1. Antihydrogen Spectroscopy David Christian Fermilab November 17, 2007

2. 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 e+e- 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 (~m e c) so a “neutral beam” is formed. Most of the atoms are formed in the ground state.

5. Fermilab E-862 (1997)

6. Antihydrogen Spectroscopy Use selective field ionization to detect antihydrogen atom AND extract information about its atomic state. Selectively excite N=2 levels using laser light (or collisions in a thin foil). Prepare a pure Stark state (in a magnet); propagate in vacuum; enter 2 nd magnet (regenerating calculable mix of Stark states); measure mix with position distribution of field ionization. Monte Carlo --> an experiment in which 100 atoms exit the 1 st magnet in N=2,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.

7. 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.

8. 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 (N=2,L), the experiment requires only ~ 2 months.

9. Interested? So far, interest has been expressed by –Mark Mandelkern (UCI) –Adrian Melissinos (UofR) –Mario Macri (Genova) Please contact me (dcc@fnal.gov) if you are interested.dcc@fnal.gov