Experiments have confirmed the Standard Model of particle physics time and again. But the model is incomplete. Among other features, it cannot explain dark matter, or the small mass of the Higgs boson or why the forces acting between particles do not unify at high energies. Give each particle a "superpartner", however, and these three problems could disappear. If such superpartners, which are predicted by an extension of the Standard Model called supersymmetry, exist and are not too weighty, then they could turn up in data from proton collisions collected by experiments at the Large Hadron Collider (LHC).
At the Large Hadron Collider Physics (LHCP) conference, taking place this week in Puebla, Mexico, the ATLAS collaboration reported new searches for three such superpartners around uncharted regions of particle masses.
The Standard Model classifies particles as either fermions or bosons depending on a property known as spin, which can be thought of as the rotation of a system around its axis. The fermions, which make up matter, all have half of a unit of spin. The bosons, which carry forces, have 0, 1 or 2 units of spin.
Supersymmetry predicts that each fermion or boson in the Standard Model has a superpartner with a spin that differs by half of a unit. That is, bosons are accompanied by superpartner fermions and vice versa. So, for example, an electron has a superpartner called selectron and a Higgs boson has a superpartner called a Higgsino; superpartners of bosons get the suffix "ino" and those of fermions get the prefix "s".
In its latest supersymmetry studies, the ATLAS collaboration has sifted through the entire proton-proton collision data collected by the experiment during the LHC's second run, which took place between 2015 and 2018, to look for signs of staus and higgsinos; staus are the superpartners of heavier versions of the electron called taus. Such superpartners are expected to be produced in very little amounts at the LHC and to be unstable, so the ATLAS team searched for them by tracking particles into which they can transform, or "decay".
In the search for staus, ATLAS looked for pairs of staus each decaying into a tau and a hypothetical "lightest supersymmetric particle", which would be invisible and a possible candidate for dark matter. Each tau further decays into composite particles called hadrons and an invisible neutrino. The invisible particles are detected by identifying missing momentum in the collisions: if the combined momentum of the particles that are produced in a proton-proton collision does not match the momentum of the two protons in the direction perpendicular to the axis of the proton beams, it is deduced that an invisible particle carried away the missing momentum.
The collaboration explored an unprecedented range of possible masses for the stau, but did not see any signs of this superpartner in the data. However, it was able to place the tightest limits yet on the stau mass.
Meanwhile, the higgsinos search focused on higgsinos transforming into pairs of electrons or muons with very low momenta; like the taus, muons are also heavier versions of the electron. Such low-momenta particles are very hard to catch, but the collaboration was able to expand this search to the lowest-yet measured muon momenta for ATLAS. Just like for the staus search, this search did not reveal any signs of higgsinos, but the results led to stronger limits on their mass than those previously obtained by ATLAS and by the LHC's predecessor the Large Electron-Positron collider.
