Migdal effect and photon Bremsstrahlung: improving the sensitivity to light dark matter of liquid argon experiments
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Springer
Received: June 10, Revised: August 28, Accepted: October 4, Published: November 10,
2020 2020 2020 2020
G. Grilli di Cortona,a A. Messinab,c and S. Piacentinib,c a
Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, PL-02-093 Warsaw, Poland b Dipartimento di Fisica, Sapienza Università di Roma, I-00185 Roma, Italy c Istituto Nazionale di Fisica Nucleare, Sezione di Roma, I-00185 Roma, Italy
E-mail: [email protected], [email protected], [email protected] Abstract: The search for dark matter weakly interacting massive particles with noble liquids has probed masses down and below a GeV/c2 . The ultimate limit is represented by the experimental threshold on the energy transfer to the nuclear recoil. Currently, the experimental sensitivity has reached a threshold equivalent to a few ionization electrons. In these conditions, the contribution of a Bremsstrahlung photon or a so-called Migdal electron due to the sudden acceleration of a nucleus after a collision might be sizable. In the present work, we use a Bayesian approach to study how these effects can be exploited in experiments based on liquid argon detectors. In particular, taking inspiration from the DarkSide-50 public spectra, we develop a simulated experiment to show how the Migdal electron and the Bremsstrahlung photon allow to push the experimental sensitivity down to masses of 0.1 GeV/c2 , extending the search region for dark matter particles of previous results. For these masses we estimate the effect of the Earth shielding that, for strongly interacting dark matter, makes any detector blind. Finally, we show how the sensitivity scales for higher exposure. Keywords: Beyond Standard Model, Dark matter, Dark Matter and Double Beta Decay (experiments) ArXiv ePrint: 2006.02453
c The Authors. Open Access, Article funded by SCOAP3 .
https://doi.org/10.1007/JHEP11(2020)034
JHEP11(2020)034
Migdal effect and photon Bremsstrahlung: improving the sensitivity to light dark matter of liquid argon experiments
Contents 1
2 Migdal effect and photon Bremsstrahlung 2.1 Migdal effect 2.2 Photon Bremsstrahlung 2.3 Rates in argon detectors 2.4 Effects of the Earth attenuation
2 3 5 5 7
3 Sensitivity calculation and LAr simulated experiment 3.1 Upper bounds and experimental sensitivity 3.1.1 90% Credible Interval upper bound 3.1.2 Prior choice 3.1.3 Experimental sensitivity and Bayes factor 3.1.4 Expected sensitivity 3.2 The tea-lab simulated LAr experiment 3.2.1 The likelihood function 3.2.2 Simplified treatment of systematic effects 3.3 Analysis model implementation
8 9 9 10 11 13 14 14 16 17
4 Sensitivity to Migdal electron and photon Bremsstrahlung 4.1 Impact of theoretical uncertainties 4.2 Impact of the experimental effects 4.3 Projected sensitivity for future experiments
17 20 22 24
5 Conclusions
26
1
Introduction
Although astronomical and cosmological observations strongly support the existence of dark matter (DM) [1–4], its nature — its mass and interactions with the Standard
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