisions, and the Large Hadron Collider beauty (LHCb), a facility specifically designed to study bottom quark physics.
A further two, much smaller, detector facilities were subsequently added. TOTal Elastic and diffractive cross-section Measurement (TOTEM) is designed to make measurements of exquisitely high precision on protons and is installed near the point where protons collide in the centre of the CMS detector. Finally, the purpose of the Large Hadron Collider forward (LHCf) detector is to study particles generated in the ‘forward’ region of proton–proton collisions, almost directly in line with the colliding beams. It nestles alongside ATLAS and shares the beam intersection point.
The general-purpose ATLAS and CMS detectors would be involved in the hunt for the Higgs boson and other ‘new physics’ that might signal the existence of supersymmetric particles and resolve the riddle of dark matter. The ATLAS detector consists of a series of ever-larger concentric cylinders around the point at which the proton beams from the LHC intersect. The function of the inner detector is to track charged particles, enable their identification, and measure their momentum. The inner detector is surrounded by a large solenoid (coil-shaped) superconducting magnet which is used to bend the paths of the charged particles.
Sitting outside this are electromagnetic and hadronic calorimeters, which absorb charged particles, photons, and hadrons, and infer their energies from the particle showers they create. A muon spectrometer measures the momentum of highly penetrating muons which pass through the other detector elements. It makes use of a toroidal (doughnutshaped) magnetic field created by large superconducting magnets formed into eight ‘barrel loops’ and two ‘end caps’. These are the largest superconducting magnets in the world (see Figure 24).
ATLAS cannot detect neutrinos, and their presence must be inferred from the energy imbalance between colliding and detected particles. The detector must therefore be ‘hermetic’: no particles other than neutrinos can escape undetected.
The ATLAS detector is about 45 metres long and 25 metres high, about half as big as Notre Dame Cathedral in Paris. It weighs about seven thousand tonnes, equivalent to the Eiffel Tower or a hundred empty 747 jumbo jets. The ATLAS collaboration is led by Italian physicist Fabiola Gianotti and consists of three thousand physicists from more than 174 universities and laboratories in 38 different countries.
CMS has a different design but similar capabilities. The inner detector is a tracking system, made of silicon pixels and silicon strip detectors which measure the positions of charged particles allowing their tracks to be reconstructed. As in the ATLAS detector, electromagnetic and hadronic calorimeters measure the energies of charged particles, photons, and hadrons. A muon spectrometer captures data on muons which penetrate the calorimeters.
FIGURE 24 The ATLAS detector makes use of a toroidal (doughnut-shaped) magnetic field created by large superconducting magnets formed into eight barrel loops and two end caps. These are the largest superconducting magnets in the world.
Source: © copyright CERN
The CMS detector is ‘compact’, meaning that it uses a single, large solenoid superconducting magnet and so it is smaller than ATLAS. But it is still large: 21 metres long, 15 metres wide, and 15 metres high (see Figure 25). The CMS website proclaims that it sits in an underground cavern ‘that could contain all the residents of Geneva; albeit not comfortably.’5 The CMS collaboration is led by Italian physicist Guido Tonelli, and also includes three thousand physicists and engineers from 183 institutes in 38 countries.
FIGURE 25 Peter Higgs (on the left) visiting the CMS detector during the construction phase. He is pictured here with CMS spokesperson Tejinder Virdee.
Source: © copyright CERN
Work had begun in 1997 and 1998 on the construction of the ATLAS and CMS detector components and the excavation of the caverns that would house them. Assembly of both detectors was completed in early 2008.
By August 2008, all 27 kilometres of the LHC had been cooled to its operating temperature. The operation had required more than 10,000 tonnes of liquid nitrogen and 150 tonnes of liquid helium to completely fill the magnets.
The LHC was ready to be switched on.
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‘It’s a fantastic moment,’ Lyndon Evans, the LHC project manager, declared on 10 September 2008. ‘We can now look forward to a new era of understanding about the origins and evolution of the universe.’6
Sadly, Evans’ delight was not to last. The LHC was switched on at 10:28 am local time. Physicists crammed into the small control room cheered as a single flash of light appeared on a monitor, signifying that high-speed protons had been steered all the way around the machine’s 27-kilometre ring at an operating temperature just two degrees above absolute zero. Though somewhat unspectacular (and something of an anti-climax for the estimated one billion people thought to have watched the moment on television), it represented the culmination of two decades of unstinting effort by armies of physicists, designers, engineers, and construction workers.
Another proton beam was sent around the ring in the opposite direction at 3 pm that day. Trouble began shortly afterwards. Just nine days later an electrical bus connection between two of the superconducting magnets short-circuited. Electricity arced, punching a hole in the magnets’ helium enclosure. Helium gas leaked into sector 3–4 of the LHC tunnel, and in the subsequent explosion 53 magnets were damaged and the proton tubes were contaminated with soot.
There was no hope of repair before the scheduled winter shut-down, and a restart was tentatively fixed for spring 2009. But there were more problems, and at a meeting in Chamonix in February 2009 CERN managers took the decision to commission further work.
The restart date was pushed back.
10
The Shakespeare Question
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In which the LHC performs better than anyone expected (except Lyn Evans), a year’s data is gathered in a few months and the Higgs boson runs out of places to hide
It was only at the beginning of September 2009, almost a year after it had first been switched on, that the last of the LHC’s eight sectors began its cool-down procedure. All eight sectors were back at their operating temperature by the end of October, and the LHC was restarted in November. Despite the increased cost of electricity during the winter months, the collider was operated through the winter of 2009–10, primarily so that CERN physicists could stay ahead of their rivals at Fermilab’s Tevatron, who had also produced tantalizing glimpses of the Higgs.
Through the first few months of 2010, protons running around the LHC in two rings travelling in opposite directions were accelerated to 3.5 TeV before being brought into head-on collision. The first 7 TeV collisions were recorded on 30 March. This collision energy was then maintained as the beam intensity and luminosity were gradually increased. Both ATLAS and CMS recorded events that could be ascribed to many familiar faces, as the panoply of Standard Model particles that had taken more than six decades to discover were registered in a matter of months. These included the neutral pion, first discovered in 1950, the eta, rho, and phi mesons (formed from various combinations of up-, down-, and strange-quarks), the J/ψ meson, the upsilon, and the W and Z bosons (see Figure 26). By July, new data on the top quark were being gathered.
On 8 July 2010, Italian physicist Tommaso Dorigo posted a blog entry reporting rumours that evidence for a light Higgs boson had been found at the Tevatron. The rumour spread rapidly around the internet and was picked up by the news media. It was almost immediately denied by Fermilab, in a ‘tweet’ which referred dismissively to ‘rumours spread by one fame-seeking blogger’.1 Dorigo subsequently sought to justify his rumour-mongering, arguing that ‘… keeping particle physics in the press with hints of possible discoveries that later die out is more important than speaking loud and clear once in ten years, when a groundbreaking discovery is actually really made, and keeping silent the rest of the time.’2
FIGURE 26 In the first few months of 7 TeV operation in 2010, both the ATLAS and CMS collaborations recorded candidate events for the entire spectrum of known Standard Model particles. This diagram from the CMS collaboration shows evidence for the J/ψ, the upsilon (Y, a meson formed from a bottom quark and its anti-particle) and the Z0, revealed through production of muon–anti-muon pairs carrying different energies.
Source: Copyright CERN, for the benefit of the CMS Collaboration.
Right or wrong, the rumours were symptomatic of the growing rivalry between Fermilab and CERN and the growing sense of expectation that something might be discovered soon. Lederman had earlier admitted that watching as CERN announced any future discovery would leave him with mixed feelings: ‘It would be a little like your mother-in-law driving off a cliff in your BMW,’ he said.3
Dorigo’s blog post had referred to rumours of ‘three-sigma’ evidence, a statistical measure reflecting the degree of confidence in the experimental data.* Three-sigma evidence would suggest a confidence level of 99.7% – in other words, a 0.3% chance that the data are in error. Although such confidence levels sound pretty convincing, to warrant declaration of a ‘discovery’, particle physicists actually demand five-sigma data, or confidence levels of 99.9999%.
Collision events leading to the production and decay of the Higgs boson were believed to be very rare, so building a five-sigma data set would require the recording of lots and lots of candidate collisions. The particle beam luminosity was therefore key. The higher the luminosity, the greater the number of collisions within a fixed period, and the greater the number of potential candidate collisions.* In fact, the integrated luminosity (the sum of the luminosity over time), is directly related to the number of candidate collisions.
The integrated luminosity is reported
in rather obscure units of inverse ‘barns’. Physicists measure the rates of nuclear reactions in the form of ‘cross-sections’, reported in units of square centimetres. The cross-section can be thought to represent the size of a hypothetical two-dimensional ‘window’ through which the reaction occurs. The larger the window, the more likely the reaction. The more likely the reaction, the faster it will occur. The reported cross-sections have atomic dimensions, typically some number multiplied by 10–24 cm2. The cross-sections for reactions involving atoms of uranium were found to be so large that one Manhattan project physicist quipped that they were as ‘big as a barn’. The barn was subsequently introduced as a unit. A cross-section reported as some number times 10–24 cm2 became some number of barns. A picobarn is a thousand billionth (10–12) of a barn, or 10–36 cm2. A femtobarn is a million billionth (10–15) of a barn, or 10–39 cm2.
At a CERN meeting in Evian, France, on 8 December 2010, Gianotti summarized the prospects for finding the Higgs and the nature of the race between the LHC and the Tevatron. Simple statistics suggested that even with an integrated luminosity building to 10 inverse femtobarns (10 times 1015 inverse barns, or 1040 cm–2) by the end of 2011, the Tevatron could do no better than report three-sigma evidence for the Higgs in certain restricted energy ranges. The more powerful LHC was in principle capable of generating three-sigma evidence with between 1 and 5 inverse femtobarns, depending on the Higgs mass.
On 17 January 2011, the US Department of Energy announced that it would not fund an extension to the Tevatron programme beyond the end of 2011. This decision did not signal the end of the race for the Higgs, but it did acknowledge the inevitable passing of custodianship of the frontier of high-energy physics from Fermilab to CERN.
The original operational plan for the LHC had included a prolonged shutdown in 2012, necessary to upgrade the proton beam energies to deliver the design collision energy of 14 TeV.* With the Higgs so tantalizingly close, in January 2011 CERN managers agreed to postpone the shutdown and continue to operate the LHC at a collision energy of 7 TeV through to December 2012. A potential upgrade to a collision energy of 8 TeV was judged to be too risky. Instead, ways to increase the beam luminosity would be implemented.
Higgs:The invention and discovery of the 'God Particle' Page 15
