IceCube, the big high-energy neutrino experiment cleverly embedded into the ice at the South Pole, announced a very interesting result yesterday, following on an already interesting result from a few weeks ago, one that I failed to cover properly. They have seen the highest-energy neutrinos ever observed, ones that, unlike previously observed high-energy neutrinos, appear not to be generated by cosmic rays hitting the top of the atmosphere. Instead, they apparently come from new sources far out in space. And as such, it tentatively appears that they’ve opened up, as long anticipated, a new era in neutrino astronomy, in which high-energy neutrinos will be used to understand astrophysical phenomena!
[The only previous example of neutrinos being used in astrophysics occurred with the discovery of neutrinos from the relatively nearby supernova, visible with the naked eye, that occurred in 1987. But those neutrinos had energies millions of times smaller than the ones discussed here. And there was hope that IceCube might see neutrinos specifically from gamma-ray bursts, including the one that occurred just two weeks ago; but that appears not to have happened.]
I don’t understand certain details well enough yet to give you a careful explanation — that will probably come next week — but here’s an early description (and expert readers are strongly encouraged to correct any errors.) Continue reading
[NOTE ADDED: A reader forwarded a message that IceCube did not see any neutrinos with energies above 1 TeV = 1000 GeV from this GRB. Maybe this is not quite the final word (there would still be sensitivity, with some effort, to neutrinos in the 100 GeV - 1000 GeV range) but clearly the neutrino signal isn't striking, and it is probably not there at all. But as I've suggested below, even a non-observation might have significant implications for the science; the question is, how many neutrinos would the standard speculations about how GRB's work have led you to expect at IceCube? If a reader can provide that info, I'd appreciate that.]
The very recent report of a powerful and long-lived gamma-ray burst (GRB), and questions and remarks by my readers (thank you!), have motivated me, both as a scientist and a blogger, to try to understand whether we should have observed neutrinos from this GRB. This is forcing me to catch up on the related subjects of GRB’s, searches for high-energy neutrinos, and the highest-energy cosmic rays. I’m certainly not caught up yet; there are decades of research out there, and I’m quite far behind on developments over the past three or four years. But here are some of the basics that I believe I understand. Still, be cautious with the content of this post, both because I’m not an expert and because this is a very active area of research in which some fraction of the more speculative stuff will surely turn out to be wrong. I will try to refine this post with a more detailed and corrected article sometime later, perhaps once we know whether neutrinos from this GRB were or were not observed.
GRBs that last more than a few seconds are widely believed to be associated with an exceptional form of Type II (or “core-collapse”) supernova, though this is not known for certain. In these types of GRBs, there are (at least) two sources of photons (everything from gamma-rays to visible light to radio waves) and two sources of neutrinos. It is important not to confuse the different sources! Continue reading
This is hot off the stellar press: as NASA announced today (with cool pictures), a brilliant, long, and rather nearby GRB, or “gamma-ray burster”, was observed on April 27th, initially by the Fermi and Swift satellites. Gamma-rays are just an old name for photons (i.e. particles of light) which have lots more energy (per photon) than the photons of visible light. And a GRB is a distant astronomical explosion that produces an enormously bright flare of these high-energy photons, typically for a short time (seconds or minutes), though this one lasted for hours. It is believed that a narrow jet of high-energy particles produced in a supernova (a powerful explosion of a star) is behind these flares, but they are still poorly understood and are under active study.
Everything about last week’s GRB is on the exceptional side. The most energetic photon detected had somewhat more energy than the photons produced in the decays of Higgs particles, a bit less than the energy of the photons that Fermi might be seeing from dark matter, and more than three times more energy than any GRB photon previously detected by Fermi. Its gamma rays were produced for many hours, setting another record. It lasted so long that several other types of telescopes were able to observe it, including those that look at visible light (it was even seen by an amateur astronomer), and those that look at radio waves (which are made from photons with vastly lower energy). And it was relatively close… well, relatively compared to most GRB’s. It occured in a galaxy 3.6 billion light years away. Now that is still a good fraction of the distance across the visible part of our universe, but still, it puts this GRB in the top 5% as far as proximity to Earth.
With such a vast amount of data to work with, it seems very likely that astronomers will learn qualitatively new things about GRBs by studying this blast. In astronomy, it sometimes takes just one spectacular event to change the scientific landscape! The next phase of the process will involve directly detecting the lesser (but still intense) glow from the (presumed) supernova that produced the GRB flare. Stay tuned! It should be a matter of a week or so…
Dark Matter, Dark Matter, Everywhere! It’s in your shoes, it’s in your coffee, it’s in the stars and even in your favorite cheese… at least, it’s widely believed to be wandering all about, mostly unnoticed. Still it’s not quite as inscrutable as its reputation would lead you to believe. It’s responsible for a galactic glow, an abundance of anti-matter, and now — three quiet little taps in an underground mine.
Or is it?
Apparent effects of dark matter have been “discovered” so many times in the last decade that you may by now feel a bit jaded, or at least dispassionate. Certainly I do. Some day, some year, one of these many hints may turn out to be the real thing. But of the current hints? We’ve got at least six, and they can’t all be real, because they’re not consistent with one other. It’s certain that several of them are false alarms; and once you open that door a crack, you have to consider flinging it wide open, asking: why can’t “several” be “all six”? All of the dark-matter search experiments are difficult, as they involve pushing the technological envelope. And as anyone with experience in science knows, most of the exciting-sounding results emerging from forefront experiments don’t survive the test of time. Never underestimate the challenge of science at the frontier of knowledge!
Still, as of two weeks ago, we have a new dark matter hint to talk about. So here’s a summary of the various hints, including the new one, exploring their implications and their consistency.
Today I’m attending the first day of a short workshop of particle theorists and experimentalists at the Princeton Center for Theoretical Science, a sort of “Where are we now and where are we going?” meeting. It’s entitled “Higgs Physics After Discovery”, but discussion will surely range more widely.
What, indeed, are the big questions facing particle physics in the short-term, meaning the next few months? Well, here are a few key ones:
- A Higgs particle of some type has been discovered by the ATLAS and CMS experiments at the Large Hadron Collider [LHC] (with some contributions from the Tevatron experiments DZero and CDF); is it the simplest possible type of Higgs particle (the “Standard Model Higgs“) or is it more complex? What data analysis can be done on the LHC’s data from 2011-2012 to shed more light on this question?
- More generally, from the LHC’s huge data set from 2011-2012 — specifically, from the data analysis that has been done so far — what precisely have we learned? (It’s increasingly important to go beyond the rougher estimates that were appropriate last year when the data was still pouring in.) What types of new phenomena have been excluded, and to what extent?
- What other types of data analysis should be done on the 2011-2012 data, in order to look for other new phenomena that could still be lurking there? (There’s still a lot to be done on this question!) And what types of work should theoretical particle physicists do to help the experimentalists address this issue?
- Several experiments from the Tevatron and the LHC, notably the LHCb experiment, have learned that newly measured decays of certain mesons (hadrons with equal numbers of quarks and anti-quarks) that contain heavy quarks are roughly consistent with the Standard Model (the equations we use to describe the known elementary particles and forces, and a simplest type of Higgs field and Higgs particle.) How do these findings constrain the possibility of other new phenomena?
- Looking ahead to 2015, when the LHC will begin running again at a higher energy per proton-proton collision, what preparations need to be made? Especially, what needs to be done to refine the triggering systems at ATLAS, CMS and LHCb, so that the maximum information can be extracted from the new data, and no important information is unnecessarily discarded?
- Which, if any, of the multiple (but mostly mutually inconsistent) experimental hints of dark matter should be taken seriously? Which possibilities do the various dark matter experiments, and the LHC’s data, actually exclude or favor?
That might be it for the very near term. There are lots of other questions in the medium- to long-term, among which is the big question of what types of experiments should be done over the next 10 – 20 years. One challenge is that the LHC’s data hasn’t yet given us a clear target other than the Higgs particle itself. An obvious possible experiment to do is to study the Higgs in more detail, using an electron/anti-electron collider — historically this has been a successful strategy that has been used on almost every new apparently-elementary particle. But there are a lot of other possibilities, including raising the LHC’s collisions to even higher energy than we’ll see in 2015, using more powerful magnets currently under development.
If there are other near-term questions I’ve forgotten about, I’m sure I’ll be reminded at the workshop, and I’ll add them in.
My Structure of Matter series has been on hold for a bit, as I have been debating how to describe protons and neutrons. These constituents of atomic nuclei, which, when combined with electrons, form atoms, are drawn in most cartoons of atoms as simple spheres. But not only are they much, much smaller than they are drawn in those cartoons, they hide within them a surprising commotion, one that cannot be anticipated from the relatively simple structures of atoms and of nuclei.
As I’ve described in my new article, along the lines of this short article and this more detailed one that I wrote some time ago in the context of the Large Hadron Collider, the story that scientists tell the public most often, that “a proton is made from two up quarks and a down quark”, is not in fact the full story — and in some ways it is deeply misleading. The structure of protons and neutrons is so entirely unfamiliar, and so complicated, that scientists neither have a simple way of calculating it, nor an entirely agreed-upon way to describe it to the public, or even to physics students. But I believe my way of describing it will be satisfactory to most particle physicists.
The new article is not entirely complete; it is perhaps only half its final length. I’ll be adding some further sections that cover some subtle issues. But since I suspect many people won’t feel the need to read those later sections, the completed part is written to stand on its own. If you like, take a look and let me know if you have questions, suggestions or corrections.