What has no thumbs and travels at the speed of light, to within experimental uncertainty?

Hi All,

I will just say this right away, the Borexino, ICARUS, LVD, OPERA, and MINOS Experiments have all independently found, within experimental uncertainty, that neutrinos travel at the speed of light. To enlighten, last September the OPERA Experiment at the Gran Sasso Laboratory, in Gran Sasso, Italy, observed what appeared to indicate that neutrinos travel faster than the speed of light. (More information available from veteran QDers Aiden and Seth).

The reported quantity is time it took neutrinos to travel from CERN to Gran Sasso minus the time it would have taken light. I should also mention that the statistical (stat.) and systematic (sys.) uncertainties are incredibly important.

δt = (Time it took neutrinos to reach GS from CERN) – (Distance between GS and CERN)/(Speed of Light)

Figure 1: Results from four Gran Sasso Laboratory experiments indicating neutrinos travel at the speed of light, to within exerpeimental uncertainty. Reported quantity is time it took neutrinos to travel from CERN to Gran Sasso minus the time it would have taken light. Credit: BERTOLUCCI, Sergio

Figure 2: Results from the MINOS Experiment indicating neutrinos travel at the speed of light, to within exerpeimental uncertainty. Reported quantity is time it took neutrinos to travel from Fermilab to MINOS minus the time it would have taken light. Credit: ADAMSON, Phil

To clarify the situation, this result was not a typical “Hey! We discovered new physics!” result. Had OPERA correctly observed a massive particle traveling faster than light, then we would truly be in the midst of a physic revolution. That is not a hyperbole either. As a result, everyone, theorists and experimentalists alike, put on their scientists hats and scrutinized the result to no end. Much drama ensued and at long last the problem has been resolved. The issue at hand were actually two very subtle effects that worked against each other. The first was that a 5.2 mi (8.3 km) cable was accidentally stretched back in 2008 and systematically introduced a 74 nanosecond delay in the system that recorded the time the neutrinos arrived at the detector. The second issue involved the highly precise master clock system for the entire experiment; it was slow by about 15 nanoseconds. 74 – 15 = 59 nanoseconds was exactly how much sooner the neutrinos were arriving than they were expected.

Figure 3: Two previously unaccounted issues regarding the OPERA Experiment. Credit: DRACOS, Marcos

In conclusion, neutrinos may still travel faster than the speed of light. It is unlikely, but still possible. Officially as of today, though, we know that all measurements of neutrinos’ speed show are consistent with the speed of light.

How many types of neutrinos are there? That was Day 3+4’s Big Question.

Hi All,

Your Day 3+4 breakdown is finally here. A lack of internet access is always an issue. At any rate, things were a very great mix of experimental results and theoretical discussions that all pointed to one question: How many types of neutrinos are there in the Universe? According to the Standard Model, which itself is founded on very rich experimental results, there are 3 flavors: electron-neutrino, muon-neutrino, and tau-neutrino. However, it is very possible there are more neutrinos that do not have any charges under the Standard Model. Such neutrinos are called sterile neutrino or singlet neutrinos.

Without further ado: MiniBooNE, Neutrino Anomalies, KamLAND-Zen, EXO-200, and Day 4.

MiniBooNE

A quick breakdown of what the Miniature Booster Neutrino Experiment, or MiniBooNE for short, is all about can be given in the following two slides. Fermilab accelerates protons into a fixed target to produce pions and kaons. The pions and kaons are then directed toward the ground and squeezed together by a system of magnetic fields called “the horn.” The pions then fly for a period of time, decaying into muons, electrons, and neutrinos. The muons further decay into more neutrinos and electrons. When the electron and neutrino beam hits the ground, the electrons are absorbed and the neutrinos pass through the planet. Finally, after popping out in a deep underground Minnesota mine, the neutrino beam flies through the MiniBooNE detector and physics is born.

One of MiniBooNE’s chief scientific goals is to confirm or refute the result of a previous experiment, LSND, which observed an excess of neutrinos. The excess was best described by introducing a single sterile neutrino and we still do not know if the result was a statistical fluke or something more serious.

Figure 1: Summary of the MiniBooNE Experiment at Fermilab with motivation for its science programme. Credit: POLLY, Chris

Figure 2: Breakdown of the MiniBooNE Experiment at Fermilab. Credit: POLLY, Chris

The experiment has announced for the first time with its full dataset, that it has observed an excess number of anti-muon-neutrinos converting to anti-electron-neutrinos. This excess is almost entirely in the lower energy range, i.e., smaller energy transfer between neutrinos and detector, and the experiment is trying vigorously to determine if this has been caused by a previously unknown background.

Figure 3: Results showing an excess in the number of low energy anti-electron neutrinos observed. Credit: POLLY, Chris

When combing the anti-neutrino and neutrino excesses, the overall excess in number of events grows in significance. The two results are consistent with each other, so there is no measurable difference between matter and anti-matter has been observed. If it is there, it is beyond the detector’s capabilities. There are a few ideas to explain the more-than-expected number of neutrinos and they are individually being studied as we speak. A VERY preliminary result (so preliminary I am choosing not the put up the plot here) is that the data is somewhat well-described by assuming the existence of two sterile neutrinos. This actually is more preferred than a single sterile neutrino, so theorists are a bit happy at the moment. 🙂

Figure 4: Results showing an excess in the number of low energy electron-neutrinos and anti-electron neutrinos observed. Credit: POLLY, Chris

Neutrino Anomalies

The prospect of adding a new neutrino to the Standard Model is a tricky issue, let alone adding two. Theoretically it is not terribly difficult but such a step would have very obvious and quickly testable predictions. The first of several theory talks (I am skipping my synopsis of all other theory talks) had a summary of known anomalies from neutrino experiments. LSND and MiniBooNE has already been discussed and the largest. A rather recently discovered discrepancy is the number of neutrinos predicted to be produced by nuclear reactors. The calculation is very well known but had not been updated in years. After recalculating the expected neutrino production rate, the predicted rate was found to be larger than the observed rate. Strictly speaking, all results ARE consistent with the Standard Model and we cannot make any definitive statements based solely on what is listed here.

Figure 5: . Credit: LASSERRE, Thierry

KamLAND-Zen

On to KamLAND-Zen, which stands for Kamioka Liquid Scintillator Anti-neutrino Detector – Zero Neutrino Double β-Decay (pronounced: beta-decay). This experiment has got to be the best example of when an experiment collaboration just stops trying to write its experiment name as a logical acronym. It is still a wicked-cool name. Nuclear β-decay is one of the most well-studied examples of radioactivity where a nucleus in an atom will disintegrate into a lighter nucleus, plus an electron (or a positron), and an anti-electron-neutrino (or a regular electron-neutrino). Some radioactive elements can also undergo the super rare double β-decay where two β-decays occur simultaneously. In the case that a sterile neutrino does indeed exist, then the even more rare neutrino-less double β-decay should be possible. In this situation, two nuclei in an element will disintegrate into two lighter nuclei and only two electrons (or positrons!). KamLAND-Zen is looking for such a decay in the gas xenon but has had no such luck. It has, however, been able to measure the rate of the still-very-rare 2-neutrino-double β-decay in xenon, an impressive feat in and of itself. The experiment was also able to disprove a previous measurement of this rate from a different experiment called DAMA. Here are the results.

Figure 6: KamLAND-Zen's measurement of the half-life of double β-decay in xenon gas. Credit: INOUE, kunio

Figure 7: Summary of KamLAND-Zen's experimental results. Credit: INOUE, kunio

EXO-200

The Enriched Xenon Observatory Experiment, or EXO-200 for short (the 200 is explained on wiki), is KamLAND-Zen’s biggest competitor in the race for finding neutrino-less double-β-decay. The first slide shows how much more data they have since the last time their results were announced. The second slide shows their background and the fact that they have observed almost 22,000 2-neutrino double β-decay events! I cannot describe how cool that is other than say just that: it is really cool that they have so many events. Consequentially, their results are in good agreement with KamLAND-Zen’s results. So sadly, no neutrino-less events.

Figure 8: Details of the EXO-200 Experiment, its analysis, and differences from its previous analysis. Credit: FARINE, Jacques

Figure 9: Results from EXO-200 Experiment. Credit: FARINE, Jacques

Figure 10: Results from EXO-200 Experiment with comparison to other experiments.. Credit: FARINE, Jacques

Day 4

Day 4 was a much needed rest for conference goers. Like most other attendees, I spent the day exploring Kyoto and then working with my adviser on a paper we are hoping to finish soon. In the evening, however, we were treated to a dance performance by real-life geisha dancers. I was unable to get too many photos but below is a good one. The two dancers are both geiko-sans (fully-fledged geisha dancers) but there were also three maikos (apprentice geisha dancers).

After the short entertainment, the main event began: a public lecture on the importance of neutrinos and their influence on how the Universe evolved, given by Prof. Hitoshi Murayama, Director of the University of Tokyo’s Institute for Physics and Mathematics of the Universe. Sadly, I was unable to find his slides online, which is especially unfortunate considering his talk was entitled, “Neutrinos May Be Our Mother.” I was able to snap this photo of Prof. Murayama discussing his recent meeting with the Prime Minister of Japan and philanthropist Fred Kavli, of the famed Kavli Foundation. Mr. Kavli’s generous contributions to physics and astronomy have led to the construction of dozens of institutes around the world to focus and have allowed us to concentrate on the most important mysteries of this universe we call home.

Figure 12: Prof. Hitoshi Murayama (Far Left), sharing a picture of his meeting with Mr. Fred Kavli (Second from Right), and Prime Minister Yoshihiko Noda (Far Right). Credit: Mine.

When it comes to neutrino experiments, they were all there Day 2: T2K, MINOS, OPERA, ICARUS, and NOνA, and LBNE!

Hi All,

Here is a run down of what happened Tuesday. I will try to post Day 3 things this afternoon, which should be Wednesday morning for the US. An update on the LBNE is at the bottom of the post.

Happy Colliding

– richard (@bravelittlemuon)

T2K

Figure 1: Update of T2K experiment after March 11, 2011 earthquake that struct Japan. Credit: NAKAYA, Tsuyoshi.

The Tokai to Kamoika Experiment, or T2K for short, is a one impressive behemoth of an experiment. Much like MINOS Experiment at Fermilab, you shoot protons into a target to make pions. The pions then decay into neutrinos, and the neutrinos travel 183 mi (295 km) through the Earth to the (Super-, Hyper-) Kamiokande detector in Kamioka, Japan.

When the March 2011 earthquake struck Japan, the proton accelerator at the J-PARC physics lab was heavily damaged, and power throughout the country was effectively shut off. Due to immense leadership of J-PARC’s director, Shoji Nagamiya, the accelerator was back online December 9, 2011, and by December 24, 2011, neutrinos were being observed in Kamioka. It is baffling that despite all this, the experiment still marched on and announced the herculean result it had observed 10 events where a muon-neutrino had converted into electron-neutrino. The predicted results were 9.07±0.93 event assuming sin²2θ₁₃=0.1, and 2.73±0.37 events assuming sin²2θ₁₃=0.0. Consequently, the experiment was able to measure θ₁₃ itself and found sin²2θ₁₃=0.104 ^+0.060_-0.045

MINOS

The MINOS Experiment at Fermilab is most simply described at the US version of T2K. It is unfair and a disservice to both MINOS and T2K to make that comparison because of the unique features of the experiments, but I have a lot to write. In 2010, MINOS caused a bit of a stir when it measured the mass difference between two of the three anti-neutrinos. The measurement itself was not at all controversial. The issue was that this result differed from the well measured mass difference for regular neutrinos. Here the Fermilab presser that can tell you all about it. On Day 2, MINOS announced that the discrepancy between neutrinos and anti-neutrinos has completely disappeared and that the previous disagreement is believed to have been a statistical fluctuation. It appears that Fermilab has released a new press release this morning explaining things in more detail. Below are the main plots. Oh, and MINOS data also slightly favors inverted hierarchy for anyone interested in that. Fun fact: In its seven years of running, MINOS has used over 1.5 sextillion protons to produce all of its neutrinos.

Figure 3: Preliminary results from the MINOS detector showing best fit value for neutrino mass splitting and mixing angle. Credit: NICHOL, Ryan

Figure 4: Preliminary results from the MINOS detector showing best fit value for anti-neutrino mass splitting and mixing angle. Credit: NICHOL, Ryan

Figure 5: Preliminary results from the MINOS detector showing best fit value for neutrino and anti-neutrino mass splitting and mixing angle. Credit: NICHOL, Ryan

OPERA

The OPERA Experiment, or Oscillation Project with Emulsion-tRacking Apparatus, is a fine and mighty experiment capable of one of the most time-consuming tasks in neutrino physics that even tests the patience of sleeping mountains: observing the conversion of tau-neutrinos into muon-neutrinos. Like T2K and MINOS, OPERA gets its neutrinos from pions, which are produced when protons strike a fixed target. Specifically, the experiment uses CERN protons in its first four years of running has used about 14.2 x 1019 protons!

Figure 6: A breakdown, by year, of how many protons the OPERA experiment has observed. Credit: NAKAMURA, Mitsuhiro

OPERA’s defining characteristic is how well it is able to extract out a signal from everything else. Below is an example of a real event in which a neutrino has collided with a nucleus, producing a charge lepton and nucleus somewhat fragments.

Figure 7: An example of a real neutrino event being extracted from the data. Credit: NAKAMURA, Mitsuhiro

The big news from OPERA on Day 2 was the second observation of a muon-neutrino converting into a tau-neutrino! 2 events in over four years; I told you this thing required patience. Here is how the event works.

Figure 8: The OPERA Experiment's second candidate event of a muon-neutrino converting into a tau-neutrino. Credit: NAKAMURA, Mitsuhiro

Here is an explanation of the event.

Figure 9: A breakdown of OPERA's second tau-neutrino candidate. Credit: NAKAMURA, Mitsuhiro

Finally, here is a summary of the status of OPERA’s search tau-neutrinos. It is worth mentioning that the experiment also announced it has observed 19 instances where a muon-neutrino has converted into an electron-neutrino!

Figure 10: A summary of the current status of the OPERA Experiment's search for appearances of tau-neutrinos. Credit: NAKAMURA, Mitsuhiro

A Few Words on ICARUS and NOνA

Due to the lack of time, I will simply say that one can expect big things from ICARUS and NOνA when they both have results. ICARUS has already started running and the gigantic, LHC-Detector-sized NOνA will start running next year when Fermilab flips on its proton beam again. NOνA will be capable of determining whether neutrinos have normal mass hierarchy or inverted mass hierarchy.

LBNE

Interesting things happen at conferences, like an impromptu talk added the morning of the second day of events. Long Baseline Neutrino Experiment co-spokesperson Robert Svoboda surprisingly gave an update of the LBNE, the first since its budget was gravely slashed. Much is still being kept internally for another few weeks when the final proposal will be submitted, so I will limit what I say. In summary, there are three options being considered for the experiment for phase 1 construction. Beyond that, it is up to the Funding Lords.

Figure 11: Update of the Long Baseline Neutrino Experiment. Credit: SVOBODA, Robert

Figure 12: Update of the Long Baseline Neutrino Experiment. Credit: SVOBODA, Robert