The Tibet air shower (AS) array and underground water-Cherenkov-type muon detector (MD) array have been successfully operating since 2014 at an altitude of 4,300 m in Tibet, China. This hybrid experiment is useful not only for the gamma-ray observation but also for the energy spectrum measurement in the knee energy region of the cosmic-ray energy spectrum. The energy and arrival directions of air showers are determined by the AS array, while the number of muons contained in air showers is counted with the MD array. In terms of gamma-ray observation, the MD array enables us to suppress more than 99.9% of background cosmic rays against gammaray signals above 100 TeV. Our gamma-ray and cosmic-ray data from the AS+MD array is well reproduced by our detailed air shower simulations. Here we report on the latest results from the Tibet AS gamma experiment, especially focusing on the world's first detection of sub-PeV gamma rays from the Crab Nebula and the world's first detection of sub-PeV diffuse gamma-ray emissions from the Galactic plane.

Gamma rays from HESS J1849−000, a middle-aged TeV pulsar wind nebula (PWN), are observed by the Tibet air shower array and the muon detector array. The detection significance of gamma rays reaches 4.0 σ and 4.4 σ levels above 25 TeV and 100 TeV, respectively, in units of the Gaussian standard deviation σ . The energy spectrum measured between 40 TeV < E < 320 TeV for the first time is described with a simple power-law function of dN / dE = ( 2.86 ± 1.44 ) × 10 − 16 ( E / 40 TeV ) − 2.24 ± 0.41 TeV − 1 cm − 2 s − 1 . The gamma-ray energy spectrum from the sub-TeV ( E < 1 TeV) to sub-PeV (100 TeV < E < 1 PeV) ranges, including the results of previous studies, can be modeled with the leptonic scenario, i.e., inverse Compton scattering by high-energy electrons accelerated by the PWN of PSR J1849−0001. On the other hand, the gamma-ray energy spectrum can also be modeled with the hadronic scenario in which gamma rays are generated from the decay of neutral pions produced by collisions between accelerated cosmic-ray protons and the ambient molecular cloud found in the gamma-ray-emitting region. The cutoff energy of cosmic-ray protons E p,cut is estimated as log 10 ( E p , cut / TeV ) = 3.73 − 0.66 + 2.98 , suggesting that protons are accelerated up to the PeV energy range. Our study thus proposes that HESS J1849−000 should be further investigated as a new candidate as a Galactic PeV cosmic-ray accelerator, or “PeVatron.”

Gamma-ray observation in the sub-PeV range helps us search for Galactic PeV cosmic-ray accelerators, PeVatrons. Measurement of the energy spectrum of gamma rays from astrophysical sources is especially crucial to discuss their hadronic or leptonic origin. In this study, gamma rays from the sky region of HESS J1843−033 are observed by the Tibet air shower array equipped with the underground muon detector array. A new gamma-ray source TASG J1844−038 is detected above 25 TeV with a 6.2 𝜎 level in the region of interest, the position of which is consistent with HESS J1843−033, eHWC J1842−035, and LHAASO J1843−0338. The gamma-ray energy spectrum of TASG J1844−038 is successfully measured in 25−130 TeV for the first time; it smoothly connects to that of HESS J1843−033 and gives consistent flux levels with those of eHWC J1842−035 and LHAASO J1843−0338 at 56 and 100 TeV, respectively. The analysis of the combined gamma-ray spectra between the H.E.S.S., LHAASO, and TASG sources indicates the cutoff at about 50 TeV. The distribution of the molecular clouds around TASG J1844−038 is investigated by analyzing public CO line-emission data; our study finds the clouds in the velocity range of 75−95 km s^−1 lying flat over the region with a 50 K km s^−1 level. The association of the observed gamma rays with a nearby supernova remnant SNR G28.6−0.1 is discussed, and it is proposed that cosmic rays accelerated by SNR G28.6−0.1 up to about 500 TeV generate the observed gamma rays from the collisions with the ambient gas found in the analysis.

The Large High Altitude Air Shower Observatory (LHAASO) has three sub-arrays, KM2A, WCDA, and WFCTA. The flux variations of cosmic ray air showers were studied by analyzing the KM2A data during a thunderstorm on June 10, 2021. The number of shower events that meet the trigger conditions increases significantly in atmospheric electric fields, with a maximum fractional increase of 20%. The variations in trigger rates (increases or decreases) were found to be strongly dependent on the primary zenith angle. The flux of secondary particles increased significantly, following a trend similar to that of shower events. To better understand the observed behavior, Monte Carlo simulations were performed with CORSIKA and G4KM2A (a code based on GEANT4). We found that the experimental data (in saturated negative fields) were in good agreement with the simulations, assuming the presence of a uniform electric field of -700 V/cm with a thickness of 1500 m in the atmosphere above the observation level. Due to the acceleration/deceleration by the atmospheric electric field, the number of secondary particles with energy above the detector threshold was modified, resulting in the changes in shower detection rate.

Introduction One of main scientific goals of the Large High Altitude Air Shower Observatory (LHAASO) is to accurately measure the energy spectra of different cosmic ray compositions around the 'knee' region. The Wide Field-of-View (FoV) Cherenkov Telescope Array (WFCTA), which is one of the main detectors of LHAASO and has 18 telescopes, is built to achieve this goal. Multiple telescopes are put together and point to connected directions for a larger FoV. Method Telescopes are deployed spatially as close as possible, but due to their own size, the distance between two adjacent telescopes is about 10 m. Therefore, the Cherenkov lateral distribution and the parallax between the two telescopes should be considered in the event building process for images crossing over the boundaries of FoVs of the telescopes. An event building method for Cherenkov images measured by multiple telescopes of WFCTA is developed. The performance of the shower measurements using the combined images is evaluated by comparing with showers that are fully contained by a virtual telescope in simulation. Results and conclusion It is proved that the developed event building process can help to increase the FoV of WFCTA by 30% while maintaining the same reconstruction quality, compared to the separate telescope reconstruction method.

Small but significant anisotropic features with amplitudes of similar to 0.1% have been reported in the arrival directions of galactic cosmic rays at TeV energies. In this presentation, we preform the modeling of the TeV cosmic-ray anisotropy outside the heliosphere using experimental data of the Tibet AS gamma experiment based on the idea of Liouville mapping. In the intensity-mapping process, we take into account for the first time the rigidity distribution of cosmic-ray particles observed by the experiment. We also improve the modeling of the cosmic-ray intensity distribution at the outer boundary outside the heliosphere to improve the reduced chi(2) of the fitting. Small structures with angular scales of similar to 10 degrees are indicated in the intensity distribution at the outer boundary.

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/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 GeV. Our results with LHAASO are robust, and have important implications for dark matter interpretations of the diffuse astrophysical high-energy neutrino emission.

The main scientific goal of the LHAASO WFCTA experiment is to measure the cosmic ray energy spectra and composition from 10 TeV to 1 EeV. Cherenkov photons in the extensive air shower measured by the SiPM camera of Cherenkov telescopes can be used to reconstruct the cosmic ray energy. The absolute calibration of the camera is a crucial step to achieve the accurate measurement of the cosmic ray energy spectrum. A multi-wavelength cylindrical illuminator based on LEDs is developed and mounted inside the telescope to calibrate and monitor the camera, and the illuminator's stability is better than 0.5% under the temperature variation from-26 to 26 degrees C. A portable probe with a single photoelectron resolution of 21.6% is developed. After calibration by National Institute of Metrology, China (NIM), the probe is taken to the LHAASO site to measure the absolute photon density of the cylindrical illuminator inside the telescope. Based on the illuminator with known photon density, the photon conversion factor of the camera can be calibrated, and the overall calibration uncertainty is less than 2.6%.

We reported on the first detection of the ultra-high-energy diffuse gamma rays from the Galactic plane [1]. The highest energy of the detected gamma rays is estimated to be unprecedentedly high, nearly 1 PeV. It is reasonable to expect that the detected gamma rays are produced by the hadronic interaction between cosmic rays escaping from the most powerful Galactic sources "PeVatrons" and the interstellar gas in the Galaxy. This experimental evidence is an important milestone to solve a long-standing mystery of cosmic-ray origins.

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 J1 843-03 3 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 degrees 34 +/- 0 degrees 12 is successfully detected above 25 TeV at (alpha, delta) = (281 degrees 09 +/- 0 degrees 10, -3 degrees 76 +/- 0 degrees 09) near HESS J1843-033 with a statistical significance of 6.2 sigma, 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 dN/dE = (9.70 +/- 1.89) x 10(-16) (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.

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 J0622+3749 with the Large High-Altitude Air Shower Observatory (LHAASO). The source is detected with a significance of 8.2σ for E>25  TeV assuming a Gaussian template. The best-fit location is (right ascension, declination) =(95.47°±0.11°,37.92°±0.09°), and the extension is 0.40°±0.07°. The energy spectrum can be described by a power-law spectrum with an index of -2.92±0.17_{stat}±0.02_{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 ∼160  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 Crab Nebula is a bright source of gamma rays powered by the Crab Pulsar's rotational energy through the formation and termination of a relativistic electron-positron wind. We report the detection of gamma rays from this source with energies from 5 × 10-4 to 1.1 peta-electron volts with a spectrum showing gradual steepening over three energy decades. The ultrahigh-energy photons imply the presence of a peta-electron volt electron accelerator (a pevatron) in the nebula, with an acceleration rate exceeding 15% of the theoretical limit. We constrain the pevatron's size between 0.025 and 0.1 parsecs and the magnetic field to ≈110 microgauss. The production rate of peta-electron volt electrons, 2.5 × 1036 ergs per second, constitutes 0.5% of the pulsar spin-down luminosity, although we cannot exclude a contribution of peta-electron volt protons to the production of the highest-energy gamma rays.

We report observations of gamma-ray emissions with energies in the 100 TeV energy region from the Cygnus region in our Galaxy. Two sources are significantly detected in the directions of the Cygnus OB1 and OB2 associations. Based on their positional coincidences, we associate one with a pulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN G75.2+0.1 with the pulsar moving away from its original birthplace situated around the centroid of the observed gamma-ray emission. This work would stimulate further studies of particle acceleration mechanisms at these gamma-ray sources.

Water Cherenkov detector array (WCDA) is one of the key parts of the Large High Altitude Air Shower Observatory (LHAASO), the construction of which was completed by the end of 2020. The WCDA covers a 78 000-m 2 area and there exist 3120 large size photomultiplier tubes (PMTs) in three ponds: 8-in PMTs are used in WCDA pond No. 1 and 20-in PMTs are used in ponds No. 2 and No. 3. The front-end electronics (FEE) system based on multigain measurement technique is designed to achieve both high-precision time and charge measurements over a large dynamic range from single photon electron (S.P.E.) to 4000 P.E. (for water pond No. 1)/1800 P.E. (for water ponds No. 2 and No. 3). To achieve a high-quality clock distribution and phase alignment as well as mixed transmission of data, clock, and commands in one fiber over a long distance, an enhanced white rabbit (WR) technique is used. Testing of all the 350 FEE modules for the WCDA is presented in this article. Test results indicate that the charge resolution is better than 20% at S.P.E. and 1% at 1800/4000 P.E. and the time resolution is better than 300 ps root mean square (rms), which successfully meets the application requirement. All the FEE modules have been fabricated and installed for the LHAASO WCDA from 2018 to 2020, and the initial commissioning operation indicates that the FEEs function well.

The Large High Altitude Air Shower Observatory (LHAASO) is a new generation hybrid cosmic ray observatory which is expected to reveal the mystery of the origin of cosmic rays. The one square kilometer array (KM2A) consisting of 5195 Electromagnetic particle Detectors (ED) and 1188 Muon Detectors (MD) is a sub-array of LHAASO. The EDs are designed to measure the density and arriving time of the secondary particles of extensive air showers. The performance of the EDs is tested for the quality control after construction in the laboratory at sea level. In this paper, the performance of more than four thousands EDs is summarized. The performance of all tested EDs meets the design requirements with the time resolution of 1.6 ns, the detection efficiency of 97.8%, charge resolution for single particle of 18.5% and count rate around 480 Hz. Furthermore, one ED is singled out for monitoring of the long-term stability over a period of nearly two years from December 2018 to November 2020.

The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 10 15 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays 1 . The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2 ). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane 3 – 6 , unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators.

The first Water Cherenkov detector of the LHAASO experiment (WCDA-1) has been operating since April, 2019. The first 10 months of data have been analyzed to test its performance by observing the Crab Nebula as a standard candle. The WCDA-1 achieves the sensitivity of 65 mCU per year with a statistical threshold of 5 $\sigma$. In order to do so, 97.7\% cosmic ray background rejection rate around 1 TeV and 99.8\% around 6 TeV with an approximately photon acceptance about 50\% by using the $compactness$ of the shower footprints to be greater than 10 as the discriminator between gamma induced showers and the cosmic ray backgrounds. The angular resolution is measured using the Crab Nebula as a point source about 0.45$^\circ$ at 1 TeV and better than 0.2$^\circ$ above 6 TeV with the pointing accuracy better than 0.05$^\circ$. They are all matching the design specifications. The energy resolution is found 33\% for gamma rays around 6 TeV. The spectral energy distribution of the Crab Nebula in the range from 500 GeV and 15.8 TeV is measured and in agreement with results of other TeV gamma ray observatories.

We report the discovery of a new unidentified extended $\gamma$-ray source in the Galactic plane named LHAASO J0341+5258 with a pre-trial significance of 8.2 standard deviations above 25 TeV. The best fit position is R.A.$=55.34^{\circ}\pm0.11^{\circ}$ and Dec$=52.97^{\circ}\pm0.07^{\circ}$. The angular size of LHAASO J0341+5258 is $0.29^\circ \pm 0.06^\circ_{stat} \pm0.02^\circ_{sys}$. The flux above 25 TeV is about $20\%$ of the flux of Crab Nebula. Although a power-law fit of the spectrum from 10 TeV to 200 TeV with the photon index $\alpha=2.98 \pm 0.19_{stat} \pm 0.02_{sys}$ is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum %($\alpha \leq 1.75$) spectrum with a cutoff at $\approx 50$ TeV. We briefly discuss the origin of UHE gamma-rays. The lack of an energetic pulsar and a young SNR inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude both the leptonic and hadronic scenarios of gamma-ray production.

We report the discovery of a UHE gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. Significant excess of gamma-ray induced showers is observed in both energy bands of 25-100 TeV and $\gt$100 TeV with 9.5 sigma and 8.5 sigma, respectively. This source is not significantly favored as an extensive source with the angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of -2.83$\pm$ 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, deeper multiwavelength observations will help to shed new light on this intriguing UHE source.

As a sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to cover a large fraction of the northern sky to hunt for gamma-ray sources at energies above 10 TeV. Even though the detector construction is still underway, a half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the pipeline of KM2A data analysis and the first observation on the Crab Nebula, a standard candle in very high energy gamma-ray astronomy. We detect gamma-ray signals from the Crab Nebula in both energy ranges of 10$-$100 TeV and $>$100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance including angular resolution, pointing accuracy and cosmic ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE =(1.13$\pm$0.05$_{stat}$$\pm$0.08$_{sys}$)$\times$10$^{-14}$$\cdot$(E/20TeV)$^{-3.09\pm0.06_{stat}\pm0.02_{sys}}$ cm$^{-2}$ s$^{-1}$ TeV$^{-1}$. It is consistent with previous measurements by other experiments. This opens a new window of gamma-ray astronomy above 0.1 PeV through which ultrahigh-energy gamma-ray new phenomena, such as cosmic PeVatrons, might be discovered.