New Fermi-LAT Dark Matter Result

The Fermi-LAT collaboration has a new dark matter result. This one looks for signs of WIMP annihilation in the isotropic gamma ray background. They consider various WIMP annihilation to various channels with gamma rays produced from these decay products. This gives a broad spectrum of gamma energies instead of a simple peak if WIMPs were to annihilate directly to photons (that process would also need to be suppressed since dark matter shouldn’t couple directly to photons).


On Walter Lewin and MIT

As many of you who may follow academia or physics may know, MIT terminated its relationship with prominent emeritus professor Walter Lewin back in December.** At the time, the official explanation was that MIT had received credible accusations that Lewin had been sexually harassing people using edX/MITx and MIT’s own online coursework. As part of the response, MIT removed Lewin’s famous online introductory physics lectures and other material from the OpenCourseWare site. This set off a firestorm across the internet with accusations that MIT must be overreacting and that the response was tantamount to Nazi book burning.

Over the past few days, a few articles have come out going into the accusations in more detail, which is what prompted me to write this. First, The Tech, MIT’s student newspaper, reported on Lewin’s supposed unprofessional behavior on Twitter. Then, Inside Higher Ed was provided with much more detailed information the person who reportedly was the one who made the complaint to MIT.

You can read the articles yourself, but I think this new information (provided that it’s true) makes it much clearer why MIT responded in the way it did. The accusations are that Lewin deliberately sought out women who were in his online classes or were interested in his classes and repeatedly sent sexually harassing messages to a number of them. Even if some of them didn’t consider this harassment (and it’s clear that at least one did think it was harassment), this kind of behavior is highly unethical and unprofessional. As far as I am aware, none of the alleged behavior is actually illegal. Regardless, it is absolutely unacceptable for a professor to exploit his (or her) position to manipulate people seeking a student-teacher type of relationship in this way. Had this been an actual MIT class, it would almost certainly expose MIT to some sort of legal liability and termination even of a tenured professor would potentially be warranted. MIT’s response of stripping Lewin of his emeritus status and removing the material he was allegedly using to harass students is simply saying that professors will also be held responsible for their behavior even in the rather new and unregulated world of online education.

**A brief disclaimer: I do know some of the people who are at least indirectly involved in this, although I don’t know Lewin or anyone who has received the kinds of messages described here and in the source articles. I don’t have any inside info or confidential info or anything beyond what has been publicly revealed so far.

Mu2e Technical Design Report Available

The Mu2e collaboration uploaded a technical design report for the project, which is planned to be built at Fermilab. It’s a huge document (almost 900 pages) but if you want to read about every aspect of the project in excruciating detail there’s a lot there. Mu2e would be part of what the DoE calls the “intensity frontier,” where lab-created high-intensity but not particularly high-energy beams are used in experiments.

As the name suggests, Mu2e is designed to look for muon conversion to electrons. While muons decay into electrons, there are also neutrinos so that flavor (lepton number) is not violated. Mu2e looks for flavor violating interactions, which could yield hints about presently unknown physics beyond the Standard Model. Their signal is to look for muons captured by an aluminum target that then convert to electrons due to their interactions with the target material. The paper mentions that in most models, a tensor interaction of an electron and muon field with a vector field and a scalar interaction with quarks are the two most important terms in an effective Lagrangian. Some models mentioned as leading to flavor violating interactions are grand unified theories and leptoquarks.

ATLAS Paper on Higgs to Tau Coupling

The ATLAS experiment at the LHC has released a lengthy paper presenting results from an analysis looking for Higgs bosons decaying to tau leptons (H→ττ). This decay can occur due to the Yukawa coupling (a 3-particle vertex, so no virtual particles are needed). The Yukawa coupling combined with the non-zero vacuum expectation value of the Higgs generates the mass of the tau. Similar couplings are expected to exist for the other fermions. Because of the relationship between the mass and the coupling, the Standard Model provides a prediction for the coupling: m_\tau / vev. Thus, the heavier the fermion, the larger the decay width and higher the branching fraction.

The tau channel is a pretty difficult channel to analyze in Higgs searches. Taus can’t be seen directly. Only their decay products (jets or other leptons) are seen, which makes it much harder to reconstruct things like the momentum. These particles are found in many other processes as well: direct production of leptons and jets, decay products of other particles like W and Z bosons, and even other decays of the Higgs. Furthermore, Z→ττ presents in irreducible background since it has the same final state. So, in order to make this measurement, these backgrounds must be accurately modeled.

The final result shows a value consistent with the Standard Model prediction. The statistical strength of the result isn’t quite high enough to call this a true measurement of the Higgs to tau Yukawa coupling, but it is enough to present the measurement as evidence of a nonzero coupling. With more data (and also an ever more thorough understanding of the background physics and detector), the analysis of this channel should continue to get stronger.