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Where are we going: Charting the Course for Elementary Particle Physics February 19, 2007

Posted by apetrov in Particle Physics, Physics, Science.

There is a very interesting paper in tomorrow’s mailings of hep-ex physics preprint archive (or as it is known, ArXive). It is a talk, “Charting the Course for Elementary Particle Physics,” that Burton Richter, Nobel Prize laureate and former director of Stanford Linear Accelerator Center (SLAC) gave at the San Franciso’s meeting of American Association for Advancement of Science (AAAS). It can be found here (there is a link to a pdf file).

This is a very compact and nice presentation of what we are doing in particle and astroparticle physics with absolutely no formulas – an excellent easy reading piece. He talks about many things: the LHC and its run plan, International Linear Collider (ILC), noting that it can be built as early as 2019 (i.e 10 years after the first physics results from the LHC) and its price tag of $8 Billion (to compare: cost of building of a new aircraft carrier is $8-$14B, cost of building the Golden Gate Bridge is $35M in 1930’s dollars — approximately $1.2B today), upcoming astrophysics and neutrino experiments.

One of his statements has caught my eye. He stated that if LHC works well and finds nothing by 2012, the ILC will not be funded. This is the point that might need some clarification. First of all, something must happen at the TeV scale. One of the reasons for it is the fact that scattering of longitudinal W-bosons violates unitarity at the center-of-mass energy of around 1 TeV if only experimentally-established particles, i.e. W-, Z-bosons and photons are taken into account. What does it mean? Well, loosely speaking, in order for a theory to conserve probability, any scattering cross section for any particles cannot grow as a power of energy. Unfortunately, scattering of longitudinally-polarized W’s does, it grows asenergy squared. This is fixed in the Standard Model by addition of a Higgs boson, which besides providing the nice parametrization of how particles acquire mass, also provides a fix for the unitarity problem by canceling the dangerous growth of that cross-section. But this is the simplest mechanism for how this cancellation happens, there are many more models that do the same job, withHiggs bosons and without. So, regardless of what exactly happens at 1 TeV, something must be there and LHC is bound to find something. But what if the Nature prepared for us the worst-case scenario? That is, LHC will find the Standard Model Higgs and nothing else, no SUSY particles, no Technicolor goldstones, no extra dimensions… What then? Would you build the ILC then?

I’ve been asking this question to many people. And the answer that I get is the one I’d probably give myself — in this case theILC would be the best machine to study much higher energy scales by carefully studying Higgs’ dynamics, kinda like current-day B-factories are looking for indirect effects of New Physics in decays and mixings of heavy flavors. But would it then be better to build a muon collider (muon’s couplings to Higgs are much larger than electron’s), even given its all technical problems today? I don’t know…


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