Particle Physics
at UPV/EHU

Quantum Chromodynamics in High-Energy Colliders

One key ingredient to the Standard Model is quantum chromodynamics (QCD), describing the interaction of particles via the strong force. Its understanding is of profound importance as, e.g., the strong interaction is predominantly responsible for the matter from which humans are formed and, in general, for most of the mass of the visible universe. Moreover, precise knowledge of hadron structure is often essential in searches for New Physics beyond the Standard Model. QCD explains the interaction of point-like quarks via the exchange of gluons, the gauge bosons of QCD. Numerous experiments have verified many aspects of perturbative QCD (pQCD) in processes involving large momentum transfers.

However, even after a roughly 50-years success story of QCD, it is still an outstanding problem how QCD works in detail on long-distance scales and/or in real-time evolution. The dynamics of quarks and gluons inside hadrons and how they shape the properties of the latter, e.g., how the inner constituents of the proton interplay to form what we know as a proton, remain to be a challenge. A prominent example is the proton’s mass or angular momentum, e.g., the spin of the proton and how it comes about from the angular momenta of the proton’s constituents. Similarly, how hadrons are formed from quarks and gluons ejected in a high energy process at the LHC or in the electron-positron annihilation process remains an equally intriguing as challenging problem.

Moreover, our understanding of QCD is primarily based on scattering experiments, whereas the dynamics of QCD matter in the cores of compact stars and during the early evolution of the Universe remain poorly understood due to the limited theoretical and experimental tools available for such problems. Such QCD matter under extreme conditions can be studied in heavy-ion collisions (HIC), which provide a window into the real-time evolution of matter at extremely large temperatures and densities—conditions similar to those encountered in cosmological and astrophysical systems. However, the lack of adequate theoretical tools still prevents the community from making decisive progress in these directions.

Our group develops new tools to extract information from experiments at e.g. CERN, JLab, BNL or KEK, and future planned ones like the fixed-target experiments at CERN or the future Electron-Ion Collider in the United States.

Complementary frontiers in neutrino and dark matter physics

Neutrinos have weak interactions and, in the Standard Model of particle physics, zero mass. This makes them a clean probe of novel physics — them having a mass is the only uncontested laboratory evidence for the existence of new interactions. It also means they have a huge penetrating power and a very long lifetime, making them a clean probe of astrophysical and cosmological environments, where they are abundantly produced.
Despite the ubiquity of neutrinos, we are just beginning to fully understand their properties. In our group, we study laboratory data, together with astrophysical and cosmological observations, to pin down neutrino properties. The rapidly improving precision of astrophysics, cosmology, and laboratory experiments is thus an opportunity. It is also a liability, as interpreting observations depends more strongly on unknown physics that must be determined elsewhere. Hence, we also study the dependence of other inferences on unknown neutrino properties.
These same tools also allow us to use neutrinos as probes of other particle physics, astrophysics, and cosmology.

Similarly, the nature of dark matter is among the most pressing questions in fundamental physics. Its existence has been confirmed by several independent observations, yet we do not know its fundamental nature nor how it was produced after the Big Bang. In our group, we study how the properties of small-scale astrophysical structures (as small as 50 light-years) inform us on the properties of dark matter. At the same time, we study how these systems may be a window to understand open problems in astrophysics such as galaxy formation or reionization.

Our research links to current and future experiments such as Super-Kamiokande, JUNO, DUNE, IceCube, Planck, DES, DESI, or the Rubin Observatory. We extract new information from existing data, and we study the reach of future experiments.