## Friday, 3 April 2015

### Friday wrap-up: LHC restart, dark photon, a drama in five acts...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

• On the LHC restart, the short circuit was fixed on Monday by melting the offending metal fragment with an injection of 400 amps of current for a few milliseconds. Now the news is that "first beams could be circulating in the machine sometime between Saturday and Monday... Particle collisions at an energy of 13 TeV could start as early as June."
• The NA48/2 Collaboration has submitted their search for the dark photon in $\pi^0\to \gamma A'$ decays, ruling out the remaining parameter space for the dark photon as an explanation for the muon $(g-2)$ anomaly.

• On the 'Evidence for dark matter in the inner Milky Way' front, we now have a 'Reply to Comment on "Evidence for dark matter in the inner Milky Way."' Will we see a 'Comment on "Reply to Comment on "Evidence for dark matter in the inner Milky Way"'? Stay tuned...
• In a blog post from Tim Head, it turns out that machine learning can do pretty well at telling the difference between interesting and uninteresting papers on the arXiv, using only the title and abstract as input.
• Arkani-Hamed and Maldacena had a preprint out on Monday: "Cosmological Collider Physics." The paper is about how we might recognise the presence of new particles with inflaton interactions by their impact on primordial cosmological fluctuations. It is quite long (49+12 pages) and far outside my area, so I will leave it at that... I wonder if Tim Head's algorithm would classify the paper as interesting?
• NPR ran a story on the CRAYFIS project aimed at detecting high-energy cosmic rays with a network of smartphones, bringing >7000 people to the service.
• An arXiv preprint from Melbourne CoEPP and collaborators has pointed out that ATLAS and CMS have been led by theorists to perform searches for an unphysical dark matter EFT. To quote from the paper:

As an example of a problem encountered with an $SU(2)_L$ violating EFT, consider the following operator:$$\frac{1}{\Lambda^2}(\overline{\chi}\gamma^\mu \chi)(\overline{u}\gamma_\mu u+\xi\overline{d}\gamma_\mu d)$$This Lagrangian violates $SU(2)_L$, unless $\xi=1$. The case of unequal $u$ and $d$ couplings was considered in Ref. [13], where a very strong constructive(destructive) “interference effect” was found for $\xi=-1(+1)$, the degree of which depends on the energy scale. The analysis of Ref. [13] was subsequently repeated by the LHC experimental collaborations ATLAS [14, 15] and CMS [16, 17]. We shall demonstrate that the large cross section enhancement for $\xi=+1$ is in fact due the production of longitudinally polarized W’s as a result of breaking gauge invariance.
• APS has spotlighted some lattice QCD results from investigators including CoEPP Adelaide node members which indicate that the Λ(1405) resonance has a molecular quark pair+triplet structure (arXiv version).
• Scientific American has a story on dark matter and the dinosaurs, inspired by the Rampino paper from February. They are refreshingly skeptical, and they mention the work of Randall-Reece and Abbas-Abbas, which was my main problem with prior articles -- overall a nice piece of science journalism!
• In video/audio media
• I have only seen one April Fools' arXiv article making the rounds this week: A Farewell to Falsifiability. From the abstract, "... some far-thinking physicists have proposed instead that we should give up on the notion of Falsifiability itself. We endorse this suggestion but think it does not go nearly far enough. We believe that we should also dispense with other outdated ideas, such as Fidelity, Frugality, Factuality and other "F" words. And we quote a lot of famous people to support this view." There is some discussion on Peter Woit's blog as to whether this article from Ashoke Sen (Milner Prize winner) is also to be taken as an April Fools' submission, or just some pop-sci...
• This week I became aware of a 1979 document on the CERN server from Ellis et al. called "Can one tell QCD from a hole in the ground? : a drama in five acts." It is indeed a drama in five acts, about QCD, with illustrations and figures!

• Lastly, some loosely science-related images from the week...

Here is a photo from César Cantú of lightning in the ash cloud of Mexico's Colima volcano (hat-tip Bad Astronomy).

Typhoon Maysak from Samantha Cristoforetti on the International Space Station.