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.