Letter

A plasma-wakefield collider at the LHC?

Earlier this year Mourou et al. made futuristic proposals on studies that could be made of the vacuum using high-intensity lasers to drive compact plasma-wakefield accelerators (CERN Courier March 2009 p21). I would like to point out further possibilities that could be pursued at the LHC using plasma-wakefields.

Caldwell et al. showed recently (2009 Nat. Phys. in press; arXiv:0807.4599) that electrons could be accelerated to tera-electronvolt (TeV) energies in plasma-wakefields driven by TeV protons. The LHC could therefore be converted to an electron–electron collider, and thence to a γγ collider, via the process e^–e^– → e^–e^–γ^*γ^* → e^–e^–X. A similar strategy was followed at LEP for e^–e⁺ → e^–e⁺γ^*γ^* → e^–e⁺X, but the e^–e^– channel is preferable for generating γγ collisions, as it suffers from less background interference.

A γγ collider at the LHC could be used to follow up unexpected results that were obtained with LEP in γγ collisions. Events similar to the multi-muon events that were reported recently at Fermilab were seen beforehand at LEP in γγ collisions. Also the cross-section for hadron production was found at LEP to exceed the prediction of QCD by 10 times at the highest transverse momenta, and still to be growing (P Yock 2009 Int. J. Mod. Phys. A in press; arXiv:0903.0434). The cross-section for the latter puzzle provides a strong constraint on the charges of quarks (E Witten 1977 Nucl. Phys. B 120 189).

These legacies from LEP are important. The first questions the particle classification of the quark model and the second questions the charges of quarks, both pillars of the Standard Model. Such questions would assume more importance if the Higgs fails to materialize at Fermilab and the LHC.

Philip Yock, University of Auckland.