This is an explainer for my recent paper with Gopolang (Gopi) Mohlabeng and Chris Murphy on the implications of recent surveys of dark matter velocity distributions from the Gaia mission on dark matter direct detection. There are a bunch of moving parts in this paper, as we’re trying to tie together some new directions from astrophysics with a long-standing problem in particle dark matter, so let me go through them.Read More
This is an explainer of my recent paper, written with my colleague here at Rutgers David Shih, and David’s graduate student, Pouya Asadi. This is a follow-up in some sense to our previous collaboration, which for various reasons I wasn’t able to write up when it came out earlier this year.
This paper concerns itself with the RDRD and RD∗RD∗ anomalies, so I better start off explaining what those are. The Standard Model has three “generations” of matter particles which are identical except for their masses. The lightest quarks are the up and down, then the charm and strange, and finally the heaviest pair, the top and bottom. The electron has the muon and the tau as progressively heavier partners. The heavier particles can decay into a lighter version only through the interaction with a WW boson — these are the only “flavor changing” processes in the Standard Model.Read More
I'm visiting the University of Oregon's theoretical physics group, and gave a talk on some of my recent work on extracting dark matter particle physics from astrophysical measurements. The field is so broad that you can't cover everything, but hopefully I gave some idea of the potential. Slides are here.Read More
The central thesis of the paper is that there is a huge potential to learn about the properties of dark matter: things like mass, interactions, production, etc using measurements from astronomy. This is not a completely novel idea: we know a great deal about dark matter from astronomy and cosmology (for example: dark matter is "cold" and not "hot"). However, there is an immense opportunity in the near future to do far more, thanks to improvements in simulation and some powerful new astronomical surveys which will be occurring.Read More
If you were made of and could "see" dark matter but had no non-gravitational interactions with the Standard Model, what could you learn about Standard Model particle physics from the gravitational imprint of the Standard Model particles on the dark matter?Read More
A few months ago, I wrote a paper with some other physicists at Rutgers: a graduate student Pouya Asadi, postdocs Anthony DiFranzo and Angelo Monteux (now a postdoc at UC Irvine), and my colleague David Shih. We had developed a new technique to sift through data from the two general purpose LHC experiments (ATLAS and CMS) to look for anomalies that could be the sign of new physics. While that paper was primarily about the technique, we had identified one possible excess, which we had dubbed the “mono-jet excess” since it was mainly found in LHC events with one jet of energy and significant amounts of missing momentum (characteristic of an energetic particle that doesn’t register in the LHC detectors). Here, we revisit that anomaly with more data and some ideas on how to determine if it is a real signal of new physics.Read More
Last night, I gave a talk to the Rutgers Society of Physics Students, the STAR group, and the Astronomical Society on dark matter: the evidence for it, how we look for it, and some thoughts about how will we be looking for it in the future. Slides are here.Read More
I'm at Fermilab for an LPC (LHC Physics Center) workshop on new ideas for LHC dark matter searches. I have been working with my fellow professor David Shih, postdocs Anthony DiFranzo and Angelo Monteux, and grad student Pouya Asadi at Rutgers on new ways to look for interesting anomalies in LHC data (see our paper and my blog post about it). Though not specifically about dark matter, it is a new idea, and so I have a talk about it. Here are the slides.Read More
I'm up in Laurentian University in Sudbury, Ontario, giving three hours of lectures on "Beyond the Standard Model physics and the LHC" for the TRISEP Summer School.
TRISEP this year is mostly experimental grad students, and mostly experimental grad students working on experiments in the underground labs (such as SNOLAB in Sudbury). I'm the only lecturer who's talking about Beyond the Standard Model physics in general (though specific topics like dark matter and neutrino physics are being covered in more detail by other lecturers), and the only one talking about the LHC. Given that, and the audience, I ended up giving a broad overview: first on the sort of things we theorists have reason to think must exist beyond the Standard Model, then how the LHC works (always entertaining to have a theorist speak on how experiments work), and then lastly on how we look for new physics at the LHC. The slides are below.Read More
Is there any new physics at the LHC?
The answer appears to be “no.” If there was obvious evidence of new physics at the LHC, trust me, you would have heard about it by now.
But how do we know? The LHC produces a truly ridiculous amount of data. For each event (and the LHC writes to permanent record 400 events per second) the LHC records all information from all the detector elements. But nowhere in that information is a little flag that says “New Physics!” Indeed, most new physics we can imagine can be aped by physics of the Standard Model. We look for new physics via statistical evidence: we hope to see more events with a particular character than we would have expected.
But we haven’t seen such an excess, correct?Read More
This is a description of a paper I’ve written with my postdoc, Anthony DiFranzo. In our paper, we consider the possibility that dark matter could form gravitationally collapsed objects, evolving from an initial state of nearly uniform distribution across the Universe into one where it forms compact objects, analogous to have the regular matter that you and I are made of eventually formed stars and galaxies. Usually, we think this is not possible for dark matter, due to evidence that, on the largest scales, dark matter forms gravitationally bound structures that are much "fluffier" than the collapsed stars and galaxies.
However, as we show in the paper, there is a way for dark matter to evolve into compact objects on small scales (say, a thousandth the size of the Milky Way), while still satisfying the constraints we've observed at larger scales. In demonstrating that it is possible for dark matter to do this, I think our paper makes an important point about some open questions in the field of dark matter research.
To explain why I started thinking about this particular project, I want to motivate it with a somewhat whimsical question.
Can there be planets and stars made of dark matter?
This is a description of my recent paper with my student David Feld.
Dark matter is a problem. We know that there is a gravitational anomaly in galaxies: the stuff we can see is moving far too fast to be held together by its own gravity. Add to this the precision measurements of the echoes of the Big Bang (the Cosmic Microwave Background), which tells us that the way the Universe was expanding and matter was clumping cannot be explained without some new stuff that didn’t interact with light, and you have very solid evidence for the existence of dark matter. Then of course, there is the Bullet Cluster, where we can see the gravitational imprint of dark matter directly.
So we know it exists. We just don’t know what it is.Read More
As a physicist, I get emails from people who get very upset that something as counterintuitive as special relativity is how the Universe works. They give lots of arguments about why things couldn't or shouldn't work that way. But, it turns out they do. More importantly, it turns out that you can pretty easily show that, in order for electromagnetism to work, special relativity is the only option. This is why Einstein's paper on special relativity is titled "On the Electrodynamics of Moving Bodies."Read More
I'm here at UC Irvine at the DM@LHC 2017 conference (that's "Dark Matter at the Large Hadron Collider"), and was asked to give a talk for a "Theory Overview of Dark Matter Searches." That's a big topic to get through in 30 minutes, so this is what I came up with.Read More
I am organizing a March for Science in Trenton, this April 22. I hope that you’ll join me there, or join one of the many other marches happening that day, around the world. As someone organizing a march, I feel it necessary to explain why I am marching: why I feel it necessary to stand up and defend science.Read More
“Keep science out of politics.”
This is the refrain I’ve been hearing a lot recently, both from scientists and non-scientists. On April 22, 2017, scientists and science-lovers around the country are planning a March for Science in DC, to protest the silencing of government scientists, the removal of scientific input into the political decision-making process, the impact of travel and immigration restrictions on the scientific and student community, and to bring attention to the climate change crisis. Given that one party has been pushing these policies (or, in the case of climate change, has been pushing to avoid setting any policy that could address the problem), this march inevitably has become embroiled in partisan politics.
This has opened a serious and important debate among scientists about the propriety and sense of marching as scientists, for political goals. Shouldn’t we keep politics out of science, and by extension, science out of politics?Read More
Relativity is profoundly unintuitive to humans. Our brain seem hardwired to visualize geometry in at most 3 dimensions, and 3 Euclidean dimensions at that. This is probably because we evolved in an environment where objects move at non-relativistic speeds. Similarly since we evolved in an environment where actions were much larger than the Planck constant, our brains just do not think naturally in terms of quantum mechanics. We are, at our core, creatures who think in classical physics. And that is good enough if you're a naked ape looking to hit a gnu with a rock, or even an engineer building the Hoover Dam, but that physical intuition falls apart when you get to the physics of the very fast, the very big, or the very small. And since the Universe is really quantum and relativistic, those limits are where things get fun.
Let's look at one of those fun things: the Twin Paradox. First in Special Relativity, and then again in General Relativity.Read More
“I am, somehow, less interested in the weight and convolutions of Einstein’s brain than in the near certainty that people of equal talent have lived and died in cotton fields and sweatshops.” -- Stephen J. GouldRead More
A few months ago, I was lucky enough to be contacted by an experimental student in the CMS collaboration, Deborah Pinna. Deborah had a question for me: in a certain set of dark matter models that I had written one of the early papers on, we only considered one particular class of final states, namely production of dark matter at the LHC along with a pair of top quarks. Why, she asked, did we not also consider the production of a single top quark, along with dark matter?
The answer was that everyone, including myself, just assumed that this channel didn’t matter. I’ll explain why in a bit, but I had just assumed that the rate at which this sort of event could occur would be so low that I never actually bothered to check. It turned out that my intuition was wrong. Deborah did check, and upon finding out that this single-top channel mattered, contacted me, assuming perhaps there was a good reason for ignoring it. There wasn’t.
I was really happy to contribute to Deborah’s project, and I want to emphasize that she and a postdoc, Alberto Zucchetta, did all of the heavy lifting on this paper.
So what was the idea? What is single versus pair production of tops, and why does it matter?Read More
This paper is part of a pair with the paper I wrote about here. We were interested in determining how constrained a particular theoretical extension of the Standard Model, supersymmetry (or "SUSY") is by the present experimental results (as of this last summer's ICHEP meeting). The previous paper was the "phenomenology" paper: we took the experimental results, reinterpreted them in the context of a number of interesting models, and calculated the amount of "tuning" that would be present in each model.
The paper we just put out is more of the "theory" paper, the paper that outlines how we did the tuning calculations we used in the phenomenology paper. The results are somewhat technical, so I will spend a bit more time describing the problem in general, and then talk in broad terms about what this paper adds to the discussion. So first I should describe a bit what we mean by "tuning," and why theoretical physicists care so much about it.Read More