### Regular Series

#### Vol. 52 (2021), No. 3, pp. 171 – 286

Isospin Breaking in Lattice QCD Computations of Decay Amplitudes

Acta Phys. Pol. B 52, 175 (2021)

page 175 •

abstract

The remarkable recent progress in the precision of Lattice QCD computations for several physical quantities relevant for flavour physics has motivated the introduction of isospin-breaking effects, including in particular electromagnetic corrections, to the computations. The isospin-breaking corrections are necessary to fully exploit this improved precision for the determination of the fundamental parameters of the Standard Model, including the CKM matrix elements, and to look for deviations from experimental measurements which might signal the presence of New Physics. Together with colleagues from Rome, we have developed and implemented a framework for including isospin-breaking corrections in leptonic decays $P\to \ell \bar \nu _\ell (\gamma )$, where $P$ is a pseudoscalar meson and $\ell$ a charged lepton, and the theoretical framework and numerical results are reviewed below. The status of our studies to extend this framework to semileptonic decays $P_1\to P_2\ell \bar \nu _\ell (\gamma )$, where $P_{1,2}$ are pseudoscalar mesons, is also presented.

Exploring the QCD Phase Diagram with Fluctuations

Acta Phys. Pol. B 52, 203 (2021)

page 203 •

abstract

We report on recent progress concerning the theoretical description of event-by-event fluctuations in heavy-ion collisions. Specifically, we discuss a new Cooper–Frye particlization routine — the subensemble sampler — which is designed to incorporate effects of global conservation laws, thermal smearing and resonance decays on fluctuation measurements in various rapidity acceptances. First applications of the method to heavy-ion collisions at the LHC energies are presented, and further necessary steps to analyze fluctuations from the RHIC beam energy scan are outlined.

From Elastic Scattering to Central Exclusive Production: Physics with Forward Protons at RHIC

Acta Phys. Pol. B 52, 217 (2021)

page 217 •

abstract

We describe a physics program at the Relativistic Heavy Ion Collider (RHIC) with tagged forward protons. The program started with the proton–proton elastic scattering experiment (PP2PP), for which a set of Roman Pot stations was built. The PP2PP experiment took data at RHIC as a dedicated experiment at the beginning of RHIC operations. To expand the physics program to include non-elastic channels with forward protons, such as Central Exclusive Production (CEP), Central Production (CP) and Single Diffraction Dissociation (SD), the experiment with its equipment was merged with the STAR experiment at RHIC. Consequently, the expanded program, which included both elastic and inelastic channels became a part of the physics program and operations of the STAR experiment. In this paper, we shall describe the physics results obtained by the PP2PP and STAR experiments to date.

Lecture on Quarkyonic Effective Field Theory

Acta Phys. Pol. B 52, 229 (2021)

page 229 •

abstract

I review the essential features of Quarkyonic Matter. I argue how such features can be included in a field theoretical description. This field theory has nucleons and quarks because close to the Fermi surface, the degrees of freedom of Quarkyonic Matter are nucleons and inside the Fermi sea, they are quarks. Ghost nucleon fields are needed to avoid over-counting degrees of freedom, and to allow the physical nucleon degrees of freedom not to extend within the quark Fermi sea.

Future Tests of Parton Distributions

Acta Phys. Pol. B 52, 243 (2021)

page 243 •

abstract

We discuss a test of the generalization power of the methodology used in the determination of parton distribution functions (PDFs). The “future test” checks whether the uncertainty on PDFs, in regions in which they are not constrained by current data, is compatible with future data. The test is performed by using the current optimized methodology for PDF determination, but with a limited dataset, as available in the past, and by checking whether results are compatible within uncertainty with the result found using a current more extensive dataset. We use the future test to assess the generalization power of the NNPDF4.0 unpolarized PDF and the NNPDFpol1.1 polarized PDF methodology. Specifically, we investigate whether the former would predict the rise of the unpolarized proton structure function $F_2$ at small $x$ using only pre-HERA data, and whether the latter would predict the so-called “proton spin crisis” using only pre-EMC data.

The Muon Abundance in the Primordial Universe

Acta Phys. Pol. B 52, 277 (2021)

page 277 •

abstract

Muon abundance is required for the understanding of several fundamental questions regarding properties of the primordial Universe. In this paper, we evaluate the production and decay rates of muons in the cosmic plasma as a function of temperature. This allows us to determine when exactly the muon abundance disappears. When the Universe cools below the temperature $kT_{\mathrm {disappear}}\approx 4.135$ MeV, the muon decay rate overwhelms production rates and muons vanish quasi-instantaneously from the Universe. Interestingly, we show that at $T_{\mathrm {disappear}}$, the muon number is nearly equal to baryon abundance.

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