We present the results of an experimental and theoretical study of the electronically excited $tripletex$ state of $^{87}$Rb$_2$ molecules. The vibrational energies are measured for deeply bound states from the bottom up to $v=15$ using laser spectroscopy of ultracold Rb$_2$ Feshbach molecules. The spectrum of each vibrational state is dominated by a 47,GHz splitting into a $cog$ and $clg$ component caused mainly by a strong second order spin-orbit interaction. Our spectroscopy fully resolves the rotational, hyperfine, and Zeeman structure of the spectrum. We are able to describe to first order this structure using a simplified effective Hamiltonian.
The inelastic dark matter scenario was proposed to reconcile the DAMA annual modulation with null results from other experiments. In this scenario, WIMPs scatter into an excited state, split from the ground state by an energy delta comparable to the available kinetic energy of a Galactic WIMP. We note that for large splittings delta, the dominant scattering at DAMA can occur off of thallium nuclei, with A~205, which are present as a dopant at the 10^-3 level in NaI(Tl) crystals. For a WIMP mass m~100GeV and delta~200keV, we find a region in delta-m-parameter space which is consistent with all experiments. These parameters in particular can be probed in experiments with thallium in their targets, such as KIMS, but are inaccessible to lighter target experiments. Depending on the tail of the WIMP velocity distribution, a highly modulated signal may or may not appear at CRESST-II.
In direct dark matter detection experiments, conventional elastic scattering of WIMPs results in exponentially falling recoil spectra. In contrast, theories of WIMPs with excited states can lead to nuclear recoil spectra that peak at finite recoil energies E_R. The peaks of such signals are typically fairly broad, with Delta E_R/E_peak ~ 1. We show that in the presence of dark matter structures with low velocity dispersion, such as streams or clumps, peaks from up-scattering can become extremely narrow with FWHM of a few keV only. This differs dramatically from the conventionally expected WIMP spectrum and would, once detected, open the possibility to measure the dark matter velocity structure with a fantastic accuracy. As an intriguing example, we confront the observed cluster of 3 events near 42 keV from the CRESST commissioning run with this scenario, and find a wide range of parameters capable for producing such a peak. We compare the possible signals at other experiments, and find that such a particle could also give rise to the signal at DAMA, although not from the same stream. Over some range of parameters a signal would be visible at xenon experiments. We show that such dark matter peaks are a very clear signal, and can be easily disentangled from potential backgrounds, both terrestrial or due to WIMP down-scattering, by an enhanced annual modulation signature in both the amplitude of the signal and its shape.
We measure and explain scintillator non-proportionality and gamma quenching of CaWO4 at low energies and low temperatures. Phonons that are created following an interaction in the scintillating crystal at temperatures of 15mK are used for a calorimetric measurement of the deposited energy, and the scintillation light is measured with a separate cryogenic light detector. Making use of radioactivity intrinsic to the scintillating crystal, the scintillator non-proportionality is mapped out to electron energies <5keV. The observed behavior is in agreement with a simple model based on Birks law and the stopping power dE/dx for electrons. We find for Birks constant $k_B=(18.5pm0.7)$nm/keV in CaWO4. Gamma lines allow a measurement of the reduced light yield of photons with respect to electrons, as expected in the presence of scintillator non-proportionality. In particular, we show that gamma-induced events in CaWO4 give only about 90 percent of the light yield of electrons, at energies between 40keV and 80keV.
The CRESST experiment monitors 300g CaWO_4 crystals as targets for particle interactions in an ultra low background environment. In this paper, we analyze the background spectra that are recorded by three detectors over many weeks of data taking. Understanding these spectra is mandatory if one wants to further reduce the background level, and allows us to cross-check the calibration of the detectors. We identify a variety of sources, such as intrinsic contaminations due to primordial radioisotopes and cosmogenic activation of the target material. In particular, we detect a 3.6keV X-ray line from the decay of 41-Ca with an activity of (26pm4)mu Bq, corresponding to a ratio 41-Ca/40-Ca=(2.2pm0.3)times10^{-16}.
The alpha decay of $ {{}^{210}Po}$ is a dangerous background to rare event searches. Here, we describe observations related to this alpha decay in the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST). We find that lead nuclei show a scintillation light yield in our $ {CaWO_4}$ crystals of $0.0142pm0.0013$ relative to electrons of the same energy. We describe a way to discriminate this source of nuclear recoil background by means of a scintillating foil, and demonstrate its effectiveness. This leads to an observable difference in the pulse shape of the light detector, which can be used to tag these events. Differences in pulse shape of the phonon detector between lead and electron recoils are also extracted, opening the window to future additional background suppression techniques based on pulse shape discrimination in such experiments.
We report here on the production of an ultracold gas of tightly bound Rb2 molecules in the ro-vibrational triplet ground state, close to quantum degeneracy. This is achieved by optically transferring weakly bound Rb2 molecules to the absolute lowest level of the ground triplet potential with a transfer efficiency of about 90%. The transfer takes place in a 3D optical lattice which traps a sizeable fraction of the tightly bound molecules with a lifetime exceeding 200 ms.
The emerging field of ultracold molecules with their rich internal structure is currently attracting a lot of interest. Various methods have been developed to produce ultracold molecules in pre-set quantum states. For future experiments it will be important to efficiently transfer these molecules from their initial quantum state to other quantum states of interest. Optical Raman schemes are excellent tools for transfer, but can be involved in terms of equipment, laser stabilization and finding the right transitions. Here we demonstrate a very general and simple way for transfer of molecules from one quantum state to a neighboring quantum state with better than 99% efficiency. The scheme is based on Zeeman tuning the molecular state to avoided level crossings where radio-frequency transitions can then be carried out. By repeating this process at different crossings, molecules can be successively transported through a large manifold of quantum states. As an important spin-off of our experiments, we demonstrate a high-precision spectroscopy method for investigating level crossings.
