Work Packages
|
WP1: The Light Dark Matter eXperiment, LDMX
At the core of this Wallenberg
Project is the Light Dark Matter eXperiment, LDMX. It is to be operated at
Stanford National Accelerator Laboratory SLAC and will provide access to
light dark matter with orders of magnitude greater sensitivity than other
experiments. We develop and build crucial parts of LDMX and take a leading
role in the data analysis and interpretation. Lund University takes a leading role
in LDMX as co-spokesperson (T. Akesson) and as the experimental physics
coordinator (R Pottgen). As experimental physics coordinator R Pottgen acts as a
liaison to WP2 and WP3 by porting the signal simulations and statistical
inference package, respectively, into the analysis work of LDMX. The hadron
calorimeter is the focus of Lund University hardware contribution to LDMX
construction. In collaboration with Caltech and Fermilab, we have designed and
optimised it to reject photonuclear reactions, and the most difficult of
those when a bremsstrahlung photon transfers its entire energy to a small
number of neutrons. As mentioned above, it is a Fe-Scintillator sandwich
calorimeter with light extracted through wavelength shifting fibres. Silicon
photomultipliers are connected directly to the fibres and read by the mu2e
readout-system. Lund University designs its readout electronics together with
Fermilab. A first version has been built and beam-tested at CERN. This work is
be done by Lennart Osterman and Geoffrey Mullier, supervised by T Akesson. |
WP2: Simulation
A crucial point for
LDMX is a detailed understanding of eN and gN reactions, both as experimental
backgrounds and for detailed understanding of the signal-process in the
target. In the following, we are going to consider both signal and background
processes and will refer to them jointly as 'reactions' or 'processes'. These
are embedded in GEANT4 [[i]] simulations that rely
on either built-in calculations of scattering probabilities, or on flexible
external event generator tools such as PYTHIA
[[ii]] for more intricate
processes. WP2 will further extend the suite of models
currently investigated in LDMX analyses and implement the extended class in MadGraph [[iii]]
(R Catena, Chalmers theory, leads this effort). During the LDMX
design-phase it was found that the GEANT4
gN model [[iv]] does not reproduce
some gN reactions, for example
moderate-angle neutron pairs and hard backscattered hadrons. Since current
experimental data is scarce for phase-space relevant for LDMX, continuous
feedback will be needed between LDMX and GEANT4. Some errors in GEANT4
have already been corrected during the experiment design, but as the
experiment proceeds, this feedback has to continue. Intensive
co-development of detailed simulations for complex signal- and background
processes is crucial, such that the current, basic, modelling does not limit
the potential of LDMX. This will be achieved by extending the reach of the
successful Lund Event Generator PYTHIA, to encompass all
relevant reactions. For signal processes, this extension is
driven by the implementation of new DM models in MadGraph
by R Catena, and PYTHIA developments by S Prestel. The resulting tools will allow rapid assessment of new
reactions in the light DM paradigm by producing realistic final states of
complex processes. Simultaneously, S Prestel will extend the simulation to
allow for complex radiative QED backgrounds. The improved PYTHIA framework
will also serve as improved cross-section module for rare or signal processes
in a full-fledged GEANT4 simulation. L Sarmiento, E Elen along with S Prestel
work to improve GEANT4 modelling and the interplay
between a GEANT4 simulation and the PYTHIA event generator to give LDMX the
best possible background estimates. This will also benefit the entirety of
the GEANT4 user and developer community, including LHC experiments. The
propagation of the above developments to LDMX as well as their evaluation and
application within the experiment will be ensured by R Pottgen. LDMX will also measure eN
and γN processes. Such reactions are needed to
better understand the neutrino nuclear response, of importance for neutrino
physics and astrophysics. The new findings will be backpropagated into
GEANT4, thus benefiting the particle physics community more widely. |
WP3: Statistical inference package to
LDMX and global data interpretation
In parallel to the experimental activities,
this project will develop optimal data interpretation strategies to maximise the
information extracted from LDMX data. We will develop the statistical
inference package that will allow the LDMX results to be presented in terms
of relevant physical quantities and compare to relevant benchmarks. This
package will be developed by one of the LDMX PhD-students in Lund supervised
by J Conrad and R Catena. Secondly, we will develop the tools to
interpret LDMX results (including null results) in combination with direct,
indirect and collider searches for DM within a general theoretical framework
for light DM. Our collaboration has made a significant contribution to this
subject: J Conrad developed the publicly available GAMBIT package [[v]]
- a global fitting code for generic Beyond the Standard Model theories, and R
Catena developed a code implementing the most general description to date of DM-nucleon
interaction and employed it to perform global fits for direct detection
experiment and neutrino telescope data [[vi]].
However, most combined analyses are still focused on WIMPs, and a global
fitting code to explore the light DM paradigm is still missing. We will
develop a fitting code exploiting the complementarity of LDMX to other
experiments by implementing the LDMX likelihood in GAMBIT, validate this
extended version of GAMBIT against mock data, and then apply it to real data
to constrain the models developed in WP2. Here, it is important to
emphasise the complementarity of LDMX to the other probes, focusing, for example, on the models
in Figure 2. Firstly, LDMX is sensitive to
light DM of all natures. In contrast, direct detection experiments can only
probe elastic scalar DM, as long as its present
cosmological density is achieved via chemical decoupling in the early
Universe. As a result, a signal at LDMX in combination with the lack of
detection at direct detection experiments would then rule out elastic scalar DM.
Secondly, a signal at LDMX can provide information on the underlying DM
model, giving an indication to other searches on the mass of the mediator
particle, as well as on the DM mass under the assumption that the signal is responsible for
the present cosmological DM. Another priority
for WP3 is to further develop LDMX data interpretation strategies focusing on
the extraction of non-trivial DM and mediator properties from data, e.g. their spins. Model-building efforts made within WP2
will contribute to this development. |
WP4:
Detector material evaluation for direct detection
In the case of a positive signal in LDMX, a
direct detection signal would be searched for as proof, to connect DM
produced in the lab to cosmological DM. The Stockholm University group of J
Conrad (Dark Matter and Astroparticle Physics, DMAP) has ten years of
experience in astrophysical detection of DM. Several groups have recently
suggested novel materials for direct detection of DM, for example so called
Dirac Materials [[vii]]
or detectors based on superconductors [[viii]].
The Stockholm group is involved in the first experiment employing Dirac
Materials (Graphene) for DM detection (PTOLEMY) [[ix]],
providing a two-dimensional target for directional detection of DM. To
exploit the possible low thresholds, however, new target materials for direct
detection are needed. The search for these has recently started in a
collaborative effort between the DMAP group and solid-state physicists.
Results from the analysis in WP3 plus the direct results from LDMX would
guide with R&D benchmarks, putting scientists at Swedish institutions in
a position to be in an internationally leading role in experiments needed for
direct detection of light DM. We will assign a PhD student working on the
interface LDMX/Theory/Direct detection/solid-state physics to (a) define
the desired properties of target material for future experiments, (b)
work with solid-state physicists to find these materials and (c) perform a proof-of-concept
theoretical sensitivity study. In this
context, Stockholm and Chalmers groups will work in tight collaboration on
the generation of the direct detection signal models required for the planned
theoretical sensitivity study. |
[[i]] Agostinelli, S.
et al, GEANT4: A Simulation toolkit, Nucl. Instrum. Meth., A506 (2003) 250-303
Allison
et al, Recent developments in GEANT4, Nucl. Instrum. Meth., A835
(2016) 186-225
[[ii]] T. Sjostrand et
al, An Introduction to PYTHIA 8.2, Comput. Phys. Commun. 191(2015), 159-177,
arXiv: 1410.3012, T. Sjostrand
et al, PYTHIA 6.4 Physics and Manual, JHEP 05(2006)026, axXiv:
0603175
[[v]]
GAMBIT Collaboration: P. Athron, C. Balazs, et. al.,
GAMBIT: The Global and Modular Beyond-the-Standard-Model Inference Tool, Eur.
Phys. J. C 77 (2017) 784, [arXiv:1705.07908]
[[vi]] R
Catena and P
Gondolo, Global limits and interference patterns in dark matter direct detection,
JCAP 1508 (2015) no.08, 022
R Catena and P Gondolo, Global fits of the
dark matter-nucleon effective interactions, JCAP
1409 (2014) no.09, 045
[[vii]]
Hochberg et al, Detection of sub-MeV Dark Matter with Three-Dimensional Dirac Materials, Phys.Rev. D97
(2018) no.1, 015004
[[viii]]
Hochberg et al, Superconducting Detectors for Superlight Dark Matter, Phys.Rev.Lett. 116 (2016) no.1,
011301
[[ix]] Baracchini et al, PTOLEMY: A Proposal for Thermal Relic
Detection of Massive Neutrinos and Directional Detection of MeV Dark Matter, http://arxiv.org/abs/arXiv:1808.01892