The Expeditious Construction of a Linear Collider

"The expeditious construction of a linear collider as the next major international High Energy Physics project" is the strong recommendation of the "physics issues" working groups at the huge meeting of particle physicists held June 30-July 21 in Snowmass, Colorado. Almost 900 scientists, including many from Europe and Asia, were present. Here are some particulars:

Particle physics views matter at the smallest possible scale (in some experiments down to a billionth of a billionth of a meter), and this often requires producing particle beams at the highest possible energies, or the shortest possible wavelength if one views an accelerator as a kind of microscope. And this tends to be expensive. Consequently numerous speakers at the meeting urged the international high-energy community to establish consensus not merely on a particular accelerator project but on a whole longterm program for making fundamental discoveries.

• The physics goals of such a program would include, first of all, a search for the Higgs boson (the particle manifestation of the ubiquitous field which, according to the standard model, endows many particles with mass); a search for super particles (a family of particles called forth by the theory of supersymmetry, according to which all known fermion particles have boson counterparts and vice versa); and a detailed study of quark and lepton properties such as mass, flavor changing (the transformation from one type to another), or CP violation.
  
  
• In extolling the virtues of constructing a new electron-positron linear collider (LC) several speakers said that such a machine would serve as a worthy complement to the Large Hadron Collider (LHC) now under construction at CERN. For example, Edward Witten (Institute for Advanced Study) drew an analogy between, on the one hand, a high-energy proton-antiproton machine like the Tevatron (where the top quark was discovered) and a high-precision electron-positron machine like LEP (where Z-boson decay modes could be carefully measured, providing information about the electroweak force, a force too subtle to be measured, as can gravity, with a torsion balance) and, on the other hand, the LHC (the high-energy proton-proton collider, now under construction at CERN, where the Higgs should be discovered, if the Tevatron does not accomplish the feat in the next few years) and the LC where, presumably, the decay modes of the Higgs would be explored in detail amid electron-positron collisions. Further components of a longterm building program might include a very large hadron collider (VLHC), a muon storage ring, or a neutrino factory.

What are the chances of finding the desired particles? David Gross (UC Santa Barbara) said that at an LC with a collision energy of 500 GeV the case for producing super particles was compelling, and the case for finding the Higgs would be very compelling.
---Determining a site for a possible LC was not on the agenda, but three detailed design proposals were showcased: TESLA, a German machine; the Japan Linear Collider (JLC); and a US project, the Next Linear Collider (NLC).

• On a sobering note, Michael Holland of the Office of Management and Budget (OMB) argued that in order to make the case to fund a new machine particle physicists would have to demonstrate that the device was important not only to their research interests but would be important for science as a whole. Luciano Maiani, Director General of CERN, and a speaker at the same panel, declared that he thought such a stringent criterion for federal support was "unfriendly to science," and an inhibition to what has come to be called "curiosity-driven" research. Also several audience members felt that certain defense and space-station research ventures were not being held to the same standard of self-justification.
  
  
• Scientists at the meeting felt that international cooperation in building a linear collider was essential. Research on LC components are proceeding at JLC and TESLA. In the US the next step involves the preparation of a report by the DOE/NSF High Energy Physics Advisory Panel (HEPAP) Subpanel on Long Range Planning for US High Energy Physics. Reports of the numerous working groups should be available on the Snowmass website soon.