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Nicole Larsen | Yale University | USA

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Anti-Matters: The Latest and Greatest from the Alpha Magnetic Spectrometer

Friday, May 8th, 2015

This past month in Geneva a conference took place bringing together the world’s foremost experiments in cosmic ray physics and indirect dark matter detection: “AMS Days at CERN”. I took a break from thesis-writing, grabbed a bag of popcorn, and sat down to watch a couple of the lectures via webcast. There was a stellar lineup, including but not limited to talks from IceCube, the Pierre Auger Observatory, H.E.S.S. and CTA, Fermi-LAT, and CREAM. The Alpha Magnetic Spectrometer (AMS) experiment was, of course, the star of the show. It is the AMS and its latest results that I’d like to focus on now.

But first, I’d like to give a brief introduction to cosmic rays, since that’s what AMS studies.

It turns out that space is not as empty as one might think. The Earth is constantly being bombarded by extremely-high-energy particles from all directions.  These cosmic rays were discovered in the early twentieth century by the Austrian physicist Victor Hess. Hess made several balloon-borne measurements of the Earth’s natural radiation at various altitudes and observed that the incidence of ionizing radiation actually increased with ascent, the exact opposite of what you would expect if all radioactivity came from the earth.

Fig. 1: An artist's rendition of cosmic rays . Image from http://apod.nasa.gov/apod/ap060814.html.

Fig. 1: An artist’s rendition of cosmic rays . Image from http://apod.nasa.gov.

The word “ray” is actually something of a misnomer – Cosmic rays are primarily charged matter particles rather than electromagnetic radiation. Their makeup goes as follows: approximately 98% are nuclei, of which 90% of are protons, 9% are alpha particles (helium nuclei), and only a small proportion heavier nuclei; and approximately 2% electrons and positrons. Only very small trace amounts (less than one ten-thousandth the number of protons) of antimatter are present, and of this, it is all positrons and antiprotons – not a single antihelium or heavier anti-nucleus has been discovered. There are two types of cosmic rays: primary rays, which come directly from extrasolar sources, and secondary rays, which come from primary rays crashing into the interstellar medium and forming new particles through processes such as nuclear spallation. Particles resulting from cosmic ray collisions with the Earth’s atmosphere are also considered secondary cosmic rays – these include particles like pions, kaons, and muons, and their decay products.

Fig. 2: Cosmic ray flux vs. particle energy.  Image from http://science.nasa.gov/science-news/science-at-nasa/2001/ast15jan_1/

Fig. 2: Cosmic ray flux vs. particle energy. Image from http://science.nasa.gov/science-news/science-at-nasa/2001/ast15jan_1/

Despite being discovered over a hundred years ago, cosmic rays remain in a lot of ways a big mystery. For one thing, we don’t know exactly where they come from. Because cosmic rays are generally electrically charged, they don’t travel to us straight from the source. Rather, they are accelerated this way and that by magnetic fields in space so that when they finally reach us they could be coming from any direction at all. Indeed, the cosmic ray flux that we see is completely isotropic, or the same in all directions.

Not only do they not come straight from the source, but we don’t even know what that source is. These particles move orders of magnitude faster than particles in our most powerful accelerators on Earth. Astronomers’ best guess is that cosmic rays are accelerated by magnetic shocks from supernovae. But even supernovae aren’t enough to accelerate the highest-energy cosmic rays. Moreover, there are features in the cosmic ray energy spectrum that we just don’t understand (see Fig. 2). Two kinks, a “knee” at about 1016 eV and an “ankle” at about 1018 eV could indicate the turning on or off of some astrophysical process. Experiments like the Pierre Auger Observatory were designed to study these ultra-high-energy particles and hopefully will tell up a little bit more about them in the next few years.

The AMS is primarily interested in lower-energy cosmic rays. For four years, ever since its launch up to the International Space Station, it’s been cruising the skies and collecting cosmic rays by the tens of billions. I will not address the experimental design and software here. Instead I refer the reader to one of my previous articles, “Dark Skies II- Indirect Detection and the Quest for the Smoking Gun”.

In addition to precision studies of the composition and flux of cosmic rays, the AMS has three main science goals: (1) Investigating the matter-antimatter asymmetry by searching for primordial antimatter. (2) Searching for dark matter annihilation products amidst the cosmic rays. And (3), looking for strangelets and other exotic forms of matter.

The very small fraction of cosmic rays made up of antimatter is relevant not just for the first goal but for the second as well. Not many processes that we know about can produce positrons and antiprotons, but as I mention in “Dark Skies II”, dark matter annihilations into Standard Model particles could be one of those processes. Any blips or features in the cosmic ray antimatter spectrum could indicate dark matter annihilations at work.

Fig. 3. The positron fraction measured by AMS.  Image from L. Accardo et al. (AMS Collaboration), September 2014.

Fig. 3. The positron fraction measured by AMS. Image from L. Accardo et al. (AMS Collaboration), September 2014.

On April 14 at “AMS Days at CERN”, Professor Andrei Kounine of MIT presented the latest results from AMS.

The first part of Kounine’s talk focused on a precise characterization of the positron fraction presented by the AMS collaboration in September 2014 and a discussion of the relevant systematics. In the absence of new physics processes, we expect the positron fraction to be smooth and decreasing with energy. As you can see in Fig. 3, however, the positron fraction starts rising at approximately 8 GeV and increases steadily up to about 250 GeV. The curve hits a maximum at about 275 GeV and then appears to begin to turn over, although at these energies the measurements are limited by statistics and more data is needed to determine exactly what happens beyond this point. Models of dark matter annihilation predict a much steeper drop-off than do models where the positron excess is produced by, say, pulsars. Five possible sources of systematic error were identified, all of which have been heavily investigated. These included a small asymmetry in positron and electron acceptance due to slight differences in some of the bits of the tracker; variations in efficiency with respect to energy of the incoming particle; binning errors, which are mitigated due to high experimental resolution; low statistics at the tails of the electron and positron distributions; and “charge confusion”, or the misidentification of electrons as positrons, which happens only in a very small number of cases.

Kounine also presented a never-before-seen, not-yet-published measurement of the antiproton-proton ratio as measured by AMS, which you can see in Fig. 4. This curve represents a total of 290,000 antiprotons selected out of total of 54 billion events collected by AMS over the past 4 years. Many of the same systematics (acceptance asymmetry, charge confusion, and so on) as in the positron measurement are relevant here. Work on the antiproton analysis is ongoing, however, and according to Kounine it’s too soon to try to match models to the data.

Fig. 4. AMS’s latest antiproton-proton ratio measurement, from Prof. Andrei Kounine’s presentation at “AMS Days at CERN”.

Fig. 4. AMS’s latest antiproton-proton ratio measurement, from Prof. Andrei Kounine’s presentation at “AMS Days at CERN”.

As a dark matter physicist, the question in my mind is, do these measurements represent dark matter annihilations? Professor Subir Sarkar of Oxford and the Niels Bohr Institute in Copenhagen thinks not. In his talk at “AMS Days”, Sarkar argues that the dark matter annihilation cross-section necessary to match the positron flux seen by AMS and other experiments such as Fermi-LAT and PAMELA needs to be so large that by all rights the dark matter in the universe should have all annihilated away already. This is inconsistent with the observed dark matter density in our galaxy. You can get around this with theoretical models that incorporate new kinds of long-range forces. However, the observed antiproton flux, according to Sarkar, is consistent with background. Therefore dark matter would have to be able to annihilate into leptons (electrons and positrons, muons, neutrinos, and so on) but not quarks. Such models exist, but now we’re starting to severely restrict our model space. Moreover, dark matter annihilating in the early universe near the time of recombination should leave visible imprints in the Cosmic Microwave Background (CMB), which have not yet been seen. CMB experiments such as Planck therefore disfavor a dark matter explanation for the observed peak in positron fraction.

Sarkar then goes on to present an alternate model where secondary cosmic ray particles such as positrons are accelerated by the same mechanisms (magnetic shocks from supernovae, pulsars, and other cosmic accelerators) that accelerate primary cosmic rays. Then, if there are invisible accelerators in our nearby galactic neighborhood, as seems likely because electrons and positrons can’t propagate very far without losing energy due to interactions with starlight and the CMB, it could be possible to get very large fluctuations in the cosmic ray flux due purely to the randomness of how these accelerators are distributed around us.

Regardless of whether or not the AMS has actually seen a dark matter signal, the data are finally beginning to be precise enough that we can start really pinning down how cosmic rays backgrounds are created and propagated. I encourage you to check out some of the webcasts at “AMS Days at CERN” for yourself. Although the event is over the webcasts are still available in the CERN document archive here.

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A New Year’s Resolution: Read More Papers!

Wednesday, January 14th, 2015

Someone once told me that if you read just one paper a week, you will become a world-class expert in your chosen topic after seven years. I’m not sure if this is true or not, but this strikes me as a terribly useful New Year’s resolution, certainly worthy of inclusion alongside getting a gym membership and learning to balance my checkbook!

Whether you are already an expert or a layperson looking to become an expert, the ability to read and digest scientific papers is an excellent skill to add to your repertoire. There are a number of great resources out there. For example, this Violent Metaphors blog post is wonderful in that it gives well-defined, step-by-step instructions for critically reading a primary source article – this is extremely useful if you really want to understand a scientific paper in depth.

Sometimes, though, you just need to read a lot of papers and assimilate a lot of new information in a short amount of time.   I’ve managed to streamline this process and would like to share with you some of my tips and tricks for reading papers as quickly as possible.

Dipping Your Toes

The first part of a paper that you will ever lay eyes upon is its abstract. This is, perhaps, the most crucial part of a paper, because it determines whether or not you should read the rest of the paper. A good abstract is short, to the point, and contains the following five ingredients.

  • – Brief background information
  • – Question/problem statement
  • – Experimental approach
  • – Results
  • – Impacts and implications of the work

I’ve found that the clearest and most easily understood abstracts are often only five sentences long: one sentence per point. Once you’ve identified these five key points, you’ll be able to make a decision as to whether or not the paper is interesting enough and useful enough to continue.

Down the Rabbit Hole

Now that you’ve decided that a certain paper is indeed worth your time, read the introduction first. If you don’t understand the background information contained in the introduction, you won’t understand the rest of the paper. This is not an indictment on your lack of knowledge; even the introduction of a paper will most likely need to be reread several times. In fact, reread the abstract at the same time. The abstract often contains the same material as the introduction, albeit in a more condensed form, and the more ways that you are exposed to the same information, the more likely it is to sink in.

While you are reading, take copious notes – even if you never read those notes again, the amount of processing that your brain has to do to in order regurgitate information back onto paper is often enough to make an idea “stick”.   In fact, my office is littered with filled-up legal pads that I will never read again for this very reason. First and foremost, while you are looking through the introduction, try to highlight and write down the background, the problem statement, the approach, the result, and the implication of the work, even if you have already identified these five points in the abstract. If you can identify these, you’re well on your way to understanding the rest of the paper. You should also write down any unfamiliar terms or jargon so that you can look them up later.

Reading a Paper is Never Just Reading ‘a’ Paper

As you read, I suggest you have available two copies of the paper in question. Chances are, this paper will cite other papers, so keep one copy open to the ‘References’ section as you read – that way you can quickly look up other papers as they are cited in the text. Reading a paper rarely if ever consists of reading only ONE paper. As you are reading, you may (actually, will) want to keep some of these new references open beside you as well.

As you begin to check out other papers, you’ll start to discover some patterns: there are generally a few key works in the field that almost everything else refers to and these are the papers that are crucial to read (or at the very least to identify so that you can skim them later). Usually there will be a couple of big review articles in the field, and a paper describing a big experimental result will often refer back to a design paper – these types of papers can be helpful to skim. And of course, knowing which papers to read will also help you figure out which papers to skip over, which is a crucial part of extracting information efficiently.

Spoiler Alert

Once you are reasonably comfortable with the introduction, skip to the conclusion. Most scientific papers have an hourglass shape. An introduction typically starts off broad in scope and hones in on a specific problem statement; a conclusion on the other hand will usually start with a concise, focused summary of the results and then zoom out to provide some broader context for the work and/or future prospects for the experiment in question. As you read the conclusion, first identify where the authors have summarized the highlights of the paper. You should now know exactly what the paper is about. Pay attention, too, if the conclusion discusses any further work to be done, especially if the paper is a couple years old. You might be able to find an updated version or more recent result.

If it’s difficult to understand the highlights of the paper as stated in the conclusion, there are a couple of things you can try. You can flip back and reread the problem statement from the introduction to see how it matches up with the conclusion.  You can do a literature search for similar works or review articles that might state the same thing but in a way that is easier to understand. Or, you can go through the results in more detail in the body of the paper.

The Meat in the Sandwich: Methods, Data, Analysis, Results

After reading the abstract, the introduction, and the conclusion, you should have the big idea down fairly well. If you still want to dig into the meat of a paper, start with the results. Results are typically self-contained with no references to other papers. Methods sections can be very technical and may or may not contain references to other papers, so consider yourself forewarned. If you absolutely have to dive into the main body of a paper, I recommend looking at the figures and tables in the results section first. A picture really is worth a thousand words. And don’t neglect figure captions – these can be very informative.

***

There you have it. I’ve found this outside-in method of reading papers to be as effective as it is quick. Hopefully this was helpful, and I wish you the best of luck in your next paper-reading endeavor!

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Dark Skies II: Indirect Detection and The Quest for the Smoking Gun

Thursday, September 25th, 2014

Dark matter is a tough thing to study. There is no getting around it: any strategy we can come up with to look for these invisible mystery particles must hinge on the sneaky little creatures interacting in some way with ordinary Standard Model particles. Otherwise we haven’t got even the slightest chance of seeing them.

One of the most popular classes of dark matter candidates is the Weakly Interacting Massive Particles (WIMPs), so called because they do not interact electromagnetically, only weakly, with ordinary matter.  In direct detection we look for WIMPs that interact by scattering off of a Standard Model particle. In contrast, indirect detection looks for interactions that consist of a dark matter particle (either a WIMP or a non-WIMP — it doesn’t matter)  annihilating with another dark matter particle or decaying on its own into Standard Model particles.  These Standard Model end products we have a good chance of detecting if we can just get our backgrounds low enough. In my last post, “Dark Skies: A Very Brief Guide to Indirect Detection,” I gave a more detailed look at the kinds of annihilation and decay products that we might expect from such a process and spoke briefly about some of the considerations that must go into a search for particles from these annihilation and decay processes. Today I will highlight three of the indirect detection experiments currently attacking the dark matter problem.

***

AMS-02

The Alpha Magnetic Spectrometer (AMS-02) is a large indirect detection experiment situated on the International Space Station. I am especially excited to talk to you about this experiment because just a couple of days ago AMS-02 released a very interesting result. Although I include a link to the press release and the relevant papers below, I intend to give away the punchline by summarizing here everything I know about the AMS-02 experiment and their result from this past week.

(Left) A 3D rendering of the AMS-02 detector, from the AMS-02 group website, ams02.org

Fig. 1: (Left) A 3D rendering of the AMS-02 detector, from the AMS-02 group website located at www.ams02.org.  (Right) A schematic of the AMS-02 detector showing all of its various subsystems [1].

First, let’s talk about the design of the experiment (Fig. 1). As the infamous Shrek once said, ogres are like onions. Well, most big particle physics experiments are like onions too. They consist of many layers of detectors interspersed with different kinds of shielding – the quintessential example being big collider experiments like ATLAS and CMS at the Large Hadron Collider. AMS-02 has just as many layers and almost as much complexity to it as something like ATLAS. In no particular order, these layers are:

  • A donut-shaped superconducting magnet surrounding most of the AMS-02 detector. Any particles traversing through the hole in the donut will be deflected by the magnet, and particles of different charges are deflected in different directions. The magnet therefore is an effective way to separate positrons from electrons, positive ions from negative ones, and antimatter from ordinary matter.
  • Veto or anticoincidence counters (ACCs) that lie just inside the superconducting magnet. The ACCs tag any particle that enters the detector through the side rather than the hole in the magnet thus allowing the AMS-02 to reject particle events that do not have well-understood trajectories.
  • A Transition Radiation Detector (TRD) consisting of twenty layers of material (that provides discrimination between extremely high-energy leptons (positrons, electrons) and hadrons (protons, etc) that are traveling near the speed of light. Each time an electron or positron passes through the TRD, it produces a shower of x-rays as it crosses the interface between layers, but a proton or other hadron does not.
  • The Time of Flight (ToF) system, which consists of 4 layers of scintillation counters, two above and two below the detector that measure the time it takes for a particle to traverse the detector and also serve as a trigger system, indicating to the other subsystems that a particle has entered the detector.
  • The tracker, which consists of eight layers of double-sided silicon sensors that record the path of any particle that enters the the detector through the magnet. By the time a particle has reached the tracker, the magnet has already done half the work by separating positively-charged from negatively-charged particles. By looking at the trajectory of each particle inside the tracker, the AMS-02 can not only determine which particles are positive and negative but also the atomic number Z of any nucleus that enters the detector, hence the “Spectrometer” part of “Alpha Magnetic Spectrometer.”
  • A Ring Imaging Cherenkov detector (RICH) which measures particle velocities by looking at the shape of the Cherenkov radiation that is produced by incident particles.
  • An electromagnetic calorimeter (ECAL) consisting of a dense piece of material inside which incident particles produce secondary showers of particles. By looking at these particle showers, the ECAL helps to discriminated between leptons (electrons, positrons, gammas) and hadrons (e.g. protons) that, if they have the same trajectory through the magnet, might be otherwise impossible to tell apart.

Although it sounds complicated, the combined power of all these various subdetectors allows AMS-02 to do a spectacular job of characterizing many different types of particles.  Here, the particles relevant to dark matter detection are antimatter particles such as positrons, antiprotons, and antideuterons. In the absence of exotic processes, we expect the spectra of these particles to be smoothly decreasing, isotropic, and generally well-behaved. Any bumps or features in, say, the positron or antiproton spectra would indicate some new process at work –like possibly WIMP annihilations [2].

(Fig. 2) The positron fraction measurement released by the AMS-02 collaboration in spring 2013.

Fig. 2: The first positron fraction measurement produced by the AMS-02 collaboration, released in April 2013 [3].

Back in April 2013, AMS-02 released its first measurement of the fraction of positrons as compared to electrons in cosmic rays (Fig 2). Clearly the curve in Fig. 2 is not decreasing – there is some other source of positrons at work here. There was some small speculation among the scientific community that the rise in positron fraction at higher energies could be attributed to dark matter annihilations, but the general consensus was that this shape is caused by a more ordinary astrophysical source such as pulsars. So in general, how do you tell what kind of positron source could cause this shape of curve? The answer is this: if the rise in positron fraction is due to dark matter annihilations, you can expect to see a very sharp dropoff at higher energies. A less exotic astrophysical source would result in a smooth flattening of this curve at higher energies [3].

Fig. 3:  The positron fraction akdsjhkajsh

Fig. 3: An updated positron fraction measurement from two years’ worth of data released by the AMS-02 collaboration on September 18, 2014 [4].

On September 18, 2014, AMS-02 released a followup to its 2013 result covering a larger range of energies in order to further investigate this positron excess (Fig. 3). The positron fraction curve does in fact begin to drop off at higher energies. Is it a smoking-gun signal of WIMP annihilations? Not yet – there are not enough statistics at high energies to differentiate between a smooth turnover or an abrupt drop in positron fraction. For now, the AMS-02 plans to investigate the positron flux at even higher energies to determine the nature of this turnover and to do a similar measurement with the antiproton fraction as compared to regular protons.

For a webcast of the official press release, you can go here. Or, to read about the AMS-02 results in more detail, check out the references [1] and [4] at the bottom of this article.

***

Fermi-LAT

Fig. x:

Fig. 4: A view of the gamma-ray sky from the Fermi-LAT instrument, from http://fermi.gsfc.nasa.gov/ssc/. The bright line is the galactic plane.

The Fermi Large Area Telescope (LAT) is another indirect detection experiment that has seen hints of something interesting. In this particular experiment, the signal of interest comes from gamma rays of energies ranging from tens of MeVs to more than 300 GeV. The science goals are to study and catalog localized gamma-ray sources, to investigate the diffuse gamma-ray background in our part of the universe, and to search for gamma rays resulting from new physics processes, such as dark matter annihilations.

Because the Earth’s atmosphere is not transparent to gamma rays, our best chance to study them lies out in space. The Fermi-LAT is a very large space-based observatory which detects gammas through a mechanism called pair conversion, where a high-energy photon rather than being reflected or refracted upon entering a medium converts into an electron-positron pair. Inside the LAT, this conversion takes place inside a tracker module in one of several thin-yet-very-dense layers of tungsten. There are sixteen of these conversion layers total, interleaved with eighteen layers of silicon strip detectors that record the x- and y- position of any tracks produced by the electron-positron pair. Beneath the tracker modules is a calorimeter, consisting of a set of CsI modules that absorb the full energy of the electrons and positrons and therefore give us a good measure of the energy of the original gamma ray. Finally, the entire detector is covered by an anticoincidence detector (ACD) consisting of plastic scintillator tiles that scintillate when charged particles pass through them but not when gamma rays pass through, thereby providing a way to discriminate the signal of interest from cosmic ray backgrounds (Fig. 5).

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Fig. 5: (Left) A 3D rendering of the Fermi spacecraft deployed above the earth, from http://fermi.gsfc.nasa.gov/. (Right) A design schematic of the Fermi-LAT instrument, also from http://fermi.gsfc.nasa.gov/.

One of the nice things about the Fermi telescope is that it not only has a wide field of view and continually scans across the entire sky, but it can also be pointed at specific locations. Let’s consider for a moment some of the places we could point the Fermi-LAT if we are hoping to detect a dark matter signal [6].

  • The probability of dark matter annihilations taking place is highest in regions of high density like our galactic center, but there is a huge gamma-ray background to contend with from many various astrophysical sources. If we look further out into our galactic halo, there will be less background, but also less statistics for our signal of interest. And there is still a diffuse gamma-ray background to contend with. However, a very narrow peak in the gamma spectrum that is present across the entire sky and not associated with any one particular localized astrophysical source would be very suggestive of a dark matter signal – exactly the kind of smoking gun we are looking for.
  • We could also look at other galaxies. Certain kinds of galaxies called dwarf spheroidals are particularly promising for a number of reasons. First of all, the Milky Way has several close dwarf neighbors, so there are plenty to choose from. Second, dwarf galaxies are very dim. They have few stars, very little gas, and few gamma-ray sources like pulsars or supernova remnants. Should a gamma signal be seen across several of these dwarf galaxies, it would be very hard to explain by any means other than dark matter annihilation.

In spring 2012, two papers came out one after another suggesting that a sharp gamma peak had indeed been found near the galactic center, which you can see in Fig. 6 [7, 8]. What is the cause of this feature? Was it some kind of instrumental effect? A statistical fluctuation? Was it the dark matter smoking gun? The Fermi-LAT team officially commented on these papers later that November, reporting a feature that was much less statistically significant and much closer to 135 GeV and consistent with gamma rays produced by cosmic rays in the earth’s atmosphere [9].

Fig. 5: The 2012 gamma-ray spectrum produced from three years of data.  The green markers represent the best-fit background-only model, the red markers represent the best-fit background + WIMP annihilation model, and the black points are the counts that were actually observed in each energy bin.  The bottom panel shows the residual. [7]

Fig. 6: The gamma-ray spectrum reported by Fermi in 2012 [7]. The black points show the number of counts observed in each energy bin.  The green markers represent the best-fit background-only model, the red markers represent the best-fit background + WIMP annihilation model, and the blue dots represent the best-fit WIMP annihilation model with the background subtracted off.

This gamma line has been an active target of study since 2012. In 2013, the Fermi-LAT group released a further investigation of this feature using over 3.7 years of data. A bump in the spectrum at about 133 GeV was again observed, consistent with the 2012 papers, but with decreased statistical significance in part because this feature was narrower than the energy resolution of the LAT and because a similar (yet smaller) feature was seen in the earth’s “limb”, or outermost edges of the atmosphere [10]. The hypothesis that this bump in the gamma-ray spectrum is a WIMP signal has all but fallen out of favor within the scientific community.

In the meantime, Fermi-LAT has also been looking for gamma rays from nearby dwarf galaxies. A survey of 25 dwarf galaxies near the Milky Way yielded no such statistically-significant signal [11]. For the next few years, Fermi will continue its search for dark matter as well continuing to catalog and investigate other astrophysical gamma-ray sources.

***

IceCube

Fig. x: The IceCube collaboration.  Image from http://news.ls.wisc.edu/.

Fig. 7: Members of the IceCube collaboration. Image from http://news.ls.wisc.edu/.

Last but certainly not least, I wanted to discuss one of my particular favorite experiments. IceCube is really cool for many reasons, not the least of which is because it is situated at the South Pole! Like LUX (my home experiment), IceCube consists of a large array of light sensors (photomultiplier tubes) that look for flashes of light indicating particle interactions within a large passive target. Unlike LUX, however, the target medium in IceCube is the Antarctic ice itself, which sounds absolutely fantastical until you consider the following: if you go deep enough, the ice is extremely clear and uniform, because the pressure prevents bubble formation; and if you go deep enough it becomes very dark, so that any flashes of light inside the ice will stand out.

In IceCube, neutrinos are the main particles of interest. They are the ninjas of the particle physics world – neutrinos interact only very rarely with other particles and are actually rather difficult to detect. However, when your entire detector consists of a giant chunk of ice 2.5 kilometers deep, that’s a lot of material, resulting in a not-insignificant probability that a passing neutrino will interact with an atom inside your detector. A neutrino interacting with the ice will produce a shower of secondary charged particles, which in turn produce Cherenkov radiation that can be detected. Neutrinos themselves are pretty awesome on their own, and there is a wealth of interesting neutrino research currently taking place. They can also tell us a lot about a variety of astrophysical entities such as gamma-ray bursts and supernovae. And even more importantly for this article, neutrinos can occur as a result of dark matter annihilations.

Unfortunately for the dark matter search, muons and neutrinos produced by cosmic ray interactions in the atmosphere are a major source of background in the detector. Muons, because they are quite heavy, tend to travel long distances in most materials. Luckily, they don’t travel nearly as far as neutrinos – we’re talking on the order of only a few meters on average before they attenuate in a medium like ice. Neutrinos travel vastly further, so a good way to discriminate between cosmic-ray muons and neutrinos is to eliminate any downwards-traveling particles. Any upwards-traveling particle tracks must be from astrophysical neutrinos, because only they can traverse the entire diameter of the Earth without getting stopped somewhere in the ground. To put it more succinctly: IceCube makes use of the entire Earth as a shield against backgrounds! Atmospheric neutrinos are a little more difficult to distinguish from the astrophysical neutrinos relevant to dark matter searches, but are neve­rtheless an active target of study and are becoming increasingly better understood.

Now that we’ve talked about the rationale of building gigantic detectors out of ice and the kinds of signals and backgrounds to expect in such detectors, let’s talk the actual design of the experiment. IceCube consists of 86 strings of 60 digital optical modules, each consisting of a photomultiplier tube and a readout board, deployed between 1.5 and 2.5 kilometers deep in the Antarctic ice. How do you get the modules down there? Only with the help of very powerful hot-water drills. The drilling of these holes and the construct­ion of IceCube is exciting enough that it probably warrants its own article.

icecube_fig_a

Fig. 8: A schematic of the IceCube experiment.  Note the Eiffel Tower included for scale.  Image from [12].

Alongside the strings that make up the bulk of the detector, IceCube also contains a couple of other subdetectors. There is a detector called IceTop on the surface of the ice that is used to help veto events that are atmospheric in origin. There is another detector called DeepCore that consists of additional strings with optical modules packed much more tightly together than the regular strings for the purpose of looking of increasing the sensitivity to low-energy events. Other special extensions designed to look for very high and very low energy events are also planned for the near future.

With regards to the quest for dark matter, the IceCube strategy is to focus on two different WIMP annihilation models: χχ to W+W- (or τ+τ- for WIMPs that are lighter than W bosons) and χχ to b b-bar. In each case, the decay products produce secondary particles, including some neutrinos. By examining neutrinos both from the sun and from other galaxies and galaxy clusters, IceCube has produced a very competitive limit on the cross section for dark matter annihilation via these and other similar annihilation modes [12, 13].

Fig. 9:

Fig. 9: IceCube’s 2012  limit on dark matter – proton spin dependent interactions.  All of the black curves correspond to different neutrino models.  Image from [15].

For more information, there is a wonderful in-depth review of the IceCube detector design and status at the Annual Review of Nuclear and Particle Science.

***

So there you have it. These are some of the big projects keeping an eye out for WIMPs in the sky. At least some of these experiments have seen hints of something promising, so over the next couple years maybe we’ll finally get that five-sigma discovery that we want so badly to see.

 

References:

[1] AMS Collaboration. “High statistics measurement of the positron fraction is primary cosmic rays of 0.5-500 GeV with the Alpha Magnetic Spectrometer on the International Space Station.” Phys. Rev. Lett. 113 (2014) 121101.

[2] AMS Collaboration. “First result from the Alpha Magnetic Spectrometer on the International Space Station: Precision measurement of the positron fraction in primary cosmic rays of 0.5-350 GeV.” Phys. Rev. Lett. 110 (2013) 141102.

[3] Serpico, Pasquale D. “Astrophysical models for the origin of the positron ‘excess’.”  Astroparticle Physics, Vol. 39, pg. 2-11 (2011). ArXiv e-print 1108.4827.

[4] AMS Collaboration. “Electron and positron fluxes in primary cosmic rays measured with the Alpha Magnetic Spectrometer on the International Space Station.” Phys. Rev. Lett. 113 (2014) 121102.

[5] Fermi-LAT Collaboration.  “The large area telescope on the Fermi Gamma-Ray Space Telescope mission.” The Astrophysical Journal, Vol. 697, Issue 2, pp. 1071-1102 (2009). ArXiv e-print 0902.1089.

[6] Bloom, Elliott. “The search for dark matter with Fermi.” Conference presentation – Dark Matter 2014, Westwood, CA. http://www.pa.ucla.edu/sites/default/files/webform/ElliottBloom_UCLADMMeeting_022614_FinalkAsPlacedOnDM2014Website.pdf.

[7] Bringmann, Torsten, et al. “Fermi LAT search for internal bremsstrahlung signatures from dark matter annihilation.” JCAP 1207 (2012) 054. ArXiv e-print 1203.1312.

[8] Weniger, Christoph. “A tentative gamma-ray line from dark matter annihilation at the Fermi Large Area Telescope.” JCAP 1208 (2012) 007. ArXiv e-print 1204.2797.

[9] Albert, Andrea. “Search for gamma-ray spectral lines in the Milky Way diffuse with the Fermi Large Area Telescope.” Conference presentation – The Fermi Symposium 2012. http://fermi.gsfc.nasa.gov/science/mtgs/symposia/2012/program/fri/AAlbert.pdf.

[10] Fermi-LAT Collaboration. “Search for gamma-ray spectral lines with the Fermi Large Area Telescope and dark matter implications.” Phys.Rev. D 88 (2013) 082002 . ArXiv e-print 1305.5597.

[11] Fermi-LAT Collaboration. “Dark matter constraints from observations of 25 Milky Way satellite galaxies with the Fermi Large Area Telescope.” Phys.Rev. D 89 (2014) 4, 042001. ArXiv e-print 1310.0828.

[12] IceCube Collaboration. “Measurement of South Pole ice transparency with the IceCube LED calibration system.” ArXiv e-print 1301.5361I.

[13] IceCube Collaboration. “Search for dark matter annihilations in the Sun with the 79-string IceCube detector.” Phys. Rev. Lett. 110, 131302 (2013). ArXiv e-print 1212.4097v2.

[14] IceCube Collaboration. “IceCube search for dark matter annihilation in nearby galaxies and galaxy clusters.” Phys. Rev. D 88 (2013) 122001. ArXiv e-print 1307.3473v2.

[15] Arguelles, Carlos A., and Joachim Kopp. “Sterile neutrinos and indirect dark matter searches in IceCube.” JCAP 1207 (2012) 016. ArXiv e-print 1202.3431.

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Dark Skies (Or, A Very Brief Guide to Indirect Detection)

Saturday, September 6th, 2014

For my inaugural post a few months ago I discussed dark matter direct detection and the search for WIMPs deep underground. As a graduate student on the Large Underground Xenon (LUX) experiment, this is the area that I am most familiar with, but it is by no means the only way to hunt for these elusive particles. The very idea of dark matter was first motivated by problems in astronomy (such as understanding the rotation curves of galaxies), so what better way to look for it than to turn our telescopes to the skies?

The best way to get an intuition for the physics behind dark matter detection is to look at the Feynman diagrams representing interactions between dark matter particles and standard model particles. For example, the relevant interactions in the direct detection of WIMPS have the general form:

qd_dark_skies_directfeyman1

Feynman diagrams are conventionally drawn with time as the horizontal axis, increasing as you go from left to right. In this particular diagram a WIMP χ and a standard model particle, which I somewhat un-creatively call sm, come in from the left, interact at the vertex of the diagram, and then a WIMP χ and a standard model particle sm, leave on the right. (Here I have deliberately obscured the vertex, since there are many possible interactions and combinations of interactions that yield Feynman diagrams with the same initial and final particle states.) More succinctly, we can think of this diagram as a WIMP χ and a standard model particle sm scattering off each other.  Direct detection experiments like LUX or the Cryogenic Dark Matter Search (CDMS) look specifically for WIMPs scattering off protons or neutrons in an atomic nucleus, so the relevant Feynman diagrams are:

qd_dark_skies_directfeyman2

Feynman diagrams are kind of beautiful in that you can draw a diagram for most any particle interaction you can think of; you can flip it, rotate it, and smoosh it around; and because of certain symmetry considerations you will in general still end up with something representing a completely valid, physically-allowed particle interaction.

Let’s do this with our direct detection diagram. If we just rotate it a quarter-turn, we end up with the following:

qd_dark_skies_directfeymanrotate

We can interpret this as a two WIMPs colliding and annihilating to form standard model particles in a way analogous to how electron-positron annihilation produces photons. WIMPs might be Majorana particles, i.e. their own antiparticles, or they might be Dirac particles, that is, distinct from anti-WIMPs, but the bottom line is still the same: the detection of the annihilation products can be used to deduce the presence of the initial WIMPs. (It might also be that WIMPs are unstable and therefore decay into standard model particles, in which case we could also look for their decay products.)

“Indirect” detection is the rather apt name for the technique of searching for WIMPs by trying to detect the products of their annihilation to standard model particles.

This strategy presents an entirely different set of challenges than direct detection. For one thing, you can’t shield against backgrounds in the same way that you can with direct detection experiments. After all, your signal consists of ordinary standard model particles, albeit standard model particles from an exotic origin, so any attempt to shield your experiment will just block out your desired signal along with the background. So where LUX is a “zero-background” experiment, indirect detection experiments look for signals that manifest themselves as tiny excesses of events over and above a large background. Additionally, indirect detection requires that WIMPs in the universe be both abundant enough and close enough together that there is a non-negligible probability for annihilation to occur. If in fact WIMPs are the answer to the dark matter problem then this was most certainly true in the early universe, but today, cosmologists estimate the local density of WIMPs to be approximately 0.3 GeV/c2/cm3. This corresponds to only about three WIMPs per cubic meter! This is a challenge indeed, but luckily there are a few places in the universe where gravity helps us out.

First of all, we can look for WIMPs in the centers of galaxies, where gravity helps coalesce both standard model and exotic massive particles into higher-density clumps. Here there are a number of annihilation processes we can search for. For instance, we can look for WIMPs annihilating directly into gamma rays, in which case the signal would be a mono-energetic peak in the gamma ray spectrum:

qd_dark_skies_gammas

Note that as in my direct detection diagrams I have deliberately obscured the vertex of this diagram. Because WIMPs by definition do not interact electromagnetically they cannot convert directly into photons. However, the interaction represented in this particular diagram could take place if it contains an intermediate step where WIMPs convert first into a non-photon standard model particle. Then this intermediate particle could produce a photon final state.

The galactic center is not the easiest place to search for rare events. Here, the hunt for gammas from WIMP annihilations is complicated by the existence of many bright, diffuse gamma backgrounds in the from astrophysical processes that are not necessarily well-understood. In addition, the density profile of our WIMP halo is not well-understood near the center of our galaxy. It might be that our dark matter halo has a very dense “cusp” near the center; on the other hand it might very well be that the dark matter density in our halo increases up to a point but then plateaus to a constant density toward the center of the galaxy. Regarding the latter, understanding this density profile is an active area of research in computational and observational cosmology today. After all, if we don’t know how much dark matter is in the center of our galaxy, then how can we predict what an annihilation signal in that location might look like?

In order to mitigate the first of these complications, we can look to galaxies other than our own. In the Milky Way’s Local Group there are a number of galaxies called “dwarf spheroidals” which have extremely low luminosities, little to no interstellar gas and dust, and as a result, much less overall background than in our own galaxy. This sort of environment might therefore be very conducive to the indirect detection of WIMPs.

We can also look for WIMPs annihilating into heavy standard model particles. Generally these decay rapidly, producing jets that in turn yield a whole continuous spectrum of gammas and other particles. Schematically, we can summarize this process as:

qd_dark_skies_jets

Perhaps the most interesting products of these annihilations are the antimatter particles produced in these jets. The matter/antimatter asymmetry in the universe is a whole other mystery to be solved, but it does provide for us a fairly conclusive smoking-gun WIMP signal. Antimatter in the universe is rare enough that a large flux of antimatter particles could suggest WIMP annihilation events are taking place. Some classes of indirect detection experiments look for positron excesses; others look for antiprotons or antideuterons. On the other hand, these experiments are also complicated by the existence of other cosmic-ray backgrounds and the diffusion of these annihilation products in the Earth’s atmosphere. Understanding and modeling the (non-WIMP-related) processes that produce cosmic rays is also a very active area of research.

Finally, we expect there to be high WIMP densities in the sun’s gravitational potential well. This means that we could conceivably hunt for WIMPs much closer to home and not have to worry about backgrounds from other sources in the galaxy. There is a catch, however. The sun is so incredibly dense that the mean free path of, say, a photon inside its center is only about a centimeter. Each particle that escapes to its surface can only do so after going through a random walk of many, many absorptions and re-emissions. On average, this can take as many as hundreds of thousands or even millions of years! Neutrinos are the sole exception: they interact so weakly with other standard model particles that for the most part they just zip straight through the sun with no problem. Searches for dark matter annihilations in the sun therefore focus on neutrino-producing processes.

qd_dark_skies_jets_sun

Neutrinos themselves are difficult to detect, but fortunately we do have technologies that are capable of doing so.

***

Over the next decade or so, I predict that indirect detection will be a very hot topic in particle physics (and not just because I really like dark matter!) There are a number of clever experiments that have already produced some interesting results, and several more scheduled to be constructed over the next few years. Stay tuned, because there will be a Part II to this article that will look at some of these experiments in detail.

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Searching for Dark Matter With the Large Underground Xenon Experiment

Thursday, April 17th, 2014

In December, a result from the Large Underground Xenon (LUX) experiment was featured in Nature’s Year In Review as one of the most important scientific results of 2013. As a student who has spent the past four years working on this experiment I will do my best to provide an introduction to this experiment and hopefully answer the question: why all the hype over what turned out to be a null result?

The LUX detector, deployed into the water tank shield

The LUX detector, deployed into its water tank shield 4850 feet underground.

Direct Dark Matter Detection

Weakly Interacting Massive Particles (WIMPs), or particles that interact only through the weak nuclear force and gravity, are a particularly compelling solution to the dark matter problem because they arise naturally in many extensions to the Standard Model. Quantum Diaries did a wonderful series last summer on dark matter, located here, so I won’t get into too many details about dark matter or the WIMP “miracle”, but I would however like to spend a bit of time talking about direct dark matter detection.

The Earth experiences a dark matter “wind”, or flux of dark matter passing through it, due to our motion through the dark matter halo of our galaxy. Using standard models for the density and velocity distribution of the dark matter halo, we can calculate that there are nearly 1 billion WIMPs per square meter per second passing through the Earth. In order to match observed relic abundances in the universe, we expect these WIMPs to have a small yet measurable interaction cross-section with ordinary nuclei.

In other words, there must be a small-but-finite probability of an incoming WIMP scattering off a target in a laboratory in such a way that we can detect it. The goal of direct detection experiments is therefore to look for these scattering events. These events are characterized by recoil energies of a few to tens of keV, which is quite small, but it is large enough to produce an observable signal.

So here’s the challenge: How do you build an experiment that can measure an extremely small, extremely rare signal with very high precision amid large amounts of background?

Why Xenon?

The signal from a recoil event inside a direct detection target typically takes one of three forms: scintillation light, ionization of an atom inside the target, or heat energy (phonons). Most direct detection experiments focus on one (or two) of these channels.

Xenon is a natural choice for a direct detection medium because it is easy to read out signals from two of these channels. Energy deposited in the scintillation channel is easily detectable because xenon is transparent to its own characteristic 175-nm scintillation. Energy deposited in the ionization channel is likewise easily detectable, since ionization electrons under the influence of an applied electric field can drift through xenon for distances up to several meters. These electrons can then be read out by any one of a couple different charge readout schemes.

Furthermore, the ratio of the energy deposited in these two channels is a powerful tool for discriminating between nuclear recoils such as WIMPs and neutrons, which are our signal of interest, and electronic recoils such as gamma rays, which are a major source of background.

Xenon is also particularly good for low-background science because of its self-shielding properties. That is, because liquid xenon is so dense, gammas and neutrons tend to attenuate within just a few cm of entering the target. Any particle that does happen to be energetic enough to reach the center of the target has a high probability of undergoing multiple scatters, which are easy to pick out and reject in software. This makes xenon ideal not just for dark matter searches, but also for other rare event searches such as neutrinoless double-beta decay.

The LUX Detector

The LUX experiment is located nearly a mile underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. LUX rests on the 4850-foot level of the old Homestake gold mine, which was turned into a dedicated science facility in 2006.

Besides being a mining town and a center of Old West culture (The neighboring town, Deadwood, is famed as the location where Wild Bill Hickok met his demise in a poker game), Lead has a long legacy of physics. The same cavern where LUX resides once held Ray Davis’s famous solar neutrino experiment, which provided some of the first evidence for neutrino flavor oscillations and later won him the Nobel Prize.

A schematic of the LUX detector.

A schematic of the LUX detector.

The detector itself is what is called a two-phase time projection chamber (TPC). It essentially consists of a 370-kg xenon target in a large titanium can. This xenon is cooled down to its condensation point (~165 K), so that the bulk of the xenon target is liquid, and there is a thin layer of gaseous xenon on top. LUX has 122 photomultiplier tubes (PMTs) in two different arrays, one array on the bottom looking up into the main volume of the detector, and one array on the top looking down. Just inside those arrays are a set of parallel wire grids that supply an electric field throughout the detector. A gate grid located between the cathode and anode grid that lies close to the liquid surface allows the electric field in the liquid and gas regions to be separately tunable.

When an incident particle interacts with a xenon atom inside the target, it excites or ionizes the atom. In a mechanism common to all noble elements, that atom will briefly bond with another nearby xenon atom. The subsequent decay of this “dimer” back into its two constituent atoms causes a photon to be emitted in the UV. In LUX, this flash of scintillation light, called primary scintillation light or S1, is immediately detected by the PMTs. Next, any ionization charge that is produced is drifted upwards by a strong electric field (~200 V/cm) before it can recombine. This charge cloud, once it reaches the liquid surface, is pulled into the gas phase and accelerated very rapidly by an even stronger electric field (several kV/cm), causing a secondary flash of scintillation called S2, which is also detected by the PMTs. A typical signal read out from an event in LUX therefore consists of a PMT trace with two tell-tale pulses. 

A typical event in LUX. The bottom plot shows the primary (S1) and secondary (S2) signals from each of the individual PMTs. The top two plots show the total size of the S1 and the S2 pulses.

A typical event in LUX. The bottom plot shows the primary (S1) and secondary (S2) signals from each of the individual PMTs. The top two plots show the total size of the S1 and the S2 pulses.

As in any rare event search, controlling the backgrounds is of utmost importance. LUX employs a number of techniques to do so. By situating the detector nearly a mile underground, we reduce cosmic muon flux by a factor of 107. Next, LUX is deployed into a 300-tonne water tank, which reduces gamma backgrounds by another factor of 107 and neutrons by a factor of between 103 and 109, depending on their energy. Third, by carefully choosing a fiducial volume in the center of the detector, i.e., by cutting out events that happen near the edge of the target, we can reduce background by another factor of 104. And finally, electronic recoils produce much more ionization than do the nuclear recoils that we are interested in, so by looking at the ratio S2/S1 we can achieve over 99% discrimination between gammas and potential WIMPs. All this taken into account, the estimated background for LUX is less than 1 WIMP-like event throughout 300 days of running, making it essentially a zero-background experiment. The center of LUX is in fact the quietest place in the world, radioactively speaking.

Results From the First Science Run

From April to August 2013, LUX ran continuously, collecting 85.3 livedays of WIMP search data with a 118-kg fiducial mass, resulting in over ten thousand kg-days of data. A total of 83 million events were collected. Of these, only 6.5 million were single scatter events. After applying fiducial cuts and cutting on the energy region of interest, only 160 events were left. All of these 160 events were consistent with electronic recoils. Not a single WIMP was seen – the WIMP remains as elusive as the unicorn that has become the unofficial LUX mascot.

So why is this exciting? The LUX limit is the lowest yet – it represents a factor of 2-3 increase in sensitivity over the previous best limit at high WIMP masses, and it is over 20 times more sensitive than the next best limit for low-mass WIMPs.

The 90% confidence upper limit on the spin independent WIMP-nucleon interaction cross section: LUX compared to previous experiments.

The 90% confidence upper limit on the spin independent WIMP-nucleon interaction cross section: LUX compared to previous experiments.

Over the past few years, experiments such as DAMA/LIBRA, CoGeNT, CRESST, and CDMS-II Si have each reported signals that are consistent with WIMPs of mass 5-10 GeV/c2. This is in direct conflict with the null results from ZEPLIN, COUPP, and XENON100, to name a few, and was the source of a fair amount of controversy in the direct detection community.

The LUX result was able to fairly definitively close the door on this question.

If the low-mass WIMPs favored by DAMA/LIBRA, CoGeNT, CRESST, and CDMS-II Si do indeed exist, then statistically speaking LUX should have seen 1500 of them!

What’s Next?

Despite the conclusion of the 85-day science run, work on LUX carries on.

Just recently, there was a LUX talk presenting results from a calibration using low-mass neutrons as a proxy for WIMPs interacting within the detector, confirming the initial results from last autumn. Currently, LUX is gearing up for its next run, with the ultimate goal of collecting 300 livedays of WIMP-search data, which will extend the 2013 limit by a factor of five. And finally, a new detector called LZ is in the design stages, with a mass twenty times that of LUX and a sensitivity far greater.

***

For more details, the full LUX press release from October 2013 is located here:

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