Quantum Diaries

This story first appeared in Fermilab Today Oct. 10.

The 1970s were a thriving time in the world of physics, heralding such milestones as the development of the Standard Model and the discovery of the bottom quark. Now scientists at Fermilab are bringing some experimental pieces of that era back – bubble chambers and fixed-target physics.

Mike Crisler, a Fermilab scientist working on COUPP, is building the chambers for the CITRE experiment. Photo: Reidar Hahn

Peter Cooper, a Fermilab physicist, is heading a new experiment calibrating the classic bubble chamber technology, which is used today to search for dark matter.

The Chicagoland Observatory for Underground Particle Physics (COUPP) collaboration looks for bubbles in chambers filled with a compound containing carbon, fluorine and iodine. The fluid is superheated beyond the boiling point but has no rough surface to form bubbles. When a specific type of particle interacts in the chamber, it can deposit enough energy to boil the fluid and make a bubble. Electrons do not produce bubbles, while a dark matter particle interacting with a nucleus can – making this the key for dark matter detection.

“When a bubble forms, we notice it in the pictures,” Cooper said of the chamber technology. “The bubbles in the fluid are slow enough that high-speed cameras will capture the changes through continuous still shots. We’re making the world’s most boring movie.”

However, scientists are uncertain about the energy it takes to create a bubble in the chamber, which directly influences the sensitivity of the experiment.

The new experiment, named COUPP Iodine Recoil Threshold Experiment (CIRTE), will calibrate the energy threshold of the COUPP bubble chambers so that the COUPP dark matter results are on a firmer foundation. Scientists will fire pions, the lightest meson, in the Fermilab Test Beam Facility at a tiny pen-sized bubble chamber to measure how much energy needs to be deposited in the chamber to form a bubble.

The CITRE collaborators will use a fixed-target technique called elastic scattering of pions. The pions interact with iodine, the target nucleus in the COUPP bubble chambers with the most sensitivity to the most popular dark matter candidates. The pions are surrogates for dark matter – the bubble chamber sees them both in the same way by observing the bubble from the recoiling iodine.

Unlike dark matter, however, pions can be easily observed with other detectors on both sides of the bubble chamber, allowing COUPP scientists to know how much energy the pion gave to a scattered iodine nucleus.

Cooper and his team are currently running preliminary beam tests on solid carbon, fluorine and iodine targets to ensure that they understand how the experiment will work, in preparation for putting an actual bubble chamber in the beam. By watching how the pions interact with each target, they can determine how the pions should behave once the bubble chamber is in place.

However, the bubble chamber will only be able to produce one measurable bubble per beam spill, or one cycle of the accelerator. After the one bubble appears, the entire chamber needs to be recompressed in order to reset the contents.

The current COUPP chamber operates at the underground SNOLAB in Canada. Being deep below the surface allows scientists to suppress background events, such as those from cosmic rays. That bubble chamber is already setting limits on dark matter interactions that approach the best in the world. But they are hampered by the uncertainty on the energy threshold. With a little help from CIRTE, the COUPP experiment will be on a solid foundation as its search for dark matter increases.

—Brad Hooker

Science is full of surprises such as this laboratory flooding at Yale. Credit: Caitlin Casey

Hi everyone, it’s been a few weeks since I did my introductory post  on who I am (my name is Hugh) and what I’m working on (dark matter detection with bubble chambers, as part of the COUPP collaboration). I fear it has been too long for there to be any continuity between the previous post and this one, so I figured I’d go in a different direction and talk a bit about the uncertainty (or unexpected challenges) of working in science.

I decided on this topic after I just spent a few minutes clicking through the past week of entries here on Quantum Diaries. There really is quite a lot of information being posted here (including a really nice, detailed description of the Higgs boson, which was useful to me).I’m particularly enjoying Bob Peterson’s description of his journey around the world. We’re certainly covering a wide range of topics.

One thing I did notice is that people are writing a lot of explanations – how things work, how their detectors work, what we’re trying to discover. I think this is really good stuff and I’m glad we’re writing about it, but it does give the impression that we generally know what is going. Unfortunately, my experience, and therefore the subject of this post, is that a lot of the time my experiments do not go as expected and I often find myself rather confused (I suspect that other people have the same experience, but I’ll be safe and speak only for myself here).

First, experimental physics never goes exactly the way you planned it. Something always goes wrong. To give a particularly disastrous example, my lab flooded once in when I was at Yale for graduate school. Most experiments have some form of basic monitoring system called “slow control.” The slow control is constantly taking measurements of the pressure or temperature of a system and reading back to some kind of database. Often, they will be set up to send the experimenter a warning of some kind when a reading is out of safe range – for example, if a pressure gets too high. On this particular occasion, I was running a chamber filled with liquid neon, which is a liquid at minus 248 degrees Celsius (or 25 Kelvin). I think it was a Friday or Saturday and I was out to dinner with a friend when I got a text message – “ALARM: T1 out of range! T1 > 28 K!” – that the temperature was rising and therefore pressures might be getting a bit high. While there was no physical danger due to the safety systems that are engineered into experiments like these, often the safety systems will not protect against the loss of the experimental run itself, which in this case would have cost me several months and delayed my graduating. And I wanted to graduate.

I broke a few traffic laws on the way to my lab (fortunately I was not too far away) and was faced with the scene shown in the picture. It may be slightly hard to tell, but normally there is a large 4-foot by 8-foot pit in the middle, where the stainless steel cylinder is sitting, but in the picture the pit is completely full of water. There is rather a lot of expensive equipment sitting around the room, also in water. I ran down the stairs, shut everything off, found the source of the flood (a plumbing connection that had burst) and prayed that it would be OK; which, thankfully, it largely was. I had managed to shut off the water before it could damage any of the more sensitive pieces of equipment. The only thing that broke was a single fuse in my cooling system, and I ended up losing only two weeks.

That’s equipment failure, and that does happen often. And we try to learn from these incidents to make our experiments safer and more robust (in the previous example, the slow control alarms worked perfectly and a freak flooding accident did no permanent damage) However, at least it’s generally pretty easy to tell when something physical in the experiment has broken down (although the cause can be elusive).

Screen shot of the slow-control system. Credit: Hugh Lippincott

If you’ve been paying attention to recent popular physics news, you’ll have noticed that various “excesses” are all over the place at the moment (in dark matter, at CDF at Fermilab, at the LHC). These are examples of data that we do not fully understand, and we really don’t know what is going on yet.

In the next post, I’ll talk a little bit about the current run of COUPP, which produced some really strange results back in November and December that we’re still trying to figure out.

—  Hugh Lippincott