Now is a good time to be a dark matter experiment. The astrophysical evidence for its existence is almost undeniable (such as gravitational lensing and the cosmic microwave background; see the “Further Reading” list if you want to know more.) Physicists are pulling out all the stops trying to pin DM down by any means necessary.
However, by its very nature, it is extremely difficult to detect; dark matter is called dark because it has no known electromagnetic interactions, meaning it doesn’t couple to the photon. It does, however, have very noticeable gravitational effects, and some theories allow for the possibility of weak interactions as well.
While there are a wide variety of experiments searching for dark matter right now, the scope of this post will be a bit narrower, focusing on a common technique used to look for dark matter at the LHC, known as ‘monojets’. We rely on the fact that a quark-quark interaction could actually produce dark matter particle candidates, known as weakly interacting massive particles (WIMPs), through some unknown process. Most likely, the dark matter would then pass through the detector without any interactions, kind of like neutrinos. But if it doesn’t have any interactions, how do we expect to actually see anything? Figure 1 shows the overall Feynman diagram of the interaction; I’ll explain how and why each of these particles comes into the picture.
The answer is a pretty useful metric used by particle physicists to measure things that don’t interact, known as ‘missing transverse energy’ or MEt. When two protons are accelerated down the beam line, their initial momentum in the transverse plane is necessarily zero. Your final state can have all kinds of decay products in that plane, but by conversation of momentum, their magnitude and direction have to add up to zero in the end. If you add up all your momentum in the transverse plane and get a non-zero value, you know the remaining momentum was taken away by these non-interacting particles. In our case, dark matter is going to be the missing piece of the puzzle.
Now our search method is to collide protons and look for… well, nothing. That’s not an easy thing to do. So let’s add another particle to our final state: a single jet that was radiated off one of the initial protons. This is a pretty common occurrence in LHC collisions, so we’re not ruining our statistics. But now we have an extra handle on selecting these events, since that radiated single jet is going to recoil off the missing energy in the final state.
An actual event display from the ATLAS detector is shown in Figure 2 (where the single jet is shown in yellow in the transverse plane of the detector).
No results have been released yet from the monojet groups with the 13 and 14 TeV data. However, the same method was using in 2012-2013 LHC data, and has provided some results that can be compared to current knowledge. Figure 3 shows the WIMP-nucleon cross section as a function of WIMP mass from CMS at the LHC (EPJC 75 (2015) 235), overlaid with other exclusions from a variety of experiments. Anything above/right of these curves is the excluded region.
From here we can see that the LHC can provide better sensitivity to low mass regions with spin dependent couplings to DM. It’s worth giving the brief caveat that these comparisons are extremely model dependent and require a lot of effective field theory; notes on this are also given in the Further Reading list. The current results look pretty thorough, and a large region of the WIMP mass seems to have been excluded. Interestingly, some searches observe slight excesses in regions that other experiments have ruled out; in this way, these ‘exclusions’ are not necessarily as cut and dry as they may seem. The dark matter mystery is still far from a resolution, but the LHC may be able to get us a little bit closer.
With all this incoming data and such a wide variety of searches ongoing, it’s likely that dark matter will remain a hot topic in physics for decades to come, with or without a discovery. In the words of dark matter pioneer Vera Rubin, “We have peered into a new world, and have seen that it is more mysterious and more complex than we had imagined. Still more mysteries of the universe remain hidden. Their discovery awaits the adventurous scientists of the future. I like it this way.“
References & Further Reading:
- Links to the CMS and ATLAS 8 TeV monojet analyses
- “Dark Matter: A Primer”, arXiv hep-ph 1006.2483
- Effective Field Theory notes
- “Simplified Models for Dark Matter Searches at the LHC”, arXiv hep-ph 1506.03116
- “Search for dark matter at the LHC using missing transverse energy”, Sarah Malik, CMS Collaboration Moriond talk
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