With their excellent energy resolution, efficiency, and intrinsic radio-purity, cryogenic calorimeters are primed for the search of neutrino-less double beta decay (0$\nu$DBD).
CUPID-0 is an array of 24 Zn$^{82}$Se scintillating bolometers used to search for 0$\nu$DBD of $^{82}$Se.
It is the first large mass 0$\nu$DBD experiment exploiting a double read-out technique: the heat signal to accurately measure particle energies and the light signal to identify the particle type.
The CUPID-0 is in data taking since March 2017 and obtained several outstanding scientific results.
The configuration of the CUPID-0 data processing environment on the CNAF computing cluster has been used for the analysis of the first period of data taking.
\end{abstract}
\section{The experiment}
Neutrino-less Double Beta Decay (0$\nu$DBD) is a hypothesized nuclear transition in which a nucleus decays emitting only two electrons.
This process can not be accommodated in the Standard Model, as the absence of emitted neutrinos would violate the lepton number conservation.
Among the several experimental approaches proposed for the search of 0$\nu$DBD, cryogenic calorimeters (bolometers) stand out for the possibility of achieving excellent energy resolution ($\sim$0.1\%), efficiency ($\ge$80\%) and intrinsic radio-purity. Moreover, the crystals that are operated as bolometers can be grown starting from most of the 0$\nu$DBD emitters, enabling the test of different nuclei.
The state of the art of the bolometric technique is represented by CUORE, an experiment composed of 988 bolometers for a total mass of 741 kg, presently in data taking at Laboratori Nazionali del Gran Sasso.
The ultimate limit of the CUORE background suppression resides in the presence of $\alpha$-decaying isotopes located in the detector structure.
The CUPID-0 project \cite{Azzolini:2018dyb,Azzolini:2018tum} was born to overcome the actual limits.
The main breakthrough of CUPID-0 is the addition of independent devices to measure the light signals emitted from scintillation in ZnSe bolometers.
The different properties of the light emission of electrons and $\alpha$ particles will enable event-by-event rejection of $\alpha$ interactions, suppressing the overall background in the region of interest for 0$\nu$DBD of at least one order of magnitude.
The detector is composed by 26 ZnSe ultra-pure $\sim$ 500g bolometers, enriched at 95\% in $^{82}$Se, the 0$\nu$DBD emitter, and faced to Ge disks light detector operated as bolometers.
CUPID-0 is hosted in a dilution refrigerator at the Laboratori Nazionali del Gran Sasso and started the data taking in March 2017.
The first scientific run (i.e.,~ Phase I) ended in December 2018, collecting 9.95 kg$\times$y of ZnSe exposure.
Such data were used to calculate a new limits on the $^{82}$Se 0$\nu$DBD~\cite{Azzolini:2018dyb,Azzolini:2018oph} and to develop a full background model of the experiment~\cite{Azzolini:2019nmi}.
Phase II will start in June 2019 with an improved detector configuration.
\section{CUPID-0 computing model and the role of CNAF}
The CUPID-0 computing model is similar to the CUORE one, being the only difference in the sampling frequency and working point of the light detector bolometers.
The full data stream is saved in root files, and a derivative trigger is software generated with a channel dependent threshold.
%Raw data are saved in Root files and contain events in correspondence with energy releases occurred in the bolometers.
Each event contains the waveform of the triggering bolometer and those geometrically close to it, plus some ancillary information.
The non-event-based information is stored in a PostgreSQL database that is also accessed by the offline data analysis software.
The data taking is arranged in runs, each run lasting about two days.
Details of the CUPID-0 data acquisition and control system can be found in \cite{DiDomizio:2018ldc}.
Raw data are transferred from the DAQ computers (LNGS) to the permanent storage area (located at CNAF) at the end of each run.
A full copy of data is also preserved on tape.
The data analysis flow consists of two steps; in the first level analysis, the event-based quantities are evaluated, while in the second level analysis the energy spectra are produced.
The analysis software is organized in sequences.
Each sequence consists of a collection of modules that scan the events in the Root files sequentially, evaluate some relevant quantities and store them back in the events.
The analysis flow consists of several key steps that can be summarized in pulse amplitude estimation, detector gain correction, energy calibration and search for events in coincidence among multiple bolometers.
The new tools developed for CUPID-0 to handle the light signals are introduced in \cite{Azzolini:2018yye,Beretta:2019bmm}.
The main instance of the database was located at CNAF and the full analysis framework was used to analyze data until November 2017. A web page for offline reconstruction monitoring was maintained.
%During 2017 a more intense usage of the CNAF resources is expected, both in terms of computing resourced and storage space.
