INTRODUCTION
The GammeV experiment is a gamma to milli-eV particle search using a “light shining through a wall” technique.
MOTIVATION
The milli-eV mass scale may be relevant to the energy frontier, neutrino properties, and astroparticle mysteries such as dark matter and dark energy. In the energy frontier, a see-saw mechanism between the electroweak symmetry scale and the Planck scale could give rise to phenomenon at the M2EW / MPL ~ 1 meV scale. Neutrino mass differences are measured in units of meV. Possible candidates for dark matter such as a gravitinos or axions may exist at the meV scale. Finally, it is noted that vacuum energy density described by dark energy is L4~ 7 x 10-30 g/cm3 ~ (2 meV)4. The GammeV experiment seeks to detect oscillations between gamma’s and meV mass scale particles to begin study in this interesting region.
Recently,
the PVLAS collaboration has reported measurements of anomalous vacuum magnetic
dichroism or polarization rotation [PRL 96, 110406 (2006)] and birefringence or
polarization ellipticity generation [ICHEP06] which, when interpreted as a new
two-photon interaction, imply new resonances of the vacuum at the milli-eV
scale with a coupling constant between 10-6 and 10-5 GeV-1
(interpretations such a milli-charged fermions also appear in the theoretical
literature). The PVLAS measurement itself and the interpretation have received
some criticism (including new results from PVLAS that no longer show the anomalous
effect). The meV scale particle would be an axion-like particle (ALP) with a
much stronger coupling than expected for the QCD axion. In addition, limits set
by the CAST experiment at CERN, which seeks to detect
The ALP interpretation of the PVLAS data can be tested in a “light shining through a wall” experiment where a photon in a high magnetic field can convert into an ALP, travel through the “wall,” and then reconvert back into a photon. Such an experiment has previously been performed by the BFRT collaboration [Z Phys. C, 56, 505 (1992)] in the early 1990s. The probability for a photon to be regenerated depends on the length of the traversal in the magnetic field and BFRT happened to use a magnet which gave a result with minimal sensitivity in the meV mass range exactly where the PVLAS signal would suggest a new resonance may occur.
PROJECT
DESCRIPTION
We intend to perform a “light shining through a wall” experiment that has sensitivity in the PVLAS region of interest. This sensitivity happens quite naturally using a 6m long Tevatron dipole magnet with a 5T field in which sits thin optically opaque material or “brick wall”, an existing pulsed Nd:YAG laser, and a single photon counting PMT. The experiment is unique in the way that the “wall” is moved to different discrete positions in order to span the mass range with maximal sensitivity. The experiment is also unique in that it is of smaller scale than other proposals, costs less, and should produce results more quickly than the competing experiments. Similar to neutrino oscillations, the conversion and reconversion probabilities oscillate with the magnetic baselines, Li, before and after the “brick wall” and the total probability is given by:
Using a 5T Tevatron magnet and an existing frequency-doubled Nd:YAG laser which produces 160 mJ, 5ns pulses of 532nm light at a frequency of 20 Hz, and assuming the central values for the mass and coupling given by PVLAS, the regeneration probability is given by:
With the “brick wall” in the center of a 6m long magnet, L1 = L2 = 3m, the signal rate including detector efficiency would be 5 x 10-3 Hz. Using a low noise (100Hz) PMT that has a 5ns pulse window, the noise rate (coincidence with the 20Hz laser pulse) is given by RNoise ~ (20 Hz) (5 ns) (100 Hz) ~ 1 x 10-5 Hz. The 95% CL limits that can be set with 5 hrs of running at a variety of “brick wall” positions are calculated. The L1=L2=3m and L1=0.8, L2=5.2m configurations cover the (purple) PVLAS region of interest and fill in the BFRT region of minimal sensitivity (black).
