Underground Prototype Water Cherenkov Muon Detector with the Tibet Air Shower Array
We are planning to build a 10,000 m2 water‐Cherenkov‐type muon detector (MD) array under the Tibet air shower (AS) array. The Tibet AS+MD array will have the sensitivity to detect gamma rays in the 100 TeV region by an order of the magnitude better than any other previous existing detectors in the world. In the late fall of 2007, a prototype water Cherenkov muon detector of approximately 100 m2 was constructed under the existing Tibet AS array. The preliminary data analysis is in good agreement with our MC simulation. We are now ready for further expanding the underground water Cherenkov muon detector.

title: Cosmic-ray energy spectrum around the knee observed with the Tibet air-shower experiment
abstract: The energy spectrum of cosmic rays around the knee measured by Tibet air-shower experiment is summarized and its characteristic features are discussed under two possible scenarios. The result of Tibet experiment measured over wide range of10−10 eV with high statistics provided details of the knee at the energy around 4×10 eV. The study of the chemical composition based on measurements of proton and helium spectra obtained from airshower core detection indicates the dominance of heavy nuclei around the knee. Such feature can be explained either by contribution of nearby sources with source composition dominated by heavy nuclei or by nonlinear effect in diffusive shock acceleration mechanism. 1 Tibet Air-shower array and core detector The Tibet air-shower (AS) experiment is characterized by high altitude of the observation site (4300 m a.s.l., 606 g/cm), which enables us to determine the primary energy less dependent upon the primary mass and models of hadronic interactions in the energy range over PeV because of the observation of the shower size at nearly maximum development irrespective of the primary mass. The second merit of the high altitude is that it is possible to select the shower events of light nuclei origin such as protons and helium through the detection of high-energy secondary particles (electromagnetic component) located at the AS core, because the probability of association of high energy secondaries depends on primary mass by the difference of the interaction cross section in the atmosphere. The study of the primary mass composition is made by simultaneous operation of the AS array and the core detector. The AS array consists of 761 fast timing (FT) counters, surrounded by 28 density (D) counters with 36 900 m coverage as shown schematically  The AS array is used to measure the energy and arrival direction of each AS. The details of the installation are described in ( Amenomori et al. , 2008a). The primary energy of each event is determined by the shower size Ne, which is calculated by fitting the lateral particle density distribution to the modified NKG structure function. The energy resolution is less than 20% for the knee energy range. The arrival direction of an air shower is determined as follows. The direction cosine of the shower axis is determined using a least square method in which the difference is minimized between the arrival time signals of each FT counter and the expected values on the assumed cone with a given direction cosine. This procedure determines the arrival angle with an accuracy smaller than 0.2◦ at energies above 10 eV, which is calibrated by observing the Moon’s shadow ( Amenomori et al., 2000a, 2003). The core detector was designed to detect the high-energy particles at the AS core by converting them into electromagnetic cascade showers (burst) using lead absorber of 160 cm×50 cm in area and 7 cm (in Phase I) or 3.5 cm (in Phase II) in thickness. The burst size under the lead plate was measured by scintillation counters of the same size as the lead plate in area and 2 cm in thickness. Four photodiodes were attached to the each corner of the scintillator whose attenuation length for the scintillation light was calibrated with electron beam. From the 4 photo-diode signals, we can estimate the burst size and the position of the burst center with spatial resolution of 10 cm. One hundred core detectors were used (total area is 80 m ) to detect burst size greater than5×10. The study of the primary mass composition was made through two experimental phases. In the first phase, 6 layers of X-ray films were placed between lead plates with 1 cm depth interval to register the development of shower spots induced by high-energy γ and electrons (hereafter abbreviated as γ-rays) over a few TeV which are incident upon the detector, and detailed analysis on the structure of the high energy core has been made with use of image scanner ( Ozawa et al. , 2004) to obtain proton and helium spectra ( Amenomori et al. , 2006). The energy determination of the individualγ-rays was made by the comparison of the transition curve of the optical density of the shower spots with electromagnetic shower theory. Such procedure of the energy determination using X-ray films was calibrated using 200 GeV electron beam at FNAL ( Hotta et al. , 1980). A bundle of the high energy γ-rays originating from successive nuclear interactions of the primary cosmic rays in the atmosphere is called γ-family, which can be detected by spatial reconstruction of the shower spots registered on X-ray films with angular resolution of 2.5◦. They are reconstructed as groups of parallel tracks with typical lateral spread of at most several cm among uncorrelated single tracks, which are reduced as background. The assignment of a γ f mily to an accompanied AS was made through the positional correlation between the X-ray film and the scintillator, and the correlation between the burst and AS was made by their time stamps. The arrival directions obtained from AS and the reconstruction of the shower spots were also used to uniquely determine the AS candidate. Suchγ-family events are more efficiently generated by light primaries than the case of heavy primaries because of the penetrating nature in the atmosphere. Hence, the AS core detector can select the air showers of light primary origin naturally. The contamination of the events of heavy nuclei origin was estimated using Artificial Neural Network (ANN) based on extensive Monte Carlo simulation of air showers (MC) assuming interaction models and primary models using CORSIKA code. Two interaction models of QGSJET and SIBYLL are used in estimating the efficiency of the γ family generation, and also two mass composition models of proton dominant (PD) and heavy dominant (HD) are used to appreciate the contamination rate of heavy primaries. The efficiency of the family generation for various primary nuclei and its energy dependence is calculated by MC and used for obtaining the primary proton and helium fluxes. For exAstrophys. Space Sci. Trans., 7, 15– 20, 2011 www.astrophys-space-sci-trans.net/7/15/2011/ M. Amenomori et al.: Cosmic-ray energy spectrum with the Tibet air-shower experiment 17  ample, the efficiencies by protons at the knee energy 4 PeV in phase I experiment are about 7% and 10% by QGSJET model and SIBYLL model, respectively. ANN is also used in procedures to obtain proton and helium spectra. Phase I experiment selected 177 γ family events with core energy ∑ Eγ >20 TeV and AS sizeNe>2×10. In the second phase experiment, X-ray films were not used and proton+helium (P+He) spectrum was studied without separating them but with higher statistics than that of phase I. The core detector was tuned to decrease the detection threshold energy of the AS core to have several times higher efficiency. The details of the analysis are described in (Amenomori et al. , 2007, 2010). 2 Experimental results The energy spectrum of all particles was obtained as shown in Fig. 2 together with other data (Cited data: Grigorov (Grigorov et al. , 1971), JACEE (Asakimori et al. , 1998), KASCADE (Antoni et al., 2005), BASJE (Ogio et al. , 2004), CASA (Fowler et al. , 2001), Akeno (Nagano et al. , 1984), Akeno array1 and Akeno array20 ( Nagano et al. , 1992)). The uncertainty of the absolute intensity due to the interaction models used in the analysis is shown by QGSJET and SIBYLL (10% difference at most). The uncertainty due to the assumed primary mass composition is also shown by HD and PD. Although the composition is fairly different between HD and PD models at energies above 10 eV, the difference of the absolute intensity is 20% at most between the two models and it decreases with increasing primary energy. Note that the shapes of the spectra from different models are almost the same and the position of the knee is clearly found at the energy around 4 PeV. From the second phase experiment of AS core observation, the energy spectrum of P+He was obtained with high statistics and in good agreement with phase I as shown in Fig. 3, in which QGSJET model was used for the analysis. Use of 10 2 10 3 10 4 10 4 10 5 10 6 10 7 10 8 10 9 E 2 .5 d J /d E [ G e V 1 .5 m − 2 s e c − 1 s r − 1 ] Primary Energy [GeV/particle] Grigorov JACEE KASCADE-QGS KASCADE-SIB BASJE2004 CASA/MIA Akeno Akeno array1 Akeno array20 Tibet3-QGSJET+HD Tibet3-QGSJET+PD Tibet3-. The results from three models of QGSJET+HD, QGSJET+PD, and SIBYLL+HD used in the analysis are shown. SIBYLL model results in about 30% lower intensity than that of the QGSJET analysis There is a problem of discrepancy between Tibet and KASCADE data as shown in Fig. 3, e.g., the flux of the light nuclei by KADCADE is higher than Tibet by more than factor 2. However, if SIBYLL model is used in the analysis of KASCADE, the difference between two data is much reduced. This fact does not necessarily mean that SIBYLL model is more preferable to explain the data at this energy range. KASCADE reported that QGSJET model explains high energy data better than SIBYLL, on the other hand, SIBYLL model is better at lower energies. This problem is related to the interaction model dependence of MC calculations and also to the different experimental methods used in two experiments. Tibet experiment observes high energy part of the air showers, namely, the most forward region of the center of momentum system (CMS) of interacting particles, while KASCADE experiment observes Ne−Nμ correlation, in which muon component is sensitive to the central region of the CMS. The model dependence in forward region www.astrophys-space-sci-trans.net/7/15/2011/  2011 18 M. Amenomori et al.: Cosmic-ray energy spectrum with the Tibet air-shower experiment 10 1 10 2 10 3 10 4 10 5 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10

title: Tibet air shower array: results and future plan
abstract: The Tibet air shower array, which has an effective area of 36,900 m2, has been in operation at Yangbajing in Tibet, China at an altitude of 4,300 m above sea level. In this paper, we will briefly introduce the recent gamma-ray observation with the present Tibet air shower array and our future plan which is called the Tibet muon detector (MD) project.

title: Cosmic-ray energy spectrum around the knee obtained by the Tibet experiment and future prospects
abstract: The observation of the extensive air showers has been carried out since 1990 by Tibet air-shower array to study the origin of the cosmic-rays. In order to investigate the chemical composition around the knee, air-shower core detector was used in two experimental phases aiming to separate light primary elements (proton and helium). In a first phase experiment made in 1996-1999, primary proton and helium spectra were measured using a hybrid detector consisted of Tibet II air-shower array, emulsion chambers, and burst detectors. The result indicated that the fraction of light elements among the all particle spectrum is less than 30%. We performed the second phase experiment in 2002-2005 to measure proton + helium together to confirm the previous result with higher statistics by improving the detection efficiency for the air-shower cores. The P + He spectrum was obtained in a good agreement with previous result by several times higher statistics. The flux of the light elements, together with the all-particle energy spectrum, indicates the dominance of nuclei heavier than helium in the energy range around the knee. We also present the outline of the next phase experiment, which is to measure the heavy elements around the knee, using low threshold air-shower core detector named YAC.

title: New Estimation of the Spectral Index of High-Energy Cosmic Rays as Determined by the Compton-Getting Anisotropy
abstract: The amplitude of the Compton-Getting (CG) anisotropy contains the power-law index of the cosmic-ray energy spectrum. Based on this relation and using the Tibet air shower array data, we measure the cosmic-ray spectral index to be –3.03 ± 0.55stat ± <0.62syst between 6 and 40 TeV, consistent with –2.7 from direct energy spectrum measurements. Potentially, this CG anisotropy analysis can be utilized to confirm the astrophysical origin of the "knee" against models for nonstandard hadronic interactions in the atmosphere.

title: Recent Results and Future Prospects on Neutrino Oscillations from Reactors

title: ALICE technical design report of the zero degree calorimeter (ZDC)

title: Study of a prototype water Cherenkov detector for the Daya Bay neutrino experiment
abstract: The Daya Bay reactor neutrino experiment is designed to precisely measure the neutrino mixing angle theta(13). The anti-neutrino detectors are shielded by highly purified water against radioactivity and spallation neutrons from surrounding rocks. The water also serves as a Cherenkov detector to tag cosmic-ray muons, which induce main backgrounds. In order to study water purification and details of the detector response, a prototype water Cherenkov detector is constructed at the Institute of High Energy Physics (IHEP), Beijing. The detector is divided by Tyvek into two sections and viewed by two 8 in. photomultipliers. Cosmic-ray muons are used for detector response studies and a Monte Carlo simulation based on the Geant4 package shows good agreement with the experimental data.

title: A water Cherenkov calorimeter for long baseline neutrino oscillation experiments
abstract: We propose a water-based large homogeneous calorimeter as the next generation neutrino detector for ν factories and/or conventional ν beams. The Cherenkov light produced in water is sufficient to have good energy resolution, and the pattern recognition is realized by a modular water tank structure. Monte Carlo simulations show that the detector performance is excellent for identifying neutrino CC events while rejecting backgrounds.

title: EXCHANGE RELATIONS OF LAX PAIR FOR NONLINEAR SIGMA MODELS
abstract: We find that the non-ultralocal term s in classical Yang-Baxter r−s matrix consist of Riemann’s connections of target manifold G/H and field-independent projection operators in local coordinates on symmetric space G/H. We give the gauge transformations formula for the Poisson bracket of Lax pair and r−s matrix, and relate these expressions in terms of moving frame with that of fixed frame by current algebra.