weakly interacting massive particles

What Is Dark Matter?

There is as yet no answer to this question, but it is becoming increasingly clear what it is not. Detailed observations of the cosmic microwave background with the WMAP satellite show that the dark matter cannot be in the form of normal, baryonic matter, that is, protons and neutrons that compose stars, planets, and interstellar matter. That rules out hot gas, cold gas, brown dwarfs, red dwarfs, white dwarfs, neutron stars and black holes.

Black holes would seem to be the ideal dark matter candidate, and they are indeed very dark. However stellar mass black holes are produced by the collapse of massive stars which are much scarcer than normal stars, which contain at most one-fifth of the mass of dark matter. Also, the processes that would produce enough black holes to explain the dark matter would release a lot of energy and heavy elements; there is no evidence of such a release.

The non-baryonic candidates can be grouped into three broad categories: hot, warm and cold. Hot dark matter refers to particles, such as the known types of neutrinos, which are moving at near the speed of light when the clumps that would form galaxies and clusters of galaxies first began to grow. Cold dark matter refers to particles that were moving slowly when the pre-galactic clumps began to form, and warm dark matter refers to particles with speeds intermediate between hot and cold dark matter.

This classification has observational consequences for the size of clumps that can collapse in the expanding universe. Hot dark matter particles are moving so rapidly that clumps with the mass of a galaxy will quickly disperse. Only clouds with the mass of thousands of galaxies, that is, the size of galaxy clusters, can form. Individual galaxies would form later as the large cluster-sized clouds fragmented, in a top-down process.

In contrast, cold dark matter can form into clumps of galaxy-sized mass or less. Galaxies would form first, and clusters would form as galaxies merge into groups, and groups into clusters in a bottom-up process.

The observations with Chandra show many examples of clusters being constructed by the merger of groups and sub-clusters of galaxies. This and other lines of evidence that galaxies are older than groups and clusters of galaxies strongly support the cold dark matter alternative. The leading candidates for cold dark matter are particles called WIMPs, for Weakly Interacting Massive Particles. WIMPs are not predicted by the so-called Standard Model for elementary particles, but attempts to construct a unified theory of all elementary particles suggest that WIMPs might have been produced in great numbers when the universe was a fraction of a second old.

A typical WIMP is predicted to be at least 100 times as massive as a hydrogen atom. Possible creatures in the zoo of hypothetical WIMPs are neutralinos, gravitinos, and axinos. Other possibilities that have been discussed include sterile neutrinos and Kaluza-Klein excitations related to extra dimensions in the universe.

The Large Hadron Collider sets its sights on dark matter

After finding the Higgs boson, the LHC has had a refit to enable it to operate at even greater extremes – and to solve more questions about the beginnings of the universe

By Robin McKie

There is no shortage of superlatives that can be applied to the Large Hadron Collider near Geneva, though many are strange and unusual. For a start, the huge underground device, which batters beams of protons into each other at colossal energies, can fairly claim to be the coolest place on Earth. Bending protons as they hurtle round the LHC’s circular 27km tunnel turns out to be a chilly business.

Thousands of huge magnets are needed to control the beams and these have to work with complete efficiency. To achieve this, the device is refrigerated to two degrees above absolute zero on the thermodynamic temperature scale: -271C, a temperature at which electric currents flow without resistance. In this way, the collider’s magnets can work to their maximum potential.

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arvit  asked:

About the dark matter thing, I have heard about a few possible candidates that are being examined more closely at the moment. One are the neutralinos, the other are black holes between 20 and 100 solar masses, which are the only size possible for dark matter and which have been discovered more often with LIGO and the other detectors for gravitational waves. What do you think about those theories? (And about MOND, that hardly ever gets coverage for the general public)

Thanks for asking! To start out, we don’t know much about dark matter, but here’s what we do know: Stars are orbiting galaxies too fast for the visible matter to be the only thing holding the galaxy together; in order for the speeds of stars orbiting to be reconciled with the amount of mass we can see, we’ve concluded that there has to be about 5-10x as much matter that can’t be seen because it doesn’t interact with light, giving it the nickname “dark matter.” Based on rotational curves (which show the speed of stars rotating around the galactic center at a given distance from the core), we know that the dark matter extends in a “halo” significantly larger than the visible galaxy. (image source)

This is a rotational curve; as you can see, the rotational curves don’t follow the path expected at all. Astrophysicists did a bunch of math and figured out that the dark matter must be in a huge halo around the galaxy: (image source)

Now for the different theories you talked about! I’m not too familiar with neutralinos; from what I (just) read, they’re theoretical particles with a lot of mass, and one of the predicted ones are WIMPs (Weakly Interacting Massive Particles - don’t you love how astronomers name things), which is a good candidate for dark matter, but hasn’t yet been proven. A lot of articles I’ve read about dark matter searches spent some time talking about WIMPs so they’re probably a good candidate and could end up being right. Another particle had the acronym MACHOs and i believe it also has to do with dark matter.

Black holes are definitely not the source of dark matter, because there is 5-10x more dark matter than luminous matter in the universe, meaning that there would have to be at least 109 or more of these black holes (essentially one black hole for every 1-10 stars), so we can write this theory off. Also, this theory wouldn’t match the observed rotational curves - whatever dark matter there is, it has to be whizzing around in the dark matter halo. 

MOND, or modified-newtonian dynamics, is almost definitely not true. The theory of gravity (general relativity) is pretty pretty solid and extremely unlikely to be wrong, so redoing our entire theory of gravity to fit dark matter is probably not the answer; this is why it gets essentially no coverage.

If you want my personal opinion, I think the most likely candidate would be a massive but very weakly (if at all) interacting subatomic particle that we haven’t discovered yet. From what I understand, we’re trying to figure out how to detect them, but it’s very very hard since these particles don’t interact with light (and only really interact gravitationally). 

If you want more information about dark matter, you should check out this post by @quantanaut about what we know (or more accurately don’t know), along with the dark matter tag on my blog. ps im sorry I literally typed you an essay but I wanted to be thorough. If i did a poor job explaining anything or you want to talk more about it or want clarification or something, feel free to shoot me another question!!

willygowild  asked:

Is it possible, or even merely,..Plausible, that black holes, "theoretically", might perhaps be more akin to, say, a super-massive quantum? They DO have a "life cycle" so to speak. If Occam's razor is more or less, universally given to prove factual,..When a caterpillar transform into moth/butterfly,..The caterpillar, did NOT die,..It simply moved to another state of being,. And merely, became, something else. Far grander than its lowly crawling beginnings.So, what do you think?

I’m not sure what you mean by ‘super-massive quantum’ BHs. So, forgive me if I misinterpreted your question. Here’s what I know:

Quantum mechanical black holes may have formed in the early stages of the universe. We call these primordial black holes (PBHs). What’s really interesting about primordial black holes is that they are not the result of collapsing stars. According to general relativity, the key ingredient is basically a region of high density matter – like the quark soup, add some intense energy density fluctuation to that region of space (inflation) and you get an increased amount of matter within a Schwarzschild radius, and voilà, a miniature event horizon is born. Of course, PBHs could span an enormous mass range; those formed in the Planck epoch would have the tiny Planck mass (10^−5g),  and those formed 1 second after the Big Bang would be as large as 10^5 solar masses - like the ones thought to reside in the center of galaxies. These PBHs may still be with us today because the rate at which a black hole evaporates (Hawking radiation) is inversely proportional to its mass; a small black hole evaporates rapidly, and a massive black hole, therefore, evaporates slowly.  The smaller ones that have evaporated left some clues behind; they produced a huge amount of radiation which affected and delayed the onset of nuclei formation (nucleosynthesis), we know this because we measured the abundance of those nuclei. There is a possibility that the observed baryon asymmetry was generated by the evaporation of PBHs. The ones that are still evaporating are actually plausible dark matter candidates, they are a bit different from the typical dark matter candidates, of course, because they are not elementary particles like the weakly interacting massive particles (WIMPs), rather massive astrophysical compact halo objects (MACHOs). The other interesting thing is that we don’t really know the end result of evaporating black holes, maybe they shrink to the Planck scale and circle around the universe as Planck-mass relics.

Dark Matter Particle Could be Size of Human Cell

Dark matter could be made of particles that each weigh almost as much as a human cell and are nearly dense enough to become miniature black holes, new research suggests.

While dark matter is thought to make up five-sixths of all matter in the universe, scientists don’t know what this strange stuff is made of. True to its name, dark matter is invisible — it does not emit, reflect or even block light. As a result, dark matter can currently be studied only through its gravitational effects on normal matter. The nature of dark matter is currently one of the greatest mysteries in science.

If dark matter is made of such superheavy particles, astronomers could detect evidence of them in the afterglow of the Big Bang, the authors of a new research study said.

Previous dark matter research has mostly ruled out all known ordinary materials as candidates for what makes up this mysterious stuff. Gravitational effects attributed to dark matter include the orbital motions of galaxies: The combined mass of the visible matter in a galaxy, such as stars and gas clouds, cannot account for a galaxy’s motion, so an additional, invisible mass must be present. The consensus so far among scientists is that this missing mass is made up of a new species of particles that interact only very weakly with ordinary matter. These new particles would exist outside the Standard Model of particle physics, which is the best current description of the subatomic world.

Some dark matter models suggest that this cosmic substance is made of weakly interacting massive particles, or WIMPs, that are thought to be about 100 times the mass of a proton, said study co-author McCullen Sandora, a cosmologist at the University of Southern Denmark. However, despite many searches, researchers have not conclusively detected any WIMPs so far, leaving open the possibility that dark matter particles could be made of something significantly different.

Now Sandora and his colleagues are exploring the upper mass limit of dark matter — that is, they’re trying to discover just how massive these individual particles could possibly be, based on what scientists know about them. In this new model, known as Planckian interacting dark matter, each of the weakly interacting particles weighs about 1019 or 10 billion billion times more than a proton, or “about as heavy as a particle can be before it becomes a miniature black hole,” Sandora said.

A particle that is 1019 the mass of a proton weighs about 1 microgram. In comparison, research suggests that a typical human cell weighs about 3.5 micrograms.

The genesis of the idea for these supermassive particles “began with a feeling of despondency that the ongoing efforts to produce or detect WIMPs don’t seem to be yielding any promising clues,” Sandora said. “We can’t rule out the WIMP scenario yet, but with each passing year, it’s getting more and more suspect that we haven’t been able to achieve this yet. In fact, so far there have been no definitive hints that there is any new physics beyond the Standard Model at any accessible energy scales, so we were driven to think of the ultimate limit to this scenario.”

At first, Sandora and his colleagues regarded their idea as little more than a curiosity, since the hypothetical particle’s massive nature meant that there was no way any particle collider on Earth could produce it and prove (or refute) its existence.


But now the researchers have suggested that if these particles exist, signs of their existence might be detectable in the cosmic microwave background radiation, the afterglow of the Big Bang that created the universe about 13.8 billion years ago.

Currently, the prevailing view in cosmology is that moments after the Big Bang, the universe grew gigantically in size. This enormous growth spurt, called inflation, would have smoothed out the cosmos, explaining why it now looks mostly similar in every direction.

After inflation ended, research suggests that the leftover energy heated the newborn universe during an epoch called “reheating.” Sandora and his colleagues suggest that extreme temperatures generated during reheating could have produced large amounts of their superheavy particles, enough to explain dark matter’s current gravitational effects on the universe.

However, for this model to work, the heat during reheating would have had to be significantly higher than what is typically assumed in universal models. A hotter reheating would in turn leave a signature in the cosmic microwave background radiation that the next generation of cosmic microwave background experiments could detect. “All this will happen within the next few years hopefully, next decade, max,” Sandora said.

If dark matter is made of these superheavy particles, such a discovery would not only shed light on the nature of most of the universe’s matter, but also yield insights into the nature of inflation and how it started and stopped — all of which remains highly uncertain, the researchers said.

For example, if dark matter is made of these superheavy particles, that reveals “that inflation happened at a very high energy, which in turn means that it was able to produce not just fluctuations in the temperature of the early universe, but also in space-time itself, in the form of gravitational waves,” Sandora said. “Second, it tells us that the energy of inflation had to decay into matter extremely rapidly, because if it had taken too long, the universe would have cooled to the point where it would not have been able to produce any Planckian interacting dark matter particles at all.”

Sandora and his colleagues detailed their findings online March 10 in the journal Physical Review Letters.

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Dark Matter

Some dark matter models are now suggesting that this cosmic substance is made of weakly interacting massive particles (WIMPs). These are thought to be about as heavy as a particle can be before becoming a miniature black hole. In a new model known as “Planckian interacting dark matter,” the upper mass limit of these particles is thought to be 1019 compared to a proton which is 1. This would mean that these particles could weigh up to 1 microgram which is almost the same weight as a human cell which is about 3.5 micrograms.

However, we still have not been able to detect what makes up most of our universe so we have no way of knowing. If we could detect dark matter and what it consists of, not only would this give us great insight into the majority of the mass in our universe, but also yield greater understanding into the nature of inflation and how it started and stopped.

CWRU Theoretical Physicists Suggest Dark Matter May Be Massive

In a new study, theoretical physicists from Case Western Reserve University suggest that dark matter may be massive and that the Standard Model may account for it.

The physics community has spent three decades searching for and finding no evidence that dark matter is made of tiny exotic particles. Case Western Reserve University theoretical physicists suggest researchers consider looking for candidates more in the ordinary realm and, well, more massive.

Dark matter is unseen matter, that, combined with normal matter, could create the gravity that, among other things, prevents spinning galaxies from flying apart. Physicists calculate that dark matter comprises 27 percent of the universe; normal matter 5 percent.

Instead of WIMPS, weakly interacting massive particles, or axions, which are weakly interacting low-mass particles, dark matter may be made of macroscopic objects, anywhere from a few ounces to the size of a good asteroid, and probably as dense as a neutron star, or the nucleus of an atom, the researchers suggest.

Physics professor Glenn Starkman and David Jacobs, who received his PhD in Physics from CWRU in May and is now a fellow at the University of Cape Town, say published observations provide guidance, limiting where to look. They lay out the possibilities in a paper listed below.

The Macros, as Starkman and Jacobs call them, would not only dwarf WIMPS and axions, but differ in an important way. They could potentially be assembled out of particles in the Standard Model of particle physics instead of requiring new physics to explain their existence.

“We’ve been looking for WIMPs for a long time and haven’t seen them,” Starkman said. “We expected to make WIMPS in the Large Hadron Collider, and we haven’t.”

WIMPS and axions remain possible candidates for dark matter, but there’s reason to search elsewhere, the theorists argue.

“The community had kind of turned away from the idea that dark matter could be made of normal-ish stuff in the late ’80s,” Starkman said. “We ask, was that completely correct and how do we know dark matter isn’t more ordinary stuff— stuff that could be made from quarks and electrons?”

After eliminating most ordinary matter, including failed Jupiters, white dwarfs, neutron stars, stellar black holes, the black holes in centers of galaxies and neutrinos with a lot of mass, as possible candidates, physicists turned their focus on the exotics.

Matter that was somewhere in between ordinary and exotic—relatives of neutron stars or large nuclei—was left on the table, Starkman said. “We say relatives because they probably have a considerable admixture of strange quarks, which are made in accelerators and ordinarily have extremely short lives,” he said.

Although strange quarks are highly unstable, Starkman points out that neutrons are also highly unstable. But in helium, bound with stable protons, neutrons remain stable.

“That opens the possibility that stable strange nuclear matter was made in the early universe and dark matter is nothing more than chunks of strange nuclear matter or other bound states of quarks, or of baryons, which are themselves made of quarks,” he said. Such dark matter would fit the Standard Model.

The Macros would have to be assembled from ordinary and strange quarks or baryons before the strange quarks or baryons decay, and at a temperature above 3.5 trillion degrees Celsius, comparable to the temperature in the center of a massive supernova, Starkman and Jacobs calculated. The quarks would have to be assembled with 90 percent efficiency, leaving just 10 percent to form the protons and neutrons found in the universe today.

The limits of the possible dark matter are as follows:

  • A minimum of 55 grams. If dark matter were smaller, it would have been seen in detectors in Skylab or in tracks found in sheets of mica.
  • A maximum of 1024 (a million billion billion) grams. Above this, the Macros would be so massive they would bend starlight, which has not been seen.
  • The range of 1017 to 1020 grams per centimeter squared should also be eliminated from the search, the theorists say. Dark matter in that range would be massive for gravitational lensing to affect individual photons from gamma ray bursts in ways that have not been seen.

If dark matter is within this allowed range, there are reasons it hasn’t been seen.

  • At the mass of 1018 grams, dark matter Macros would hit the Earth about once every billion years.
  • At lower masses, they would strike the Earth more frequently but might not leave a recognizable record or observable mark.
  • In the range of 109 to 1018, dark matter would collide with the Earth once annually, providing nothing to the underground dark matter detectors in place.

lizzieashwood  asked:

Hi there, another female physicist here! I was wondering what your research focuses on? I didn't get to attend cuwip this year, so I missed out on hearing from all the women in the field.

Sure!

My research group is working on constructing a liquid organic scintillator fast neutron spectrometer for the purpose of creating a detector that can filter out thermalized neutron signals from an actual dark matter detector. I’m too early on in physics to have the proper nuclear/particle physics background, but uncharged Weakly Interacting Massive Particles (WIMPs) are a supposed candidate for dark matter. Thus, one thing of great importance for our detector is that we are ruling out false positive signals from uncharged neutrons that can enter the detector.

My part of the research specifically focuses on the chemistry of the detector (for now; I will be moving onto software soon!).

First of all, what even is a scintillator? It’s a mixture of an aromatic solvent, wave shifters, and other chemicals that allow us to detect the presence of particles through light signals produced within the scintillating solvent. In our case, a fast neutron enters the scintillator and collides with nuclei (any nucleus, but most likely the solvent nuclei). This collision, based on classical kinematics, kicks off a proton of similar mass that travels through the scintillator and interacts with pi-bonding orbitals in the aromatic solvent, exciting then de-exciting those electrons, which gives off a pulse of light that can be detected by the electronics of the detector via PhotoMultiplier Tubes (PMTs).

So we get a light pulse signal from this nuclear recoil reaction of the solvent. But that signal can be ambiguous; other types of particles can produce similar signals which could lead to false positives. So we enrich the scintillator with an element like Li-6, because in the nuclear recoil reaction that occurs with Li-6 a very specific pulse in the MeV range is given off. When we have these two pulses happen in coincidence (one after the other) we can say that the particle we detected was indeed a fast neutron.

I spent most of the summer working on synthesizing three different chain lengths of the lithium carboxylates I wanted to use, only because they cannot be bought. From there I confirmed my structures via IR Spectroscopy, making sure there was no water in my samples. That’s incredibly important because in the case of a completely hydrophobic scintillator cocktail, any additional water would form an emulsion with the surfactants in the scintillator which would lead to reduced optical clarity of the scintillator.

The surfactants in the scintillator I was using were only there because it is more commonplace to use something like LiCl to dissolve the lithium into the scintillator. Another guy in my research group has done this. It works okay, but there are issues with proper dispersion of the LiCl in the tube (due to solubility, even with the surfactants) and discoloration from quenching of the Cl.

Optical clarity (i.e., how clear the scintillator is) is an important aspect of our detector because the signal used to detect the presence of the neutrons we’re looking for is light. If the scintillator is foggy (not optically clear) then the light can’t travel through the scintillator and get picked up by the PMTs. Having perfect clarity isn’t as important for a small detector like we’re making (it’s much more important in detectors that are meters across) but we want the best we can get so long as we can also have good light output and pulse shape discrimination. Because if we can’t get enough light from the scintillator and it doesn’t create clear enough pulses, our signals will be lost in the background noise.

An additional problem that optical clarity poses is in relation to our PMTs. The ones we have are refurbished from another project, so we got what we got. Their sensitive range is between 290-650 nm, which might seem fine if you look at the UV-Vis spectra below. But the highest sensitivity of our PMTs is a bell curved peaking around 420 nm, where those big drop offs on the UV-Vis plot occur. So even if, in principle, it’s not important for a small detector to be perfectly clear, given the specific sensitivity of our PMTs, we need better optical clarity to successfully detect a signal in the first place.

I’ve made some solutions of the various lithium carboxylates in the scintillator fluid Ultima Gold AB (which is diisopropyl naphthalene-rich) and so far the only one that dissolves clearly is lithium dodecanoate (12C chain), with the highest transmittance of light given by a 0.1M sample. We need a lithium-loading of closer to 1M for this to even compare to the LiCl scintillator. So for the spring semester my PI ordered some new scintillator that’s similar to Ultima Gold AB but without the surfactants. Which is a great idea because I don’t even need the surfactants. That’ll hopefully allow me to dissolve more lithium into the scintillator given the freed up space (which is especially important because my compound is kinda big) and allow for more optical clarity.

We’ll see how it goes this semester!

NASA's Fermi mission expands its search for dark matter

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA’s Fermi Gamma-ray Space Telescope, have broadened the mission’s dark matter hunt using some novel approaches.

“We’ve looked for the usual suspects in the usual places and found no solid signals, so we’ve started searching in some creative new ways,” said Julie McEnery, Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it.”

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos – in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

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Dark-matter search considers exotic possibilities

As observations fail to pin down the stuff, explanations once considered fringe are getting another look.

Ever since astronomers realized that most of the matter in the universe is invisible, they have tried to sort out what that obscure stuff might be. But three decades of increasingly sophisticated searches have found no sign of dark matter, causing scientists to question some of their basic ideas about this elusive substance.

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NASA’s Fermi mission expands its search for dark matter

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA’s Fermi Gamma-ray Space Telescope, have broadened the mission’s dark matter hunt using some novel approaches.
“We’ve looked for the usual suspects in the usual places and found no solid signals, so we’ve started searching in some creative new ways,” said Julie McEnery, Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it.”

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos – in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

The leading candidates for dark matter are different classes of hypothetical particles. Scientists think gamma rays, the highest-energy form of light, can help reveal the presence of some of types of proposed dark matter particles. Previously, Fermi has searched for tell-tale gamma-ray signals associated with dark matter in the center of our galaxy and in small dwarf galaxies orbiting our own. Although no convincing signals were found, these results eliminated candidates within a specific range of masses and interaction rates, further limiting the possible characteristics of dark matter particles.

Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields.

These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

Manuel Meyer at Stockholm University led a study to search for these effects in the gamma rays from NGC 1275, the central galaxy of the Perseus galaxy cluster, located about 240 million light-years away.

High-energy emissions from NGC 1275 are thought to be associated with a supermassive black hole at its center. Like all galaxy clusters, the Perseus cluster is filled with hot gas threaded with magnetic fields, which would enable the switch between gamma rays and axion-like particles. This means some of the gamma rays coming from NGC 1275 could convert into axions – and potentially back again – as they make their way to us.

Meyer’s team collected observations from Fermi’s Large Area Telescope (LAT) and searched for predicted distortions in the gamma-ray signal. The findings, published April 20 in Physical Review Letters, exclude a small range of axion-like particles that could have comprised about 4 percent of dark matter.

“While we don’t yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses,” Meyer said. “Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi.”

Another broad class of dark matter candidates are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC’s appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC’s rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. In a paper published in Physical Review D on March 22, Caputo and her colleagues modeled the dark matter content of the SMC, showing it possessed enough to produce detectable signals for two WIMP types.

“The LAT definitely sees gamma rays from the SMC, but we can explain them all through conventional sources,” Caputo said. “No signal from dark matter annihilation was found to be statistically significant.”

In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA’s Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays.

Some models predict that EGB gamma rays could arise from distant interactions of dark matter particles, such as the annihilation or decay of WIMPs. In a detailed analysis of high-energy EGB gamma rays, published April 14 in Physical Review Letters, Ajello and his team show that blazars and other discrete sources can account for nearly all of this emission.

“There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small,” Ajello said. “This information may help us place limits on how often WIMP particles collide or decay.”

Although these latest studies have come up empty-handed, the quest to find dark matter continues both in space and in ground-based experiments. Fermi is joined in its search by NASA’s Alpha Magnetic Spectrometer, a particle detector on the International Space Station.

ericvilas  asked:

You want weird naming schemes? Look at astrophysics: two of the proposed candidates for kinds of dark matter are "weakly interacting massive particles" and "massive compact halo objects". That's right, dark matter is made up of WIMPs and MACHOs.

goddammit astrophysics 

NEXT-GENERATION DARK MATTER EXPERIMENTS GET THE GREEN LIGHT

Berkeley Lab to lead new underground project in hunt for dark matter.

Last week, the U.S. Department of Energy’s Office of Science and the National Science Foundation announced support for a suite of upcoming experiments to search for dark matter that will be many times more sensitive than those currently deployed.

These so-called Generation 2 Dark Matter Experiments include the LUX-Zeplin (LZ) experiment, an international collaboration formed in 2012, managed by DOE’s Lawrence Berkeley National Lab (Berkeley Lab) and to be located at the Sanford Underground Research Facility (SURF) in South Dakota. With the announcement, the DOE and NSF officially endorsed LZ and two other dark matter experiments.

“The great news is we’ve been given the go-ahead,” says William Edwards, LZ project manager and engineer in Berkeley Lab Physics Division. “We’re looking forward to making what has been a proposal into a real, operational, first-rate experiment.”

The LZ experiment was first proposed two years ago to search for and advance our understanding of dark matter, a mysterious substance that makes up roughly 27 percent of the universe. The experiment will build on the current dark matter experiment at SURF called the Large Underground Xenon detector, or LUX.

Dark matter, so named because it doesn’t emit or absorb light, leaves clues about its presence via gravity: it affects the orbital velocities of galaxies in clusters and distorts light emitted from background objects in a phenomenon known as gravitational lensing. But direct detection of dark matter has so far been elusive.

Physicists believe dark matter could be made of difficult-to-detect particles called Weakly Interacting Massive Particles or WIMPs, which usually pass through ordinary matter without leaving a trace. The current LUX experiment consists of a one-third ton liquid xenon detector that sits deep underground where it is shielded from cosmic rays and poised to find WIMPs. When one of these particles passes through the xenon detector, it should occasionally produce an observable flash of light.

“When completed, the LZ experiment will be the world’s most sensitive experiment for WIMPs over a large range of WIMP masses,” says Harry Nelson, physicist at the University of California, Santa Barbara and current spokesperson of the LZ Collaboration. The international LZ collaboration includes scientists and engineers from 29 institutions in the United States, Portugal, Russia and the United Kingdom.

The next-generation detector, LZ, will consist of a 7-ton liquid xenon target and an active system for suppressing the rate of non-WIMP signals known as background events, both located inside the same water – tank shield used by LUX. This significant increase in detection capability will increase the sensitivity to WIMPs by more than a hundred times.

Another DOE- and NSF-approved project called SuperCDMS-SNOLAB will also be looking for WIMPs, but with a focus on those that are lighter and less energetic than those primarily detectable by the LZ detector. A third project called ADMX-Gen2 is tuned specifically for axions, and will watch for them by monitoring signals stimulated by a strong magnetic field.

“By picking a combination of these WIMP detection techniques that balance the potential sensitivity, the technical readiness, and the cost, the idea is to have the broadest dark-matter detection program possible,” says Murdock “Gil” Gilchriese, LZ project scientist and physicist in Berkeley Lab’s Physics Division.

“This is great news in the hunt for dark matter,” says Kevin Lesko, senior physicist with LUX/LZ, SURF operations manager and from Berkeley Lab’s Physics Division. “With our new detector at SURF, we plan on getting the experiment up and running by 2018 and will continue searching with LUX in the interim.”

Dark matter looks more and more likely after new gamma-ray analysis

Scientists describe as ‘extremely interesting’ new analysis that makes case for gamma rays tracing back to Wimp particles

Not long after the Fermi Gamma-ray SpaceTelescope took to the sky in 2008, astrophysicists noticed that it was picking up a steady rain of gamma rays pouring outward from the center of the Milky Way galaxy.

This high-energy radiation was consistent with the detritus of annihilating dark matter, the unidentified particles that constitute 84% of the matter in the universe and that fizzle upon contact with each other, spewing other particles as they go. If the gamma rays did in fact come from dark matter, they would reveal its identity, resolving one of the biggest mysteries in physics. But some argued that the gamma rays could have originated from another source.

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