Tag Archives: atlas

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.

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

At the Naturalness 2014 Conference

Greetings from the last day of the conference “Naturalness 2014“, where theorists and experimentalists involved with the Large Hadron Collider [LHC] are discussing one of the most widely-discussed questions in high-energy physics: are the laws of nature in our universe “natural” (= “generic”), and if not, why not? It’s so widely discussed that one of my concerns coming in to the conference was whether anyone would have anything new to say that hadn’t already been said many times.

What makes the Standard Model’s equations (which are the equations governing the known particles, including the simplest possible Higgs particle) so “unnatural” (i.e. “non-generic”) is that when one combines the Standard Model with, say, Einstein’s gravity equations. or indeed with any other equations involving additional particles and fields, one finds that the parameters in the equations (such as the strength of the electromagnetic force or the interaction of the electron with the Higgs field) must be chosen so that certain effects almost perfectly cancel, to one part in a gazillion* (something like 10³²). If this cancellation fails, the universe described by these equations looks nothing like the one we know. I’ve discussed this non-genericity in some detail here.

*A gazillion, as defined on this website, is a number so big that it even makes particle physicists and cosmologists flinch. [From Old English, gajillion.]

Most theorists who have tried to address the naturalness problem have tried adding new principles, and consequently new particles, to the Standard Model’s equations, so that this extreme cancellation is no longer necessary, or so that the cancellation is automatic, or something to this effect. Their suggestions have included supersymmetry, warped extra dimensions, little Higgs, etc…. but importantly, these examples are only natural if the lightest of the new particles that they predict have masses that are around or below 1 TeV/c², and must therefore be directly observable at the LHC (with a few very interesting exceptions, which I’ll talk about some other time). The details are far too complex to go into here, but the constraints from what was not discovered at LHC in 2011-2012 implies that most of these examples don’t work perfectly. Some partial non-automatic cancellation, not at one part in a gazillion but at one part in 100, seems to be necessary for almost all of the suggestions made up to now.

So what are we to think of this? Continue reading

Day 2 At CERN

Day 2 of my visit to CERN (host laboratory of the Large Hadron Collider [LHC]) was a pretty typical CERN day for me. Here’s a rough sketch of how it panned out:

  • 1000: after a few chores, arrived at CERN by tram. Worked on my ongoing research project #1. Answered an email about my ongoing research project #2.
  • 1100: attended a one hour talk, much of it historical, by Chris Quigg, one of the famous experts on “quarkonium” (atom-like objects made from a quark or anti-quark, generally referring specifically to charm and bottom quarks). Charmonium (charm quark/antiquark atoms) was discovered 40 years ago this week, in two very different experiments.
  • 1200: Started work on the talk that I am giving on the afternoon of Day 3 to some experimentalists who work at ATLAS. [ATLAS and CMS are the two general-purpose experimental detectors at the LHC; they were used to discover the Higgs particle.] It involves some new insights concerning the search for long-lived particles (hypothesized types of new particles that would typically decay only after having traveled a distance of at least a millimeter, and possibly a meter or more, before they decay to other particles.)
  • 1230: Working lunch with an experimentalist from ATLAS and another theorist, mainly discussing triggering, and other related issues, concerning long-lived particles. Learned a lot about the new opportunities that ATLAS will have starting in 2015.
  • 1400: In an extended discussion with two other theorists, got a partial answer to a subtle question that arose in my research project #2.
  • 1415: Sent an email to my collaborators on research project #2.
  • 1430: Back to work on my talk for Day 3. Reading some relevant papers, drawing some illustrations, etc.
  • 1600: Two-hour conversation over coffee with an experimentalist from CMS, yet again about triggering, regarding long-lived particles, exotic decays of the Higgs particle, and both at once. Learned a lot of important things about CMS’s plans for the near-term and medium-term future, as well as some of the subtle issues with collecting and analyzing data that are likely to arise in 2015, when the LHC begins running again.

[Why triggering, triggering, triggering? Because if you don’t collect the data in the first place, you can’t analyze it later!  We have to be working on triggering in 2014-2015 before the LHC takes data again in 2015-2018]

  • 1800: An hour to work on the talk again.
  • 1915: Skype conversation with two of my collaborators in research project #1, about a difficult challenge which had been troubling me for over a week. Subtle theoretical issues and heavy duty discussion, but worth it in the end; most of the issues look like they may be resolvable.
  • 2100: Noticed the time and that I hadn’t eaten dinner yet. Went to the CERN cafeteria and ate dinner while answering emails.
  • 2130: More work on the talk for Day 3.
  • 2230: Left CERN. Wrote blog post on the tram to the hotel.
  • 2300: Went back to work in my hotel room.

Day 1 was similarly busy and informative, but had the added feature that I hadn’t slept since the previous day. (I never seem to sleep on overnight flights.) Day 3 is likely to be as busy as Day 2. I’ll be leaving Geneva before dawn on Day 4, heading to a conference.

It’s a hectic schedule, but I’m learning many things!  And if I can help make these huge and crucial experiments more powerful, and give my colleagues a greater chance of a discovery and a reduced chance of missing one, it will all be worth it.

Off to CERN

After a couple of months of hard work on grant writing, career plans and scientific research, I’ve made it back to my blogging keyboard.  I’m on my way to Switzerland for a couple of weeks in Europe, spending much of the time at the CERN laboratory. CERN, of course, is the host of the Large Hadron Collider [LHC], where the Higgs particle was discovered in 2012. I’ll be consulting with my experimentalist and theorist colleagues there… I have many questions for them. And I hope they’ll have many questions for me too, both ones I can answer and others that will force me to go off and think for a while.

You may recall that the LHC was turned off (as planned) in early 2013 for repairs and an upgrade. Run 2 of the LHC will start next year, with protons colliding at an energy of around 13 TeV per collision. This is larger than in Run 1, which saw 7 TeV per collision in 2011 and 8 TeV in 2012.  This increases the probability that a proton-proton collision will make a Higgs particle, which has a mass of 125 GeV/c², by about a factor of 2 ½.  (Don’t try to figure that out in your head; the calculation requires detailed knowledge of what’s inside a proton.) The number of proton-proton collisions per second will also be larger in Run 2 than in Run 1, though not immediately. In fact I would not be surprised if 2015 is mostly spent addressing unexpected challenges. But Run 1 was a classic: a small pilot run in 2010 led to rapid advances in 2011 and performance beyond expectations in 2012. It’s quite common for these machines to underperform at first, because of unforeseen issues, and outperform in the long run, as those issues are solved and human ingenuity has time to play a role. All of which is merely to say that I would view any really useful results in 2015 as a bonus; my focus is on 2016-2018.

Isn’t it a bit early to be thinking about 2016? No, now is the time to be thinking about 2016 triggering challenges for certain types of difficult-to-observe phenomena. These include exotic, unexpected decays of the Higgs particle, or other hard-to-observe types of Higgs particles that might exist and be lurking in the LHC’s data, or rare decays of the W and Z particle, and more generally, anything that involves a particle whose (rest) mass is in the 100 GeV/c² range, and whose mass-energy is therefore less than a percent of the overall proton-proton collision energy. The higher the collision energy grows, the harder it becomes to study relatively low-energy processes, even though we make more of them. To be able to examine them thoroughly and potentially discover something out of place — something that could reveal a secret worth even more than the Higgs particle itself — we have to become more and more clever, open-minded and vigilant.

Some Higgs News from the LHCP Conference

Some news on the Higgs particle from the ATLAS and CMS experiments, the two general purpose experiments at the Large Hadron Collider. I just mention a few highlights. Continue reading