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Neutrinoless double beta decay

LIP

Carlos Roxo, Universidade de Coimbra

Francisco Pais, Instituto Superior Técnico

Supervisors: Alexandre Lindote & Paulo Brás

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0νββ Decay

Double β-decay is a process wherein a nucleus emits two electrons and two electron antineutrinos. If neutrinos are Majorana fermions, a neutrinoless mode is possible: 0νββ.

Detecting 0νββ would mean the neutrino is it’s own antiparticle; this would lead to a violation of lepton number invariance and perhaps pose an explanation of the matter-antimatter asymmetry observed in the universe. So far, 0νββ has not been detected.

Stefano Dell’Oro et al. «Neutrinoless Double Beta Decay: 2015 Review». In:Advances in High Energy Physics2016 (2016),pp. 1–37.ISSN: 1687-7365.DOI:10.1155/2016/2162659.URL:http://dx.doi.org/10.1155/2016/2162659.

Diagram of the first mechanism (β^-β^-).

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0νββ Decay

Schematic view of double β-decay spectra. The X-Axis represents the combined energy of the emitted electrons.

In 0νββ all the energy in the process (Q_ꞵꞵ) is taken by the electrons, so we expect to find a peak in the energy spectrum. This is unlike what happens in 2νββ decay, in which the energy is distributed between the electrons and the neutrinos (producing a continuous spectrum).

If we detect an excess of events in the ROI, it is possible to relate the 0νββ half-life with background events using

where M is the xenon mass (in the selected volume), t the exposure time (taken to be 1000 days), ΔE the ROI’s energy interval (in keV), f_ROI the fraction of signal inside the ROI, ⍺ the Xe-136 abundance and ϵ the fraction of signal that survives the cuts.

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The LZ Detector

The detector consists of a 7t liquid Xenon chamber surrounded by a Xenon skin and a Gadolinium scintillator cylindrical shell. The skin and the shell are used to exclude events that interact in the main target and in either of them. These multiple scatters cannot be singal events, which produce highly localized interactions in the xenon.

Photomultipliers are distributed throughout the detector: 494 inside the main Xe chamber, 131 in the skin and 120 in the scintillator.

(Xenon is used due to it’s high density, purity and the fact it is transparent to its own ionization light. Its high atomic mass (which leads to a high cross section) makes it a good target for WIMP dark matter.)

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The LZ Detector

A particle interacts with the detector and deposits energy, producing scintillation and ionization. The scintillation produces the S1 signal, while the electrons that do not recombine are drifted by an electric field onto the liquid surface and extracted to the gas, generating the S2 signal.

A full 3D reconstruction of the event is obtained using the temporal difference between signals and the light pattern produced by S2.

Vitaly A. Kudryavtsev. «Recent Results from LUX and Prospects for Dark Matter Searches with LZ». In:Universe5.3 (2019).ISSN:2218-1997.DOI:10.3390/universe5030073.URL: https://www.mdpi.com/2218-1997/5/3/73.

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Objectives and Data

Optimize and characterize the detector’s sensitivity to 0νββ decay, by varying a set of parameters:

Detection Volume (vetoes outermost region with higher background rate)
Energy Resolution
Single Scatter Cut and Efficiency (rejection of events that interact multiple times with the detector)

D. S. Akerib et al. «Projected sensitivity of the LUX-ZEPLIN experiment to the 0νββ decay of 136Xe». In: Physical Review C102.1(2020). ISSN: 2469-9993.DOI:10.1103/physrevc.102.014602.URL:http://dx.doi.org/10.1103/PhysRevC.102.014602.

The data is taken from a simulated run of LZ with 1000 days of data taking. It includes most backgrounds for the neutrinoless sensitivity analysis:

Radioactivity from detector materials (like U-238 and Th-232 contaminants)
Contaminants in the liquid xenon (like Bi-214)
𝛾’s emitted from the cavern rocks (which also contain the contaminants mentioned above)
Bananas

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Results (Volume, 1st Iteration)

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Results (Volume, 2nd Iteration)

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Results (Volume)

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Results (Energy Resolution and Single Scatter Cut)

Although the energy resolution and the ability to separate multiple scatters will only be known when the detector starts operating, we can study the how the sensitivity to the decay depends on these parameters.

We expect to obtain a 1% energy resolution and a 0.3 cm single scatter cut. With current data, the best sensitivity is obtained for a 0.2 cm SS_Cut. The decay seen for SS_Cut< 0.2 cm is due to the loss of events that occurs when the cut is too small.

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Conclusion

For an energy resolution of 1% the maximum estimated sensitivity is 7.2*10²⁵ years, obtained using the parameter values R_max= 42cm, Z_min= 36cm, Z_max= 92cm and SS_Cut= 0.2cm. In addition:

Xe mass in the fiducial volume is M_Xe = 899 kg.
For an abundance of 8.9%, that corresponds to M_Xe-136 = 80 kg.

Xe-136 is currently the most sensitive isotope used to measure the Majorana mass and, therefore, to verify the neutrino mass hierarchy. The expected LZ results are as good as the current best result in the area (KamLAND-Zen). This shows that LZ is ideal to search for these rare events, since it attains high sensitivities (even though this isn’t its main goal!). The tech used in the experiment (dual phase xenon TPC’s) is perhaps the best in the search of 0νββ decay.

(Estimated value ~ 1.06*10²⁶ years for 1000 live days of the experiment, in a 90% confidence interval)