Last Update: October 2000

BOREXINO

Collaboration:

Dipartimento di Fisica Università e I.N.F.N di Genova

L.N.G.S.

Dipartimento di Fisica Università' e I.N.F.N. di Milano

Dipartimento di Fisica Università e I.N.F.N. di Pavia

Dipartimento di Chimica dell'Università e I.N.F.N di Perugia

Belgium (I.R.M.M. European Joint Research Center - Geel)

Canada (Queen's University - Kingston)

France (College de France)

Germany (Max-Planck-Institut fuer Kernphysik Heidelberg, Technische Universitaet Muenchen )

Hungary (KFKI-RMKI Research Institute for Particle & Nuclear Physics Budapest)

Poland (Institute of Physics, Jagellonian University, Cracow.)

Russia (J.I.N.R. DUBNA, Kurchatov Institute - Moscow)

United States ( Bell Laboratories - Lucent Technologies, Massachusetts Institute of Technology, Princeton University, Virginia Polytechnic Institute)

Laboratory : Laboratorio Nazionale del Gran Sasso (LNGS)

1. Goal of the experiment

Borexino, a solar neutrino real time experiment at LNGS, makes use of the neutrino-electron scattering reaction to detect neutrinos emitted from the Sun. It is the only experiment in real time planned for the next few years, having a threshold below 1 MeV.

In Borexino we are mainly interested in the observation of the 7Be neutrinos, which is a monochromatic line at 863 keV.

The detection reaction is the electroweak elastic scattering on electrons; the recoil electron spectrum will have a maximum of 664 keV kinetic energy. The detection threshold for observing such process will be of 250 keV, so that a rate of about 50 events/day is predicted by the Standard Solar Models in the 100 tons Borexino fiducial volume.

The feasibility of this program depends critically on the expected background in the 250/700 keV energy window. This background was extensively studied with the CTF detector.

The 7Be neutrino line is predicted by all standard solar models to be the second most important neutrino production reaction (after the basic pp reaction) in the sun. The flux of 7Be neutrinos is predicted much more accurately (uncertainty less than 10%) and is about a 1000 times larger than the 8B neutrino flux that is measured by SuperKamiokande and SNO. Also, since 7Be decay produces only neutrino lines, the theoretical predictions of neutrino oscillations are more unique for 7Be than for the 8B neutrinos, which have a broad energy spectrum (0-15 MeV).

From the data of the solar neutrino experiments operating so far, the 7Be rate is estimated to be much lower than the prediction of the Standard Solar Model. There are many possible combinations of explanations for the observed rates in the radiochemical and Cerenkov experiments, if neutrino oscillations are assumed to occur, but only BOREXINO can provide a measurement of the specific 7Be contribution.

In summary, the solar 7Be neutrino line intensity and its time (seasonal, day-night) variations will be the main goal of the Borexino experimental program. In some aspects of the study of the time variations, Borexino is unique both for MSW and Vacuum scenarios.

In addition to 7Be neutrino physics, other goals under study by the collaboration (potential Borexino goals) are:

- More solar neutrino studies: 8B and CNO studies.

- Neutrino magnetic moment can be studied using a 51Cr source. An indicative sensitivity of 10-11 Bohr magnetons is expected.

- Antineutrino searches can be performed through the Reines-Cowan detection reaction. This can be used to search for neutrino/antineutrino transitions in the case of solar neutrinos or to a measurement of the Earth emitted neutrinos.

2. Physics achievements during 2000

The major accomplishment in the year 2000 concerns the installation of important subsytems and parts of the detector and of the ancillary plants:

- final qualification of the sealing of the phototubes in the dedicated experimental set-up (Two liquid test tank);
- completion of the storage area for the scintillator;
- start up of the new measurement phase with the Counting Test Facility, for scintillator qualification;
- electronics installation in Hall C;
- completion of the Liquid Handling and of the scintillator purification system;
- finalization of the support structures inside the sphere, and start up of the PMT's and cables installation.

3. INFN contribution to the experiment in terms of manpower and financial support

-Manpower: 24 physicts (equivalent to 17 full time researchers), 10 experts in technology (equivalent to 8 full time persons),8 technicians (equivalent to 6 full time persons)

-The financial support for Borexino in 2000 was of 4158 ML of lire.

4. Number of publications in refereed journals: 2

5. Number of talks to conferences: 11

6. Number of undergraduate and doctoral thesis on the experiment:

-n.2 Ph.D.

-n. 3 Laurea Diplomas

7. Leadership of the experiment

G. Bellini, member of the Milano group, is the spokesman of the experiment.

G. Manuzio, member of the Genova group, is in the Executive Committee.

Several physicists of the italian part of the collaboration are responsible of some of the main subsystems and plants of the experiment:

M. Giammarchi - liquid plants;

G. Ranucci, M. Pallavicini - Electronics, Daq, and PMT's;

D. Giugni - Engineering;

R. Tartaglia - Site manager

S. Malvezzi - Calibrations

E. Meroni - Software

G. Testera - Scintillator

8. Innovative instruments

The Counting Test Facility has been demonstrated to be one of the lowest background facilities in the World.

The collaboration has developed measurement techniques for radioassay of constructing materials, water, and nitrogen at unprecedented sensitivity levels.

The levels of purification of liquid scintillator and the sensitivities in the low radioactivity measurements reached by the collaboration represent a record in this field.

9.Competing experiments

There are no direct competitors in the near future for the 7Be neutrino measurements.

However, the Japanese experiment Kamland, after a first measurement phase for reactor antineutrinos, that aims to explore the LMA solution at larger mass, could be upgraded to attempt the 7Be measurement.

10. International committee which has reviewed the experiment

The experiment is continuously reviewed by the Committee II of INFN, by the NSF, by the German funding agencies, and by the International Scientific Committee of the Gran Sasso Laboratory.