Knocking on Heaven's Door - Lisa Randall [121]
Though visible, taus are not quite so simple to find. The tau is a charged lepton like the electrons and the muon, but it is even heavier. Like most heavy particles, it too is unstable, which is to say it decays—leaving only other particles in its wake. A tau rapidly decays into a lighter charged lepton and two particles called neutrinos or into a single neutrino along with a particle called a pion that experiences the strong force. Experimenters study these decay products—the particles the initial particle decayed into—to figure out whether a heavy decaying particle was responsible for their presence and if so, what its properties are. Even though the tau doesn’t directly leave a track, all the information the experiments record about the decay products helps identify it and its properties.
The electron, muon, and the even heavier tau lepton have charge—1, the opposite charge of a positively charged proton. Colliders also produce the antiparticles associated with these charged leptons—the positron, antimuon, and antitau. These antiparticles carry charge +1, and leave similar-looking tracks in the detectors. However, because of their opposite charges, they curve in the opposite direction in the presence of a magnetic field.
In addition to the three types of charged leptons just described, the Standard Model also includes neutrinos, which are leptons that don’t carry electric charge at all. Whereas the three charged leptons experience both the force of electromagnetism and the weak nuclear force, neutrinos have zero charge and are therefore impervious to the electric force. Until the 1990s, experimental results indicated that neutrinos had zero mass. One of the most interesting discoveries in that decade was the extremely tiny but nonvanishing masses of neutrinos, which provided important information about the structure of the Standard Model.
Although neutrinos are very light and therefore well within the energy reach of colliders, they are impossible to directly detect at the LHC because they have no electric charge and therefore interact only weakly—so weakly that although more than 50 trillion neutrinos from the Sun pass through you every second, you really have no idea until someone tells you.
In spite of their invisibility, the physicist Wolfgang Pauli conjectured neutrinos existed as a “desperate way out” to explain where the energy went when neutrons decay. Without the neutrino carrying off some of the energy, it appeared that energy conservation was violated by this process, since the proton and the electron that were detected after the decay didn’t add up to the same energy as the neutron that went in. Even well-established physicists such as Niels Bohr were willing at the time to sacrifice their principles and accept that energy could be lost. Pauli was more faithful to known physical premises and conjectured instead that energy is indeed conserved, but experimenters just couldn’t see the charge-neutral particle that carried the remaining energy off. He turned out to be right.
Pauli named his then-hypothetical particle the neutron, but the name has since been used for other purposes—namely, the neutral partner of the proton that sits inside a nucleus. So Enrico Fermi, the Italian physicist who developed the theory of the weak interactions but is perhaps best known for helping develop the first nuclear reactor, gave it the cutish name neutrino, which in Italian means “little neutron.” It’s of course not a little neutron, but—like a neutron—it carries no electric charge. And a neutrino is indeed much lighter than a neutron.
As with all the other types of Standard Model particles, three types of neutrinos exist. Each charged lepton—the electron, muon, and tau—has an associated neutrino that it interacts with via the weak nuclear force”54
We have already seen how to find electrons, muons, and taus. So the remaining