Be Careful What You Publish

I'm a little late in posting this, but I thought this ought to be discussed. We've talked a lot about the report of superluminal neutrinos and how the erroneous measurement was apparently caused by a loose cable and some clock drift. Well, apparently Dario Auterio and Antonio Ereditato both resigned from their positions as OPERA team leaders after the final investigations concluded and OPERA's collaboration board gave them a vote of "no confidence." I was personally just surprised at the direction all of this took.

Neutrinos Behaving Badly

The OPERA Collaboration has announced what they claim is a 6-sigma measurement that neutrinos move faster than the speed of light in a vacuum (see stories by Nature and the BBC).  OPERA is an Italian experiment designed to measure neutrino oscillations in a stream of the elusive sub-atomic particles coming from CERN.  To date they have detected the arrival of over 16,000 neutrinos (which is quite a few considering how hard it is to stop a neutrino) and now they claim that they have good evidence that those particles move just a hair faster than what was thought to be the universal speed limit.

The OPERA folks are being very cautious with their claims and the point out that this could simply be some systematic error for which they haven't accounted.  Interestingly, Nature is reporting that there was some evidence for similar speeding neutrinos at the MINOS neutrino experiment, but that the distance from Fermilab (where the neutrinos were being made) to the MINOS detector wasn't know to sufficient precision to be able to make such a claim.

So what does this mean?  The joke that half of all 3-sigma results are false rings in my ears.  If I had to bet, my money would be on some sort of error or some effect of a previous theory that accounts for the apparent effect.  I guarantee that there will be a lot of very smart experimentalists thinking about systematic errors in measuring neutrino speeds and that there will be a whole slew of papers from theorists showing that super-luminal neutrinos are predicted by their brand of quantum gravity, super-symmetry, or f(R) theory of gravity.  What does it mean if this is true?  Well, we can start by going back to the drawing board on relativity.

UPDATE:  The paper is up on the arXiv here.  I am not a particle physicist, but it looks like they have done a very nice, careful job in not over-selling their result.  The measured velocity was about 7430 +/- 1740 m/s faster than the speed of light.

One other interesting note:  When the famous supernova 1987A went off in our friendly neighborhood Large Magellanic Cloud, neutrino bursts were measured at three different detectors around the globe about 3 hours before the first light from the supernova was seen.  This is generally explained because 1987A was a core-collapse supernova and while the neutrinos it produced could simply stream outward through the outer layers of the collapsing star, the light could not and therefore was delayed slightly.  That supernova was about 168,000 light-years away.  Assuming that neutrinos actual do travel faster than light by the amount measured by the OPERA team, those neutrinos should have arrived 4 years prior to the first light from the supernova.  Things like this are why most physicists are looking for a problem with the experiment rather than throwing relativity out the window.

Cosmology Can Possibly Solve the Neutrino Hierarchy Problem.

There are three neutrino species in the standard model, hereafter refereed to as 1, 2, and 3, that we know have mass from atmospheric and solar neutrino oscillation experiments. Furthermore, data from these experiments put constraints on the mass-splittings between these three neutrinos.  From atmospheric experiments we know the mass differences between 2 and 3 is |M²₂₃| ~ 1.4x10^-3 eV² and from the solar neutrino experiments we know the mass splitting between 1 and 2 is M²₁₂ ~ 7.9x10^-5 eV².

So here is the problem: We know that the neutrinos have mass and we know what their mass splittings are but we don't know their hierarchy or in other words the order of their masses as shown in the figure to the right. For example, it could be that neutrino 3 is the most massive of the three... but it can also be the case that it is the least massive.  This is what I mean by the neutrino hierarchy problem I used in the title.

Cosmology To The Rescue!

Fortunately, there are cosmological measurements that can be made that may solve this issue in the future. In this post I will discuss a wonderful paper that pioneered these details was written by Jimenez et al.  The solution goes like this:

1.  CMB and large scale structure experiments give us a bound on the sum of the neutrino masses denoted as Σ = m₁+m₂+m₃.  The current bound is that  Σ  is between 0.05eV and 0.3eV.

2. After Σ is better constrained in the future, the mass splitting Δ = m₃ - m₁, importantly with sign!, can be be measured from the matter power spectrum of large scale structure for the given Σ.  The plot above shows how the matter power spectrum, P(k), is altered by the different values of Δ.  Once Δ is known with confidence, including sign, the problem is solved.



3. The plot above shows forecasts for how well we will be able to tell the difference between the normal inverted hierarchy given future experiments. (Normal being where m₃ is larger and invereted when it is smaller.)

4.  Furthermore, cosmolgy should be able to shed light on whether neutrinos are Dirac or Majorana particles. (If Majorana they are their own anti-particle and if Dirac they are not.) The below flow chart shows how this works.  First, double beta-decay experiments may be able to determine directly if neutrinos are their own anti-particle.  But if future experiments fail to see a signal, cosmology may help answer if this is because the signal is just too weak or whether it is because neutrinos really are Dirac. As you can see, if Σ is just right and if the hierarchy is inverted or degenerate, cosmology will be able to demonstrate neutrinos are in fact Dirac.

So in conclusion: It appears cosmology may be able to provide a wealth of insight into neutrino physics in the coming years.  Through cosmology we may solve the neutrino hierarchy problem and even possibly say whether or not neutrinos are Dirac.

Jimenez, R., Kitching, T., Peña-Garay, C., & Verde, L. (2010). Can we measure the neutrino mass hierarchy in the sky? Journal of Cosmology and Astroparticle Physics, 2010 (05), 35-35 DOI: 10.1088/1475-7516/2010/05/035

Could The Planck Satellite Discover A New Species Of Neutrino?

It has been known for some time that the WMAP data is more consistant with the existence of four neutrino species than three. Nevertheless, most cosmologists shrug this off as three is by no means ruled out. However, Hamann et al. 2010 demonstrate that such a dismissal may be a mistake.

It turns out, when WMAP 7 year data is combined with Sloan data, the three neutrino species model is ruled out by nearly two sigma. The best fit number of neutrino species becomes 4.78 +/- 1.79 at 95% confidence. Furthermore, big bang nucleosynthesis (BBN) data involving Heluim abundances seems to confirm that such an excess better fits the data.

With this in mind, Hamann et al. 2010 decides to test just how many extra neutrinos are needed to fit the combined data of "the WMAP 7-year data release, small-scale CMB observations from ACBAR, BICEP and QuAD, the 7th data release of the Sloan Digital Sky Survey, and measurement of the Hubble parameter from Hubble Space Telescope observations".  Their findings are plotted above.  They confirm that when all data is added together, the existence of one or two extra neutrinos provides a much better fit than only the standar three.

If this is real it would be major news! On one hand such a "4th" or even "5th" neutrino would have to exist at low energies as it has clearly affected both BBN and CMB physics.  However, extra neutrinos at such low energies have alluded modern particle accelerators.  Therefore, such much neutrinos must be "sterile" in that they do not couple to the rest of the standard model the way normal neutrinos do.  Furthermore, they must not have a lepton partner the same way other neutrinos do. (Example: like the electron neutrino does with the electron.)

Interestingly, if there are an extra one or two of such neutrinos in nature, the Planck satellite has a good chance of making a 5-sigma discovery!  (See plot below).  If this happens, the discovery of such interesting low energy neutrinos could well go down as one of Planck's greatest contributions to science.

Jan Hamann, Steen Hannestad, Georg G. Raffelt, Irene Tamborra, & Yvonne Y. Y. Wong (2010). Cosmology seeking friendship with sterile neutrinos Eprint arXiv: 1006.5276v1

Low energy solar neutrinos have finally been detected. This confirms current theory that neutrinos oscillate between three types on their journey from the sun to earth. From the NSF:

An international team of researchers has detected low-energy solar neutrinos--subatomic particles produced in the core of the sun--and measured in real-time the rate the particles hit our planet.

The researchers also obtained fresh evidence that neutrinos oscillate (transform from one state to another) before arriving at Earth, adding weight to present theories about the nature of neutrinos and the inner workings of the sun and other stars.

The team of more than 100 researchers, including National Science Foundation (NSF)-supported investigators at Princeton University and Virginia Tech, have operated the so-called Borexino experiment in one of the deepest laboratories in the world, the Gran Sasso Laboratory of the Istituto Nazionale di Fisica Nucleare (INFN, the Italian National Institute of Nuclear Physics), near the town of L'Aquila, Italy.

These are the first results from the Borexino experiment that has been under construction since the late 1990s with the support of INFN as the lead agency, NSF in the United States, and institutions in Germany, France and Russia.

"In making these first direct measurements of low-energy neutrinos coming from the sun, Borexino represents a convergence of our present understanding of neutrino properties and the physics of solar energy generation," said Brad Keister, program director for nuclear physics in NSF's mathematical and physical sciences directorate.

"The great depth of the laboratory and the incredible purity of the materials used in the detection were critical to the discovery and demonstrated the impact of eliminating background radiation from such experiments," added Keister.

Produced in the Big Bang, and more recently in stars and nuclear reactors, neutrinos are everywhere. They constantly bombard the Earth, but because they interact very weakly, chances are slim a neutrino will hit anything. More than 100,000,000,000,000 pass through each of us every second without our noticing them.

The 18-meter (59-foot) diameter Borexino detector lies more than a kilometer (almost a mile) underground in one of the planet's deepest laboratories. The depth blocks out cosmic rays and other radiation sources that could create additional background signals.

The detector is comprised mainly of concentric layers of radiation shielding. Within an external tank filled with 2,400 tons of water, an enormous stainless steel sphere lies anchored. Within the sphere are two nested nylon vessels, each containing successively purer detector fluids.

Neutrinos knock electrons out of atoms in the detector fluid, and in turn, the electrons generate photons as they travel further through the liquid environment...

The research preprint is now available online at the arXiv server, a leading pre-publication posting site for physics discoveries.

What Would I Like to See in Graduate School?

As many of you may know I hope to be off to graduate school next year. If I am like the average graduate student I will be there 5-6 years meaning I may be there until 2012-2013.(Yikes!) I happen to be going at perhaps the best time in history. In the next 5-6 years we may come up with some of our most profound discoveries ever. Here is what I hope to see.

1. The Higgs field and Supersymmetry: We all have been thankful for the success of the Standard Model, but I believe we are ready to move on. One major particle that I am desperately hoping CERN finds is the Higgs Boson. Come 2007, the LHC, at CERN will be operating at energies higher then currently being achieved anywhere else; on the order of of 14 TeV. In addition to finding the Higgs, I am really crossing my fingers that we will find strong evindence for supersymmetry. Not only will supersymmetry give us a huge slew of new particles to explore, it will hopefully resolve some of our issues such as the vacuum energy problem and candidates for dark matter.

2. Gravitational Waves: Being able to detect light we cannot see visually, like infrared and microwaves, has greatly furthered our understanding of nature. I hope we will soon add to that gravitational waves. Gravitational waves will greatly enhance our ability to study the universe. We will better understand centers of Galaxies, neutron stars and black holes. In addition, many are very excited on what light gravitational waves will shed on the initial stages of the universe. That will help us further understand not only large stellar objects, but also the small particles which formed in the early stages of the universe. Hopefully LIGO and LISA will be successful which I am at graduate school.

3. Neutrino Background: Many are familiar with the CMB or cosmic microwave background radiation but not as many people are familiar with the theoretical neutrino background. It turns out, there should be a huge collection of neutrinos left over from the big bang era which haven't interacted with anything. We should be able to study this collection much as we can study the CMB to determine truths about the universe. Surely there will be much learned from the neutrino background, and this is yet another discovery I hope we make soon.

4. Pop III Objects: Stars and planets today have lots of elements heavier then hydrogen and helium. During the initial stages of the universe, it was practically all hydrogen and helium. Because of this the physics of the cosmos was very different then it is today. These differences make all the difference in the world if you are trying to understand the universes history. Understanding the universe's history is very important in uncovering how and why the universe is the way it is. Hopefully the Hubble, or more realistically the new James Webb Telescope which will launch in 2013, will detect these objects.

5. Dark Matter and Dark Energy: This may be asking a little much, but with some luck the above things will set straight what is dark matter and dark energy. Maybe supersymmetry is responsible, or maybe something else.

6. The Stage Set for the Theory of Everything: Okay, I have to realistically believe quantum gravity, etc... will not be solved in the next 5-6 years. However, if we can understand the basics of supersymmetry, information coming from gravitational waves, the neutrino background, early population III objects and the higgs field, we might be able to have enough cosmological data to really get somewhere. I've said it before, and I will keep saying it, string theorists are demanding particle accelerators the size of the solar system to test things that will result in understanding quantum gravity. We may never have such accelerators, but we do have an event about 13.7 Billion years ago with energies high enough that if we could examine it close enough we may be knocking on the door to understanding quantum gravity and more. These are the events I hope to see which I believe will lead up to that.