symmetry day: crossing, permutation

Today's brown-bag talk, by Grant Remmen (NYU), was about (in part) crossing symmetry. This is the symmetry that any Feynman diagram can be rotated through 90 degrees (converting time into space and vice versa) and the interaction will have the same scattering amplitude. This symmetry relates electron–positron annihilation to electron–electron scattering. The symmetry has an important role in string theory, because it is a constraint on any possible fundamental theory. This symmetry has always seemed incredible to me, but it is rarely discussed outside very theoretical circles.

After the talk, and in the Blanton–Hogg group meeting, I brought up things about invariant functions that I learned from Soledad Villar (JHU) that are really confusing me: It is possible (in principle, maybe not in practice) to write any permutation-invariant function of N objects as a function of a sum of universal functions of the N objects (that's proven). How does that relate to k-point functions? Most physicists believe that any k-point function estimate will require a sum over all N-choose-k k-tuples. That's a huge sum, way bigger than a sum over N. What gives? I puzzled some of the mathematical physicists with this and I remain confused.

the stability of the vacuum

The research highlight of my day today was our weekly lunchtime blackboard talk, as it often is on Mondays. TOday it was Isabel Garcia Garcia (NYU), talking about the stability of the vacuum. She was specifically talking about the stability of a false vacuum, and specifically when there are large extra dimensions. The weird thing is, in all string-like models for the fundamental particle physics model there are both large extra dimensions and an exceedingly low probability that we live in the true vacuum state. That means a decay to a different state is possible (inevitable?). Why has this vacuum lived so long?

Pheno 2019, day 3

I spent the day at Pheno 2019, where I gave a plenary about Gaia and dark matter. It was a fun day, and I learned a lot. For example, I learned that when you have a dark photon, you naturally get tiny couplings between the dark matter and the photon, as if the dark matter has a tiny charge. And there are good experiments looking for milli-charged particles. I learned that deep learning methods applied to LHC events are starting to approach information-theoretic bounds for classifying jets. That's interesting, because in the absence of a likelihood function, how do you saturate bounds? I learned that the Swampland (tm) is the set of effective field theories that can't be represented in any string theory. That's interesting: If we could show that there are many EFTs that are incompatible with string theory, then string theory has strong phenomenological content!

In the last talk of the day, Mangano (CERN) talked about the future of accelerators. He made a very interesting point, which I have kind-of known for a long time, but haven't seen articulated explicitly before: If you are doing a huge project to accomplish a huge goal (like build the LHC to find the Higgs), you need to design it such that you know you will produce lots and lots of interesting science along the way. That's an important idea, and it is a great design principle for scientific research.

the measure problem

Not a high-research day. Conversations with Bedell (Flatiron) about papers we need to finish, and a plan to sprint in one week's time. Also a great Brown-Bag talk by Matt Kleban (NYU) about the many vacuum states available in a string-theory-like model with lots of axions. There will always be lots of volume that has a cosmological constant near our observed value, no matter that it is so damned low. But figuring out what this means for generic observers is hard, both because of the anthropic issues, but also because there is no measure for the spacetime. That's crazy and led to yet another discussion of the measure problem. It always surprises me that this is an unsolved problem.

talks all day; Dr Gorbenko

Roberto Sanchis-Ojeda (Berkeley) arrived for a few days of hacking. He is responsible for finding some very short-period transiting exoplanets. We didn't get a chance to set our goals for the day, because the day was packed with talks:

Victor Gorbenko (NYU) gave an absolutely excellent PhD defense. Congratulations Dr Gorbenko. Among other things, he was considering the difference between the theoretical (analytic) action of massless modes on the 1+1 worldsheet of a string (not a 0+1 wordline but s 1+1 worldsheet) and a lattice calculation. In this comparison, he found massive modes. He very nicely connected this work to larger questions in string theory, but also to the origins of string theory in QCD.

At lunch, Guido D'Amico (NYU) spoke about axions, axion–photon interactions, and cosmic transparency. This is related to work I did with Bovy and with More in the optical. He advertised some clever ideas he has about about using the Planck data to constrain this sector using millimeter wavelengths. He has been implementing some of these ideas at #astrohackny.

At the end of the day, Neil Lawrence (Sheffield) spoke about deep machine-learning models built from nested Gaussian Processes. The methods show a lot of promise and connect well to things we are thinking about in CampHogg.

radio-telescope calibration to string theory

At #NYCastroML, Josh Peek and I had a public conversation about how to self-calibrate single-dish radio observations of the Milky Way HI gas. We were able to cast it into a linear-ish form and develop the hope that he could use a sparse-coding method advocated (effectively) in the Ivezic et al book.

In the afternoon, Kilian Walsh presented the work he has done on cosmology to his PhD committee. He can use the void probability function to constrain the halo model beyond the abundance and clustering constraints usually in use. He is now a PhD candidate; congratulations!

At lunch time, Roberto Gobbetti (NYU) defended his PhD. He presented a mechanical model for slow-roll inflation based on flux discharge around compact dimensions in the string landscape. It is a beautiful model in which a great many things are computable. Let's hear it for the observational consequences of string-like theories! And let's congratulate Dr Gobbetti.

Dr Schillo

Marjorie Schillo (NYU) defended her PhD today. She showed that eternal inflation implies a non-zero spatial curvature for the Universe. More specifically, it links the CMB low-ell amplitudes to the spatial curvature. Since all string-based inflation models are eternally inflating, she can construct hard tests of string theory by measurement of the spatial curvature. Right now there are just upper limits, but there is some chance her tests will be executed. She gave a great talk and handled some heavy interrogation from the floor. Congratulations, Dr Schillo.

Very late in this low-research day, I did some work on the 2014 Kentucky Derby. As of writing, I like Dance With Fate, Wicked Strong, and Medal Count. But that's not investment advice!

the Higgs, stellar modeling

Neil Weiner gave a great brown-bag (all chalk) talk about the Higgs at lunch today. He started from scratch: He started at the Lagrangian in field theory and ended up saying what the implications are for new physics (especially supersymmetry-like theories) of various different Higgs masses! And all in 50 minutes. It was a masterpiece, and hilarious to boot. Ask him for his joke about string theorists.

Before and after that, Hou showed us the results of fitting a stochastic stellar oscillation model to K-giant radial velocity data. We are hoping to show that including a physical model for surface variations will improve the results of exoplanet fitting. We can show this for fake data; now onto real data.