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-====== CUPID: the CUORE Upgrade with Particle IDentification  ======+====== CUPID  ======
  
-The project **CUPID** 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> 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 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 ===== ===== Physics Goal =====
  
-Neutrinos  have been known for a century, and in nature are produced by several sources, like Sun, Supernovae or nuclear decay in the Earth mantel. Neutrino interactions with other  elementary particles are well described by the Standard Model of particle physics, the best theory we  have to date describing the electromagnetic, weak and strong interactions.+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.
  
-Despite neutrinos have been studied for more than a 60 years, some of their fundamental properties (e.g the mass) are still unknown and the intrinsic nature of neutrino itself  is up to debate.   +0νββ decay is a process in which a nucleus (A,Z) decays to (A,Z+2) with the simultaneous emission of two electrons, 
-Neutrino can be Dirac ( neutrinos are  distinct by antineutrinos) or Majorana particles (neutrinos and antineutrinos are the same particles ). +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.
  
-If neutrinos are Majorana particles (or have a Majorana component)  an extremely rare nuclear decay could be measured: the Neutrinoless Double Beta Decay (0υ2β).+===== 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.
  
-=====The 0υ2β decay===== +On the other hand,  2νββ decay is a 5 body decay, with again a negligible nuclear recoil. 
-The measurement of the neutrinoless double beta decay (see figure below, the bottom part of the figure shows the 0υ2β)  +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:doppio_beta.png?400 |}} +
- +
-in addition to  probe the Majorana nature of the neutrino, would give hints to explain the neutrino tiny mass with  respect of the charged leptons mass  and it would be the first evidence for lepton number violation. +
- +
- +
-The experimental signature of such decay  is the presence of  a mono-energetic peak at the Q-value of the reaction, at the end of the energy spectrum of the double decay with neutrino as shown in the following  figure (the magnitude of the 0υ2β is magnified to make it visible).   +
- +
- +
-{{ :cupid_pub:spectrum.png?450 |}} +
  
 +|{{ 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.|
  
  
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 =====The Experiment===== =====The Experiment=====
-The first experiments trying to measure  0υ2β  are dated back to the end of the  '40s. During the last decades  many experiments have been proposed to measure the neutrinoless double beta decay. 
- 
-Among the operating  neutrinoless double beta experiment, CUORE is one of the most promising experiment able to measure the 0υ2β for the first time.  
-CUORE is located at the Laboratori Nazionali del Gran Sasso (LNGS) in the  Abruzzo region, beneath Monte Aquila, surrounded by at least 1400 meters (~3800 m.w.e.) of rock in all the direction. 
- 
- 
-{{ :cupid_pub:campo-imperatore.jpg?400 |}} 
- 
- 
-The CUORE detector is composed by 988 crystals operated at cryogenic temperature and equipped with sensitive thermometers (bolometers) made by TeO<sub>2</sub> searching for the 0υ2β  in<sup>130</sup>Te.  
- 
-The future of the CUORE experiment is the  CUPID project, whose  goal is  to measure the  0υ2β decay in the <sup>100</sup>Mo isotope instead of <sup>130</sup>Te used in CUORE using 1500   Li<sub>2 </sub>MoO<sub>4</sub> crystals  . The main reason for the change in the target material is due to the scintillating properties of the  Li<sub>2 </sub>MoO<sub>4</sub> which are necessary for the particle identification. 
- 
- 
-The CUPID crystal will be placed inside  the CUORE cryostat arranged as in the sketch shown below. 
- 
-{{ :cupid_pub:image2.jpeg?200|}} 
-{{ :cupid_pub:cryostat2.jpg?200 |}} 
- 
- 
- 
- 
-==== The Detector ==== 
- 
- 
-The CUPID crystals are operated as cryogenic calorimeters, each equipped with a cryogenic light detector. A particle interaction in the crystal produces a phonon  and light signal (see figure below), the latter one is used to discriminate  α background from the electrons events.  
-{{ :cupid_pub:cupid_detector.png?450 |}} 
- 
- 
- 
- 
- 
- 
  
 +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.|