Implications of LUNA for BBN and CMB constraints on MeV-scale Thermal Dark Sectors

Very recently, the LUNA collaboration has reported a new measurement of the d+p→ 3He+γ reaction rate, which plays an important role in the prediction of the primordial deuterium abundance at the time of BBN. This new measurement has triggered a new set of global BBN analyses within the context of the Standard Model. In this addendum to JCAP 01 (2020) 004 (arXiv:1910.01649), we consider the implications of these new results for our constraints on MeV-scale dark sectors. Importantly, we find that our bounds in the BBNonly and Planck-only analyses are insensitive to these updates. Similarly, we find that our constraints derived using BBN and CMB data simultaneously are not significantly modified for neutrinophilic particles. The bounds on electrophilic dark sector states, however, can vary moderately when combining BBN and CMB observations. We present updated results for all the relevant light dark sector states, calculated using the rates obtained by the leading groups performing standard BBN analyses. ORCID: 0000-0002-7924-546X ORCID: 0000-0003-2020-0803 ORCID: 0000-0002-4487-8742 ORCID: 0000-0002-0566-4127 ORCID: 0000-0003-2646-0112 ar X iv :2 10 7. 11 23 2v 1 [ he pph ] 2 3 Ju l 2 02 1 Recent LUNA Results. The LUNA collaboration has recently reported a very precise measurement of the d + p → 3He + γ cross-section at the energies relevant for Big Bang Nucleosynthesis (BBN) (E ∼ 30 − 300 keV) [1]. This quantity is particularly important for the prediction of the primordial deuterium abundance [2]. As a result, the three leading groups currently performing Standard BBN analyses have updated their predictions for the primordial deuterium abundance [3–5]. These three groups agree on the form of the d+ p→ 3He+γ reaction rate as a function of temperature, and its contribution to the total deuterium error budget, as it is now dominated by the small error bars from the LUNA measurement1. However, these groups use different parametrisations and experimental measurements to fit the two other relevant reaction rates for the prediction of D/H|P, namely d + d → n + 3He and d + d → p + 3H. In particular, Ref. [3] fits these rates to theoretical models, Ref. [4] uses a polynomial function to fit the rates, and Ref. [5] uses the rates from the NACREII compilation [6]. As a result of these differences in approach, the various groups report different predictions for D/H|P at a fixed value of the baryon density Ωbh. Using the bestfit value reported by the Planck collaboration for a ΛCDM cosmology, Ωbh 2 = 0.02236 [7], the three groups obtain the following abundances: D/H|P = (2.49± 0.11)× 10−5 , [Yeh et al. ’21] (1.1) D/H|P = (2.52± 0.07)× 10−5 , [Pisanti et al. ’21] (1.2) D/H|P = (2.45± 0.04)× 10−5 . [Pitrou et al. ’21] (1.3) To assess the concordance between BBN and CMB determinations of the baryon density, these predictions should then be compared to the measured value of D/H|P, e.g. the one as recommended by the PDG [8]: D/H|obs P = (2.547 ± 0.025) × 10−5. We can clearly appreciate that the results of Yeh et al. [5] and Pisanti et al. [4] are in agreement with this observed value. On the other hand, the results of Pitrou et al. [3] show a slight tension between the predicted value of D/H|P in Eq. (1.3) and D/H|obs P , which can in turn be rephrased as a 1.6σ tension on the reconstructed value of Ωbh 2 from BBN observations and the CMB [3]. Updates in our BBN Analysis. In Ref. [9], we used the BBN code PRIMAT [10, 11] linked to the cosmological code NUDEC BSM [12, 13] to calculate the evolution of the primordial element abundances in the presence of light thermal dark sectors. Ref. [9] was submitted before the recent LUNA measurements were released, and as such, the calculations were carried out using the rates and uncertainties from Ref. [10]. Here, we comment on how our results are modified with the inclusion of the updated rates from each of the three groups. For this purpose, we again use PRIMAT, but we change the relevant nuclear reaction rates to those outlined in [3], [4], and [5]. Then, at the level of our data analysis, we take the theoretical uncertainties in the predicted value of D/H|P to be 4.4%, 2.8% and 1.6% respectively, see Eqs. (1.1)−(1.3). In addition, we also use the latest recommended value of the observed deuterium abundance from the PDG, which has been updated from D/H|obs P = (2.569 ± 0.027) × 10−5 to D/H|obs P = (2.547 ± 0.025) × 10−5. We note that this shift in the measured value is only at the level of 0. 8σ, and that the recommended value of the primordial helium abundance is the same in the 2018 and 2020 editions of the PDG review on BBN. Some of these results were discussed in the Latest Advances in the Physics of BBN and Neutrino Decoupling Workshop held virtually on 12-13 April 2021, see https://indico.ph. tum.de/event/6798/.