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Tag Archives: neutralinos

Axions, Inflation and Baryogenesis: It’s a SMASH (pi)

Searches for direct detection of dark matter have focused primarily on WIMPs (weakly interacting massive particles) and more precisely on LSPs (the lightest supersymmetric particle). These are hypothetical particles such as neutralinos that are least massive members of the hypothesized family of supersymmetric partner particles.

But supersymmetry may be dead. There have been no supersymmetric particles detected at the Large Hadron Collider at CERN as of yet, leading many to say that this is a crisis in physics.

At the same time as CERN has not been finding evidence for supersymmetry, WIMP dark matter searches have been coming up empty as well. These searches keep increasing in sensitivity with larger and better detectors and the parameter space for supersymmetric WIMPs is becoming increasingly constrained. Enthusiasm unabated, the WIMP dark matter searchers continue to refine their experiments.

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LUX dark matter detector in a mine in Lead, South Dakota is not yet detecting WIMPs. Credit: Matt Kapust/ Sanford Underground Research Facility

What if there is no supersymmetry? Supersymmetry adds a huge number of particles to the particle zoo. Is there a simpler explanation for dark matter?

Alternative candidates under consideration for dark matter, including sterile neutrinos, axions, and primordial black holes, and are now getting more attention.

From a prior blog I wrote about axions as dark matter candidates:

Axions do not require the existence of supersymmetry. They have a strong theoretical basis in the Standard Model as an outgrowth of the necessity to have charge conjugation plus parity conserved in the strong nuclear force (quantum chromodynamics of quarks, gluons). This conservation property is known as CP-invariance. (While CP-invariance holds for the strong force, the weak force is CP violating).

In addition to the dark matter problem, there are two more outstanding problems at the intersection of cosmology and particle physics. These are baryogenesis, the mechanism by which matter won out over antimatter (as a result of CP violation of Charge and Parity), and inflation. A period of inflation very early on in the universe’s history is necessary to explain the high degree of homogeneity (uniformity) we see on large scales and the near flatness of the universe’s topology. The cosmic microwave background is at a uniform temperature of 2.73 Kelvins to better than one part in a hundred thousand across the sky, and yet, without inflation, those different regions could never have been in causal contact.

A team of European physicists have proposed a model SMASH that does not require supersymmetry and instead adds a few particles to the Standard Model zoo, one of which is the axion and is already highly motivated from observed CP violation. SMASH (Standard Model Axion Seesaw Higgs portal inflation) also adds three right-handed heavy neutrinos (the three known light neutrinos are all left-handed). And it adds a complex singlet scalar field which is the primary driver of inflation although the Higgs field can play a role as well.

The SMASH model is of interest for new physics at around 10^11 GeV or 100 billion times the rest mass of the proton. For comparison, the Planck scale is near 10^19 GeV and the LHC is exploring up to around 10^4 GeV (the proton rest mass is just under 1 GeV and in this context GeV is short hand for GeV divided by the speed of light squared).

smashcmb

Figure 1 from Ballesteros G. et al. 2016. The colored contours represent observational limits from the Planck satellite and other sources regarding the tensor-to-scalar power ratio of primordial density fluctuations (r, y-axis) and the spectral index of these fluctuations (ns, x-axis). These constraints on primordial density fluctuations in turn constrain the inflation models. The dashed lines ξ = 1, .1, .01, .001 represent a key parameter in the assumed slow-roll inflation potential function. The near vertical lines labelled 50, 60, 70, 80 indicate the number N of e-folds to the end of inflation, i.e. the universe inflates by a factor of e^N in each of 3 spatial dimensions during the inflation phase.

So with a single model, with a few extensions to the Standard Model, including heavy right-handed (sterile) neutrinos, an inflation field, and an axion, the dark matter, baryogenesis and inflation issues are all addressed. There is no need for supersymmetry in the SMASH model and the axion and heavy neutrinos are already well motivated from particle physics considerations and should be detectable at low energies. Baryogenesis in the SMASH model is a result of decay of the massive right-handed neutrinos.

Now the mass of the axion is extremely low, of order 50 to 200 μeV (millionths of an eV) in their model (by comparison, neutrino mass limits are of order 1 eV), and detection is a difficult undertaking.

There is currently only one active terrestrial axion experiment for direct detection, ADMX. It has its primary detection region at lower masses than the SMASH model is suggesting, and has placed interesting limits in the 1 to 10 μeV range. It is expected to push its range up to around 30 μeV in a couple of years. But other experiments such as MADMAX and ORPHEUS are coming on line in the next few years that will explore the region around 100 μeV, which is more interesting for the SMASH model.

Not sure why the researchers didn’t call this the SMASHpie model (Standard Model Axion Seesaw Higgs portal inflation), because it’s a pie in the face to Supersymmetry!

stooges_heavenlydaze

It would be wonderfully economical to explain baryogenesis, inflation, and dark matter with a handful of new particles, and to finally detect dark matter particles directly.

Reference

“Unifying inflation with the axion, dark matter, baryogenesis and the seesaw mechanism” Ballesteros G., Redondo J., Ringwald A., and Tamarit C. 2016  https://arxiv.org/abs/1608.05414

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Dark Axion Stars

In a post from 4 years ago I discussed “Dark Matter Powered Stars”.

The context here was neutralino dark matter, which is a possible explanation for very massive stars in the early universe. The idea is that the very first stars could be thousands of solar masses, much greater than is possible with ordinary matter dominated stars. They would be powered by dark matter annihilation in their cores during the early part of their life. They would eventually collapse to black holes and could be candidates to seed supermassive black holes found at the center of many galaxies.

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Hubble Space Telescope image of Sirius A and Sirius B (lower left) 

NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)

Another dark matter candidate apart from the neutralino is the axion. While the neutralino is expected to have masses in the several to tens of GeV (Giga-electron-Volts), the axion mass is a tiny fraction of an eV, at least a trillion times smaller than the expected neutralino mass. So there would be many more of them, of course, to explain the amount of dark matter we detect gravitationally.

Neither neutralinos nor axions have been discovered to date. The axion does not require supersymmetry beyond the Standard Model of particle physics, so in that sense it is a more conservative proposed candidate.

Currently we detect dark matter only through its gravitational effects – in galaxies, in clusters of galaxies, and at the very large scale by looking at thermal variations in the cosmic microwave background radiation.

In addition there are three main direct methods to try to ‘see’ these elusive particles. One is to directly detect dark matter (e.g. neutralinos) here on Earth when it collides with ordinary matter – or in the case of axions – generates photons in the presence of a magnetic field. Another is to attempt to create it at the Large Hadron Collider, and the third is to look in space for astrophysical signals resulting from dark matter. These could include gamma rays produced in the galactic center when dark matter mutually annihilates.

In a paper recently published in the journal Physical Review Letters and titled “Accretion of dark matter by stars”, Richard Brito, Vitor Cardoso and Hirotada Okawa discuss a different kind of dark star, one whose dark matter component is axions. The paper is available here.

There are two formation scenarios envisaged. The first is that dark matter (axion) stellar cores form and then these accrete additional dark matter and ordinary matter. In the second scenario, a star forms primarily from ordinary matter, but then accretes a significant amount of dark matter.

We are talking about dark matter fractions which may be say 5% or 20% of the total mass of the star.

The authors find that stable configurations seem to be possible and that the axion dark matter may lead to stellar oscillations in the microwave band. So looking for stellar oscillations in the Gigahertz range may be another astrophysical detection method for dark matter. They intend to explore the idea more deeply in future research.


AMS Positron Excess: Due to Dark Matter or not?

The first results from the Alpha Magnetic Spectrometer (AMS), which is an experiment operating in orbit on the International Space Station (ISS), have been released.

ImageIt’s been two years since the delivery via Space Shuttle to the ISS (in May, 2011) of the AMS-02 instrument, which was especially designed to explore the properties of antimatter. And it’s been a long time coming to get to this point, since the experiment was first proposed in 1995 by the Nobel Prize-winning M.I.T. physicist, professor Samuel Ting. A prototype instrument, the AMS-01, flew in a short-duration Space Shuttle mission in 1998, and had much lower sensitivity.

Over 16 countries across the globe participate in the AMS mission, and the instrument underwent testing at the CERN particle physics research center near Geneva and also in the Netherlands before being launched from Cape Canaveral, Florida.

The lifetime of the mission is expected to extend for over 10 years. In this first data release, with over 30 billion cosmic rays detected, the AMS has detected among these over 400,000 positrons, the positively charged antiparticles to electrons. This is the most antimatter that has ever been directly measured in space.

It is hoped that the AMS can shed light on dark matter, since one of the possible signatures of dark matter is the production of positrons and electrons when dark matter particles annihilate. This assumes that some type of WIMP is the explanation for dark matter, WIMP meaning a “weakly interacting massive particle”. Weakly interacting signifies that dark matter particles (in this scenario highly favored by many physicists) would interact through the weak nuclear force, but not the electromagnetic force. Which basically explains why we can’t easily detect them except through their gravitational effects. Massive particle refers to a particle substantially more massive than a proton or neutron, which have rest masses of just under 1 GeV (1 giga volt) in energy terms. A WIMP dark matter particle mass could be 10 to 1000 times or more higher. The lightest member of the neutralino family is the most-favored hypothesized WIMP dark matter candidate.

The figure below shows the positron relative abundance versus energy, based on 18 months of AMS operational data. The energy of detected positrons ranges from 1/2 GeV to over 300 GeV. The abundance, shown on the y-axis, is the fraction of positrons relative to total electrons and positrons detected at a given energy. The spectrum shows a clear trend to relatively fewer positrons as the energy grows to 10 GeV and then a substantially increasing relative number of positrons at higher energies. This general shape for the spectrum was seen with previous experiments including Fermi, Caprice94 and Pamela, but is much clearer with the AMS due to the higher resolution and significantly greater number of positrons detected. It is particularly this increase in positrons seen above 10 GeV that is suggestive of sources other than the general cosmic ray background.

PositronspectrumFigure: Positron relative fraction (y-axis) versus energy (x-axis)

So what is the source of the energetic positrons detected by the AMS? Some or all of these could be produced when two dark matter WIMP particles meet one another. In the WIMP scenario the dark matter particle such as the neutralino is neutrally charged (no electromagnetic interaction, remember) and also its own antiparticle. And when a particle meets its antiparticle what happens is that they mutually annihilate. The energy of the pair of colliding dark matter particles is transformed into lighter particles, including electron-positron pairs and energetic photons including gamma rays.

Another likely source is pulsars, which are rotating neutron stars with magnetic fields. Since neutron stars are compact and rotate quickly, and their magnetic field strengths are high, electrons and positrons can be accelerated to very high energy. In particular, the Geminga pulsar is the closest energetic pulsar and has been suggested as a major source of these extra positrons.

More data is needed, especially at higher energies above 100 GeV. Over the next few years as AMS continues to operate and the number of positrons detected climbs to 1 million and above, this spectral shape will be better determined. And as the shape of the high-energy portion of the spectrum becomes clearer, it will help elucidate whether dark matter or pulsars or something else are the primary source of the positrons.

You can follow AMS-02 on Facebook here.

References:

http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/

http://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html

http://press.web.cern.ch/backgrounders/first-result-ams-experiment

http://physicsworld.com/cws/article/news/2009/aug/10/excess-positrons-are-linked-to-geminga-pulsar

https://darkmatterdarkenergy.com/2012/06/05/antimatter-is-not-dark-matter-antimatter-being-uncovered-with-nuclear-reactors/


Dark Matter Powered Stars

Gamma Ray Burster 070125

GRB (gamma ray burster) 070125. Credit: B. Cenko, et al. and the W. M. Keck Observatory.

So what is a “dark star”? It is not a Newtonian black hole as suggested by John Michell in the 18th century who used the term while postulating that gravity could prevent light escaping from a very massive, compact star. It is not a “dark energy star”, which is related to a black hole, but rather than having a singularity at the center, quantum effects cause infalling matter to be converted to vacuum state energy, dark energy. It is not a comic book, science fiction comedy film, or Grateful Dead song.

In this blog entry we are writing about dark matter powered stars. These would be the very first stars, formed within the first few hundred million years of the universe’s existence. The working assumption is that dark matter consists of WIMPs – weakly interacting massive particles, in particular the favored candidate is the neutralino. The neutralino is the lightest particle among the postulated supersymmetric companions to the Standard Model suite of particles. As such it would be stable, would not ordinarily decay and is being searched for with XENON, CDMS, DAMA, AMS-02 and many other experiments.

The first stars are thought by astrophysicists to have been formed from clouds of ordinary hydrogen and helium as well as dark matter, with dark matter accounting for 5/6 of the total mass. These clouds, called “dark matter halos” are considered to have contained from about one million to 100 million times the Sun’s mass. The ordinary matter would settle towards the center as it cooled via radiation processes and the dark matter (which does not radiate) would be more diffuse. The stars forming at the center would be overwhelmingly composed of ordinary matter (hydrogen and helium nuclei and electrons).

Without any dark matter at all, ordinary matter stars up to about 120 to 150 solar masses could form; above this limit they would have very hot surfaces and their own radiation would inhibit further growth by infall of matter from the halo. But if as little as one part in a thousand of the protostar’s mass was in the form of dark matter this limitation goes away. The reason is that the neutralino WIMPs will, from time to time, meet one another inside the star and mutually annihilate since the neutralino is its own anti-particle. The major fraction of the energy produced in the annihilation remains inside the star, but some escapes in the form of neutrinos (not neutralinos).

Annihilation of these neutralinos is a very efficient heating mechanism throughout the volume of the star, creating a great amount of heat and pressure support, basically puffing up the star to a very large size. The stellar surface is, as a result, much cooler than in the no dark matter case, radiation pressure is insignificant, and accretion of significantly more material onto the star can occur. Stars could grow to be 1000 solar masses, or 10,000 solar masses, potentially even up to one million solar masses. Their sizes would be enormous while they were in the dark matter powered phase. Even the relatively small 1000 solar mass star, if placed at the Sun’s location, would extend through much of our Solar System, beyond the orbit of Saturn.

We have mentioned the neutralino meets neutralino annihilation mechanism. A second mechanism for heating the interior of the star would be direct impact of neutralinos onto protons and helium nuclei. This second mechanism could help sustain the duration as a dark matter powered star potentially even beyond a billion years.

Eventually the dark matter fuel would be exhausted, and the heat and pressure support from this source lost. The star would then collapse until the core was hot enough for nuclear fusion burning. Stars of 1000 solar masses would burn hydrogen, and later helium, and evolve extremely rapidly because of the high density and temperature in their cores. After their hydrogen and helium fusion cycles completed there would be no source of sufficient pressure support and they would collapse to black holes (or maybe dark energy stars).

It is calculated with detailed simulations that the dark star mechanism allows for much more massive stars than could be formed otherwise, and this provides a potentially natural explanation for the creation of massive black holes. Our own Milky Way has a black hole around 3 million solar masses at its center, and it appears a majority of galaxies have large black holes. The image at the top of this blog is of a gamma ray burst detection that may have come from a large black hole formation event.

References:

en.wikipedia.org/wiki/Dark_star_(dark_matter)

Freese, K. et al 2008, Dark Stars: the First Stars in the Universe may be powered by Dark Matter Heating, http://arxiv.org/pdf/0812.4844v1

Freese, K. et al 2010, Supermassive Dark Stars, http://arxiv.org/abs/1002.2233

http://news.discovery.com/space/did-dark-stars-spawn-supermassive-black-holes.html


Dark Matter

Dark matter is like the hidden part of an iceberg found below the water line. The hidden part is the dominant portion of the mass and supports the structure apparent from the visible portion above. Dark matter couples to ordinary matter through the gravitational force. The ordinary visible matter, which we detect through light from galaxies and stars, is analogous to the portion of an iceberg above the water line.

Why is dark matter important? It dominates the mass-energy density of the universe during the early part of its lifetime. Just after the epoch of the cosmic microwave background (CMB) the universe is composed of mostly dark matter (dark energy comes to dominate much later, during the most recent 5 billion years). But also there is the ordinary matter, which at that time is a highly uniform gas of hydrogen and helium atoms, with slightly overdense and slightly underdense regions.

The existence of a large amount of dark matter promotes much more efficient gravitational collapse of the overdense regions. This is a self-gravitational process in which regions that are slightly denser than the critical mass density (which is also the average mass density of the universe) at a given time will collapse away from the overall expansion that continues around them. Both the dark and ordinary matter in such a region collapse together, but it is the ordinary matter that forms the first stars since it interacts via various physical processes (think friction, radiation, etc.) to a much greater degree allowing for collapse. The dark matter, interacting only via gravity and perhaps the weak nuclear force, but not through electromagnetism, remains more spread out, more diffuse. The dark matter promotes the collapse process though, through increasing the self-gravity of a given region and this results in more efficient formation of stars and galaxies. There are many more stars and galaxies formed at early times than would be the case in the absence of substantial dark matter.

From cosmological observations including the CMB and high redshift (distant) supernovae, we find dark matter is about 1/4 of the mass-energy density of the universe. Dark matter is composed of either faint ordinary matter or, more likely, exotic matter that interacts through the weak and gravitational forces only. Dark matter is clearly detected by its gravitational effects on galaxy rotation curves, and is inferred from the kinematics of clusters of galaxies and the temperature measures of X-ray emission from very hot gas between galaxies in these same clusters. Dark matter is also detected through gravitational lensing effects in our galactic halo and in very large-scale cosmological structures. The abundance of deuterium, which is produced from Big Bang nucleosynthesis, in conjunction with the universe’s now well-known expansion rate, severely constrains the density of baryons (amount of ordinary nucleonic matter, i.e. protons and neutrons) in the universe and leads to the conclusion that over 80% of matter is nonbaryonic.

The MACHO (MAssive Compact Halo Objects) alternative, which refers to potential ordinary matter, is thus limited. The WIMP (Weakly Interacting Massive Particles) contribution is dominant. Hot WIMPS (e.g. neutrinos) are ruled out because they inhibit clumpiness and galaxy formation in the early universe. Cold nonbaryonic dark matter is the best candidate, with the primary candidate being an hypothesized, undiscovered particle. Neutralinos are thought by many particle physicists to be the best candidate for this, and have an expected mass of order 50 to 250 times the mass of the proton. It must be emphasized that no neutralino or similar particles (known as supersymmetric particles) have ever been detected directly. It is hoped that the Large Hadron Collider newly operational at CERN near Geneva may do this.

There may be direct detection of an annual modulation of the WIMP wind in a large scintillation array. There is also a possible indirect detection which manifests as an excess of 1 GeV gamma rays in our galactic halo. Significantly more sensitive detectors are needed to find these elusive particles and to provide a stronger foundation for supersymmetric physics, which postulates many new and heavy particles. An important experiment AMS-02 (Alpha Magnetic Spectrometer, 2nd generation) is scheduled to be carried to the International Space Station on the last Endeavour Shuttle mission scheduled for April 19, 2011. See http://www.ams02.org and http://cosmiclog.msnbc.msn.com/_news/2011/04/04/6403905-will-space-jam-delay-shuttle-launch

The next decade should allow us to shed new light on dark matter. Whatever it is made of, without the existence of substantial amounts of dark matter, there wouldn’t be nearly as many stars and galaxies in the universe, and we very likely wouldn’t be here.