Research Topics
The research performed in the IMPRS on precision tests of fundamental symmetries covers broad aspects in Particle Physics, Nuclear Physics, Atomic Physics and Astroparticle Physics, for instance:Electroweak Symmetry Breaking and New Physics
Future Neutrino Experiments and their Physics Potential
Test of CPT-Symmetry and Special Relativity
New techniques for rare Event Experiments in Astroparticle Physics
Experiments for Astroparticle Physics
Lepton Flavor Violation and Universality
Machine Learning for Fundamental Physics
Global Symmetries in Theory and Phenomenology
Electroweak Symmetry Breaking and New Physics
(Goertz, Dunford, Jaeckel, Lindner, Plehn, Rodejohann, Schultz-Coulon)The electroweak symmetry breaking sector of the Standard Model (SM) is motivated only by minimality, and is nowadays tested experimentally with increasing precision. Theoretical implications of the observed Higgs particle at 125 GeV for the SM and beyond need to be studied, for instance with regard to vacuum stability bounds. The scalar doublet of the SM is modified by extensions of the SM in various ways, offering unique insights into new physics. It is the main goal of the CERN Large Hadron Collider (LHC) to identify the mechanism behind electroweak symmetry breaking, which involves a very broad theoretical and experimental research programme. Searches for Higgs pair production or anomalous triple and quartic gauge boson couplings, in particular at the high-luminosity run (HL-LHC), directly probe the Higgs potential or many theories modifying standard symmetry breaking. The many unexplained features of the SM imply that it is just the low-energy limit of a general and more comprehensive theory. From the many approaches to this theory (e.g. supersymmetry, extra dimensions, compositeness, etc.), there are several common features, such as novel particles possibly observable at colliders, or Lepton Flavour Violation (LFV). Many of the ultraviolet completions of the SM also include a dark matter candidate. The LHC therefore also has the excellent prospect of testing the existence of weakly interacting dark matter and other new physics. Phenomenological studies require a solid quantitative understanding of the detector performance, signals and backgrounds at the LHC, including QCD effects as the dominant background. Moreover, the structure of the underlying models and the scope of their predictions must be understood. And last but not least, the extraction of new physics parameters has to be statistically sound. Scientists in the IMPRS are involved in the experimental programme at the LHC as well as in studying predictions of theories beyond the SM. (back to top)
Future Neutrino Experiments and their Physics Potential
(Blaum, Gastaldo, Lindner, Rodejohann, Schwenk)The precision measurements by upcoming next generation neutrino experiments will put the many models and mechanisms for neutrino mass generation and lepton flavor physics to the test. This includes searches for the Majorana nature of the neutrino in neutrinoless double beta decay experiments (e.g. in LEGEND), and precision studies of neutral currents involving neutrinos with coherent elastic neutrino-nucleus scattering (e.g. in CONUS). In addition, the minimal picture of three active neutrinos whose mixing with each other is described by a unitary matrix can be checked, and the presence of light sterile neutrinos can be tested for (e.g. in STEREO). The presence of alternative neutrino physics beyond the textbook paradigm (non-standard interactions, light sterile neutrinos, etc.) can also be investigated. Searches for such effects probe energy scales that are compatible with collider searches, and allow for attractive complementarity tests. A remarkable breakthrough in the relative kinematic precision of ion traps (values of 1 in 1011 have already been reached) has happened in recent years. For a nucleon (with a mass of order GeV) undergoing beta decay, this implies that a sensitivity to (sterile) neutrino masses of order 10 keV can be achieved, if the energy and momentum of the emitted beta particle and the final nucleus can be measured with sufficient accuracy. Such a measurement would cover exactly the range in which neutrinos are warm dark matter candidates. Furthermore, endpoint measurements in order to reduce the uncertainty of neutrino mass experiments with double beta decay and beta decay experiments, in particular KATRIN and ECHo, are a fruitful contribution of ion traps to neutrino physics. The ECHo experiment is currently performing extensive R&D towards reaching eV-scale neutrino mass sensitivity and below. On the theory side, the physics potential and the implications of upcoming experimental searches for observables related to neutrino mass, indirect searches, neutrinoless double beta decay and cosmology are studied. Models and mechanisms being able to explain the incoming data will be analyzed, in particular with regard to testability outside pure neutrino physics. (back to top)
Test of CPT-Symmetry and Special Relativity
(Blaum, Hinton, Schmelling)The nowadays achievable accuracy in Penning-trap experiments on short-lived exotic as well as stable nuclides sets the stage for precision fundamental tests. High-precision measurements of the mass and magnetic moment of the proton and the antiproton aim at testing the fundamental symmetry between matter and antimatter. This is the CPT-symmetry, where C stands for charge conjugation, P for parity inversion and T for time reversal. CPT symmetry is a property of Lorentz-invariant local quantum-field theories like the Standard Model. Theories beyond the Standard Model often involve novel concepts beyond local fields, and there is thus a common expectation that CPT symmetry is violated at some level. However, there are no specific predictions in which systems and at what level CPT violation should be observed. The challenge from the experimental side is therefore to make optimal use of a few antimatter systems which can be handled for precision experiments and to test the symmetry between matter and antimatter at ever-increasing levels of precision. For instance, the BASE collaboration performs comparison measurements of antiproton and proton magnetic g-factors. By comparing particle and antiparticle masses of hadrons containing strange, charm and bottom quarks, the LHC allows for tests of CPT invariance also in heavy flavor systems. In addition, the energy scales accessible at the world's highest energy particle accelerator boost the sensitivity of searches for Lorentz symmetry violating couplings as parameterized in the SME framework, which e.g. lead to diurnal modulations of the corresponding observables. Another complementary test of fundamental symmetries can be performed by H.E.S.S., CTA or MAGIC, via gamma-ray travel time differences from objects at cosmological distances, thus checking Lorentz invariance. Any deviation in these experiments would have profound consequences for our understanding of Nature. (back to top)
New Techniques for rare Event Experiments in Astroparticle Physics
(Blaum, Degenkolb, Gastaldo, Lindner, Marrodan, von Krosigk)Upcoming experiments searching for fundamental discoveries in astroparticle physics face various technological challenges. These include the unprecedented suppression of intrinsic and external backgrounds, in addition to significantly improving old or to developing completely new detector technologies. In the school, various techniques will be developed which help to improve several on-going or planned experiments and which may also lead to new internationally fore-front projects. Choosing the site of an experiment is often a compromise between the signal to be detected and accessibility, depth and size restrictions. Smart shielding techniques ("virtual depth" ) of the experimental setup are here very powerful since they allow experiments at or close to the surface (e.g. close to a power reactor as neutrino source) which would otherwise not be possible since the background would require deep underground labs. Students in the school will contribute to the development of virtual depth shielding techniques by compact multi-layering and active neutron vetoing. Furthermore, analysis techniques can be refined aiming at a reduction of the threshold of an experiment or improving background rejection. Pulse shape analysis is an example that can be employed in low threshold germanium detectors to enhance their capabilities. Such studies will be beneficial, for instance, for experiments looking for coherent neutrino scattering. In searches for new phenomena, the suppression of backgrounds is in general of great importance. Radon contributes to the natural radioactivity as it originates in the uranium and thorium chains, i.e. the decays of these elements which are naturally occurring in the Earth's crust and also are present in all materials. Being a noble gas, radon is notoriously difficult to avoid. Indeed, 222Rn emanated from detector materials is one of the largest contributions to background in dark matter search experiments such as XENONnT or DARWIN, and is also dangerous in experiments looking for rare events like the search for neutrinoless double-beta decay in LEGEND. Besides the selection of detector construction materials with a low radon emanation, new developments aiming to build radon barriers at the material's surface are of interest. Several rare events searches as for neutrinoless double beta decay (for example CUORE and AMoRE) and for dark matter (for example CRESST) are based on low temperature detectors. The performance achieved by metallic magnetic calorimeters (MMCs), a particular type of this class of detectors, makes them suitable to be used in such experiments. MMCs are operated below 100 mK and can be used to readout the increase of temperature in large crystals following the interaction of a particle. The already demonstrated very good energy resolution will allow not only for achieving low energy thresholds, but also for precise studies of spectra at energies below 1 keV. Students participating to the school will contribute to the optimization of detectors based on MMC technology for the application in a new direct dark matter detection experiment (DELight) and for rare events searches in general. In addition to the techniques directly motivated by astroparticle physics, detectors for rare events are developed in closely related fields such as ultracold neutron spectroscopy and electric dipole moment searches (PanEDM and EDM^n). These efforts represent a novel application of quantum sensing technology to the field of low-energy particle physics, and promise to increase measurement sensitivity against the irreducible backgrounds present at modern neutron sources or in spin-precession experiments with hyperpolarized noble gases. Finally, ultra-high-precision mass data of isotopic chains in combination with high-resolution laser spectroscopy studies will allow in the near future for novel searches for dark matter and fifth forces. (back to top)
Experiments for Astroparticle Physics
(Gastaldo, Hansmann-Menzemer, Hinton, Lindner, Marrodan, von Krosigk)In the search for physics beyond the standard model of particle physics, a wealth of experiments are performed at intermediate energies (keVs to MeVs) and new experiments are running or upcoming at low energies (eVs to keVs). Most of these experiments aim to detect rare events in an energy region where natural radioactivity dominates the detector's rate. An example would be the hunt for keV-nuclear-recoil energies from the elastic scattering of dark matter on target nuclei as exploited in the ongoing XENONnT experiment. Similarly in neutrino physics: the search for lepton number violation in neutrinoless double beta decays (LEGEND), the investigation of sterile neutrino signals in reactor neutrino experiments (STEREO) or the search for elastic coherent neutrino-nucleus scattering with ultra-sensitive germanium detectors (CONUS). To unambiguously identify signals from these new phenomena, a suppression of the background from natural radioactivity is crucial. Furthermore, the remaining background level needs to be precisely measured in order to have a proper assessment of the experiment's sensitivity. Low background techniques using high purity germanium detectors, mass spectrometry and radon emanation devices allow to asses very low activities of natural, human-made or cosmogenically-produced radioisotopes. Students in the IMPRS learn world-leading techniques which will be essential for future experiments in the field. One example is DARWIN, an up-scaling of XENONnT to a target of 50 t, where it becomes possible to detect solar neutrinos, supernova neutrinos, and coherent neutrino scattering in addition to the search for dark matter and for neutrinoless double beta decay. DELight is an upcoming experiment using 10 l of superfluid helium-4 as target for a high sensitivity to sub-GeV dark matter particles. In both cases, the detector target and construction materials can be evaluated and selected to achieve lowest activity levels required to find new physics. (back to top)
CP Violation at LHCb
(Hansmann-Menzemer, Schmelling, Uwer)Precision measurement of loop dominated decays in flavor physics are a powerful tool to test the Standard Model. Potential new physics phenomena far beyond the direct accessible energy range can be revealed indirectly in these processes. CP violation for our purposes is an interference phenomena of different decay amplitudes, which is sensitive to loop contributions and thus to potential new physics contributions. As the CP violation in the Standard Model does not suffice to generate the observed baryon asymmetry of the Universe, the presence of additional sources is a must. The LHCb experiment at CERN is dedicated to study bottom and charm decays in the forward region in pp collision at the LHC. The analysis of the data taken in LHC Run 1 and Run 2 is ongoing while in parallel the detector undergoes an upgrade for Run 3 (starting at the end of 2021), to cope with a significantly higher luminosity. Several interesting avenues exist which can be followed in the school. For instance, the Heidelberg LHCb group is one of the key players in the measurement of the CP violating Bs mixing phase phis, which is studied in the decay B0s to J/Psi KK. The experimental measurement is not yet systematically limited, and will therefore directly profit from increased statistics. For precise tests of the Standard Model and the search of new phenomena exact theory predictions are necessary. However, the theory calculation is limited, among others, by the unknown size of penguin contributions to this decay. Thus we plan to work closely with theory colleagues to provide necessary input to improve the theory predictions. Furthermore we plan to exploit our expertise in time-dependent analysis in the Bs system to perform a time-dependent amplitude analysis to determine the CKM angle gamma together with phis in the decay B0s to D+/-s K+/- pi+ pi-. The recent discovery of direct CP violation in the decays D0 to pi pi and D0 to KK motivates the search for further CP violation in charm hadron decays, which we plan to pursue. (back to top)
Lepton Flavor Violation and Universality
(Hansmann-Menzemer, Schmelling, Schöning, Uwer)Lepton Flavour is an accidental symmetry of the Standard Model. Neutrino oscillation phenomena have proven that this symmetry is violated, which implies effects in the charged sector as well. Those, when mediated by light neutrinos, are however far beyond any experimental sensitivity. Therefore, an observation of violation of charged-lepton flavor (such as a decay mu to 3 e) or universality (e.g. different meson decay rates in muon and electron pairs) would be a clear sign of physics beyond the Standard Model. Indeed, several well-motivated extensions of the Standard Model predict measurable effects in the charged sector. Hence, experiments sensitive to violations of lepton flavor and universality play a significant role in the search for BSM physics. Current limits for charged lepton flavor violating mu to e transitions are in the 10-12 ...-13 range. Next-generation experiments at MEG, Mu3e, Mu2e, and COMET expect to improve these sensitivities by as much as four orders of magnitude on the timescale of the mid–2020s. The outstanding sensitivities that can be achieved provide access to new physics mass scales in the 103 - 4 TeV range, well beyond what can be directly probed at colliders. In particular, students in the school can contribute to the \myunderline{Mu3e experiment} at the Paul Scherrer Institute in Switzerland, which will start data taking in 2021. It has a sensitivity to the decay mu to 3 e at the level of 10-16, four orders of magnitude beyond the current limit. Other processes that could be searched for include the search for familons or lepton flavor violation with displaced vertices. The experiment is a main player in the world-wide search for charged lepton flavor violation. This programme also includes tau to \mu and tau to e transitions. The Heidelberg LHCb group is involved in the search for lepton flavor violating tau decays of the types tau to 3mu and tau to mu phi. The later one is especially interesting because, for instance, leptoquark models, which could explain the recent B anomalies observed in multiple observables of decays involving b to s mu mu transitions, predict decay rates in reach with the current data set in hand. The aforementioned anomalies include indications for violation of lepton flavor universality in the ratio of branching ratios of B to K mu mu and B to K ee decays. Our group aims for complementary tests of the same underlying b to s transition by studying B+ to K*+ mumu decays and by studying the angular distribution of the decay B0 to K*0 ee in the very high and very low ee invariant mass range. These decays are in general a powerful probe of flavor universality. (back to top)
Fundamental Constants
(Blaum, Degenkolb, Gastaldo, Schwenk)The fine-structure constant, the masses and magnetic moments of elementary particles like the electron or proton, and others are fundamental quantities which determine the basic structure of the Universe. They can not be predicted by theory, but their precise determination is required to enable the comparison of theoretical models with experimental observations at the highest possible level. The improvement of these quantities beyond the present level of accuracy represents a significant challenge for modern metrology. Within the IMPRS-PTFS, ambitious experiments and measurements to substantially improve the precision of a number of fundamental constants will be carried out and their impact on theoretical studies investigated. These include high-precision studies of cooled and stored exotic ions (antimatter, radionuclides and highly charged ions) in Penning traps or storage rings where our students get access to world-leading facilities like ISOLDE at CERN (Geneva, Switzerland) and GSI/FAIR (Darmstadt, Germany). Measurements include among other the most accurate determination of the masses of the constitutes of matter, i.e. electron, proton and neutron. Measurements of permanent electric dipole moments (EDMs) are key precision tests of the Standard Model, which predicts finite but tiny values far below the sensitivity of today's experiments. Joint analysis of the experimental constraints on permanent electric dipole moments (EDMs) from many systems is a powerful tool to constrain CP-violation. New CP-violating physics, required to explain the cosmological baryon asymmetry, is now strongly constrained by the extremely precise null results from EDM measurements in several systems. Further advancing the sensitivity of new experiments, in particular PanEDM at the ILL's world-leading neutron source (Grenoble, France) and measurements with diamagnetic atoms at Heidelberg, will clarify how CP-violating interactions could impact Standard Model observables. (back to top)
Machine Learning for fundamental Physics
(Butter, Dunford, Plehn, Schöning, Schultz-Coulon)In recent years, unprecedented large data samples have become increasingly available in many areas of fundamental physics. This includes the experiments at the Large Hadron Collider (LHC), astrophysics and cosmology experiments, many dark matter searches, as well as global analyses of the parameters of the Standard Model. In these fields we can benefit from new data processing concepts and techniques, for instance image recognition or simulation-driven inference based on the exploitation of these large data samples. While the big-data industry has been a spearhead for many new machine learning methods, precision measurements in fundamental physics require machine learning applications that include statistical or systematic uncertainties, frequentist or likelihood approaches, or the implementation of known structures and symmetries. Based on information provided by LHC experiments, it has been shown that jet classification machine learning tools vastly outperform classic approaches and that, using Bayesian networks, allows for an improved control of systematic uncertainties, thereby increasing the precision of measurements. In addition, new machine learning methods allow us to perform searches based on anomaly detection, thereby reducing our dependence on specific theoretical models. As a final example, generative adversarial networks or normalizing flow networks could help control systematic uncertainties related to our theoretical modeling and detector simulations. Here we will follow promising aspects of modern machine learning with an emphasis on applications to measurements in fundamental physics. The ITP has a strong foundation in innovation machine learning methods for high-energy physics applications. The KIP and PI have long-standing expertise in precision experiments, including LHC experiments, the Mu3e experiment, future dark matter experiments, and future collider detectors to which to apply these methods. (back to top)
Global Symmetries in Theory and Phenomenology
(Hebecker, Jaeckel)Global symmetries represent an interesting and active field where phenomenology and research in fundamental theoretical structures interact in a fruitful way. On the one hand, such symmetries play a central role both in our established particle physics Lagrangians and in almost all model building efforts going beyond them. On the other hand, there are strong and (due to recent research in part quantitative) arguments that global symmetries must always be broken if one embeds a model in a consistent quantum gravity framework. Both in string theory and in quantum gravity more generally, recent discussions within the popular Swampland paradigm promise to constrain specifically those global symmetries which are nonlinearly realized, leading to light Goldstone bosons or axions. Their role in phenomenology (from QCD to dark matter to inflation and ultralight particles in the early Universe) can not be overestimated. In addition, there is a significant and growing number of experiments searching for them. There are also more recent efforts, including research in Heidelberg, to extend this logic to linearly realized global symmetries, which protect particle numbers such as the lepton and baryon numbers of the Standard Model or quantum numbers arising in flavor model building. Phenomenologically this usually leads to particles with couplings that have a non-trivial flavor structure providing interesting avenues to exploit the sensitivity of quark and lepton flavor experiments. Moreover, there is recent widely recognized progress associated with the use of holography to prove the absence of global symmetries in quantum gravity. It is so far unclear to which extent such theoretical advances can be translated in phenomenological statements about real-world Lagrangians, but this is certainly a very important challenge to be confronted. Apart from each of these distinct fields being interesting on their own, there exists a variety of apparent cross relations (intra-field or theory/experiment). Examples are global symmetries and searches for lepton number violation (LEGEND) or familons (Mu3e), lepton flavor universality and non-standard neutrino physics, Penning trap measurements for neutrino physics, or developments of new astroparticle physics detectors in the light of theoretical progress and complementary searches. Those connections link also topics on which only UH faculty works, to fields covered at the MPIK and vice versa. (back to top)