Antimatter, Dark Matter and Helium

Postcards from the Energy Frontier by Prof Jon Butterworth

A new measurement at CERN tells us something about the way particles travel through interstellar space. Which in turn may help a satellite on the International Space Station find evidence for Dark Matter.

One of the satisfying and sometimes wonderful things about science is the way information from very different experiments can combine to tell us something new. A couple of weeks ago Michael Winn, from the LHCb experiment at the CERN Large Hadron Collider near Geneva, Switzerland, gave another example of how this can work.

Let’s start, though, with the Antimatter Spectrometer (AMS-02), a particle detector installed on the International Space Station.

High-energy particles arrive at the Earth continually. Most get stopped by our friendly atmosphere, which is a good thing from our point of view. However, that means if you want to get good measurements of them, you have to go above the atmosphere. AMS-02 is the most sensitive particle detector ever deployed in space, and is making those measurements now.

Apart from “being in space”, an important feature of AMS-02 is the “spectrometer” aspect. AMS-02 has a magnetic field, and the way electrically charged particles curve in this field allows their electric charge to be measured. Positively charged particles will curve in the opposite direction to negatively charged particles. Combined with other measurements that AMS-02 can make, this means it can tell the difference between matter and antimatter.

This is important and interesting, because the universe seems to be made mostly of matter not antimatter.

Antiparticles do exist, and can be created quite easily if you have enough energy. Positrons, which are the antiparticle of the electron, are even produced naturally in some radioactive decays, and are used routinely in medicine. Anti-protons have more mass, so are harder to produce¹, but are created quite often at particle colliders like those at CERN.

But where would anti-protons in space come from, and what might they tell us?

One of the ways they might be produced is when Dark Matter particles collide with each other and annihilate. Dark Matter is an unknown kind of particle postulated to explain why the motions of the stars in galaxies, and the distributions of galaxies in space, are as they are. If our understanding of gravity is correct, then Dark Matter has to make up 84.5% of the mass of the universe, according to data from the Planck satellite and others. As you might imagine, seeing whether this is correct and, if so, identifying what kind of particle Dark Matter might be, is high on the “to do” list for both particle physics and astrophysics. So, the fact that they might be a tell-tale sign of Dark Matter is an important motivation for measuring anti-protons.

We are getting back to LHCb now.

Dark Matter annihilation is not the only possible source of anti-protons. High-energy protons travel through the universe, and if they collide with the tenuous interstellar gas they may have enough energy to produce anti-protons. 

Interstellar gas is essentially hydrogen and helium. From a nuclear physics point of view, that means protons (hydrogen nuclei) and alpha particles (two protons and two neutrons bound together; helium nuclei). To work out whether or not any anti-protons seen in AMS-02 are due to Dark Matter annihilation, one thing we need to know is how often protons collide with this gas and produce an anti-proton.

The probability of a proton colliding with another proton and producing an anti-proton² is reasonably well known, because it has been measured in accelerator experiments. But the probability of a proton colliding with helium and making an anti-proton has big theoretical uncertainties, and had not been measured. A bit of a cosmic shambles, you might say.

Enter LHCb.

LHCb is one of the particle detector experiments at the Large Hadron Collider at CERN. The other big detectors³ each surround one of the points where the LHC particle beams collide, but LHCb is a little different. Most of the detector is “downstream” of the beams, detecting particles which are produced and travel with a relatively small angle to the beams themselves. That’s because most particles containing bottom quarks go that way, and LHCb is optimised for detecting them. But this unique geometry gives LHCb another capability.

LHCb detector, showing a particle collision on the left with the debris (red lines) spreading through the detector to the right.

Gas can be injected into the point where normally the proton beams would collide, and one of the proton beams will crash into it. Since the gas is effectively stationary compared to the proton beam (which is moving almost at the speed of light), all the debris from such a collision will be boosted along the beam direction, at a relatively small angle. Just where LHCb has its best detection capability. 

So by injecting helium, smashing protons into it, and looking for anti-protons, LHCb can measure the unknown probability of a proton colliding with helium and making anti-protons, something that AMS-02 needs to understand whether it is seeing signs of Dark Matter or not.

It is this measurement that Winn presented in August. The full analysis is available here. Below is one of the data plots from that paper. It shows the cross section (which is a way we express the probability) for protons colliding with Helium and producing anti-protons, as a function of the momentum of the anti-protons.

Figure from the LHCb paper here.

The black points are the data, with the shaded band showing the uncertainty in the measurements. All the other lines are different theoretical models for the cross section. The key point is that the spread between the different theory predictions is very wide, and now we know the answer we can adjust the models to agree with the data, making the spread smaller. This will reduce the uncertainty in the expected “background” number of anti-protons in AMS-02, and once that is fed back in, will improve its sensitivity to any possible Dark Matter signal.

A satisfying connection between a tunnel in Geneva and a box on a satellite. And, if it helps solve the mystery of Dark Matter, pretty wonderful.

¹ The energy needed is equal to the mass multiplied by the square of the speed of light, as any fule kno.

² Plus other stuff. Usually written p + p → pbar + X


Images are copyright CERN/LHCb 

Professor Jon Butterworth is a physics professor at University College London and a researcher on the ATLAS experiment at CERN involved with, amongst other things, the discovery of the Higgs Boson. He is the author of two popular science books Smashing Physics and A Map of the Invisible. Postcards From the Energy Frontier is the successor to Jon’s hugely successful blog for The Guardian, Life and Physics.  He is @jonmbutterworth on Twitter.

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