Search

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 $>10\mathrm{years}$.

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.

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.

The diffuse Galactic $\gamma$-ray emission, mainly produced via interactions between cosmic rays and the interstellar medium and/or radiation field, is a very important probe of the distribution, propagation, and interaction of cosmic rays in the Milky Way. In this Letter, we report the measurements of diffuse $\gamma$ rays from the Galactic plane between 10 TeV and 1 PeV energies, with the square kilometer array of the Large High Altitude Air Shower Observatory (LHAASO). Diffuse emissions from the inner ($15°<l<125°$, $|b|<5°$) and outer ($125°<l<235°$, $|b|<5°$) Galactic plane are detected with $29.1\sigma$ and $12.7\sigma$ significance, respectively. The outer Galactic plane diffuse emission is detected for the first time in the very- to ultra-high-energy domain ($E>10\mathrm{TeV}$). The energy spectrum in the inner Galaxy regions can be described by a power-law function with an index of $-2.99\pm 0.04$, which is different from the curved spectrum as expected from hadronic interactions between locally measured cosmic rays and the line-of-sight integrated gas content. Furthermore, the measured flux is higher by a factor of $\sim 3$ than the prediction. A similar spectrum with an index of $-2.99\pm 0.07$ is found in the outer Galaxy region, and the absolute flux for $10\lesssim E\lesssim 60\mathrm{TeV}$ is again higher than the prediction for hadronic cosmic ray interactions. The latitude distributions of the diffuse emission are consistent with the gas distribution, while the longitude distributions show clear deviation from the gas distribution. The LHAASO measurements imply that either additional emission sources exist or cosmic ray intensities have spatial variations.

The attenuation length of the muon content in extensive air showers provides important information regarding the generation and development of air showers. This information can be used not only to improve the description of such showers but also to test fundamental models of hadronic interactions. Using data from the LHAASO-KM2A experiment, the development of the muon content in high-energy air showers was studied. The attenuation length of muon content in the air showers was measured from experimental data in the energy range from 0.3 to 30 PeV using the constant intensity cut method. By comparing the attenuation length of the muon content with predictions from high-energy hadronic interaction models (qgsjet-II-04, sibyll 2.3d, and epos-lhc), it is evident that LHAASO results are significantly shorter than those predicted by the first two models (qgsjet-II-04 and sibyll 2.3d) but relatively close to those predicted by the third model (epos-lhc). Thus, the LHAASO data favor the epos-lhc model over the other two models. The three interaction models confirmed an increasing trend in the attenuation length as the cosmic-ray energy increases.

We report the first detection of gamma-ray emission up to ultrahigh-energy (UHE; $>100\mathrm{TeV}$) emission from the prototypical gamma-ray binary system LS I $+61°$ 303 using data from the Large High Altitude Air Shower Observatory. It is detected with significances of $9.2\sigma$ in the Water Cherenkov Detector Array (WCDA, $1.4-30.5\mathrm{TeV}$) and $6.2\sigma$ in the Kilometer Square Array (KM2A, $25-267\mathrm{TeV}$); in KM2A alone we identify 16 photonlike events above 100 TeV against an estimated 5.1 background events, corresponding to a $3.8\sigma$ detection. These results provide compelling evidence of extreme particle acceleration in LS I $+61°$ 303. Furthermore, we observe orbital modulation at $3.9\sigma$ confidence level, between 25 and 100 TeV, with a hint that the orbital modulation is energy dependent. These features can be understood in a composite scenario in which leptonic and hadronic processes jointly contribute.

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.

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.

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 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.

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.

In this Letter we try to search for signals generated by ultraheavy dark matter at the Large High Altitude Air Shower Observatory (LHAASO) data. We look for possible $\gamma$ rays by dark matter annihilation or decay from 16 dwarf spheroidal galaxies in the field of view of the LHAASO. Dwarf spheroidal galaxies are among the most promising targets for indirect detection of dark matter that have low fluxes of astrophysical $\gamma$-ray background while having large amount of dark matter. By analyzing more than 700 days of observational data at LHAASO, no significant dark matter signal from 1 TeV to 1 EeV is detected. Accordingly we derive the most stringent constraints on the ultraheavy dark matter annihilation cross section up to EeV. The constraints on the lifetime of dark matter in decay mode are also derived.

We present the measurements of all-particle energy spectrum and mean logarithmic mass of cosmic rays in the energy range of 0.3–30 PeV using data collected from LHAASO-KM2A between September 2021 and December 2022, which is based on a nearly composition-independent energy reconstruction method, achieving unprecedented accuracy. Our analysis reveals the position of the knee at $3.67\pm 0.05\pm 0.15\mathrm{PeV}$. Below the knee, the spectral index is found to be $-2.7413\pm 0.0004\pm 0.0050$, while above the knee, it is $-3.128\pm 0.005\pm 0.027$, with the sharpness of the transition measured with a statistical error of 2%. The mean logarithmic mass of cosmic rays is almost heavier than helium in the whole measured energy range. It decreases from 1.7 at 0.3 PeV to 1.3 at 3 PeV, representing a 24% decline following a power law with an index of $-0.1200\pm 0.0003\pm 0.0341$. This is equivalent to an increase in abundance of light components. Above the knee, the mean logarithmic mass exhibits a power law trend towards heavier components, which is reversal to the behavior observed in the all-particle energy spectrum. Additionally, the knee position and the change in power-law index are approximately the same. These findings suggest that the knee observed in the all-particle spectrum corresponds to the knee of the light component, rather than the medium-heavy components.

The diffuse Galactic gamma-ray emission is a very important tool used to study the propagation and interaction of cosmic rays in the Milky Way. In this Letter, we report the measurements of the diffuse emission from the Galactic plane—covering Galactic longitudes from 15° to 235° and latitudes from $-5°$ to $+5°$, in an energy range of 1 to 25 TeV—made with the Water Cherenkov Detector Array (WCDA) of the Large High Altitude Air Shower Observatory. After the sky regions of known sources are masked, the diffuse emission is detected with $24.6\sigma$ and $9.1\sigma$ significance in the inner Galactic plane ($15°<l<125°$, $|b|<5°$) and outer Galactic plane ($125°<l<235°$, $|b|<5°$), respectively. The WCDA spectra in both regions can be well described by a power-law function, with spectral indices of $-2.67\pm 0.05_{\mathrm{stat}}$ in the inner region and $-2.83\pm 0.19_{\mathrm{stat}}$ in the outer region, respectively. Combined with the Square Kilometer Array (KM2A) measurements at higher energies, a clear softening of the spectrum is found in the inner region, with change of spectral indices by $\sim 0.5$ at a break energy around 30 TeV. The fluxes of the diffuse emission are higher by a factor of 1.5–2.7 than the model prediction assuming local cosmic ray spectra and the gas column density, which are consistent with those measured by the KM2A. Along the Galactic longitude, the spatial distribution of the diffuse emission shows deviation from that of the gas column density. The spectral shape of the diffuse emission may vary in different longitude regions. The WCDA measurements bridge the gap between the low-energy measurements by space detectors and the ultra-high-energy observations by KM2A and other experiments. These results suggest that improved modeling of the wideband diffuse emission is required.

Cosmic-ray electrons and positrons (CREs) offer valuable insights into the origins of local cosmic rays, as they rapidly cool in interstellar magnetic and radiation fields, limiting their propagation range. The CRE spectrum reveals a spectral break around 1 TeV, followed by a continuous power-law behavior that possibly extends beyond 40 TeV. Deviations from a pure power-law in this spectrum could provide important clues about the local origins of CREs. However, due to the rapidly falling flux of CRE, the ratio of hadron/CRE increases with energy, making the rejection more and more challenging. Equipped with a large detection aperture and strong background rejection capability, LHAASO has the potential to extend CRE spectrum measurements beyond 25 TeV, which is almost the highest energy measured by existing experiments. In this work, we study the detector performance and explore the optimized background rejection of cosmic nuclei background specifically for CRE observations in LHAASO. Our analysis reveals that muon-poor nuclei showers have a strong dependence on the hadronic model when estimating the cosmic nuclei background and rejection capability. Consequently, we derived a 90% CL upper limit for the CRE spectrum from 25 to 160 TeV, providing a new and restrictive constraint in the hundreds of TeV range for the first time.

Primordial black holes (PBHs) are hypothetical black holes with a wide range of masses that formed in the early Universe. As a result, they may play an important cosmological role and provide a unique probe of the early Universe. A PBH with an initial mass of approximately ${10}^{15}\mathrm{g}$ is expected to explode today in a final burst of Hawking radiation. In this Letter, we conduct an all-sky search for individual PBH burst events using the data collected from March 2021 to July 2024 by the Water Cherenkov Detector Array of the Large High Altitude Air Shower Observatory (LHAASO). Three PBH burst durations, 10, 20, and 100 s, are searched in the gamma-ray energy range from about 1 to 15 TeV, with no significant PBH bursts observed. The upper limit on the local PBH burst rate density is set to be as low as $181{\mathrm{pc}}^{-3}{\mathrm{yr}}^{-1}$ at 99% confidence level, representing the most stringent limit achieved to date.

We present a precise measurement of the westward rigidity-dependent shift of the Moon’s shadow using three and a half years of cosmic ray data collected by the kilometer square array (KM2A) of the Large High Altitude Air Shower Observatory (LHAASO) experiment. These measurements enable us to calibrate the detector energy response in the range of 20 to 260 TeV, with results showing excellent agreement with the energy response derived from Monte Carlo (MC) simulations of the KM2A detector. We also measure a best-fit parameter $\epsilon =0.015\pm 0.08$, corresponding to a 95% CI of $[-14\%,+17\%]$ for the energy scale estimation. This result establishes the exceptional accuracy of the KM2A-MC in simulating the detector’s response within this energy range.