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Roll over Einstein: Pillar of physics challenged,


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On Thursday, the world's biggest physics lab unveiled a shocking finding: that one type of subatomic particle was clocked going faster than the speed of light. If true - a big if, even the scientists there concede - it could undercut Einstein's theories. Physicist Michio Kaku of City College of New York called it "the biggest challenge to relativity in 100 years."


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The CERN Neutrinos to Gran Sasso project aims to unravel some of the mysteries surrounding neutrinos - very light, neutral particles that interact very little with matter. Three types - or flavours - of neutrino exist: the electron neutrino, the muon neutrino and the tau neutrino. But it seems that neutrinos are the chameleons of the particle world: they can change from one flavour into another. This phenomenon, called ‘oscillation’, occurs as the neutrinos travel long distances through matter. It is important to investigate it further as it is directly related to the tiny mass of the neutrinos.


The CNGS project sends muon neutrinos from CERN to the Gran Sasso National Laboratory (LNGS), 732 km away in Italy. There, two experiments, OPERA and ICARUS, wait to find out if any of the muon neutrinos have transformed into tau neutrinos. To create the neutrino beam, a proton beam from the Super Proton Synchrotron at CERN is directed onto a graphite target, creating particles called pions and kaons. These particles are fed into a system of two magnetic lenses that focus the particles into a parallel beam in the direction of Gran Sasso. Next, in a 1 km long tunnel, the pions and kaons decay into muons and muon neutrinos. At the end of this decay tunnel, an 18 m thick block of graphite and metal absorbs the remaining protons, and pions and kaons that did not decay. The muons are stopped by the rock beyond. Only, the muon neutrinos remain to streak through the rock on their journey to Italy.

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Neutrinos rarely interact with anything, and are consequently rarely observed. In the early 1900s, theories predicted that the electrons resulting from beta decay should have been emitted at a specific energy. However, in 1914, James Chadwick showed that electrons were instead emitted in a continuous spectrum. In 1930, Wolfgang Pauli theorized that an undetected particle was carrying away the observed difference between the energy, momentum, and angular momentum of the initial and final particles.


Pauli originally named his proposed light particle a neutron. When James Chadwick discovered a much more massive nuclear particle in 1932 and also named it a neutron, this left the two particles with the same name. Enrico Fermi, who developed the theory of beta decay, coined the term neutrino in 1934 as a clever way to resolve the confusion. It was a pun on neutrone, the Italian equivalent of neutron.


Upon the prediction and discovery of a second neutrino, it became important to distinguish between different types of neutrinos. Pauli's neutrino is now identified as the electron neutrino, while the second neutrino is identified as the muon neutrino.


The muon neutrino is a subatomic lepton elementary particle which has the symbol ν

μ and no net electric charge. Together with the muon it forms the second generation of leptons, hence its name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by Leon Lederman, Melvin Schwartz and Jack Steinberger. The discovery was rewarded with the 1988 Nobel Prize in Physics.


The tau (τ), also called the tau lepton, tau particle or tauon, is an elementary particle similar to the electron, with negative electric charge and a spin of 1⁄2. A tau can be thought of as a much heavier version of the electron. Because of their greater mass, tau particles do not emit as much bremsstrahlung radiation as electrons; consequently they are potentially highly penetrating, much more so than electrons. However, because of their short lifetime, the range of the tau is mainly set by their decay length, which is too small for bremsstrahlung to be noticeable: their penetrating power appears only at ultra high energy (above PeV energies). The tau was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC-LBL group.


The tau neutrino or tauon neutrino is a subatomic elementary particle which has the symbol ν

τ and no net electric charge. Together with the tau, it forms the third generation of leptons, hence its name tau neutrino. Its existence was immediately implied after the tau particle was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC–LBL group. The discovery of the tau neutrino was announced in July 2000 by the DONUT collaboration.


DONUT (Direct Observation of the NU Tau, E872) was an experiment at Fermilab dedicated to the search for tau neutrino interactions. Even though the detector operated only during a few months in the summer of 1997, it was largely successful. In DONUT, protons accelerated by the Tevatron were used to produce tau neutrinos via decay of charmed mesons. After eliminating as many unwanted background particles as possible by a system of magnets and bulk matter (mostly iron and concrete), the beam passed through several sheets of nuclear emulsion. In very rare cases one of the neutrinos would interact in the detector, producing electrically charged particles which left visible tracks in the emulsion and could be electronically registered by a system of scintillators and drift chambers.


The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) is an experiment to test the phenomenon of neutrino oscillations. It exploits CERN Neutrinos to Gran Sasso (CNGS), a high-intensity and high-energy beam of muon neutrinos produced at the CERN Super Proton Synchrotron in Geneva and pointing to the Laboratori Nazionali del Gran Sasso (LNGS) underground laboratory, 733 km (455 mi) away at Gran Sasso in central Italy (Abruzzo region).


The Gran Sasso National Laboratory (LNGS) is the largest underground laboratory in the world for experiments in particle physics, particle astrophysics and nuclear astrophysics. It is located between the towns of L'Aquila and Teramo, about 120 km from Rome.


The underground facilities are located on a side of the ten kilometres long freeway tunnel crossing the Gran Sasso Mountain. They consist of three large experimental halls, each about 100 m long, 20 m wide and 18 m high and service tunnels, for a total volume of about 180,000 cubic metres. The average 1400 m rock coverage gives a reduction factor of one million in the cosmic ray flux; moreover, the neutron flux is thousand times less than on the surface, thanks to the smallness of the Uranium and Thorium content of the dolomite rocks of the mountain.


OPERA needs an intense and energetic beam of muon neutrinos traveling a distance of hundreds of kilometers to detect the appearance of oscillated tau neutrinos. A beam of this type is generated by collisions of accelerated protons with a graphite target after focusing the particles produced (pions and kaons in particular) in the desired direction. The products of their decays, muons and neutrinos, continue to travel in generally the same direction as the parent particle. Muon neutrinos produced in this way at CERN pass through the Earth's crust reaching OPERA after a 730 km journey.


OPERA is located in Hall C of the Gran Sasso underground labs. Construction started in 2003, and the apparatus was completed in summer 2008. The taus resulting from the interaction of tau neutrinos will be observed in "bricks" of photographic films (nuclear emulsion) interleaved with lead sheets. Each brick has an approximate weight of 8.3 kg and the two OPERA supermodules contain about 150,000 bricks arranged into parallel walls and interleaved with plastic scintillator counters. Each supermodule is followed by a magnetic spectrometer for momentum and charge identification of penetrating particles. During the data collection, a neutrino interaction is tagged in real time by the scintillators and the spectrometers, which also provide the location of the bricks where the neutrino interaction occurred. These bricks are extracted from the walls asynchronously with respect to the beam to allow for film development, scanning and for the topological and kinematic search of tau decays.


The OPERA detector at LNGS, designed for the study of neutrino oscillations in

appearance mode, has provided a precision measurement of the neutrino velocity over the 730 km

baseline of the CNGS neutrino beam sent from CERN to LNGS through the Earth’s crust. A time

of flight measurement with small systematic uncertainties was made possible by a series of

accurate metrology techniques. The data analysis took also advantage of a large sample of about

16000 neutrino interaction events detected by OPERA.


The analysis of internal neutral current and charged current events, and external νμ CC

interactions from the 2009, 2010 and 2011 CNGS data was carried out to measure the neutrino

velocity. The sensitivity of the measurement of (v-c)/c is about one order of magnitude better

than previous accelerator neutrino experiments.


The results of the study indicate for CNGS muon neutrinos with an average energy of 17

GeV an early neutrino arrival time with respect to the one computed by assuming the speed of

light in vacuum:

δt = (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns.

The corresponding relative difference of the muon neutrino velocity and the speed of light


(v-c)/c = δt /(TOF’c - δt) = (2.48 ± 0.28 (stat.) ± 0.30 (sys.)) ×10-5.

with an overall significance of 6.0 σ.



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