Colliders of the future: LHeC and FCC-he
In this decade, CERN is exploiting and upgrading the LHC – but not constructing “the next big machine”.
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For many years, one of the world’s most powerful scopes, as in a microscope, was the Hadron-Elektron Ring Anlage (HERA) particle accelerator in Germany. Where scopes bounce electromagnetic radiation – like visible light – off surfaces to reveal information hidden to the naked eye, accelerators reveal hidden information by bombarding the target with energetic particles. At HERA, those particles were electrons accelerated to 27.5 GeV. At this energy, the particles can probe a distance of a few hundredths of a femtometer (earlier called fermi) – 2.5 million times better than the 0.1 nanometers that atomic force microscopy can achieve (of course, they’re used for different purposes).
The electrons were then collided head on against protons accelerated to 920 GeV.
Unlike protons, electrons aren’t made up of smaller particles and are considered elementary. Moreover, protons are approx. 2,000-times heavier than electrons. As a result, the high-energy collision is more an electron scattering off of a proton, but the way it happens is that the electron imparts some energy to the proton before scattering off (this is imagined as an electron emitting some energy as a photon, which is then absorbed by the proton). This is called deep inelastic scattering: ‘deep’ for high-energy; ‘inelastic’ because the proton absorbs some energy.
One of the most famous deep-inelastic scattering experiments was conducted in 1968 at the Stanford Linear Accelerator Centre. Then, the perturbed protons were observed to ’emit’ other particles – essentially hitherto undetected constituent particles that escaped their proton formation and formed other kinds of composite particles. The constituent particles were initially dubbed partons but later found to be quarks, anti-quarks (the matter/anti-matter particles) and gluons (the force-particles that held the quarks/anti-quarks together).
HERA was shut in June 2007. Five years later, the plans for a successor at least 100-times more sensitive than HERA were presented – in the form of the Large Hadron-electron Collider (LHeC). As the name indicates, it is proposed to be built adjoining the Large Hadron Collider (LHC) complex at CERN by 2025 – a timeframe based on when the high-luminosity phase of the LHC is set to begin (2024).
On December 15, physicists working on the LHC had announced new results obtained from the collider – two in particular stood out. One was a cause for great, yet possibly premature, excitement: a hint of a yet unknown particle weighing around 747 GeV. The other was cause for a bit of dismay: quantum chromodynamics (QCD), the theory that deals with the physics of quarks, anti-quarks and gluons, seemed flawless across a swath of energies. Some physicists were hoping it wouldn’t be so (because its flawlessness has come at the cost of being unable to explain some discoveries, like dark matter). Over the next decade, the LHC will push the energy frontier further to see – among other things – if QCD ‘breaks’, becoming unable to explain a possible new phenomenon.
Against this background, the LHeC is being pitched as the machine that could be dedicated to examining this breakpoint and some other others like it, and in more detail than the LHC is equipped to. One helpful factor is that when electrons are one kind of particles participating in a collision, physicists don’t have to worry about how the energy will be distributed among constituent particles since electrons don’t have any. Hadron collisions, on the other hand, have to deal with quarks, anti-quarks and gluons, and are tougher to analyse.
So, to accomplish this, the team behind the LHeC is considering installing a pill-shaped machine called the energy recovery linac (ERL), straddling the LHC ring (shown above), to produce a beam of electrons that’d then take on the accelerated protons from the main LHC ring – making up the ‘linac-ring LHeC’ design. A first suggestion to install the LHeC as a ring, to accelerate electrons, along the LHC ring was rejected because it would hamper experiments during construction. Anyway, the electrons will be accelerated to 60 GeV while the protons, to 7,000 GeV. The total wall-plug power to the ERL is being capped at 100 MW.
The ERL has a slightly different acceleration mechanism from the LHC, and doesn’t simply accelerate particles continuously around a ring. First, the electrons are accelerated through a radio-frequency field in a linear accelerator (linac – the straight section of the ERL) and then fed into a circular channel, crisscrossed by magnetic fields, curving into the rear end of the linac. The length of the circular channel is such that by the time the electrons travel along it, their phase has shifted by 180º (i.e. if their spin was oriented “up” at one end, it’d have become flipped to “down” by the time they reached the other). And when the out-of-phase electrons reenter the channel, they decelerate. Their kinetic energy is lost to the RF field, which intensifies and so provides a bigger kick to the new batch of particles being injected to the linac at just that moment. This way, the linac recovers the kinetic energy from each circulation.
Such a mechanism is employed at all because the amount of energy lost in a form called synchrotron radiation increases drastically as the particle’s mass gets lower – when accelerated radially using bending magnetic fields.
Keeping in mind the need to explore new areas of physics – especially those associated with leptons (elementary particles of which electrons are a kind) and quarks/gluons (described by QCD) – the energy of the electrons coming out of the ERL is currently planned to be 60 GeV. They will be collided with accelerated protons by positioning the ERL tangential to the LHC ring. And at the moment of the collision, CERN’s scientists hope that they will be able to use the LHeC to study:
- Predicted unification of the electromagnetic and weak forces (into an electroweak force): The electromagnetic force of nature is mediated by the particles called photons while the weak force, by particles called W and Z bosons. Whether the scientists will observe the unification of these forces, as some theories predict, is dependent on the quality of electron-proton collisions. Specifically, if the square of the momentum transferred between the particles can reach up to 8-9 TeV, the collider will have created an environment in which physicists will be able to probe for signs of an electroweak force at play.
- Gluon saturation: To quote from an interview given by theoretical physicist Raju Venugopalan in January 2013: “We all know the nuclei are made of protons and neutrons, and those are each made of quarks and gluons. There were hints in data from the HERA collider in Germany and other experiments that the number of gluons increases dramatically as you accelerate particles to high energy. Nuclear physics theorists predicted that the ions accelerated to near the speed of light at the [Relativistic Heavy Ion Collider] would reach an upper limit of gluon concentration – a state of gluon saturation we call colour glass condensate.”
- Higgs bosons: On July 4, 2012, Fabiola Gianotti, soon to be the next DG of CERN but then the spokesperson of the ATLAS experiment at the LHC, declared that physicists had found a Higgs boson. Widespread celebrations followed – while a technical nitpick remained: physicists only knew the particle resembled a Higgs boson and might not have been the real thing itself. Then, in March 2013, the particle was most likely identified as being a Higgs boson. And even then, one box remained to be checked: that it was the Higgs boson, not one of many kinds. For that, physicists have been waiting for more results from the upgraded LHC. But a machine like the LHeC would be able to produce a “few thousand” Higgs bosons a year, enabling physicists to study the elusive particle in more detail, confirm more of its properties – or, more excitingly, find that that’s not the case – and look for higher-energy versions of it.
A 2012 paper detailing the concept also notes that should the LHC find that signs of ‘new physics’ could exist beyond the default energy levels of the LHeC, scientists are bearing in mind the need for the electrons to be accelerated by the ERL to up to 140 GeV.
The unique opportunity presented by an electron-proton collider working in tandem with the LHC goes beyond the mammoth energies to a property called luminosity as well. It’s measured in inverse femtobarn per second, denoting the number of events occurring per 10-39 squared centimetres per second. For example, 10 fb-1 denotes 10 events occurring per 10-39 sq. cm s-1 – that’s 1040 events per sq. cm per second (The luminosity over a specific period of time, i.e. without the ‘per seconds’ in the unit, is called the integrated luminosity). At the LHeC, a luminosity of 1033 cm-2 s-1 is expected to be achieved and physicists hope that with some tweaks, it can be hiked by yet another order of magnitude. To compare: this is 100x what HERA achieved, providing an unprecedented scale at which to explore the effects of deep inelastic scattering, and 10x the LHC’s current luminosity.
It’s also 100x lower than that of the HL-LHC, which is the configuration of the LHC with which the ERL will be operating to make up the LHeC. And the LHeC’s lifetime will be the planned lifetime of the LHC, till the 2030s, about a decade. In the same period, if all goes well, a Chinese behemoth will have taken shape: the Circular Electron-Positron Collider (CEPC), with a circumference 2x that of the LHC. In its proton-proton collision configuration – paralleling the LHC’s – China claims it will reach energies of 70,000 GeV (as against the LHC’s current 14,000 GeV) and luminosity comparable to the HL-LHC. And when its electron-positron collision configuration, which the LHeC will be able to mimic, will be at its best, physicists reckon the CEPC will be able to produce 100,000 Higgs bosons a year.
As it happens, some groups at CERN are already drawing up plans, due to be presented in 2018, for a machine dwarfing even the CEPC. Meet the Future Circular Collider (FCC), by one account the “ultimate precision-physics machine” (and funnily named by another). To be fair, the FCC has been under consideration since about 2013 and independent of the CEPC. However, in sheer size, the FCC could swallow the CEPC – with an 80-100 km-long ring. It will also be able to accelerate protons to 50,000 GeV (by 2040), attain luminosities of 1035 cm-2 s-1, continue to work with the ERL, function as an electron-positron collider (video), and look for particles weighing up to 25,000 GeV (currently the heaviest known fundamental particle is the top quark, weighing 169-173 GeV).
And should it be approved and come online in the second half of the 2030s, there’s a good chance the world will be a different place, too: not just the CEPC – there will be (or will have been?) either the International Linear Collider (ILC) and Compact Linear Collider (CLIC) as well. ‘Either’ because they’re both linear accelerators with similar physical dimensions and planning to collide electrons with positrons, their antiparticles, to study QCD, the Higgs field and the prospects of higher dimensions, so only one of them might get built. And they will require a decade to be built, coming online in the late 2020s. The biggest difference between them is that the ILC will be able to reach collision energies of 1,000 GeV while the CLIC (whose idea was conceived at CERN), of 3,000 GeV.
FCC-he = proton-electron collision mode; FCC-hh = proton-proton collision mode; SppC = CEPC’s proton-proton collision mode.
