Introduction to Large High Altitude Air Shower Observatory (LHAASO)

CHINESE ASTRONOMY AND ASTROPHYSICS Chinese Astronomy and Astrophysics 43 (2019) 457–478

Introduction to Large High Altitude Air Shower Observatory (LHAASO)†  CAO Zhen1,2

CHEN Ming-jun1

LIU Cheng1

LIU Ye3

SHENG Xiang-dong1 YIN Li-qiao1

CHEN Song-zhan1 MA Ling-ling1

WU Han-rong1 ZHA Min1

HU Hong-bo1,2

MA Xin-hua1

XIAO Gang1

YAO Zhi-guo1

ZHANG Shou-shan1

(On behalf of the LHAASO collaboration) 1

Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 2

3

University of Chinese Academy of Sciences, Beijing 100049

School of Management Science and Engineering, Hebei University of Economics and Business, Shijiazhuang 050061

Abstract Since the century discovery of cosmic ray, the origin of cosmic ray is always a mystery. The study on the origin of high-energy cosmic ray is in an interdiscipline between the very high-energy (VHE) gamma-ray astronomy and the cosmic ray physics. The Large High Altitude Air Shower Observatory (LHAASO) is a unique and new generation cosmic-ray station with the advantages of high altitude, all-weather, and large-scale. It takes the function of hybrid technology to detect cosmic rays and to upgrade greatly the resolving power between gamma rays and cosmic rays. The LHAASO is expected to make the full-sky survey to find new gamma-ray sources, to obtain the highest sensitivity of gamma-ray detection at the high energy band of > 30 TeV, and to make the very high precision measurement on the component energy spectra of cosmic rays in a broad energy range of 5 orders of magnitude, in order to provide the evidence for revealing the mystery of the origin of cosmic ray. This paper de†

Supported by the National Natural Science Foundation (11675187, 11575203, U1731136, 11505190,

11635011), and the key project of the Bureau of International Co-operation, Chinese Academy of Sciences (113111KYSB20170055) Received 2019–02–02; revised version 2019–03–19  

A translation of Acta Astronomica Sinica Vol. 60, No. 3, pp. 19.1–19.16, 2019 [email protected]

0275-1062/19/$-see front matter © 2019 B. V. AllScience rights reserved. c Elsevier 0275-1062/01/$-see front matter  2019 Elsevier B. V. All rights reserved. doi: 10.1016/j.chinastron.2019.11.001 PII:

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scribes the detector structure, performance superiority and scientific motivation of the LHAASO. Key words cosmic ray—VHE gamma ray—extensive air shower—LHAASO 1.

A BRIEF HISTORY

In a broad sense, cosmic ray means the particles and radiations from the space outside the Earth, including the charged particles like atomic nuclei, electrons, positrons, antiprotons etc., and the electro-neutral particles like neutrons, high-energy gamma rays, neutrinos etc. In a narrow sense, cosmic ray refers to the atomic nuclei from light (such as protons and helium nuclei) to heavy (such as iron nuclei) that occupy ninety-nine percent. The definition of cosmic ray in a narrow sense is accepted in this paper. The energy span of cosmic ray covers 109 –1020 eV (by contrast, the artificially produced particles with the highest energy are the protons generated by the Large Hadron Collider (LHC) experiment, the highest energy of protons in a single beam can only be accelerated to 7 TeV, the energy in the laboratory reference frame generated by the proton collider can only reach 1017 eV). The discovery of cosmic ray has been over one hundred years, but the origin of cosmic ray is still a mystery. There are two important routes for the study of origin of cosmic ray: (1) The accurate measurement of cosmic-ray component energy spectrum, by studying the variations of energy spectrum with the energy and component to explore the mechanisms of generation, acceleration, and propagation of cosmic ray. The cosmic ray moves with a velocity close to to the speed of light, under the constraint of the Galactic magnetic field its moving direction is deflected as well, and it can stay in the Galaxy for about ten million years in average. There are two ways to observe the cosmic ray, one is the direct measurement, i.e., by the detectors carried by spacecrafts or high-altitude balloons to detect directly the cosmic ray outside the atmosphere. Because the detectors can not be too large and too heavy as the payloads, and the flight time is limited, while the flux intensity of the cosmic ray above 100 TeV is very low, so that it is impossible to obtain sufficient statistical quantities that necessary for studying the energy spectrum of 100 TeV and above. Another way is the indirect measurement, i.e., to build a large-scale detector array on the ground, and by detecting the extensive air shower (EAS), i.e., the thousands of secondary particles and radiations including the electrons, positrons, muons, gamma rays, hadrons, Cherenkov light, fluorescent light, radio emission etc. generated by the collisions between cosmic rays and atomic nuclei in the Earth atmosphere, to deduce the energy and composition of cosmic ray. (2) To search for the gamma-ray sources, and accurately measure the energy spectra of gamma-ray sources, and by studying the radiation mechanism of gamma ray, to find the source of cosmic ray. The advantage of studying gamma ray is that it is not affected by the interstellar magnetic field, through the accurate measurement of the direction of gamma ray, the celestial body from which the gamma ray is generated can be detected directly. The very

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high-energy (VHE) gamma ray astronomy (with energy >100 GeV) is a scientific branch for studying the astrophysical process at the highest energy state, which has become one of the main subdivisions of particle astrophysics and the hot spot in the present international research field. The VHE gamma ray is originated from the nonthermal radiation, which is closely related to the acceleration mechanism, and it may impose a very strong constraint to the theoretical models. In the meantime, the VHE gamma ray can be considered as a probe for studying the extreme astrophysical processes, and it plays a very important role in the studies of the origin of gamma-ray burst (GRB), and of the physical behaviors in the extreme environments (such as the neutron star, supernova, active galactic nucleus etc.). The experimental observation of VHE gamma rays is also helpful for studying some more fundamental physical topics, such as for searching the signals of high-energy gamma rays generated by the annihilation of dark matter particles in galaxies. The study on the origin of cosmic ray has been devoted for a long time in China, and rich experiences have been accumulated. In the end of 1970s, the Chinese scientists in cooperation with the Japanese scientists carried out the emulsion chamber experiment of cosmic ray at the Kanbala Mountain of 5000 m altitude above the sea level, and in the 1970s, in cooperation with the Japanese and Italian scientists they successfully performed the internationally famous ground-based EAS experiment at Yangbajing in Tibet. The Yangbajing experiment has accurately measured the energy spectrum of cosmic ray in the energy range of 1014 –1017 eV[1,2] , the 2D anisotropy of cosmic ray in the energy range of 1012 –1015 eV[3,4] , and made the observation of gamma ray of 1012 –1013 eV[5] , obtained important physical results. But due to the limited scale of the array, and the single type of the detector, it is difficult to recognize whether the primordial particles belong to gamma ray or cosmic ray, also due to the low sensitivity, it is failed to find some new gammaray sources, thus the advantage of high altitude has not been fully displayed. Based on these previous studies, aiming at the frontier subjects in the relevant fields, the Large High Altitude Air Shower Observatory (LHAASO) is being established in China, in combination with multiple detecting methods, to carry out the total component and 3D observations of the ESA, the various performances, such as the angular resolution, energy resolution, and the recognition ability of primordial component will be enhanced significantly in comparison with the previous similar experiments. This paper will briefly introduce the experimental device, performance superiority, and physical objectives of LHAASO. 2.

EXPERIMENTAL EQUIPMENT OF LHAASO

The site of LHAASO is selected to be the Haizi Mountain in Daocheng of Sichuan Province with a altitude up to 4400 m above the sea level, which is close to the provincial road 217, and only 8 km from the Daocheng Yading airport, hence the transportation is very convenient. The LHAASO experiment mainly includes three detector arrays[6] (see Fig.1): the groundbased particle detector array KM2A of one square kilometer has the largest distributed

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NORTH

area, where the ground-based electromagnetic particle detectors (ED) and underground muon detectors (MD) are uniformly distributed in an area of 1.3 km2 ; the central part is the Water Cherenkov light Detector Array (WCDA) with a full coverage, low energy threshold, and the total area of 7800 m2 ; there is also a telescope array (WFCTA), which is composed of 20 Wide Field Cherenkov light Telescopes (WFCTs) that may be distributed flexibly to fit the different physical needs. These detector systems will be introduced respectively as follows.

WFCTA ED MD

150 m WCDA

Fig. 1 The layout of LHAASO detectors

2.1 KM2A The KM2A array is composed of 5195 EDs and 1171 MDs, together with the subsequent functional systems of electronics, timing, data collection, trigger discrimination, data processing, calibration etc. The ED (Fig.2) is a plastic scintillation detector of one square meter, from which the generated photons are collected and propagated through optical fibers to a photomultiplier (PMT) of 1.5 inch, and transformed into an electric signal as the readout. There is a lead plate of 5 mm thickness covered on the surface of each ED, which is used to convert the photons in the EAS into electrons in order to enhance the detecting efficiency and angular resolution. There is an equilateral triangular distribution among the EDs, the interval is 15 m in the central area of one square kilometer. The distance between two adjacent EDs in the outer area is 30 m, which are used to judge whether the shower core is located inside or outside the array, and the total distributed area reaches 1.3 km2 . The EDs are used to reconstruct the energy and direction of primordial particles. The MD (Fig.3) is the WCDA of 36 m2 , and one PMT of 8 inch is used for the readout. There is a soil layer of 2.5-meter thickness covered on the surface of each MD, which is used to absorb the electrons and photons, the energy threshold for detecting the muons is 1.3 GeV. The MDs exhibit also an equilateral triangular distribution with an interval of 30 m in the

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central area. The distribution of all the arrays is shown in Fig.1, the missing area of which is limited by the geological condition that impossible to build the MD. Because there are rich muons contained in the secondary particles of the hadron cosmic ray, while there are very few muons in the secondary particles of gamma ray, which can be used to distinguish the gamma ray and cosmic ray. Fig.4 shows the muon number and electron number in the gamma ray and cosmic ray observed by the KM2A, from which it can be seen that there is an evident difference between the two cases, the cosmic ray can be completely excluded above a definite energy range[7] . 








  















   



Fig. 2 Schematic view of ED

PMT

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pure water 2.5 m

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1.2 m side view

6.8 m

top view

Fig. 3 Schematic view of MD

2.2 WCDA The WCDA (Fig.5) is divided into 3 water pools, which are composed of 3120 unit detectors and subsequent functional systems of electronics, timing, data collection, trigger, data processing, calibration and so on. The unit detector (Fig.6) is a water area of 5 m×5 m, with a depth of 4.4 m, there is a layer of light insulation between two units, so that to avoid the crosstalk of the signal from the same secondary particle, especially the muon. The WCDA adopts totally 3120 large-size PMTs (in which, 900 pieces of 8 inch and 2220 pieces of 20 inch), which are respectively located in the center of each unit and the water bottom for observing upward. In addition, there are 3120 small-size PMTs in the water pools (in which,

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900 pieces are of 1.5 inch and 2220 pieces are of 3 inch), the small-size PMTs are located beside the large-size PMTs, in order to enlarge the dynamic range for measuring the particle number of the shower, thus to realize the high-accuracy measurement of high-energy cosmic ray.

105 104 103

Nμ

102 10 1 10−1 10−2

102

103

Ne

104

105

106

Fig. 4 Separation of the primordial gamma ray (black dots) from the cosmic ray (blue dots) by using the electron number (Ne ) and the muon number (Nμ ) measured by KM2A. The black line is the separation line (from a simulation calculation)[7]

150 mˈ30 cells 110 mˈ22 cells

300 mˈ60 cells

150 mˈ30 cells

150 mˈ30 cells

Fig. 5 The effect diagram and schematic diagram of the overall layout of WCDA

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WFCTA

The WFCTA (Fig.7) is composed of at most 20 WFCTs, each WFCT is composed of 6 parts, namely the optical system, silicon PMT (SiPM) array, electronic readout system, the slow control system and detection system, calibration system, and mechanical system[8,9] . All the components of a single WFCT are located in one shipping container put on the chassis, in order for conveniently moving and lifting between the elevation angles of 0◦ –90◦ . After the high-energy cosmic ray enters in the atmosphere, the extensive air shower can be caused, and the charged particles in the shower will emit the Cherenkov light or the fluorescence due to the deexcitation of excited nitrogen molecules. Meanwhile, the detection of cosmic ray is realized just by these photons detected by the WCDA. The Cherenkov light or fluorescence is collected by the optical system composed of multiple reflectors, and focused on the SiPM array, to form the image of Cherenkov light or fluorescence on the SiPM array. The optical system of each WFCT is jointed by the 20 hexagonal spherical sub-mirrors and 5 corresponding half sub-mirrors, the total light-gathering area is 5 m2 . Each reflector has a radius of curvature of 5.8 m. In order to reduce the inconsistence of the speckle energy distribution caused by the abberation of the optical system in the field of view, by through an optical optimization, the distance from the SiPM to the reflector center is 2.87 m. The design of wide field of view is adopted for each telescope, there are totally 1024 SiPMs arranged in the SiPM array according to the configuration of 32×32. The field of view corresponding to each SiPM is 0.5◦ ×0.5◦ , the range of field of view is 16◦ ×16◦ for an individual telescope. 









 




    



  


Fig. 6 Schematic view of the WCDA detector

Besides the respective structures of the above sub-arrays, the LHAASO possesses the following whole-body features: (1) Though the number of detector units of LHAASO reaches ten thousands, but each unit transforms the light signal into the electric pulse signal through a light-sensitive device (PMT or SiPM), the signal type is identical, and the signal feature is consistent, thus convenient to deal with uniformly.

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(2) The detector units of LHAASO all adopt the little-rabbit clock system to record the arrival time of each signal, the accuracy is up to 0.5 ns and above, and the optical fibers are used to transmit the data, to ensure the reliability of data transmission. (3) Both the WCDA and KM2A adopt the trigger mode without hardware, i.e., to transmit all the over-threshold signals to an online data platform for a temporary buffer, where a software trigger discrimination is performed timely, then the selected event is transmitted to an off-line platform for permanent saving. The WFCTA also makes the online software triggering after the first-order trigger of hardware. This mode makes the triggering have a high flexibility and diversity, the triggering design can be made in respect to different physical objectives, and only the useful data are saved, to ensure the sufficient, high-efficient and rapid usage on the huge amount of data.

mechanical system SiPM Winston cone

divider SiPM frame

HV connector board analog board digital board

heat sink Sub-Cluster mounting system

Fig. 7 Schematic view of WFCTA

3.

SCIENTIFIC OBJECTIVES OF LHAASO

The core scientific objective of LHAASO is to explore the origin of high-energy cosmic ray and carry out the relevant studies on the basic sciences, such as the high-energy radiation, celestial body evolution, dark matter distribution, and so on. The particular scientific objectives of LHAASO are given as follows: (1) To explore the origin of high-energy cosmic ray. Through the accurate measurement of the broad-range energy spectra of gamma-ray sources, especially, by searching the cosmic Pevatrons in the energy range above 100 TeV, to study the particle features of high-energy radiation sources, to explore the evidence for the existence of hadron accelerator in the

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Galaxy, and to make a breakthrough in the aspect of cosmic ray source detection. To measure accurately the energy spectrum and composition of cosmic ray, and to study the acceleration and propagation mechanisms. (2) To make the full sky gamma-ray source scanning and searching, to detect a large number of new gamma-ray sources, especially the extragalactic sources, to accumulate the statistical sample of various sources, to explore their high-energy radiation mechanisms, including the mechanism to produce the violent time-variation phenomena, to study the evolutionary rules of active galactic nuclei with the central super-massive black holes, to catch out the high-energy GRB events in the universe, and to explore their eruption mechanisms. (3) To search the new physical phenomena, such as the dark matter, quantum gravity, or Lorenz invariance defect etc., and to find new rules. The various scientific objectives are respectively introduced as follows. 3.1

VHE Gamma Ray Astronomy

In the past 20 years, the application of Image Atmosphere Cherenkov Telescope (IACT) has broken through the bottleneck of excluding the cosmic ray background under the premise of greatly enhancing the angular resolution, and initiated the VHE gamma-ray astronomy. A great amount of gamma-ray sources have been detected by the IACT experiments of the Whipple atmosphere Cherenkov telescope (WHIPPLE), High-Energy Solid atmosphere Cherenkov telescope System (H.E.S.S.), VHE Radiation Image Telescope Array System (VERITAS), gamma-ray atmosphere Cherenkov imaging telescope (MAGIC) etc., it is definitely indicated that each of the supernova remnant, pulsar, super-massive black hole in the galactic center, and starburst galaxy etc. is a TeV cosmic accelerator (Tevatron), and at least electrons have already been accelerated in these Tevatrons to the energy not lower than 100 TeV. However, the electrons at ∼1 GeV only occupy 1% of the normal flux intensity of hadron cosmic ray, and 0.1% at ∼1 TeV, moreover, the gamma-ray emission is not necessary to be directly correlated with the acceleration of cosmic ray in the source region, the already discovered TeV gamma-ray sources are suitable for the model of inversely Compton (IC) scattering, i.e., the electron origin. The energy spectrum measurement made by the H.E.S.S. experiment is limited between 100 GeV and several ten TeV, which is not enough to determine finally the cosmic ray origin. At present, the key problem in the field of VHE gamma-ray astronomy is that on the one hand, the sample with enough number of gamma-ray sources has not been collected, it is impossible to make classification according to the acceleration behavior; while the known sources with detailed energy spectra and multi-waveband observations are less, it is difficult to make a correct judgement between the general rule and a specific individual behavior. Hence, it is required at present to make the all-sky survey and to detect a great amount of gamma-ray sources. On the other hand, it is necessary to make the in-depth imaging observations on the gamma-ray sources, the energy spectrum measurement in a broad range, and the multi-waveband observation as widely as

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possible, to completely make clear of the radiation mechanism of gamma ray, and to select the celestial bodies really accelerated by hadrons. It is worthy to emphasize that among the electron acceleration mechanisms that can produce the gamma ray with the highest energy is the IC scattering of electrons in the soft photons, however this process almost cannot produce the gamma ray above 100 TeV, if it exists, then it can be determined as the origin of cosmic ray unambiguously, thus the search of Pevatron is a new hot spot of the gamma-ray astronomy. The earlier ARGO-YBJ (Astrophysical Radiation with Ground-based Observatory at YangBaJing) experiment with a high altitude and full coverage has successfully reduced the energy threshold of gamma ray detection to 300 GeV, while, the MILAGRO experiment not only can detect the electrons, gamma ray, and muons in the shower, but also can effectively distinguish the gamma-ray shower and cosmic-ray shower due to the adopted water Cherenkov technique. Hence, the water Cherenkov experiment built in a high altitude has the advantage of both the low energy threshold and high sensitivity for the gamma ray detection. The HAWC (High Altitude Water Cherenkov) experiment of USA-Mexico cooperation is such an example to first practice this combination, in 2013, the HAWC experiment successfully built an experimental array of 22500 m2 at the 4100 m altitude, the sensitivity is 15 times higher than that of AGRO-YBJ or MILAGRO, and detected over 10 new gamma-ray sources in the Galaxy very soon[10] . Gamma-ray sources are numerous in number and have multiple types, including: (1) The supernova remnant (SNR) is an extended celestial body generated by the interaction of the ejected material of supernova during its outward expansion with the interstellar medium. The SNR is considered to be the main cosmic ray source in the Galaxy for a long time. In 2013, the two cosmic-ray nucleus sources IC443 and W44 were identified by the Fermi Large Area Telescope (Fermi-LAT) based on the energy spectrum feature under 200 MeV[11] , and they just belong to this kind of SNRs, hence this discovery was evaluated by the journal Science as one of the ten large scientific breakthroughs in 2013. In addition, the supernova remnant W51 was also found to be a cosmic-ray nucleus source. However, its gamma-ray energy is rather low, the required cosmic-ray energy is below TeV, which is much lower than the “knee region”, furthermore, the number of identified nucleus sources is too small, which is far away from solving the origin problem of the Galactic cosmic ray. (2) The pulsar wind nebula (PWN) is the type with the most Galactic sources in the energy band of TeV, at present, there are 34 PWNe at the energy band of TeV, in which there are 13 PWNe in the field of view of LHAASO. The PWNe at various wavebands are always considered to be stable radiation sources, but the discovery of the explosive phenomenon of Crab Nebula in 2010[12,13] has changed the idea of people, and become the most important progress in the recent observations of PWNe. In the large explosion of 2011 April, the flux intensity was enhanced over 30 times, the smallest timescale of light variation was shorter than 1 h, which means that the source size is very small, to estimate with the magnetic field

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strength of mGs, the corresponding electron energy of synchrotron radiation is required to be the order of magnitude of PeV, such a high energy is a challenge for the present electron acceleration mechanism. (3) The Galactic diffuse gamma-ray emission is mainly originated from the decay of 0 π caused by the strong interaction of cosmic ray with the interstellar medium, and the IC scattering of electrons in the interstellar radiation field, which can be used to study the cosmic ray propagation in the Galaxy, and to study the distributions and interactions of the cosmic ray and interstellar medium gas in different regions. The observed results of Fermi-LAT and ARGO-YBJ from GeV to TeV are basically consistent with the prediction of the cosmic-ray propagation model, the energy spectrum is also a power law approximately, and no truncation has been found. The H.E.S.S. detected the diffuse gamma-ray emission nearby the Galactic center[14] , which was considered to be the evidence for the acceleration of the PeV cosmic ray nearby the Galactic center, but the energy measured by the H.E.S.S. is mainly below 30 TeV, and its cutoff energy requires to be deduced under the assumption of a certain energy spectrum shape (the exponential truncation model). (4) The gamma ray is generated from the interaction between energetic particles and interstellar material or radiation field, and these energetic particles are escaped from the acceleration region, thus many VHE gamma-ray sources exhibit as the extended sources. In addition, a large part of the diffuse gamma ray in the Galactic equator is generated from the radiation caused by the interaction between cosmic ray and interstellar material or radiation field, and the observation of gamma ray is the most direct probe for detecting the energy spectrum and flux intensity of cosmic ray. The neutral π meson is originated from the interaction between the cosmic-ray hadron and the interstellar medium, and the decay of π meson produces the gamma-ray emission; while the cosmic-ray electrons produce the high-energy gamma-ray emission by the IC scattering. By comparing the configurations of the hydrogen column density and diffuse gamma-ray emission in the Galaxy, it is possible to obtain the contributions respectively from these two gamma-ray emission processes or the corresponding constraints. Inversely, for a given model of the cosmic-ray distribution in the Galaxy, it is possible to use the composition of neutral π mesons for detecting the column densities of molecular cloud and hydrogen, thus it is possible to measure the proportions of CO and molecular hydrogen in the different regions of the Galaxy. (5) In the over 70 extragalactic sources detected already in the energy band above 100 GeV, most of them belong to the active galactic nuclei (AGNs), with an evident feature of flux intensity varying with time, it has an important significance to understand the evolutions of these very active galactic nuclei (commonly the massive black holes) and their interactions with the surrounding materials. Because the source distance is very far (commonly > 8 pc), through the combined observations with other wavebands, the timevariation phenomena can be used to explore multiple fundamental astrophysical problems, including the large-scale material distribution, the distribution of extragalactic background

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light (EBL), the quantum gravitational effect, and so on. Obviously, the best observational tool is a large-scale detector able to continuously monitor the whole sky in 24 h with a rather high sensitivity. Besides the fast-varying gamma-ray sources, also exist another kind of spatially extended sources. Taking the source in the direction of Cygnus detected by the American MILAGRO experiment as an example, this source and Geminga are all the quite bright gamma-ray sources in the sky, and their flux intensities much exceed the minimum flux intensity detectable by the Cherenkov telescopes, but they were not discovered by these telescopes with a narrow field of view. This sufficiently embodies the advantage of a survey detector with a wide field of view to explore the extended sources, and also leaves a broad space for the ground-based detector array like LHAASO or HAWC to discover the stronger sources, while an older SNR always has a more extended radiation region, of course, the radiation flux intensity in a unit area of the source region will have a corresponding reduction, which requires the detector with a higher sensitivity. (6) The GRB is the most violent star-class high-energy explosive phenomenon in the universe, in observations, the GRB appears as a sudden enhancement of gamma-ray flux with a short timescale from the cosmic space, the typical GRB timescale is 0.01–1000 s, it possesses an inflected power-law spectrum of nonthermal radiation[15] , the light curve is complicated and varied fast, with multiple irregular pulses. The isotropic radiation energy released once by the GRB may reach 1046 J, or even higher. The high-energy radiation of a GRB commonly means the part from several ten to a hundred MeV. Since the launch of the Fermi gamma-ray space telescope in 2008, about one hundred GRBs with high-energy radiations have been discovered successively, there are several GRBs with an energy above 10 GeV[16−18] , GRB130427A[19] reaches 94–126 GeV (after the redshift revision), it was recently detected by MAGIC that the gamma-ray energy generated by GRB 190114C > 300 GeV[20] , which means that the GRB can produce the VHE photons. However, we still don’t know where the energy spectrum can be extended to, or how high the highest energy of the high-energy photon produced by the GRB can reach. Whereas the study on the high-energy radiations of GRBs has a crucial importance for fully understanding this kind of violent explosive phenomena, which may provide clues for the physics relevant to black holes and compact stars, and for the problems about the generation of high-energy cosmic ray, neutrinos etc., and also can be used to constrain some key parameters of GRBs, such as the Lorenz factor, magnetized extent and so on. Limited by the effective area of the Fermi gamma-ray space telescope, the detected gamma ray energies are mainly concentrated in the lower energies, the high-energy radiation only occupies a small part of the total energy. Due to the insufficient confidence for detecting the high-energy photons of GRBs, it is impossible to get a definite conclusion about the physical mechanisms of energy spectrum and so on. Meanwhile, the IACT can only work on clear and moonless nights, the observation time is only 10%, and the observed field of view is narrow, it can only make the fixed-point observations, even an alarm appears for the GRBs, a definite time is still needed to turn the

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device, hence it is deficient for observing such kind of transient explosive phenomena. Furthermore, electrons only occupy a very small part in cosmic ray, but electrons are the well-known most stable fundamental particles, and a kind of important probe for exploring the new physics. Since 2008, the space experiments of Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), Advanced Thin Ionization Calorimeter (ATIC), Fermi satellite, and Alpha Magnetic Spectrometer (AMS-02) etc. have measured the electron spectrum in the energy region below 1 TeV, and found the new phenomena beyond the prediction of the propagation model of cosmic rays, thus to trigger the upsurge of dark matter studies. On 2015 December 17, the DArk Matter Particle Explorer (DAMPE) ‘Wukong” satellite was launched in China, its core subject is to search the evidence of dark matter by the high-accuracy measurement of the electron energy spectrum of 5 GeV–10 TeV. The WCDA and KM2A combination of LHAASO will focus on the VHE gamma-ray astronomy. The LHAASO has the features of large field of view, all weather, low energy threshold, and high sensitivity, among the known gamma-ray sources, there are 103 sources of > 100 GeV and 187 sources of > 50 GeV existed in the field of view of LHAASO (Fig.8). The WCDA will make the survey observation on the gamma-ray sky of 100 GeV–30TeV, in the energy region of TeV it can attain the best sensitivity of < 0.1 flux intensity of Grab nebula (Fig.9), to form an advantage complement with the next generation of imaging Cherenkov Telescope Array (CTA) in Europe. Besides the observation of Galactic gammaray sources, the WCDA will discover and monitor the extragalactic time-varying sources like GRBs and AGNs, this has an important significance for studying the problems of the origin and acceleration of cosmic ray, the multi-waveband radiation mechanisms of GRBs and AGNs, the EBL, intergalactic magnetic field, galactic evolution, and so on. Meanwhile, the WCDA has the advantage for observing the extended sources, which is hopeful to detect more extended sources that difficult to observe by the CTA. The sensitivity of KM2A in the energy region of > 30 TeV is much higher than that of the previous IACT experiment and the future large-scale CTA project (Fig.9). The KM2A will focus on the gamma-ray observation with the energy above 30 TeV, to explore the high-energy radiation behaviors of gamma-ray sources, and to study the acceleration mechanism of the Galactic gamma-ray sources and the evolution of energetic celestial bodies up to the energy of PeV. For instance, it can be seen from the predicted energy spectra of IC443 and W51C, which belong to SNRs, to be observed by KM2A (Fig.10) that there is a very evident difference of over 5 σ at the key point of energy above 50 TeV between the energy spectra predicted by the hadron and lepton models. In addition, through measuring the Galactic diffuse gamma-ray and electron distributions and energy spectra from several hundred GeV to one hundred TeV, the LHAASO will extend the results measured by the Fermi satellite experiment and the carpet experiments of ARGO-YBJ and MILAGRO to higher energies, to give a strong constraint to the propagation model of Galactic cosmic ray.

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Fig. 8 The spatial distribution of the known gamma-ray sources with emission energies > 0.1 GeV[21] , >

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LHAASO leptonic model 5yr LHAASO AD model 5yr

LHAASO AD model 5yr

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Fig. 10 The expected energy spectrum measurement of LHAASO for the SNR IC 443 (left) and W51C (right)[25]

3.2

Cosmic Ray Physics

In the energy range spanning over 10 orders of magnitude, the full energy spectrum of cosmic ray (regardless of composition) basically follows the power law decreasing monotonously with energy, but there are 4 evident deflections: (1) there is a steepened deflection at about 4× 1015 eV, which is called the “knee”; (2) there is a more steepened deflection in the 300–400 PeV range, which is called the second “knee”; (3) there is a smoothed deflection at 4×1018 eV, which is called the “ankle”; (4) there is a truncation at 1020 eV, which is called the “GZK cutoff”. There are multiple assumptions of cosmic-ray origin theories for the cause of deflections, different predictions are proposed by these assumptions for the deflections of the energy spectra of various cosmic-ray components in the knee region (i.e., the component energy spectrum), the magnitudes of flux intensity before and after the deflections, the power-law index, the proportions of various components, and the turning points of various components. Only by the accurate measurement of the component energy spectrum, can an effective evaluation on the theoretical models be made, answering the question about the cause of energy spectrum deflections, and solving the mystery of the origin of cosmic rays. There are a lot of ground-based arrays for observing the air showers focused on exploring the energy spectrum and composition in the knee region of cosmic ray[26−31] . These experiments respectively explore one kind or multi-kind of secondary particles of air showers, some experiments have a small scale, and the statistical number of high-energy cosmic-ray events is not high enough; some experiments have rough detectors, and the performances, such as the energy resolution and the discrimination of primordial component, are not good enough; some experiments only have a single technical measure that can not deal with the complexities of cosmic ray, such as the multiple energies, multiple flux intensities, and multiple components. At present, there is a systematic difference up to 30% existed among different experimental results, thus it is impossible to make an effective judgement for the various theoretical assumptions about the cause of the knee region. Similarly, in the energy band of 1017 –1018 eV, the Akeno Giant Air Shower Array (AGASA), Fly’eye cosmic exper-

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iment, High Resolution Echelle Spectrometer (HiRes) etc. all observed the “second knee” in the cosmic-ray energy spectrum, but due to the defect of relative energy calibration among these detectors, the measured position of the ”second knee” differs from one another. The VHE cosmic ray can only be detected indirectly by the ground-based experiments, it is impossible to make an absolute calibration for the detectors, also because of the defect of relative energy calibration among the different experiments, in the measurement of energy spectrum, even the corresponding deflection of energy spectrum is observed by all the experiments, but the position and structure differ from one another, from which it is impossible to obtain a consistent conclusion. The direct measurement of energy spectrum below 100 TeV in space provides a unique reference energy standard for all the ground-based experiments, by comparing with the directly measured energy spectrum, and by the crossed calibration, the energy scale can be gradually transferred to the high-energy band, this is the unique effective measure to realize a continuous and consistent measurement for the VHE cosmic ray. With the priority of high altitude, the LHAASO possesses multiple detecting measures to perform the complete and 3D observations on the longitudinal and transverse developments of air showers, and to enhance greatly the energy measurement accuracy and component discriminating ability of cosmic ray, which include: the total photon number (Npe) measured by the WFCTA telescope can be used to measure the cosmic-ray energy; the shape of the EAS Cherenkov image observed by the EFCTA telescope, including the ratio between the major and minor image axes and the angular distance from the image centroid to the arrival direction of the shower, are closely related to the position when the EAS is developed longitudinally to the maximum; the energy of the core region recorded by the WCDA contains the hadron information in the early development of the EAS; the muon component recorded by the MD reflects the hadron information in the EAS. The WFCTA is an array of Cherenkov/fluorescence telescopes, a movable design idea is adopted, different array configurations are used for the different observation modes (Cherenkov observation mode and fluorescence observation mode) and different observing energy regions, to realize the accurate measurement of component energy spectrum of 1013 –1018 eV, and the energy scale transfer from the direct space measurement to the extremely high energies. Totally 4 stages are divided by the WFCTA to realize the component energy spectrum measurement of cosmic ray, in which the stages 1, 2, and 3 are the Cherenkov light mode, the WFCTA detects the directly incident Cherenkov light faced to the EAS direction, the stage 4 is the fluorescence mode, the WFCTA detects the diffuse-scattering fluorescence from the EAS side (Fig.11). At the first stage, 6 WFCTs are used to measure the component energy spectrum of cosmic ray in the range of 1013 –5×1014 eV, and to have the energy scale calibrated by the WCDA through the moon shadow transfer to the Cherenkov telescopes, and by comparing with the results obtained by the direct measurement experiments such as the DAMPE,

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Elevation /°

AMS02, and CREAM (Cosmic Ray Energetics And Mass), to study the error of energy scale transfer and to verify the correctness of the energy scale transfer. Because the statistical number of the cosmic-ray events in the low-energy band is enough large, only 6 WFCTs are needed to join in, which are close to a water pool distribution of the WCDA, in order to realize the effective overlap with the directly measured energy spectrum and extend to high energies. 90 80 70 60 50 40 30 20 10 0

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Fig. 11 Left: The field of view of WFCTA in the northern sky at the third stage. The azimuth angle 90◦ is the due north direction. The curve group for the elevations of 40–50 degree indicates the trajectories of the moon in one year. Right: The layout of WFCTA at the fourth stage

At the second stage, 6 WFCTs are used to measure the component energy spectrum of cosmic ray in the range of 1014 –1016 eV. The WFCTs are close to a water pool distribution of the WCDA. For the air shower with the energy higher than 1014 eV, the effective acceptance of the WFCTA in combination with a water pool of WCDA is approximately 9000 m2 ·sr. The combination of WFCTA, WCDA and KM2A can realize the purity up to 90% for selecting protons, and the purity up to 95% for selecting protons and helium nuclei, and can obtain the high-accuracy energy spectrum of the light component in the knee region (the left panel of Fig.12), the “knee” structure of the light component can be seen clearly. At the third stage, 18 WFCTs are used to measure the component energy spectrum of cosmic ray in the range of 5×1015 –1017 eV. All the centers of WFCTs are pointed to the 45◦ zenith angle, and arranged along a circle around the azimuthal direction to cover the azimuth angles of 240◦ . The 18 telescopes are located in the center of the KM2A array. The advantage of this layout is able to make a full usage of the EDs and MDs, and the effective spatial coverage area of the telescopes can reach the maximum (the left panel of Fig.11). The number of Cherenkov photons generated in this energy band is enormous, in order to prevent the PMT from saturation, there is an UV-band filter plate installed at the entrance of the telescope tube, to reduce the number of photons that entered the telescope, as well as the number of background photons of the night sky. It is worthy to emphasize that because the aging effect of the SiPM can be neglected, and even at the time of full moon the threshold value is higher than the background light at night, thus the WFCTs can work in all cases except for directly viewing the moon, hence, while the WFCTs that directly view the moon are closed alternately, the other WFCTs can be fully opened to increase the

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Proton Proton+Helium 104

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observing time obviously. For the air showers with an energy above 1016 eV, the effective acceptance of each telescope is about 18000 m2 · sr. To calculate according to the effective working time of 10% each year, there are totally 500 cosmic-ray events of above 100 PeV can be detected by the 18 telescopes, so as to obtain the high-accuracy energy spectrum for the heavy components in the knee region (the right panel of Fig.12), and the knee structure of iron nuclei can be clearly identified. 10−1 iron+MgAISi iron

10−2 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8 lg (E/GeV)

Fig. 12 Left: The expected energy spectra of pure light nuclei at the knee region to be measured at the second stage in one year of observation. Right: The expected energy spectra of pure heavy nuclei to be measured at the third stage in one year of observation

At the fourth stage, 20 WFCTs are used to measure the component energy spectrum of cosmic ray in the range of 1017 –1018 eV. The observation is performed by using the fluorescence emitted in the deexcitation of nitrogen molecules in the atmosphere excited by the secondary particles of the EAS. The distance between the 20 telescopes of WFCTA and the central array of LHAASO is about 5 km (the right panel of Fig.11), in which a main array of 4×4 and the auxiliary arrays in the two sides are included. This may realize the 3D observation of a fluorescence event of cosmic ray, to enhance the reconstruction accuracy of the cosmic-ray direction and core position, and the measurement accuracy of the maximum position Xmax of the air shower development. Under the fluorescence observation mode, the WFCTA can measure Xmax , while Xmax is a parameter containing the composition information. In addition, a combined observation can be performed by the WFCTA and the KM2A array. Thus the KM2A may provide the information of the muons in the air shower, so as to enhance the ability of discriminating the components of cosmic ray. The preliminary simulation shows that the telescopes of WFCTA have a rather flat acceptance at the energies higher than 0.2 EeV, which is about 79 km2 · sr, the corresponding event rate is about 2000 events each year. Furthermore, multiple ground-based cosmic-ray experiments have detected that there exists a large scale anisotropy with an intensity of about 1/1000 in the direction distribution of cosmic ray, which provides the new and important information for the Galactic cosmicray propagation and the interaction with the Galactic magnetic field. The ASγ (Yangbajing cosmic ray station) experiment has detected firstly in the world the 2D anisotropic distribution of cosmic rays of 4–300 TeV in the northern sky area, and proved that the Galactic

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cosmic rays, interstellar gas materials and stars rotate altogether around the Galactic center by using the almost disappeared anisotropy at 300 TeV. The ARGO-YBJ experiment also measured the anisotropy of cosmic rays in the energy region of 1–30 TeV with a high accuracy, the result showed that the anisotropy below 4 TeV increases with the increase of energy, but decreases gradually above 10 TeV. The newest result of ARGO-YBJ for the anisotropy has extended to 520 TeV, and confirmed that the anisotropic structure above 100 TeV is changed. In recent years, the IceCube neutrino (neutrino observatory) experiment and ASγ experiment have measured the anisotropy in the energy range of PeV respectively in the southern and northern sky areas, the new study showed that in the spatial distribution of anisotropy an evident change appears at 100 TeV[32] . The KM2A has a detecting area of 1 km2 , and has a great amount of underground muon detectors, which can distinguish the different components of cosmic ray, thus the KM2A can measure the anisotropy for the different components of cosmic ray above several ten TeV and the variation with the energy in the future, which will provide important clues for understanding the cause of the cosmic-ray anisotropy. 3.3

New Physical Phenomena

Besides the TeV gamma-ray astronomy and cosmic-ray physics, the LHAASO can play a special role for exploring some new physical phenomena by its advantages, including: (1) To detect the dark matter indirectly. The most sensitive gamma-ray detector with a large field of view in the world at present is the Fermi satellite, both its angular resolution and sensitivity for the gamma ray have reached an unprecedented level. However, due to the limitation of the satellite experiment itself, the volume detector area is only about 1 m2 , thus for the gamma ray detection, the highest energy is only about 300 GeV. If a much higher energy and sensitivity are required, it is necessary to build a ground-based detector with an even larger scale, LHAASO is just such a detector. There is a complementary role between Fermi and LHAASO in the energy band, while it is generally required that the mass of dark matter particle is around 1 TeV for explaining the results of ATIC and PAMELA, the ground-based detector is more superior for the signals from such heavy dark matter particles. (2) To explore the effect of quantum gravity or Lorenz symmetry defect by using some transient phenomena. In the relativity and quantum field theory, the Lorenz symmetry is a basic symmetry. But in the high-energy condition of the Planck scale and the condition of quantum gravity, the Lorenz symmetry may have a definite defect. The development of modern science has made the experimental accuracy enhance greatly, while the accumulation of cosmological scale makes the small Lorenz defect become an observable effect, all this has made the experimental study on the Lorenz defect become possible. An important prediction of the Lorenz defect is that the propagation speed of high-energy photons has a decrease in respect to the low-energy photons. If two photons with different energies emit simultaneously from one point, through a long distance propagation with a cosmological scale, the Lorenz

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defect will be accumulated to make the high-energy photon reach the observation point with a time delay in respect to the low-energy photon. Hence, if the difference of the arrival times of the high and low energy photons from a gamma-ray source of cosmological scale is measured on the Earth, then the magnitude of the Lorenz defect effect can be measured or used to make constraint on the relevant theoretical parameters. (3) To observe the phenomena of particle physics at the new energy scale above the LHC energy. The discovery of intergalactic cosmic-ray sources will open a new era for studying the particle physics by using the cosmic accelerator. The photons and neutrinos with the energy up to several hundred TeV can be generated in the target material around the cosmic accelerator, and propagated to our detectors, with the increasingly enriched multiband observations of the source region and adjacent region, the spatial distributions of the matter and magnetic field in which, as well as the time-variation behavior of the radiation will be measured more clearly, which will produce a strong constraint on the interaction model, and provide an effective experimental verification on the models of particle physics at the new energy scale. 4.

CONCLUSION AND PROSPECT

The LHAASO has started its construction since 2017, and it is progressed smoothly at present, it is predicted to be built completely in 2021. Through the joint observation with multiple detecting measures, and combined with the detecting area of one square kilometer, the LHAASO will have an unprecedented sensitivity of gamma-ray detection, it is hopeful to discover the acceleration source of Galactic cosmic ray, to make a great contribution to the studies on the high-energy astrophysical radiation, cosmology, and new physical rules. Meanwhile, due to the feature of low threshold energy of the high-altitude and full-coverage detector, the LHAASO will realize the transfer of energy scale from the direct space measurement to the extremely high energies, and provide important evidence for the cause of the two “knees” in the cosmic-ray energy spectrum. Through the platform of LHAASO, the Chinese cosmic-ray career will step on a new stage, to make new and important contributions to the scientific career of mankind, and to lay a solid foundation for the future development. On the basis of LHAASO, the studies of various new techniques are undertaking, such as the water-lens Cherenkov telescope with a large aperture and wide angle[33] , the thermal neutron detector of high-energy cosmic ray[34] , radio antenna array[35] , SiPM and the photomultiplier of the micro-channel plate type (MCP-PMT)[36] and so on. Depending on the large-scale scientific facility, new techniques can be cultivated only by inputting a small amount of seed fund, which is the practice of Chinese Academy of Sciences in the “135” strategy, and the guarantee for a continuous and long-term development of the scientific experimental work. After the LHAASO, the options of Chinese ground-based cosmic-ray research include the aspects of the observation of high-energy Gamma bursts in the energy

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region of 100 GeV, the neutrino astronomy, and the VHE cosmic ray and so on. China has a vast territory with various kinds of terrain, and possesses an innate geographical priority to realize these programs. The higher altitude makes the experiments acquire a lower threshold energy, and therefore enables the effective study in the fields of low energy band, in favor of the GRB observation in the energy region of 100 GeV. The present altitude of LHAASO is 4410 m, where is only the edge area of the Qinghai-Tibet Plateau, while there are a lot places with a altitude of 5000 m even 6000 m in the depth of the Qinghai-Tibet Plateau, where the requirements for the experimental site are satisfied. A lower altitude is helpful for observing the cosmic ray with the highest energy. Whereas, if the extremely rare τ neutrinos are planned to be detected, it is necessary to find out a massif with a relatively high altitude and large span as the conversion body, such as the Barkol mountains and Tianshan mountains in Xinjiang. Furthermore, the perpendicular cutoff rigidity of cosmic ray is close to zero in the south and north poles of the Earth, thus the cosmic ray can travel straightly almost without the influence of the geomagnetic field, hence there is a geographical advantage to monitor the solar charged particles in the polar regions. Meanwhile, it has been proven that the polar glacier is a very reliable detecting medium, very suitable for the neutrino astronomical observation. The polar scientific investigation and its peaceful usage possess a very important strategic significance for a country.

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