Are the constituents of Dark Matter in the same mass range as the constituents of known matter?
Our coming experiment LDMX aims to answer that question, and a positive answer would open the door to a new, currently hidden, sector of physics.
A major question in science is the particle nature of dark matter. There is ample evidence for dark matter on various astronomical scales. In addition, dark matter is a dominant component of the mechanism explaining structure formation in the early universe. Dark matter accounts for 85% of all matter, yetit is unknown what it is made of.
The only evidence we have for dark matter (DM) is from its gravitational influence, and that does not give much indication on the mass of its constituents; they could have masses ranging from a tiny fraction of an eV to many solar masses. But, under the assumption of the maybe most natural origin of DM, the thermal origin, the DM masses are constrained to within about 1 MeV to 10 TeV.
The region ~MeV to ~GeV of the allowed thermal origin mass range is largely unexplored. Intriguingly, most of the stable constituents of known matter have masses in this range. Moreover, hints for physics beyond the Standard Model (BSM) are found at low energy (the decay of 8Be*, g-2 and the neutrino masses). It is therefore a priority to explore this mass range. This scenario requires an additional interaction between DM and the Standard Model which can be realized by DM being part of a hidden sector of physics that would additionally explain the DM stability, mediate interactions with the Standard Model and therefore realize the thermal origin in the ~MeV to ~GeV mass range.
LDMX will have a unique potential to explore this mass range.
If there is an interaction between light DM and ordinary matter as there has to be in the case of a thermal origin, then there necessarily is a production mechanism in accelerator-based experiments. The most sensitive way to search for this production is to use an electron beam to produce DM in ﬁxed-target collisions. This is what LDMX will do when built and under operation at SLAC/Stanford.
We have designed LDMX guided by extensive simulations for a 4 GeV electron beam. LDMX will also get an 8 GeV electron beam, and maybe even a 16 GeV beam in a later phase at CERN. These master works are to explore the LDMX performance at 8 GeV and 16 GeV using the same simulation program as was used for 4 GeV. The different topics include:
- Check and validate the physics models used by the simulation package Geant-4, for the higher 16 GeV beam energy
- Estimate the performance at 8 GeV following the path previously performed at 4 GeV
- Optimize the detector geometries for 8 GeV beam energy
- A prototype of the hadron calorimeter, HCal, will be tested in a beam at CERN in the fall 2021; simulate the prototype response to the testbeam particles
- Machine learning studies for reconstruction and background rejection
- Tracking of minimum ionizing particles in the hadron calorimeter
The work will be performed in the context of the LDMX collaboration with many universities in the USA in addition to Lund University. This means that the master students have to be prepared to participate in evening (because of time zones) Zoom meetings roughly once a week, or every second week, to discuss the progress and problems with our collaborations, in particular in California.