Survey of the Sun in the Lake Baikal Neutrino Experiment

Since long, galactic rotation curves indicate the existence of non-radiating dark matter. This evidence has been hardened also by evidences on larger cosmic scales, ending up with the largest scales, surveyed with the help of the cosmic microwave background radiation. In particular, the data suggest a dominant contribution of non-baryonic, non-relativistic (cold) dark matter. Among the favoured DM candidates are weakly interacting massive particles (WIMPs), in particular neutralinos as the lightest particles in many minimal supersymmetric models.

One strategy to search for a neutralino signal is to search for neutrinos produced in annihilation processes inside a large gravitational mass like the Sun. This method is well established since the nineties using underground and underwater(ice) neutrino telescopes. The Baikal Neutrino Experiment has published results of a search for neutrinos from WIMP annihilations in the core of the Earth [1], while the analysis with respect to the Sun is now presented the first time. We are looking for high energy neutrinos from the Sun in excess of the expected atmospheric neutrinos. The analysis is based on data taken with the NT200 neutrino telescope between April 1998 and February 2003.

Located in Lake Baikal, South-East Siberia, the Neutrino Telescope NT200 is operated underwater at a depth of 1.1 km since 1998. The telescope detects Cherenkov light from upward and downward-going relativistic muons. NT200 consists of 192 optical modules (OMs) arranged pair-wise on 8 strings, with 12 pairs per string. The height of the detector is 72 m, its diameter is 42 m. Each OM contains a 37-cm photomultiplier tube (PMT). To suppress background from bioluminescence and dark noise, the two PMTs of a pair are switched in coincidence. Since 2005, the NT200 configuration was upgraded by additional 3 strings, each 100 m away from the center. This upgraded detector of about 10 Mton (named NT200+) serves as a prototype cell for a later Gigaton volume detector. The status of the Baikal Neutrino Telescope is presented at this conference [2].

We have analyzed NT200 data collected between April, 1998 and February, 2003, with a total of 1007 live days. Calibration methods and methods to reconstruct muon tracks have been described elsewhere [3], [4], [5]. Our analysis is based on data taken with the muon trigger. It requires N hit ≥ n within 500 ns, where hit refers to a pair of OMs coupled in a channel. CORSIKA [6] and MUM [7], the Bartol atmospheric ν flux [8] and the neutrino cross-sections from [9].

The offline filter which requires at least 6 hits on at least 3 strings (“6/3”) selects about 40% of all triggered events. To distinguish upward and downward going muons on a one-per-million mis-assignment level, a filter with several levels of quality cuts was developed for the atmospheric neutrino (ν at ) analysis [10]. The atmospheric muons which have been misreconstructed as upward-going particles are the main source of background in a search for neutrino induced upward-going muons. To get the best possible estimator for the direction, we use multiple start guesses for the χ 2 minimization. For the final choice of the local minimum of χ 2 we use quality parameters which are not related to the time information. At the offline filter level (“6/3”) the angular resolution (Ψ -r.m.s. mismatch angle) is about 14.1 • for the ν at -sample. The present analysis defines two samples -sample A and sample B -which are optimized for the low and high WIMP-mass region, respectively. They use further differently tight quality cuts, resulting in different background contaminations and sligthly different angular resolution. The quality cuts are applied to variables like the number of hit channels, the probability of fired channels to have been hit or not, the actual position of the track with respect to the detector centre and χ 2 /d.o.f.. To improve the signal-tobackground ratio we used only events with reconstructed zenith angle Θ > 100 • . This results in 2376 and 510 upward going muons for sample A and B, respectively (with ν at -angular resolutions Ψ = 5.3 • and Ψ = 3.9 • ). Both Ψ values are much bigger than the visible size of the Sun. However the angular window for a signal search may be even larger, since at least at energies below 100 GeV the kinematical angle between neutrino and muon dominates.

A map of the ecliptic plane centered at the Sun is shown in Fig. 1 with 510 upward going muons. No clustering toward the Sun is observed. The distribution of the correlation angles between muons and the Sun is shown in Fig. 2. The dots refer to angles with the real position of the Sun, the histogram to angles with “fake Suns”, defining the expected background behaviour.

No excess is observed, resulting in upper limits on number of muons from the Sun. Table I gives the upper limits at 90% confidence level (c.l.) for the two selected data samples. We give numbers for three values of the half cone to the Sun

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