IceCube: An Antarctic View of the Neutrino Universe
08 May 2006 - Interviews, Logistics
Francis Halzen
© IPF / IPF
Sciencepoles interviewed Professor Francis Halzen, from the Department of Physics at the University of Wisconsin, at the recent BEPOLES workshop (23 March).
Professor Halzen is involved at the very edge of astronomical discovery as he works on an exciting new neutrino telescope, ICECUBE, gathering information about "the universe's most violent events". ICECUBE, which sits far beneath the Antarctic ice, is already capturing some data about high-energy neutrinos from deep space, and when the planned cubic kilometre of sensors is fully operational by 2010/2011, will provide insight into the origins of large numbers of these subatomic particles.
SciencePoles: Let's talk first a little about the history of the neutrino " for a long time nobody thought this virtually massless, chargeless particle really existed?
The idea of a subatomic particle which had no charge and just about no mass had been around for some time but it wasn't possible to prove neutrinos existed until the creation of nuclear reactors " so they were only detected for the first time in 1956. Before that many physicists thought they were just some mathematical entity in the theory. As soon as their existence was proved it was realised that they could be terrific for astronomy.
You see, the fact that neutrinos have no charge is their strength, from an astronomical perspective. The particles we're most interested in right now reach us from very far away in space " we can only learn about their origins if they don't interact with the things they pass through on the way. Any physical or electromagnetic interaction would alter their course and/or their nature. The great thing about neutrinos is they travel in straight lines " like light.
Having proved they existed, then there was the problem of working out how to detect naturally occurring neutrinos and how to harness their useful information. That's where ICECUBE has come in.
You have described neutrinos as "cosmic messengers from the most violent processes in the universe". What can they tell us about those processes?
Neutrinos interact with their surroundings less than light " they can bring us more information from deeper inside black holes. Black holes are where the really violent cosmic processes take place. The theory is that gamma ray bursts there give rise to "cosmic rays", consisting of high-energy particles, including neutrinos, which can reach us here on Earth. The neutrinos point back to their origins and enable us to look for the gamma ray bursts.
The theories don't really cover the facts very well though and particles of inexplicably high energy levels have been observed. We don't know where they've come from and we don't know how they've been accelerated to the energy levels they demonstrate: the hope is that through analysing neutrino information we'll learn something new about how all this really works.
Light doesn't give us any information about these cataclysmic events as it can't escape from where they're all going on. Nor can we use positively charged protons to point us back to their point of origin as they've been affected by interstellar objects on their way here.
You've also tantalised the BEPOLES workshop with the prospect of learning about "telltale signals of additional dimensions" ...
One of the aims of the Large Hadron Collider (LHC) at the Centre Europ&ecute;en pour la Recherche Nucleaire (CERN) in Geneva is to try to detect the additional dimensions called for under particle "string theory", whereby some 10-11 dimensions are needed to make sense of Einsteinian curved space. We're doing the same but with our "universal accelerator". The neutrinos ICECUBE detects are much higher in energy than those we can produce through an accelerator like that at CERN. We are using the universe as an accelerator to collect millions of high-energy neutrinos.
Let's turn to the technology itself. ICECUBE involves kilometres-long "fishing lines" of detectors dropped into very deep holes in the Antarctic ice. Why does it need to be in such an isolated part of the world?
ICECUBE represents a breakthrough from previous approaches. Some of these involved building detectors in very deep disused mine shafts " trying to get rid of background "cosmic noise" " including from our own sun. For similar reasons, others involve putting detectors deep in the ocean and these face challenges as the sea surface has to be very flat for the sensor roll-out.
ICECUBE is very effective for several reasons. It is relatively straightforward to deploy sensors at around 2 kilometres deep in the earth " or ice I should say! " and the sensor array is large, much bigger than those previously constructed in mines. The other critical thing is the transparency of the medium. Very deep, very dark, crystal-clear ice turns out to be ideal for our purpose. We can't detect neutrinos directly. We have to do it when they crash into other particles. What you actually detect is the blue light emitted when there is a nuclear collision and you therefore need a medium which will transmit that light very well.
When we started building AMANDA, the prototype telescope for ICECUBE, in 1993 we discovered that blue light, in fact, travelled hundreds of metres in the deep ice. This meant we could go ahead to build ICECUBE, a cubic kilometre array of some 5,000 detectors. The data is already coming in and by 2010 we will have collected information from literally millions of neutrinos.
How do you know that light emitted on collision will not be emitted at an angle unrelated to the direction from which the neutrinos have come?
Let me explain how we do it. We look for well-defined patterns of light which occur when certain particles collide with other particles. In particular, muons, particles which can only be generated when neutrinos collide with other particles, travel many kilometres, leaving behind them a conical "wake" indicating the direction they've come from. And so we can reconstruct the direction of the muon and the original neutrino's direction is aligned with that.
Can you tell us a little about the Belgian involvement in this project?
Several Belgian institutions are involved: l'Universit&ecute; de Mons-Hainaut, Vrije Universiteit Brussel (VLB), l'Universit&ecute; Libre de Bruxelles (ULB) and recently the Universiteit Gent. Many of our Belgian partners are particle physicists with an interest in looking for string theory signatures. They, like all of our international collaborators, are excited by the possibility that we will find out something really new. It has generally been the case in the past that each time mankind invents a new type of telescope we break through to new astronomy.
By: Richard de Ferranti
