Project C - Search for candidates for Dark Matter, new particles and symmetries

H. Fischer, B. Heinemann, G. Herten, K. Jakobs, M. Schumacher, M. Schumann, C. Weiser

The existence of dark matter (DM) is well established by experimental observations, but the standard model of particle physics (SM) does not contain any particles that qualify as a DM candidate. An extension of the SM is therefore essential to explain the observations. There are several approaches to finding DM. In direct searches one use detectors with the highest sensitivity and lowest background rates to detect DM particles from the galactic halo through interactions in the detector. This approach is followed with the XENON experiments at LNGS and with the CAST experiment.

Another possibility is to generate candidates for DM particles in collisions at the LHC. This allows a complementary and systematically completely different access to their properties. The joint interpretation of the results of both approaches represents a significant scientific added value. The searches at the LHC are not limited to DM particles. A wealth of models of New Physics predicts the existence of other (heavy) particles, some of which have very interesting properties, such as lepton-number-violating decays. One of the most attractive classes of models are models with supersymmetry (SUSY); the search for SUSY particles is therefore one of the main focuses of our activities.

C.1 Search for supersymmetric partner particles and candidates of Dark Matter 

One central question of our research program is whether supersymmetric (SUSY) partner particles are realized in nature and whether they can be detected at the LHC. SUSY might help to naturally explain the hierarchy between the electroweak and the Planck scale, and the lightest supersymmetric particle (LSP) is an excellent candidate for Dark Matter assuming R-parity conservation. In most models considered here the lightest neutralino is the LSP.

Since the start of the LHC the constraints on the SUY parameter space have been tightened significantly and lower mass bound on the masses of supersymmetric partner particles have sometimes been pushed by an order of magnitude. The searches performed within the RTG can be devided in three areas: inclusive searches mainly for partner of the gluon andquark of the first two generations; exclusive searches for partners of the top-quark (stop quarks); and searches for electroweak production of neutralinos and charginos, the partners of the electroweak gauge and Higgs bosons.

Inclusive searches aim to search rather model-independently in particular for partners of 1st and 2nd generation quarks and gluon in final states with leptons, jets and missing transverse energy and to cover many different SUSY scenarios.The challenge is to suppress the huge background from e.g. top-quark pair production and electroweak gauge boson production in association with jets using multivariate techniques and to estimate the background yields in a data-driven by defining control regions in data and exploiting precision predictions from theory. The larger data set and new available calculations will allow to significantly increase the sensitivity. The findings will be interpreted in simplified and complete SUSY models with and without R-parity conservation.

Explicit searches for partner particles of the third generation are of special interest as a low mass of the stop-quark is mandatory to solve the hierarchy problem in supersymmetric theories. The experimental signature is rather diverse depending on many parameters of SUSY theories e.g. the masses of squarks of the 1st and 2nd generations and of the neutralinos. The most sensitive topology for many scenarios arises from stop-quarks pair production with subsequent decay chain stop  top neutralio, top  W b, Wq q / l ν yielding a final state of 2 b-jets, four light jets, one lepton and missing transverse energy. Challenges are scenarios where the top quarks are highly boosted and the decay products are reconstructed as a single "fat" jet or scenarios where transverse momentum of final state objects are below reconstruction threshold due to small difference between, stop-quark, top-quark, W-boson and neutralino. Search strategies for 0 and 1 lepton final states will be optimized in particular for such scenarios exploiting deep neural networks. The results will be interpreted in simplified models, complete SUSY models and generic models for DM to allow the interplay with astroparticle physics and cosmology.

In many SUSY models some of the four neutralinos and two charginos are lighter than the strongly interacting squarks and gluinos. However the production cross sections are reduced by orders of magnitude and hence the searches are more difficult. Hence the mass limits are in several cases only slightly stronger than those obtained at LEP and in particular if the 2nd lightest neutralino decays to a Higgs boson only very loose constraints have been obtained from analysis of LHC data. Searches for pair production pp Chargino1 + Neutralio2 W Neutralino1 H Neutralino1 via the decays Hτ τ and HWW. Final states with two or three leptons will be exploited to cover in particular regions of SUSY parameter space with small mass splittings. In addition searches for long-lived charginos with lifetimes exceeding 0.1 ns will performed utilizing ionization loss in the pixel detector and time-of flight measurements from calorimeters and muon spectrometer.

Thesis topics:

  • Inklusive searches for SUSY  in final states with two or more leptons, jets and missing transverse energy 
  • Search stop-quark pair production in final states with jets and missing transverse energy 
  • Search for electroweak production of charginos and neutralinos in the decay via Higgs bosons and for longlived charginos   

 C.2 Search for resonant Higgs-boson pair production and for lepton flavour violating decays of new bosons

The production rate for di-Higgs-boson production ist enhanced significantly with respect to the SM prediction in many of its extensions. The deviation is localized for particular di-Higgs boson masses as the Higgs-boson pair is produced in the decay of new, heavy particles (e.g. heavy neutral Higgs bosons, Kaluza-Klein excitation of gravitons etc.). Scenarios exist in which the search for di-Higgs boson production is the only way to discover these new resonances at the LHC.
So far no hints for resonant di-Higgs boson production have been observed. The final state of two tau-leptons and two b-quarks originating from the Higgs boson decays is again one of the most sensitive for discovering such new resonances In contrast  to the searches for non-resonant di-Higgs boson production the Higgs bosons receive a large boost for high masses of the new resonance yielding pairs of collimated b-quarks and tau-leptons. An optimized search for such topologies requires the development of new reconstruction and identification algorithms.  Machine learning techniques will be employed to optimize the search for  different spin nature and mass values of the new hypothetical resonance. The findings will be interpreted in a model-independent way,  in specific extended models and in the SMEFT.
Lepton flavour violating (LFV) decays of additional Higgs boson and new U(1) gauge bosons occur in a natural way in many extensions of the SM and also in the decay of partner particles of neutrinos in supersymmetric extensions with R-parity violation. It might well be that the observation of LFV decays is the only way to detect such new particles at the LHC. Past searches have shown to hints for such LVF decays. The symmetry method (see A.2) has not been utilized in these searches up to know. The final states are similar to the search for LFV decays of the observed Higgs boson (A.2) but different production processes might be dominant and also the background composition will change with the considered mass of the new resonance.  The search will be optimized for various mass values and spin natures using the symmetry method and machine learning techniques.  The results will yield model-independent measurements of or limits on production cross sections, which are then interpreted in specific extended models 

Thesis topics: 

  • Search for resonant di-Higgs boson production in ppHH b b τ τ
  • Search for new particles in lepton flavour violating decays X  τ e (τ μ) 


C.3 Direct searches for Dark Matter candidates (WIMPS and Axions) 

Dark Matter Candidates are predicted by many BSM models. The most prominent ones are WIMPs at the GeV/c²-scale, produced by freeze-out in the expanding Universe, and axions and axion-like-particles (ALPs) with masses well below 1 eV, produced by spontaneous breaking of an U(1)-symmetry. They couple to two photons (Primakoff-effect) or interact with electrons via the axio-electric effect.

It is possible to search for dark matter or mediators to the dark sectors at the LHC, however, it is not possible to find out whether the "produced" particle makes up the cosmological dark matter component. This is possible with the direct search for the dark matter in the solar system. This approach is fully complementary to the LHC searches.

The direct WIMP search employs low-background detectors installed in underground labs, i.e., a statistically significant detection is already possible with a few signal events. The currently most sensitive detectors searching for WIMPs with masses >3.5 GeV/c² are dual-phase liquid xenon (LXe) TPCs, searching for WIMP-induced nuclear recoils based on the detection of light and charge signals. The ration light/charge allows for the rejection of beta- and gamma-induced background electron recoil events. Once interpreted in the framework of effective field theories (EFTs) or more complex "simplified models", the results from direct detection and the LHC searches can be directly compared. The extremely low electron recoil background of LXe TPCs expands the science reach of direct WIMP searches to axions/ALPs via the axio-electric effect, neutrinoless double-beta decay and supernova neutrinos. Freiburg has a leading role in the XENON dark matter project which recently placed the world's leading WIMP search results. The project is currently being upgraded to the XENONnT phase with an active 5.9t LXe target. The Freiburg responsibilities are design and construction of the TPC, the data acquisition system, low-background material identification and data analysis.

Two generic detector concepts are used for the dedicated axion search: haloscopes convert galactic dark matter axions to microwaves, helioscopes convert axions produced in the sun into X-rays. Both concepts rely on the converson in strong magnetic fields. Helioscopes also allow searching for other hypothetical particles, e.g., the scalar Chameleon, a dark energy candidate. Freiburg is a member of the CAST helioscope operated at CERN which has recently been equipped with four phased-locked, tunable resonantors to also operate as haloscope.

Thesis topics: 

  • Search for WIMP dark matter with XENONnT
  • Search for exotic dark-matter nucleon couplings
  • Search for halo-axions with CAST
  • Search for solar axions with CAST


Benutzerspezifische Werkzeuge