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CERN | Geneva | Switzerland

View Blog | Read Bio

Grey matter confronted to dark matter

After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron.  To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars.  How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.

Addendum:

Here are two plots to show how different the positron fraction spectrum (i.e. the curve showing the fraction of positrons as a function of energy) would differ at high energy (the rightmost part of the plot) if the positrons come from the sum of all pulsars around or if it comes from dark matter annihilation. Note they are not on the same scale and difficult to compare, but they still give some idea. It will be easier once theorists update their plots with the new AMS-02 data points on them and of course, once AMS-02 releases further information at high energy.

This is one theoretical prediction of what the positron fraction spectrum should look like if the positrons come from dark matter particles like neutralinos (represented by the symbol χ). Two curves are shown, depending on the hypothetical mass of the neutralino (mχ) at 400 GeV or 800 GeV. In each case, the maximum energy the positrons can get is roughly equal to the the mass of the neutralino, such that the curve ends close to the neutralino mass. Note the logarithmic scale on both axes.

Here is the expected spectrum if the positrons come from the sum of all pulsars. Three hypotheses were shown but only the middle one seemed to fit the PAMELA experimental results. The important feature is that this curve comes down smoothly, and not sharply at neutralino mass as with the dark matter hypothesis. Again, this curve only represents one theoretical prediction as done by Dan Hooper and his colleagues. The data point in red are from the PAMELA experiment and stop around 100 GeV. The hope is that AMS-02 will be able to provide accurate measurements at higher energies, up to several hundred GeV.

Pauline Gagnon

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