LDMX HCal prototype data analysis

Abstract

The Light Dark Matter eXperiment (LDMX) is a planned missing momentum and energy experiment at SLAC, the purpose of which is to produce dark matter through production at an accelerator. A key theoretical process making this possible is called "dark bremsstrahlung," where an electron scattering off a nucleus produces a dark photon instead of a Standard Model photon. LDMX aims to detect dark matter production through the invisible signature: an electron with missing energy and a significant transverse momentum change, with no other particles present. Dark bremsstrahlung has multiple background processes that could result in the production of neutral hadrons, making a hadronic calorimeter (HCal) necessary for vetoing these events. However, detection through visible signatures, such as the dark photon decaying back into Standard Model particles, is also possible. As this decay has a high chance of occurring in the HCal, identifying the decay products and reconstructing their energies would be important to be able to characterize the dark photon. A test beam was conducted in 2022 at CERN with a prototype version of the HCal. The electron energy response and resolution, as well as shower shapes, were determined using data taken at the test beam and found to be consistent with expectations.

Popular Abstract

Some say physicists are useless, since we don’t even know where 85% of the matter in the universe is. Those people are incorrect. We know reasonably well where it is: dark matter can be observed by seeing the effect of its gravitational pull. It is spread throughout the whole universe, congregating especially around the outer parts of galaxies, forming dark matter halos. All this to say, we know where that 85\% is. What we don't know, is what it is. The Light Dark Matter eXperiment (LDMX) will try to find out.

The effects of dark matter have been observed for close to a century today. The most important observations concerned the rotational velocities of galaxies - the outer regions are expected to rotate much slower than the inner region. However, what is being observed is that the velocities are roughly the same. This points towards the existence of "extra" mass in those outer regions. This "extra mass" is what we call dark matter, has just two must-have properties. One: it needs to have mass. Two: it must not interact with light. If it did, we would have figured out what it is by now.

Unsurprisingly, this leaves an enormous number of possibilities open on the precise nature of
dark matter. LDMX is built on the idea that it might be particles. Specifically, the idea is that dark matter particles were in the early universe freely interacting with "normal", Standard Model matter. However, as the universe expanded and cooled, this interaction became weaker and weaker, effectively "freezing" the amount of dark matter that exists. If this is true, dark matter could be produced today, although only very rarely.

LDMX will try to make dark matter by shooting electrons at a tungsten target. As they scatter, they might have a chance of producing dark photons - a mediator between dark matter and the Standard Model. Since neither the dark photon nor dark matter particles would be visible to our detectors, LDMX looks for an invisible signature: an electron that lost energy and had a significant change in transverse momentum, with no other particle present. However, electrons will most often do something else instead of producing dark matter, such as knocking out a neutron from a tungsten nucleus, for example. Neutrons, on account of being neutrally charged, are a bit difficult to detect. Because of this, a so-called hadronic calorimeter (HCal) needs to be part of the LDMX detector setup, which can detect neutral particles and therefore veto these events. Another thing that might happen is that the dark photon decays back into Standard Model particles, producing a visible signature. If this does happen, it is likely to do so in the HCal, due to its large volume. Then, if we can identify and measure the energy of the decay products, we would learn something about the properties of the dark photon and dark matter in turn.

There was a test beam done with a prototype version of the HCal, where particles were shot directly into it at different energies, using this data, the HCal's response to electrons was studied in this thesis by determining electron shower shapes (i.e., how does it look when an electron is found in the HCal) and the electron energy response and resolution (i.e. how well can we measure electron energies with the HCal). The results show good initial promise that the HCal prototype is working as expected and the final HCal could be used to detect visible signatures.

• author: Gajdán, Greta
• supervisor: Hannah Herde; Erik Wallin
• organization: Particle and nuclear physics; Department of Physics
• course: FYSM64 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 9203836
• date added to LUP: 2025-06-24 08:25:52
• date last changed: 2025-06-24 08:25:52