Seeing the invisible

When scientists were first studying beta decay, they expected the electron to be emitted with a single unique energy, as depicted in red. However, they measured instead a range of energies for the emitted electron, shown in yellow, all lower than the expected energy, which the electron would carry if neutrinos didn’t exist. In the lower right hand corner, we see a closeup of the spectrum near the expected energy. The dashed line is what we see if the neutrino has no mass, while the magenta curve is what we’d see if the neutrino had a small but non-zero mass. CMS scientists employed this technique to study top quark production to validate the method.

The world of particle physics and cosmology is full of invisible phenomena like dark matter, neutrinos and that latest spiffy object predicted by the theory of the week. When you think about it, it’s really quite hard to measure some of the properties of these invisible particles. So scientists had to come up with some clever ways to determine things like the mass of something that cannot be detected directly. One such way involves careful accounting of the energy observed in the experiment.

This technique has been used in the past. A type of radioactivity called beta decay occurs when a neutron in the nucleus of an atom converts to a proton and emits an electron. Following the principle of energy conservation, scientists predicted the electron to be emitted with a single energy, but measurements showed that the energy of the electron can have many different values. In fact, it turned out that the predicted value of the electron’s energy was actually the maximum it could be. The measured values were always lower.

In 1930 Wolfgang Pauli proposed a solution to this curious situation: Not only were a proton and an electron emitted in beta decay, but a neutrino was emitted as well. Neutrinos are particles that interact only via the weak nuclear force and are therefore very, very hard to detect. Clyde Cowan and Frederick Reines showed the idea to be correct in 1955 when the neutrino was detected.

Once it was understood that beta decay involved three—not two—emitted particles, scientists could use energy conservation to determine the mass of the third, the neutrino. The energy of the beta decay can go into six things. Given that energy and mass are equivalent, three of those things are the masses of the proton, electron and neutrino. The other three are their motion energies. We can measure the mass and motion energy for the proton and electron, so the unknowns are the mass and motion energy of the neutrino. To determine the mass of the neutrino, we determine the maximum motion energy carried by the electron, which occurs when the neutrino has no motion energy. Any leftover energy from the decay is the mass of the neutrino.

This technique is not uniquely applicable to beta decay. At CMS, we search for collisions in which supersymmetric particles are created. Many supersymmetric particles are invisible in ways similar to the neutrinos of beta decay, leading researchers to apply this old idea to a new situation.

In order to evaluate how well this technique works in the CMS environment, scientists tried to validate it by using the large sample of top quarks produced at the LHC. Top quarks decay into W bosons and bottom quarks. In some instances, the ephemeral W bosons decay into a neutrino and an associated electron or muon. In such events, the top quark, W boson and neutrinos are not observed. This study was able to extract values for the mass of the top quark, W boson and neutrino and validated the technique. Scientists now have some insight into how well it would work for a search for new physics.

—Don Lincoln

These US CMS scientists contributed to this analysis.
Fermilab’s Jim Hanlon will retire after spending about two decades working on the CMS project, most recently as the US CMS Resource Manager. He will be sorely missed.