The Search for 0Νββ Decay: a Look at Four Experiments

The Search for 0νββ Decay: A Look at Four Experiments

Cameron Sumner

April 30, 2018

Abstract

Background: Double beta (2νββ) decay is a process in certain isotopes where two neutrons decay into two protons and emit two electrons and two electron-neutrinos. If neutrinos are their own antiparticles

(called Majorana particles), a theoretical neutrinoless beta decay (0νββ), where the two neutrinos annihilate each other, should be possible. Four experiments are searching for evidence of this decay.

Purpose: Searching for lepton number violation via neutrinoless beta decay (0νββ), which would prove that neutrinos are Majorana particles. Methods: Each experiment observes the decay of certain isotopes that undergo 2νββ decay in the search for 0νββ decay. The methods and isotopes used are all slightly different, but they all measure energy absorption from the decay. If 0νββ decay is observed, the resulting energy absorption would include the energy of the missing neutrinos. Results: No evidence of

0νββ decay has been found. Each experiment found a lower limit of the decay ranging from

푇/( 푋푒) > 1.0 푥 10 푦푟 to 푇/( 퐺푒) > 8.0 푥 10 푦푟 (all at 90% confidence level). An upper limit on the mass of Majorana neutrinos is shown to be within the range 푚 < (120 − 520) 푚푒푉. Conclusion:

While there has not been evidence of 0νββ decay yet, continued technological advancements will increase the sensitivity of the detectors and the total mass of the isotope allowed while still being protected from background, and decrease the background noise.

The current interpretation of the standard model is incomplete. Currently, it is believed that each charged lepton (the electron, muon, and tauon, and their antiparticles) each have their own corresponding neutrino or antineutrino, which helps conserve lepton number in reactions. For example, consider the decay of the pion:

휋 →μ + 휈̅

The pion decays into an antimuon and a muon antineutrino. Each generation of leptons has a different lepton number (denoted as Lx, where x is the specific lepton generation) attached to it. As the

Lµ for the pion is zero, the creation of the antimuon (whose Lµ=-1) must be accompanied by the muon neutrino to conserve Lµ (whose Lµ=1).

However, the discovery of neutrino oscillations (where neutrinos oscillate between the three generations) shows the absolute nature of this to be untrue. And while the standard model says total lepton number is conserved, the doubt cast by neutrino oscillations, and the overall lack of knowledge on the nature of neutrinos has inspired several experiments in search of violation of total lepton number.

The debate currently centers around whether neutrinos are Dirac particles (the current interpretation) or Majorana particles (particles that are their own antiparticles) and the search for

neutrinoless double beta decay (0νββ).

Observations of this process would prove

that total lepton number is violated and

that neutrinos are Majorana particles,

giving us proof of physics beyond the

Fig. 1: An artistic representation of 0νββ decay. Left: a normal 2νββ decay. Middle: 0νββ decay. Right: A possible example of 0νββ decay with a new, standard model. “Ordinary two-neutrino heavier element. [Credit: APS/Alan Stonebraker] “Viewpoint: The Hunt for No Neutrinos” https://physics.aps.org/articles/v11/30 (2018) double-beta decay (2νββ) is an uncommon

2 process in which two neutrons decay into two protons, two electrons, and two antineutrinos at the same time (shown in Fig. 1, left). However, if neutrinos and antineutrinos are identical, then the two antineutrinos can annihilate each other, resulting in a neutrinoless decay (shown in Fig. 1, middle) antineutrinos at the same time.” [6]

These four experimental collaborations have reported new lower limits of the decay’s half-life within the past two months. The purposes of these experiments are all nearly identical, to find a 0νββ decay, and determine the lower bounds of the half-life of the decay. They all do it in a similar fashion.

Each experiment monitors a large number of atoms of a given isotope that supports 2νββ decay, and searches for peaks in the energy of the electrons involved in the decay that would indicate the two neutrinos have annihilated.

These experiments, however, differ in the isotope used, and how they measure electron energies. GERDA (the GERmanium Detector Array), located at the underground Laboratori Nazionali del

Gran Sasso (LNGS) below Gran Sasso mountain in L’Aquila, Italy [2-3], and the Majorana Demonstrator

[1], located at the Sanford Underground Research Facility in Lead, South Dakota, use 76Ge. CUORE (the

Cryogenic Underground Observatory for Rare Events), also located at the LNGS [4], uses 130Te. EXO-200

(the Enriched Xenon Observatory), located at the Waste Isolation Pilot Plant near Carlsbad, New Mexico

[5], uses 136Xe.

Each experiment found the half-life of 0νββ decay to be on the order of 1025 years. This is why such a large amount of the isotope is necessary. Half-life is not “the time required for exactly half of the entities to decay.” It’s a measurement of the average time for half of the entities to decay. So, to simulate a half-life on that scale, hundreds of kilograms of the isotopes (which translates to ~1025 atoms) are used with the intention of only a relatively few of them to decay.

The experiments all use similar methods to shield the experiment from outside background.

Lead is copiously used, and, except for the CUORE experiment, a device is used to veto muon-induced events. Each experiment carefully selects events that could possibly be candidates for 0νββ decay.

The CUORE experiments focuses on the decay in 130Te to the ground state of 130Xe. CUORE is

3 composed of 988 5 x 5 x 5 cm TeO2 crystals, each having a mass of 750 g (equaling 741 kg of TeO2), which can be cooled to temperatures as low as 7 mK. The crystals are arranged into 19 copper-framed towers, with each tower containing 13 floors of 4 crystals. These crystals are held in the frame by polytetrafluoroethylene (PTFE) supports. The towers are thermally connected to the mixing chamber of a 3He—4He dilution refrigerator, which is precooled by five two-stage pulse tube coolers (at approximately 40 and 4 K) and an expansion valve. Two lead shields are integrated to suppress external

γ-ray backgrounds. In addition, an external lead shield surrounded by PTFE mixed with boric acid provide additional shielding.

When a crystal absorbs energy, the resulting temperature increase is used to measure that energy. To accomplish this, each crystal is equipped with a thermistor (an electrical resistor whose resistance changes when heated) that is used to record thermal pulses.

The data are from two month-long data collection periods which ran from May to June (dataset

1) and August to September (dataset 2) of 2017. Between the two collection periods, an active noise cancellation system was implemented on the coolers. Each collection period ended with periods devoted to calibration with 232Th γ-ray sources. This was done to verify the stability of the detector response through the collection period.

In the end, it was concluded that there was no evidence for 0νββ decay of 130Te. A lower limit of

the half-life was placed at 푇/( 푇푒) > 1.3 푥 10 푦푟. This combined with previous experiments from

the CUORE collaboration (Cuoricino and CUORE-0) show a lower limit of 푇/( 푇푒) > 1.5 푥 10 푦푟, both of these at 90% confidence level.

EXO-200 (200 kg on xenon at the Enriched Xenon Observatory) is searching for 0νββ decay in

136Xe. It uses a time projection chamber (TPC), which is a particle detector that uses electric and magnetic fields in a volume of gas or liquid (in this case, a liquid), to perform reconstructions of particle interactions. The TPC is filled with liquid xenon (LXe) enriched to 80.6% 136Xe. The TPC has two drift regions split by a common cathode, each with radii of about 18 cm, and drift lengths of about 20 cm.

Xenon is a scintillator, meaning it flashes light when excited by ionizing radiation. Decay particles produce light. This fact is exploited by arrays of avalanche photodiodes (APDs), which are highly sensitive and convert this light to electricity. This light is used to determine event time. A large electric field drives ionization electrons to wires for collection. The time between the light and first collection determines the z coordinate of the event, while a grid of wires determines the x and y coordinated. The total energy deposited is determined from the combination of the charge and light signals.

The LXe is surrounded by several layers of passive and active shielding, including HFE-7000 cryofluid, lead, and a plastic scintillator muon veto. In addition, the Waste Isolation Pilot Plant provides an overburden of water. Like the other experiments, there is a minimum time between events required to avoid mixing between events. In this case, they are required to be 1 s from all other events.

In this experiment, the data were collected in two phases. 80% of the data came Phase I, which ran from Sep. 2011 to Feb. 2014. This phase ended when EXO-200 was forced to suspend operations, because of accidents at the WIPP facility. The other 20% came from Phase II, which began in May 2016

(There isn’t an end date stated in the paper, but the manuscript was first received 1 August 2017, so I believe it ended before that.). Between the two phases, the detector was upgraded with new electronics to improve the background and readout noise. Because of these upgrades, the phase II sensitivity is comparable to that of Phase I.

“No statistically significant evidence for 0νββ was observed.” Individually, Phase I sets a lower

limit of 푇/( 푋푒) > 1.0 푥 10 푦푟, and phase II 푇/( 푋푒) > 4.4 푥 10 푦푟. The next phase in this

5 experiment is called nEXO, “a tonne-scale LXe TPC being designed to reach half-life sensitivity of ~1028 yr.”

The GERDA experiment in Italy is one of the two current experiments studying the decay of 76Ge.

It searches for 76Ge 0νββ decay using Ge detectors. High-purity germanium (HPGe) detectors are made from material enriched with ~87% 76Ge. The Germanium detectors are mounted in low-mass copper supports and operated in a 64 m3 liquid argon (LAr) bath. The argon cryostat is inside a tank of 590 m3 of high-purity water. The LAr and water are used to shield against the external radioactivity. In addition, the water tank contains photomultipliers to act as a Cherenkov detector to find and veto residual muon- induced events.

The HPGe detectors search for energy depositions above the trigger threshold (40-100 keV).

The HPGe detector signals are read out with charge sensitive amplifiers. These analog signals are then digitized and analyzed off-line. When a detector reaches the trigger threshold, all channels are analyzed for possible coincidences.

Phase I ran from Nov. 2011 until May 2013. It detected no 0νββ decay and derived a half life of

푇/( 퐺푒) > 2.1 푥 10 푦푟. Phase II ran from Dec. 2015 until April 2017. Once again, no 0νββ decay

was detected. A new lower limit on the half-life was placed at 푇/( 퐺푒) > 8.0 푥 10 푦푟. Like the other experiments, this one also reports their data at a 90% confidence level.

The Majorana Demonstrator is the last of the four recent experiments into 0νββ decay, also using 76Ge. Once again, it takes advantage of “well-known benefits of HPGe” detectors to detect these events. These include “intrinsically low-background source material, understood enrichment process, excellent energy resolution, and sophisticated event reconstruction.” C. E. Aalseth et al. also state that

“a primary technical goal of the experiment is the development and use of ultralow activity materials and methods to suppress backgrounds to a low enough level to motivate the construction of a tonne- scale experiment.”

The Majorana Demonstrator is made up of 44.1 kg of Ge detectors split between two modules contained a low background shield. Each detector has a mass of 0.6-1.1 kg. Thirty five of the detectors are made up 29.7 kg fabricated with Ge material enriched to ~88.1% 76Ge. The other 23 detectors have a total mass of 14.4 kg fabricated from natural Ge, including 7.6% 76Ge.

Going from the inside out two cryostat modules are surrounded by a layer of electroformed copper, then a layer of commercially obtained C10100 copper, high-purity lead, an active muon veto, borated polyethylene, and polyethylene. The cryostats, copper, and leads are all enclosed in a radon exclusion box.

Once again, no 0νββ decay has been observed, and a lower limit on the half-life has been placed

at 푇/( 퐺푒) > 1.9 푥 10 푦푟 (90% confidence level).

In addition to the lower limits on the half-life, each experiment placed upper limits on the

Majorana neutrino masses, due to the limits on the sensitivity of the experiments. CUORE placed it at

푚 < (147 − 520) 푚푒푉. EXO-200 placed it at 푚 < (147 − 398) 푚푒푉. GERDA gave the figure 푚 <

(120 − 260) 푚푒푉. And finally, the Majorana demonstrator constrained it to 푚 < (240 − 520) 푚푒푉.

The next generation of experiments in 0νββ decay will have increased sensitivity to small energy changes and will hopefully get to the tonne-scale while still reducing background as much as possible.

These two goals will probe deeper for the incredibly neutrino mass, and will be able to simulate longer and longer half-lives.

References

1. “Search for Neutrinoless Double-β Decay in Ge76 with the Majorana Demonstrator.” C. E.

Aalseth et al. (Majorana Collaboration). Phys. Rev. Lett. 120, 132502 (2018). Published March

26, 2018.

2. “Results on Neutrinoless Double-β Decay of 76Ge from Phase I of the GERDA Experiment.” M.

Agostini et al. (GERDA Collaboration). Phys. Rev. Lett. 111, 122503. Published 19 September

2013.

3. “Improved Limit on Neutrinoless Double-β Decay of Ge76 from GERDA Phase II.” M. Agostini et

al. (GERDA Collaboration). Phys. Rev. Lett. 120, 132503 (2018). Published March 26, 2018.

4. “First Results from CUORE: A Search for Lepton Number Violation via 0νββ Decay of Te130.” C.

Alduino et al. (CUORE Collaboration). Phys. Rev. Lett. 120, 132501 (2018). Published March 26,

2018.

5. “Search for Neutrinoless Double-Beta Decay with the Upgraded EXO-200 Detector.” J. B. Albert

et al. (EXO-200 Collaboration). Phys. Rev. Lett. 120, 072701 (2018). Published February 15, 2018

6. “Viewpoint: The Hunt for No Neutrinos.” J. Engel and P. Vogel. Published March 26, 2018.

https://physics.aps.org/articles/v11/30