Project leader: Heikki Mäntysaari Personnel
The This theory project focuses on studying quantum chromodynamics (QCD), the fundamental theory of strong interactions between the quarks and gluons. The complicated non-abelian nature of QCD requires different approaches to access QCD dynamics in different kinematical domains.
Perturbative calculations can be used to describe rare processes with large momentum transfers. In the collinear factorization approach, these scattering processes are expressed as convolutions of perturbatively calculated hard subprocesses and non-perturbative parton distribution functions that describe the partonic structure of protons and nuclei. In the high parton density region effective field theories, such as Color Glass Condensate and hard thermal loop resummations, become useful. Full space-time evolution of hadronic collisions can be simulated using event generators such as PYTHIA, or in heavy ion collisions where Quark Gluon Plasma is produced simulations are possible using real time lattice calculations combined with subsequent hydrodynamical evolution. In the soft sector where the QCD coupling is non-perturbatively large AdS/CFT correspondence is used to understand the QCD dynamics.
QCD evolution: In the CGC framework we focus on NLO calculations for LHC and future EIC. In particular, we aim to complete the NLO calculation of diffractive structure functions to be measured at the EIC. We will also perform the first consistent CGC calculation of particle production in forward rapidities at the LHC at NLO accuracy. We additionally initiate a process to perform a global analysis including HERA and LHC small longitudinal momentum fraction x data to extract the non-perturbative initial condition for the small-x evolution. This analysis can be used to determine if saturation effects are visible in current collider energies.
To further constrain nuclear parton distribution functions (a specialty of our group) we calculate exclusive vector meson production in different photon-induced processes studied at the LHC with different nuclei in collinear factorization. A simultaneous global analysis of proton and nuclear PDFs is pursued, and we will also investigate the advantages of performing the DGLAP evolution in the physical structure function basis without the need to introduce actual quark and gluon distribution functions.
In the PYTHIA event generator development, we focus on improving support for photon-nucleus interactions important for the future EIC.
Properties of the Quark-Gluon Plasma: We develop a global Bayesian analysis setup that enables us to determine the QGP properties such as viscosities. Hydrodynamical simulations of QGP evolution with event-by-event fluctuations are performed using a Monte Carlo implementation of the EKRT initial state description which we will finalize in 2023. This enables us to include observables sensitive to event-by-event fluctuations to the global analysis. The potential to use correlation measurements in proton-nucleus and nucleus-nucleus collisions to constrain the event-by-event fluctuating nucleon substructure is also explored.
Equation of State: We calculate the QCD pressure in the high baryon density region at zero temperature at the order g6 in QCD coupling. This development brings the high-density perturbative calculation to the level of accuracy that even exceeds that achieved in corresponding studies of high-temperature QGP.
Holographic approach to QCD matter: We start to calculate non-perturbative contributions to total DIS cross section at small-x, determine how such a contribution should be included to the perturbative calculations also performed within this project. The applied holographic framework we constrain by fitting the model parameters to precise lattice data in a manner that is consistent with the perturbative results that are available from the high baryon density region.
QCD theory groups from Jyväskylä and Helsinki participate in this project, and the Jyväskylä node is integrated to the Centre of Excellence in Quark Matter. In phenomenological applications we maintain close connection with the Jyväskylä ALICE group and Helsinki CMS groups. At the international level we have a wide network of collaborators in various top institutes. The group of C. Salgado (Santiago de Compostela) actively participates in nuclear PDF related projects and hydrodynamical analyses (H. Niemi is leading the Jyväskylä node of the YoctoLHC ERC AdG of Salgado). The groups of D. Rische (Frankfurt), G. Denicol (Rio de Janeiro) and C. Shen (Wayne state) are other important collaborators in hydrodynamical simulations. PYTHIA development takes place in close connection to the Lund group. In small-x physics we work together e.g. with B. Schenke (BNL), F. Salazar (UCLA/LBNL) and A. Dumitru (CUNY). Real time lattice simulations are performed with K. Boguslavski (TU Wien), A. Kurkela (Stavanger U.) and D. Mueller (TU Wien). We also maintain close connection to groups planning future nuclear DIS experiments EIC (BNL: T. Ullrich, E. Aschenauer) and LHeC/FCC-eh, e.g. N. Armesto). The EOS project will be led by R. Paatelainen at UH. The project will be carried in close collaboration with the strong internationally competitive local theory group in Helsinki and with several other internationally well-known scientists, in particular with T. Gorda (TU Darmstadt), S. Säppi (TU Munich) and A Kurkela (Stavanger U.). The research that involves holographic methods will be led by N. Jokela at UH. Here we collaborate closely with holography experts worldwide, in particular with the group members of Prof. Matti Järvinen (APCTP) and of Prof. Carlos Hoyos (Oviedo U.).