Backgrounds, statistics and calibrationin direct detection experiments

07 and 14/01/2016Teresa Marrodan Undagoitia

• Program:

– Sources of background

∗ External background: gammas, neutrons and neutrinos∗ Internal and surface backgrounds

– Detector signals and background reduction

– Statistical treatment of data

– Detector calibration

• Literature:

– G. Heusser, Low-radioactivity background techniques, Annu. Rev. Nucl. Part. Sci. 45(1995) 543

– D.-M. Mei and A. Hime, Muon-induced Background Study for Underground Laboratories,Phys. Rev. D 73 (2006) 053004, arXiv:astro-ph/0512125

– L. E. Strigari, Neutrino Coherent Scattering Rates at Direct Dark Matter Detectors, New J.Phys. 11 (2009) 105011, arXiv:0903.3630.

– S. Cebrian et al., Cosmogenic activation in germanium and copper for rare event searches,Astropart. Phys. 33 (2010) 316.

– G. J. Feldman and R. D. Cousins, A Unified Approach to the Classical Statistical Analysis ofSmall Signals, Phys. Rev. D57 (1998) 3873 , arXiv:physics/9711021

– S. Yellin, Finding an upper limit in the presence of unknown background, Phys. Rev. D66(2002) 032005, arXiv:physics/0203002.

– G. Cowan, K. Cranmer, E. Gross, and O. Vitells, Asymptotic formulae for likelihood-basedtests of new physics, Eur. Phys. J. C71 (2011) 1554, arXiv:1007.1727.

– W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer-Verlag, 1987second revision 1994

– T. Marrodan Undagoitia and L. Rauch, Dark matter direct-detection experiments, J. Phys.G: 43 (2015) 1, arXiv:1509.08767

• Material for the lecture:

Figure 1: Left: Background spectra of a Ge detector without shield (top), with 15 cm lead shield(middle), and with shield and at 500 m.w.e. (bottom). Right: flux of cosmic ray secondaries andtertiary-produced neutrons in a typical Pb shield as function of shielding depth. Figures from G.Heusser, Annu. Rev. Nucl. Part. Sci. 45 (1995) 543.

Figure 2: (Left) Schematic of the GIOVE Ge-detector at MPIK. Figure from G. Heusser et al. Eur.Phys. J. C75 (2015) 11, 531. (Right) Total muon flux measured for the various underground sites asfunction of the equivalent vertical depth relative to flat overburden. Figure from arXiv:1509.08767.

Figure 3: Examples of detector shielding, DAMA detector (left) and XENON100 detector (right).

Figure 4: Neutrino background, example for liquid xenon detectors. (Left) Contribution of ERfrom solar neutrinos (green) in comparison with other backgrounds. (Right) Spectral shape ofcoherent neutrino scattering in xenon. Figures from XENON Coll., arXiv:1512.07501.

Figure 5: Illustration of different statistical methods used to derive results from direct detectionexperiments. Figure from arXiv:1509.08767.

Figure 6: Statistical methods to derive upper limits or two-sided confidence intervals. (Left)Feldman and Cousins generic confidence-belt construction. Figure from G. J. Feldman and R. D.Cousins, arXiv:physics/9711021. (Right) Yellin’s illustration of the maximum gap method. Figurefrom S. Yellin, Phys. Rev. D66 (2002) 032005.

Figure 7: Schematic of signals measured in different detection technologies. Figure fromarXiv:1509.08767.

Figure 8: Schematic representation for the calibration of signal and background regions fordifferent types of detectors: germanium (left), LXe (middle) and LAr (right). Figures fromarXiv:1509.08767.

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Figure 9: Nuclear-recoil energy scale for different target materials relevant for dark matter detec-tion. Figures from arXiv:1509.08767.