Saturday, November 25, 2006

Glimpses of Solar light


Ramallah sunset : A minaret is silhouetted as the sun sets over the West Bank city of Ramallah. (AFP/Abbas Momani)

Northern lights dance across the sky near Palmer, Alaska, Wednesday, Nov. 22, 2006. (AP Photo/Bob Martinson)

See More spectacular pictures of
Northern Lights

Into the LIGHT
Rehearsal in Seville : Mares perform during a rehearsal of Sicab 2006, the International Horse Show of Spain, in Seville. (AFP/Cristina Quicler)

Where visible light cannot travel thru

Six years after its construction began, the CNGS facility at CERN has sent its first batch of neutrinos 732 km to Gran Sasso in Italy in a highly successful commissioning run.

The CERN Neutrinos to Gran Sasso (CNGS) facility was built to create a neutrino beam to search for oscillations between muon-neutrinos and tau-neutrinos. An intense, almost 100% pure beam of muon-neutrinos is produced at CERN in the direction of the Laboratorio Nazionale del Gran Sasso (LNGS), almost 732 km away in Italy . There, the OPERA experiment (see CERN Courier November 2006 p24) is being constructed to find interactions of tau-neutrinos among those of other neutrinos.

The production of the CNGS beam of muon-neutrinos follows the "classic" scheme that was first used in the 1960s at Brookhaven and CERN, and has been refined ever since. An intense proton beam from CERN's Super Proton Synchrotron (SPS) is sent to strike a target, in this case graphite. Protons that interact with nuclei in the target produce many particles, mostly unwanted, but including positively charged pions and kaons – mesons that decay naturally into pairs of muons and muon-neutrinos.

Two magnetic lenses – the horn and the reflector – collect these mesons within a selected momentum range and focus them into a parallel beam towards LNGS. After a decay tube nearly 1 km long, all the hadrons – i.e. protons that have not interacted in the target, pions and kaons that have not yet decayed, and so on – are absorbed in a hadron stopper; only neutrinos and muons can traverse this solid block of graphite and iron.

The muons, which are ultimately absorbed downstream in around 500 m of rock, are measured first in two detector stations. Only the neutrinos are left to travel onwards through the top layer of the Earth's crust towards LNGS.

Konrad Elsener, Edda Gschwendtner and Malika Meddahi @ CERN courier

The neutrino is of scientific interest because it can make an exceptional probe for environments that are typically concealed from the standpoint of other observation techniques, such as optical and radio observation.
[+/-] Click here to expand

The first such use of neutrinos was proposed in the early 20th century for observation of the core of the Sun. Direct optical observation of the solar core is impossible due to the diffusion of electromagnetic radiation by the huge amount of matter surrounding the core. On the other hand, neutrinos generated in stellar fusion reactions are very weakly interacting and therefore pass right through the sun with few or no interactions. While photons emitted by the solar core may require 1,000 years to diffuse to the outer layers of the Sun, neutrinos are virtually unimpeded and cross this distance at nearly the speed of light.

Neutrinos are also useful for probing astrophysical sources beyond our solar system. Neutrinos are the only known particles that are not significantly attenuated by their travel through the interstellar medium. Optical photons can be obscured or diffused by dust, gas and background radiation. High-energy cosmic rays, in the form of fast-moving protons and atomic nuclei, are not able to travel more than about 100 megaparsecs due to the GZK cutoff. Neutrinos can travel this distance, and greater distances, with very little attenuation.

The galactic core of the Milky Way is completely obscured by dense gas and numerous bright objects. However, it is likely that neutrinos produced in the galactic core will be measurable by Earth-based neutrino telescopes in the next decade.

The most important use of the neutrino is in the observation of supernovae, the explosions that end the lives of highly massive stars. The core collapse phase of a supernova is an almost unimaginably dense and energetic event. It is so dense that no known particles are able to escape the advancing core front except for neutrinos. Consequently, supernovae are known to release approximately 99% of their energy in a rapid (10 second) burst of neutrinos. As a result, the usefulness of neutrinos as a probe for this important event in the death of a star cannot be overstated.

Determining the mass of the neutrino is also an important test of cosmology. Many other important uses of the neutrino may be imagined in the future. It is clear that the astrophysical significance of the neutrino as an observational technique is comparable with all other known techniques, and is therefore a major focus of study in astrophysical communities.

In particle physics the main virtue of studying neutrinos is that they are typically the lowest mass, and hence lowest energy examples of particles theorized in extensions of the Standard Model of particle physics. For example, one would expect that if there is a fourth class of fermions beyond the electron, muon, and tau generations of particles, that a fourth generation neutrino would be the easiest to generate in a particle accelerator.

Neutrinos are also obvious candidates for use in studying quantum gravity effects. Because they are not affected by either the strong interaction or electromagnetism, and because they are not normally found in composite particles (unlike quarks) or prone to near instantaneous decay (like many other standard model particles) it is easier to isolate and measure gravitational effects on neutrinos at a quantum level.

Neutrinos for beginners & Icecream by Sabine Hossenfelder
Tunnelling in Faster than Light by Plato @ Dialogues of Eide