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.
The Alpha Magnetic Spectrometer [AMS] finally reported its first scientific results today. AMS, a rather large particle physics detector attached to the International Space Station, is designed to study the very high-energy particles found flying around in outer space. These “cosmic rays” (as they are called, for historical reasons) have been under continuous study since their discovery a century ago, but they are still rather mysterious, and we continue to learn new things about them. They are known to be of various different types — commonly found objects such as photons, electrons, neutrinos, protons, and atomic nuclei, and less common ones like positrons (antiparticles of electrons) and anti-protons. They are known to be produced by a variety of different processes. It is quite possible that some of these high-energy particles come from physical or astronomical processes, perhaps very exciting ones, that we have yet to discover. And AMS is one of a number of experiments designed to help us seek signs of these new phenomena.
The plan to build AMS was hatched in 1995, and the detector was finally launched, after various delays, in 2011, on a specially-ordered Space Shuttle mission. Today, Sam Ting, winner of the Nobel Prize for a co-discovery of the charm quark back in 1974, presented AMS’s first results — a first opportunity to justify all the time, effort and money that went into this project. And? The results look very nice, indicating the AMS experiment is working very well. Yet the conclusions from the results so far are not very dramatic, and, in my opinion, have been significantly over-sold in the press. Despite what you may read, we are no closer to finding dark matter than we were last week. Any claims to the contrary are due to scientists spinning their results (and to reporters who are being spun).
A busy week and a computer crash has delayed my report on a number of new results on the Higgs particle from the current Moriond conference on particle physics, but the quiet not only on my blog but on some others should be a clue: the new results shown do not significantly change what we have previously known, and to the extent they do, they do not point to anything unexpected.
The rate for events with two lepton/anti-lepton pairs (data from CMS is the black dots, with uncertainties given by the black bars) as a function of the mass-energy of a particle that might have produced them; the Z particle is the bump around 90 GeV. The bump near 125 GeV due to the Higgs-like particle is now difficult to miss, even if one ignores the blue and red lines which are there to guide the eye.
As a summary before I mention a few details, let me say that all in all, I think it is pretty safe now to award the Nobel prize to the theoretical physicists behind this story; last year was too early, but this year is not. Confidence is steadily growing that this “Higgs-like” particle really is a type of Higgs (Brout-Englert) (Guralnik-Hagen-Kibble) boson [what's a boson?], and most alternatives are now significantly disfavored. Whether it is the one and only type of Higgs particle in nature, and whether it is exactly of Standard Model type (the simplest possible type of Higgs particle), we cannot yet be sure, but its properties are more or less in line with what Higgs and friends proposed, enough to give them credit for having correctly imagined (to greater and lesser degrees) how nature might provide mass to force-carrier particles like the W and Z particles, and how we might test this notion experimentally. We should also remember some theorists who came before them and some who came after, but that’s a story for another day.
As many of you will have already noticed, today’s Science Times section of the New York Times newspaper is devoted to articles by Dennis Overbye on the search for the Higgs particle. At first read, the articles seem pretty good; several key players are interviewed (though inevitably, given page constraints, a number of important players in the experiments are not mentioned) and the science seems mostly accurate, with a few small errors, omissions, or misleading ways of saying things in the glossary and elsewhere. I’m busy preparing a new public talk for tomorrow, so I’ll have to reserve any detailed comments for later in the week.
But one thing you will notice, if you read the long article which describes the ins and outs of the search process, is that several of the responsible scientists quoted indicate, directly or indirectly, that the December 2011 data did not convince them that a Higgs particle had yet been found. That was the position I took on this blog, and I reported to you that most responsible scientists I had spoken to (which didn’t happen to include any of the ones quoted in the Science Times today) viewed the December data as inconclusive — meaning that it was still quite possible that the apparent signal of a Higgs particle might evaporate. Almost every other major particle physics blogger disagreed with me, both on my opinion and on my characterization of others’ opinions. But I stand by my statements: that though the data reported in July 2012 was essentially definitive, the data in December 2011 was, not only from my perspective but from that of many serious scientists, suggestive yet inconclusive. And you can now read that in the New York Times.
Posts have been notably absent, due mainly to travel with very limited internet; apologies for the related lack of replies to comments, which I hope to correct later this week.
Meanwhile I’ve been working on a couple of articles related to the nuclei of atoms, part of my Structure of Matter series, which serves to introduce non-experts to the basics of particle physics. The first of these articles is done. In it I describe why it was so easy (relatively speaking) to figure out that nuclei are made from certain numbers of protons and neutrons, and how it was understood that nuclei are very small compared to atoms. Comments welcome as always!
A related article, which should appear later this week, will clarify why nuclei are so tiny relative to atoms, and describe the force of nature that keeps them intact.