abstract

The results of an analysis of the invariant mass spectra of photon pairs produced in \(d\)C, \(p\)C, and \(d\)Cu interactions at momenta of 2.75, 5.5, and 3.83 GeV/\(c\) per nucleon respectively, are presented. Signals in the form of enhanced structures at invariant masses of about 17 and 38 MeV/\(c^2\) are observed. The results of testing the observed signals, including the results of the Monte Carlo simulation are presented. The test results support the conclusion that the observed signals are the consequence of detection of the particles with masses of about 17 and 38 MeV/\(c^2\) decaying into a pair of photons.

abstract

The SIDDHARTA-2 Collaboration aims to measure for the first time the shift and width induced on the \(1s\) level of kaonic deuterium by the strong interaction. In the preliminary phase to the experiment, a test run using a Helium-4 target was performed to optimize the performance of the full experimental apparatus. This preliminary study highlighted the possibility to measure transition lines coming from intermediate-mass kaonic atoms, such as kaonic carbon and kaonic aluminum. In order to measure transitions where strong interaction is manifesting at higher energies, out of the energy range of the SIDDHARTA-2 apparatus, the collaboration is testing a new detector system which exploits a novel compound semiconductor, the cadmium–zinc–telluride. Tests are now running at DA\(\Phi \)NE to study the performance of this detector, exploring the possibility to build a dedicated setup.

abstract

We studied the SU(2) version of the PNJL model with a chiral imbalance at finite temperature, exploring different regularization schemes. We also argued about the missing ingredients for chiral models to obtain results for the pseudocritical temperatures for chiral and deconfinement transitions in agreement with Lattice simulations.

abstract

We study the pseudoscalar glueball and its first excited state by constructing three interaction Lagrangians. One produces the two- and three-body decays of the pseudoscalar glueball, \(J^{PC} = 0^{-+}\), into (axial-)vector mesons, scalar glueball, and (pseudo)scalar mesons. The other two describe the two- and three-body decays of the first excited pseudoscalar glueball, \(J^{PC} = 0^{*-+}\), into (pseudo)scalar mesons, a scalar, and a pseudoscalar glueball. We compute the decay widths by fixing the mass of the ground state of a pseudoscalar glueball to \(2.6\) GeV and of its first excited state to \(3.7\) GeV, as predicted by lattice QCD in the quenched approximation. We present the results as branching ratios with a parameter-free prediction, which are interesting and relevant for the PANDA experiment at the upcoming FAIR facility experiment and running BESIII and Belle-II experiments.

abstract

We put forward the idea that a medium-dependent coupling can be introduced in the context of the Nambu–Jona-Lasinio model and the Linear Sigma Model with quarks to yield a non-monotonic behavior for the speed of sound of isospin asymmetric matter as recently found by lattice QCD simulations.

abstract

In this paper, we overview main results indicating the existence in QCD of three qualitatively different regimes connected by smooth crossovers upon heating: a hadron gas, a stringy fluid, and a quark–gluon plasma. In the combined large-\(N_{c}\) and chiral limit, these regimes likely become distinct phases separated by phase transitions: a chiral restoration phase transition around \(T_{\mathrm {ch}} \sim 130\) MeV and a deconfinement phase transition around \(T_{\mathrm {d}} \sim 300\) MeV. It should be an important task to verify this issue on the lattice. We will introduce a chiral spin symmetry, which is a symmetry of the electric part of electrodynamics and of QCD with light quarks. It is realized approximately in QCD above the chiral restoration crossover and disappears in the QGP regime. The center symmetry of the pure glue action and the chiral spin symmetry of the electric part of the QCD Lagrangian with light quarks are complementary to distinguish the confining regime and its disappearance. We also address other lattice evidence for the stringy fluid: hadron resonances extracted from the lattice correlators; a breakdown of the thermal perturbation theory at \(T \lt 600\) MeV and fluctuations of conserved charges that point out the \(N_{c}\) scaling above \(T \sim 155\) MeV.

abstract

We test a method for computing static hybrid quark–antiquark potentials in lattice QCD, which is not based on Wilson loops, but where the trial states are formed by eigenvector components of the covariant lattice Laplace operator and their covariant derivatives. The main advantage of the new method is that we can compute off-axis distances without much extra work and by introducing a basis of Gaussian profiles in distillation space and solving a GEVP, we can also access excited states. We present a static hybrid spectrum with ground and excited \({\mit \Sigma }\) and \({\mit \Pi }\) states, and perform a basic Born–Oppenheimer approximation on a QCD-like ensemble with two dynamical quarks of half the charm quark mass.

abstract

The study of Quantum Chromodynamics (QCD) at ultra-relativistic energies can be performed in a controlled environment through lepton–hadron deep inelastic scatterings (DIS), as in the future Electron Ion Collider (EIC). In such collisions, the high-energy QCD emissions that follow the ejection of hard partons are accurately described by perturbation theory. However, the lower-energy scales at which quarks and gluons experience colour confinement — hadronization — fall out of the validity regions for perturbative calculations, requiring phenomenological models to describe it. As such, hadronization physics cannot be currently derived from first principles alone. The Monte Carlo (MC) event generators are useful tools to describe these processes as they simulate both the perturbative (pQCD) and the non-perturbative (npQCD) interactions, with a model-dependent energy scale at which parton dynamics transition from one to the other. This work aims to use jets — experimental reconstructions of final-state particles likely to have a common partonic origin — to inspect this transition further. Although originally proposed to circumvent hadronization effects, we show that jets have great potential as probes of non-perturbative phenomena. The charge correlation ratio was recently shown to be sensitive to hadronization effects and our work shows that jet substructure selections, namely on formation time and the depth in the clustering tree at which the leading charged particles are resolved, can improve this sensitivity.

abstract

At high energies, the production of light mesons induced by a photon beam interacting with a nucleon target is governed by the exchange of Regge trajectories in the \(t\)-channel. In charge-exchange reactions at small momentum transfer, unnatural parity exchanges, such as pion exchange, dominate. The point-like nature of the photon–pion interaction makes the pion photoproduction reaction an excellent way to study the pion exchange mechanism. However, the nucleon Born terms are required to ensure that the scattering amplitude is gauge invariant. The gauge invariance of the pion exchange amplitude is crucial in approaching its reggeization properly. We discuss a novel strategy to reggeize the pion pole which considers explicitly the exchange in the \(t\)-channel of all the mesons in the pion trajectory. Each of these exchanges contributes with a pole in spin, known as a Regge pole, and we perform their summation analytically.

abstract

We study the production rate of charm quarks in hot QCD matter within the effective quasiparticle approach. The quark–gluon plasma is composed of the quasiparticles carrying the dynamical masses which are linked to the lattice QCD equation of state. Describing the evolution of the deconfined medium as a perfect fluid propagating longitudinally, we compute the production rate of charm quarks as a function of temperature and time and show that it significantly depends on the effective masses of the quasiparticles, as well as on the initial conditions of the fireball.

abstract

The doubly charmed exotic state \(T_{cc}^+\) observed recently by the LHCb Collaboration is studied in the Effective Field Theory framework. The free parameters of the theory are fixed from the fit either to the experimental line shape or to the lattice \(DD^*\) scattering amplitude. The position of the \(T_{cc}^+\) pole is extracted and its dependence on the charm-quark mass is investigated using five lattice sets with \(m_c\) deviating from its physical value and \(m_\pi \approx 280\) MeV.

abstract

When a resonance lies near the threshold of a heavier channel, an interesting feature can occur. The paradigmatic example employed here is the scalar–isoscalar \(f_0(980)\) resonance that couples to the lighter \(\pi \pi \) and heavier \(K\bar {K}\) channels. It is shown that the decay width is given by the sum or subtraction of the partial decay widths depending on whether the pole lies in the Riemann sheet that is contiguous with the physical one above or below the \(K\bar {K}\) threshold, respectively. Next, we show that the usually disregarded renormalization of bare parameters in the Flatté or energy-dependent Breit–Wigner parameterizations is essential to extract physical information. The compositeness of the \(f_0(980)\) by using a Flatté parameterization matched to reproduce the pole properties obtained from the Roy equations and other analytic constraints is evaluated.

abstract

We report on our recent works on dispersive analyses of \(\pi \pi \to \pi \pi \) and \(\pi \pi \to K \bar K\) scattering data and their use to address two controversial aspects of the \(f_0(1370)\) and \(f_0(980)\) scalar mesons. First, we show with model-independent techniques that the \(f_0(1370)\) pole does indeed appear in meson–meson scattering data, although there is tension between its values in the \(\pi \pi \) and \(K \bar K\) channels. Second, we explain the proper interpretation of the \(f_0(980)\) pole residue, which would otherwise lead to branching ratios larger than one. We have also provided simple \(\pi \pi \to \pi \pi \) data parameterizations that implement both features together with other resonances while respecting various dispersive constraints.

abstract

LHCb has observed giant CP violation in localized regions of the Dalitz plots of \(B\) to three charmless light mesons. This has been interpreted as an enhancement due to strong two-body final-state interactions. In this paper, we show how such interactions, described with dispersive analyses of data, can be implemented beyond the leading order expansion in the two-body rescattering amplitude.

abstract

The decays of the \(Z\) boson and CP-even or CP-odd scalar bosons into quark–antiquark pairs have been calculated at NLO in the framework of Implicit Regularization (IReg), which operates strictly in the physical dimension and complies with the BPHZ procedure. The presence of the \(\gamma _5\) matrix is dealt without the need of gauge symmetry restoring counterterms and the Kinoshita–Lee–Nauenberg (KLN) theorem is verified. The results are compared to the ones obtained in the Dimensional Reduction scheme (DRed).

abstract

Resonance pole positions of the \(f_0(500)\) alias \(\sigma (500)\) meson are computed and plotted as a continuous function of pion mass in the framework of a unitary and analytic coupled-channel model for scalar mesons as dynamical \(q\bar {q}\) states. The \(\sigma \) is described with a light and a strange \(q\bar {q}\) seed, mixing with each other mainly through the common \(\pi \pi \), \(K\bar {K}\), and \(\eta \eta \) meson–meson channels. The few model parameters are fitted to experimental \(S\)-wave \(\pi \pi \) phase shifts up to 1 GeV, yielding, in the case of the physical pion mass, resonance poles at \((460-i222)\) MeV for the \(\sigma (500)\) and \((978-i37)\) MeV for the \(f_0(980)\). Resonance, bound-state, and virtual-state pole trajectories are shown as a function of \(m_\pi \) running from 139.57 MeV to 1 GeV. These are compared to recent lattice QCD computations that use interpolating fields corresponding to the model’s channels, i.e. , for a few discrete \(m_\pi \)values.

abstract

A narrow structure, reported by the Belle Collaboration in the \(\gamma \psi (2S)\) channel, is an excellent candidate for the spin-2 partner of \(X(3872)\) in the hadronic molecular picture. This paper explores the decay processes of \(X_2\to \gamma \psi \) (\(\psi =J/\psi ,~\psi (2S)\)) and \(X_2 \to e^+e^-\), evaluating the direct production of \(X_2\) in \(e^+e^-\) collisions. Our results indicate that the ratio \({\mit \Gamma }[X_2\to \gamma \psi (2S)]/{\mit \Gamma }[X_2\to \gamma J/\psi ]\) is less than \(1.0\). Additionally, based on the estimation of the process \(X_2\to e^+e^-\), we expect that the planned Super \(\tau \)-Charm Facility provides an opportunity to search for this new state in \(\gamma J/\psi \) and \(\gamma \psi (2S)\) final states.

abstract

The NA62 experiment at CERN is a high-intensity kaon decay experiment with a very broad physics program. Between 2016 and 2018, it collected the world’s largest dataset of \(K^+\) decays, which led to the first measurement of the branching ratio of the rare \(K^+ \rightarrow \pi ^+ \nu \overline {\nu }\) (PNN) decay, observed with a significance of \(3.4\sigma \). At the same time, several other new-physics (NP) studies are carried out. For example, the experiment is very active in the search for lepton-flavour/number-violating kaon decays, the most recent result concerning the \(K^+ \rightarrow \mu ^- \nu e^+ e^+\) channel. Moreover, limits were recently set on the existence of evidence of a dark (pseudo-) scalar in the PNN dataset and hidden-sector mediators are sought in the \(K^+ \rightarrow \pi ^+ e^+ e^+ e^- e^-\) channel. In 2021, the experiment was run in beam-dump mode for the first time. This allows for new searches for exotic particles — for instance, a massive dark photon which decays into a \(\mu ^+\mu ^-\) pair is sought.

abstract

We briefly review the general procedure for building the Born–Oppenheimer Effective Field Theory Lagrangian at next-to-leading order and focus on the hyperfine splitting of heavy-quarkonium hybrids as an example. We use an interpolation between the known forms of the short- and long-distance potentials. We correct an error in one of the long-distance potentials, which leads to very small modifications of the previous results.

abstract

We report on the measurement of the \(pp \ \rightarrow \ p h^+ h^- p\) central exclusive production process in proton–proton collisions with the STAR detector at RHIC at two different center-of-mass energies \(\sqrt {s} = 200\) GeV and \(\sqrt {s} = 510\) GeV. At these energies, the process is dominated by a double Pomeron exchange mechanism. The charged particle pairs were constructed by combining oppositely charged tracks within the central detectors of STAR, the Time Projection Chamber, and the Time of Flight. The pairs were identified using the ionization energy loss and the time-of-flight method. Diffractively scattered protons, which remain intact inside the RHIC beam pipe after the collision, were measured in the Roman Pots system allowing for full control of the interaction’s kinematics and verification of its exclusivity. In these proceedings, we present differential cross sections for centrally produced \(\pi ^+ \pi ^-\), \(K^+ K^-\), and \(p \bar {p}\) pairs measured within the STAR acceptance at \(\sqrt {s} = 200\) GeV together with the preliminary results on the measurement of the same physics process at the higher center-of-mass energy, \(\sqrt {s} = 510\) GeV.

abstract

Glueball spectroscopy in the presence of dynamical quarks faces a number of obstacles. Not only is there a signal-to-noise problem heavily affecting purely gluonic operators, but also flavour-singlet energy eigenstates of the theory are a mixture of mesonic and gluonic degrees of freedom. Tackling these issues requires large statistics and operators with large overlap onto energy eigenstates of interest. We present results for the glueball spectrum with and without dynamical quarks using a wide variety of Wilson loop shapes, which work well for the quenched case but are heavily affected by excited-state contamination when dynamical quarks are introduced. For the latter case, flavour-singlet meson operators based on the framework of optimized distillation are included and show that energy eigenstates have significant contributions from both types of operators.

abstract

In this paper, we study a scalar \(O(N)\) model with a so-called moat regime — a regime with negative bosonic wave function renormalization — using lattice field theory. For negative bare wave function renormalization, inhomogeneous condensates are solutions of the classical equations of motions. Using hybrid Monte Carlo simulations, we demonstrate how bosonic quantum fluctuations disorder the inhomogeneous condensate. Instead, one finds a so-called Quantum Pion Liquid, where bosonic correlation functions are spatially oscillating, but also exponentially decaying.