Ultra High Energy Cosmic Rays： Spectral Signatures and Observations

The observations of Ultra High Energy Cosmic Rays （UHECR） are renewed, focusing on the energy spectra as measured by HiRes, Telescope Array （TA） and Auger detectors （PAO）. It is found that highest energy Auger steepening does not agree with GZK cutoff, which is most probably explained by the nuclei mass composition detected by Auger. At present the difference in mass composition in Auger and HiRes/TA data remains the main unsolved problem of UHECR origin.

Study on the separation of 100 TeV γ-rays from cosmic rays for the Tibet ASγ experiment

A γhadron separation analysis is described for the observed air shower events with primary energy above 100 TeV based on the Tibet ASγ detector configuration. The shower age and size parameters are fitted from the measured lateral density distribution and used as discrimination variables. According to the MC simulation while taking into account the systematic uncertainty estimated from data and MC comparison, it is found that 70% of the cosmic ray （CR） background can be rejected while more than 78% of the T-rays can be retained. Sensitivity for 100 TeV γ-rays observation can thus be improved by at least 40%.

Origin of hardening in spectra of cosmic ray nuclei at a few hundred GeV using AMS-02 data

Many experiments have confirmed spectral hardening at a few hundred GeV in the spectra of cosmic ray(CR)nuclei.Three different origins have been proposed:primary source acceleration,propagation,and the superposition of different kinds of sources.In this work,a broken power law has been employed to fit each of the spectra of cosmic ray nuclei from AMS-02 directly,for rigidities greater than 45 GeV.The fitting results of the break rigidity and the spectral index differences less than and greater than the break rigidity show complicated relationships among different nuclear species,which cannot be reproduced naturally by a simple primary source scenario or a propagation scenario.However,with a natural and simple assumption,the superposition of different kinds of sources could have the potential to explain the fitting results successfully.Spectra of CR nuclei from a single future experiment,such as DAMPE,will provide us the opportunity to do cross checks and reveal the properties of the different kinds of sources.

Design of the LHAASO detectors

Introduction The Large High Altitude Air Shower Observatory plans to build a hybrid extensive air shower array with an area of about 1 km^(2) at an altitude of 4,410 m a.s.l.in Sichuan province,China,to explore the origin of high-energy cosmic rays.KM2A LHAASO-KM2A will detect gamma ray sources with a sensitivity of about 1%Crab Unit at 100 TeV.It covers an area of 1 km^(2) with a total of 5195 scintillation detectors.Its angular resolution reaches about 0.3 degrees,and the energy resolution is better than 25%.With the help of 1171 muon detectors,cosmic nuclei background will be rejected to a level of 10-4 at 50 TeV.The design and performances of the scintillation detectors and muon detectors are described in detail.WCDA LHAASO-WCDA focuses on surveying the northern sky for steady and transient sources from 100 GeV to 20 TeV,with a very high background rejection power and a good angular resolution.The WCDA consists of three water ponds with a total area of 78,000 m^(2),and the effective water depth is 4 m.Each water pond is divided into 5m×5m cells partitioned by black plastic curtains to prevent penetration of the light from neighboring cells.An 8-inch PMT sits at the bottom center of each cell,looking upward to collect Cherenkov light generated by shower secondary particles in water.WFCTA LHAASO-WFCTA is composed of 12 wide-field-of-view Cherenkov/fluorescence telescopes.Each telescope consists of a spherical light collector of about 4.7 m^(2) and focal plane camera of 32×32 pixels with a pixel size of 0.5 degree.LHAASO prototype arrays A prototype array about 1%of LHAASO has been constructed at Yangbajing Cosmic Ray Observatory and has been in operation for more than 2 years.Its performance fully meets the design requirements.Conclusion The LHAASO detectors are designed to fulfill the physical goals in gamma ray astronomy and cosmic ray physics.One-fourth of LHAASO will be constructed and put into operation to produce physical data by the end of 2018.The whole array will be finished in the beginning of 2021.