The history, current status, and prospects for the development of the Baikal Neutrino Project are considered. The main physical results obtained with the help of NT200 and NT200+ neutrino telescopes are presented.

A new analysis of the data from the NT200 neutrino telescope based on the reconstruction of parameters for high-energy showers generated in neutrino interactions has yielded new upper limits on the diffuse neutrino fluxes predicted by a number of theoreticalmodels. The upper limit on the all-flavor neutrino flux with an energy spectrum E −2 is E 2Φ ν < 2.9 × 10−7 GeV cm−2 s−1 sr−1.

A multipurpose deep-water experimental string has been developed for testing new promising techniques and performing methodical tasks of deep-water muon and neutrino detection in Lake Baikal. The design of the setup is described, and some experimental results obtained with the pilot sample of the string under field conditions on Lake Baikal are presented.

The main physical results obtained with the Baikal neutrino telescope NT200 during the period 1998–2003 are reviewed: the limits for the diffuse flux of high-energy neutrinos, high-energy muons, and magnetic monopoles and the results of search for neutrinos from the center of the Earth due to annihilation of weakly interacting massive particles and from local neutrino sources. In April, 2005, the neutrino telescope NT200 was extended by introduction of three new strings, located at a distance of 100 m from the NT200 center. The new deep-water complex NT200+ has an effective volume for detecting cascades from high-energy neutrinos larger than that of NT200 by a factor of 4. At a cascade energy of 10 PeV, the effective volume of the new complex is 107 m3. Further development of the Baikal neutrino experiment is related to the design and fabrication of a detector with a volume of about 1 km3.

We review the present status of the Baikal Neutrino Experiment and present results of a search for upward-going atmospheric neutrinos and magnetic monopoles obtained with the detector NT200. The results of a search for very high energy neutrinos are presented and an upper limit on the extraterrestrial diffuse neutrino flux is obtained. We describe the strategy of upgrading the NT200 to NT200+ and creating a detector on the Gigaton scale at Lake Baikal. The first results obtained with the new NT200+ detector as a basic cell of a future Gigaton detector are presented.

We present the design of a device for detection of acoustic signals from high energy particle showers. The module will be stationary installed above the Baikal Neutrino Telescope NT-200+.

High-frequency noise of Lake Baikal is investigated using a submersible self-contained instrument to determine the noise background for the acoustic detection of superhigh-energy neutrinos. It is found that, under stationary and uniform meteorological conditions, the integral noise power in the frequency band 1–50 kHz is virtually independent of depth and is 10–200 mPa or more, depending on the specific conditions. The noise itself contains multiple short pulses of different amplitudes and shapes.

New results of 300 hours of operation of the TUNKA array are presented. An improved parametrization of the Cherenkov light lateral distribution function (LDF), based on CORSIKA Monte Carlo simulations and the experiment QUEST, has been used for the reconstruction of EAS parameters. The corrected energy spectrum in the knee region is obtained. The mean depth of the EAS maximum has been derived both from the analysis of LDF steepness and the FWHM of Cherenkov light pulse. The mean mass composition around the knee is estimated.

The project of an EAS Cherenkov array in the Tunka Valley/Siberia with an area of about 1 km 2 is presented. The new array will have a ten times bigger area than the existing TUNKA-25 array and will permit a detailed study of the cosmic ray energy spectrum and the mass composition in the energy range from 10 15 to 10 18 eV.

One of the possible ways to study high energy neutrino is to detect acoustic pulses from showers in water. First results of the study of high frequency acoustic noise in Lake Baikal are presented. Many short pulses with different amplitudes and shapes were observed, they should be considered as a background for acoustic neutrino detection. However, most of the short excesses are explained as a result of noise sound waves interference and can be eliminated by a correlation analysis. An algorithm for separation of acoustic signals with definite shape, which probably were produced by quasi local sources, is presented. The efficiency of the method was tested on real data, obtained with a 2-channels acoustic module. This algorithm is now used for online data filtering in the 4-channels acoustic device, which was designed and will be installed close to the Baikal Neutrino telescope NT-200+.

The hydroacoustic coordinate-measuring system of the NT-200 Baikal neutrino telescope is described. It is a ranging long-base hydroacoustic system constantly operating in an automated or interactive mode and capable of measuring the coordinates of the detecting modules of the NT-200 to within 20 cm. Special attention is given to the justification of the estimate of the coordinate measurement errors. As an illustration, some results of measuring the coordinates of the elements of the NT-200 and the hydrophysical characteristics of lake Baikal are presented.

The results from a comparative assessment of the applicability of various vacuum photodetectors with different types of photocathodes to next-generation neutrino telescopes are presented. It is shown how the spectrum of Cerenkov radiation is altered during its propagation through the waters of Lake Baikal and the Mediterranean Sea. The effect exerted by the dispersion of the medium on the duration of a Cerenkov light pulse is studied.

The present status of the Baikal Neutrino Experiment and the present results of a search for upward going atmospheric neutrinos, WIMPs, and magnetic monopoles obtained with the NT-200 detector are reviewed. The results of a search for very high-energy neutrinos are presented as well. An upper limit on the ν e +ν e +ν τ neutrino diffuse flux of E 2 Φ( E )<1.3×10 −6 cm −2 s −1 sr −1 GeV within a neutrino energy range of 10 4 –10 7 GeV is obtained, assumingan E −2 behavior of the neutrino spectrum and a flavor ratio ν e :ν π :ν τ =1:1:1. We also describe the moderate upgrade of the NT-200 planned for the next few years and present a possible detector on the Gigaton scale.

The effect of hydrostatic pressure and its gradient on the optical parameters of fiber-optic cables for the calibration system of the НТ-200 Baikal neutrino telescope was investigated. The mean delay time of the light signal in the fiber-optic cables of the system is shown to be virtually constant at absolute pressures varying from 0 to 150 atm; incidentally, the average signal amplitude decreases by 5%. When the pressure difference changes from 0 to 7 atm, the average signal delay time increases by 0.5 ns, and the average signal amplitude decreases by 17%.

We rewiew the present status of the Baikal Neutrino Project and present the results of a search for high energy neutrinos with the detector intermediate stage NT-96.

The results of direct measurements of group velocity of light in the lake Baikal water at the depth of 1100 m are presented. The lake Baikal water dispersion has been measured at three wavelengths: 370 nm, 470 nm and 525 nm. The results are in a rather good agreement with theoretical predictions.

A two-channel photomultiplier tube is developed to use as part of a QUASAR-370 hybrid photodetector. The test results of the amplitude and time responses are described. The time resolution is ≤340 ps; the peak-to-valley ratio of the charge distribution of single photoelectron pulses is ∼1.5.