Ken Peach: The Gran Sasso Laboratory Scientific Program
Gran sasso is one of a network of underground science laboratories. It is the largest if not the deepest. The big reason to go deep is to reduce cosmic-ray background. One muon per square meter per hour arrives at the cavern.
The experiments carried out in the facility are important at the precision frontier. They test the experimenters’ skill as well as the theoreticians’ ingenuity. Even if these experiments fail to make a discovery, they place severe constraints on new physics.
The neutrino has a colourful history, so it is interesting to spend a little while remembering it. In the 50’s the nature and existence of the neutrino was probed. In the following thirty years the focus was on the structure of the nucleon and weak currents. But now, and in the future, the focus has returned to be on the nature of the neutrino, and we found that this particle is a quite interesting and complex fundamental particle.
Neutrino oscillations are now well established and consistent with the 3-flavour mixing scheme. We can now subject to experimental test many parameters.
Peach discussed Opera, praising the revival of emulsion technique and its offering us the chance to really “see” the produced interactions. About ICARUS, he said that its huge scale took a while to develop. This “electronic bubble chamber” concept is not a mature technology yet, but one which is producing already beautiful data. What is nice about the icarus technique is that it gives in principle all the charge collection.
Solar neutrinos also have an interesting story – in the beginning we hoped to learn something about the sun, but this rapidly changed into learn something about neutrinos; but now we are back to trying to learn about the sun. The Borexino experiment has been producing stunning data on that.
Twenty years ago it was doubtful that we could study the energy balance of the Earth using neutrinos. But you can now place constraints and make measurements in this field. So there has been an enormous development and the opening of a new field of investigation there.
Neutrinos are being used to monitor the core collapse of supernovaes. The neutrinos again will give some information on themselves as well as on their source. For all these studies you need large masses and high availability – high duty factors.
The measurement of neutrino velocity emphasizes the need to cross-calibrate, make sure that there are no systematics.
The controversy on the existence of more neutrinos is intriguing. Ken agrees with Carlo: first of all one needs to resolve the experimental inconsistencies. Peach quoted Sherlock Holmes and asked: the “impossible” been eliminated ? Sox can over-constrain the measurement and make sure we understand the systematics the way we claim.
We spent the morning today discussing double-beta decay experiments. If we do see a signal, we then know that the neutrino is much more complex than we imagine, and this would mark a revolution. But one would need to measure in more than one experiment. To extract the parameter of interest, the effective neutrino mass, all these experiments are important.
The idea of COBRA is to use a large amount of CdZnTe semiconductor detectors. They have a first indication of the 2-neutrino double beta decay for now, and it will scale up. You can get a visual representation of the potential decays. Finally there’s Lucifer, another one of the crystal bolometers setting up. It has a tight time scale, should start taking data in 2015.
Dark matter searches are important because there are eight or nine indications for why dark matter is needed. The simplest explanation is very often the better one, and dark matter has the merit of linking astrophysical phenomena in various ranges, and there is sufficient motivation to look very seriously for it. It should exist and be detectable, with sensitive searches. At Gran Sasso there are four searches ongoing. Dama-Libra, using ultra-pure Na(TI), looking at the annual modulation. They see it very clearly. There are some features very hard to interpret without a signal, but the claim is controversial.
Cresst is a very different DM search technique. They do claim a signal, which is unfortunately incompatible with the DAMA signal. Looking at these numbers it is hard to believe that the excess has the >4-sigma significance that is claimed; however it is certainly intriguing.
A major enterprise is XENON, which will eventually reach an incredible level of sensitivity, 10^-47 cm^2. They are scaling up to 1 ton. XENON-100 is taking data now. Already, on the basis of XENON-10, the data is world-beating, which are incompatible with the claimed signals. There are ways around, but it is a confusing situation. Supersymmetric “preferred” regions might not be taken that seriously, but the experimental game is to carry on looking. In 2017 they expect to reach their limit sensitivity.
Darkside is an ambitious program to look for dark matter with liquid argon. It is a detector designed for discovery. Their unique selling point is their use of underground Argon, where they can see it as a source.