Antihydrogen Spectroscopy

1
Antihydrogen Spectroscopy

• David Christian
• Fermilab
• November 17, 2007

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CPT

• The atomic spectrum of hydrogen is extremely well
  measured and well understood theoretically.
• Measurements of the antihydrogen spectrum could
  potentially provide exceedingly stringent tests
  of CPT invariance.

3
Cold antihydrogen

• Cold antihydrogen has been formed in Penning
  traps at CERN.
• However, the atoms are formed in highly excited
  (n 65) Rydberg states.
• To date, efforts to form cold antihydrogen in
  lower level states have been unsuccessful, and no
  plausible scheme yet exists for driving
  transitions to the ground state and trapping the
  atoms.

4
Hot antihydrogen

• An atomic beam of antihydrogen was formed at
  Fermilab in an experiment done using the
  Antiproton Accumulator.
• The formation reaction occurs when a positron,
  created as a member of an ee- pair by a beam
  antiproton in the Coulomb field of a target
  nucleus, is captured by the beam antiproton.
• The momentum transfer in the reaction is small
  (mec) so a neutral beam is formed.
• Most of the atoms are formed in the ground state.

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Fermilab E-862 (1997)
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Antihydrogen Spectroscopy

• Use selective field ionization to detect
  antihydrogen atom AND extract information about
  its atomic state.
• Selectively excite N2 levels using laser light
  (or collisions in a thin foil).
• Prepare a pure Stark state (in a magnet)
  propagate in vacuum enter 2nd magnet
  (regenerating calculable mix of Stark states)
  measure mix with position distribution of field
  ionization.
• Monte Carlo --gt an experiment in which 100 atoms
  exit the 1st magnet in N2,L will yield a 1
  measurement of the fine structure and a 5
  measurement of the Lamb shift. Assuming that
  only the 2S level is shifted by a CPT violating
  force, the 1? sensitivity is 50 parts per billion
  of the 2S binding energy.

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Formation cross section

• The formation cross section is small (1pb for
  formation on H at 5.7 GeV/c).
• The cross section increases slowly with
  antiproton energy and depends on the square of
  the target nucleus charge.
• Estimate of formation on Xenon at 8 GeV KE (AA
  injection energy) is 4.4nb.

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Rate

• Assuming a stacking rate of 3E11/hr
• Stack for 1 hour, collect data for 1 hr.
• Ignore cooling during data collection.
• Set the target density so that the beam lifetime
  is 30 minutes.
• Integrated luminosity in 1 hour 1.8 nb-1.
• Implies 8 antihydrogen atoms per 2 hours.
• 10 cycles per day implies 80 antihydrogen per
  day.
• Even if only 2 can be excited to (N2,L), the
  experiment requires only 2 months.

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Interested?

• So far, interest has been expressed by
• Mark Mandelkern (UCI)
• Adrian Melissinos (UofR)
• Mario Macri (Genova)
• Please contact me (dcc_at_fnal.gov) if you are
  interested.