Direct detection of dark matter
The search for candidate particles for dark matter, in direct detection experiments and through axion telescopes, is one of the lines of research of the Center for Astroparticles and High Energy Physics. The discovery of a new fundamental particle, a potential component of the dark matter of the universe, would constitute a first-rate discovery in particle physics. The experimental establishment of physics beyond the standard model would have revolutionary consequences for particle physics, astrophysics and cosmology, and would redefine the research strategy in these fields. The results in this line may have a great impact on the positioning of Spanish astroparticle physics in the international context.
Much of this experimental work is carried out at the Canfranc Underground Laboratory (LSC), using techniques specific to nuclear physics and, in particular, working in ultra-low background radiation conditions. The LSC is one of the ICTSs in Aragon, where members of the Nuclear Physics and Astroparticles Research Group (GIFNA), which currently holds the status of Associated Scientific Unit, have been working for 30 years. Their technical and scientific collaboration with the LSC and their direct participation in some of the experiments carried out there represent a substantial support for an ICTS in the Aragonese system. Currently, the group’s research in this laboratory is focused on the search for WIMPs (Weakly Interacting Massive Particles), which are the preferred candidate to constitute the dark matter of the universe. The group, which is a pioneer in dark matter studies in Spain, directs two experiments (ANAIS-112 and TREX-DM) strategically chosen to complement each other and cover two of the most promising experimental approaches to the direct detection of dark matter: annual modulation and low-mass WIMPs. ANAIS-112, approved as an LSC experiment in 2010, aims to confirm or refute the controversial result of the DAMA/LIBRA experiment at the Gran Sasso National Laboratory in Italy, which has observed an annual modulation in the detection rate compatible with that expected for WIMPs. The experiment, consisting of 112.5 kg of ultrapure sodium iodide scintillators, has started data collection in August 2017 and has a high potential for dark matter discovery with five years of data if the modulation signal observed by DAMA/LIBRA is due to WIMPs. The TREX-DM experiment is aimed at the detection of low-mass WIMPs, and consists of a detector with Micromegas technology and components of maximum radiopurity, resulting from developments carried out at the University of Zaragoza as part of the TREX project (ERC StG 2009-2015). TREX-DM was approved as an LSC experiment in early 2017 and is being assembled in 2018 before entering the data collection phase to obtain the first physical results.
Dark Matter Modelling in Galaxies
The first argument for the existence of non-baryonic dark matter came from the description of the rotation curves observed in galaxies. New evidence was later added to this from primordial nucleosynthesis, the angular power spectrum of the cosmic microwave background, the large structures of the universe and gravitational lenses produced by galaxy clusters.
But dark matter is not enough. Another new component is also needed to explain the age of the oldest stars and the accelerated expansion of our universe. This new component, called “dark energy”, would present, from a hydrodynamic point of view, a negative pressure that would be a function of its energy density. The simplest model of dark energy is that of the cosmological constant.
The standard model with which the universe has been treated is the cosmological constant and cold dark matter model (LCDM). In this model, the cosmological constant has a constant pressure equal to minus its energy density, and cold dark matter is considered as non-relativistic particles (negligible pressure) that only interact gravitationally.
Following this model, the effects of the cosmological constant on dark matter clusters have been studied. In particular, the collapse region of these clusters has been investigated. The results obtained are consistent with observational results of galaxy clusters studied in the CAIRNS (Cluster and Infall Region Nearby Survey).
On the other hand, the LCDM model, on large scales, accounts for observations of groups and clusters of galaxies. However, on small scales, large discrepancies are being found. In simulations of galaxy groups, the number of dwarf galaxies in these groups exceeds by more than a factor of 10 the number of those observed. Furthermore, simulations for these galaxies give very sharp dark matter densities around their centre, while results from measurements of the velocities of their stars give constant densities. All this would indicate that dark matter could be different from that considered in the standard model.
In this regard, some years ago we proposed dark halos composed of Bose-Einstein condensates, we studied dark clusters of degenerate fermions, and we presented their rotation curves, which could account for those observed in galaxies. In the future we plan to estimate values for the masses of these particles, both in the case of degenerate fermions, and in the case of dark halos composed of Bose-Einstein condensates, by studying dwarf spheroidal stars (dSph) of the Milky Way group (the smallest and weakest galactic systems and the objects most dominated by dark matter in the universe).
Axion Physics: Theory and Detection
Although the most relevant phenomena of the Standard Model of fundamental particles can be described by this theory, there are numerous indications that there are new sectors of matter and interactions that do not fit into it and that can presumably also be described within the framework of quantum field theories. In particular, it is possible that dark matter also admits a description within this framework. The most interesting proposal at present is based on axions, which are the hypothetical particles predicted by the Peccei-Quinn mechanism, formulated within the framework of Quantum Chromodynamics (QCD) to solve the so-called Strong CP problem. Several lines of research focus on the phenomenology of axionic dark matter and its detection, as well as on the search for solar axions in the recently proposed International Axion Observatory (IAXO).
The international IAXO collaboration is made up of 17 institutions, including CERN and DESY, and is led by Igor García Irastorza, a researcher at the Centre. IAXO is a next-generation axion helioscope, successor to the CAST helioscope that has been operating at CERN for the past 15 years, and will search for solar axions with unprecedented sensitivity. IAXO is currently in the technical design phase. The collaboration envisages as a first step the realisation of an intermediate version of the experiment, called BabyIAXO, which will constitute a test for the final subsystems, as well as providing relevant physics results in itself. The group from the University of Zaragoza will contribute with the realisation of one of the X-ray detectors located at the focal point of the optics, and will build a first prototype of such a detector, called IAXO-D0. In addition, a review of the physical potential of IAXO is being worked on, as well as a strategy to measure the mass of the axion in case IAXO detects a signal.
A key theme of the project is the study of the scenario in which dark matter is highly inhomogeneous on a small scale (smaller than a parsec). In this line, we will complete the study of the formation of axitons and miniclusters of axions in the universe through numerical simulations and analytical extrapolations. In collaboration with the group of Jens Niemeyer (University of Goettigen), we are studying the gravitational collapse that will describe the density of these clusters. In collaboration with the Max Planck Institute in Munich, we are working on axastronomy (axion astronomy) with axionic dark matter detectors, as well as on the local velocity distribution and the identification of miniclusters or tidal streams. In the detection of axionic dark matter, the group is part of the RADES collaborations (in the Spanish area) and MADMAX (international collaboration based mainly in Germany). With these groups, we will participate in optimizing the design and characterization of the signal in non-ideal situations.
Neutrino Physics: Double Beta Decay
The search for neutrinoless double beta decay of nuclei is one of the most important challenges in astroparticle physics. Its detection would provide unique information about the nature of the neutrino and its mass, and about the role of this particle in the early universe. As mentioned in the background section of this report, the University of Zaragoza has a very important track record in this line of research, with widely recognized historical contributions in the field, such as the realization of the IGEX experiment in the 1990s. Subsequently, members of the group have participated in other double beta experiments such as CUORE or SuperNEMO, as well as in the BiPo assembly, carried out at the Canfranc Laboratory in support of this last experiment. Members of the proposing groups have played a central role in the conception, proposal and initial stage of the NEXT experiment, currently part of the LSC scientific program. Although there is currently no formal participation in it, support activity is maintained in aspects of radiopurity. More recently, the PandaX-III collaboration, composed mainly of Chinese institutions, has initiated an ambitious experiment for the detection of double beta decay of the isotope 136Xe with time projection cameras read with Micromegas planes based largely on concepts developed in Zaragoza as part of the TREX project. In this context, a support relationship is maintained with this collaboration in aspects of advice on the conception and design of the Micromegas planes of the experiment.
Radioactivity and ultra-low background techniques
At the LABAC (Low Activity Laboratory) the study of environmental radioactivity is carried out by characterising organic and inorganic samples. Part of this study is included within the National Environmental Radiological Surveillance Network (REVIRA) of the Nuclear Safety Council (CSN), in which periodic measurements of aerosols, soil, drinking water and food are carried out. Measurements are also made for other public bodies (including the Government of Aragon and local councils) and private companies, including, in addition to the above samples, raw materials, manufactured products, industrial waste, etc., which are managed through projects and contracts. In addition, in collaboration with the LSC, monitoring of 222Rn in the laboratory air is carried out to verify that it is below the legally established limits in Spain, the determination of a set of physical and chemical parameters of the LSC facilities and the joint use of equipment to measure the radiopurity of materials. LABAC is an officially recognised service by the University of Zaragoza.
In relation to the above, there is also a collaboration in R&D with institutions and companies, both national and international, for the development and construction of highly sensitive radiopure radiation detectors that can be applied in experiments with ultra-low radioactive background or in measurements of radioactivity levels much lower than those corresponding to natural radioactivity.
Development of new particle detectors
The experimental activity mentioned in the previous points involves an important instrumental aspect, derived from the fact that it requires very special specifications for the detection equipment (expressed in terms of low noise levels, extreme radiopurity of the materials, etc.), for which there are no industrial standards. Obviously, this implies going beyond the mere operation or use of commercial particle detectors, and often entails the replacement, improvement or complete development of parts of the detectors. This implies knowledge almost at the construction level of the detector and its associated technology. The history of experimental particle physics activities has allowed us to accumulate this type of know-how for various detection technologies, such as germanium detectors, scintillators, bolometers and time projection cameras. This has provided an ideal breeding ground for developments beyond the state-of-the-art in these technologies, always in conjunction with low-background and radiopurity techniques in which the University of Zaragoza is a leader. An outstanding example of this is the TREX project, funded with an ERC Starting Grant during 2009-2015, in which new time projection cameras (TPCs) have been developed, and, more specifically, TPCs read by Micromegas planes, for their application in searches for unlikely events such as those described in the previous points. This has allowed the University of Zaragoza to position itself as an international reference in this detection technology. TREX developments have been important in the participation in a good number of the investigations mentioned in the previous points; in particular in the CAST, NEXT, PandaX-III, TREX-DM or IAXO experiments.
Gauge theories on the lattice
In 1974, Kenneth Wilson introduced the lattice as a non-perturbative regularization of Field Theories. In 1980, Mike Creutz demonstrated, using Wilson’s formulation and relatively small lattices, that QCD can be simulated on a computer, obtaining the correct results from Continuum Physics. Since the publication of these pioneering works, the application of lattice techniques to the study of non-perturbative effects in the Standard Model has achieved great success. The first contributions of Spanish scientists in this field appeared in the early 1980s, and the groups from the University of Zaragoza and the Autonomous University of Madrid were the pioneers in Spain. The activity of the group from the University of Zaragoza in the field of the “Lattice” has continued uninterruptedly since then. Members of the group have been invited to give plenary talks at various Lattice Symposium conferences and have participated in the organization of the last of these symposia, held in Granada in 2017, which was attended by more than 400 delegates from all over the world.
Currently, within the HPQCD (High Precision QCD) collaboration, the calculation from first principles (using the lattice formulation) of quantities of phenomenological interest in QCD is being developed. This is important as a test of precision in the non-perturbative regime, as well as being crucial in the search for physics beyond the Standard Model. Recently, we have developed very precise methods to deal with systems with charm quarks, which have traditionally been difficult to include in lattice simulations. We have calculated the leptonic decay constants of the D and DS mesons with a much higher precision than had been possible until now, as well as the value of the charm quark mass.
On the other hand, understanding the role of the parameter ‘theta’ (θ) in QCD and its connection with the strong CP problem, axion physics and their implications in the early universe is one of the greatest challenges for high-energy theoretical physicists. Researchers from the group at the University of Zaragoza have extensive experience in developing efficient algorithms to simulate systems with a theta vacuum term overcoming the severe sign problem, as well as in applying them to the calculation of the vacuum energy and topological charge in several exceptional physical systems. One of our main objectives is to write a code to simulate QCD with a topological term and calculate the theta dependence. There is an added interest in this line because for the first time the behaviour of hadronic matter under extreme temperature and density conditions can be verified by heavy ion collisions that are being carried out at the Brookhaven and CERN laboratories. Our group also has extensive experience in simulating QCD at finite baryonic density, where the severe sign problem is also present.
Applications of field theory in quantum information and topological materials
Particle physics techniques have recently found unexpected applications in two fields of physics with important technological prospects: quantum information theory and the development of topological materials. The plethora of new materials with interesting conductor/insulator-type properties, due to their internal topological structure, opens new perspectives for the application of field theory methods in high-energy physics. Although at a fundamental level topological materials are not relativistic, in certain regimes they present edge states in which pseudoparticles exist that behave like particles in quantum field theories. Using particle physics techniques, new effects and types of conductor/insulator behavior can be predicted that may lead to interesting applications in quantum computing and spintronics.
Phenomenology of the Standard Model of Particle Physics and new physics beyond the Standard Model
The Standard Model of Particle Physics (SM) has been confirmed in high-energy accelerators with a precision of the order of thousandth, its most recent success being the discovery of the Higgs particle at the Large Hadron Collider (LHC). However, there are theoretical reasons to believe that the SM might not be the last word in the theory of fundamental interactions. Some theoretical problems remain unanswered, such as: the large difference between the electroweak scale and the Planck scale, the origin of dark matter, the existence of three generations of fermions, etc. Consequently, extensions of the SM that try to solve these problems are projected and investigated, called New Physics theories or models. Currently, the new particles proposed in these models have not yet been observed in current experiments, imposing restrictions on their masses and parameters, without totally excluding the possible observation of New Physics particles in current or future experiments.
There are many extensions of the Standard Model that aim to describe high-energy physics in higher energy regimes than the current ones, where discrepancies are expected to appear in the near future, requiring a change of model. Among these proposals, the models that incorporate supersymmetry as a basic symmetry complementary to gauge and Poincaré symmetry stand out. The phenomenological predictions of these models have been widely discussed in previous works by our group. Currently, these models have been greatly restricted by the recent results of the ATLAS and CMS experiments at CERN. However, there are still parameter regions that are not excluded and it is essential to have all the data and predictions of the possible supersymmetric models up to date in order to compare them with the experimental results and enable the definitive discarding or not of these models. The calculations require powerful analysis tools and the development of appropriate software. Our group is actively working in this area, being part of the “LHC Higgs Cross Section Working Group”, created with the aim of investigating in a coordinated manner between theoretical and experimental physicists, all the precision observables and pseudo-observables relevant to Higgs boson physics, both in the SM and in its supersymmetric extensions. This work requires the active collaboration of the main theoretical physicists who develop the theoretical frameworks and calculation tools, with the different experimental collaborations. This is one of the lines that our group will develop with great interest in the coming years.
On the other hand, with the avalanche of new experimental measurements at the LHC, increasingly precise theoretical predictions are needed, either by developing strategies to obtain information on particles that may even be too heavy to be discovered, but whose existence would be manifested through quantum corrections (measurable experimentally), or by using the method of effective Lagrangian, which allows us to describe the physics of a system in terms of the light degrees of freedom of the theory (those detected in the experiments) and thus encode the effects of New Physics in effective couplings between these fields. The differences between these effective couplings and the SM couplings provide us with information about possible New Physics, and/or confirm the validity of the SM and its possible extensions. In this framework of study, the physics of flavor observables has gained special interest in recent years motivated by the discrepancies with respect to the SM predictions obtained in the LHCb experiment at CERN. Our group also has experience in this field of work, and doctoral theses are currently being directed in this field of research on current topics.
Quantum gravity theory and phenomenology
The challenge of constructing a theory of quantum gravity compatible with observational data is unresolved and is one of the objectives of our group. On the one hand, in light of the results of LIGO/VIRGO on gravitational waves, we intend to analyse the effects of alternatives to Einstein’s theory of gravitation that may be important for solving the main cosmological problems of the Standard Model: the problem of information loss, dark energy, dark matter, etc. The multitude of alternative models that explain some of these phenomena has been greatly constrained by the recent results of LIGO/VIRGO. The theories that are being analysed at the Centre pass this filter and are also compatible with the results of the PLANCK satellite.
On the other hand, we intend to explore the possibility of going beyond the framework of relativistic quantum field theory to incorporate quantum gravity effects that may be observable. One of the possible consequences of a quantum gravity theory is the modification of space-time symmetries, which leaves open the question of identifying traces of possible modifications to relativistic invariance. Astroparticle physics (gamma rays, neutrinos and very high energy cosmic rays) is the ideal place to study the traces of a modification of special relativity. Future data from the AUGER, IceCube, Fermi or CTA collaborations will be vital for this study. We intend to carry out a detailed analysis of the phenomenological consequences of the possible ways in which special relativity symmetries may be broken or deformed at high energies, including a multi-messenger approach (combined analysis of the different astroparticles) to the problem. We will also consider more formal consequences at the level of field theory, such as a modification of the usual concept of locality and its observational implications, or the study of the different decoupling mechanisms necessary to make the presence of additional scalar fields (dilaton, etc.) compatible with the experimental tests of general relativity.