Helium systematic uncertainties

Finally, we wish to mention that, at Harvard University, another laser spectroscopy experiment was carried out in a discharge cell, and not in a clean environment as the metastable helium beam of the other experiments. The uncertainty of this unpublished measurement is 8 kHz [11], probably due to systematic effects from collisions in the helium cell [13]. 

To further improve the accuracy of our FS measurements, the frequency stability of the master laser and the S/N of the 1083 nm helium transitions must be increased. In this way, not only the statistical uncertainty will be improved, but it will also allow a precise experimental check of systematic effects. 

Two measurements [31,10] were conducted at a low-inductance vacuum spark plasma and a tokamak plasma respectively. In both cases only the w line was reported. The first study used a double Johann spectrograph and characteristic K lines were used for calibration [31]. The energy of the w was 5.20558(55) keV or a 105 ppm result The second study [10] used a tokamak plasma and claimed an uncertainty of 40 ppm. Close lying Lyman series lines were used for calibration so this was a relative measurement chain assuming one-electron QED. Shorter wavelength calibration lines and helium-like resonances were observed in second order diffraction suggesting the significant systematic shifts discussed above. The third study [11] was a relative measurement to the w line and, as such, can not be compared to absolute measurements. 

It seems clear that until new data address the unresolved systematic errors afflicting the derivation of the primordial helium abundance, the true errors must be much larger than the statistical uncertainties. For the comparisons between the predictions of SBBN and the observational data to be made in the next section, I will adopt the Olive, Steigman Walker (2000 OSW) compromise Yp = 0.238 0.005 the inflated errors are an attempt to account for the poorly-constrained systematic uncertaintiess. 

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