Jul 12, 2013
John Ellis, King’s College London and CERN.
The discovery of a Higgs boson by the ATLAS and CMS collaborations at the LHC has opened new perspectives on accelerator-based particle physics. While much else might well be discovered at the LHC as its energy and luminosity are increased, one item on the agenda of future accelerators is surely a Higgs factory capable of studying this new particle in as much detail as possible. Various options for such a facility are under active consideration and circular electron–positron (e+e–) colliders are now among them.
In a real sense, a Higgs factory already exists in the form of the LHC, which has already produced millions of Higgs bosons and could produce hundreds of millions more with the high-luminosity upgrade planned for the 2020s. However, the experimental conditions at the LHC restrict the range of Higgs decay modes that can be observed directly and measured accurately. For example, decays of the Higgs boson into charm quarks are unlikely to be measurable at the LHC. On the one hand, decays into gluons can be measured only indirectly via the rate of Higgs production by gluon–gluon collisions and it will be difficult to quantify accurately invisible Higgs decays at the LHC. On the other hand, the large statistics at the LHC will enable accurate measurements of distinctive subdominant Higgs decays such as those into photon pairs or ZZ*. The rare decay of the Higgs into muon pairs will also be accessible. The task for a Higgs factory will be to make measurements that complement or are even more precise than those possible with the LHC.
Cleaner experimental conditions are offered by e+e– collisions. Prominent among other contenders for a future Higgs factory are the design studies for a linear e+e– collider: the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). In addition to running at the centre-of-mass energy of 240 GeV that is desirable for Higgs production, these also offer prospects for higher-energy collisions, e.g. at the top–antitop threshold of 350 GeV and at 500 GeV or 1000 GeV in the case of the ILC, or even higher energies at CLIC. These would become particularly attractive options if future, higher-energy LHC running reveals additional new physics within their energy reach. High-energy e+e– collisions would also offer prospects for determining the triple-Higgs coupling, something that could be measured at the LHC only if it is operated at the highest possible luminosity.
There has recently been a resurgence of interest in the capabilities of circular e+e– colliders being used as Higgs factories following a suggestion by Alain Blondel and Frank Zimmermann in December 2011 (Blondel and Zimmermann 2011). It used to be thought that the Large Electron–Positron (LEP) collider would be the largest and highest-energy circular e+e– collider and that linear colliders would be more cost-efficient at higher energies. However, advances in accelerator technology since LEP was designed have challenged this view. In particular, the development of top-up injection at B factories and synchrotron radiation sources, as well as advances in superconducting RF and in beam-focusing techniques at interaction points, raise the possibility of achieving collision rates at each interaction point at a circular Higgs factory that could be more than two orders of magnitude larger than those achieved at LEP. Moreover, it would be possible to operate such a collider with as many as four interaction points simultaneously, as at LEP.
One attractive option would be to envisage a future circular e+e– collider as part of a future, very large collider complex. For example, a tunnel with a circumference of 80–100 km could also accommodate a proton–proton collider capable of collisions at 80–100 TeV in the centre of mass, which would also open up the option of very-high-energy electron–proton collisions. This could be an appealing vision for accelerator particle physics at the energy frontier for much of the 21st century. Such a complex would fit naturally into the updated European Strategy for Particle Physics, which has recently been approved.”
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