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Phil Richerme | CERN | Switzerland

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Antiproton cooling and 100-year-old physics

One of the great things about physics is its universality: theory developed to describe a certain phenomenon can often be widely applied in a multitude of situations. A century ago, the electron was a recent discovery, and the “plum-pudding” model of the atom had just been felled. Protons (and indeed, antiprotons), ion traps, and the rest of our modern toolkit remained unknown. Yet, the methods used at ATRAP to cool antiprotons in an ion trap some 6 orders of magnitude can be traced back to ideas formulated around the turn of the century.

Why cool antiprotons at all? Well, when we make antihydrogen, its temperature is dominated by the temperature of the incoming antiproton, since the mass of the positron is so comparatively small. The fraction of trappable antihydrogen atoms decreases dramatically as the temperature goes up, so it’s important to start with the coldest possible antiprotons.

ATRAP Magnetic Field

Proof of our large magnetic field (yes, small Euro coins are slightly magnetic)

We start by noting that in our ion trap, we have a large, uniform background magnetic field. A charged particle in a magnetic field is confined to move in circles, constantly changing direction and therefore accelerating. If we reach back to 1897, we come across Larmor’s derivation that accelerating charges radiate away energy. Exactly how quickly depends on the magnetic field and the mass of the particle; in the ATRAP experiment, the antiproton radiates its energy away with a time constant of 36 years.

But, there’s good news. For the same magnetic field, the much lighter electron radiates much more quickly – 2/10 of a second. Even better, the electron and antiproton have the same sign of charge (negative). They can be trapped simultaneously in the same voltage well, and allowed to interact (there are no annihilations, since the antiproton and electron do not form an antimatter-matter pair). So, we exploit the quick cooling of electrons by letting them collide with antiprotons in our trap. After only a minute or so, the electrons and antiprotons have come into thermal equilibrium with each other, and with their 4 Kelvin surroundings. The final antiproton temperature is actually closer to 20 Kelvin, though, because unwanted electrical noise makes its way down into our trap and acts as a heat source. Nonetheless, electron cooling successfully reduces the antiproton energy by a factor of 100000.

(Side note: the same sort of physics explains why the LHC has to be so Large. The tighter the loop, the larger the energy loss due to radiation; at some point, the energy losses make the whole process wildly inefficient).

We’ve recently published a paper describing how we can further cool antiprotons to 3.5 Kelvin. We use the technique of adiabatic cooling, which is a fancy way to say that an expanding gas gets colder (provided nothing external puts in or takes away energy). Examples can be found in surprising places – it’s the reason why compressed air sprayed out of a can feels cold, why water vapor condenses into clouds as it rises and expands, and why a refrigerator can keep food cold. And, in keeping with the theme of this post, it’s all well described by thermodynamics worked out in the late 19th century. (Incidentally, the related process of adiabatic heating – compressing a gas makes it hotter – forms the heart of a diesel engine).

Larmor, Carnot, Boltzmann

Larmor, Carnot, and Boltzmann - 3 guys who never heard of an antiproton

At ATRAP, our “gas” is a cloud of antiprotons, which we can let expand in a controlled way by reducing the trapping electric field. We demonstrate that the measured temperature decreases as the volume increases – the hallmark of adiabatic cooling. It’s worth mentioning that we measure the final temperature of our antiprotons by observing the number that escape our trap as a function of trap depth; this traces out the tail of a Boltzmann distribution, from which we can determine the temperature – another 100+ year old invention.

It amazes me that here we are doing cutting edge research, and concepts from the 19th century are still being put to good use. As experimentalists, we should consider ourselves lucky that the well from which we draw our ideas runs so deep.