Final Report Summary - TREX (Novel Developments in Time Projection Chambers (TPCs) for Rare Event Searches in Underground Astroparticle EXperiments)
Research at the frontier of particle physics often requires the search for phenomena of extremely low probability of occurrence, generically called "rare events". Under this category falls the search for new hypothetical particles potentially composing the mysterious dark matter (DM) halo of our galaxy, like e.g. the Weakly Interacting Massive Particles (WIMPs) or the axions. Similar experimental requirements are shared by experiments searching the neutrinoless double beta decay (DBD), a very rare decay of some nuclei that, if detected, would provide precious information on the mass and nature of the neutrino. Being low energy events, they are buried under much higher levels of background events from environmental radiation. The task is challenging: to reduce background levels to extremely low rates and/or to ingeniously devise strategies to get distinctive signal features that help "see the needle in the haystack". Both DM and DBD experiments are among the highest priorities in current modern particle physics roadmaps and are typically carried out by large international collaborations. It could be that a breakthrough discovery in one or more of these searches is awaiting us, and it may happen already during the next decade.
The T-REX project has explored the concept of Gaseous Time Projection Chamber (GTPC) to search for these rare events. Our claim has been that recent developments in GTPC readouts, especially the micromesh gas structures or Micromegas, allow cost-effectively building large size GTPCs of improved performance, with enhanced robustness and extreme cleanliness from the point of view of radioactivity. Within these perspectives, the tracking capability of GTPCs can make them very competitive detectors for signal identification and background discrimination. The goal of T-REX has been to demonstrate these claims with the construction of several small and medium scale prototypes that would tune and prepare the technique for eventual application in current and future full scale experiments. A number of such prototypes have proven an outstanding performance and have attracted the attention of several collaborations in the community. In some cases they have already been implemented in real experiments with physics results, in other cases concrete plans are ongoing for their implementation in the near future.
A generic result of T-REX has been the confirmation that the radioactivity level of Micromegas planes can be indeed extremely low (below 0.1 microBq/cm2 of the natural Th and U chains), making them suitable components of very low background setups. In the field of axion searches, several Micromegas x-ray detectors developed as part of T-REX have been installed and operated in the CAST experiment at CERN, currently looking for solar axions. Record background values, below 1 count/keV/s/cm2, have been achieved, the lowest ever achieved in any detector exposed to an external x-ray flux. This has allowed the experiment to improve its sensitivity in the most recent data taking campaigns. CAST has been the first axion helioscope entering unexplored axion and axion-like-particle parameter space. The final result of the experiment from 2014 and 2015 data, still not released, will slightly touch the range of the axion-photon coupling values hinted by some astrophysical observations, so a surprise is not excluded. A next generation axion helioscope, the International Axion Observatory (IAXO) has been proposed as a future larger-scale follow-up of CAST. IAXO will surpass CAST signal-to-noise ratio by five orders of magnitude and will deeply enter into new territory. The Micromegas detectors developed within T-REX stand as one of the most promising detection technique candidates for IAXO.
In the field of WIMP searches a very interesting window of opportunity for the GTPC technique has been identified. After more than a decade of impressive improvement in sensitivity in “mainstream” WIMP experiments without a positive signal, attention is shifting to less standard WIMP models. Among these are the WIMPs with particularly low mass (i.e. below 10 GeV) that would leave energy deposits of too low energy in normal experiments. Thanks to the amplification in gas (and the correspondingly low energy threshold), and their flexibility in target gases, GTPCs may be especially effective to search for these type of WIMPs. The TREX-DM prototype has been developed particularly to explore low-background and low-threshold performance in a relatively large volume of detection. The detector has been successfully commissioned on surface, and prospects anticipated from an exhaustive screening campaign are that very competitive background levels, maybe below 1 count/keV/kg/day could be achievable, together with a threshold well below 1 keV. These prospects must be experimentally realized by operating the prototype underground, something that is now being planned. If this is confirmed, this technique could be sensitive to signals of WIMPs of low mass (2 to 8 GeV) well beyond current best limits in the field and, again, with possibility for discovery. In case no WIMP is detected, there will be motivation to increase the size of the detector and to further reduce its background. The technique has good scalability prospects and there is a priori no evident showstopper to further reduce its intrinsic radioactivity.
In the field of DBD searches, high pressure Xenon GTPC can offer a truly unique handle for background reduction that is not at reach of any other detection technique. The DBD event in gas offers a particular topology that, if properly imaged, can be used for efficient identification against background events. In order to be competitive, however, at least 100 kg of enriched Xe must be instrumented. If the DBD is not detected in current generation experiments, a ton-scale experiment will be needed as the next milestone in the field. With the TREX-DBD prototypes we have demonstrated that Micromegas readout planes can operate in high pressure Xe with extremely good performance, provided a proper additive is used like e.g. trimethylamine (TMA) The addition of ~1% TMA improves the gain, stability, energy resolution of the Micromegas planes. In addition, the electron diffusion reduces dramatically (a factor 10 better than pure Xe) leading to exquisite spatial definition (a blurring of ~1 mm for ~20 cm long events having drifted 1 meter). In overall, the technique shows promise to reach very competitive background levels corresponding to less than 1 count per year in the region of interest for a 100 kg detector. These activities have served to provide technological feedback during an early stage of the NEXT DBD experiment. More recently, a new international collaboration, PandaX-III, has put forward an ambitious schedule aiming to construct a 200 kg Xe GTPC fully based on the results obtained by the TREX-DBD prototypes. Thanks to the results of T-REX, the GTPC technology for DBD is now among the few under consideration in international roadmaps for the future ton-scale experiment, a decision that will happen in the coming 2-3 years, after the results of the current generation of experiments.
To conclude, the challenges ahead in the “rare event frontier” are posing strong demands on detector technologies. T-REX has demonstrated that Gas TPCs can play an important role in next generation rare event experiments, identifying particular physics cases for which they can make a definite impact. In axion physics, they have already improved current best sensitivities in the CAST experiment. Several other international initiatives, both regarding DM and DBD experiments, have been triggered by T-REX results and will apply these detection concepts at the necessary scale to improve current state-of-the-art. It is possible that a positive detection in one or more of these types of experiments happens in the coming years. Such a discovery would have groundbreaking consequences for our knowledge of the foundations of particle physics and the nature of the dark Universe.
The T-REX project has explored the concept of Gaseous Time Projection Chamber (GTPC) to search for these rare events. Our claim has been that recent developments in GTPC readouts, especially the micromesh gas structures or Micromegas, allow cost-effectively building large size GTPCs of improved performance, with enhanced robustness and extreme cleanliness from the point of view of radioactivity. Within these perspectives, the tracking capability of GTPCs can make them very competitive detectors for signal identification and background discrimination. The goal of T-REX has been to demonstrate these claims with the construction of several small and medium scale prototypes that would tune and prepare the technique for eventual application in current and future full scale experiments. A number of such prototypes have proven an outstanding performance and have attracted the attention of several collaborations in the community. In some cases they have already been implemented in real experiments with physics results, in other cases concrete plans are ongoing for their implementation in the near future.
A generic result of T-REX has been the confirmation that the radioactivity level of Micromegas planes can be indeed extremely low (below 0.1 microBq/cm2 of the natural Th and U chains), making them suitable components of very low background setups. In the field of axion searches, several Micromegas x-ray detectors developed as part of T-REX have been installed and operated in the CAST experiment at CERN, currently looking for solar axions. Record background values, below 1 count/keV/s/cm2, have been achieved, the lowest ever achieved in any detector exposed to an external x-ray flux. This has allowed the experiment to improve its sensitivity in the most recent data taking campaigns. CAST has been the first axion helioscope entering unexplored axion and axion-like-particle parameter space. The final result of the experiment from 2014 and 2015 data, still not released, will slightly touch the range of the axion-photon coupling values hinted by some astrophysical observations, so a surprise is not excluded. A next generation axion helioscope, the International Axion Observatory (IAXO) has been proposed as a future larger-scale follow-up of CAST. IAXO will surpass CAST signal-to-noise ratio by five orders of magnitude and will deeply enter into new territory. The Micromegas detectors developed within T-REX stand as one of the most promising detection technique candidates for IAXO.
In the field of WIMP searches a very interesting window of opportunity for the GTPC technique has been identified. After more than a decade of impressive improvement in sensitivity in “mainstream” WIMP experiments without a positive signal, attention is shifting to less standard WIMP models. Among these are the WIMPs with particularly low mass (i.e. below 10 GeV) that would leave energy deposits of too low energy in normal experiments. Thanks to the amplification in gas (and the correspondingly low energy threshold), and their flexibility in target gases, GTPCs may be especially effective to search for these type of WIMPs. The TREX-DM prototype has been developed particularly to explore low-background and low-threshold performance in a relatively large volume of detection. The detector has been successfully commissioned on surface, and prospects anticipated from an exhaustive screening campaign are that very competitive background levels, maybe below 1 count/keV/kg/day could be achievable, together with a threshold well below 1 keV. These prospects must be experimentally realized by operating the prototype underground, something that is now being planned. If this is confirmed, this technique could be sensitive to signals of WIMPs of low mass (2 to 8 GeV) well beyond current best limits in the field and, again, with possibility for discovery. In case no WIMP is detected, there will be motivation to increase the size of the detector and to further reduce its background. The technique has good scalability prospects and there is a priori no evident showstopper to further reduce its intrinsic radioactivity.
In the field of DBD searches, high pressure Xenon GTPC can offer a truly unique handle for background reduction that is not at reach of any other detection technique. The DBD event in gas offers a particular topology that, if properly imaged, can be used for efficient identification against background events. In order to be competitive, however, at least 100 kg of enriched Xe must be instrumented. If the DBD is not detected in current generation experiments, a ton-scale experiment will be needed as the next milestone in the field. With the TREX-DBD prototypes we have demonstrated that Micromegas readout planes can operate in high pressure Xe with extremely good performance, provided a proper additive is used like e.g. trimethylamine (TMA) The addition of ~1% TMA improves the gain, stability, energy resolution of the Micromegas planes. In addition, the electron diffusion reduces dramatically (a factor 10 better than pure Xe) leading to exquisite spatial definition (a blurring of ~1 mm for ~20 cm long events having drifted 1 meter). In overall, the technique shows promise to reach very competitive background levels corresponding to less than 1 count per year in the region of interest for a 100 kg detector. These activities have served to provide technological feedback during an early stage of the NEXT DBD experiment. More recently, a new international collaboration, PandaX-III, has put forward an ambitious schedule aiming to construct a 200 kg Xe GTPC fully based on the results obtained by the TREX-DBD prototypes. Thanks to the results of T-REX, the GTPC technology for DBD is now among the few under consideration in international roadmaps for the future ton-scale experiment, a decision that will happen in the coming 2-3 years, after the results of the current generation of experiments.
To conclude, the challenges ahead in the “rare event frontier” are posing strong demands on detector technologies. T-REX has demonstrated that Gas TPCs can play an important role in next generation rare event experiments, identifying particular physics cases for which they can make a definite impact. In axion physics, they have already improved current best sensitivities in the CAST experiment. Several other international initiatives, both regarding DM and DBD experiments, have been triggered by T-REX results and will apply these detection concepts at the necessary scale to improve current state-of-the-art. It is possible that a positive detection in one or more of these types of experiments happens in the coming years. Such a discovery would have groundbreaking consequences for our knowledge of the foundations of particle physics and the nature of the dark Universe.