Data analysis

Synchronised sensors send digital signals to a database. The data acquisition system processes the information via software, applying procedures to reconstruct from the data the type of interactions and their position inside the detector.

A key component of the experiment

The experimental devices used for research in modern physics always include a data acquisition system (Data Acquisition , DAQ). It is a set of electronic tools, managed by software, which records signals (for example those generated in a dark matter detector) and sometimes also carries out a preliminary processing of the data. The structure of a data acquisition system can be very complex and its design requires broad spectrum skills (in addition to a good knowledge of physics) such as electronics, computer science and statistics.

The XENON1T detector

The data acquisition system is based on the characteristics of the detector. The DAQ system of the experiment on dark matter in the Gran Sasso videogame draws inspiration from that of the XENON1T experiment, one of the most impressive experiments underway at the INFN Gran Sasso National Laboratory (see Dark Matter). Basically, the XENON1T detector is a large cylinder filled with xenon (see xenon) which is used as a scintillator (see scintillator ), i.e. it emits faint light signals as consequences of particle interactions. The light signals are detected by photomultipliers (see photomultiplier ) arranged on two arrays, at the top and at the bottom of the cylindrical tank, which when "illuminated" generate an electrical signal proportional to the intensity of the light. Each interaction causes two signals (S1 and S2), which have different amplitudes and are produced one after the other: putting this data together with the position of the "illuminated" photomultipliers allows us to reconstruct the type of interaction that took place and its position in the detector. The cylindrical tank containing xenon (called Time Projection Chamber , TPC) is placed in the center of another even larger tank filled with water. This constitutes the veto of muons: here light is generated by cosmic rays (the muons, in fact, see muon veto), which are undesired signals to be discarded.

The DAQ system of Xenon1T

The logic of the DAQ system is as follows: acquisition of the signals of all the photomultipliers, storage of the data within a database, grouping and selection of potentially interesting events (the interactions being searched). Each of the 332 photomultipliers (248 in the TPC and 84 in the veto of muons) is connected to an analogical digital converter, that is an electronic card that transforms the electrical impulse produced by the photomultiplier when it detects light in a digital signal. These signals travel in separate and independent "channels", all of which are (see Photomultiplaier calibration) temporally synchronized. Whenever a photomultiplier produces a signal, its characteristics are recorded (together with the exact time it took place) in a database, which is continuously inspected by a software whose job is to find and reconstruct the relevant signals from the data. The software carries out the reconstruction through five consecutive steps. First of all it checks that the signal is not due to the intrinsic "noise" of the channel, caused by the instrument itself, checking that the intensity of the pulse is higher than a certain threshold, for a certain interval of time (in the order of hundreds of nanoseconds). Subsequently, the software compares the signals recorded by other channels to relatively close times with respect to the signal in question, grouping them according to a procedure that takes into account their spatial distribution and their different creation times (remember that the signals are produced by the arrival of photons on two flat grids of the photomultipliers, at the top and at the bottom of the TPC). Then the software calculates various properties of each group of signals, such as area and width, and uses this information to reconstruct the position on the horizontal plane of the interaction that caused the light pulses. Subsequently the groups are classified as S1 and S2 (first signal and second signal produced by a single interaction) considering again the shape of the signals, but this time looking at the time in which the signal reaches its maximum intensity. Finally, the software calculates every possible coupling between the groups of S1 and S2, reconstructing, for each pair, the height at which the interaction took place in the TPC, using the information previously obtained on the horizontal positions and integrating them with the time delay between S1 and S2. The signals of the muon veto arrive at the same DAQ system that manages the signals coming from the TPC. However, muon veto data is sent to the database (which is used in data processing to reject TPC signals) only when a certain number of signals coming from its photomultipliers are recorded in a given time interval. From this processing we obtain a set of files with the information of the significant reconstructed signals. Once a sufficient volume of data is acquired, we proceed with the statistical analysis, to establish whether there have been interactions of dark matter particles. This is the phase in which the data obtained from the DAQ system is processed taking into consideration in a quantitative manner all possible sources of uncertainty connected to the measurements carried out. To this end, the knowledge acquired during the calibration phase of the experimental apparatus is exploited: for example, how many and which interactions are expected to be found, in a given period of time, due to "background sources" that create undesired signals in the detector, such as the presence of traces of radioactive contaminants in xenon.