Cupid Collaboration

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
====== CUPID  ======

The project **CUPID** ((CUPID is a 2<sup>nd</sup> order acronym that stands for CUORE Upgrade with Particle IDentification.)) is an upgrade of the CUORE experiment, aiming at searching for neutrinoless double beta (0νββ) decay with Li<sub>2</sub>MoO<sub>4</sub> scintillating crystals enriched in <sup>100</sup>Mo. The crystals are operated as cryogenic detectors in the zero-background condition for its entire life cycle, which provides the fastest increase of sensitivity over the data collection time. The zero background condition is achievable via the particle discrimination in the scintillation channel.

===== Physics Goal =====

Neutrinos have been postulated almost a century ago, and over 60 years have passed since their discovery.
Over the past decades, neutrinos of different origins have been measured, including solar, atmospheric, reactor and supernova neutrinos, providing an impressive amount of information on their sources, as well as on the Standard Model of particle physics.
Nevertheless, the experimental evidence of non-zero neutrino masses provided by the oscillation experiments
unambiguously indicates the existence of new physics beyond the Standard Model.
In parallel, several open questions tantalize the physics community since decades:
  * What are the fundamental symmetries of the Standard Model, and their corresponding conserved quantities?
  * What is the origin of the asymmetry between the amount of Matter and that of Anti-Matter in the Universe?
  * Why is the mass of neutrinos much smaller than that of the other leptons?
A possible answer to these questions can be provided by 0νββ decay.

0νββ decay is a process in which a nucleus (A,Z) decays to (A,Z+2) with the simultaneous emission of two electrons,
without any counterbalancing emission of antiparticles. Since the introduction of the Fermi theory of β decay,
this would be the first example of a process that does not conserve
the difference between the number of particles and antiparticles.
In the framework of the Standard Model, 0νββ decay is a process that violates the lepton number L,
and, more fundamentally, the difference between the baryon and lepton number B-L.
Hence, the observation of 0νββ decay would simultaneously prove that the conservation of B-L is only approximate
and that the Standard Model is thus just an approximate version of a broader theory,
and provide an unequivocal evidence for the existence of matter creating processes.
0νββ is only possible if neutrinos have a Majorana mass component: in this framework,
the mass of the three known (light) left-handed neutrinos could be generated by some heavy right-handed partner
via the so-called see-saw mechanism, which naturally accounts for the smallness of neutrino masses
in comparison to the Higgs-generated masses of all other known particles.

===== The 0νββ decay signature =====

0νββ decay is a 3 body decay, with the available energy shared between the daughter nucleus
and the two emitted electrons. Given its much larger mass, the daughter nucleus is subject to a negligible recoil.
Moreover, for most detector technologies the two emitted electrons are not distinguishable,
so only their sum energy is measurable.
The experimental signature of 0νββ decay is thus an excess at the Q-value of the reaction,
corresponding to the mass difference between the parent and daughter atom.

On the other hand,  2νββ decay is a 5 body decay, with again a negligible nuclear recoil.
In this case, the two antineutrinos escape undetected, so only a fraction of the Q-value
is shared between the electrons. The signature in this case is therefore a continuum from zero
to the Q-value.

|{{ cupid_pub:double_beta.png?330 }} |{{ cupid_pub:screenshot_20210522_144702.jpeg?330 }}|
|Decay scheme of 2νββ (top) and 0νββ decay (bottom). The two processes share the same parent and daughter nucleus, but differ for the number of emitted particles, and consequently their energy.|The measurable sum electron spectrum is a continuum for 2νββ decay, and an excess at Q<sub>ββ</sub> for 0νββ decay.|






=====The Experiment=====

The first experimental attempts to detect 0νββ decay are date back to the 1940s. Over the decades, many different technologies have been developed to search for this process on a variety of candidate isotopes.
Presently, [[https://cuore.lngs.infn.it/|CUORE]]((CUORE is again an acronym, standing for Cryogenic Underground Observatory for Rare Events.)) is one of the most sensitive experiments in the field.
It is located at the [[https://www.lngs.infn.it|Laboratori Nazionali del Gran Sasso (LNGS)]]  of INFN, in the Abruzzo region in central Italy.
CUORE is composed of 988 TeO<sub>2</sub> crystals operated as cryogenic calorimeters at a temperature of 10-15 mK. The crystals act simultaneously as detectors and source of 0νββ decay: in fact, they contain the ββ decay isotope <sup>130</sup>Te, that contributes to ~27% of the crystal mass.
The CUORE detectors are operated in the largest dilution refrigerator ever build((J. Ouellet, [[https://arxiv.org/abs/1410.1560|arXiv:1410.1560]])), capable of cooling down ~1.5 tons of material to base temperature about a month, and operating it stably for years. 

|{{cupid_pub:campo-imperatore.jpg?450}}|{{cupid_pub:cuore_clean_room.jpg?350}}|
|The Gran Sasso National Park, below which the underground lab of LNGS is located.|The CUORE detectors right after their installation in the cryostat.|

CUPID will profit of the established CUORE cryogenic infrastructure, and deploy ~1500  Li<sub>2 </sub>MoO<sub>4</sub> crystals in place of the TeO<sub>2</sub> ones.
Thus, CUPID will not only change the crystal, but also the candidate isotope. The reason for this choice is twofold:
on the one hand, Li<sub>2 </sub>MoO<sub>4</sub> is a scintillating material with a particle-dependent light yield,
on the other hand the candidate isotope <sup>100</sup>Mo has a Q-value of 3034 keV
(compared to 2527 keV of <sup>130</sup>Te), which lies above most of the γ background from environmental radioactivity. Special attention is paid to the minimization of the radioactive contamination levels of all employed materials. Using the information from the predecessor experiments CUORE, [[https://cupid-0.lngs.infn.it/|CUPID-0]], and [[https://cupid-mo.mit.edu|CUPID-Mo]], the projected background at Q<sub>ββ</sub> is expected to be at the level of 10<sup>-4</sup> counts/keV/kg/yr.
CUPID-Mo  has  robustly  demonstrated  that Li<sub>2</sub>MoO<sub>4</sub> scintillating bolometers
meet the requirement for CUPID. CUPID-Mo was an array of 20 elements that took data until 2020 in the Modane underground laboratory in France, as a follow-up  of  the LUMINEU  project.
It  has shown  the  maturity  reached  by  the proposed CUPID  technology  and  the  high  standard  of the Li<sub>2</sub>MoO<sub>4</sub> detectors  in  terms  of  energy resolution, α/β rejection capabilities, internal radiopurity, and overall reproducibility of the results.


|{{cupid_pub:cuore_cryostat_3.jpg?nolink&545}}|{{cupid_pub:cupidrendering.jpg?nolink&300}}|
|The CUORE/CUPID cryostat during its construction, with the distillation unit visible in the middle.|CUPID geometry in Geant4 Monte Carlo simulation.|


===== The Detector =====

In CUPID, the Li<sub>2</sub>MoO<sub>4</sub> crystals are operated as cryogenic calorimeters, and coupled to a light detector. The light detectors are germanium wafers, and are also instrumented as calorimeters.
A particle interaction in the crystal produces phonons and scintillation light.
The heat from recombining phonons is read by a Neutron Transmutation Doped (NTD) germanium thermistor
glued to the crystal. The light escapes the crystal, inducing a phonon signal in the light detector, which is also read by an NTD.

|{{ cupid_pub:cupid_detector.png?350 }}|{{cupid_pub:screenshot_20210522_221509.jpeg?450}}|
|Schematic of a cryogenic calorimeter, with the heat channel (blue) and a light detector (gray).|Installation of crystals for a CUPID test run. A light detector is visible in the bottom right.|
Page Tools
