Science Festival About to Start in Cambridge, MA

It’s a busy time here in Cambridge, Massachusetts, as the US’s oldest urban Science Festival opens tomorrow for its 2015 edition.  It has been 100 years since Einstein wrote his equations for gravity, known as his Theory of General Relativity, and so this year a significant part of the festival involves Celebrating Einstein.  The festival kicks off tomorrow with a panel discussion of Einstein and his legacy near Harvard University — and I hope some of you can go!   Here are more details:


First Parish in Cambridge, 1446 Massachusetts Avenue, Harvard Square, Cambridge
Friday, April 17; 7:30pm-9:30pm

Officially kicking off the Cambridge Science Festival, four influential physicists will sit down to discuss how Einstein’s work shaped the world we live in today and where his influence will continue to push the frontiers of science in the future!

Our esteemed panelists include:
Lisa Randall | Professor of Physics, Harvard University
Priyamvada Natarajan | Professor of Astronomy & Physics, Yale University
Clifford Will | Professor of Physics, University of Florida
Peter Galison | Professor of History of Science, Harvard University
David Kaiser | Professor of the History of Science, MIT

Cost: $10 per person, $5 per student, Tickets available now at

More on Dark Matter and the Large Hadron Collider

As promised in my last post, I’ve now written the answer to the second of the three questions I posed about how the Large Hadron Collider [LHC] can search for dark matter.  You can read the answers to the first two questions here. The first question was about how scientists can possibly look for something that passes through a detector without leaving any trace!  The second question is how scientists can tell the difference between ordinary production of neutrinos — which also leave no trace — and production of something else. [The answer to the third question — how one could determine this “something else” really is what makes up dark matter — will be added to the article later this week.]

In the meantime, after Monday’s post, I got a number of interesting questions about dark matter, why most experts are confident it exists, etc.  There are many reasons to be confident; it’s not just one argument, but a set of interlocking arguments.  One of the most powerful comes from simulations of the universe’s history.  These simulations

  • start with what we think we know about the early universe from the cosmic microwave background [CMB], including the amount of ordinary and dark matter inferred from the CMB (assuming Einstein’s gravity theory is right), and also including the degree of non-uniformity of the local temperature and density;
  • and use equations for known physics, including Einstein’s gravity, the behavior of gas and dust when compressed and heated, the effects of various forms of electromagnetic radiation on matter, etc.

The output of the these simulations is a prediction for the universe today — and indeed, it roughly has the properties of the one we inhabit.

Here’s a video from the Illustris collaboration, which has done the most detailed simulation of the universe so far.  Note the age of the universe listed at the bottom as the video proceeds.  On the left side of the video you see dark matter.  It quickly clumps under the force of gravity, forming a wispy, filamentary structure with dense knots, which then becomes rather stable; moderately dense regions are blue, highly dense regions are pink.  On the right side is shown gas.  You see that after the dark matter structure begins to form, that structure attracts gas, also through gravity, which then forms galaxies (blue knots) around the dense knots of dark matter.  The galaxies then form black holes with energetic disks and jets, and stars, many of which explode.   These much more complicated astrophysical effects blow clouds of heated gas (red) into intergalactic space.

Meanwhile, the distribution of galaxies in the real universe, as measured by astronomers, is illustrated in this video from the Sloan Digital Sky Survey.   You can see by eye that the galaxies in our universe show a filamentary structure, with big nearly-empty spaces, and loose strings of galaxies ending in big clusters.  That’s consistent with what is seen in the Illustris simulation.

Now if you’d like to drop the dark matter idea, the question you have to ask is this: could the simulations still give a universe similar to ours if you took dark matter out and instead modified Einstein’s gravity somehow?  [Usually this type of change goes under the name of MOND.]

In the simulation, gravity causes the dark matter, which is “cold” (cosmo-speak for “made from objects traveling much slower than light speed”), to form filamentary structures that then serve as the seeds for gas to clump and form galaxies.  So if you want to take the dark matter out, and instead change gravity to explain other features that are normally explained by dark matter, you have a challenge.   You are in danger of not creating the filamentary structure seen in our universe.  Somehow your change in the equations for gravity has to cause the gas to form galaxies along filaments, and do so in the time allotted.  Otherwise it won’t lead to the type of universe that we actually live in.

Challenging, yes.  Challenging is not the same as impossible. But everyone one should understand that the arguments in favor of dark matter are by no means limited to the questions of how stars move in galaxies and how galaxies move in galaxy clusters.  Any implementation of MOND has to explain a lot of other things that, in most experts’ eyes, are efficiently taken care of by cold dark matter.

Dark Matter: How Could the Large Hadron Collider Discover It?

Dark Matter. Its existence is still not 100% certain, but if it exists, it is exceedingly dark, both in the usual sense — it doesn’t emit light or reflect light or scatter light — and in a more general sense — it doesn’t interact much, in any way, with ordinary stuff, like tables or floors or planets or  humans. So not only is it invisible (air is too, after all, so that’s not so remarkable), it’s actually extremely difficult to detect, even with the best scientific instruments. How difficult? We don’t even know, but certainly more difficult than neutrinos, the most elusive of the known particles. The only way we’ve been able to detect dark matter so far is through the pull it exerts via gravity, which is big only because there’s so much dark matter out there, and because it has slow but inexorable and remarkable effects on things that we can see, such as stars, interstellar gas, and even light itself.

About a week ago, the mainstream press was reporting, inaccurately, that the leading aim of the Large Hadron Collider [LHC], after its two-year upgrade, is to discover dark matter. [By the way, on Friday the LHC operators made the first beams with energy-per-proton of 6.5 TeV, a new record and a major milestone in the LHC’s restart.]  There are many problems with such a statement, as I commented in my last post, but let’s leave all that aside today… because it is true that the LHC can look for dark matter.   How?

When people suggest that the LHC can discover dark matter, they are implicitly assuming

  • that dark matter exists (very likely, but perhaps still with some loopholes),
  • that dark matter is made from particles (which isn’t established yet) and
  • that dark matter particles can be commonly produced by the LHC’s proton-proton collisions (which need not be the case).

You can question these assumptions, but let’s accept them for now.  The question for today is this: since dark matter barely interacts with ordinary matter, how can scientists at an LHC experiment like ATLAS or CMS, which is made from ordinary matter of course, have any hope of figuring out that they’ve made dark matter particles?  What would have to happen before we could see a BBC or New York Times headline that reads, “Large Hadron Collider Scientists Claim Discovery of Dark Matter”?

Well, to address this issue, I’m writing an article in three stages. Each stage answers one of the following questions:

  1. How can scientists working at ATLAS or CMS be confident that an LHC proton-proton collision has produced an undetected particle — whether this be simply a neutrino or something unfamiliar?
  2. How can ATLAS or CMS scientists tell whether they are making something new and Nobel-Prizeworthy, such as dark matter particles, as opposed to making neutrinos, which they do every day, many times a second?
  3. How can we be sure, if ATLAS or CMS discovers they are making undetected particles through a new and unknown process, that they are actually making dark matter particles?

My answer to the first question is finished; you can read it now if you like.  The second and third answers will be posted later during the week.

But if you’re impatient, here are highly compressed versions of the answers, in a form which is accurate, but admittedly not very clear or precise.

  1. Dark matter particles, like neutrinos, would not be observed directly. Instead their presence would be indirectly inferred, by observing the behavior of other particles that are produced alongside them.
  2. It is impossible to directly distinguish dark matter particles from neutrinos or from any other new, equally undetectable particle. But the equations used to describe the known elementary particles (the “Standard Model”) predict how often neutrinos are produced at the LHC. If the number of neutrino-like objects is larger that the predictions, that will mean something new is being produced.
  3. To confirm that dark matter is made from LHC’s new undetectable particles will require many steps and possibly many decades. Detailed study of LHC data can allow properties of the new particles to be inferred. Then, if other types of experiments (e.g. LUX or COGENT or Fermi) detect dark matter itself, they can check whether it shares the same properties as LHC’s new particles. Only then can we know if LHC discovered dark matter.

I realize these brief answers are cryptic at best, so if you want to learn more, please check out my new article.

The LHC restarts — in a manner of speaking —

As many of you will have already read, the Large Hadron Collider [LHC], located at the CERN laboratory in Geneva, Switzerland, has “restarted”. Well, a restart of such a machine, after two years of upgrades, is not a simple matter, and perhaps we should say that the LHC has “begun to restart”. The process of bringing the machine up to speed begins with one weak beam of protons at a time — with no collisions, and with energy per proton at less than 15% of where the beams were back in 2012. That’s all that has happened so far.

If that all checks out, then the LHC operators will start trying to accelerate a beam to higher energy — eventually to record energy, 40% more than in 2012, when the LHC last was operating.  This is the real test of the upgrade; the thousands of magnets all have to work perfectly. If that all checks out, then two beams will be put in at the same time, one going clockwise and the other counterclockwise. Only then, if that all works, will the beams be made to collide — and the first few collisions of protons will result. After that, the number of collisions per second will increase, gradually. If everything continues to work, we could see the number of collisions become large enough — approaching 1 billion per second — to be scientifically interesting within a couple of months. I would not expect important scientific results before late summer, at the earliest.

This isn’t to say that the current milestone isn’t important. There could easily have been (and there almost were) magnet problems that could have delayed this event by a couple of months. But delays could also occur over the coming weeks… so let’s not expect too much in 2015. Still, the good news is that once the machine gets rolling, be it in May, June, July or beyond, we have three to four years of data ahead of us, which will offer us many new opportunities for discoveries, anticipated and otherwise.

One thing I find interesting and odd is that many of the news articles reported that finding dark matter is the main goal of the newly upgraded LHC. If this is truly the case, then I, and most theoretical physicists I know, didn’t get the memo. After all,

  • dark matter could easily be of a form that the LHC cannot produce, (for example, axions, or particles that interact only gravitationally, or non-particle-like objects)
  • and even if the LHC finds signs of something that behaves like dark matter (i.e. something that, like neutrinos, cannot be directly detected by LHC’s experiments), it will be impossible for the LHC to prove that it actually is dark matter.  Proof will require input from other experiments, and could take decades to obtain.

What’s my own understanding of LHC’s current purpose? Well, based on 25 years of particle physics research and ten years working almost full time on LHC physics, I would say (and I do say, in my public talks) that the coming several-year run of the LHC is for the purpose of

  1. studying the newly discovered Higgs particle in great detail, checking its properties very carefully against the predictions of the “Standard Model” (the equations that describe the known apparently-elementary particles and forces)  to see whether our current understanding of the Higgs field is complete and correct, and
  2. trying to find particles or other phenomena that might resolve the naturalness puzzle of the Standard Model, a puzzle which makes many particle physicists suspicious that we are missing an important part of the story, and
  3. seeking either dark matter particles or particles that may be shown someday to be “associated” with dark matter.

Finding dark matter itself is a worthy goal, but the LHC may simply not be the right machine for the job, and certainly can’t do the job alone.

Why the discrepancy between these two views of LHC’s purpose? One possibility is that since everybody has heard of dark matter, the goal of finding it is easier for scientists to explain to journalists, even though it’s not central.  And in turn, it is easier for journalists to explain this goal to readers who don’t care to know the real situation.  By the time the story goes to press, all the modifiers and nuances uttered by the scientists are gone, and all that remains is “LHC looking for dark matter”.  Well, stay tuned to this blog, and you’ll get a much more accurate story.

Fortunately a much more balanced story did appear in the BBC, due to Pallab Ghosh…, though as usual in Europe, with rather too much supersymmetry and not enough of other approaches to the naturalness problem.   Ghosh also does mention what I described in the italicized part of point 3 above — the possibility of what he calls the “wonderfully evocatively named `dark sector’ ”.  [Mr. Ghosh: back in 2006, well before these ideas were popular, Kathryn Zurek and I named this a “hidden valley”, potentially relevant either for dark matter or the naturalness problem. We like to think this is a much more evocative name.]  A dark sector/hidden valley would involve several types of particles that interact with one another, but interact hardly at all with anything that we and our surroundings are made from.  Typically, one of these types of particles could make up dark matter, but the others would unsuitable for making dark matter.  So why are these others important?  Because if they are produced at the LHC, they may decay in a fashion that is easy to observe — easier than dark matter itself, which simply exits the LHC experiments without a trace, and can only be inferred from something recoiling against it.   In other words, if such a dark sector [or more generally, a hidden valley of any type] exists, the best targets for LHC’s experiments (and other experiments, such as APEX or SHiP) are often not the stable particles that could form dark matter but their unstable friends and associates.

But this will all be irrelevant if the collider doesn’t work, so… first things first.  Let’s all wish the accelerator physicists success as they gradually bring the newly powerful LHC back into full operation, at a record energy per collision and eventually a record collision rate.

How Evidence for Cosmic Inflation Was Reduced to Dust

Many of you will have read in the last week that unfortunately (though to no one’s surprise after seeing the data from the Planck satellite in the last few months) the BICEP2 experiment’s claim of a discovery of gravitational waves from cosmic inflation has blown away in the interstellar wind. [For my previous posts on BICEP2, including a great deal of background information, click here.] The BICEP2 scientists and the Planck satellite scientists have worked together to come to this conclusion, and written a joint paper on the subject.  Their conclusion is that the potentially exciting effect that BICEP2 observed (“B-mode polarization of the cosmic microwave background on large scales”; these terms are explained here) was due, completely or in large part, to polarized dust in our galaxy (the Milky Way). The story of how they came to this conclusion is interesting, and my goal today is to explain it to non-experts.  Click here to read more.

Giving Public Talk Jan. 20th in Cambridge, MA

Hope all of you had a good holiday and a good start to the New Year!

I myself continue to be extraordinarily busy as we move into 2015, but I am glad to say that some of that activity involves communicating science to the public.  In fact, a week from today I will be giving a public talk — really a short talk and a longer question/answer period — in Cambridge, just outside of Boston and not far from MIT. This event is a part of the monthly “CafeSci” series, which is affiliated with the famous NOVA science television programs produced for decades by public TV/Radio station WGBH in Boston.

Note for those of you have gone before to CafeSci events: it will be in a new venue, not far from Kendall Square. Here’s the announcement:

Tuesday, January 20th at 7pm (about 1 hour long)
Le Laboratoire Cambridge (NEW LOCATION)
650 East Kendall St, Cambridge, MA

“The Large Hadron Collider Restarts Soon! What Lies Ahead?”

Speaker: Matthew Strassler

“After a long nap, the Large Hadron Collider [LHC], where the Higgs particle was discovered in 2012, will begin operating again in 2015, with more powerful collisions than before. Now that we know Higgs particles exist, what do we want to know about them? What methods can we use to answer our questions? And what is the most important puzzle that we are hoping the LHC will help us solve?”

Public Transit: Red line to Kendall Square, walk straight down 3rd Street, turn right onto Athenaeum Street, and left onto East Kendall

Parking: There is a parking deck – the 650 East Kendall Street Garage – accessible by Linskey Way.

How a Trigger Can Potentially Make or Break an LHC Discovery

Triggering is an essential part of the Large Hadron Collider [LHC]; there are so many collisions happening each second at the LHC, compared to the number that the experiments can afford to store for later study, that the data about most of the collisions (99.999%) have to be thrown away immediately, completely and permanently within a second after the collisions occur.  The automated filter, partly hardware and partly software, that is programmed to make the decision as to what to keep and what to discard is called “the trigger”.  This all sounds crazy, but it’s necessary, and it works.   Usually.

Let me give you one very simple example of how things can go wrong, and how the ATLAS and CMS experiments [the two general purpose experiments at the LHC] attempted to address the problem.  Before you read this, you may want to read my last post, which gives an overview of what I’ll be talking about in this one.

Click here to read the rest of the article…

Final Days of Busy Visit to CERN

I’m a few days behind (thanks to an NSF grant proposal that had to be finished last week) but I wanted to write a bit more about my visit to CERN, which concluded Nov. 21st in a whirlwind of activity. I was working full tilt on timely issues related to Run 2 of the Large Hadron Collider [LHC], currently scheduled to start early next May.   (You may recall the LHC has been shut down for repairs and upgrades since the end of 2012.)

A certain fraction of my time for the last decade has been taken up by concerns about the LHC experiments’ ability to observe new long-lived particles, specifically ones that aren’t affected by the electromagnetic or strong nuclear forces. (Long-lived particles that are affected by those forces are easier to search for, and are much more constrained by the LHC experiments.  More about them some other time.)

This subject is important to me because it is a classic example of how the trigger systems at LHC experiments could fail us — whereby a spectacular signal of a new phenomena could be discarded and lost in the very process of taking and storing the data! If no one thinks carefully about the challenges of finding long-lived particles in advance of running the LHC, we can end up losing a huge opportunity, unnecessarily. Fortunately some of us are thinking about it, but we are small in number. It is an uphill battle for those experimenters within ATLAS and CMS [the two general purpose experiments at the LHC] who are working hard to make sure they have the required triggers available. I can’t tell you how many times people within the experiments — even at the Naturalness conference I wrote about recently — have told me “such efforts are hopeless”… despite the fact that their own experiments have actually shown, already in public and in some cases published measurements (including this, this, this, this, this, and this), that it is not. Conversely, many completely practical searches for long-lived particles have not been carried out, often because there was no trigger strategy able to capture them, or because, despite the events having been recorded, no one at ATLAS or CMS has had time or energy to actually search through their data for this signal.

Now what is meant by “long-lived particles”? Continue reading