1. 2013

    EXTRA EXTRA (positrons)! Read all about it!

    Last week, I wrote about the announcement of the first results from the Alpha Magnetic Spectrometer: a measurement of the positron fraction in cosmic rays. Although AMS-02 wasn’t the first to make this measurement, it was nevertheless a fairly exciting announcement because they confirm a drastic deviation from the theoretical prediction based on known astrophysical sources.

    Unfortunately, most of what you can read about it is pretty light on details. News articles and blog posts alike tend to go (1) Here’s what AMS measured, (2) DARK MATTER!!!1!1!! All the attention has been focused on the experimental results and the vague possibility that it could have come from dark matter, but there’s precious little real discussion of the underlying theories. What’s a poor theoretical physics enthusiast to do?

    Well, we’re in luck, because on Friday I attended a very detailed presentation on the AMS results by Stephane Coutu, author of the APS Viewpoint about the announcement. He was kind enough to point me to some references on the topic, and even to share his plots comparing the theoretical models to AMS (and other) data, several of which appear below. I never would have been …

  2. 2013

    Positrons in space!!

    A fair amount of what I write about here is about accelerator physics, done at facilities like the Large Hadron Collider. But you can also do particle physics in space, which is filled with fast-moving particles from natural sources. “All” you need to do is build a particle detector, analogous to ATLAS or CMS, and put it in Earth orbit. That’s exactly what the Alpha Magnetic Spectrometer (AMS) is. Since 2011, when it was installed on the International Space Station, AMS has been detecting cosmic electrons and positrons, looking for anomalous patterns, and today they presented their first data release.

    Let’s jump straight to the results:

    This plot shows the number of positrons with a given energy as a fraction of the total number of electrons and positrons with that energy, \(\frac{N_\text{positrons}}{N_\text{electrons} + N_\text{positrons}}\). The key interesting feature, which confirms a result from the previous experiments PAMELA and Fermi-LAT, is that the plot rises at energies higher than about \(\SI{10}{GeV}\). That’s not what you’d normally expect, because most physical processes produce fewer particles at higher energies. (Think about it: it’s less likely that you’ll accumulate …

  3. 2012

    Dark matter on the mind

    There are a few interesting experimental results and analyses from the physics world this week, mostly having to do with dark matter. Probably the biggest of these is a fairly detailed paper on the local density of dark matter by the team of Moni Bidin, Carraro, Méndez, and Smith. As you may know, dark matter is astrophysicists’ favorite method to explain how the tangential velocity of stars in large galaxies can be nearly constant all the way from the center out to the (visible) edge, despite the fact that a simple model would tell you that the velocity should be slower for stars further out. It explains a bunch of other observations too, including measurements of gravitational lensing by large galaxy clusters, so we’re pretty confident that dark matter exists.

    With that in mind, it’s kind of surprising that the analysis done by Moni Bidin, Carraro, Méndez, and Smith finds no dark matter at all within a few kiloparsecs of the solar system! Basically, what they’ve done is apply Newtonian gravity (which applies fairly well on these scales), along with ten reasonable-sounding assumptions, to find a formula which relates the velocities of stars in some region of …