Monday, July 6, 2015

Acronym in experimental physics: MINOS for Main Injector Neutrino Oscillation Search

Neutrinos are subatomic particles (leptons) with half-integer spin and without electric charge. They come in three types or flavors: electron-neutrino, muon-neutrino and tau-neutrino. The flavor of a neutrino oscillates, while the neutrino is flying through space—for example, after being generated by beta-decay of a radioactive atom. Different neutrino flavors have slightly different masses [1].

Neutrino oscillation has been observed by different experiments.  The oscillation probability of  neutrinos and the exact tiny neutrino masses have not yet been measured with the desired accuracy. The MINOS experiment has the aim to do this: the abbreviation MINOS stands for Main Injector Neutrino Oscillation Search [1-3] .

MINOS is based on two detectors: the first being stationed at the neutrino source at the Fermilab and the second being located 450 miles (735 km) away at the Soudan Underground Mine, a former iron mining site, in norther Minnesota. Frank Close explains the MINOS design in his acclaimed “neutrino cracker” [3]:

A huge 5000 tonne detector was built in a new, bigger, cavern in the Soudan mine. This utises yet another detection method. Charged particles passing through plastic, which had been loaded with small quantities of special chemicals, emit flashes of light (scintillate). These scintillations can be collected and delivered to phototubes which are similar in principle to those used to detect the Cerenkov light in the water detectors. By forming the plastic into narrow strips, sandwiched between plates of steel, the path of the charged particles through the detector can be followed, and by magnetising the steel plates, the curvature of the paths and thus the energy of the produced particles can be measured. From all this information, the details of the neutrino interaction, and in particular its energy, can be reconstructed. Then both the distance travelled (the 735 km from Fermilab) and the neutrino energy are known. A very similar (but smaller) detector was also built at Fermilab, so that by comparing the energy distribution of the neutrinos measured at Fermilab with that measured at Soudan, they could measure how any deficit depended on the energy of the neutrinos. If, as expected, this showed an oscillatory pattern, it would measure the difference in mass between the produced and oscillated neutrino.

What are the special chemicals loaded into the steel-plate-sandwiched plastic strips?

Neutrino mass is not included in the Standard Model of particle physics: neutrinos are assumed to have zero mass. But the phenomenon of neutrino oscillation suggests non-zero masses. MINOS is expected to clarify the neutrino mass conundrum and, thus, add new insight to particle physics and beyond.

Keywordselementary particles, Cerenkov radiation, oscillation pattern, nuclear physics, cosmology.

References and more to explore
[1] Cambridge MINOS Group:
[2] Fermilab: The MINOS Experiment and NuMI Beamline:
[3] Frank Close: Neutrino. Oxford University Press, Oxford, U.K., 2010.

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