Chapter IX High-Energy and Nuclear Physics
IX.1 Leading up to the LHC
This is a good moment to take advantage of the five years that separate our first and second editions to see how physics has succeeded in stimulating the development of new accelerator projects. In Chapter VI of our first edition we described the situation at that time. Until then, the needs of physics had prompted the construction of large proton - proton colliders such as the ISR followed by proton-antiproton colliders (at CERN and Tevatron at Fermilab). Physicists were eager to establish the precise properties of the new particles (W and Z) uncovered at the SPpbarS. .
In the history of high energy particle research, discoveries with hadron colliders, such as SPpbarS and the Tevatron, are usually followed by the construction of electron-positron colliders. When two hadrons collide, each contains three quarks. To put it simply, only one quark in each hadron is involved in the collision. Together, the two colliding quarks only carry about 10 per cent of the kinetic energy of the hadrons. In addition there is a certain amount of ambiguity: the colliding objects could be either up or down quarks. Electrons and positrons, on the other hand, though harder to accelerate to high energy, are not composite objects. They carry the whole available kinetic energy to the collision and are unambiguous in nature. It therefore seemed logical to complement SPpbarS with the large positron-electron collider LEP. Although this was limited to and beam energy of just above 100 GeV it was equivalent to a hadron collider of roughly ten times this energy. The need for a lepton collider had been foreseen well in advance at CERN and LEP was already waiting in the wings to continue the research once SPpbarS had finished its work.
Between them, this generation of colliders had established what is perhaps the most important major advance in physics during the last 50 years: the theoretical framework called “The Standard Model,” (See sidebar on the Standard Model.)
The next piece of theory that needed to be tested to complement the Standard Model and explain the mystery of the masses of its particles predicted yet another particle: the “Higgs”. The Higgs was predicted to be massive and outside the range of existing colliders (See sidebar on the Search for the Higgs.)
The US response to the discovery of the W and Z at CERN had been to start design and construction of the SSC, a proton-proton collider 20 times more powerful than the Tevatron but this had proved too costly even for the US science budget and was terminated in mid-construction. We were left with the “Large Hadron Collider” or LHC as the way forward.
IX.2 The Large Hadron Collider (LHC)
The sheer size and cost of this enterprise was to be without parallel and had it been built on a “green field site” it is likely that it would have shared the same fate as the SSC. LEP was shut down and removed from its tunnel on the CERN site to make room for the new collider. The LHC could be fed by the chain of accelerators that already existed on the site. Nevertheless it is an ambitious machine for, in order to reach 7 TeV in the 27km LEP tunnel, the magnetic field had to be close to 9 Tesla. This is five times the field at which a normal magnet saturates and more than 50% higher than that for which previous accelerators such as the SSC had been designed. Only superconducting magnets can provide this field (See sidebar for Superconducting Magnets in Chapter V).
LHC is a conventional two ring interlaced proton-proton collider like the ISR, but rather than two separate magnet rings and their cryostats it uses the more ingenious solution of putting both beam pipes in the same magnet and cryostat. The magnet cross section of the 7 TeV hadron collider is shown in Fig 9.2. The proton beams have the same polarity but go around in opposite directions. Thus, although they share a magnetic yoke, they circulate in separate pipes, guided by separate superconducting coils, providing a vertical magnetic field of opposite sign in the two pipes. The construction project was led by Lyn Evans (See sidebar for Evans) and the repair, exploitation and operation of the machine by Steve Myers (See sidebar for Myers).
The LHC started up in spectacular fashion on 10 September 2008. With the confidence of many successful projects behind them, the CERN team had decided to press the button to the machine under the eyes of the world's media. Although it was a common practice for the launch of space vehicles, this was a "first" in the world of particle physics. The ensuing public relations event, second to none in the world of particle physics, had been prepared in the days before in broadcasts by the world’s most eloquent journalists. As the start-up approached TV viewers could not claim to be ignorant of the aims of the project or of its importance for our understanding of the origins of the universe. Every available expert in particle physics was wheeled out to face cameras. Some, less expert than others and possessing only “that little knowledge that is a dangerous thing”, predicted that LHC collisions might trigger a black hole to swallow up the planet. Though CERN was quick to reassure everyone that this could not happen, the uncertainty served only to intensify the interest and to ensure that a huge audience was perched on the edge of their chairs for the countdown.
Throughout the day at CERN, regular live action from the control room was broadcast by TV channels spanning the globe. To everyone’s relief the first beam made the full 27km journey around the LHC, travelling in a clockwise direction, on schedule, and to be followed just after breakfast by its opposing counterpart. The beam intensity was very low and the energy just that of injection, but the first step was over. It seemed to be a simple matter of increasing the injected intensity and putting more current into the magnet system before the machine would produce its first results. But in the cheers and applause that followed few could have guessed that it would still take many long and expensive months before high energies were collided and new particles discovered.
All was well for a few days and then tests were made to raise the current in the superconducting magnets. The magnets had all of course been tested, and they were protected in case slight warming of a magnet would start to change its superconducting coils back into normal resistive windings. If such a thing happened, the thousands of Amps of current flowing in the coils and which produced the magnetic field would cause a catastrophic heating of the now resistive windings. To stop this happening, the magnet’s current would be diverted with diodes into an external resistor. Here the heating could be handled safely at the slightest hint of a magnet warming up. The weak link in the circuit was the place where the coils of one magnet were connected to the next with a kind of soldered lap-joint. The soldering had to be done within the cryostat as magnets were put in the ring. Unfortunately, some of these joints proved less than perfect and as the current was raised one of these became normally conducting. At this point the current could not be diverted to an external load and went on to produce an intense heat source warming neighboring magnets and causing the liquid helium in which the magnets were bathed to boil. So much helium gas was evolved that the panels designed to blow out when such an event happened were unable to deal with the gas flow. A shock wave of gas pressure surged through about half a kilometer of enclosed cryostat and like a tsunami swept the delicate tube lining the beam pipe before it literally blowing many magnets off their supports.
Opening up a magnet ring enclosed in its cryostat is itself a lengthy procedure. The remaining liquid helium has to be allowed to evaporate and the gas, either stored in huge reservoirs, or re-liquefied so that it can be transported away from the site. Cutting open the cylindrical cryostat with its connecting pipe work to expose and remove magnet units is a delicate procedure. In the months that followed more and more of the ring was investigated. It became clear that it would take the best part of a year to repair and reassemble the LHC together with better joints, larger escape panels and more precise monitoring equipment to detect every slight hiccup in the temperature around the ring. Just slightly over a year later, and after what the reader can imagine a long series of careful testing procedures, the restart finally got underway with the injection of both beams into the LHC on Friday 20November 2009. Again, as on the famous start-up day in September 2008, beam was gingerly threaded around the ring in both directions. The ATLAS detector was first, with a collision event recorded at around 2.22p.m. on 27th November. The other detectors were quick to follow suit.
At first the LHC operated as a storage ring and as a collider, but at beam energy of only 450GeV – the injection energy from the SPS. An important next step was to begin tests to ramp the current and hence the fields in all magnets in synchronism with increasing beam energy (supplied by the RF) and on 29November the LHC accelerated Beam1 from 450GeV to 1.04TeV. This exceeded the previous world-record beam energy of 0.98TeV, which had been held by Fermilab's Tevatron collider since 2001.
During the next three years the LHC team operated a “once bitten twice shy” approach and for a long time limited the circulating beams to a small number of particles which, if lost, could not provoke another thermal catastrophe. It was also decided that the full 7 TeV excitation current would have to wait until after a yearlong shut down in 2013 to replace all of the delicate interconnections with a more robust solution. The current was raised to about half the peak design value in cautions steps – all the time the joints were monitored electrically to stave off the slightest indication of an impending quench. Meanwhile the thousands of researchers made do with a gradually increasing luminosity and energy. At first they had only enough events to make interesting routine observations but in the last year (2011) LHC has reached an intensity where one might dare to claim to have seen the shadow of a Higgs. Now at last, as we write, the greatest of all accelerator projects to begin to fulfill its promise and to feel that we stand on the threshold of a decade or two of important discoveries.
The LHC is not only used to study elementary particle physics, but also to study nuclear matter under extreme conditions (such as shortly after the Big Bang). To this end, a significant amount of time on the LHC is devoted to the collisions of heavy ions with heavy ions. Of course such studies have been carried out at lower energies and one can find a discussion of this in Chapter VI.
IX.3 The Origins of Linear Colliders
Both LEP and SLC were electron-positron colliders: the result of a swing of the pendulum from protons and antiprotons to leptons. The pendulum’s swing followed the success of the antiproton-proton collider that had first created the W and Z bosons. Once the euphoria of the discovery was over there was a need to collide electrons and positrons for more detailed studies. We explained in Chapter VI that leptons, being point-like particles, are ideal for pinning down the precise properties of the newly discovered bosons. Hadrons, such as protons and antiprotons, though easier than electrons to accelerate to high energy, are rather blunt instruments of science. They are bundles of three quarks of different kinds, and their collisions may be thought of as encounters between two of the six quarks involved. This is not only an ambiguous situation when it comes to interpreting the results but provides a much lower energy of effective collision than one might think for production of new massive particles. Only about ten percent of that carried by proton and antiprotons is available when the two quarks collide. Neither of these problems arises for colliding leptons. Consequently, the lower energy of lepton colliders is not as serious a problem as it at first seems. In fact two of LEP’s 100 GeV leptons were as effective as a proton and antiproton colliding in the Tevatron at close to 1 TeV.
Inevitably, once LEP had completed its precision work, the pendulum had to swing back again as the brute force of a hadron collider of even higher energy was once more needed to search for a new and heavier particle: the Higgs —the key to explaining the strange variety of masses among fundamental particles.
Of course the pendulum analogy is an illusion, and such decisions lie in the hands of the world community of particle-physicists and accelerator-builders — not to mention the governments of the many states who must band together to pay for the next step. The community is diverse and many interests come into play. Research and development programs investigating new methods spawn workshops and international meetings to recruit the worldwide interest of accelerator scientists to their cause. The International Committee for Future Accelerators (ICFA) considers these various interests.
Naturally, there is a tendency for the continent that failed to host the last large machine to expect the next in order to sustain its own community of experts, and bring the research to be done by its PhD students closer to home. For example, it is strongly argued that the US should make a bid to site the next linear collider to maintain high energy physics in the USA now that the Tevatron and the B-Factory have been closed. Laboratories that have acquired experience in one particular kind of accelerator or collider — whether electron or hadron, linear or circular — hope that their expertise is used. The problems of hosting a truly international laboratory with ease of access by nationals of all member states, and with a future protected from unilateral budgetary decisions among these states, must also be addressed.