Here are some highlights from my research in the last few years. See my publications for a full list of papers.
All the charged matter we know of is quantized in units of the electron charge (quarks are fractional, but confined). Nevertheless, the possibility of milli-charged particles is a natural one within the theory of Abelian gauge theories. That does not mean they exist, but the model describing such particles is well-defined, simple, and compelling enough to attempt searching for them. Many searches were done in the past, and strong limits exists. But, the mass range of 0.1 – 100 GeV/c^2 remains relatively unexplored. In arXiv:1410.6816 we proposed a new detector to be installed at the LHC to look for the production of milli-charged particles in high-energy collisions at this mass range. In a follow-up paper arXiv:1506.04760 we described the general theory of such objects based on the ideas of Holdom and others. Whether the detector would actually get built would ultimately depend on funding, but there are reasons to be cautiously optimistic. Our study suggests sensitivity for milli-charged particles in the 0.1 – 100 GeV/c^2 mass range and charges as small as a thousand of the electron charge.
Neutrino Trident Production
Neutrinos are the least interactive particle in the Standard Model. Nevertheless, beams of neutrinos have been used now for a couple of decades to study some of the rarest scattering processes. One of them, barely observed, is known as neutrino trident production: incoming neutrino scatters against a stationary nucleus to produce an outgoing muon and anti-muon pair, as shown in the diagram. In arXiv:1406.2332 we pointed out that the weakness of this process also makes it a good venue through which to look for new physics that may contribute more strongly to the same outgoing final states. In particular, it can be used to search for new gauge bosons that may be responsible for other observed anomalies in flavor physics (see our work in arXiv:1403.1269 and arXiv:1508.07009). We are now exploring with experimental colleagues whether this process is in fact observable in the planned facility at Fermilab.
RECAST is a framework we proposed in arXiv:1010.2506 to extend the impact of existing experimental searches through a technique we dubbed ‘recasting’. Experimental searches for new phenomenon have always been reused to place constraints and learn about models beyond those considered in the original analysis. Unfortunately, the complexity of modern searches in high energy colliders often make such future reuse of the analysis difficult, if not impossible. Given the significant investment in time and resources such searches represent it is clearly desirable to be able to extend their impact beyond the original publication. RECAST is a framework to do exactly that. It is not a cure-all, it cannot help discover something that was not discovered in the original analysis. It simply allows for future testing of alternative hypothesis against the data, taking into account the complex connection between a theoretical model and its experimental signatures including proper statistical analysis.
Potassium and Dark Matter
I have been interested in the DAMA experiment and their claim of detecting dark matter for some time now. At first I thought that the failure of other experiments to see a signal might point to a more intricate dark matter model than the vanilla WIMP scenario (for example our paper on MiDM model in arXiv:1007.4200). A few years ago in arXiv:1111.4222 we even showed why it is unlikely that their signal is due to atmospheric effects. But, I am now pretty convinced there is something wrong with their results. In the process of trying to understand their backgrounds, we have shown in arXiv:1210.5501 and arXiv:1210.7548 how we can beautifully account for all of the unmodulated background. Such levels of unmodulated background leave little space to any additional putative signal due to Dark Matter. It doesn’t mean that it’s not there, is just means that the modulation fraction has to be close to 100%, something that is hard to come by for dark matter models. I was also rather taken back by the amateurish treatment of background on the part of the DAMA collaboration (the red line in the figure is their “best fit”). The whole thing seems very unconvincing to me, although it is true that I still don’t know what is the origin of the modulations they are seeing.
One positive outcome of all this is that we identified an interesting decay branch of K40, a long-lived isotope of potassium that has never been measured before (see the figure above for the decay schema). It turns out to be of interest to nuclear physicists as well since it is the only known of its kind, so called “third-forbidden unique”. Together with an experimental colleague I am now pursuing a measurement of this decay branch. It is not easy.
Most subatomic particles we know of are unstable. We can nevertheless detect their presence by the imprint they leave on their decay products: special relativity tells us that if we square the sum of the energies of all the decay products and subtract from that the square of the sum of all the momenta of the decay products, that should be the mass (squared) of the original unstable particle. That’s the resonance, and this is how the Higgs boson was discovered for example. However, when the unstable particle decays into quarks (as do all the heavy bosons of the Standard Model), life gets more complicated. The final states are not quarks, but jets. It is not easy to separate such decays from background processes that also give jets. In arXiv:1407.7037 we proposed a new observable that enhances the sensitivity to signal over background. At least theoretically it can lead to an enhancement of 3-5 in signal over background. The observable was recently used for the first time in an experimental analysis by CMS.
Dark Matter and Electromagnetic Interactions
Dark Matter is called dark matter because we do not detect its presence anywhere in the electromagnetic spectrum. This means that dark matter, if it is indeed a new particle, has little if any interactions with the electromagnetic field. That is not too novel, many known particles have only very weak interactions with electromagnetism, for example the neutron. While it is overall charge neutral, the neutron does interact with light through its magnetic dipole moment. In a series of papers (arXiv:1007.4200, arXiv:1206.2910, and arXiv:1209.1093) we have been exploring simple models of dark matter with very weak interactions with the photon. This led to several new proposal for experimental searches in high-energy collider (arXiv:1303.4404) and underground detectors (arXiv:1312.1363). It is a speculation, but the experimental signatures are straightforward and it can be searched for efficiently.