Of Particular Significance

Author: Matt Strassler

Looking for Signs of Dark Matter at the Milky Way’s Center

There is going to be some amount of debate regarding dark matter in the next few weeks, so I’ve written an article on one of the best ways to go looking for new signs of dark matter out in space.

The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects — for instance, its effect on the motions of stars within galaxies, its ability to bend light a la Einstein, etc. It’s almost certain that most of a galaxy is dark matter. And over the years we’ve convinced ourselves this dark matter almost certainly can’t be made from any type of particle that we already know about.

But to learn more about what it is, we need to find signs of some of its non-gravitational effects, if it has any. One possibility is that dark matter particles, if and when they collide, might annihilate into ordinary known particles. If those known particles are photons, we might be able to detect them. A good way to look for them would be to point a suitable telescope toward the center of the Milky Way, our galaxy, which is one place where we expect dark matter particles to be especially numerous, and collisions among them to be especially common.

In the article I just finished, I explain how this can be done. One goes looking for photons from the galactic center, makes a plot of the number of photons observed at a particular energy, and looks for a bump in the plot — an exceptional number of photons with the same energy.

And the reason I’m doing this now is that there is a new paper claiming that a signal of this type may have been seen (with a claimed significance of 3.3 standard deviations, after including the look-elsewhere effect.) This is a paper by a theorist, analyzing publicly available data taken by the experimental group that operates the Fermi Large Area Telescope satellite. One should note that the record of theorists making discoveries using experimentalists’ data is very poor. Typically there are either detector-related or statistics-related issues that theorists screw up. And there are risks of bias — I am not yet sure whether the rather sophisticated analysis method used by this theorist was chosen in a blinded fashion. [For instance, did he choose his method first and then look at the data, or did he already know there was a hint of a peak in the data before he started designing his method?] So I would be skeptical of this claim for now. (And the theorist, knowing he’s out on a limb, was careful [and wise] to put the word “Tentative” in his title.) However, stranger things have happened, so I wouldn’t dismiss this claim out of hand either, at least not until the Fermi experimentalists tell us that in their opinion the theorist over-estimated the statistical significance of this particular bump. We’ll be looking forward to what they have to say.

I’ll have a few more details about this for you soon.

POSTED BY Matt Strassler

ON April 19, 2012

A Visit to Brandeis

Yesterday, I was visiting Brandeis University, where I gave a colloquium on the Large Hadron Collider [LHC]. It was such a gorgeous ~~June~~ April day outside that I felt quite lucky to see a substantial audience, which I think reflects a sense in the wider physics community that the excitement surrounding the LHC no longer reflects merely its future, but also its present.

I also enjoyed a research talk given by a Harvard postdoc, Matt Reece. He had some things to say about which variants of supersymmetry can now largely be excluded by LHC data, both directly from existing searches for superpartner particles, and indirectly from the search for the Standard Model Higgs particle (the simplest possible form that the Higgs particle might take), assuming the hints of a Higgs particle with a mass of 125 GeV/c² turn out to be the real deal. He also made a nice little back-of-the-envelope calculation in his introduction, showing how that the hierarchy (the same one for which we have a hierarchy problem) between the incredible weakness of gravity and the strengths of the other forces is required for there to be large objects (i.e. planets, stars) that are held together by gravity, but prevented from collapse into a black hole by the effect of electromagnetism.

These days, at every place I visit — and Brandeis was no exception — the question of the Higgs particle always comes up. No surprise; it’s the hottest topic in particle physics right now. And the quality of the evidence always gets discussed. Dr. Reece was of course asked his opinion by someone in the room. So I get a chance to hear a lot of viewpoints.

One of the things that I have found puzzling is that almost all of the other particle physics and string theory bloggers not only are of the opinion that the Higgs particle has definitely been found, but also claim that almost all other particle physicists think so too. I honestly just can’t understand how they can say this. I find plenty of theorists who say things like “well, if I didn’t already have a strong theoretical reason to believe the Higgs particle exists, I wouldn’t be very confident in the evidence that is in the current data.” Meanwhile, many senior experimenters regale me with stories of past errors and biases, some of which you find in the history books, and some of which you can’t. All of this is anecdotal; I can’t tell you how opinion is really distributed. But clearly a substantial fraction of the community — maybe a minority, but not a small one — are much less confident than most of the bloggers. It’s not a question of nay-saying — I haven’t heard anyone argue there’s no evidence at all for a Higgs at 125 — but many physicists feel that the evidence is too weak at this stage for any certainty. Of course we all expect the uncertain situation to be resolved in 2012.

One of the other great pleasures of visiting other universities is that I always get to hear about interesting research directions being pursued that I’d not been following. (The theorists at Brandeis were all doing neat stuff that unfortunately would take way too long to describe here. ) And then, as the day comes to a close, I always hear some good stories. My favorite this time was of an ATLAS experimentalist describing how easy it is to get lost while crawling around, installing or fixing things, inside the vast muon system of the ATLAS detector (one of the two general purpose detectors at the LHC.) Can you imagine getting lost inside your own experiment?! 🙂 Well, this one’s the size of an eight-story office building, but hasn’t got hallways, elevators, big EXIT signs, or even an obvious THIS WAY UP. Meanwhile, your GPS device doesn’t work down there either!

POSTED BY Matt Strassler

ON April 18, 2012

The Large Hadron Collider Accelerates

Every time I turn around, the Large Hadron Collider [LHC] is breaking another record. Since 2012 data-taking began April 5th, the news has been improving steadily and rapidly. Over the weekend, the accelerator physicists brought proton-proton collision rates — the total number of collisions per second — back up to the same level as last year’s record, and even a bit beyond. The LHC produced over these two days more than six times as much data as was obtained in all of 2010, and a quarter of what was obtained in the first three months of 2011. Granted, the accelerator physicists won’t keep up this torrid pace every day; there are times (typically weekends and overnights) where the LHC produces lots of data, and other times where the focus is on improving the accelerator and much less data is gathered. But so far the signs are very good indeed. Some experts were unsure whether the move from 7 TeV to 8 TeV energy per collision would be smooth, but fortunately it seems that the accelerator is performing very well at the new energy.

Two differences from last year that are important to know:

The energy per collision is higher: 8 TeV of energy for each proton-proton collision, rather than 7 TeV last year.
The simultaneous collision rate (or “pile-up”) is higher: the current high collision rate per second is being obtained with a smaller number of bunches of protons per beam than at the end of last year, by about 25%, which means the number of simultaneous collisions has gone up by about 25%, to as many as 30 collisions or so happening at virtually the same instant. (Pile-up will be a recurrent theme (and perhaps thorn) in 2012.) But the use of fewer bunches at the present time means that more bunches can be added later, which will allow another increase in the collision rate per second sometime this year (I’m not sure when.)

The challenges ahead for the ATLAS and CMS experiments to handle this firehose of data, especially the pile-up, are not to be minimized. I’ve written about one of the big challenges here; there are others. But better to have to face these difficulties than to be sitting idly, twiddling their thumbs and waiting for data to start showing up! Also, the more data they get in the early part of 2012, the sooner Phase 1 of the search for the Higgs particle will come to a close — and the more likely that July 2012 will see significant news, perhaps fairly convincing news, concerning the hint of a Higgs particle with a mass of about 125 GeV/c².

POSTED BY Matt Strassler

ON April 17, 2012

How Einstein Trumped Newton

Sometimes I encounter people whose impression is that what Einstein’s 1905 theory of special relativity (the one that said no object’s speed can exceed the speed of light in vacuum, etc.) did in “overthrowing” the ideas of the past was somehow like what the Bolsheviks did to the Czars twelve years later– out with the old order, in with the new, and let nothing remain behind. The widespread notion, inherited from philosopher and historian Thomas Kuhn, is that a new paradigm arose, and the old was swept away. The truth is far different. Important parts of the conceptual superstructure of 19th century physics had to be replaced, but the predictive mathematical core was not replaced, but rather was extended. It was like tearing the roof and facade off a building while keeping the interior beams and columns, then extending the structure to make it much larger than before, and finally giving it a very novel external appearance. That’s why the Einsteinian revolution was possible at all! Newton’s equations had already been used to design all sorts of real-world 18th and 19th century technology. If Einstein’s equations hadn’t contained those of Newton and his descendants as a special case, they would have been in conflict with the real world… a no-no for a scientific theory.

Sine this is so important to understand, I’ve written an article illustrating how Einstein’s equations relating energy, momentum, mass and speed were an extension, not a replacement, of the equations that were previously in use. It describes how Einstein unified two separate classes of equations, one set that could be used for massive objects moving slowly compared to the speed of light, and the other for light itself, into a single class of equations, one that not only included the two previous classes but made predictions for massive objects moving at speeds comparable to that of light.

POSTED BY Matt Strassler

ON April 16, 2012

Getting (Un)Confused About Matter and Energy

In the last couple of weeks I produced an article on Mass and Energy (along with a bit on Momentum). But I warned you in that article not to confuse “Mass” (which is somewhat related to Energy, after Einstein’s work) and “Matter” (which is not related to energy, despite the fact that the phrase “matter and energy” can be found all over the place.)

I’ve just finished a new article “Matter and Energy: A False Dichotomy”. This article points out that “matter” is defined differently in different contexts. And in each of those contexts, matter and energy aren’t opposites, partners, or in any other way crisply related. The situation is inherently confusing — but I hope the article itself cuts through some of that confusion and helps clear up the matter.

POSTED BY Matt Strassler

ON April 12, 2012

LHC Producing 8 TeV Data

Still early days in the 2012 data-taking run, which just started a couple of weeks ago, but already the Large Hadron Collider [LHC] accelerator wizards, operating the machine at 8 TeV of energy per proton-proton collision (compared to last year’s 7 TeV) have brought the collision rates back up nearly to where they were last year. This is very good news, in that it indicates there are no significant unexpected technical problems preventing the accelerator from operating at the high collision rates that are required this year. And the experiments are already starting to collect useful data at 8 TeV.

The challenges for the experiments of operating at 8 TeV and at the 2012 high collision rate are significant. One challenge is modeling. To understand how their experiments are working, well enough that they can tell the difference between a new physical phenomenon and a badly understood part of their detector, the experimenters have to run an enormous amount of computer simulation, modeling the beams, the collisions, and the detector itself. Well, 8 TeV isn’t 7 TeV; all of last year’s modeling was fine for last year’s data, but not for this year’s. So a lot of computers are running at full tilt right now, helping to ensure that all of the needed simulations for 8 TeV are finished before they’re needed for the first round of 2012 data analysis that will be taking place in the late spring and early summer.

Another challenge is “pile-up.” The LHC proton beams are not continuous; they consist of up to about 1300 bunches of protons, each bunch containing something like 100,000,000,000 protons. Collisions in each detector occur whenever two bunches pass through each other, every 50 nanoseconds (billionths of a second). With the beam settings that were seen late in 2011 and that will continue to intensify in 2012, every time two bunches cross at the center of the big experiments ATLAS and CMS, an average of 10 to 20 proton-proton collisions occur essentially simultaneously. That means that every proton-proton collision in which something interesting happens is doused in the debris from a dozen uninteresting ones. Moreover, some of the debris from all these collisions hangs around for a while, creating electronic noise that obscures measurements of future collisions. One of the questions for 2012 is how much of a nagging problem the increasing pile-up will pose for some of the more delicate measurements — especially study of Higgs particle decays, both expected ones and exotic ones, and searches for relatively light-weight new particles with low production rates, such as particles created only via the weak nuclear force (e.g. supersymmetric partners of the W, Z and Higgs particles.)

But I have a lot of confidence in my colleagues; barring a really nasty surprise, they’ll manage pretty well, as they did last year. And so far, so good!

POSTED BY Matt Strassler

ON April 11, 2012

A Neutrino Success Story

Almost all the news on neutrinos in the mainstream press this past few months was about the OPERA experiment, and a possible violation of Einstein’s foundational theory of relativity. That the experiment turned out to be wrong didn’t surprise experts. But one of the concerns that scientists have about how this story turned out and was reported in the press is that perhaps many non-experts may get the impression that science is so full of mistakes that you can’t trust it at all. That would be a very unhappy conclusion — not just unhappy but in fact a very dangerous conclusion, at least for anyone who would like to keep their economy strong, their planet well-treated and their nation well-defended.

So it is important to balance the OPERA mini-fiasco with another hot-off-the-presses neutrino story that illustrates why, even though mistakes in individual scientific experiments are common, collective mistakes in science are rare. A discipline such as physics has intrinsic checks and balances that significantly reduce the probability of errors going unrecognized for long. In the story I’m about to relate, one can recognize how and why scientists start to come to consensus. Though quite suspicious of any individual experiment, scientists generally take a different view of a group of experiments that buttress one another.

The context of this story, though much less revolutionary than a violation of Einstein’s speed limit, still represents a milestone in our understanding of neutrinos, which has been advancing very rapidly over the past fifteen years or so. When I was a starting graduate student in the late 1980s, almost all we knew about neutrinos was that there were at least three types and that they were much lighter than electrons, and perhaps massless. Today we know much, much more about neutrinos and how they behave. And in just the last few months and weeks and days, one of the missing entries in the Encyclopedia Neutrinica appears to have been filled in. (more…)

POSTED BY Matt Strassler

ON April 9, 2012

Question to Laypersons: Your Views on the Neutrino Saga

So, many of you have probably been following, to a greater or lesser degree, the story of the OPERA experiment. This is the one that found that neutrinos sent from the CERN lab near Geneva, Switzerland to the Gran Sasso lab in Italy (where OPERA is located) arrived earlier than they expected. Of course there were, from the beginning, two natural explanations:

Einstein was wrong and neutrinos travel faster than light, or
OPERA made a mistake, and their expectations were off.

The news media made a huge deal out of the first possibility, while the vast majority of professional physicists assumed, for various reasons we can discuss, that the second possibility was almost certainly correct. It is now pretty clear that possibility #2 was right; first OPERA admitted it had found two mistakes which made its previous results invalid; then its competitor down the lab, ICARUS, announced it had seen neutrinos arriving just as expected from the same CERN neutrino beam; and finally OPERA itself revealed that it had managed to characterize its errors in detail and now, re-analyzing its data, finds (preliminarily) that neutrinos do in fact arrive as expected.

Now, with this backdrop, I would like to ask YOU a question or two. And by “you”, I mean non-scientists. I would like to know how seeing this episode unfold changed (or did not change) your view of science, or physics, or particle physics. Or of science journalism. What’s your perspective on all of this? What surprised you most? What annoyed you or turned you off or excited you? Are you disappointed in or pleased with the scientific process as you saw it unfold? Are you more suspicious of or less suspicious of scientists and/or of science now that you’ve seen this happen? I think these are things that many scientists would be curious to learn.

Granted, since you’re reading this blog, you’re a member of a non-representative sample of the public. But I still think it would be useful to hear what you have to say. So, please. Comment.

[p.s. As BBC reports today, the LHC now has stable data-quality proton-proton collisions at 8 TeV of energy per collision; data taking will start at slow collision rates and ramp up over the year. Here’s a post and a following article on why 8 TeV is better than last year’s 7 TeV. As usual, BBC says correctly that 2012 will be a crucial year for the search for the Higgs particle, but say incorrectly that this will be the year that the Higgs is found or not found; that statement is true only of the Standard Model Higgs particle, the simplest possible form of Higgs particle. For an overview of what I mean by this, read my guest post at the Cosmic Variance blog.]

POSTED BY Matt Strassler

ON April 5, 2012

Final Installment of the Mass and Energy Article is Up

I’ve now finished the last installment of the Mass and Energy article, whose first two installments were completed last week. If you’ve read the first two, skip down past Figure 5 to the section entitled “Where We Are So Far”. The third installment summarizes what’s been explained about energy, momentum and mass, introduces the concept of the mass of a system of two or more objects, and then goes on to illustrate all of the issues in a very famous system: a Higgs particle decaying to two photons (particles of light), as viewed by three different observers.

More articles providing a bit of additional side information, and others addressing more subtle questions for which the Mass and Energy article was a prerequisite, are coming soon.

POSTED BY Matt Strassler

ON April 4, 2012

Denouement: How OPERA’s Mystery Was Solved

On Friday I learned, and reported to you, that the OPERA experiment’s investigations into its early-arriving-neutrino anomaly (widely reported as `faster-than-light neutrinos’), performed with help from the nearby LVD experiment, have basically confirmed that a combination of (1) an optical fiber within the main timing system that was incorrectly screwed in, and (2) a timing drift in OPERA’s main synchronizing clock, together caused the observed 60 nanosecond early-arrival time. The fiber provides the main effect, with the clock drift playing a subsidiary role.

However, in Friday’s post I only gave you the main idea of how this was done. I have now finished an article that goes through the OPERA story in detail, to the extent I understand it, from the initial discovery and diagnosis of the two problems through the scientific investigation that demonstrated that the two problems probably caused all of the effect that OPERA observed. On the one hand, the solution of the mystery is a classic scientific detective story, instructive and interesting, and for its rather convincing and successful conclusion, the OPERA team deserves applause. On the other hand, it leaves one wondering if this whole episode could have been avoided; why didn’t some of these investigations, which don’t seem exceptionally subtle, happen before OPERA announced its results?

Be that as it may, preliminarily (which means unofficially in this context — OPERA still has more work to do before they can announce a result officially) the revised result from OPERA-2 (the short-pulse version of OPERA) agrees with Einstein’s prediction that neutrinos at these high energies should travel at a speed unmeasurably close to the speed of light. And thus it agrees with the ICARUS experiment’s recent result. So we now have two preliminary confirmations that the neutrinos coming to the Gran Sasso lab from CERN obey Einstein’s speed limit.

POSTED BY Matt Strassler

ON April 2, 2012

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