OPERA experiment: Difference between revisions

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{{Short description|Italian neutrino detector (2011–2012)}}

[[File:CNGS_layout.jpg|right|thumb|350px|alt=CERN Neutrinos to Gran Sasso Underground Structures|[[CERN Neutrinos to Gran Sasso]] Underground Structures]]

{{redirect|OPERA|other uses|Opera (disambiguation)}}

The '''Oscillation Project with Emulsion-tRacking Apparatus''' ('''OPERA''') is an experiment to test the phenomenon of [[neutrino oscillation]]s. It exploits [[CERN Neutrinos to Gran Sasso]] (CNGS), a high-intensity and high-energy beam of [[muon neutrino]]s produced at the [[CERN]] [[Super Proton Synchrotron]] in Geneva and pointing to the [[Laboratori Nazionali del Gran Sasso]] (LNGS) underground laboratory, {{convert|733|km|abbr=on}} 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 neutrino]]s from the oscillation of muon neutrinos during their 3 millisecond travel from Geneva to Gran Sasso. [[Tau (particle)|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.<ref name="autogenerated1">{{cite web |url=http://operaweb.lngs.infn.it/spip.php?rubrique39 |title=Opera – The Opera Detector |accessdate=25 Sep 2011}}</ref> Its construction was completed in spring 2008 and the experiment is currently collecting data.<ref>{{cite web |url=http://operaweb.lngs.infn.it/spip.php?rubrique14 |title=Opera – News and Updates |accessdate=25 Sep 2011}}</ref>

{{Use dmy dates|date=August 2021}}

The '''Oscillation Project with Emulsion-tRacking Apparatus''' ('''OPERA''') was an instrument used in a scientific experiment for detecting [[tau neutrino]]s from [[muon neutrino]] [[neutrino oscillation|oscillations]]. The experiment is a collaboration between [[CERN]] in [[Geneva]], [[Switzerland]], and the [[Laboratori Nazionali del Gran Sasso]] (LNGS) in [[Gran Sasso d'Italia|Gran Sasso]], [[Italy]] and uses the [[CERN Neutrinos to Gran Sasso]] (CNGS) [[Accelerator neutrino|neutrino beam]].

On 31 May 2010, OPERA researchers announced the observation of a first tau neutrino candidate event in a muon neutrino beam.<ref>

The process started with protons from the [[Super Proton Synchrotron]] (SPS) at CERN being fired in pulses at a carbon target to produce [[pion]]s and [[kaon]]s. These particles decay to produce [[muon]]s and [[neutrino]]s.<ref>

{{cite web

|last = Al-Khalili

|first = J.

|date = 18 November 2011

|title = Faster than the speed of light?

|url = http://www.jimal-khalili.com/blog/faster-than-the-speed-of-light.html#more-1055

|work = Jim's Blog

|access-date = 2013-08-31

|url-status = dead

|archive-url = https://web.archive.org/web/20140201230116/http://www.jimal-khalili.com/blog/faster-than-the-speed-of-light.html#more-1055

|archive-date = 1 February 2014

}}</ref>

The beam from CERN was stopped on 3 December 2012,<ref>{{cite web |title=[OPERA]: News and Updates |date=21 May 2018 |url=http://operaweb.lngs.infn.it/spip.php?rubrique14 |quote=3 December 2012 After 5 years of operation the CNGS beam was stopped. In total it was {{val|17.97|e=19}} protons on target provided by the CNGS. |access-date=25 September 2011 |archive-date=25 January 2021 |archive-url=https://web.archive.org/web/20210125003644/http://operaweb.lngs.infn.it/spip.php?rubrique14 |url-status=dead }}</ref> ending data taking, but the analysis of the collected data has continued.

== Detector ==

[[File:OPERA.jpg|thumb|267x267px|View of the OPERA detector at the CNGS facility]]

OPERA, in Hall C of the Gran Sasso underground labs, was built in 2003–2008. The [[tau lepton|taus]] resulting from the interaction of tau neutrinos are observed in "bricks" of photographic films ([[nuclear emulsion]]) interleaved with lead sheets. Each brick weighs 8.3&nbsp;kg; the two OPERA supermodules contain 150,000 bricks arranged into parallel walls interleaved with plastic [[scintillator]] counters. Each supermodule is followed by a magnetic [[spectrometer]] for momentum and charge identification of penetrating particles. During data collection, a neutrino interaction and its corresponding brick are tagged in real time by the scintillators and spectrometers. These bricks are extracted from the walls asynchronously with respect to the beam for film development, scanning and for the topological and kinematic search of tau decays.<ref name="autogenerated1">{{cite web

|title=The Opera Detector

|url=http://operaweb.lngs.infn.it/spip.php?rubrique39

|publisher=OPERA

|access-date=2011-09-25

|archive-date=11 February 2019

|archive-url=https://web.archive.org/web/20190211183031/http://operaweb.lngs.infn.it/spip.php?rubrique39

|url-status=dead

}}</ref>

== Tau neutrinos ==

[[File:OPERA_neutrino.jpg|thumb|First neutrino events seen by the core of the OPERA experiment]]

In total, five tau neutrinos were detected. On 31 May 2010, OPERA researchers observed the first [[tau neutrino]] candidate event in a muon neutrino beam.<ref>

{{cite journal

|authorlast1=N. Agafonova ''et al|first1=N.'' (OPERA Collaboration)

|author2=et al. (OPERA Collaboration)

|year=2010

|date=2010

|title=Observation of a first ν_τ candidate event in the OPERA experiment in the CNGS beam

|journal=[[Physics Letters B]]

|volume=691 |issue=3 |pages=138–145

|arxiv=1006.1623

|bibcode = 2010PhLB..691..138A

|doi=10.1016/j.physletb.2010.06.022

|s2cid=119256958

}}</ref> In September 2011, CERN and OPERA announced that time of flight measurements made by their collaboration had indicated muon neutrinos traveling at [[faster-than-light|faster]] than [[lightspeed]]. While acknowledging that such a measurement would be a major discovery if correct, many physicists,<ref name="sciam_luminarydoubt">http://www.scientificamerican.com/article.cfm?id=ftl-neutrinos</ref><ref>http://news.xinhuanet.com/english2010/sci/2011-09/27/c_131161271.htm</ref><ref name="sciam_luminarydoubt"/><ref name=Challenge>{{cite web|title=Challenging Einstein is usually a losing venture|url=http://news.yahoo.com/challenging-einstein-usually-losing-venture-214054440.html|publisher=AP/Yahoo News|accessdate=26 September 2011|author=Jordans, Frank; Borenstein, Seth|date=24 September 2011}}</ref> and the OPERA team itself, have expressed skepticism that OPERA's measurements are sufficiently free of error. Experimental groups such as the [[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.<ref name="LA Times">

}}</ref> On 6 June 2012, OPERA announced the observation of a second tau neutrino event.<ref>

{{cite press release

|date=6 June 2012

|title=OPERA Observes the Second Tau Neutrino

|url=http://www.interactions.org/cms/?pid=1031794

|publisher=[[Istituto Nazionale di Fisica Nucleare]]

|access-date=2012-06-08

}}</ref> On 26 March 2013, the experiment caught for the third time a muon neutrino oscillating into a tau neutrino during travel from [[CERN]] to [[LNGS]].<ref>

{{cite web

|last=Jepsen |first=K.

|author=E. Brown, A. Khan

|date=2326 SeptemberMarch 20112013

|title=OPERA snags third tau neutrino

|title=Faster than light? CERN findings bewilder scientists

|url=http://www.latimessymmetrymagazine.comorg/newsarticle/sciencemarch-2013/laopera-scisnags-0923third-speedtau-of-light-20110923,0,497738.storyneutrino

|work=Los[[Symmetry Angeles TimesMagazine]]

|access-date=2013-03-27

|accessdate=24 September 2011

}}</ref> The fourth one was found in 2014, and the fifth was seen in 2015.<ref>

|quote=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.

{{cite web

}}</ref><ref name="Scientists question">{{cite web|title=Scientists Question Faster-Than-Light Neutrinos|url=http://www.wired.com/wiredscience/2011/09/scientists-question-neutrinos/|publisher=Condé Nast Publications|accessdate=26 September 2011|author=Ars Technica|date=23 September 2011|quote=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.}}</ref>

|last=O'Luanaigh|first=C.

|date=6 October 2015

|title=OPERA detects its fifth tau neutrino

|url=https://home.cern/about/updates/2015/06/opera-detects-its-fifth-tau-neutrino

|access-date=2017-02-08

}}</ref>

== NeutrinoTime-of-flight beammeasurements ==

{{main|Faster-than-light neutrino anomaly|Measurements of neutrino speed}}

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 [[proton]]s with a [[graphite]] target after focusing the particles produced ([[pion]]s and [[kaon]]s 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&nbsp;km journey.<ref name=OPERA_20110922/>

In September 2011, OPERA researchers observed [[muon neutrino]]s apparently traveling faster than the [[speed of light]].<ref name=op>

== Detector ==

{{cite arXiv

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&nbsp;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.<ref name="autogenerated1"/>

|last=Adam |first=T.

|author2=''et al.'' (OPERA collaboration)

==Neutrino time-of-flight anomaly==

|date=17 November 2011

[[File:OPERA experiment.png|right|thumb|320px|alt=Result of the OPERA experiment|Result of the [[OPERA experiment]]: The left plot shows δt as a function of the energy for νμ CC internal events, where δt (low-energy: <span style="text-decoration: overline">13.9</span> GeV) = (53.1 ± 18.8 ([[Statistical_error|stat.]]) ± 7.4 ([[Systematic_error|sys.]])) ns and δt (high-energy: <span style="text-decoration: overline">42.9</span> GeV) = (67.1 ± 18.2 ([[Statistical_error|stat.]]) ± 7.4 ([[Systematic_error|sys.]])) ns. The right plot shows the global result of the analysis including both internal and external events (for the latter the energy cannot be measured), where δt (average-energy: <span style="text-decoration: overline">17</span> GeV) = (60.7 ± 6.9 ([[Statistical_error|stat.]]) ± 7.4 ([[Systematic_error|sys.]])) ns.]]

On 23 September 2011, the OPERA Collaboration announced that 17-[[electronvolt|GeV]] neutrinos had been observed travelling from CERN in Geneva to the OPERA detector at [[faster-than-light]] speed. Similar results were obtained using 28-GeV Neutrinos, which were observed to test energy dependence.<ref name="CERN Press Release">

{{cite press release

|date=23 September 2011

|title=OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso

|url=http://press.web.cern.ch/press/PressReleases/Releases2011/PR19.11E.html

|publisher=[[CERN]]

|accessdate=24 September 2011

|archiveurl=http://www.webcitation.org/61vtgK592

|archivedate=24 September 2011

|deadurl=no

}}</ref> The particles were measured arriving at the detector by a factor of (''v''&nbsp;−&nbsp;''c'')/''c''&nbsp;=&nbsp;(2.48&nbsp;±&nbsp;0.28 ([[Errors_and_residuals_in_statistics|stat.]])&nbsp;±&nbsp;0.30 ([[Systematic_error|sys.]])){{E|-5}} (approximately 1 in 40,000) prior to the time expected if they were travelling at lightspeed, with a [[Statistical significance#Use as "σ" (sigma)|significance of 6.0 sigma]] (or {{val|99.9999998|u=%}}, if the distribution is [[Normal distribution|normal]]). This measure includes estimates for both statistical and instrumentation error, though it may leave additional sources of systematic error unaccounted for.<ref name=OPERA_20110922>

{{cite arxiv

|author=T. Adam ''et al.'' (OPERA collaboration)

|date=22 September 2011

|title=Measurement of the neutrino velocity with the OPERA detector in the CNGS beam

|class=hep-ex

|eprint=1109.48974897v1

}}</ref> In February and March 2012, OPERA researchers blamed this result on a loose fibre optic cable connecting a GPS receiver to an electronic card in a computer. On 16 March 2012, a report announced that an independent experiment in the same laboratory, also using the CNGS neutrino beam, but this time the [[ICARUS (experiment)|ICARUS]] detector found no discernible difference between the speed of a neutrino and the speed of light.<ref name=nature2012b>

}}</ref> For particle physics experiments involving collision data, the standard baseline for a discovery announcement is 5-sigma significance.<ref>

{{cite journal

|last=Brumfiel |first=G.

|author=C. Seife

|date=16 March 2012

|year=2000

|title=Neutrinos not faster than light

|title=CERN's Gamble Shows Perils, Rewards of Playing the Odds

|url=http://www.nature.com/news/neutrinos-not-faster-than-light-1.10249

|journal=[[Science (journal)|Science]]

|journal=[[Nature News]]

|volume=289 |issue=5488 |page=2260

|doi=10.11261038/sciencenature.2892012.5488.226010249

|s2cid=119620081

}}</ref>

|access-date=2013-03-12

|url-access=subscription

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."<ref name="CERN Press Release" /> 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."<ref name="roll over">

}}</ref> In May 2012, the Gran Sasso experiments BOREXINO, ICARUS, LVD and OPERA all measured neutrino velocity with a short-pulsed beam, and obtained agreement with the speed of light, showing that the original OPERA result was mistaken.<ref>

{{cite web

{{cite press release

|author=F. Jordans, S. Borenstein

|date =22 September8 June 20112012

|title = Neutrinos sent from CERN to Gran Sasso respect the cosmic speed limit

|title=Roll over Einstein: Law of physics challenged (Update 3)

|url = http://press-archived.web.cern.ch/press-archived/PressReleases/Releases2011/PR19.11E.html

|url=http://www.physorg.com/news/2011-09-cern-faster-than-light-particle.html

|workpublisher = [[PhysOrgCERN]]

|access-date = 2012-06-08

|accessdate=24 September 2011

|url-status = dead

}}</ref>

|archive-url = https://web.archive.org/web/20140222165941/http://press-archived.web.cern.ch/press-archived/PressReleases/Releases2011/PR19.11E.html

|archive-date = 22 February 2014

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 3-GeV neutrinos yielding a speed exceeding that of light by 1.8 sigma.<ref name=fermilab /> Those measurements were considered statistically consistent with neutrinos traveling at lightspeed.<ref>

}}</ref> Finally in July 2012, the OPERA collaboration updated their results. After the instrumental effects mentioned above were taken into account, it was shown that the speed of neutrinos is consistent with the speed of light.<ref name=op4>

{{cite news

{{cite journal

|author=D. Overbye

|last=Adam |first=T.

|date= 22 September 2011

|author2=et al. (OPERA collaboration)

|title=Tiny neutrinos may have broken cosmic speed limit

|date=2012

|url=http://www.nytimes.com/2011/09/23/science/23speed.html

|title=Measurement of the neutrino velocity with the OPERA detector in the CNGS beam

|newspaper=[[New York Times]]

|journal=[[Journal of High Energy Physics]]

|quote= That group found, although with less precision, that the neutrino speeds were consistent with the speed of light.

|volume=2012 |issue= 10|page=93

}}</ref>

|arxiv=1109.4897

|bibcode=2012JHEP...10..093A

In a much lower energy range, a limit of |''v''&nbsp;−&nbsp;''c''|/''c'' < 2{{E|-9}} was set by the observation of 10-MeV anti-neutrinos detected in connection with the [[SN 1987A]] supernova.<ref name="OPERA_20110922"/> 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.<ref>{{cite web|url=http://blogs.discovermagazine.com/badastronomy/2011/09/22/faster-than-light-travel-discovered-slow-down-folks/ |title=Faster-than-light travel discovered? Slow down, folks &#124; Bad Astronomy &#124; Discover Magazine |publisher=Blogs.discovermagazine.com |accessdate=25 September 2011}}</ref> The measurement from SN 1987A cannot 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.<ref>{{cite web|url=http://news.discovery.com/space/reality-check-what-are-those-naughty-neutrinos-really-up-to-110924.html |title=Naughty 'Faster Than Light' Neutrinos a Reality? |publisher=Discovery News|accessdate=25 September 2011}}</ref><ref name=sciam_luminarydoubt />

|doi=10.1007/JHEP10(2012)093

|s2cid=17652398

Notable physicists unaffiliated with the experiment, such as [[George Smoot]],<ref>http://news.xinhuanet.com/english2010/sci/2011-09/27/c_131161271.htm</ref> [[Steven Weinberg]],<ref name="sciam_luminarydoubt"/> [[Martin Rees]],<ref name="sciam_luminarydoubt">http://www.scientificamerican.com/article.cfm?id=ftl-neutrinos</ref> [[Lawrence Krauss]],<ref name="sciam_luminarydoubt"/> [[Brian Greene]], and [[Michio Kaku]]<ref name=Challenge>{{cite web|title=Challenging Einstein is usually a losing venture|url=http://news.yahoo.com/challenging-einstein-usually-losing-venture-214054440.html|publisher=AP/Yahoo News|accessdate=26 September 2011|author=Jordans, Frank; Borenstein, Seth|date=24 September 2011}}</ref> have been skeptical of the result's accuracy, with some, such as [[Sheldon Lee Glashow|Sheldon Glashow]], offering formal refutations of the result. <ref> A. G. Cohen and S. L. Glashow, ''New Constraints on Neutrino Velocities'', [http://arxiv.org/abs/1109.6562 preprint]</ref><ref>http://arxiv.org/abs/1109.6160</ref><ref name="arxiv opera link" /> Others have been more receptive, and some papers have been published discussing the result's possible validity, and the implications thereof.<ref name="arxiv opera link">{{cite web|title=Papers Discussing the OPERA result at Arxiv.org|url=http://arxiv.org/find/all/1/all:+Opera/0/1/0/all/0/1|accessdate=2 October 2011}}</ref> Physicists affiliated with OPERA have remained skeptical, however, stating in their paper: {{quote|Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results.{{citation needed}}}}

}}</ref> This was confirmed by a new, improved set of measurements in May 2013.<ref>

{{Cite journal

Following OPERA and CERN's request for confirmation, spokespeople for both Fermilab and the [[T2K experiment]] have confirmed their intentions to test the OPERA result in coming months.<ref name="LA Times" /> 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.<ref name=fermilab>

|last=Adam |first=T.

{{cite web

|author2=et al. (OPERA collaboration)

|title=OPERA experiment reports anomaly in flight time of neutrinos

|date=2013

|url=http://www.fnal.gov/pub/today/archive_2011/today11-09-23.html

|title=Measurement of the neutrino velocity with the OPERA detector in the CNGS beam using the 2012 dedicated data

|work=Fermilab Today

|journal=[[Journal of High Energy Physics]]

|publisher=[[Fermilab]]

|volume=1 |page=153

|accessdate=24 September 2011

|arxiv=1212.1276

}}</ref> A result based on already recorded data should be available within 4 to 6 months.<ref>http://idealab.talkingpointsmemo.com/2011/09/was-einstein-wrong-us-accelerator-lab-to-test-cerns-results.php</ref>

|bibcode=2013JHEP...01..153A

|doi=10.1007/JHEP01(2013)153

|s2cid=119258343

}}</ref>

==References==

{{Reflist|2}}

==Further reading==

*{{cite web

|author=J. Ouellette

|date=24 September 2011

|title=Naughty 'Faster Than Light' Neutrinos a Reality?

|url=http://news.discovery.com/space/reality-check-what-are-those-naughty-neutrinos-really-up-to-110924.html

|work=[[Discovery News]]

|accessdate=24 September 2011

}}

== External links ==

*{{Commons category-inline}}

* [http://operaweb.lngs.infn.it Opera Main Page]

<!--* [http://proj-cngsoperaweb.weblngs.cerninfn.ch/proj-cngs/it CNGSOpera NeutrinoMain beam at CERNPage]-->

* [httphttps://www.youtube.com/user/CERNTV?ob=5#p/search/0/ufCyDHSAvLE CERN: First Appearance Ofof Tau Neutrino ]

* [https://web.archive.org/web/20110825051935/http://prometeo.sif.it/papers/online/sag/026/03-04/pdf/03-scienza-in-primo-piano-1.pdf The appearance of the tau-neutrino]

* [http://www.nu.to.infn.it/exp/all/opera/ OPERA publications ]

{{Neutrino detectors}}

{{Use dmy dates|date=September 2011}}

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