LISTSERV 16.5 - SNOWMASS-EF Archives

Energy Frontier. The mysteries of the newly discovered Higgs boson were a major theme at Snowmass.  The properties of the Higgs boson raise crucial questions that guide large parts of the future particle physics program. These questions call for a three-pronged research program at high energy accelerators:  first, to search for new particles with TeV masses predicted by models of electroweak symmetry breaking; second, to make precise measurements of the heavy particles $W$, $Z$, and the top quark, which can carry the imprint of the Higgs; and, third, to measure the properties of the Higgs boson itself to very high precision.  Questions about the Higgs boson also inspire the search for the dark matter particles and for flavor-changing rare decays, since in both cases, the motivating theory often comes from models of the Higgs and its role in symmetry-breaking.

For at least the next fifteen years, the experiments at the Large Hadron Collider at CERN will drive the energy frontier program forward. Especially in its high-luminosity phase the LHC is expected to explore deeply for new particles produced through either the strong or the electroweak interactions.  The LHC will study rare decays using a sample of billions of top quarks and probe for new dynamics of $W$, $Z$, and Higgs at TeV energies.  It will measure Higgs boson couplings at the few-percent level and provide the first measurement of the Higgs self-coupling.  The LHC experiments have already proven their ability to work as global collaborations.  Technology, insights, and leadership from the US have played indispensible roles in these experiments.

There is strong scientific motivation for continuing this program with lepton colliders. Experiments at lepton colliders allow searches for new particles with unequivocal discovery or exclusion, complementing those at the LHC. They can improve the precision of our knowledge of the $W$, $Z$, and top properties by an order of magnitude, potentially bringing these measurements into confrontation with theory. They can reach sub-percent precision in the Higgs boson properties in a unique, model-independent way, allowing discoveries of percent-level deviations predicted in theoretical models. A global effort has now completed the technical design of the International Linear Collider (ILC), an accelerator that will provide these capabilities.  The Japanese high energy physics community has named this facility as its first priority.

The Snowmass study considered many other options for high-energy colliders that might be realized over a longer term.  These included linear and circular e+e- colliders, muon colliders, and photon colliders.  A longer term option with great promise is a 100 TeV hadron collider, which has unprecedented potential reach for new physics associated with electroweak symmetry breaking, naturalness, and dark matter. Further investigations of the physics and technical issues would be opportune at this time, leading to conceptual and technical design reports.

There is unanimous agreement that maintaining US leadership and continuing experience in experiment design and construction --- especially in accelerator R&D and construction --- is critical to achieving our universal particle physics' scientific goals. To this end a balanced program of construction in the next decade and R&D towards new instrumentation and accelerator technologies targeting the decade after is every bit as important as our pure science goals.

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Chip