Index:
QCD
The strong interaction is one of the four fundamental forces in nature. This extremely powerful force acts over very short distances, overcoming the electric repulsion between protons and allowing the existence of stable atomic nuclei. The theory that describes the strong force is Quantum Chromodynamics (QCD), a building block of the Standard Model of particles. In this theory, the strong force is described in terms of particles called quarks and gluons, which are the carriers of the color charge (analogously to the electric charge in electromagnetism). Besides its intrinsic interest, a profound understanding of QCD is crucial for the success of numerous experimental programs in high-energy physics, whether in accelerator facilities like the LHC or RHIC, or astroparticle observatories such as Pierre Auger. Dealing with hadronic collisions means that the strong interactions of quarks and gluons initiate most observed reactions. One of the main goals of the phenomenology group at LIP is to improve the understanding and predictive power of QCD. For that, its members conduct cutting-edge research that bridges the gap between theory and experiment across the broad areas of high-energy physics. This includes —but is not limited to — Heavy Ion Collisions, Forward Physics, and perturbative QCD corrections.
Heavy-Ion Physics
One area of strong focus for the Pheno group at LIP is high-energy heavy-ion physics, which explores the hot and dense corners of the QCD phase space. Members of this research area are proficient at exploring grounded heavy-ion theory and phenomenology aspects on hard probes under extreme conditions ruled by QCD in its perturbative regime, a study whose ultimate aim is the uncovering of the strongly interacting quark-gluon plasma (QGP) brought forth by colliding heavy atomic nuclei. Our focus is on theoretically motivated studies on how jets — spray of collimated particles originated by quarks and gluons that traverse the QGP — propagate, modify and lose energy through the plasma, while simultaneously accounting for the medium’s back-reaction to jet propagation. In pursuit of this research, our group provides expertise on covering the parton cascading scenario via the development of Monte Carlo parton shower generators designed to suitably run a broad spectrum of processes at hadronic colliders such as the LHC and RHIC. This work serves as a vital link between heavy-ion theory modelling and experimental observables. Furthermore, we dedicate ourselves to the development of novel phenomenological observables suitable for a heavy-ion environment aimed at scrutinizing the inner workings of the QGP.
Forward Physics
In order for the LHC physics program to be successfully concluded in every run, it is crucial to have a deep understanding of the complete set of final states. Normally, searches for new physics take place in the central rapidity region, where factorization theorems allow for the use of parton densities (PDFs), and hard partonic subprocesses can be calculated by using (fixed order) perturbation theory. However, the forward kinematic region (as close as possible to the beam direction) needs to be studied as well, as it offers many interesting physics prospects. In the high-energy small-angle scattering limit of QCD, the presence of a large scale — usually the logarithm of the colliding energy — in every order of the perturbative expansion of partonic hard processes could potentially break down the convergence. As a consequence, a resummation scheme to resum these large contributions from diagrams to all orders is needed. In such a framework, new QCD concepts emerge, e.g. the perturbative Pomeron and Odderon, or the notion of parton saturation. Key examples of studies in the forward region include final states with high forward multiplicities, as well as those with rapidity gaps. Our group is involved in a number of important projects within forward physics, such as: the application of the Monte Carlo code BFKLex to the study of angular correlations in multi-jet production, the evaluation of the non-forward BFKL Green’s function for diffractive processes, and the study of the complexity of the Feynman diagrams that appear in the perturbative Pomeron and Odderon.
Precision QCD
Testing the Standard Model of Particle Physics at high energy particle colliders such as the LHC can result in data/theory discrepancies which can either be attributed to an imprecise Standard Model calculation, or to a failure of the model itself, indicating its breakdown. If theoretical errors (and their definition) are not under control, we won’t be able to distinguish between these two scenarios. For this reason, a precise theoretical description of hadronic collisions is fundamental for a reliable analysis of LHC data, to probe the validity of the Standard Model at the TeV energy scale and to search for new physics. A crucial ingredient in the theoretical predictions for an hadron collider such as the LHC, is the computation of radiative corrections in QCD. Our group conducts research to push the frontier of the precision of such calculations, through the inclusion of high-order QCD effects in cross sections and related observables measured at the LHC, with state of the art Monte Carlo generators, including next-to-next-to-leading order (NNLO) radiative corrections in the perturbative expansion. Within this approach a significant reduction of the theoretical error can be achieved. As a result, these predictions allow the theoretical uncertainty at the LHC to match or better the experimental error, allowing us to study with unprecedented accuracy the Standard Model and search for new effects signalling new physics.
BSM
The discovery of the Higgs boson at the Large Hadron Collider (LHC), marked as one of the greatest scientific achievements of the 21st century, was the last missing piece of the Standard Model (SM) of particle physics to be confirmed experimentally, and crowned the theory as one of the most successful within physics. However, there are still many questions that the SM has left unanswered, such as the nature of dark matter and the origin of neutrino masses. To tackle this challenge, the Pheno group at LIP studies scenarios that go beyond the Standard Model (BSM). The scientific community has proposed a multitude of different BSM theories to address the shortcomings of the SM, and, in order to assess their validity, they need to be confronted with experimental data. This task often requires us to scan parameter spaces in search of the tiny fraction of points yielding predictions in agreement with experimental data. The traditional methods deployed for these scans are, in general, computationally expensive and extremely time-consuming. Our group make use of powerful Artificial Intelligence and Machine Learning techniques to improve the performance of BSM research, this way helping to solve nature’s theory puzzle. Given the myriad of different proposed BSM models, it seems impractical to perform dedicated searches for each one. In that sense, the inclusion of model-independent approaches is an essential strategy in searching for New Physics (NP) at colliders. The Pheno group at LIP is working on one of these strategies, namely the use of anomaly detection (AD) techniques, where ML algorithms are trained only on SM events, thus learning to detect deviations (outliers) which might come from any possible BSM signal.