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OPERA experiment

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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). OPERA is located in Hall C of LNGS and is aimed at detecting for the first time the appearance of tau neutrinos from the oscillation of muon neutrinos during their 3 millisecond travel from Geneva to Gran Sasso. Tau particles resulting from the interaction of tau neutrinos will be observed in "bricks" of photographic emulsion films interleaved with lead plates. The apparatus contains about 150,000 bricks, for a total mass of 1300 tons, and is complemented by electronic detectors (trackers and spectrometers) and ancillary infrastructure.[1] Its construction was completed in spring 2008 and the experiment is currently collecting data.[2]

The OPERA Detector

On 31 May 2010, OPERA researchers announced the observation of a first tau neutrino candidate event in a muon neutrino beam.[3] In September 2011, CERN and OPERA announced that time of flight measurements made by their collaboration had indicated muon neutrinos traveling at faster than lightspeed. While acknowledging that such a measurement would be a major discovery if correct, many physicists,[4][5][4][6] and the OPERA team itself, have expressed skepticism that OPERA's measurements are sufficiently free of error. Experimental groups such as MINOS Experiment at Fermilab and the T2K experiment are planning to attempt to replicate the result, while others in the physics community search for any experimental errors which might account for it.[7][8]

Neutrino beam

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.[9]

Detector

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.[1]

Neutrino time-of-flight anomaly

The result of OPERA experiment
The result of OPERA experiment

On 23 September 2011, the OPERA Collaboration announced that neutrinos had been observed travelling from CERN in Geneva to the OPERA detector at faster-than-light speed.[10] The particles were measured arriving at the detector by a factor of (v − c)/c = (2.48 ± 0.28 ± 0.30)×10−5 (roughly 1 part in 40,000) prior to the time expected if they were travelling at lightspeed, at significance of 6.0 sigma (or 99.9999998%). However, it should be noted that this represents a statistical significance on the data set, which itself may be prone to systematic measurement error which is unaccounted for by this method.[9] For particle physics experiments involving collision data, the standard baseline for a discovery announcement is 5-sigma significance.[11]

OPERA collaboration scientist and spokesperson Antonio Ereditato explained that the OPERA team has "not found any instrumental effect that could explain the result of the measurement."[10] James Gillies, a spokesman for CERN said on 22 September that the scientists are "inviting the broader physics community to look at what they've done and really scrutinize it in great detail, and ideally for someone elsewhere in the world to repeat the measurements."[12]

Previous experiments have not detected statistically significant faster-than-light motion; for instance, in 2007 Fermilab's MINOS collaboration reported results measuring the flight-time of neutrinos yielding a speed exceeding that of light by 1.8 sigma.[13] Those measurements were consistent with neutrinos traveling at lightspeed.[14]

In a much lower energy range, a limit of |v − c|/c < 2×10−9 was set by the observation of anti-neutrinos detected in connection with the SN 1987A supernova.[9] Had neutrinos emitted by SN 1987A been travelling with a speed corresponding to the speed reported by the OPERA experiment, the particles would have arrived at Earth almost four years before light from the event, while light from the supernova was in fact detected at roughly the same time as the neutrinos, consistent with the neutrinos travelling at the same speed as light.[15] However, the measurement from SN 1987A can not immediately be seen as contradicting the measurement from the OPERA experiment, as it is not currently known how neutrino velocity may depend on particle energy, distance traveled, or other factors.[16]

Physicists unaffiliated with the experiment, such as Martin Rees,[4] George Smoot,[17] Steven Weinberg, Lawrence Krauss[4] Brian Greene, and Michio Kaku[18][6] have been generally skeptical of OPERA's result. Spokespeople for both Fermilab and the T2K experiment confirmed their intentions to test the OPERA result in coming months.[7] Fermilab noted in reaction to the OPERA announcement that the detectors for the MINOS project are being upgraded, and new results are not expected until at least 2012.[13] A result based on already recorded data should be available within 4 to 6 months.[19]

References

  1. ^ a b "Opera – The Opera Detector". Retrieved 25 September 2011.
  2. ^ "Opera – News and Updates". Retrieved 25 September 2011.
  3. ^ N. Agafonova et al. (OPERA Collaboration) (2010). "Observation of a first ντ candidate event in the OPERA experiment in the CNGS beam". Physics Letters B. 691 (3): 138–145. arXiv:1006.1623. Bibcode:2010PhLB..691..138A. doi:10.1016/j.physletb.2010.06.022.
  4. ^ a b c d http://www.scientificamerican.com/article.cfm?id=ftl-neutrinos
  5. ^ http://news.xinhuanet.com/english2010/sci/2011-09/27/c_131161271.htm
  6. ^ a b Jordans, Frank; Borenstein, Seth (24 September 2011). "Challenging Einstein is usually a losing venture". AP/Yahoo News. Retrieved 26 September 2011.{{cite web}}: CS1 maint: multiple names: authors list (link)
  7. ^ a b E. Brown, A. Khan (23 September 2011). "Faster than light? CERN findings bewilder scientists". Los Angeles Times. Retrieved 24 September 2011. MINOS scientists may perform experiments of their own in as soon as six months, said particle physicist and MINOS co-spokesperson Jenny Thomas. Plans to test the CERN results in Japan's multinational T2K (Tokai-to-Kamioka) experiment are in the works, said neutrino physicist and T2K spokesman Chang Kee Jung.
  8. ^ Ars Technica (23 September 2011). "Scientists Question Faster-Than-Light Neutrinos". Condé Nast Publications. Retrieved 26 September 2011. In the meantime, the physics community will be looking through the paper, trying to spot unaccounted for sources of error. There are two other similar neutrino detectors in use—T2K and MINOS—and they'll undoubtedly be looking into working out the timing of their hardware with the same sort of thoroughness OPERA has.
  9. ^ a b c T. Adam et al. (OPERA collaboration) (22 September 2011). "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam". arXiv:1109.4897 [hep-ex].
  10. ^ a b "OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso" (Press release). CERN. 23 September 2011. Archived from the original on 24 September 2011. Retrieved 24 September 2011. {{cite press release}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  11. ^ C. Seife (2000). "CERN's Gamble Shows Perils, Rewards of Playing the Odds". Science. 289 (5488): 2260. doi:10.1126/science.289.5488.2260.
  12. ^ F. Jordans, S. Borenstein (22 September 2011). "Roll over Einstein: Law of physics challenged (Update 3)". PhysOrg. Retrieved 24 September 2011.
  13. ^ a b "OPERA experiment reports anomaly in flight time of neutrinos". Fermilab Today. Fermilab. Retrieved 24 September 2011.
  14. ^ D. Overbye (22 September 2011). "Tiny neutrinos may have broken cosmic speed limit". New York Times. That group found, although with less precision, that the neutrino speeds were consistent with the speed of light.
  15. ^ "Faster-than-light travel discovered? Slow down, folks | Bad Astronomy | Discover Magazine". Blogs.discovermagazine.com. Retrieved 25 September 2011.
  16. ^ "Naughty 'Faster Than Light' Neutrinos a Reality?". Discovery News. Retrieved 25 September 2011.
  17. ^ http://news.xinhuanet.com/english2010/sci/2011-09/27/c_131161271.htm
  18. ^ http://news.yahoo.com/challenging-einstein-usually-losing-venture-214054440.html
  19. ^ http://idealab.talkingpointsmemo.com/2011/09/was-einstein-wrong-us-accelerator-lab-to-test-cerns-results.php

Further reading

42°28′N 13°34′E / 42.46°N 13.57°E / 42.46; 13.57

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