Posts Tagged ‘neutrinos’

Le LSM, le laboratoire bas-bruit qui fait du bruit dans les guides touristiques!

Thursday, August 23rd, 2012

Le LSM(1) est un laboratoire insolite par sa situation géographique, situé à 1700 mètres sous la roche pour une meilleure observation de l’univers. Ce n’est pas sa seule particularité …

Entre la Savoie et l’Italie, dans l’atmosphère étouffante et assourdissante du tunnel du Fréjus, rien n’indique la présence du laboratoire au kilomètre 6,5. Puis, en pénétrant dans l’antre, et à la vue de cette grande caverne bardée d’instruments scientifiques dans laquelle s’affairent des chercheurs aux accents russe, grec ou chinois, c’est une excitante sensation d’être au beau milieu d’un film de James Bond qui vous saisit. A l’extérieur, dans la vallée, comme un écho à cette impression, les rumeurs vont bon train et parlent même d’expériences secrètes ! Pourtant, il n’en est rien, car l’intérêt à s’installer sous la montagne est purement scientifique. En effet, le but n’est pas de se soustraire aux regards indiscrets, mais de s’abriter du flux des rayons cosmiques qui bombardent la surface de la Terre en permanence. L’objectif est de mener des recherches sur la matière noire ou le neutrino et procéder à des mesures d’ultra faible radioactivité grâce à un niveau de bruit de fond très bas. Une quête au moins aussi palpitante qu’un scénario de James Bond !

C’est ainsi que depuis 30 ans, le laboratoire aiguise la curiosité des habitants de Modane et des vacanciers… Un lieu propice à l’échange avec les chercheurs s’est donc révélé nécessaire et a été créé en 2009 dans le bâtiment Carré Sciences situé à Modane. Près de 3000 personnes découvrent chaque année “les petits secrets de l’univers” et environ 300 chanceux visitent le laboratoire lui-même.

Tubes de Geissler-Plücker, découverte de l ionisation – photo : lsm

A l’entrée de l’exposition se trouve un cosmophone qui révèle en direct le passage des rayons cosmiques et les transforme en une mélodie de l’univers. Conçu par le Centre de Physique des Particules de Marseille (CPPM), cet instrument ludique aide à comprendre pourquoi le laboratoire cherche à se mettre à l’abri des rayons cosmiques.

Suivent ensuite des vidéos, l’exposition d’objets remarquables, des panneaux et des jeux ou encore le petit train de la radioactivité naturelle. Une chambre à brouillard, instrument fascinant, donne une touche artistique et permet de voir concrètement la trace laissée par le passage d’une particule de radioactivité venant de l’air, de la Terre, du cosmos… ou bien même de notre propre corps !

De quoi aiguiser la matière grise en attendant de percer les secrets de la matière noire…

Avec l’essor du tourisme scientifique, la qualité de cette exposition permanente et l’intérêt du laboratoire sont désormais reconnus et mis en avant par les professionnels du tourisme. L’exposition du LSM et le laboratoire lui-même sont cités dans le Guide du Routard(2), le Guide Vert Michelin(3) ou encore le Petit Futé(4). Un coup de pouce qui nous aide à partager la science avec le public. Pas mal non ?

(1) LSM : laboratoire souterrain de Modane – UMR6417 du CNRS/IN2P3 et du CEA/IRFU
(2) Guide du Routard Savoie Mont-Blanc, page 121
(3) Guide Vert Michelin Alpes du Nord – Savoie Dauphiné, page 422
(4) Petit Futé France souterraine, page 14 – Petit Futé Savoie, page 322 – Petit Futé Alpes

– Article envoyé par le Laboratoire souterrain de Modane –

Physicists discover large body orbiting Earth!

Sunday, April 1st, 2012

After gathering a huge amount of data the physicists at the Ice Cube experiment in Antarctica have come to an inescapable and startling conclusion. There is a massive body orbiting the Earth, and the scientists can see its “shadow” in their data. They can even trace its path across the sky.

This body is called “Luno” by some scientists and it seems to be cross the sky once every 29.5 days. The mass of Luno is estimated to be quite staggering- about 1% of the mass of the Earth! Despite its large size there seems to be little danger posed by this body, It seems to be orbiting happily, showing no sign deviating from its course. Taking a look at the data once the movement of Luno is taken into account gives a striking pattern, confirming that its orbit is indeed stable over long periods of time:

The position of neutrinos in the sky respect to Luno (Ice Cube) (Link to pdf)

The Ice Cube experiment is a neutrino observatory that searches for high energy neutrinos from outer space. These are thought to be given off by gamma ray bursts, neutron stars and alien TV broadcasts. (Some controversial theories also state that we can expect high energy neutrinos from malfunctioning microwave ovens and vacuum cleaners. But it would have to be some extreme form of malfunction.) As the neutrinos cosmic rays hit Luno they interact and the associated neutrinos don’t make it to Ice Cube. This is how Ice Cube see the “shadow” of Luno:

Other observations of Luno

This is not the first time that a particle physics experiment has speculated about a massive extra terrestrial body. The experiments at LEP postulated the existence of a massive body outside the Earth that changed their centre of mass energies. The assumptions went as far as to say that Luno was responsible for huge tidal forces that changed the shape of the Earth subtly around LEP. Then again, the LEP experiments were also sensitive to the TGV train timetables and meetings of the CERN Yoga Club.

Scientists at NASA have been studying Luno and they have come to some interesting conclusions. The most striking prediction is that Luno should be visible to the naked eye. Luno should reflect electromagnetic radiation from the sun, making it particularly visible at night. It is also thought that Luno is largely responsible for the tides we see in the seas and oceans across the world, a phenomenon which had been a mystery for centuries. Luno could even block the line of sight between the sun and the Earth, causing nightfall for a brief period of time. This could cause panic for people from scientifically illiterate cultures, nocturnal animals and biochemists. After much study there have been a number of artist’s impressions to help with identification of Luno:

Artist's impression of Luno to aid identification (NASA)

Ancient prophecy

Although Ice Cube has only discovered Luno recently, there are several examples of prophecy of Luno in various forms. Several ancient civilizations drew pictograms that represented Luno in some way with some examples, such as the Tarot deck, surviving to the present day. Some cultures even had a Luno deity, such as Khonsu of the ancient Egyptians. His pictogram includes a large figure, which carries Luno. Given the size of Luno, we should be able to see the large figure as well, but all searches have been fruitless. Some people think that this figure may be even harder to find than SUSY, or even extra dimensions (outside of the Terry Pratchett universe.)

Khonsu (discovered portions shown in gray) (Wikipedia)

Whatever Luno is, it should be heralded as one of the greatest discoveries of 2012, and I wouldn’t be surprised if it won the Nobel Prize!

A Grumpy Note on Statistics

Tuesday, March 13th, 2012

Last week’s press release Fermilab about the latest Higgs search results, describing the statistical significance of the excess events, said:

Physicists claim evidence of a new particle only if the probability that the data could be due to a statistical fluctuation is less than 1 in 740, or three sigmas. A discovery is claimed only if that probability is less than 1 in 3.5 million, or five sigmas.

This actually contains a rather common error — not in how we present scientific results, but in how we explain them to the public. Here’s the issue:

Wrong: “the probability that the data could be due to a statistical fluctuation”
Right: “the probability that, were there no Higgs at all, a statistical fluctuation that could explain our data would occur”

Obviously the first sentence fragment is easier to read — sorry![1] — but, really, what’s the difference? Well, if the only goal is to give a qualitative idea of the statistical power of the measurement, it likely doesn’t matter at all. But technically it’s not the same, and in unusual cases things could be quite different. My edited (“right”) sentence fragment is only a statement about what could happen in a particular model of reality (in this case, the Standard Model without the Higgs boson). The mistaken fragment implies that we know the likelihood of different possible models actually being true, based on our measurement. But there’s no way to make such a statement based on only one measurement; we’d need to include some of our prior knowledge of which models are likely to be right.[2]

Why is that? Well, consider the difference between two measurements, one of which observed the top quark with 5 sigma significance and the other of which found that neutrinos go faster than light with 5 sigma significance. If “5 sigma significance” really meant “the probability that the data could be due to a statistical fluctuation,” then we would logically find both analyses equally believable if they were done equally carefully. But that’s not how those two measurements were received, because the real interpretation of “5 sigma” is as the likelihood that we would get a measurement like this if the conclusion were false. We were expecting the top quark, so it’s a lot more believable that the excess is associated with the top quark than with an incredibly unlikely fluctuation. But we have many reasons to believe neutrinos can’t go faster than light, so we would sooner believe that an incredibly unlikely fluctuation had happened than that the measurement was correct.[3]

Isn’t it bad that we’d let our prior beliefs bias whether we think measurements are right or not? No, not as long as we don’t let them bias the results we present. It’s perfectly fair to say, as OPERA did, that they were compelled to publish their results but thought they were likely wrong. Ultimately, the scientific community does reach conclusions about which “reality” is more correct on a particular question — but one measurement usually can’t do it alone.

———————————

[1] For what it’s worth, I actually spent a while thinking and chatting about how to make the second sentence fragment simpler, while preserving the essential difference between the two. In this quest for simplicity, I’ve left off any mention of gaussian distributions, the fact that we really give the chance of a statistical fluctuation as large or larger than our excess, the phrase “null hypothesis,” and doubtless other things as well. I can only hope I’ve hit that sweet spot where experts think I’ve oversimplified to the point of incorrectness, while non-expert readers still think it’s completely unreadable. 😉

[2] The consensus among experimental particle physicists is that it’s not wise to include prior knowledge explicitly in the statistical conclusions of our papers. Not everyone agrees; the debate is between Frequentist and Bayesian statistics, and a detailed discussion is beyond the scope of both this blog entry and my own knowledge. A wider discussion of the issues in this entry, from a Bayesian perspective, can be found in this preprint by G. D’Agostini. I certainly don’t agree with all of the preprint, but I do owe it a certain amount of thanks for help in clarifying my thinking.

[3] A systematic mistake in the result, or in the calculation of uncertainties, would be an even likelier suspect.

But what if they are faster than light?

Friday, February 24th, 2012

This week the OPERA experiment released a statement about their famous “faster than light” neutrino measurement. In September scientists announced that they had measured the speed of neutrinos traveling from CERN to Gran Sasso and they found that they arrived slightly sooner than they should do according to special relativity. There was a plethora of scientific papers, all kinds of rumors and speculation, and most physicists simply refused to believe that anything had traveled faster than light. After months of diligent study, OPERA announced that they may have tracked down two sources of experimental error, and they are doing their best to investigate the situation.

But until we get the results of OPERA’s proposed studies we can’t say for sure that their measurement is right or wrong. Suppose that they reduce the lead time of the neutrinos from 60ns to 40ns. That would still be a problem for special relativity! So let’s investigate how we can get faster than light neutrinos in special relativity, before we no longer have the luxury of an exciting result to play with.

The OPERA detector (OPERA Collaboration)

Special relativity was developed over a hundred years ago to describe how electromagnetic objects act. The electromagnetic interaction is transferred with electromagnetic waves and these waves were known to travel extremely quickly, and they seemed to travel at the same speed with respect to all objects, no matter how those objects were moving. What Einstein did was to say that the constancy of the speed of light was a fundamental law of nature. Taking this to its logical conclusion meant that the fastest speed possible was the speed of light. We can call the fastest possible speed $$s$$ and the speed of light $$c$$. Einstein then says $$c=s$$. And that’s how things stood for over a century. But since 1905 we’ve discovered a whole range of new particles that could cast doubt on this conclusion.

When we introduce quantum mechanics to our model of the universe we have to take interference of different states into account. This means that if more than one interaction can explain a phenomenon then we need to sum the probabilities for all these interactions, and this means we can expect some strange effects. A famous example of this is the neutral kaon system. There two lightest neutral kaons are called $$K^0$$ and $$\bar{K}^0$$ and the quark contents of these mesons are $$d\bar{s}$$ and $$s\bar{d}$$ respectively. Now from the “outside” these mesons look the same as each other. They’ve got the same mass, they decay to the same particles and they’re made in equal numbers in high energy processes. Since they look identical they interfere with each other, and this gives us clues about why we have more matter than antimatter in the universe.

Since we see interference all over the place in the Standard Model it makes sense to ask if we see interference with a photon. It turns out that that we do! The shape of the Z mass peak is slightly asymmetric because of interference between virtual Z bosons and virtual photons. There are plenty of other particles that the photon can interfere with, including the $$J/\psi$$ meson, and the $$\rho$$ meson. In fact, any neutral vector meson with no net flavor will do. Einstein didn’t know about any of these particles, and even if he did he never really accepted the conclusions of quantum mechanics, so it’s no surprise that his theory would require that the speed of light is the fastest speed (that is, $$c=s$$.) But if the photon interferes with other particles then it’s possible that the speed of light is slightly lower than the fastest possible speed ($$c<s$$). Admittedly, the difference in speed would have to be very small!

In terms of quantum mechanics we would have something like this:
$|light>_{Einstein} = |\gamma>$
$|light>_{reality} = a_\gamma |\gamma> + a_{J/\psi} |J/\psi> + a_Z |Z> + \ldots$

As you can see there are a lot of terms in this second equation! The contributions would be tiny because of the large difference in mass between the massive particles and the photon. Even so, it could be enough to make sure that the speed of light is ever so slightly slower than the fastest possible speed.

At this point we need to make a few remarks about what this small change in speed would mean for experiments. It would not change our measurements of the speed of light, since the speed of light is still extremely fast and no experiment has ever showed a deviation from this extremely fast speed. Unless somebody comes up with an ingenious experiment to show that the difference between the speed of light and the fastest possible speed is non-zero we would probably never notice any variation in the speed of light. It’s a bit unfortunate that since 1983 it’s been technically impossible to measure the speed of light, since it is used in the definition of our unit of length.

Now we know that photons can interfere with other particles it makes sense to ask the same question about neutrinos. Do they interfere with anything? Yes, they can interfere, so of course they do! They mix with neutrinos of other flavors, but beyond that there are not many options. They can interfere with a W boson and a lepton, but there is a huge penalty to pay in the mass difference. The wavefunction looks something like this:
$|\nu_e>(t) = a(t)_{\nu_e}|\nu_e> + a(t)_{\nu_{\mu}}|\nu_\mu> + a(t)_{\nu_{\tau}}|\nu_\tau> + a(t)_{We}|We>$
(I’ve had to add a time dependence due to neutrino mixing, but it’s essentially no more complicated than what we had for the photon.)

That means that the photon could get slowed down slightly by the interference with other particles (including particles in the vacuum) and that neutrinos could get slowed down more slightly by their interference terms with other particles. And that way we could get neutrinos traveling faster than the speed of light and special relativity could remain intact. (In this description of the universe we can do what used to seem impossible, we can boost into the rest frame of a photon. What would it mean to do that? Well I suppose it would mean that in this frame the photon would have to be an off-shell massive particle at rest.)

The SN 1987 supernova, a rich source of slower than light electron neutrinos (Hubble, ESA/NASA)

Now I’ll sit back and see people smarter than I am pick holes in the argument. That’s okay, this isn’t intended to be a serious post, just a bit of fun! There are probably predictions of all kinds of weird effects such as shock waves and time travel that have never been observed. And there are plenty of bits I’ve missed out such as the muon neutrinos traveling faster than electron neutrinos. It’s not often we get an excuse to exercise our analytic muscles on ideas like this though, so I think we should make the most of it and enjoy playing about with relativity.

New Information on “FTL Neutrinos”

Thursday, February 23rd, 2012

We have new information, but my position on the OPERA experiment’s FTL neutrino measurement hasn’t changed.

First, here’s what we know. Members of the OPERA experiment has been working diligently to improve their measurement, better understand their uncertainties, and look for errors. Yesterday, the discovery of some possible problems was leaked anonymously (and vaguely) in Science Insider. This compelled OPERA to release a statement clarifying the status of their work: there are two possible problems, which would have opposite effects on the results. (Nature News has a good summary here.)

The important thing to learn here, I think, is that the work is actually ongoing. The problems need further study, and their overall impact needs to be assessed. New measurements will be performed in May. What we’ve gotten is a status update whose timing was forced by the initial news article, not a definitive repudiation of the measurement.

Of course, we already knew with incredible confidence that the OPERA result is wrong. I wrote about that last October, but I also wrote that we still need a better understanding of the experiment. Good scientific work can’t be dismissed because we think it must have a mistake somewhere. I’m standing by that position: it’s worth waiting for the final analysis.

Innovation at Fermilab: Liquid Argon Test Facility

Tuesday, January 24th, 2012

Despite the biting cold and snow, scientists and Fermilab personnel gathered outside to break ground for Fermilab’s new Liquid Argon Test Facility. The facility, expected to be completed spring 2013, will house liquid-argon based experiments.

Scientists have speculated since the 1980s that liquid argon could be used as a crash pad for high-energy neutrinos and have subsequently constructed several liquid-argon neutrino detectors; the largest and most prominent being ICARUS, the Imaging Cosmic And Rare Underground Signals, detector in Italy. The design of the new MicroBooNE experiment improves upon technology developed for ICARUS and will allow scientists to observe neutrinos with greater precision and resolution.

Regina Rameika is the project manager for the construction of the MicroBooNE detector.

“The MicroBooNE detector that will first use this facility is smaller than ICARUS, but incorporates some advanced designs,” Rameika said.

MicroBooNE will use liquid argon as a target for neutrinos generated in the Booster neutrino beam. When the neutrinos hit the argon nuclei, they generate showers of charged particles that then drift to an electrical detector. The purer the argon, the further the particles are able to drift. MicroBooNE will use ultrapure argon to maximize the distance these particles drift. This model is more efficient, cost effective, and has the potential to be scaled-up to a much larger size than previous detectors.

The MicroBooNE experiment will provide another layer of data for using the Booster neutrino beam. Not only will scientists be able to observe particles with the existing MiniBooNE detector, but now they will be able to measure neutrinos from the Booster neutrino beam with a second, higher-resolution detector.

“The MicroBooNE experiment will be focused on understanding some anomalies observed in the data from the MiniBooNE experiment,” Rameika said. This project will also provide valuable insight into different designs for liquid-argon detectors that could be located in the LArTF once MicroBooNE is complete.

—Sarah Charley

Fermilab planning a busy 2012

Tuesday, January 3rd, 2012

This column by Fermilab Director Pier Oddone first appeared in Fermilab Today Jan. 3 .

We have a mountain of exciting work coming our way!

In accelerator operations, we need to give enough neutrinos to MINERvA to complete their low-energy run, enough anti-neutrinos to MiniBooNE to complete their run and enough neutrinos to MINOS to enable their independent neutrino velocity measurement that will follow up on last year’s OPERA results. We need to provide test beams to several technology development projects and overcome setbacks due to an aging infrastructure to deliver beam to the SeaQuest nuclear physics experiment. And we need to do all of this in the first few months of the year before a year-long shutdown starts. During the shutdown, we will modify the accelerator complex for the NOvA era and begin the campaign to double the number of protons from the Booster to deliver simultaneous beams to various experiments.

In parallel with accelerator modifications, we will push forward on many new experiments. The NOvA detector is in full construction mode, and we face challenges in the very large number of detector elements and large mechanical systems. Any project of this scale requires a huge effort to achieve the full promise of its design. We have the resources in our FY2012 budget to make a lot of progress toward MicroBooNE, Mu2e and LBNE. We will continue to work with DOE to advance Muon g-2. All these experiments are at an important stage in their development and need to be firmly established this year.

At the Cosmic Frontier, we will commission and start operation of the Dark Energy Survey at the Blanco Telescope in Chile, where the camera has arrived and is being tested. In the dark matter arena we will commission and operate the 60 kg COUPP detector at Canada’s SNOLAB and continue the run of the CDMS 15 kg detector in the Soudan Mine while carrying out R&D on future projects. We continue to have a major role in the operation of the Pierre Auger cosmic-ray observatory. In addition we should complete the first phase of the Fermilab Holometer, which will study the properties of space-time at the Planck scale.

At the Energy Frontier, we play a major role in the LHC detector operations and analysis. It should be a fabulously exciting year at the LHC as we push on the hints that we already see in the data.

Beyond construction and operation of facilities we continue our R&D efforts on the superconducting RF technology necessary for Project X and other future accelerators. We will be building the Illinois Accelerator Research Center and moving forward to connect our advanced accelerator program with industry and universities. Our rich program on theory, computation and detector technology will continue to support our laboratory and the particle physics community.

If we accomplish all that is ahead of us for 2012, it will be a year to remember and celebrate when we hit New Year’s Day 2013!

Science with ties to Fermilab top year-end ‘best of’ lists

Wednesday, December 28th, 2011

Christmas time brings not only presents and pretty cookies but an outpouring of media lists proffering the best science stories of the year and predicting those that will top the list in 2012.

While the lists varied wildly everyone seemed excited by a few of the same things: upsetting Einstein’s theory of special relativity, a hint of the ‘god particle’ and finding planets like our own.

Several of the stories that made nearly every media outlet’s list, though in various rankings, have a connection, directly or indirectly, to Fermilab. Here’s a sampling with the rankings from the publications.

Discover magazine had the largest list, picking the top 100 science stories.

1: A claim by researchers at the OPERA experiment at CERN that they had measured neutrinos traveling faster than the speed of light, something disallowed by Einstein’s Theory of Special Relativity. Now the scientific community is looking for another experiment to cross-check OPERA’s claim.

That brought renewed interest to a 2007 measurement by the MINOS experiment based at Fermilab that found neutrinos skirting the cosmic speed limit, but only slightly. The MINOS collaboration always planned to study this further when it upgrades its detector in early 2012 but the OPERA result added new urgency.

Look in 2012 for MINOS to update the time of flight of neutrinos debate in three stages. First, MINOS is analyzing the data collected since its 2007 result to look for this phenomena. Results should be ready in early 2012. This likely will improve the MINOS  precision in this area by a factor of three from its 2007 result. Second, MINOS is in the process of upgrading its timing system within the next few months using a system of atomic clocks to detect when the neutrinos arrive at the detector. The atomic clock system will progressively improve resolution, which is needed to make the MINOS analysis comparable to the OPERA result and improve precision from the 2007 MINOS result by as much as a factor of 10. That will tell us if OPERA was on the right track or not, but may not be the definitive answer. That answer will come with the upgrades to the MINOS experiment  and a more powerful neutrino beam, producing a larger quantity of neutrino events to study. The upgraded MINOS experiment will be in many ways a more precise system than OPERA’s and could produce a result comparable with OPERA’s precision likely by January 2014.

4: Kepler’s search for Earth-like planets that could sustain life produces a bounty of cosmic surprises, fueled, in part, by the computing skills of a Fermilab astrophysicist.
32: The on-again, off-again rumor of finding the Higgs boson particle.  Physicists working with experiments at Fermilab’s Tevatron experiments and CERN’s Large Hadron Collider expect to answer the question of whether a Standard Model version of the Higgs exists in 2012.
65: The shutdown of the Tevatron at Fermilab after 28 years and numerous scientific and technological achievements.
82: Fermilab physicist Jason Steffen’s frustration with slow airplane boarding drives him to figure out a formula to speed up the aisle crawl.

﻿﻿Nature’s year in review didn’t rank stories but started off by mentioning the Tevatron’s shutdown after 28 years and following up shortly with the puzzling particle news of potentially FTL neutrinos and a Higgs sighting.

For science — as for politics and economics — 2011 was a year of upheaval, the effects of which will reverberate for decades. The United States lost three venerable symbols of its scientific might: the space-shuttle programme, the Tevatron particle collider and blockbuster profits from the world’s best-selling drug all came to an end.

﻿Cosmos magazine rankings:

The MINOS far detector in the Soudan Mine in Minnesota. Credit: Fermilab

1: Kepler’s exoplanet findings
2: FTL neutrinos
3: Higgs

Scientific American‘s choices:

3: FTL neutrinos
5: Higgs

ABC News asked science radio and TV host physicist Michio Kaku for his top 10 picks. They include:

3: Hint of Higgs
5: Kepler’s exoplanet findings
10: Nobel Prize for the discovery that the expansion of the universe is accelerating, which laid the groundwork for the today’s search for dark energy. Fermilab has several connections to to this work. The latest tool in dark energy survey experiments, the Dark Energy Camera,  was constructed at Fermilab in 2011. One of the three prize winners, Saul Perlmutter, is a member of the group that will use the camera, the Dark Energy Survey collaboration. Adam Riess, another of the winners, is a member of the SDSS-II experiment, a predecessor to DES that Fermilab was key in building and later operating its computing system.

Live Science

5: FTL neutrinos
4: Kepler’s exoplanet findings
2: Higgs

If the Higgs boson’s mass is high, it is expected to decay predominantly into two W bosons. Plushies images from the Particle Zoo.

To make the ﻿﻿Ars Technica list stories had to be awe inspiring in 2011 AND have a chance of making the 2012 list as well.

1: FTL neutrinos
2: ﻿﻿﻿Kepler’s exoplanet findings
6: Higgs hunt

Science magazine chose the best scientific breakthrough of the year. Kepler’s exoplanet hunt made it into the runner up list.

Tell us who you agree with or, better, yet give us your own top 10 science stories of the year.

— Tona Kunz

Can neutrino experiments predict earthquakes?

Tuesday, November 1st, 2011

The March earthquake and subsequent tsunami in Japan killed 20,352 people. Finding ways to reduce the death toll from such natural disasters has captured the interest of scientists in many fields.

There exists several ways to predict an earthquake such as the detection of radon gas emission or electromagnetic changes. However short-term predictions (hours todays) are, in general, unlikely.

At least for now.

Neutrino physics aims to answer some fundamental questions in physics, such as neutrino mass, and matter-antimatter asymmetry, but some think it could also answer fundamental questions about the Earth’s most volatile activities.

Using giant neutrino detectors physicists may be able to predict earthquakes and/or volcano eruptions by detecting geoneutrinos.

Geoneutrinos were first found by the Kamioka Liquid-scintillator Anti-Neutrino Detector (KamLAND) experiment in 2005 and recently by the Borexino Collaboration at the Gran Sasso National Laboratory of the Italian Institute of Nuclear Physics. Geoneutrinos are electron antineutrinos – the antimatter counterparts of electron neutrinos. Geoneutrinos are produced by the radioactive decay of uranium, thorium and potassium in Earth’s crust and mantle.

Geoneutrinos provide us another way to better understand the Earth’s interior besides the usual way of seismology by analyzing the vibrations produced by earthquakes and sensed by thousands of instrument stations worldwide. Geoneutrinos can provide crude information about chemistry, that is to say, how much uranium and how much thorium there is. This will help us to better understand deep-Earth processes which will affect events on the surface such as earthquakes and volcanoes.

A group of Chinese physicists proposed another method for earthquake prediction via neutrino tomography. The idea is to use antineutrinos emitted from nuclear reactors as a probe. As the antineutrinos traverse through a region prone to earthquakes, observable variations in the matter effect on the antineutrino oscillation would provide a tomography of the vicinity of the region. Although they concluded that it is a difficult task with the present technology, “there is hope that a medium-term earthquake forecast would be feasible” with the development of geology, and new detection technology.

Can the NOvA detector being built at Fermilab and in northern Minnesota detect geoneutrinos and make a prediction about earthquakes? The answer is probably no. While the 1,000 ton KamLAND detector sits in an old mine with 2,700 meters, or a little less than 1 ½ miles, of shielding to reduce cosmic ray interference, the 14-kiloton NOvA far detector in Minnesota has only three meters, or about 10 feet, of shielding.

The 222-ton NOvA near detector at Fermilab has 105 meters, or 344 feet, of shielding and will detect a much lower cosmic- ray rate, but it is not big enough to detector the geoneutrinos.

With more and more advanced technologies and geoneutrino-detecting facilities, we may expect that we can have a detailed understanding of Earth’s interior and the source of its internal heat in the near future. Someday, we may be able to predict the occurrence of events such as earthquakes, tsunami and volcano eruptions using our neutrino detectors.

– Xinchun Tian

Neutrinos: The Great Asterisk of the Standard Model

Monday, October 31st, 2011

For what it’s worth, neutrinos are weird. They are probably the strangest bits of matter in the Universe, and I do not mean in the quark sense either. Assuming that neutrinos are not actually trans-dimensional beings in search of a new home, there is probably no particle in Physics Past, Present, & Future that has bore more brunt of physicists’ creativity. On the other hand, as far as I know, there is no other particle that has solved as many problems in physics as neutrinos. The higgs boson is a good contender, but I still think neutrinos take the cake due to the fact that they have been around longer. Well, that and actually having been found to exist.

Figure 1: The (Left) Electron-, (Center) Muon-, and (Right) Tau-Neutrino, in plushie representation, brought to you by ParticleZoo. [Images: ParticleZoo]

I am sure by now you are wondering, “What are you talking about?”, and in all fairness, that is a very good question. In physics, neutrinos have a long history of being either the particle that broke the mold or the particle that saved physics. In doing so, neutrinos have developed this reputation for being the go-to particle for a new theory. In all fairness though, neutrinos are not doing themselves any favors if experiments keep finding contradictions with known laws of physics *cough*. I am sure for every flavor of ice cream at Baskin-Robbins or Ben & Jerry’s, there is a neutrino that has either been discovered or hypothesized.

Figure 2: The (Left) Electron-, (Center) Muon-, and (Right) Tau-Antineutrino, in plushie representation, also brought to you by ParticleZoo. [Images: ParticleZoo]

For today’s post, I though I would share with you a few of the many flavors of neutrinos. It is also my secret goal to mention “neutrinos” so often in this post that it will be at the top of Google’s queue. The table of contents is just below with the full list today’s neutrino flavors. Believe it or not, there are still plenty of types omitted. I suppose I have to write a future post to include these. 😀

Happy Halloween & Happy Colliding!

– richard (@bravelittlemuon)

1. The First Neutrino: Pauli’s Neutron

Back in the days when particle physics was still a young field in physics, about a decade before the discovery of Quantum Mechanics, experimentalists studying radioactive decay discovered something very startling: When a radioisotope decayed and emitted a high speed electron, then energy & momentum were not conserved. This was a very worrisome result because these conservation laws were, and still are, pillars of physics. In 1930, Wolfgang Pauli, after whom the famed Pauli-Exclusion Principle is named, made an audacious suggestion that perhaps radioactive decay involving electron emission also involved the production of an additional particle. Pauli’s stated that his neutrino, then named the neutron (different from today’s neutron), that was (1) electrically neutral and (2) massless, or nearly massless, (3) did not travel at the speed of light, and (4) virtually undetectable by contemporary, experimental standards.

Figure 3. The Nobel Foundation’s official portrait of Prof. Pauli (Nobel 1945). Yes, this is the man responsible for suggesting the existence of the neutrino. As father of all hypothetical particles, Pauli would later come to regret (mid-page) proposing an undetectable objects. [Image: Nobel Foundation]

At the end of the day Pauli was spot on with his suggestion. Radioactive decay involving electron emission does, indeed, require a very light, electrically neutral particle. In fact, the following generation of neutrino detectors were able to discover it without a problem. It turns out, all someone needed was a nuclear reactor and patience.

Figure 4: The (real) neutron is composed of one up-flavor quark and two down-flavor quarks. [Image: Internet]

James Chadwick‘s discovery of the neutron proved one thing very, very well: that the Universe has an odd sense of humor and likes to confuse those to attempt to understand it. Having uses from nuclear power to cancer therapy, at the end of the day neutrons have been a boon for the scientific community and society as a whole. When first discovered, however, Chadwick initially misidentified it as Pauli’s neutron (a.k.a. the real neutrino). Today, the names we have for many particles are really artifacts of the confusion in particle physics through the 1930s & 40s. (For those of the physics history persuasion, this is just like the discovery of the “μ” meson.) Here is a time line the discovery of Chadwick’s neutrino (a.k.a. the fake neutrino):

• 1911 – The gold foil experiment is carried showing that the atom consists of a dense center. It is later found that an atom’s nucleus is too heavy to be composed only of protons. Fifty years later, gold foil is also discovered to be a source of unlimited amounts of chocolate.
• 1911β-decay, the mechanism through which some radioisotopes decay, appears initially to violate the Law of Conservation of Energy.
• 1930 – Pauli proposes, in his famous “Dear Radioactive Ladies and Gentlemen” letter, the existence of a massless (0r near massless), electrically neutral particle, called the “neutron” (actually the electron-neutrino), to resolve the apparent energy non-conservation in radioactive β-decay.
• 1932 – Chadwick claims possible discovery of a massive, electrically neutral, particle within the nucleus of an atom. Believing it to be Pauli’s neutron (actually the electron-neutrino), he calls it the “neutron” (actually the real neutron).
• 1934Enrio Fermi, using the newly created framework of Quantum Field Theory, proposes a simple four-particle interaction to describe β-decay (See 3. Fermi’s Neutrino). With known experimental results, Fermi was able to determine that Chadwick’s neutron (real neutron) was much too heavy to be Pauli’s neutron (fake neutron; real neutrino) and renamed Pauli’s neutron the “neutrino,” which is Italian for “little neutral one.” The only thing more impressive than the accuracy to which this model actually describes Nature is how short the paper is.
• 1942 – Pauli’s neutrino is discovered. In full disclosure, the particle he proposed to solve the problems of β-decay and what was actually discovered first is really the anti-electron-neutrino.

The real neutron is not really a neutrino; it just stole the real neutrino’s name. That jerk (the neutron not Chadwick).

[Note: It is really hard to write “neutrino,” “neutron,” and embed hyperlinks, all while focusing on the historical context.]

3.Fermi’s Neutrino: The Key to the Weak Nuclear Force

The mathematical and physical description of radioactive decay is, by far, one of the most beautiful things I have every seen in either Mathematics or Physics. (The second is probably the metric structure in Special Relativity.) What is so amazing about it is how it changes at higher energies. On one end of the energy spectrum, you have everyday radioactive decay; somewhere near the middle, you have the restoration of electroweak symmetry and higgs boson production; and on the far end, you have the grand unification of all forces.

In attempt to explain a type of radioactive decay known as β-decay, Enrico Fermi, in 1934, supposed that during this process a radioisotope will decay into a more stable isotope, a high speed electron (β-particle), and a hypothetical particle predicted to exist by Pauli, called the neutrino (See 2. Chadwick’s Neutrino). They Feynman diagram that illustrates this interaction is just below. I should note now that what Pauli really predicted is a neutrino’s antimatter equivalent call the anti-neutrino.

Figure 5: Enrico Fermi’s 4-fermion interaction model to describe β-decay. n represents an incoming neutron, p represents an outgoing proton, e is an outgoing electron, and note the outgoing anti-electron-neutrino (νe). [Image: Mine]

Being a fermion, a neutrino has an antimatter partner called an anti-neutrino. Under the rules of Quantum Field Theory, one can then induce β-decay by directing a beam of neutrinos into a bunch of heavy nuclei, like a thick plate of steel. Such a process would be drawn like this:

Figure 6: Enrico Fermi’s 4-fermion interaction model to describe neutrino scattering. n represents an incoming neutron, p represents an outgoing proton, e is an outgoing electron, and note the incoming electron-neutrino (νe). [Image: Mine]

Though the probability of inducing β-decay is very small but it becomes larger with higher energy. If you extrapolate this to very high energies, you find out that eventually the probability of inducing β-decay becomes larger than 100%, which is total nonsense. You can never have a 101% of your interactions result in anything. In particle physics, the sum of all probabilities must add up to 100%; in such cases where they do not, we say that “unitarity has been violated.” This terminology originates from the fact that the matrix containing all possible interaction outcomes is a unitary matrix, implying that total probability is (1) conserved and (2) identically equal to 1 (or 100%).

How does Nature avoid breaking math at high energies? Well at around 100 GeV, rather than two particles smashing into each other to produce two different particles, a neutrino will radiate a W boson and become the high speed electron (β-particle). This W boson is then absorbed by a neutron (Chadwick’s neutron) and is turned into a proton, thereby transmuting one isotope into another isotope. Since producing a W boson (mW = 80.399 GeV/c2) is not cheap and requires a lot of energy, the probability of scattering a neutrino off a nucleus is driven down and prevents unitarity from being violated.

In summary, Fermi’s neutrino & Weak Nuclear Theory model is the  foundation for the Electroweak component of the Standard Model.

Figure 7: Tree-level diagram of the neutrino scattering process in which (1) a neutrino will emit a W and become an electron, and is followed by (2) a down-type quark absorbing the W boson and becoming an up-type quark. The 4-fermion model is the low-energy approximation of this description. Color represents the QCD charge held by the quarks in a nuclei. Color also makes things look nicer. [Image: Mine]

4. Majorana’s Neutrino

Antimatter, the destroyer of basilicas, the stuff of warp drives, and just all around fascinating piece of science, was predicted to exist in 1928 by the great Paul Dirac, and discovered shortly thereafter (1932) by Caltech’s Carl Anderson. This is the same Anderson who is discovered the muon, and so he probably qualifies to be my hero. One way to describe antimatter is to imagine regular, ordinary matter, but for each charge a piece of matter has its antimatter partner has the opposite charge. For example, the top quark has a number of charges: +2/3 electric charge; it can have a red, blue, or green charge from the Strong Nuclear force (QCD); and it also has a “topness” (or “truthfulness”) charge under the Weak Nuclear force. An anti-topquark then must have: a -2/3 electric charge; an anti-red, anti-blue, or anti-green “color” charge; and has “anti-topness” (or “anti-truthfulness”… does that make anti-topquarks liars?).

Well, I suppose one has to wonder if it is possible for a particle to ever be its own anti-particle. The answer is yes. Such particles are called Majorana particles. Italian physicist Ettore Majorana speculated and determined a number of constraints, namely to conserve all the various types of charges (electric, color, weak) a Majorana particle must be neutral under all its charges. To get this right, I need an electrically neutral, colorfully neutral, and weakly neutral. To me, this sounds just like a neutrino! If it smells like a neutrino, looks like a neutrino, and tastes like a neutrino, then clearly it must be a duck neutrino.

What is the problem? Well, if neutrinos are their own antiparticle then physicists expect to see something called neutrino-less double β-decay (or 0νββ for short). In this process, a radioisotope will undergo β-decay and emit a high speed electron and an anti-electron neutrino. If neutrinos are indeed Majorana particles, then the anti-electron-neutrino is also an electron-neutrino and can force a second radioisotope to also emit a high speed electron.

To date, 0νββ has not been observed but that does not mean it does not exist. It is possible that 0νββ does exist, it must just be a really, really rare process.

Figure 8: Feynman diagram demonstrating how neutrino-less double β can occur if neutrinos are also Majorana particles. [Image: Wikipedia]

5. The Super Massive Neutrino

According to the Standard Model of Particle Physics, there are only three “light” neutrinos. “Light” is defined as less than 1/2 the mass of the Z boson, which mZ = 91.1876 GeV/c2. We have observed this empirically by producing Z bosons in copious amounts at the large electron positron collider and looking at all possible ways we can observe a Z boson can decay. The total number of observed Z decays is then used to calculate the Z boson’s average lifetime (or rate of decay). The observed decay rate is subtracted from the Standard Model’s prediction for the total decay rate. The difference between the theoretical prediction and the experimental observation is then compared to the situation where the Z boson were able to decay into 1, 2, 3, … different pairs of particles that could not be observed with our detectors. These sorts of decays are called “invisible decays” or “invisible decay modes.” From this data, all signs point to three different invisible decay modes, which correspond to the three neutrino flavors in the Standard Model (electron, muon, tau).

Time for caveat number 4,321: Z bosons can only decay into particles lighter than itself, otherwise all sorts of bad things would happen. By bad things, I mean violations of conservation laws. If any particle were to decay into two (almost) identical particles, then at most each daughter particle could weight half of the mother particle. This means, according to invisible decay searches of the Z boson, there are only three types of neutrinos with mass less than 1/2 the mass of the Z boson. It is fair game for neutrinos to be heavier than half the Z mass; in fact, it is possible for a neutrino to be as heavy as ten top quarks! (The top quark is currently the most heavy particle known to exist.)

The most recent experimental results have found that for a stable (non-decaying) neutrino, its mass must be at least 45.0 GeV/c2 (39.5 GeV/c2) for an ordinary (Majorana) neutrino. For a short-lived (decaying) neutrino, it must have a mass of at least 90.3 GeV/c2 (80.5 GeV/c2) for an ordinary (Majorana) neutrino.

6. The Extra, Extra Neutrino

Neutrinos can oscillate. What do I mean by that? Well, if you make a beam of neutrinos and look at the beam composition (% of electron-neutrinos v.s. % of muon-neutrinos, v.s. % of tau-neutrinos),  as a function of distance, then one will notice that the relative composition changes.

For example: If I measure the beam to be made of 100% electron-neutrinos & 0% muon-neutrinos, and a few football pitches away I find that it is now 50% electron-neutrinos, 50% muon-neutrinos, then a few football pitches away from that I can expect to see 100% electron-neutrinos & 0% muon-neutrinos once again. I made up the exact numbers, but I hope you get the idea. It has only been recently (1,2) that all oscillation permutations have been observed.

Figure 9: To measure neutrino oscillations, a neutrino beam is typically shot into the Earth (right), measured by a detector close to the beam’s origin (near detector), and then detected by a detector on the opposite side of the planet (left). Yes, we literally shoot a beam a particles into the Earth and wait for them to come out the other side. PHYSICS. IS. AWESOME. [Image: Interactions]

Well, back in 2001 (that was over 10 years ago, weird…) a Los Alamos experiment LSND (Liquid Scintillator Neutrino Detector) saw a signal that could be explained if neutrinos were also oscillating into a fourth type of neutrino. The MiniBooNE experiment at Fermilab tried to verify this result and was unable to make a conclusive determination. In other words, the jury is still out on the existence of a 4th type of neutrino.

7. The Sterile Neutrino: Type I

I like sterile neutrinos; they are fun. According to the Standard Model, all observed neutrinos are (1) colorless (no interactions via the Strong Nuclear Force), (2) electrically neutral (no interactions via Electromagnetism), and (3) are left handed (Weak charge). This means that Standard Model neutrinos can only interact with the W bosons and sometimes with the Z boson. Well, suppose there were a right-handed neutrino (opposite Weak charge from left-handed neutrino). It is still invisible to the Strong Nuclear Force, the Electromagnetic Force, and the W± bosons (because all W‘s are left-handed). In principle right-handed neutrinos can interact with the Z boson, trying to separate the corresponding signal from background data is like trying to find a find a needle, in a haystack, at a fair. Did I mention this fair is a tri-state fair?

Right-handed neutrinos and other neutrinos that are invisible to the Standard Model forces are examples of what physicists call “sterile neutrinos.” (Personally, I like to qualify these sorts of little tykes with the title “Type I.” See 8. The Sterile Neutrino: Type II why I do so.) If right-handed neutrinos do exist, then there is no way to see detect them given our current understanding of physics. However, this does not mean they cannot interact through some new, undiscovered force.

To date, there is no confirmed evidence, direct or indirect, of the existence of a right-handed or any other type  sterile neutrino. To date, there is no evidence for a new fundamental force either. Though interestingly enough, since sterile neutrinos, in principal, cannot be detected, then it is logical that there could be hundred or even thousands of slightly different sterile neutrinos. Alternatively, we can also a universe filled with a single type of neutrino and we would not be able to detect them outside of gravity (assuming they have mass), which brings me to mention that sterile neutrinos have even been proposed as a dark matter candidate. Neutrinos are resourceful, I will give them that.

Figure 10: A snow-covered hay bale at Fermilab. Imagine trying to find a needly in that field. [Image: FNAL]

8. The Sterile Neutrino: Type II

Sterile neutrino type II (again, I made up the “type” nomenclature) is very much like type I but with one glaring difference. Even if there are are new forces in the Universe, these types of neutrinos will still not interact with anything. The only possible forces through which these neutrinos might interact are gravity and whatever unified force that produced these oddballs.

9. The Tachyon Neutrino

In September, the Italian neutrino experiment OPERA (Oscillation Project with Emulsion-tRacking Apparatus) shocked the world when the collaboration announced it had observed neutrinos traveling at a speed faster than that at which light travels. My colleagues have blogged about it here, here, here, and more recently here. This is a huge deal because, according to Special Relativity, the speed of light (numerically c = 299,792, 458 m/s or 983, 571, 056 ft/s) is pretty much a cosmic speed limit that no real particle can surpass. So I am not sure which makes me happier, the fact that tachyons are seriously being floated as an explanation for this claim or that #FTLneutrinos is a thing. (“FTL” stands for “faster than light.”)

Metaphorically, tachyons are interesting sorts of creatures. I do not know too much about them beyond the fact that they have (in the mathematical sense) a purely imaginary mass. The last time I checked quantum mechanics, we cannot observe strictly imaginary quantities, but I digress. What I do know is that special relativity implies that having a purely imaginary mass should then enable tachyons to permanently travel at speeds faster than c. If neutrinos do travel at speeds faster than the speed of light, then they may also be tachyons. I think it is a perfectly reasonable argument. However, there is a very big elephant in the room that I have to address. Having imaginary mass means that all tachyons always travel at superluminal speeds. If some neutrinos are found to travel at subluminal speeds then the idea that neutrinos are tachyons is tossed out. End of story.

So in light of the considerable implications of any particle traveling faster than the speed of light, it is very appropriate to remain cautious and wait for OPERA to reproduce their results and independent verification, possibly by Fermilab’s MINOS Experiment or KEK’s T2K Experiment.

Figure 11: A real life tachyon. [Image: ParticleZoo]