New particle links dark matter with missing antimatter

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Nov 29, 2010

Physicists in the US and Canada have proposed a new particle that could solve two important mysteries of modern physics: what is dark matter and why is there much more matter than antimatter in the universe?

The yet-to-be-discovered "X" particle is expected to decay mostly to normal matter, whereas its antiparticle is expected decay mostly to "hidden" antimatter. The team claims that its existence in the early universe could explain why there is more matter than antimatter in the universe – and that dark matter is in fact hidden antimatter.

Dark matter is a mysterious substance that appears to make up about 80% of the material universe. Although its existence can be inferred from its gravitational pull on normal matter, physicists have yet to detect it directly and therefore don't know what it is made of. Antimatter, on the other hand, is easy to create and study in the lab. However, the Standard Model of particle physics cannot explain why antimatter is so rare in a universe that is dominated by matter – a mystery called baryon asymmetry.

Now, Hooman Davoudiasl of Brookhaven National Laboratory and colleagues at TRIUMF and the University of British Columbia have proposed a new particle dubbed X that could solve both of these mysteries. X has a mass of about 1000 GeV – making it about a thousand times heavier than a proton. This particle can decay to a neutron or to two hypothetical hidden particles called Y and F. Both hidden particles would have masses of about 2–3 GeV. Its antiparticle, anti-X, decays to an antineutron or to the pair anti-Y and anti-F.

Physicists have tried to try to explain the baryon asymmetry by invoking a violation of the charge–parity (CP) symmetry – the result being that decaying particles are more likely to generate matter than antimatter. CP violation has been observed in laboratories, but the preference for matter is far too small to account for the proportion of matter in the universe.

X also commits CP violation in a way that author Kris Sigurdson of the University of British Columbia calls a "yin yang" decay pattern. While X decays to neutrons more often than anti-X decays to antineutrons, it is balanced by anti-X, which decays to anti-Y and anti-F more often than Y and F. When almost all particles with an available antiparticle annihilated one another in the early universe, these discrepancies left a chunk of visible matter and a heavier chunk of dark antimatter to form the cosmos.

The team has also thought of how the anti-Y and anti-F particles could be detected. Unlike weakly interacting massive particles (WIMPs) – which dominate many theories of dark matter – anti-Y and anti-F do not annihilate each other. However, the antiparticles would cause protons to decay, which is forbidden by the Standard Model. If an anti-Y particle collides with a proton, for instance, a virtual interaction with particle X can break the proton apart, transforming it into a positively charged kaon, and turning the anti-Y particle into a F particle.

A detector looking for proton decays, such as SuperKamiokande in Japan's Kamioka mine, could catch the kaon. Kaons produced this way would have much higher energies than those generated by proton decays allowed by other theories that go beyond the Standard Model. Although protons are expected to be fairly resilient to this decay process, Sigurdson says, "This scenario could be on the boundary of detectability."

"It looks like a very interesting model," says Dan Hooper of Fermilab. Although at least three more models linking the production of dark matter to the baryon asymmetry are in development, he says that the proton-decay signature sets this scenario apart.

Matthew Buckley of Fermilab says that there is a sudden interest in linking dark matter with the baryon asymmetry because of recent experiments that have tried (unsuccessfully) to detect dark matter. Although WIMP models prefer dark-matter particles with masses around 100 GeV, the experiments suggest that dark-matter particles have masses nearer 7 or 8 GeV.

Having such a large mass "definitely isn't what a WIMP is 'supposed to look like'," says Buckley. However, dark matter that also explains the baryon asymmetry seems to be more in line with recent experimental results – which is why Buckley believes it deserves further exploration.

The work is reported in Phys. Rev. Lett. 105 211304.

Kate McAlpine is a science communicator based in the US

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'Best evidence yet' for dark matter comes from Milky Way centre

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Oct 29, 2010

Energetic radiation pulsing from the belly of the Milky Way is the clearest signal yet of dark matter. That is according to a pair of astrophysicists in the US who reach this conclusion after scrutinising the public data collected by NASA's orbiting Fermi Observatory. "I certainly think it's the best evidence we've seen so far," says Dan Hooper, one half of the team, based at the University of Chicago.

It is a huge claim because for over 70 years astrophysicists have debated the existence of dark matter, which is thought to make up 80% of the universe's mass, yet they have failed to gather any definitive evidence, either direct or indirect, for its existence. But with several hints for dark matter published in recent years – all received with scrutiny by the wider astrophysics community – the US pair will have a hard time convincing others that their signal is what they think it is.

Hooper and his colleague Lisa Goodenough of New York University have analysed the spectra of gamma rays coming from the centre of our galaxy, as collected by the Large Area Telescope onboard the Fermi observatory. Although dark matter does not couple to light, it should annihilate with itself to produce gamma rays, and the amount of annihilation should increase rapidly towards the galactic centre as dark-matter density increases.

Last year Hooper and Goodenough compared the Fermi spectra of gamma rays with a simple computer model of dark matter, and suggested that an excess of gamma rays coming from the galactic centre might be evidence of dark-matter annihilation. At that time other researchers weren't convinced because there were other possible origins for the signal, such as high-energy photons striking interstellar gas. In their latest analysis, however, Hooper and Goodenough have tried to allay these concerns using a far more complex methodology that looks at specific components making up the background of gamma rays.

The US pair break down the gamma-ray background into three parts: a narrow emission from the galaxy's disc; an emission from known point sources; and a spherical or "bulge" emission around the galactic centre. According to their model, no matter what parameters one chooses for dark matter, there should always be a threshold within the bulge emission where dark-matter annihilation begins to outshine other gamma-ray sources. This is because – unlike other sources – emission from dark-matter annihilation follows a square law, so that doubling the density increases the annihilation four-fold.

Hooper and Goodenough examined the Fermi spectra at many regions inside the gamma-ray bulge, and found the data always matched the model's prediction of normal emission – except right at the galactic centre. Here, in a narrow region spanning less than one-quarter of a degree, the emission was far stronger than the model predicted, and had a more lopsided spectrum. Those characteristics, the US pair claims, point to a dark-matter particle – a weakly interacting massive particle, or WIMP – in a mass range of 7.3–9.2 GeV.

This light mass is partly what lends the analysis credence. For years physicists working on the DAMA experiment in Italy claim to have found WIMPs colliding with sodium-iodide nuclei, while those working on the CoGeNT collaboration in the US have tentatively revealed similar WIMP signals coming from germanium detectors – and many believe the only way to reconcile these signals is to assume a WIMP with a mass around 8 GeV.

"Until I had seen this latest paper from Hooper and Goodenough, I was kind of thinking with the light WIMP scenario – nah," says Alex Murphy, a particle astrophysicist who works on the ZEPLIN-III dark-matter experiment in the UK. "But now I've seen it, I'm starting to think – hmm, maybe. Perhaps now we should be looking at other ways to confirm or disprove this proposal."

Murphy voices scepticism about the strength of the claim, however, because he is not convinced Hooper and Goodenough understand the idiosyncrasies of the Fermi instrumentation sufficiently well. Although the Fermi team has published its own preprint revealing an excess of gamma rays near the galactic centre, it has so far stopped short of interpreting this as dark matter.

Ronaldo Bellazzini, the principal investigator on Fermi's Italian team, warns that Hooper and Goodenough's analysis of the galactic centre could still be prone to misinterpretation. "Unfortunately, this region, and whatever [Fermi] observes along the line of sight to it, is rich with astrophysical sources that can mimic signals similar to dark-matter annihilation, like pulsars and supernovae remnants" he says.

Meanwhile, Michael Kuhlen, a dark-matter theorist at the University of California at Berkeley, believes there is "probably a good reason" why the Fermi collaboration has held back from making conclusions on the gamma-ray excess. "They're certainly aware of it, but probably just haven't been able to convince themselves that they fully understand the instrument's behaviour, or the backgrounds, or the kinds of possible astrophysical sources that could produce the signal," he says.

But Kulen adds: "Really they're just trying to stir the pot, and get people to seriously consider the possibility that Fermi may have already detected a dark-matter annihilation signal. This is a good thing."

A preprint of the paper is available at arXiv: 1010.2752.

Jon Cartwright is a freelance journalist based in Bristol, UK

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'Best evidence yet' for dark matter comes from Milky Way centre

To enjoy free access to all high-quality "In depth" content, including topical features, reviews and opinion sign up

Oct 29, 2010

Energetic radiation pulsing from the belly of the Milky Way is the clearest signal yet of dark matter. That is according to a pair of astrophysicists in the US who reach this conclusion after scrutinising the public data collected by NASA's orbiting Fermi Observatory. "I certainly think it's the best evidence we've seen so far," says Dan Hooper, one half of the team, based at the University of Chicago.

It is a huge claim because for over 70 years astrophysicists have debated the existence of dark matter, which is thought to make up 80% of the universe's mass, yet they have failed to gather any definitive evidence, either direct or indirect, for its existence. But with several hints for dark matter published in recent years – all received with scrutiny by the wider astrophysics community – the US pair will have a hard time convincing others that their signal is what they think it is.

Hooper and his colleague Lisa Goodenough of New York University have analysed the spectra of gamma rays coming from the centre of our galaxy, as collected by the Large Area Telescope onboard the Fermi observatory. Although dark matter does not couple to light, it should annihilate with itself to produce gamma rays, and the amount of annihilation should increase rapidly towards the galactic centre as dark-matter density increases.

Last year Hooper and Goodenough compared the Fermi spectra of gamma rays with a simple computer model of dark matter, and suggested that an excess of gamma rays coming from the galactic centre might be evidence of dark-matter annihilation. At that time other researchers weren't convinced because there were other possible origins for the signal, such as high-energy photons striking interstellar gas. In their latest analysis, however, Hooper and Goodenough have tried to allay these concerns using a far more complex methodology that looks at specific components making up the background of gamma rays.

The US pair break down the gamma-ray background into three parts: a narrow emission from the galaxy's disc; an emission from known point sources; and a spherical or "bulge" emission around the galactic centre. According to their model, no matter what parameters one chooses for dark matter, there should always be a threshold within the bulge emission where dark-matter annihilation begins to outshine other gamma-ray sources. This is because – unlike other sources – emission from dark-matter annihilation follows a square law, so that doubling the density increases the annihilation four-fold.

Hooper and Goodenough examined the Fermi spectra at many regions inside the gamma-ray bulge, and found the data always matched the model's prediction of normal emission – except right at the galactic centre. Here, in a narrow region spanning less than one-quarter of a degree, the emission was far stronger than the model predicted, and had a more lopsided spectrum. Those characteristics, the US pair claims, point to a dark-matter particle – a weakly interacting massive particle, or WIMP – in a mass range of 7.3–9.2 GeV.

This light mass is partly what lends the analysis credence. For years physicists working on the DAMA experiment in Italy claim to have found WIMPs colliding with sodium-iodide nuclei, while those working on the CoGeNT collaboration in the US have tentatively revealed similar WIMP signals coming from germanium detectors – and many believe the only way to reconcile these signals is to assume a WIMP with a mass around 8 GeV.

"Until I had seen this latest paper from Hooper and Goodenough, I was kind of thinking with the light WIMP scenario – nah," says Alex Murphy, a particle astrophysicist who works on the ZEPLIN-III dark-matter experiment in the UK. "But now I've seen it, I'm starting to think – hmm, maybe. Perhaps now we should be looking at other ways to confirm or disprove this proposal."

Murphy voices scepticism about the strength of the claim, however, because he is not convinced Hooper and Goodenough understand the idiosyncrasies of the Fermi instrumentation sufficiently well. Although the Fermi team has published its own preprint revealing an excess of gamma rays near the galactic centre, it has so far stopped short of interpreting this as dark matter.

Ronaldo Bellazzini, the principal investigator on Fermi's Italian team, warns that Hooper and Goodenough's analysis of the galactic centre could still be prone to misinterpretation. "Unfortunately, this region, and whatever [Fermi] observes along the line of sight to it, is rich with astrophysical sources that can mimic signals similar to dark-matter annihilation, like pulsars and supernovae remnants" he says.

Meanwhile, Michael Kuhlen, a dark-matter theorist at the University of California at Berkeley, believes there is "probably a good reason" why the Fermi collaboration has held back from making conclusions on the gamma-ray excess. "They're certainly aware of it, but probably just haven't been able to convince themselves that they fully understand the instrument's behaviour, or the backgrounds, or the kinds of possible astrophysical sources that could produce the signal," he says.

But Kulen adds: "Really they're just trying to stir the pot, and get people to seriously consider the possibility that Fermi may have already detected a dark-matter annihilation signal. This is a good thing."

A preprint of the paper is available at arXiv: 1010.2752.

Jon Cartwright is a freelance journalist based in Bristol, UK

View the original article here