Studies of Dark Showers for Present and Future Colliders

Abstract

In this work we explore a strongly-interacting dark sector with a Z’ portal to the Standard Model. We focus on the dark shower phenomenology and their semi-visible jet signatures with a strong emphasis on first principles of the theory. We use Pythia and Delphes for event generations and detector simulations, and consider both pp and e+e− collision events at 13 TeV. The kinematic studies are based on jet objects with radius R = 0.4 or R = 0.8. This work highlights important model parameters and studies correlations between several of these parameters and the kinematics of dark shower models. Additionally, we take a closer look at the r_inv parameter that is commonly used in semi-visible jet studies and propose a new variable, r_rec, based on event energy. This new variable is realistically observable and shows a strong correlation to r_inv in e+e− collision events.

Popular Abstract

Beyond the Standard Model physics is an extensive field that attempts to provide answers to fundamental questions that cannot be found within the Standard Model. The Standard Model is the most complete description of the fundamental particles and forces of the Universe that we have to date. It explains a vast amount of phenomena that we have observed and agrees with data across a large energy scale range. However, the Standard Model still leaves several fundamental questions unanswered and observed phenomena unexplained. One of the main questions is that of dark matter. We know that it exists from observing its gravitational interactions, and we know that it is approximately 5 times more abundant than ordinary matter, i.e. matter that we can explain with the Standard Model, but we don't know what dark matter is made of.

The motivation behind many Beyond the Standard Model theories is to provide a dark matter candidate, i.e. a new particle or phenomenon that can account for the dark matter abundance. Some of the most popular dark matter theories only require the introduction of a single new kind of particle, such as the popular WIMP candidate (Weakly Interacting Massive Particle) which appears in several different theories, or the sterile neutrino which are similar to our Standard Model neutrinos but are completely invisible to our detectors. However, there is no reason why the real explanation behind dark matter can't be more complicated than so. In fact, there could be a whole sector of new dark particles, and perhaps even dark forces, that we have not observed yet.

One such dark sector theory is the strongly-interacting dark sector. It is named so because the interactions of the dark particles are similar to the strong interactions of quarks in the Standard Model. Quarks are the particles that make up hadrons like the proton and neutron, and the strong force is responsible for keeping the quarks together. The strong force is special in that the interaction between two quarks grows stronger as the two quarks move apart. Imagine the quarks as the two ends of an elastic ribbon; when you pull at the ends in opposite directions the ribbon becomes tighter and the further you pull them apart, the more energy is stored in the ribbon (this can easily be tested by letting go of one end of the ribbon at different distances and feeling the effect). At one point, the energy stored between the two quarks is so high that it is energetically favorable to create a new pair of quarks rather than keeping the original two connected. This can be imagined as cutting the ribbon in half so there are two pieces of ribbon, each with two ends or two quarks. This process creates new hadrons from energy and is called hadronization. If there is enough energy in the system the process can go on for a while and essentially create a shower of new hadrons. Such high energies are achieved at particle collision experiments at colliders such as the Large Hadron Collider (LHC) at CERN. If there exists dark particles that interact strongly, then they may also hadronize in a dark shower. Furthermore, if we can create dark particles in particle collisions, dark showers could happen right under our noses at e.g. the LHC.

Is it realistic to think we could create dark particles at collider experiments? That depends entirely on the theory, so we can choose to consider theories that allow for the creation of dark particles. In the case of the strongly-interacting dark sector, we can introduce a new particle that can be created by Standard Model particles in a collision and subsequently decay to dark particles. Furthermore, the reverse may happen afterwards and some dark particles may decay back to Standard Model particles. This type of dark shower, called the semi-visible dark shower, is particularly interesting, because the dark shower will not be entirely invisible and it leads to phenomenology that we may indeed be able to observe.

This small branch of Beyond the Standard Model theories has achieved more attention in recent years, and a dedicated search for such particles within experimental data was conducted for the first time in 2021 by the CMS collaboration. Semi-visible dark showers at collider experiments is the subject of this thesis. The focus is on the kinematics of various dark shower models, including the model used in the CMS search. Additionally, this thesis strives to motivate a discussion of how we handle dark shower model parameters and how we can quantify the visibility or invisibility of a dark shower.

• author: Hemme, Nicoline Krogh
• supervisor: Caterina Doglioni; Suchita Kulkarni
• organization: Particle and nuclear physics; Department of Physics
• course: FYSM60 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: high-energy particle physics, dark matter, collider physics, beyond the standard model
• language: English
• id: 9093940
• date added to LUP: 2022-07-06 07:40:37
• date last changed: 2022-07-06 07:40:37