Search For Events Research Articles

Alpha -event characterization and rejection in point-contact HPGe detectors

P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector’s response to alpha particles incident on the sensitive passivated and p^+ surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the MajoranaDemonstrator experiment, a search for neutrinoless double-beta decay (0nu beta beta ) in ^{76}Ge. alpha decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of alpha identification, reliably identifying alpha background events on the passivated surface of the detector. We demonstrate effective rejection of all surface alpha events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the 0nu beta beta region of interest window by an order of magnitude in the MajoranaDemonstrator and will be used in the upcoming LEGEND-200 experiment.

Measurements of gamma ray, cosmic muon and residual neutron background fluxes for rare event search experiments at an underground laboratory

Ambient radiation background contributed by the penetrating cosmic ray particles and the radionuclides present in the rock materials have been measured at an underground laboratory at Jaduguda, Jharkhand, India, located inside a mine at 555 m depth. The laboratory is being set up to explore rare event search processes, such as direct dark matter search, neutrinoless double beta decay, axion search, supernova neutrino detection, etc., that require specific knowledge of the nature and extent of the radiation environment in order to assess the sensitivity reach and also to plan for its reduction for the targeted experiment. The gamma ray background, which is mostly contributed by the primordial radionuclides and their decay chain products, have been measured inside the laboratory and found to be dominated by rock radioactivity for Eγ≲3MeV. Shielding of these residual gamma rays for the experiment was also evaluated. The cosmic muon flux, measured inside the laboratory using large area plastic scintillator telescope, was found to be: (2.257±0.261±0.042)×10−7cm−2s−1, which agrees reasonably well with simulation results. The neutron background flux has been measured for the radiogenic neutrons and found to be: (1.61±0.03)×10−4cm−2s−1 for no threshold cut. Detailed GEANT4 simulation for the radiogenic neutrons and the cosmogenic neutrons have been carried out. Effects of multiple scattering of both the types of neutrons within the surrounding rock and the cavern walls were studied and the results for the radiogenic neutrons are found to be in reasonable agreement with experimental results. Neutron fluxes contributed by those neutrons of cosmogenic origin have been reported as function of the energy threshold.

Searching for Low Probability Opening Events in a DNA Sliding Clamp.

The subunit of E. coli DNA polymererase III is a DNA sliding clamp associated with increasing the processivity of DNA synthesis. In its free form, it is a circular homodimer structure that can accomodate double-stranded DNA in a nonspecific manner. An open state of the clamp must be accessible before loading the DNA. The opening mechanism is still a matter of debate, as is the effect of bound DNA on opening/closing kinetics. We use a combination of atomistic, coarse-grained, and enhanced sampling strategies in both explicit and implicit solvents to identify opening events in the sliding clamp. Such simulations of large nucleic acid and their complexes are becoming available and are being driven by improvements in force fields and the creation of faster computers. Different models support alternative opening mechanisms, either through an in-plane or out-of-plane opening event. We further note some of the current limitations, despite advances, in modeling these highly charged systems with implicit solvent.

Performance of a radial time projection chamber with electroluminescence in liquid xenon

The dual-phase xenon time projection chamber (TPC) is a leading detector technology in rare event searches for dark matter and neutrino physics. The success of this type of detector technology relies on its capability to detect both primary scintillation and ionization signals from particle interactions in liquid xenon (LXe). The ionization electrons are converted into electroluminescence in the gas xenon (GXe), where a single electron can be amplified by more than 100 times in number of photons in a strong electric field. Maintaining a strong and uniform electric field in the small gas gap in large diameter TPCs is challenging. One alternative solution is to produce the electroluminescence in the LXe directly to overcome the gas gap uniformity problem. Here we report on the design and performance of a single-phase Radial TPC (RTPC) which can create and detect the electroluminescence directly in LXe. It simplifies the design and operation of the LXe TPC by using a single wire in the axial center to create the strong electric field. We present the performance of such an RTPC and discuss its limitations for potential applications.

Italian Research Facilities for Fundamental Physics

This Special Issue of Universe addresses the international community working at the Italian Research Facilities for Fundamental Physics, Italian labs and facilities playing a pivotal role in the core fields of this journal, such as gravitational waves, dark matter and rare event searches, neutrino astronomy, and underground physics [...]

A forensic-driven data model for automatic vehicles events analysis

Digital vision technologies emerged exponentially in all living areas to watch, play, control, or track events. Security checkpoints have benefited also from those technologies by integrating dedicated cameras in studied locations. The aim is to manage the vehicles accessing the inspection security point and fetching for any suspected ones. However, the gathered data volume continuously increases each day, making their analysis very hard and time-consuming. This paper uses semantic-based techniques to model the data flow between the cameras, checkpoints, and administrators. It uses ontologies to deal with the increased data size and its automatic analysis. It considers forensics requirements throughout the creation of the ontology modules to ensure the records’ admissibility for any possible investigation purposes. Ontology-based data modeling will help in the automatic events search and correlation to track suspicious vehicles efficiently.

REST-for-Physics, a ROOT-based framework for event oriented data analysis and combined Monte Carlo response

The REST-for-Physics (Rare Event Searches Toolkit for Physics) framework is a ROOT-based solution providing the means to process and analyze experimental or Monte Carlo event data. Special care has been taken to the traceability of the code and the validation of the results produced within the framework, together with the connectivity between code and stored data, registered through specific version metadata members.The framework development was originally motivated to cover the needs of Rare Event Searches experiments (experiments looking for phenomena having extremely low occurrence probability, like dark matter or neutrino interactions or rare nuclear decays). The framework components naturally implement tools to address the challenges in these kinds of experiments. The integration of a detector physics response, the implementation of signal processing routines, or topological algorithms for physical event identification are some examples. Despite this specialization, the framework was conceived thinking in scalability. Other event-oriented applications could benefit from the data processing routines and/or metadata description implemented in REST, being the generic framework tools completely decoupled from dedicated libraries.REST-for-Physics is a consolidated piece of software already serving the needs of different physics experiments - using gaseous Time Projection Chambers (TPCs) as detection technology - for detector data analysis and characterization, as well as generic R&D. Even though REST has been exploited mainly with gaseous TPCs, the code could be easily applied or adapted to other detector technologies. We present in this work an overview of REST-for-Physics, providing a broad perspective to the infrastructure and organization of the project as a whole. The framework and its different components will be described in the text.

Production and validation of scintillating structural components from low-background Poly(ethylene naphthalate)

Poly Ethylene Naphthalate (PEN) is an industrial polymer plastic which is investigated as a low background, transparent, scintillating and wavelength shifting structural material. PEN scintillates in the blue region and has excellent mechanical properties both at room and cryogenic temperatures. Thus, it is an ideal candidate for active structural components in experiments for the search of rare events like neutrinoless double-beta decay or dark matter recoils. Such optically active structures improve the identification and rejection efficiency of backgrounds events, like this improving the sensitivity of experiments. This paper reports on the production of radiopure and transparent PEN plates These structures can be used to mount germanium detectors operating in cryogenic liquids (LAr, LN). Thus, as first application PEN holders will be used to mount the Ge detectors in the Legend-200 experiment. The whole process from cleaning the raw material to testing the PEN active components under final operational conditions is reported.

The search for baseline events for assessing the safety of nuclear refueling operations at nuclear power plants

The relevance of the topic of the safety of nuclear refueling operations is associated with the specificity of exploitation of RBMK units. One of the most hazardous, from the perspective of accidents at modern nuclear power plants, is the process of nuclear fuel reloading. The operations on rearrangement of fuel cartridges entail the risk of fuel damage, and thus, the likelihood of the release of radioactive substances exceeding the permissible limits. The process of reloading RBMK, if the reactor is at full capacity, consists of the vast number of complex operations characterized by a range parameters. Non-observance of the criteria for carrying out operations, or if the parameter values exceed permissible limits, with high probability leads to an accident. This article explores the possibility of application of formalized approach towards determination of the baseline events that may cause accidents for the purpose of the development of essential protection instruments. The formal approach would allow detecting the excessiveness in protection instruments on the existing blocks, as well as revealing the accident situations that cannot be prevented using these protection instruments. The author formulated systemic approach towards comprehensive assessment of the accident rate of structurally complex systems. Adaptation of this method relative to REM allows systematizing the search for baseline vents that entail uncontrolled situations, as well as optimizing the protection instruments that would ultimately enhance reliability of the system, simplify the exploitation process, and reduce the time of operating cycle of the controller for processing of the protection.

Toverline{t}toverline{t} signatures through the lens of color-octet scalars

We reinterpret two recent LHC searches for events containing four top quarks left(toverline{t}toverline{t}right) in the context of supersymmetric models with Dirac gauginos and color-octet scalars (sgluons). We explore whether sgluon contributions to the four-top production cross section sigma left( ppto toverline{t}toverline{t}right) can accommodate an excess of four-top events recently reported by the ATLAS collaboration. We also study constraints on these models from an ATLAS search for new phenomena with high jet multiplicity and significant missing transverse energy left({E}_{mathrm{T}}^{mathrm{miss}}right) sensitive to signals with four top quarks. We find that these two analyses provide complementary constraints, with the jets + {E}_{mathrm{T}}^{mathrm{miss}} search exceeding the four-top cross section measurement in sensitivity for sgluons heavier than about 800 GeV. We ultimately find that either a scalar or a pseudoscalar sgluon can currently fit the ATLAS excess in a range of reasonable benchmark scenarios, though a pseudoscalar in minimal Dirac gaugino models is ruled out. We finally offer sensitivity projections for these analyses at the HL-LHC, mapping the 5σ discovery potential in sgluon parameter space and computing exclusion limits at 95% CL in scenarios where no excess is found.

Growth and spectroscopic investigations of Na2W2O7 single crystals doped with cerium and chromium ions

Alkaline tungstate crystals are promising scintillation materials for rare events search, especially for WIMP investigations. Na2W2O7 single crystals doped with 1.0 mol.% chromium and cerium ions were grown from melt by low-thermal-gradient Czochralski technique (LTG Cz) developed at NIIC SB RAS (Novosibirsk, Russia) for obtaining large oxide scintillating crystals of high optical quality. Obtained doped Na2W2O7 single crystals 25 mm in length and 30 mm in diameter. The excitation and emission spectra of the photoluminescence, as well as the decay curves in the temperature range 77—300 K were measured and colorimetric parameters were estimated. Comparative studies of the photoluminescence properties of undoped and doped with Ce3+ and Cr3+ ions Na2W2O7 crystals in the temperature range of 77—300 K have been carried out. It was shown that while the introduction of cerium and chromium ions in the Na2W2O7 crystal lattice didn¢t significantly effect the colorimetric parameters and optical properties, it did increase mechanical hardness of the crystals and decreased the tendency to cracking along cleavage planes. The emission color chromaticity coordinates, сorrelated color temperature and color purity for Na2W2O7:Ce and Na2W2O7:Cr crystals were calculated. The lifetimes of photoluminescence were calculated and a strong temperature dependence of the lifetimes in the range 77—300 K was shown.

Determination of Double Beta Decay Half-Life of 136Xe with the PandaX-4T Natural Xenon Detector.

Precise measurement of two-neutrino double beta decay (DBD) half-life is an important step for the searches of Majorana neutrinos with neutrinoless double beta decay. We report the measurement of DBD half-life of 136Xe using the PandaX-4T dual-phase Time Projection Chamber (TPC) with 3.7-tonne natural xenon and the first 94.9-day physics data release. The background model in the fiducial volume is well constrained in situ by events in the outer active region. With a 136Xe exposure of 15.5 kg-year, we establish the half-life as 2.27 ± 0.03(stat.) ± 0.10(syst.) × 1021 years. This is the first DBD half-life measurement with natural xenon and demonstrates the physics capability of a large-scale liquid xenon TPC in the field of rare event searches.

Astroparticle physics in DUNE with the X-ARAPUCA detectors

DUNE is a long-baseline accelerator experiment in construction at Fermilab and SURF (South Dakota). The scientific program encompasses the investigation of long-baseline neutrino oscillation, with a focus on the study of the CP violation and neutrino mass hierarchy, along with high sensitivity searches for rare events, such as the observation of supernova neutrino bursts and the search for proton decay. The DUNE far detector consists of four liquid Argon TPCs located deep underground, paired with a Photon Detection System. In the present article we will discuss the role of the PDS in the search of the aforementioned rare events and the latest results in terms of technological achievements obtained by the PDS Consortium.

Search for pairs and groups in the 2006 Geminid meteor shower

Context. The existence of pairs and groups of meteors during meteor showers has been an open question for a long time. The double-station video observation of the 2006 Geminid meteor shower, one of the most active annual showers, is used for the search of such events. Aims. The goal of the paper is to determine whether the observed pairs of Geminid meteors are real events or cases of random coincidence. Methods. The atmospheric trajectories of the observed meteors, photometric masses, and both time and spatial distances of meteoroids in the atmosphere were determined using a double-station video observation. The time gaps between them were analysed statistically. The Monte Carlo simulation was used for the determination of the probability of random pairings. Results. A higher than expected number of candidates for pairs was found among 2006 Geminids. An evaluation of the Poisson distribution shows that a significant fraction of them may be real cases. However, the Monte Carlo simulation did not confirm this result and provided a different view. Analysis of geometrical positions of candidate pairs also did not support the presence of real pairs and groups. Although we cannot exclude that some of them may be physically connected pairs, all the observed cases can be explained as the coincidental appearance of unrelated meteors.

Search for Multi-Coincidence Cosmic Ray Events over Large Distances with the EEE MRPC Telescopes

The existence of independent, yet time correlated, Extensive Air Showers (EAS) has been discussed over the past years, with emphasis on possible physical mechanisms that could justify their observation. The detector network of the Extreme Energy Events (EEE) Collaboration, with its approximately 60 cosmic ray telescopes deployed over the Italian territory, has the potential to search for such events, employing different analysis strategies. In this paper, we have analyzed a set of EEE data, corresponding to an approximately five month observation period, searching for multi-coincidence events among several far telescopes, within a time window of 1 ms. Events with up to 12 coincident telescopes have been observed. Results were compared to expectations from a random distribution of events and discussed with reference to the relativistic dust grain hypothesis.

Identification of neutron sources and background levels in the polyethylene room of the China Jinping Underground Laboratory

The neutron backgrounds induced by supplementary experimental materials can result in contaminations in rare event search experiments. To address this, we present the neutron background levels arising from ambient materials in the polyethylene room of the China Jinping Underground Laboratory; particularly, we compare simulated spectra with measured neutron spectra unfolded using a genetic algorithm. The genetic algorithm optimizes the continuity of the energy spectra and obtains a reasonable spectral result. A good agreement between the unfolded and simulated spectra is achieved. Moreover, estimated neutron background levels of representative ambient materials such as polyethylene, aluminum, and lead are obtained using an exposure time of 511.27 days via a 28 liter 0.5%-gadolinium-doped liquid scintillator detector. The identification of rare neutron sources can aid in background reduction in next-generation large-scale rare event experiments.

Measurements and Simulation of background radiation for rare event search experiments at an underground laboratory

Experimental measurements and simulation of various radiation backgrounds at a new underground site have been presented. The experimental cosmic muon flux, measured at this site with the help of a muon telescope set-up, agree well with simulation results after accounting for the geometric effect of the detector assembly. The gamma ray background inside the laboratory space was measured to assess and optimize the required thickness of Lead for shielding. Neutrons, produced in the surrounding rock due to radioactive decay, followed by nuclear reactions with the rock constituents, and the faster neutrons, produced through interaction of the charged cosmic particles, propagate through the rock material to reach inside the laboratory. The neutron background flux was measured and GEANT4 simulations were carried out. The results were found to be in reasonable agreement with measurements.

A measurement of the mean electronic excitation energy of liquid xenon

Detectors using liquid xenon as target are widely deployed in rare event searches. Conclusions on the interacting particle rely on a precise reconstruction of the deposited energy which requires calibrations of the energy scale of the detector by means of radioactive sources. However, a microscopic calibration, i.e. the translation from the number of excitation quanta into deposited energy, also necessitates good knowledge of the energy required to produce single scintillation photons or ionisation electrons in liquid xenon. The sum of these excitation quanta is directly proportional to the deposited energy in the target. The proportionality constant is the mean excitation energy and is commonly known as W-value. Here we present a measurement of the W-value with electronic recoil interactions in a small dual-phase xenon time projection chamber with a hybrid (photomultiplier tube and silicon photomultipliers) photosensor configuration. Our result is based on calibrations at mathcal {O}(1{-}10,{hbox {keV}}) with internal {^{37}hbox {Ar}} and {^{83text {m}}hbox {Kr}} sources and single electron events. We obtain a value of W={11.5}{} , ^{+0.2}_{-0.3} , mathrm {(syst.)} , hbox {eV}, with negligible statistical uncertainty, which is lower than previously measured at these energies. If further confirmed, our result will be relevant for modelling the absolute response of liquid xenon detectors to particle interactions.

GERDA: Final Results and Physics Beyond Neutrinoless Double-Beta Decay

The GERDA experiment searched for the lepton number violating neutrinoless double-beta 0υββ decay of 76 Ge. Observation of this decay would provide answers to fundamental problems in particle physics and cosmology, including the origin of neutrino masses and baryon asymmetry in the universe. The GERDA experiment achieved the most stringent lower limit on the half-life of the 0υββ decay of 1.8 • 1026 yr at 90% C.L. (which coincides with the sensitivity) by operating high-purity germanium (HPGe) detectors enriched in 76Ge submerged in liquid argon (LAr). The collaboration could achieve this breakthrough by reducing the background event rate to 5.2 • 10–4 counts/(keV kg yr) at the end-point energy. This unprecedented background index could be achieved by developing unique technologies like utilizing the scintillation light of the LAr to reject efficiently background events that deposit energy simultaneously in the HPGe detectors and in LAr, and the pulse shape discrimination which exploits specific event topologies of backgrounds and signal candidates. Due to the ultra-low background approach the GERDA data is also suited for other rare event searches beyond the 0υββ decay like the search for super-WIMPs.

The cosmic muon-induced background for the LEGEND-1000 alternative site at LNGS

The in-situ production of long-lived radio-isotopes by cosmic muon interactions may generate a non-negligible background for rare event searches in the deep subsurface. The delayed decay of 77(m)Ge has been identified as the dominant in-situ cosmogenic contributor for a neutrinoless double-beta decay search with 76Ge. The future ton-scale LEGEND-1000 experiment requires a total background of ≤ 10−5 cts/(keV-kg-yr). Dedicated Monte Carlo studies of the 77(m)Ge background at the alternative LNGS site were performed. The addition of passive neutron moderators, in combination with a delayed coincidence strategy, results in a background contribution of 8.6 × 10−7 cts/(keV.kg.yr) with an additional dead time of < 9%.