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S. Ragazzi: the Gran Sasso Physics Program

March 3, 2015

The gran sasso laboratory was born in the early eighties, with a remarkable program for those times: “not only proton lifetime”, as appears in a note by A.Zichichi of the time, but also a beam from CERN.

It is the largest underground science laboratory in the world, with easy access although at a depth of 3400 meters of water equivalent. The flux of muons is of 1.1 per square meter per hour, and the neutron flux is of 3×10^-6 per cm^2 per second. The surface is of 17800 square meters, the volume of 180,000 cubic meters. Amusingly, since December last year google street view maps the lab. So you can take your own virtual tour of the lab if you wish !

There are 20 experiments manned by a hundred staff personnel, over 900 users, and an average of 220 persons per day at the lab. Core activities in LNGS include neutrino astrophysics, neutrino physics, dark matter.

A supernova observatory looking for neutrinos from core collapse started in 1994. It has more than 1000 tons of liquid scintillator and has a high duty cycle.

Solar neutrinos are covered by Borexino, a remarkable detector. It is a large vessel of liquid scintillator inside shielding made of water and liquid scintillator. In 2014 they achieved a record reduction in radioactive contaminants, and detected neutrinos from the main nuclear reaction that powers the Sun. They were also able to collect evidence of antineutrinos from the earth (geo-neutrinos), a signal of 14.3+-4.4 events.

In the future, Borexino will try to address the CNO neutrino rate, which is a fraction of the background from beta decays of 210Bismuth. Since 210Bi has a lifetime of some motnhs, one can monitor the background with the alpha decay rate of 210Polonium.

Another pillar of gran sasso physics is neutrinoless double beta decay. This is a clear signature of a Majorana mass component in the neutrino mass, and is sensitive to the effective Majorana neutrino mass. The Gerda detector disproved an early claim for 0vbb decay in germanium. They are preparing for their phase 2. Carla Macolino will discuss it at this conference later.

Cuore is another experiment looking for double beta decays in 130Tellurium, 200kg of tellurium oxyde crystals, cubes of 5cm side. The principle is to detect energy deposits by measuring the increase of temperature of crystals. It is a very compact system, kept at 10 millikelvin. Reached 6mK, coldest cubic meter in the universe (ever existed!). They used lead free of 210Pb, found in ancient ship wreckages 2000 years old.

Lucifer is a tower of scintillating bolometers, again looking for double beta decay. Background from alpha decays is separated by the energy measurement and the detected light in scintillation diodes. Other non-scintillating crystals measure cherenkov emission from beta decays. Lucifer has very encouraging results by measuring the Cuore crystals where they could separate alpha from electron particles. Most likely, development of R&D from Lucifer, will define the next generation of double beta decay experiments.

Dark matter searches are performed by Dama/Libra, Cresst, Xenon, and DarkSide. Dama is made of ultra-pure sodium iodide crystals. They have been reporting an annual modulation attributed to the motion of the sun with respect to the average speed of dark matter in the galaxy. A puzzling result, not confirmed by other experiments. Trivial explanations have been ruled out.

Cresst is made by a low-temperature scintillating crystals where both energy deposit is measured by temperature increase, and scintillating light is seen. This way they discriminate nuclear recoils from gamma compton scattering and other backgrounds. In the recent run with a single upgraded crystal they disclaimed a possible indication of a signal from the previous run. Cresst is interesting in the search for WIMP dark matter at low mass, where it has better sensitivity than other experiments.

Another class of DM detectors are liquified noble gases. Xenon will be described by Elena Aprile later.

DarkSide, still in R&D phase, is a liquid argon detector performing pulse shape discrimination on the primary scintillation, measuring the ionization to scintillation ratio. The Darkside-g2 program (3600 kg fiducial) has been stopped, so they are now thinking at a new generation detector.

The Gran Sasso has a plan encompassing four pillars: precision measurements, including Borexino and Luna; double beta decay experiments; dark matter searches in high purity environments, especially testing the puzzling Dama result.

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