Searching for Displaced Signals of Naturalness

• Applicant: Professor Dr. Gregor Kasieczka
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 392528336

The Standard Model (SM) of particle physics is an extremely successful theory, describing the interactions of elementary particles and unifying three of the four fundamental forces. However, a number of unsolved problems make it clear that the SM cannot be the final theory of nature.An especially intriguing question is the so-called hierarchy problem: the mass (125 GeV) of the recently discovered Higgs boson is relatively small although it is subject to quantum corrections from virtual contributions by other particles. These corrections are proportional to the highest energy cut off up to which the theory is assumed to be valid and quickly become larger than the observed mass. This implies either that the parameters of the theory are unreasonably fine-tuned or that new particles exist which cancel these contributions.Traditional solutions either invoke a new symmetry (supersymmetry) or suggest that the Higgs boson is not elementary but composite. Both approaches predict new particles that should be produced copiously at the Large Hadron Collider (LHC) and should have been observed by now. Unfortunately, no compelling evidence has been found.Recently, a class of so-called hidden valley theories has been proposed which introduce partner states for all SM particles, but no interactions between them. Only the Higgs boson couples to its partner, connecting the SM and hidden sector, and solving the hierarchy problem. These are particular examples for models where the Higgs boson serves as a portal to new physics.These theories predict pairs of mirror particles to be produced in decays of the SM Higgs boson at the LHC. The mirror states travel unobserved for a macroscopic distance and then decay to SM particles. We propose to search for the striking displaced vertex signature of two pairs of SM particles appearing inside the Compact Muon Solenoid (CMS) detector and to interpret the results in terms of hidden valley and other models with similar phenomenology. Standard reconstruction techniques are blind to this signature. We will use novel deep learning methods to build flexible algorithms to identify these decays anywhere inside the CMS detector, between 0.1 mm and 5 m away from the primary interaction vertex. In the next few years the amount of data collected by the LHC will increase by a factor ten. Searches with pairs of displaced vertices have low backgrounds and will directly gain from the increase in integrated luminosity. To make most of this, we will develop online event selection (trigger) criteria for the upgrade of the CMS experiment.Searches for displaced vertices are urgently needed to ensure the discovery potential at uncharted energies. By using new reconstruction techniques, our work will reach unprecedented sensitivity at the lifetime frontier and test a large class of models that potentially would solve the hierarchy problem. Discovering this signature would be groundbreaking for our fundamental understanding of nature.

• DFG Programme: Independent Junior Research Groups