Tag Archives: antimatter

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.

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Antimatter is not Dark Matter: Antimatter secrets uncovered with nuclear reactors

Dark matter is much more abundant than ordinary matter. On the other hand, antimatter is much rarer in the universe than ordinary matter. Antimatter is not dark matter. In general dark matter refers to something other than antimatter, although it would be possible to have dark matter that is anti-dark matter. See this prior post

Antimatter refers to matter that is similar to ordinary matter but has the opposite electrical charge from what is seen in regular matter. Electrons have a negative charge of -1, positrons, which are anti-electrons, have a positive charge of +1. Similarly, protons posses a charge of +1, and antiprotons have a charge of -1. Inside protons and neutrons there are quarks. There can also be antiquarks and so on.

When a particle and its associated anti-particle get too close to one another they mutually annihilate and all of their rest mass energy is converted to radiation or other particles, in accordance with E = mc2. For example, the electron has a rest mass of 511 keV (1 keV is one thousand electron-Volts, where the energy of 1 eV is that of moving an electron through a potential of one Volt.) When an electron and positron (anti-electron) annihilate, two gamma rays are produced each with energy around 511 keV. See the figure below, which is the Feynman diagram for the interaction. In the case of electron-positron annihilation, this is the only outcome possible due to the low energy of the two annihilating particles.


Mutual annhilation of an electron and positron yielding two gamma rays at 511 keV each

The big mystery is why there is matter in the universe at all! Why did not the Big Bang produce equal amounts of matter and anti-matter? In such a case the matter and anti-matter mutual annihilation process could have left little or no matter behind, and stars, galaxies, planets and people could not have formed. Cosmologists and particle physicists believe there was some small excess of matter over anti-matter, such that our present amount of matter remained after all the annihilation processes were finished.

This excess of matter over anti-matter is thought to be due to some asymmetry in the laws of physics. In general the laws are highly symmetric. Particle physicists look to understand the degree and nature of any putative asymmetries. One way to do this is by studying neutrinos, very low mass electrically neutral particles which are signatures of the weak nuclear force and products of radioactive decay. The neutrino mass is less than 2 eV, much, much less than the already small electron mass. There are believed to be 3 types of neutrinos – electron neutrinos, muon neutrinos and tau neutrinos – which are in turn associated with the electron, muon and tau particles; the muon and tau are ‘heavy’ members of the electron family.

If the neutrino has non-zero mass, then through a quantum effect known as “neutrino oscillation”, the different types of neutrinos mix together. This is due to the wave nature of all particles in quantum mechanics. Neutrinos have been detected from the Sun for many years, but at a much lower rate than initially expected, which was an outstanding puzzle. The “neutrino oscillation” mechanism resolves the discrepancy. Also, differences in neutrino and antineutrino interactions, which are due to neutrino oscillation, are thought by many particle physics to be related to the excess of matter over antimatter in the universe.

There are 3 parameters of the “neutrino oscillation” theory, which are known as ‘mixing angles’, and two of these, θ12 and θ23, have been reasonably well measured. The third mixing angle, known as θ13, is has not been well measurable until very recently.

Particle physicists working as part of the US-Chinese collaboration at the Daya Bay experiment have announced in March 2012 a positive result for the third mixing angle. It is based on measurements made near two nuclear reactors in China, one at Daya Bay and one at Ling Ao. Nuclear reactors are strong sources of antineutrinos. Another similar experiment, known as RENO, is based at a six-reactor nuclear power site in Korea. As of April 2012 the RENO physicists are also claiming a positive measurement of the θ13 mixing angle parameter, with a similar level of statistical confidence in excluding the zero value hypothesis.

Both experiments are indicating a value of around 0.10 for the mixing angle parameter, satisfying the expression sin2 (2θ13 ) = 0.1.

Other experiments include T2K in Japan, MINOS in the US and the Double Chooz international collaboration based in France. All three are seeing hints of a positive value of θ13 as well, but none have reached the statistical confidence level of the Daya Bay and RENO experiments.

The value being measured is surprisingly large, and thus very supportive of the neutrino oscillation theory for the matter vs. anti-matter discrepancy. These are exciting times for oscillating neutrinos and these experiments are moving us to closer to solving the antimatter quandry!

References: – RENO neutrino experiment, Korea – Presentation on RENO results

Wikipedia articles on antimatter, annhilation and the neutrion oscillation: