Periodic Reporting for period 1 - Si4DM (Silicon for Direct Detection of Dark Matter in DarkSide)
Berichtszeitraum: 2021-09-01 bis 2023-02-28
Many astrophysical observations suggest that ordinary matter- which makes up galaxies, the Earth, us- only accounts for less than 5% of the Universe. Most of the matter in the universe, which is non-luminous and thus referred to as dark matter (DM), is unknown. Unveiling the nature of DM is one of the most fundamental open questions in Physics today.
The search for WIMPs (weakly interacting massive particles), one of the most favoured class of DM candidates, with predicted mass in the range of 1 to 10000 proton masses, is among the highest priorities of astroparticle physics for the next decade. An increasingly important area of the field is searches for DM candidates beyond the WIMP paradigm, most notably at lower masses. This complementary region of the parameter space is extraordinarily challenging to address experimentally, due to the low detection thresholds required, the difficult background mitigation and challenging detector calibration.
Silicon detectors are one of the most promising techniques to overcome the experimental challenges. The DarkSide experiment aims to discover DM by observing its interactions with ordinary matter in an ultra-sensitive liquid argon detector. DarkSide's major technological advance is to employ a novel silicon-based light detector system (silicon photo-multiplier, SiPMs), specifically developed for the experiment, to observe tiny signals that may be created in the argon following a DM particle interaction. While these devices enable a projected 100x increase in sensitivity over current results for WIMP DM candidates, the full range of potential applications is yet unexplored.
My proposed research leverages the unique opportunities of the novel DarkSide SiPM and aims at further developing this technology, with potential application in future particle astrophysics experiments and industry. The main goal of the programme is to perform new searches for light DM interactions in the SiPMs themselves, a novel approach profiting from the large mass of ultra-radio-pure silicon deployed in the DarkSide detector. This will significantly broaden discovery reach, to a yet-uncovered and compelling mass range.
The search for WIMPs (weakly interacting massive particles), one of the most favoured class of DM candidates, with predicted mass in the range of 1 to 10000 proton masses, is among the highest priorities of astroparticle physics for the next decade. An increasingly important area of the field is searches for DM candidates beyond the WIMP paradigm, most notably at lower masses. This complementary region of the parameter space is extraordinarily challenging to address experimentally, due to the low detection thresholds required, the difficult background mitigation and challenging detector calibration.
Silicon detectors are one of the most promising techniques to overcome the experimental challenges. The DarkSide experiment aims to discover DM by observing its interactions with ordinary matter in an ultra-sensitive liquid argon detector. DarkSide's major technological advance is to employ a novel silicon-based light detector system (silicon photo-multiplier, SiPMs), specifically developed for the experiment, to observe tiny signals that may be created in the argon following a DM particle interaction. While these devices enable a projected 100x increase in sensitivity over current results for WIMP DM candidates, the full range of potential applications is yet unexplored.
My proposed research leverages the unique opportunities of the novel DarkSide SiPM and aims at further developing this technology, with potential application in future particle astrophysics experiments and industry. The main goal of the programme is to perform new searches for light DM interactions in the SiPMs themselves, a novel approach profiting from the large mass of ultra-radio-pure silicon deployed in the DarkSide detector. This will significantly broaden discovery reach, to a yet-uncovered and compelling mass range.
The Action aims are the characterization of the energy measurement in silicon photomultipliers and the development of background rejection techniques, to enable an event-by-event and a sidereal modulation dark matter searches.
Part of the work realised during the first part of the Action involved the upgrade and operation of a the LEIM test setup at TRIUMF. LEIM enables to expose a SiPM sample, operated in a light tight enclosure and at cryogenic temperature, to a collimated LASER light at different wavelength (visible and NIR). The pulsed and collimated LASER beam can be directed on a single SIPM pixel (a SPAD of 30x30 um2) and the response of the SiPM is recorded. Incident light at different wavelengths is used to generate electron-hole pairs at different depths in the bulk of the silicon. A complete dataset, corresponding to several combinations of wavelengths and operating voltages, has been acquired, to study the response of the SiPM to charges generated in the bulk. This data will enable the measurement of the hole lifetime in the bulk of the sensors.
During the secondment phase of the Action, I commissioned at RHUL a second test setup to operate SiPM devices in a liquid nitrogen bath, to test long term stability of the DarkSide SiPMs and for the determination of PDFs for internal and external backgrounds. In addition, a SiPM sample was exposed to a 10 keV X-ray source, to generate ionization in the bulk of the silicon and measure the corresponding detector response. This data will serve as input for the dark matter searches, providing background pdfs and assessing the resolution of the energy estimation.
While two publications are in preparation, the status of the data analysis and the prospects of this Action have been discussed in seminars (at TRIUMF and GSSI, IT) with members of the scientific community.
Part of the work realised during the first part of the Action involved the upgrade and operation of a the LEIM test setup at TRIUMF. LEIM enables to expose a SiPM sample, operated in a light tight enclosure and at cryogenic temperature, to a collimated LASER light at different wavelength (visible and NIR). The pulsed and collimated LASER beam can be directed on a single SIPM pixel (a SPAD of 30x30 um2) and the response of the SiPM is recorded. Incident light at different wavelengths is used to generate electron-hole pairs at different depths in the bulk of the silicon. A complete dataset, corresponding to several combinations of wavelengths and operating voltages, has been acquired, to study the response of the SiPM to charges generated in the bulk. This data will enable the measurement of the hole lifetime in the bulk of the sensors.
During the secondment phase of the Action, I commissioned at RHUL a second test setup to operate SiPM devices in a liquid nitrogen bath, to test long term stability of the DarkSide SiPMs and for the determination of PDFs for internal and external backgrounds. In addition, a SiPM sample was exposed to a 10 keV X-ray source, to generate ionization in the bulk of the silicon and measure the corresponding detector response. This data will serve as input for the dark matter searches, providing background pdfs and assessing the resolution of the energy estimation.
While two publications are in preparation, the status of the data analysis and the prospects of this Action have been discussed in seminars (at TRIUMF and GSSI, IT) with members of the scientific community.
One of the main outcome of the first year of project is the measurement of delayed avalanches generated with a pulsed LASER source and an X-ray source in the DarkSide SiPMS. This is done for the first time and it is a key enabler for the planned dark matter searches in the SiPM silicon.