Underground Muon Research Articles

Reconstruction of air shower muon lateral distribution functions using integrator and binary modes of underground muon detectors

The investigation of cosmic rays holds significant importance in the realm of particle physics, enabling us to expand our understanding beyond atomic confines. However, the origin and characteristics of ultra-high-energy cosmic rays remain elusive, making them a crucial topic of exploration in the field of astroparticle physics. Currently, our examination of these cosmic rays relies on studying the extensive air showers (EAS) generated as they interact with atmospheric nuclei during their passage through Earth’s atmosphere. Accurate comprehension of cosmic ray composition is vital in determining their source. Notably, the muon content of EAS and the atmospheric depth of the shower maximum serve as the most significant indicators of primary mass composition. In this study, we present two novel methods for reconstructing particle densities based on muon counts obtained from underground muon detectors (UMDs) at varying distances to the shower axis. Our methods were analyzed using Monte Carlo air shower simulations. To demonstrate these techniques, we utilized the muon content measurements from the UMD of the Pierre Auger cosmic ray Observatory, an array of detectors dedicated to measuring extensive air showers. Our newly developed reconstruction methods, employed with two distinct UMD data acquisition modes, showcased minimal bias and standard deviation. Furthermore, we conducted a comparative analysis of our approaches against previously established methodologies documented in existing literature.

Muon counting with the Underground Muon Detector of The Pierre Auger Observatory

The Underground Muon Detector (UMD) aims to extend the detection range of the Pierre Auger Observatory to observe cosmic rays of energies greater than 1016.6 eV and performs a direct measurement of the muon component (sensitive observable to the mass composition) of the air showers. In this work, we will present an overview of the final design of the UMD and its reconstruction techniques alongside the observations obtained during the engineering array phase. First results suggest that the current hadronic interaction models have a muon deficit at energies between 2×1017 eV and 2×1018 eV.

Understanding the Phase Reversals of Galactic Cosmic-Ray Anisotropies

Energy spectra and anisotropies are very important probes of the origin of cosmic rays. Recent measurements show that complicated but very interesting structures exist at similar energies in both the spectra and energy-dependent anisotropies, indicating a common origin of these structures. A particularly interesting phenomenon is that there is a reversal of the phase of the dipole anisotropies, which challenges theoretical modeling. In this work, for the first time, we identify that there might be an additional phase reversal at ∼100 GeV energies of the dipole anisotropies as indicated by a few underground muon detectors and the first direct measurement by the Fermi satellite, coincident with the hundreds of GV hardening of the spectra. We propose that these two phase reversals, together with the energy evolution of the amplitudes and spectra, can be naturally explained with a nearby source overlapping onto the diffuse background. As a consequence, the spectra and anisotropies can be understood as the scalar and vector components of this model, and the two reversals of the phases characterize just the competition of the cosmic-ray streamings between the nearby source and the background. The alignment of the cosmic-ray streamings along the local large-scale magnetic field may play an important but subdominant role in regulating the cosmic-ray propagation. More precise measurements of the anisotropy evolution at both low energies by space detectors and high energies by air shower experiments for individual species will be essential to further test this scenario.

Status of the AugerPrime upgrade of the Pierre Auger Observatory

The Pierre Auger Observatory consists of a detector system to study ultrahigh-energy cosmic rays. These cosmic rays can be detected only through the observation of extensive air showers. The hybrid detection of air showers at the Observatory is based on the Surface Detector (SD) – an array of about 1660 water-Cherenkov detectors, and the Fluorescence Detector, with 27 telescopes overlooking the SD. Recently, an upgrade of the Observatory was initiated, called AugerPrime. The main purpose of the upgrade is to improve the mass composition sensitivity of the SD through precise measurements of the muonic and electromagnetic components of extensive air showers. For this purpose, additional scintillator and radio detectors are being installed on top of SD stations. The upgrade also includes updated SD electronics, and underground muon detectors. In this talk, the motivation and the current status of the upgrade will be reviewed.

Estimation of the number of counts on a particle counter detector with full time resolution

We present a general method for estimating the number of particles impinging on a segmented counter or, in general, on a counter with sub-units. We account for unresolved particles, i.e., the effect of two or more particles hitting the same sub-unit almost simultaneously. To achieve full time resolution we account for the dead time that occurs after the first time-bin of a particle signal. This general counting method can be applied to counting muons in existing detectors like the Underground Muon Detector of the Pierre Auger Observatory. We therefore use the latter as a study case to test the performance of our method and to compare it to other methods from literature. Our method proves to perform with little bias, and also provides an estimate of the number of particles as a function of time (as seen by the detector) to a single time-bin resolution. In this context, the new method can be useful for reconstructing parameters sensitive to cosmic ray mass, which are key to unveiling the origin of cosmic rays.

Muon detector for underground tomography

We utilize cosmic ray muons to explore hidden underground archaeological structures. Presented here is the design, simulation studies and first laboratory results of a compact, scintillators based, cosmic ray muon telescope for underground muon radiography.

Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843–033 Region

HESS J1843–033 is a very high energy gamma-ray source whose origin remains unidentified. This work presents, for the first time, the energy spectrum of gamma rays beyond 100 TeV from the HESS J1843–033 region using the data recorded by the Tibet air shower array and its underground muon detector array. A gamma-ray source with an extension of 0.°34 ± 0.°12 is successfully detected above 25 TeV at (α, δ) = (281.°09 ± 0.°10, −3.°76 ± 0.°09) near HESS J1843–033 with a statistical significance of 6.2σ, and the source is named TASG J1844–038. The position of TASG J1844–038 is consistent with those of HESS J1843–033, eHWC J1842–035, and LHAASO J1843–0338. The measured gamma-ray energy spectrum in 25 TeV < E < 130 TeV is described with (E/40 TeV)−3.26±0.30 TeV−1 cm−2 s−1, and the spectral fit to the combined spectra of HESS J1843–033, LHAASO J1843–0338, and TASG J1844–038 implies the existence of a cutoff at 49.5 ± 9.0 TeV. Associations of TASG J1844–038 with SNR G28.6–0.1 and PSR J1844–0346 are also discussed in detail for the first time.

On the Accuracy of Underground Muon Intensity Calculations

Abstract Cosmic-ray muons detected by deep underground and underwater detectors have served as an information source on the high-energy cosmic-ray spectrum and hadronic interactions in air showers for almost a century. The theoretical interest in underground muons has nearly faded away because space-borne experiments probe the cosmic-ray spectrum more directly, and accelerators provide precise measurements of hadron yields. However, underground muons probe unique hadron interaction energies and phase space, which are still inaccessible to present accelerator experiments. The cosmic-ray nucleon energies reach the hundred-TeV and PeV ranges, which are barely accessible with space-borne experiments. Our new calculation combines two modern computational tools: mceq for surface muon fluxes and proposal for underground transport. We demonstrate excellent agreement with measurements of cosmic-ray muon intensities underground within estimated errors. Beyond that, the precision of historical data turns out to be significantly smaller than our error estimates. This result shows that the sources of high-energy atmospheric lepton flux uncertainties at the surface or underground can be significantly constrained without taking more data or building new detectors. The reduction of uncertainties can be expected to impact data analyses at large-volume neutrino telescopes and be used for the design of future ton-scale direct dark matter detectors.

Underground muon flux measured by EEE students

The Extreme Energy Events experiment (EEE) is a cosmic ray observatory made of about 60 muon telescopes based on Multigap Resistive Plate Chamber (MRPC) detectors. The EEE experiment has two main targets: a scientific and a dissemination. The EEE collaboration has also developed a large set of portable scintillator-based detectors, named Cosmic Box (CB), mainly used for educational purposes. The CB allows students to perform cosmic ray counting measurements in several environments. CBs are made of two 15 × 15 × 1 cm scintillators read by two 3 × 3 mm2 SiPMs operated in coincidence. Three CBs were deployed in Nuraxi Figus and Seruci coal mine to perform an underground measurement of the cosmic muon flux attenuation. High school and university students were directly involved in all the stages of the measurements: from the preliminary measurements to the on-site work and data analysis.

Development and validation of the signal simulation for the underground muon detector of the Pierre Auger Observatory

The underground muon detector of the Pierre Auger Observatory is aimed at attaining direct measurements of the muonic component of extensive air showers produced by cosmic rays with energy from 1016.5 eV up to the region of the ankle (around 1018.7 eV). It consists of two nested triangular grids of underground scintillators with 433 m and 750 m spacings, and a total of 71 positions, each with 192 scintillator strips (30 m2) deployed 2.3 m underground. The light produced by impinging muons in the scintillators is propagated with optical fibers towards an array of silicon photomultipliers. In this work, we present the development, validation, and performance of an end-to-end tool for simulating the response of the underground muon detector to single-muon signals, which constitutes the basis for further simulations of the whole array. Laboratory data and simulation outcomes are found consistent, showing that with the underground muon detector we can measure single muons, with an efficiency of 99%, up to about 1050 particles arriving at exactly the same time in 30 m2 of scintillator.

Dynamic range extension of the Tibet underground muon detectors for measurements up to the knee energy region with 3-inch photomultipliers

Aiming to study the primary cosmic-ray mass composition up to the knee energy region with the Tibet underground water-Cherenkov muon-detector array (MD), we propose to extend the dynamic range of each MD detector with an 8-inch PMT (Beijing Hamamatsu CR365) and a 3-inch PMT (HZC XP72B22). We carried out a series of tests on the candidate 3-inch XP72B22 PMT to examine whether it can meet our requirements. The measurement results showed that the XP72B22 has a wide dynamic range of 3 orders of magnitude and a linear response up to ∼105 photoelectrons (PEs) at the supply voltage of 700 V. Combined with previous test results of the CR365, these two types of PMT can achieve an equivalent dynamic range from 30 to 106 PEs. As a result, an MD cell is able to detect cosmic rays around the knee energies by adding the CR365 and the XP72B22.

Muon Tracker with Unsegmented Plastic Scintillator Panels

A muon tracker was developed using three polyvinyl toluene scintillator panels instrumented with photomultiplier tubes (PMTs) mounted at the corners. Panels are mounted in parallel on an aluminum frame which allows for simple adjustment of angle, orientation and separation distance between the panels. The responses of all PMTs in the system are digitized simultaneously at sub-nanosecond sample spacing. Software was developed to adjust settings and implement event rejection based on the number of panels that detected a scintillation event within a 400-nanosecond record. The relative responses of the PMTs are used to calculate the position of scintillation events within each panel. The direction of the muons through the system can be tracked using the panel strike order. Methods for triangulation by both time-of-flight (TOF) and PMT magnitude response are reported. The time triangulation method is derived and experimentally demonstrated using parallel cables of differing length. The PMTs used in this experiment are only optimized for amplitude discrimination, not for time spread jitter as would be required to implement TOF methods into the scintillator panels. A Gaussian process regression machine learning tool was implemented to learn the relationship between PMT response features and positions from a calibration dataset. Resolution is analyzed using different numbers of PMTs and low-versus-high PMT sensitivities. Muons traveling in forward and reverse directions through the detector system were counted in all six axis orientations. The muon detector was deployed for 28 days in an underground tunnel and vertical muon counts were recorded.

Calibration of the underground muon detector of the Pierre Auger Observatory

To obtain direct measurements of the muon content of extensive air showers with energy above 1016.5 eV, the Pierre Auger Observatory is currently being equipped with an underground muon detector (UMD), consisting of 219 10 m2-modules, each segmented into 64 scintillators coupled to silicon photomultipliers (SiPMs). Direct access to the shower muon content allows for the study of both of the composition of primary cosmic rays and of high-energy hadronic interactions in the forward direction. As the muon density can vary between tens of muons per m2 close to the intersection of the shower axis with the ground to much less than one per m2 when far away, the necessary broad dynamic range is achieved by the simultaneous implementation of two acquisition modes in the read-out electronics: the binary mode, tuned to count single muons, and the ADC mode, suited to measure a high number of them. In this work, we present the end-to-end calibration of the muon detector modules: first, the SiPMs are calibrated by means of the binary channel, and then, the ADC channel is calibrated using atmospheric muons, detected in parallel to the shower data acquisition. The laboratory and field measurements performed to develop the implementation of the full calibration chain of both binary and ADC channels are presented and discussed. The calibration procedure is reliable to work with the high amount of channels in the UMD, which will be operated continuously, in changing environmental conditions, for several years.

Measurement of the cosmic muon annual and diurnal flux variation with the COSINE-100 detector

We report measurements of annual and diurnal modulations of the cosmic-ray muon rate in the Yangyang underground laboratory (Y2L) using 952 days of COSINE-100 data acquired between September 2016 and July 2019. A correlation of the muon rate with the atmospheric temperature is observed and its amplitude on the muon rate is determined. The effective atmospheric temperature and muon rate variations are positively correlated with a measured effective temperature coefficient of αT = 0.82 ± 0.10. This result is consistent with a model of meson production in the atmosphere. We also searched for a diurnal modulation in the underground muon rate by comparing one-hour intervals. No significant diurnal modulation of the muon rate was observed.

Estimation of the number of muons with muon counters

The origin and nature of the cosmic rays is still uncertain. However, a big progress has been achieved in recent years due to the good quality data provided by current and recent cosmic-rays observatories. The cosmic ray flux decreases very fast with energy in such a way that for energies ≳1015 eV, the study of these very energetic particles is performed by using ground based detectors. These detectors are able to detect the atmospheric air showers generated by the cosmic rays as a consequence of their interactions with the molecules of the Earth’s atmosphere. One of the most important observables that can help to understand the origin of the cosmic rays is the composition profile as a function of primary energy. Since the primary particle cannot be observed directly, its chemical composition has to be inferred from parameters of the showers that are very sensitive to the primary mass. The two parameters more sensitive to the composition of the primary are the atmospheric depth of the shower maximum and the muon content of the showers. Past and current cosmic-rays observatories have been using muon counters with the main purpose of measuring the muon content of the showers. Motivated by this fact, in this work we study in detail the estimation of the number of muons that hit a muon counter, which is limited by the number of segments of the counters and by the pile-up effect. We consider as study cases muon counters with segmentation corresponding to the underground muon detectors of the Pierre Auger Observatory that are currently taking data, and the one corresponding to the muon counters of the AGASA Observatory, which stopped taking data in 2004.

Composition determination of cosmic rays from the muon content of the showers

The origin and nature of ultra high energy cosmic rays remains being a mystery. However, great progress has been made in recent years due to the observations performed by the Pierre Auger Observatory and Telescope Array. In particular, it is believed that the composition information of the cosmic rays as a function of the energy can play a fundamental role for the understanding of their origin. The best indicators for primary mass composition are the muon content of extensive air shower and the atmospheric depth of the shower maximum. In this work we consider a maximum likelihood method to perform mass composition analyses based on the number of muons measured by underground muon detectors. The analyses are based on numerical simulations of the showers. The effects introduced by the detectors and the methods used to reconstruct the experimental data are also taken into account through a dedicated simulation that uses as input the information of the simulated showers. In order to illustrate the use of the method, we consider AMIGA (Auger Muons and Infill for the Ground Array), the low energy extension of the Pierre Auger Observatory that directly measures the muonic content of extensive air showers. We also study in detail the impact of the use of different high energy hadronic interaction models in the composition analyses performed. It is found that differences of a few percent between the predicted number of muons have a significant impact on composition determination.

100 TeV diffuse $$\gamma $$-rays observation by YangBaJing Hybrid Array

Cosmic rays up to PeV energy are believed to be generated in our galaxy. $$\gamma $$ -rays at 100 TeV energies are important probe to identify PeVtrons. Besides, $$\gamma $$ -rays contain additional information on the propagation of the galactic cosmic rays. Diffuse $$\gamma $$ -rays originating from the Galactic plane from 20 to 200 TeV region are studied with early 160-day data collected by YangBaJing Hybrid Array (YBJ-HA), which is a hybrid array and consists of scintillation detectors and underground muon detectors. All-distance equi-zenith angle and time-swapping methods are used to analyze the number of $$\gamma $$ -rays excess. Because of no significant observation on $$\gamma $$ -ray emission, Helene’s method is adopted to derive the $$90\%$$ confidence level upper limits on the flux of diffuse galactic $$\gamma $$ -rays. And the limit to the ratio of the flux of $$\gamma $$ -rays to CRs is obtained at the order of $$10 ^{-4}$$ , which is the current best result at these energies.

Direct measurement of the muonic content of extensive air showers between mathbf { 2times 10^{17}} and mathbf {2times 10^{18}}~eV at the Pierre Auger Observatory

The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between 2times 10^{17} and 2times 10^{18} eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at 10^{{17.5}}, {mathrm{eV}} and 10^{{18}}, {mathrm{eV}} . We find that, for the models to explain the data, an increase in the muon density of 38%pm 4% (12%)pm {}^{21%}_{18%} for EPOS-LHC, and of 50% (53%)pm 4% (13%)pm {}^{23%}_{20%} for QGSJetII-04, is respectively needed.

B. V. Sreekantan (1925–2019)

The new face of India after independence could be seen not only in dams and industries, but it was also apparent in the experiments in fundamental research undertaken with zeal by its youngsters. One of those youngsters was B. V. Sreekantan who passed away on 27 October 2019 at the ripe age of 95 years. He pioneered experiments in three fields: (1) Cosmic ray interactions and extensive air showers; (2) Underground muon and neutrino experiments; and (3) High energy astrophysics experiments. His experiments of those times are interesting for several reasons: (a) These are pioneering efforts of an individual as also of a new institution looking for new ways and means in research. (b) These would set up the roads to be travelled by the next generation of researchers. (c) The ingenuity in the face of scarcity of science instruments was very important in a newly independent and not an affluent country.

Investigation of Energy Spectrum of Semidiurnal Variations of Cosmic Rays

An energy spectrum of semidiurnal variations of cosmic rays observed at the Earth is investigated. The data of registration of cosmic rays by the ground and underground multidirectional muon telescopes (stations Yakutsk, Nagoya, Hobart, Kuwait, Sao Martinho da Serra, Misato, Sakashita, Matsushiro), which covers the time period from 1971 to 2018 and the median energy range from 55 to 686 GeV, are used. Average annual values of semidiurnal variations of cosmic rays observed by the mentioned stations and free from atmospheric and geomagnetic effects are obtained. Based on the analysis of the obtained data, a form and temporal dynamics of their energy spectrum are investigated. It is established that the energy spectrum reveals dependence on the solar activity cycle.