Periodic Reporting for period 2 - POKER (Positron resonant annihilation into darK mattER)
Berichtszeitraum: 2022-06-01 bis 2023-11-30
Multiple, independent cosmological and astrophysical observations allow us to conclude that 85% of the mass of the Universe is made by an unknown type of matter, called “Dark Matter” (DM), that does not emit or absorb light. Unveiling the nature of DM is one of the open questions in contemporary physics, by answering to fundamental questions such as “what is DM made of?” and “does it interact with ordinary matter through any non-gravitational mechanism?”. Up to now, despite the large number of laboratory experiment, no evidences for DM have been found, and the aforementioned questions are still open. Among the different theories that have been proposed to explain the DM microscopic nature, the “Light Dark Matter” hypothesis (LDM) assumes that it is made by new light particles, with masses comparable to that of the protons and neutrons. To maintain consistency with the astrophysical observations, reproducing the fraction of DM mass in our Universe, it is necessary to introduce a new interaction mechanism between LDM and ordinary matter, different from those included in the Standard Model. In the simplest hypothesis, the new interaction is mediated by a new massive particle, the “dark photon”, with properties similar to that of the ordinary photon, fleebly interacting with ordinary charged particles.
POKER aims to search for LDM through an accelerator-based experiment, exploiting an innovative technique. In the experiment, a high-energy positron beam is focused toward a thick active target, capable of measuring the energy deposited by each impinging particle. LDM could be produced by the annihilation of the positron with an atomic electron. Since LDM would then escape from the detector without any further interaction, the corresponding signature would be the observation of a large “missing energy”, defined as the difference between the nominal beam energy and that measured by the detector. The existence of other “traditional” processes that may mimic the signal, such as the production of neutrons in the target, requires the use of a sophisticated detector, with enhanced signal efficiency and background rejection capabilities.
The final goal of POKER is to run a pilot measurement at CERN, exploiting the high-energy positrons available at the H4 beamline, to demonstrate this technique and pave the road to a future high-statistics experiment, capable of confirming or ruling out the LDM hypothesis.
POKER aims to search for LDM through an accelerator-based experiment, exploiting an innovative technique. In the experiment, a high-energy positron beam is focused toward a thick active target, capable of measuring the energy deposited by each impinging particle. LDM could be produced by the annihilation of the positron with an atomic electron. Since LDM would then escape from the detector without any further interaction, the corresponding signature would be the observation of a large “missing energy”, defined as the difference between the nominal beam energy and that measured by the detector. The existence of other “traditional” processes that may mimic the signal, such as the production of neutrons in the target, requires the use of a sophisticated detector, with enhanced signal efficiency and background rejection capabilities.
The final goal of POKER is to run a pilot measurement at CERN, exploiting the high-energy positrons available at the H4 beamline, to demonstrate this technique and pave the road to a future high-statistics experiment, capable of confirming or ruling out the LDM hypothesis.
The first part of the POKER project was devoted to the evaluation of the expected pilot run performances, in terms of signal and background yields. This was performed by running Monte Carlo numerical simulations. We found that current Monte Carlo softwares used in particle physics describe reasonably the most critical background processes that could affect the experiment. On the other hand, we had to design and implement an ad-hoc program to simulate the signal production, the “DMG4” software, and to integrate it in the Monte Carlo codes.
Starting from these results, we designed the optimized detector for the POKER pilot run measurement. We plan to use part of the existing equipment at CERN developed by the NA64 collaboration, currently performing a similar experimental program (based on the use of an electron beam) to search for LDM, but to design and construct a new active target, with improved performances. This has to fulfill very strong requirements to support the physics measurement: it must be capable of detecting each impinging particle at very large (MHz) rate, with excellent energy resolution, and to operate in a high radiation dose environment. The calorimeter will be made by lead tungstate (PbWO4) scintillating crystals, with SiPM used for readout. We went through an intense R&D program to characterize each component of the detector (the crystals, the photosensors, the readout electronics, the mechanics and cooling), after identifying for each of these the most appropriate solution. To validate the overall design, we constructed a small-scale prototype, “POKERINO”, that will be tested at CERN in July 2023.
In preparation to the pilot run at CERN, in 2022 we performed a dedicated measurement to characterize the properties of the H4 positron beam, in particular regarding the purity. Our measurement showed that the contamination of hadrons, when operating in positive-charge mode, is a factor approximately ten higher than during standard, negative charge operations; this was also confirmed by simulations. To solve this, we are currently investigating how to improve the existing NA64 positrons identification system. A backup solution will be to slightly lower the beam energy, by about 60-80%, since our simulations predict that, in this configuration, the hadronic contamination will be again within a tolerable level for the measurement.
Starting from these results, we designed the optimized detector for the POKER pilot run measurement. We plan to use part of the existing equipment at CERN developed by the NA64 collaboration, currently performing a similar experimental program (based on the use of an electron beam) to search for LDM, but to design and construct a new active target, with improved performances. This has to fulfill very strong requirements to support the physics measurement: it must be capable of detecting each impinging particle at very large (MHz) rate, with excellent energy resolution, and to operate in a high radiation dose environment. The calorimeter will be made by lead tungstate (PbWO4) scintillating crystals, with SiPM used for readout. We went through an intense R&D program to characterize each component of the detector (the crystals, the photosensors, the readout electronics, the mechanics and cooling), after identifying for each of these the most appropriate solution. To validate the overall design, we constructed a small-scale prototype, “POKERINO”, that will be tested at CERN in July 2023.
In preparation to the pilot run at CERN, in 2022 we performed a dedicated measurement to characterize the properties of the H4 positron beam, in particular regarding the purity. Our measurement showed that the contamination of hadrons, when operating in positive-charge mode, is a factor approximately ten higher than during standard, negative charge operations; this was also confirmed by simulations. To solve this, we are currently investigating how to improve the existing NA64 positrons identification system. A backup solution will be to slightly lower the beam energy, by about 60-80%, since our simulations predict that, in this configuration, the hadronic contamination will be again within a tolerable level for the measurement.
The main result expected by the end of the POKER project will be the completion of the positron-beam, missing-energy measurement at CERN with the optimized detector; the corresponding results will be presented in one or more scientific publications.
Nevertheless, two main achievements have been already obtained by the project. First, we extended the idea of exploiting the electron-positron annihilation mechanism for LDM production to electron-beam missing energy experiments. Specifically, we observed that the contribution of secondary positrons produced in the target is crucial for the evaluation of the LDM signal. We thus re-analyzed past results from NA64, obtaining update exclusion limits and exploring a so-far unknown region of the LDM parameters space; this result has been summarized in a dedicated publication, “Improved exclusion limit for light dark matter from e+e- annihilation in NA64”.
Second, thanks to the enthusiastic support that POKER obtained at CERN by NA64, in 2022 we run a first low-statistics missing-energy measurement to search for light dark matter with a high-energy positron beam, using the existing, NA64 experimental setup. We could identify the most critical challenges for the measurement, and optimize the POKER detector for these. The data analysis is in progress, and a scientific publication is expected by the end of 2023.
Nevertheless, two main achievements have been already obtained by the project. First, we extended the idea of exploiting the electron-positron annihilation mechanism for LDM production to electron-beam missing energy experiments. Specifically, we observed that the contribution of secondary positrons produced in the target is crucial for the evaluation of the LDM signal. We thus re-analyzed past results from NA64, obtaining update exclusion limits and exploring a so-far unknown region of the LDM parameters space; this result has been summarized in a dedicated publication, “Improved exclusion limit for light dark matter from e+e- annihilation in NA64”.
Second, thanks to the enthusiastic support that POKER obtained at CERN by NA64, in 2022 we run a first low-statistics missing-energy measurement to search for light dark matter with a high-energy positron beam, using the existing, NA64 experimental setup. We could identify the most critical challenges for the measurement, and optimize the POKER detector for these. The data analysis is in progress, and a scientific publication is expected by the end of 2023.