Paper Explainer: Collapsed Dark Matter Structures

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Paper Explainer: Collapsed Dark Matter Structures

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?

 

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Paper Explainer: Hiding Thermal Dark Matter with Leptons

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Paper Explainer: Hiding Thermal Dark Matter with Leptons

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.

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How to Show Relativity Must be True

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How to Show Relativity Must be True

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."

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The Twin Paradox in Special and General Relativity.

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The Twin Paradox in Special and General Relativity.

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. 

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Paper Explainer: Two is not always better than one: Single Top Quarks and Dark Matter

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Paper Explainer: Two is not always better than one: Single Top Quarks and Dark Matter

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?

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Paper Explainer: Precision Corrections to Fine Tuning in SUSY

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Paper Explainer: Precision Corrections to Fine Tuning in SUSY

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.

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Paper Explainer: Cornering Natural SUSY at LHC Run II and Beyond

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Paper Explainer: Cornering Natural SUSY at LHC Run II and Beyond

This is a blog post on my most recent paper, written with my fellow Rutgers professor David Shih, a Rutgers NHETC postdoc Angelo Monteux, and two Rutgers theory grad students: David Feld (my student) and Sebastian Macaluso (David’s student). It was a pretty big project, as the large (for a theory paper) author list indicates, and in fact the end result was split into two papers for publication, with the 2nd paper coming along shortly. 

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Conference Talk: Tops and Dark Matter

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Conference Talk: Tops and Dark Matter

I was asked to give a talk at the 2016 TOP Conference in Olomouc, Czech Republic. The TOP Conference is, as the name implies, a conference about the top quark. It was mostly experimentalists, with only a few theorists. I was asked to talk about possible connections between the top quark and dark matter. Since there weren't many theorists, I decided to give a relatively broad overview of the topic, rather than drilling down on one particular paper of mine. Here's the talk as I gave it.

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Why FTL implies time travel

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Why FTL implies time travel

In science fiction, it is pretty standard fare to introduce some form of faster-than-light communication or travel. After all, space is big, and you can't write your swashbuckling Hornblower-in-space novel if you have to wait for a generation ship to crawl painfully slowly between the nearest stars, much less try to cross a galaxy.

However, faster-than-light communication (which includes travel) breaks something very fundamental about physics, something that is often ignored by sci-fi, and difficult for non-physicists to understand. If you allow faster-than-light (FTL), then you break causality: you are allowing time-travel.

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Equivalence

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Equivalence

I want to talk a little about the central idea that led Einstein to the concept of General Relativity. To get there, I want you to perform a little experiment.

Take a moment to think carefully about the forces you feel on yourself right now. If you’re sitting, you feel the chair pushing on your back, which pushes on the rest of your body. You feel the floor pushing your feet up. You might feel the muscles and tendons in your shoulder holding your arm up, or strain your neck holding your head up. 

But do you feel the force of gravity?

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Workshop Slides: Combinations fits of ATLAS and CMS data

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Workshop Slides: Combinations fits of ATLAS and CMS data

I'm at a workshop (hosted by the theorists at U Oregon in Eugene) on recent LHC anomalies, most notably the diphoton excess of which there has been so much noise of late. I was fortunate enough to be asked to give the opening talk, showing my theorist-level fits to the CMS and ATLAS diphoton data. I thought it might be nice to put the slides I used up here. Enjoy.

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Paper Explainer: Vector Boson Fusion Searches for Dark Matter at the LHC

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Paper Explainer: Vector Boson Fusion Searches for Dark Matter at the LHC

Here, I describe a recent paper I wrote with a group of experimentalists (Jim Brooke, Patrick Dunne, Bjoern Penning, and Miha Zgubic) and a Rutgers undergrad, John Tamanas. We investigated the ability of the Large Hadron Collider (LHC) to find dark matter using a particular type of event, one called “vector boson fusion,” or VBF.

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