Remarks on the Physics Case for the Linear Collider

1
Remarks on the Physics Case for the Linear
Collider

• John Womersley
• Fermi National Accelerator Laboratory, Batavia,
  Illinois
• http//www-d0.fnal.gov/womersle/womersle.html

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Preamble

• I have worked on hadron colliders for many years.
  I am a member of CMS and have played some role
  in setting up the US-CMS physics effort. My
  personal plan is to work on the Tevatron and then
  the LHC.
• Nonetheless, I believe a linear collider should
  be built.
• Why?
• Because we need to think in terms of a program
• A program is broader than one persons interests
• Not just discoveries, precision measurements too
• 10-3 level of SM
• The LC is needed because it is unlikely that the
  LHC will tell us enough about physics at the EW
  scale to make it desirable or even possible to
  jump ahead to the next energy scale
• The LC complements the capabilities of the LHC

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Higgs

• The linear collider will play the role for the
  Higgs that LEP and SLC did for the Z
• The Z was discovered at the SPS collider, but to
  convince ourselves that the EW Standard Model was
  correct required a whole program of precision
  measurements
• The LHC (if not the Tevatron) will discover the
  Higgs, but its ability to characterize it is
  limited
• In contrast, the LC can
• separate all decays including gg, cc, and even
  invisible
• Measure all couplings at few level
• In the case of SUSY Higgses, there are some well
  known gaps in the LHC coverage
• Charged H only visible in top decays for
  moderate tan ?
• Pseudoscalar higgs A not visible at all for
  moderate tan ?
• At the LC, one can see
• HH- ? ttbb and HA ? 4b up to the kinematic limit

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SUSY

• At the LHC, the problem is not to discover SUSY
  but to separate all the states and elucidate the
  mass spectrum. In addition, charginos/neutralinos
  and sleptons are hard to see except in cascade
  decays.
• At the LC
• Can tune energy to avoid producing all together
• Can see sleptons and charginos/neutralinos
  cleanly
• Can separate L and R sleptons
• Can measure spin of sleptons and
  charginos/neutralinos
• Can measure lepton flavor of sleptons
• Can understand stop mixing (important for Higgs
  sector)
• Can measure mass of lightest neutralino
• I am sure we will need both LHC and LC data to
  understand SUSY and SUSY breaking
• If you are disinclined to believe in EW scale
  SUSY, consider this
• New particles at LHC are almost certain to be
  interpreted in this framework
• LC results are able to actually verify if it is
  SUSY or not

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Worst cases

• What if LHC sees strong WW scattering and nothing
  else
• LHC will see cross section increase in WW final
  state
• Linear Collider can probe WW masses far beyond
  ?s, measure real and imaginary parts of form
  factor
• LC can explore other final states hard to see at
  LHC
• WW ? tt, WW ? ZZ
• What if LHC sees nothing except a light SM-like
  Higgs
• Maybe an inverted hierarchy SUSY model
• but how can we tell?
• I cant imagine going to the funding agencies to
  ask for a higher energy machine without at least
  some evidence that there are additional particles
  within reach
• Such evidence can only come from LC measurements
  of the Higgs
• Worst cases are just that a bad outcome for all
• The LC makes a bad outcome less bad by providing
  additional information that we will need in order
  to choose the next steps

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A call to arms

• What I have said so far has been addressed more
  to skeptics than supporters
• Finally I would like to say something to the
  enthusiasts, also based on my experience
• To make a project like this into a reality is
  going to require that a large number of people
  commit a significant fraction of their careers to
  it
• I will have worked 20 years on the Tevatron by
  the end of Run 2
• I will have worked 10 years on CMS by the time it
  starts data taking (and 5 years on the SSC before
  that)
• No complaints, I did this because its fun
• But unless enough people show this kind of decade
  long commitment to the linear collider, all our
  meetings and declarations of consensus are not
  going to get us this machine

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Summary

• A linear collider complements and completes the
  physics goals of the LHC
• Both in the best cases and the worst cases
• It does not threaten the development of the next
  generation of hadron colliders
• In fact its results will help guide and motivate
  that development
• Just as LEP/SLC results do for us now
• By ensuring continued broad progress in the field
  it makes future colliders more likely
• A linear collider is part of a balanced
  international particle physics program and should
  be built