The announcement of the discovery at the LHC in July of “that new boson” at approximately 125 GeV has everybody excited (and I’m sure more than a few quite relieved). As a humble accelerator builder I stood in wonder as I observed the world media event that followed. Following my particle physics colleagues to Cracow for the recent European Strategy Preparation Group open symposium, I was also caught up in the excitement of this process; compared to its predecessor five years ago in Orsay, this meeting was focused, positive and lively: the LHC was here! And that new boson does look awfully like the H thing.
The linear collider physics community is at least as invigorated, and possibly more so. This piece of the jigsaw adds real weight to the physics case for the ILC which is an excellent precision tool for pulling apart the intricacies of this new particle, its quantum numbers and its couplings.
Perhaps inevitably then, there is suddenly a lot of attention being paid to the concept of a Higgs Factory. Indeed accelerator physicists the world over seem to be opening that bottom drawer and pulling out designs for just such a machine based on all sorts of concepts: storage rings that fit in the LHC tunnel; storage rings that sit in a very big tunnel; all variants of gamma-gamma colliders; and not forgetting the muon collider.
What does this all mean for the ILC, and in particular for the Technical Design Report (TDR) which is close to completion?
The GDE’s mandate was to produce a cost-effective and mature design for an electron-positron linear collider in the centre-of-mass energy range from 200 to 500 GeV, with a possible upgrade to 1 TeV. Understandably, the primary focus of the design and costing work has been on the more demanding scope of the initial 500-GeV centre-of-mass machine, for which we believe we have a realistic and defendable design, with a reliable cost estimate and few remaining technical issues. As per the mandate, however, running at low centre-of-mass energies is included in the baseline: at 250 GeV centre-of-mass energy the TDR baseline parameter tables quote a luminosity of approximately 7×1033 cm-2s-1 (as opposed to 1.8 x 1034 cm-2s-1 at 500 GeV). To turn the beam energy down, one simple turns down the voltage in the linacs.
But much of the discussion in the physics community appears to be on a possible 250-GeV machine as a first stage, or indeed as a standalone “Light Higgs Factory”. This is a slightly different question to the one the GDE has answered. There is fundamentally no reason why we cannot construct a machine with half the length of linac – the technology is the same, and there is no difference to the other accelerator subsystems such as damping rings. There are certainly “beyond current baseline” concepts that could be considered, such as an increase of the luminosity by a factor of two by increasing the number of bunches per pulse (a scenario already included in the TDR baseline as a possible luminosity upgrade). Or indeed one could revisit the parameter space in the hope of optimising the “Higgs physics”. But I expect that these things would be small perturbations to the overall design and cost of the machine. The only real questions are “how much money do you save?” “what’s the reduction in construction time?” and — probably most important – “is it worth just to consider a light Higgs factory?” While I leave the last question to my particle physics colleagues, the GDE can at least make some ball-park estimates on the first two questions, although a thorough design study at this point is beyond our means, and will need to be left to the post-TDR organisation.
Based on some rather simplistic scaling, the cost of a dedicated 250-GeV machine would be ~70% of the cost of the 500-GeV machine. This may seem surprising until you realise that only about 60% of the total baseline cost is actually the linacs; the remaining 40% is for the sources, damping ring, beam delivery system and IR hall. A first look at the construction schedule also shows a relatively modest saving, possibly 12 to 18 months. This is because the main tunnel is constructed in parallel segments, so reducing the length saves little time. In addition, the central region and detector halls, sized for two detectors in a push-pull arrangement, drive the schedule and the detectors themselves need many years for construction.
However, much of the discussion within the GDE is not focused on a limited 250-GeV centre-of-mass machine, but more on this energy as a first stage of the 500-GeV project. The preferred scenario is to construct the tunnels and infrastructure required for the entire 500-GeV baseline machine, but to initially only install half the linacs. This scenario comes in at about 75% of the TDR baseline cost. Increasing the centre-of-mass energy up to the maximum 500 GeV is then relatively straightforward, by simply adding more cryomodules, RF and cryogenics to the tunnel. Such an approach lends itself to a rather more adiabatic upgrade over several years. Furthermore, we can imagine scenarios where after the initial construction cryomodule production would ramp down to a lower rate rather than stopping altogether, thus avoiding the issue of shutting down production lines, only to having to start them up again at a later date.
There are still may details that need to be considered in such a staged approach, and the relative cost figures are likely to change slightly as further technical studies are made. Nonetheless they can be taken as a very good indication of the scope.
Finally, a comment on the final question that I left to my physics colleagues: “is it worth it just to consider a light Higgs factory?” My personal opinion is that it makes little sense to discuss an ILC just as a Higgs factory, since one of the most attractive features of a linear machine is its inherent extendibility in energy, compared to the “dead end” inherent in a storage ring solution. Indeed it was primarily this reason and the associated cost scaling that lead to the suggestion of a linear collider in the first place. Whether we start at 250 GeV or 500 GeV, the same mature technology is available and will work at either energy. We can — and probably should — start at 250 GeV, but there is a strong case all the way up to ~500 GeV. And of course we don’t need to stop there: if it looks like there is interesting physics above 500 GeV, we can just make it longer — at least until the space runs out …