Potential contribution from gamma-ray sources to the Galactic diffuse gamma rays observed above 100 TeV (sub-PeV energy range) by the Tibet AS γ experiment is an important key to interpreting recent multimessenger observations. This paper reveals a surprising fact: none of the 23 Tibet AS γ diffuse gamma-ray events above 398 TeV within the Galactic latitudinal range of ∣ b ∣ < 10° come from the 43 sub-PeV gamma-ray sources reported in the 1LHAASO catalog, which proves that these sources are not the origins of the Tibet AS γ diffuse gamma-ray events. No positional overlap between the Tibet AS γ diffuse gamma-ray events and the sub-PeV LHAASO sources currently supports the diffusive nature of the Tibet AS γ diffuse gamma-ray events, although their potential origin in the gamma-ray sources yet unresolved in the sub-PeV energy range cannot be ruled out.

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.”

The fraction of the contribution from yet-unresolved gamma-ray sources in the Galactic diffuse gamma rays observed by the Tibet air shower array is an important key to interpreting recent multi-messenger observations. This paper shows a surprising fact: no Tibet diffuse events above 398TeV come from the gamma-ray sources newly detected above 100 TeV by LHAASO. Based on this observational fact, the contribution of sources unresolved by LHAASO to the Tibet diffuse events is estimated to be less than 31% above 398TeV with a 99% confidence level. Our result shows that unresolved sources make only a sub-dominant contribution to the Tibet diffuse events above 398 TeV and a large fraction of the events are truly a diffusive nature.

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.

aFaculty of Engineering, Yokohama National University, Yokohama, Japan bNational Astronomical Observatories, Chinese Academy of Sciences, Beijing, China cFaculty of Engineering, Utsunomiya University, Utsunomiya, Japan dKey Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China eInstitute for Cosmic Ray Research, University of Tokyo, Kashiwa, Japan f Tokyo Metropolitan College of Industrial Technology, Tokyo, Japan E-mail: kurashige-daichi-ts@ynu.jp

The interpretation of the EAS data relies on the employment of hadronic interaction models, which are subject to theoretical and experimental uncertainties that may hamper composition studies of cosmic rays. To check the reliability of such models at the energies relevant for EAS studies, the predictions of the models can be compared with data from air shower observatories. In this regard, the study of the number of muons becomes extremely useful, since they are sensitive to the hadronic interactions that occur in the early phases of the EAS development. In the paper, we propose a Monte Carlo study to test the number of muons in hadronic interaction models by the hybrid experiment (YAC-II + Tibet-III + MD). For an air-shower event, the Tibet air-shower array (Tibet-III) provides the arrival direction and the air-shower size which are interrelated to primary energy, the Yangbajing Air shower Core detector (YAC-II) array measures the high energy electromagnetic particles in the very forward region so as to obtain the characteristic parameters of air-shower cores, at the same time, the underground MDs record the number of high-energy muons above 1 GeV. Since we can select proton events with high accuracy by YAC-II almost independently of the hadronic interaction models, the accompanying number of muons induced by proton events can be fed out. With the unique settle of YAC-II, our results show that the description of muon numbers in different hadronic interaction models can be well systematics-checked to avoid the ambiguity of the primary cosmic-ray mass composition around the knee energy region by the Tibet hybrid experiment (YAC-II + Tibet-III + MD).

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 have built a new hybrid detector system in Tibet, China, in 2014, it consists of the Tibet air-shower array (Tibet-AS), the air-shower core-detector array (YAC) and the underground water-Cherenkov muon-detector array (Tibet-MD). In this paper, the muon multiplicity will be discussed in detail by analyzing the muon components of the secondary particles of extensive air showers (EAS) in the primary energy range of 10$^{14}$ to 10$^{16}$ eV recorded by Tibet-MD array. We have carried out detailed Monte Carlo simulation by using CORSIKA (ver.7.35), which includes QGSJET01c, EPOS-LHC and SIBYLL2.1 hadronic interaction models. The simulated air-shower events are reconstructed with the real detector configuration as the (Tibet-AS+MD) hybrid experiment, and all detector responses are calculated using Geant4 (ver. 9.5). The data observed by the Tibet-AS+MD has been used to study the correlation between the air-shower size (Ne) and the number of muons (Nmuon). Here, we will also report their primary mass sensitivity and the interaction model dependence using our new hybrid experimental result.

Cosmic-ray experiments including ground-based air-shower arrays as well as underground muon telescopes have reported the existence of anisotropy in the arrival directions of galactic cosmic rays. The origin of the anisotropy, however, has not been fully understood yet. At sub-TeV energies, the anisotropy does not exhibit a 22-year cycle variation expected from the reverse of the solar dipole magnetic field, although the amplitude of the anisotropy becomes attenuated by solar modulation effects. At TeV energies, there is not an established model for two distinct large-scale structures reported by a lot of experiments: a deficit region called the "loss-cone" and an excess region called the "tail-in". Above similar to 100TeV, it has been pointed out that the amplitude and the phase of the anisotropy begin to change dramatically compared with those at TeV energies. The anisotropy reflects how cosmic rays propagate through the magnetic field in the heliosphere and the surrounding interstellar medium. In this presentation we study the influence of the heliosphere on the anisotropy by tracking particle trajectories in a heliosphere reconstructed from MHD simulations. We also discuss the anisotropy outside the heliosphere that can be derived from the experimental data obtained by the Tibet AS array.

The Tibet air shower (AS) array and underground water-Cherenkov-type muon detector (MD) array have been successfully operated since 2014, at an altitude of 4,300 m in Tibet, China. we observed 24 gamma-ray events with energy greater than 100 TeV against 5.5 background events, which corresponds to 5.6$\sigma$ statistical significance (Amenomori et al. 2019). The highest energy of the detected gamma rays is estimated to be 450 TeV. This is the first detection of gamma rays beyond 100 TeV from an astrophysical source, and a pioneering work opening a new higher energy window in the astronomy and astrophysics.

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.

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 similar to 10(5) photoelectrons (PEs) at the supply voltage of 700V. 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.

We have upgraded the new Tibet AS g experiment in China since 2014 to measure the chemical composition of cosmic rays around the knee. This hybrid experiment consists of an air-shower-core detector array (YAC) to detect high energy electromagnetic component and an air-shower array (Tibet-III). We have carried out a detailed air-shower Monte Carlo (MC) simulation to study the performance of the hybrid detectors by using CORSIKA (version7.5000), which includes EPOS-LHC, QGSJETII-04, SIBYLL2.1 and SIBYLL2.3 hadronic interaction models. The preliminary results of the interaction model checking above 50 TeV energy region are reported in this paper, and the primary proton and helium spectra at energy range from 50 TeV to 10(16) eV was derived from YAC data and it is smoothly connected to direct observation data at lower energies and also to previously reported our works at higher energies within statistical errors.

We have built a large water Cherenkov muon detector array (Tibet-MD) under the existing Tibet air shower array (Tibet-AS) at 4,300 m above sea level, to observe 10-1000 TeV gamma rays from cosmic-ray accelerators in our Galaxy with wide field of view at very low background level. The Tibet-MD array will improve the sensitivity to gamma-ray sources by an order of magnitude around 100 TeV. In this paper, we will report on the gamma-ray emission from the TeV pulsar wind nebula MGRO J1908+06 observed in the energy region from multi TeV to sub PeV with the Tibet-AS+MD array using data accumulated from 2014.

Aiming at observing cosmic gamma rays in the energy region beyond several tens of TeV, a water-Cherenkov-type muon detector (MD) array was built under the Tibet air shower (AS) array. The Tibet AS+MD array, which possess high sensitivity to gamma rays beyond 10 TeV, has been operated since 2014. We performed analysis of gamma rays from the Geminga PWN using data collected by the Tibet AS+MD array. We report on the gamma-ray energy spectrum beyond 10 TeV and the morphology of >10 TeV gamma rays.

A new hybrid experiment consisting of a high-energy air-shower-core array(YAC), a high-density air-shower array (Tibet-III) and a large underground water-Cherenkov muon-detector array (MD) has been operated by the Tibet AS gamma collaboration since 2014. In this hybrid experiment, YAC is used to observe high-energy core events induced by cosmic rays, Tibet-III provides the total energy and the direction of air showers while MD is used to measure the number of muons contained in the air showers. The first step of YAC, called YAC-I that consists of 16 EAS core detectors located near the center of the Tibet-III, has been carried out since May, 2009. In this paper, we will report on the check of hadronic interaction models in the forward region from 10 TeV to 1 PeV by observing EAS cores using (YAC-I+Tibet-III) hybrid experimental data.

The Tibet ASγ experiment located at 4300 m above sea level, Tibet, China, has a wide field of view and large effective area. It consists of the Tibet air-shower array (Tibet-AS), the air-shower core-detector array (YAC) and the underground water-Cherenkov muon-detector array (Tibet- MD). The Tibet-MD array significantly improves its gamma-ray sensitivity in the 10-1000 TeV energy region by an order of magnitude better than any other previous existing experiments in the world. In this paper we will search the γ-ray sources using data taken from 2014. The result shows the direction to the Crab has the most excess in the number of candidate primary γ rays from 3 TeV to 500 TeV. This is the first detection of the highest energy photons beyond 100 TeV from an astrophysical source, and thus opens up the sub-PeV window in astronomy.

We report, for the first time, the long-awaited detection of diffuse gamma rays with energies between 100 TeV and 1 PeV in the Galactic disk. Particularly, all gamma rays above 398 TeV are observed apart from known TeV gamma-ray sources and compatible with expectations from the hadronic emission scenario in which gamma rays originate from the decay of π^{0}'s produced through the interaction of protons with the interstellar medium in the Galaxy. This is strong evidence that cosmic rays are accelerated beyond PeV energies in our Galaxy and spread over the Galactic disk.

We present the results of a search for pulsed gamma-ray emission in the 100 TeV region from several pulsars using data taken with the Tibet air shower (AS) array and a water Cherenkov type muon detector (MD) array. The Tibet AS gamma experiment has improved significantly gamma-ray sensitivity by the constructed MD array since 2014. Based on the observational data with the AS + MD array, we will report data on several famous pulsars in the northern hemisphere in this presentation.