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A correlation between the secondary cosmic ray flux and the near-earth electric field intensity, measured during thunderstorms, has been found by analyzing the data of the ARGO-YBJ experiment, a full coverage air shower array located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China). The counting rates of showers with different particle multiplicities ($m=1$, 2, 3, and $\ge 4$) have been found to be strongly dependent upon the intensity and polarity of the electric field measured during the course of 15 thunderstorms. In negative electric fields (i.e., accelerating negative charges downwards), the counting rates increase with increasing electric field strength. In positive fields, the rates decrease with field intensity until a certain value of the field ${\mathrm{EF}}_{\min }$ (whose value depends on the event multiplicity), above which the rates begin increasing. By using Monte Carlo simulations, we found that this peculiar behavior can be well described by the presence of an electric field in a layer of thickness of a few hundred meters in the atmosphere above the detector, which accelerates/decelerates the secondary shower particles of opposite charge, modifying the number of particles with energy exceeding the detector threshold. These results, for the first time to our knowledge, give a consistent explanation for the origin of the variation of the electron/positron flux observed for decades by high altitude cosmic ray detectors during thunderstorms.

Cosmic rays are hampered by the Moon and a deficit in its direction is expected (the so-called Moon shadow). The Moon shadow is an important tool to determine the performance of an air shower array. Indeed, the westward displacement of the shadow center, due to the bending effect of the geomagnetic field on the propagation of cosmic rays, allows the setting of the absolute rigidity scale of the primary particles inducing the showers recorded by the detector. In addition, the shape of the shadow permits to determine the detector point spread function, while the position of the deficit at high energies allows the evaluation of its absolute pointing accuracy. In this paper we present the observation of the cosmic ray Moon shadowing effect carried out by the ARGO-YBJ experiment in the multi-TeV energy region with high statistical significance (55 standard deviations). By means of an accurate Monte Carlo simulation of the cosmic rays propagation in the Earth-Moon system, we have studied separately the effect of the geomagnetic field and of the detector point spread function on the observed shadow. The angular resolution as a function of the particle multiplicity and the pointing accuracy have been obtained. The primary energy of detected showers has been estimated by measuring the westward displacement as a function of the particle multiplicity, thus calibrating the relation between shower size and cosmic ray energy. The stability of the detector on a monthly basis has been checked by monitoring the position and the deficit of the Moon shadow. Finally, we have studied with high statistical accuracy the shadowing effect in the day/”night” time looking for possible effect induced by the solar wind.

The ${}^{18}$Ne($\alpha$,$p$)${}^{21}$Na reaction provides a pathway for breakout from the hot CNO cycles to the $rp$ process in type-I x-ray bursts. To better determine this astrophysical reaction rate, the resonance parameters of the compound nucleus ${}^{22}$Mg have been investigated by measuring the resonant elastic scattering of ${}^{21}$Na+$p$. An 89 MeV ${}^{21}$Na radioactive ion beam was produced at the CNS Radioactive Ion Beam Separator and bombarded an 8.8 mg/cm${}^2$ thick polyethylene target. The recoiled protons were measured at scattering angles of ${\theta }_{\mathrm{c}.\mathrm{m}.}\approx 175{}^{\circ }$ and 152${}^{\circ }$ by three $\Delta E$-$E$ silicon telescopes. The excitation function was obtained with a thick-target method over energies $E_x$(${}^{22}$Mg) = 5.5–9.2 MeV. The resonance parameters have been determined through an $R$-matrix analysis. For the first time, the $J^{\pi }$ values for ten states above the $\alpha$ threshold in ${}^{22}$Mg have been experimentally determined in a single consistent measurement. We have made three new $J^{\pi }$ assignments and confirmed seven of the ten tentative assignments in the previous work. The ${}^{18}$Ne($\alpha$,$p$)${}^{21}$Na reaction rate has been recalculated, and the astrophysical impact of our new rate has been investigated through one-zone postprocessing x-ray burst calculations. We find that the ${}^{18}$Ne($\alpha$,$p$)${}^{21}$Na rate significantly affects the peak nuclear energy generation rate and the onset temperature of this breakout reaction in these phenomena.

The Wide Field-of-View Cherenkov Telescope Array (WFCTA) and the Water Cherenkov Detector Array (WCDA) of LHAASO are designed to work in combination for measuring the energy spectra of the cosmic ray species over a very wide energy range from a few TeV to 10 PeV. The energy calibration can be achieved with a proven technique of measuring the westward shift of the Moon shadow cast by galactic cosmic rays due to the geomagnetic field. This deflection angle $\mathrm{\Delta }$ is inversely proportional to the cosmic ray rigidity. The precise measurement of the shifts by WCDA allows us to calibrate its energy scale for energies as high as 35 TeV. Through a set of commonly triggered events, the energy scales can be propagated to WFCTA. The energies of the events can be derived both by WCDA-1 and WFCTA with the median energies $23.4\pm 0.1\pm 1.3\mathrm{TeV}$ and ($21.9\pm 0.1\mathrm{TeV}$), respectively, which are consistent within uncertainties. In addition, the propagation of the energy scale is also validated by the Moon shadow based on the same data selection criteria of the commonly triggered events. This paper reports, for the first time, an observational measurement of the absolute energy scale of the primary cosmic rays generating showers observed by air Cherenkov telescopes.

On 9 October 2022, the Large High Altitude Air Shower Observatory (LHAASO) reported the observation of the very early TeV afterglow of the brightest-of-all-time gamma-ray burst 221009A, recording the highest photon statistics in the TeV band ever obtained from a gamma-ray burst. We use this unique observation to place stringent constraints on the energy dependence of the speed of light in vacuum, a manifestation of Lorentz invariance violation (LIV) predicted by some quantum gravity (QG) theories. Our results show that the 95% confidence level lower limits on the QG energy scales are $E_{\mathrm{QG},1}>10$ times the Planck energy $E_{\mathrm{Pl}}$ for the linear LIV effect, and $E_{\mathrm{QG},2}>6\times {10}^{-8}{E}_{\mathrm{Pl}}$ for the quadratic LIV effect. Our limits on the quadratic LIV case improve previous best bounds by factors of 5–7.

The kilometer square array (KM2A) of the large high altitude air shower observatory (LHAASO) aims at surveying the northern $\gamma$-ray sky at energies above 10 TeV with unprecedented sensitivity. $\gamma$-ray observations have long been one of the most powerful tools for dark matter searches, as, e.g., high-energy $\gamma$ rays could be produced by the decays of heavy dark matter particles. In this Letter, we present the first dark matter analysis with LHAASO-KM2A, using the first 340 days of data from $1/2$-KM2A and 230 days of data from $3/4$-KM2A. Several regions of interest are used to search for a signal and account for the residual cosmic-ray background after $\gamma /\text{hadron}$ separation. We find no excess of dark matter signals, and thus place some of the strongest $\gamma$-ray constraints on the lifetime of heavy dark matter particles with mass between ${10}^5$ and ${10}^9\mathrm{GeV}$. Our results with LHAASO are robust, and have important implications for dark matter interpretations of the diffuse astrophysical high-energy neutrino emission.

The solar composition problem has puzzled astrophysicists for more than 20 years. Recent measurements of carbon-nitrogen-oxygen (CNO) neutrinos by the Borexino experiment show a $\sim 2\sigma$ tension with the “low-metallicity” determinations. ${}^{14}\mathrm{N}(p,\gamma {)}^{15}\mathrm{O}$, the slowest reaction in the CNO cycle, plays a crucial role in the standard solar model (SSM) calculations of CNO neutrino fluxes. Here we report a direct measurement of the ${}^{14}\mathrm{N}(p,\gamma {)}^{15}\mathrm{O}$ reaction, in which $S$-factors for all transitions were simultaneously determined in the energy range of $E_{\mathrm{p}}=110–260\mathrm{keV}$ for the first time. Our results resolve previous discrepancies in the ground-state transition, yielding a zero-energy $S$-factor $S_{114}(0)=1.93\pm 0.10\mathrm{keV}\mathrm{b}$, which is 15% higher than the $1.68\pm 0.14\mathrm{keV}\mathrm{b}$ recommended in Solar Fusion III (SF-III). With our $S_{114}$ values, the SSM B23-MB22p, and the latest global analysis of solar neutrino measurements, the C and N photospheric abundance determined by the Borexino experiment is updated to $N_{\mathrm{CN}}=(4.42_{-0.63}^{+0.70})\times {10}^{-4}$. This new $N_{\mathrm{CN}}$ value agrees well with latest “high-metallicity” composition; however, it is also consistent with the “low-metallicity” determination within $\sim 1\sigma$ CL, indicating that the solar metallicity problem remains an open question. In addition, the significant reduction in the uncertainty of $S_{114}$ paves the way for the precise determination of the CN abundance in future large-volume solar neutrino measurements.

Recently, the LHAASO Collaboration published the detection of 12 ultrahigh-energy $\gamma$-ray sources above 100 TeV, with the highest energy photon reaching 1.4 PeV. The first detection of PeV $\gamma$ rays from astrophysical sources may provide a very sensitive probe of the effect of the Lorentz invariance violation (LIV), which results in decay of high-energy $\gamma$ rays in the superluminal scenario and hence a sharp cutoff of the energy spectrum. Two highest energy sources are studied in this work. No signature of the existence of the LIV is found in their energy spectra, and the lower limits on the LIV energy scale are derived. Our results show that the first-order LIV energy scale should be higher than about ${10}^5$ times the Planck scale $M_{\mathrm{Pl}}$ and that the second-order LIV scale is $>{10}^{-3}{M}_{\mathrm{Pl}}$. Both limits improve by at least one order of magnitude the previous results.

We report the discovery of an extended very-high-energy (VHE) gamma-ray source around the location of the middle-aged (207.8 kyr) pulsar PSR $\mathrm{J}0622+3749$ with the Large High-Altitude Air Shower Observatory (LHAASO). The source is detected with a significance of $8.2\sigma$ for $E>25\mathrm{TeV}$ assuming a Gaussian template. The best-fit location is (right ascension, declination) $=(95.47°\pm 0.11°,37.92°\pm 0.09°)$, and the extension is $0.40°\pm 0.07°$. The energy spectrum can be described by a power-law spectrum with an index of $-2.92\pm {0.17}_{\mathrm{stat}}\pm {0.02}_{\mathrm{sys}}$. No clear extended multiwavelength counterpart of the LHAASO source has been found from the radio to sub-TeV bands. The LHAASO observations are consistent with the scenario that VHE electrons escaped from the pulsar, diffused in the interstellar medium, and scattered the interstellar radiation field. If interpreted as the pulsar halo scenario, the diffusion coefficient, inferred for electrons with median energies of $\sim 160\mathrm{TeV}$, is consistent with those obtained from the extended halos around Geminga and Monogem and much smaller than that derived from cosmic ray secondaries. The LHAASO discovery of this source thus likely enriches the class of so-called pulsar halos and confirms that high-energy particles generally diffuse very slowly in the disturbed medium around pulsars.

The proton-air cross section in the energy range 1–100 TeV has been measured by the ARGO-YBJ cosmic ray experiment. The analysis is based on the primary cosmic ray flux attenuation for different atmospheric depths (i.e. zenith angles) and exploits the detector capabilities of selecting the shower development stage by means of hit multiplicity, density and lateral profile measurements at ground. The effects of shower fluctuations, the contribution of heavier primaries and the uncertainties of the hadronic interaction models, have been taken into account. The results have been used to estimate the total proton-proton cross section at center-of-mass energies between 70 and 500 GeV, where no accelerator data are currently available.

Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain, and SNRs’ contribution to CRs around PeV energies is unclear. In this Letter, we present observations of high-energy $\gamma$-ray emission from the SNR IC 443 using the Large High Altitude Air Shower Observatory (LHAASO). The morphological analysis reveals a pointlike source whose location and spectrum are consistent with those of the Fermi-LAT-detected compact source with ${\pi }^0$-decay signature, and a more extended source that is associated with a newly discovered Fermi source. The spectrum of the point source can be described by a power-law function with an index of $\sim 3.0$, extending beyond $\sim 30\mathrm{TeV}$ without apparent cutoff. Assuming a hadronic origin of the $\gamma$-ray emission, the 95% lower limit of accelerated protons reaches about 300 TeV. The extended source might be associated with IC 443, SNR $\mathrm{G}189.6+3.3$, or the putative pulsar wind nebula CXOU $\mathrm{J}061705.3+222127$, and can be explained by either a hadronic or a leptonic model with particles reaching hundreds of TeV. These LHAASO results provide compelling evidence that sub-PeV CRs can be accelerated efficiently by shocks of SNRs.

The geomagnetic field causes not only the east-west effect on primary cosmic rays but also affects the trajectories of the secondary charged particles in the shower, causing their lateral distribution to be stretched. Thus, both the density of the secondaries near the shower axis and the trigger efficiency of detector arrays decrease. The effect depends on the direction of the showers, thus, introducing a modulation in the measured azimuthal distribution. The azimuthal distribution of the events collected by the ARGO-YBJ detector is deeply investigated for different zenith angles in light of this effect.

The ARGO-YBJ experiment detects extensive air showers in a wide energy range by means of a full-coverage detector which is in stable data taking in its full configuration since November 2007 at the YBJ International Cosmic Ray Observatory (4300 m a.s.l., Tibet, People’s Republic of China). In this paper the measurement of the light-component spectrum of primary cosmic rays in the energy region $(5÷200)\mathrm{TeV}$ is reported. The method exploited to analyze the experimental data is based on a Bayesian procedure. The measured intensities of the light component are consistent with the recent CREAM results and higher than that obtained adding the proton and helium spectra reported by the RUNJOB experiment.

Angular distributions of the differential cross sections for the quasi-elastic scattering from a ${}^{\mathrm{nat}}\mathrm{Pb}$ target by ${}^{10}\mathrm{C}$ at 226 and 256 MeV, by ${}^{11}\mathrm{C}$ at 222 MeV and 226 MeV, and by ${}^{10}\mathrm{B}$ at 173 MeV were measured at the Heavy Ion Research Facility in Lanzhou, Radioactive Ion Beam Line in Lanzhou (HIRFL-RIBLL). Contributions from the inelastic scattering channels to these data are found to be negligibly small with coupled-channel calculations within the angular range covered by this experiment. These data can be well reproduced by optical model calculations with systematic nucleus-nucleus potentials. The reduced total reaction cross sections were compared with other existing data.

The ${}^{18}$Ne($\alpha$,$p$)${}^{21}$Na reaction is thought to be one of the key breakout reactions from the hot CNO cycles to the $rp$ process in type I x-ray bursts. In this work, the resonant properties of the compound nucleus ${}^{22}$Mg have been investigated by measuring the resonant elastic scattering of ${}^{21}$Na + $p$. An 89-MeV ${}^{21}$Na radioactive beam delivered from the CNS Radioactive Ion Beam Separator bombarded an 8.8 mg/cm${}^2$ thick polyethylene (CH${}_2$)${}_n$ target. The ${}^{21}$Na beam intensity was about $2\times 10$${}^5$ pps, with a purity of about 70% on target. The recoiled protons were measured at the center-of-mass scattering angles of ${\theta }_{\mathrm{c}.\mathrm{m}.}\approx 175.2$${}^{\circ }$, 152.2${}^{\circ }$, and 150.5${}^{\circ }$ by three sets of $\Delta E$-$E$ telescopes, respectively. The excitation function was obtained with the thick-target method over energies $E_x{(}^{22}\text{Mg})=5.5$–9.2 MeV. In total, 23 states above the proton-threshold in ${}^{22}$Mg were observed, and their resonant parameters were determined via an $R$-matrix analysis of the excitation functions. We have made several new $J^{\pi }$ assignments and confirmed some tentative assignments made in previous work. The thermonuclear ${}^{18}$Ne($\alpha$,$p$)${}^{21}$Na rate has been recalculated based on our recommended spin-parity assignments. The astrophysical impact of our new rate has been investigated through one-zone postprocessing x-ray burst calculations. We find that the ${}^{18}$Ne($\alpha$,$p$)${}^{21}$Na rate significantly affects the peak nuclear energy generation rate, reaction fluxes, and onset temperature of this breakout reaction in these astrophysical phenomena.

Small deviations from isotropy in the arrival directions of Galactic cosmic rays serve as a unique probe of the local magnetic environment. In this Letter, we report observations of medium-scale anisotropies at energies above 10 TeV using the LHAASO-KM2A array. Our analysis identifies four regions of excess and four regions of deficit, each spanning angular scales of approximately 10°. Crucially, we detect significant energy-dependent shifts in the centroids of two excess regions: Region B and the newly identified Region $\tilde{\mathrm{D}}$. We also characterize the energy evolution of the fractional relative intensity across both excess and deficit regions. These findings imply that the observed anisotropies are shaped by the specific realization of the local turbulent magnetic field within the cosmic ray scattering length. Such energy-dependent behaviors impose strict constraints on local turbulence models and cosmic ray propagation theories.

In the Large High Altitude Air Shower Observatory (LHAASO), the square kilometer array, with 5249 electromagnetic particle detectors (EDs) and 1188 muon detectors, is deployed to explore the gamma-ray sources above 30 TeV with unprecedented sensitivity and to measure primary cosmic rays in the energy range from 10 TeV to 100 PeV. The energetic particles produced by extensive air showers can serve as a continuously available source for calibration of the numerous EDs over a large area. In this study, the detector untriggered probability is first proposed to estimate the particle density at different distances from the shower core and distinguish the characteristic single-particle signal detected by each ED. This method uses science data directly, and does not require prior knowledge of the cosmic-ray elemental composition or hadronic interaction model. Experimental results show that this self-calibration can be used to determine the number of particles detected by each ED with an accuracy better than 2% within a time scale of hours, which is adequate to meet the physics requirements of the LHAASO experiment. With this high efficiency and accuracy, this calibration also provides an ideal method to monitor the detector performance throughout an expected lifetime of $>10\mathrm{years}$.

We report a measurement of the cosmic ray helium energy spectrum in the energy interval 0.16–13 PeV, derived by subtracting the proton spectrum from the light component (proton and helium) spectrum obtained with observations made by the Large High Altitude Air Shower Observatory (LHAASO) under a consistent energy scale. The helium spectrum shows a significant hardening centered at $E\simeq 1.1\mathrm{PeV}$, followed by a softening at $\sim 7\mathrm{PeV}$, indicating the appearance of a helium “knee.” Comparing the proton and helium spectra in the LHAASO energy range reveals some remarkable facts. In the lower part of this range, in contrast to the behavior at lower energies, the helium spectrum is significantly softer than the proton spectrum. This results in protons overtaking helium nuclei and becoming the largest cosmic ray component at $E\simeq 0.7\mathrm{PeV}$. A second crossing of the two spectra is observed at $E\simeq 5\mathrm{PeV}$, above the proton knee, when helium nuclei overtake protons to become the largest cosmic ray component again. These results have important implications for our understanding of the Galactic cosmic ray sources.

Large- or medium-scale cosmic ray anisotropy at TeV energies has not previously been confirmed to vary with time. Transient anisotropy changes have been observed below 150 GeV, especially near the passage of an interplanetary shock and coronal mass ejection containing a magnetic flux rope ejected by a solar storm, which can trigger a geomagnetic storm with practical consequences. Here, we report the first observation of transient large-scale anisotropy in TeV cosmic ray ions using data from the Large High-Altitude Air Shower Observatory. We analyze hourly skymaps of the transient cosmic ray intensity excess or deficit, the gradient of which indicates the direction and magnitude of transient large-scale anisotropy across the field of view. We observe enhanced anisotropy above typical hourly fluctuations with $>5\sigma$ significance during some hours of November 4, 2021, in separate datasets for four primary cosmic ray energy ranges of median energy from $E=0.6$ to 2.8 TeV. The gradient varies with energy as $E^{\gamma }$, where $\gamma =-0.5$. At a median energy $\le 1.0\mathrm{TeV}$, this gradient corresponds to a dipole anisotropy of at least 0.9%, or possibly a weaker anisotropy of higher order. This new type of observation opens the opportunity to study interplanetary magnetic structures using air shower arrays around the world, complementing existing in situ and remote measurements of plasma properties.