Non-perturbative Strong Interaction Research Articles

Renormalization-Group Evolution for the Three-Particle B -Meson Soft Function

We determine for the first time the renormalization-group equation for the three-particle B-meson soft function dictating the nonperturbative strong interaction dynamics of the long-distance penguin contributions to the double radiative B-meson decays. The distinctive feature of the ultraviolet renormalization of this fundamental soft function consists in the pattern of mixing positive into negative support for an arbitrary initial condition. The exact solution to this integrodifferential evolution equation is then derived with the Laplace transform technique, allowing for the model-independent extraction of the asymptotic behavior at large and small partonic momenta. Published by the American Physical Society 2024

Exploring the internal structure of exotic resonances with ALICE

The investigation of the quark content of hadrons has been a major goal of nonperturbative strong interaction physics. In the last decade, several resonances in the mass range 1000-2000 MeV/c2 have emerged that cannot be explained by the quark model. The internal structure of exotic resonances such as f0, f1, and f2 is currently unknown. Different scenarios are possible ranging from twoquark, four-quark, molecule, a hybrid state, to glueballs. A modification of the measured yields of these exotic hadrons in A–A and p–A collisions as compared to pp collisions has been proposed as a tool to investigate their internal structure. In this proceeding, the first-ever measurement of f1 production in pp collisions and measurements of f0 and f2 production both in pp and p–Pb collisions will be discussed. The measurements of their mass, width, yields and their sensitivity to the internal structure of these exotic resonances will be shown. These results will pave the way for future experimental investigations on the internal structure of other exotic hadrons.

STCF conceptual design report (Volume 1): Physics & detector

The super τ-charm facility (STCF) is an electron–positron collider proposed by the Chinese particle physics community. It is designed to operate in a center-of-mass energy range from 2 to 7 GeV with a peak luminosity of 0.5 × 1035 cm−2·s−1 or higher. The STCF will produce a data sample about a factor of 100 larger than that of the present τ-charm factory — the BEPCII, providing a unique platform for exploring the asymmetry of matter-antimatter (charge-parity violation), in-depth studies of the internal structure of hadrons and the nature of non-perturbative strong interactions, as well as searching for exotic hadrons and physics beyond the Standard Model. The STCF project in China is under development with an extensive R&D program. This document presents the physics opportunities at the STCF, describes conceptual designs of the STCF detector system, and discusses future plans for detector R&D and physics case studies.

Implementation of ACTS for STCF track reconstruction

With an electron-positron collider operating at center-of-mass-energy 2–7 GeV and a peak luminosity above 0.5 × 1035 cm-2 s-1, the STCF physics program will provide an unique platform for in-depth studies of hadron structure and non-perturbative strong interaction as well as probing new physics beyond the Standard Model in the τ-Charm sector, succeeding the present Beijing Electron-Positron Collider. To fulfill the physics targets and further maximize the physics potential at STCF, the STCF tracking software should have capability to reconstruct charged particles with high efficiency and excellent momentum resolution, especially for the charged particles with low transverse momentum down to 50 MeV. A Common Tracking Software (ACTS) providing a set of detector-independent tracking algorithms is adopted for reconstructing charged tracks with the information of two sub-detectors, a μRWELL-based inner tracker and a drift chamber, at STCF. This is the first demonstration of ACTS for a drift chamber. The implementation details and performance of track reconstruction are presented.

Recent advances in charm mixing and CP violation at LHCb

After playing a pivotal role in the birth of the Standard Model in the 70s, the study of charm physics has undergone a revival during the last decade, triggered by a wealth of precision measurements from the charm and B factories, and from the CDF and especially the LHCb experiments. In this paper, we sum up how the unique phenomenology of charmed hadrons can be used to test the Standard Model and we review the latest measurements performed in this field by the LHCb experiment. These include the historic first observations of CP violation and of a nonzero mass difference between the charmed neutral-meson mass eigenstates, the most precise determination of their decay-width difference to date, and a search for time-dependent CP violation reaching the unprecedented precision of [Formula: see text]. These results challenge our comprehension of nonperturbative strong interactions, and their interpretation calls for further studies on both the theoretical and experimental sides. The upcoming upgrades of the LHCb experiment will play a leading role in this quest.

Discovery of the doubly charmed Tcc+ state implies a triply charmed Hccc hexaquark state

The doubly charmed exotic state ${T}_{cc}$ recently discovered by the LHCb Collaboration could well be a $D{D}^{*}$ molecular state long predicted in various theoretical models, in particular, the $D{D}^{*}$ isoscalar axial vector molecular state predicted in the one-boson-exchange model. In this work, we study the $DD{D}^{*}$ system in the Gaussian expansion method with the $D{D}^{*}$ interaction derived from the one-boson-exchange model and constrained by the precise binding energy of $273\ifmmode\pm\else\textpm\fi{}63\text{ }\text{ }\mathrm{keV}$ of ${T}_{cc}$ with respect to the ${D}^{*+}{D}^{0}$ threshold. We show the existence of a $DD{D}^{*}$ state with a binding energy of a few hundred keV, isospin $1/2$, and spin-parity ${1}^{\ensuremath{-}}$. Its main decay modes are $DDD\ensuremath{\pi}$ and $DDD\ensuremath{\gamma}$. The existence of such a state could in principle be confirmed with the upcoming LHC data and will unambiguously determine the nature of the ${T}_{cc}^{+}$ state and of the many exotic states of similar kind, thus deepening our understanding of the nonperturbative strong interaction.

Do we need to use regularization for the thermal part in the NJL model? * *Supported by the Natural Science Foundation of Jiangsu Province (BK20140523), the start-up funding of Jiangsu University (15JDG042, 4111190010), the National Natural Science Foundation of China (11735007)

The Nambu–Jona-Lasinio (NJL) model is one of the most useful tools for studying non-perturbative strong interactions in matter. Because it is a nonrenormalizable model, the choice of regularization is a subtle issue. In this paper, we discuss one of the general issues regarding regularization in the NJL model, which is whether we need to use regularization for the thermal part by evaluating the quark chiral condensate and thermal properties in the two-flavor NJL model. The calculations in this work include three regularization schemes that contain both gauge covariant and invariant schemes. We found that, regardless of the regularization scheme we choose, it is necessary to use regularization for the thermal part when calculating physical quantities related to the chiral condensate and to not use regularization for the thermal part when calculating physical quantities related to the grand potential.

Super Tau-Charm Facility of China

The tau-charm region is one of the most important energy regions in the research of particle physics due to its unique properties, rich frontier topics and great potential for scientific discovery. In order to maintain our country's pioneering position in the tau-charm research field, we propose to build a new generation electron-positron collider—the Super Tau-Charm Facility (STCF), based on the Hefei National Synchrotron Radiation Laboratory, the State Key Laboratory of Particle Detection and Electronics, and other research platforms. The STCF would have a luminosity greater than 0.5×1035 cm-2 s-1, and a center-of-mass energy region of 2 to 7 GeV. Its performance is expected to be significantly better than the currently running BEPCII, and will provide a key platform for exploring the asymmetry of matter-antimatter (charge parity violation), in-depth studies of the internal structure of hadrons and the nature of non-perturbative strong interactions, as well as the quest for exotic particles and physics beyond the standard model. The construction of STCF poses a great challenge to the existing accelerator and detector technologies in China, but is very important scientifically and strategically not only for fundamental scientific research but also for the development of new technology and the cultivation of comprehensive expertise.

Pion-mass dependence of the nucleon-nucleon interaction

Nucleon-nucleon interactions, both bare and effective, play an important role in our understanding of the non-perturbative strong interaction, as well as nuclear structure and reactions. In recent years, tremendous efforts have been seen in the lattice QCD community to derive nucleon-nucleon interactions from first principles. Because of the daunting computing resources needed, most of such simulations were still performed with larger than physical light quark masses. In the present work, employing the recently proposed covariant chiral effective field theory (ChEFT), we study the light quark mass dependence of the nucleon-nucleon interaction extracted by the HALQCD group. It is shown that the pion-full version of the ChEFT can describe the lattice QCD data with mπ=469 MeV and their experimental counterpart reasonably well, while the pion-less version can describe the lattice QCD data with mπ=672,837,1015,1171 MeV, for both the S01 and S13-D13 channels. The slightly better description of the single channel than the triplet channel indicates that higher order studies are necessary for the latter. Our results confirmed previous studies that the nucleon-nucleon interaction becomes more attractive for both the singlet and triplet channels as the pion mass decreases towards its physical value. It is shown that the virtual bound state in the S01 channel remains virtual down to the chiral limit, while the deuteron only appears for a pion mass smaller than about 400 MeV. It seems that proper chiral extrapolations of nucleon-nucleon interaction are possible for pion masses smaller than 500 MeV, similar to the mesonic and one-baryon sectors.

From triply charmed dibaryons to pentaquark states: A model-independent way to determine the spins of the Pc(4440) and Pc(4457)

The LHCb collaboration has recently observed three narrow pentaquark states --- the $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$ ---that are located close to the $\bar{D} \Sigma_c$ and $\bar{D}^* \Sigma_c$ thresholds. Among the so-far proposed theoretical interpretations for these pentaquarks, the molecular hypothesis seems to be the preferred one. Nevertheless, in the molecular picture the spins of the $P_c(4440)$ and $P_c(4457)$ have not been unambiguously determined yet. In this letter we point out that heavy antiquark-diquark symmetry induces a model-independent relation between the spin-splitting in the masses of the $P_c(4440)$ and $P_c(4457)$ $\bar{D}^* \Sigma_c$ pentaquarks and the corresponding splitting for the $0^+$ and $1^+$ $\Xi_{cc} \Sigma_c$ triply charmed dibaryons. This is particularly relevant owing to a recent lattice QCD prediction of the $1^+$ triply charmed dibaryon, which suggests that a calculation of the mass of its $0^+$ partner might be within reach. This in turn would reveal the spins of the $P_c(4440)$ and $P_c(4457)$ pentaquarks, providing a highly nontrivial test of heavy-quark symmetry and the molecular nature of the pentaquarks. Furthermore, the molecular interpretation of the hidden-charm pentaquarks implies the existence of a total of ten triply charmed dibaryons as $\Xi_{cc}^{(*)} \Sigma_c^{(*)}$ molecules, which, if confirmed in the lattice, will largely expand our understanding of the non-perturbative strong interaction in the heavy-quark sector.

Transverse Single-Spin Asymmetry for Very Forward Neutral Pion Production in Polarized p+p Collisions at s=510 GeV

Transverse single-spin asymmetries of very forward neutral pions generated in polarized p+p collisions allow us to understand the production mechanism in terms of perturbative and nonperturbative strong interactions. During 2017, the RHICf Collaboration installed an electromagnetic calorimeter in the zero-degree region of the STAR detector at the Relativistic Heavy Ion Collider (RHIC) and measured neutral pions produced at pseudorapidity larger than 6 in polarized p+p collisions at sqrt[s]=510 GeV. The large nonzero asymmetries increasing both in longitudinal momentum fraction x_{F} and transverse momentum p_{T} have been observed at low transverse momentum p_{T}<1 GeV/c for the first time, at this collision energy. The asymmetries show an approximate x_{F} scaling in the p_{T} region where nonperturbative processes are expected to dominate. A non-negligible contribution from soft processes may be necessary to explain the nonzero neutral pion asymmetries.

CLAS $$\varvec{N^*}$$ Excitation Results from Pion and Kaon Electroproduction

The study of the structure of excited nucleon $$N^*$$ states using the electroproduction of exclusive reactions is important for exploring the non-perturbative strong interaction. The electrocoupling amplitudes of $$N^*$$ states for masses below $$W=1.8$$ GeV have been determined from analyses of CLAS $$\pi N$$ , $$\eta N$$ , and $$\pi \pi N$$ data at four momentum transfers $$Q^2$$ up to 5 GeV $$^2$$ . These studies have made it apparent that consistency of the results from independent analyses of multiple exclusive channels with different couplings and non-resonant backgrounds but the same $$N^*$$ electroexcitation amplitudes, is necessary to have confidence in the results. For the hadronic couplings, many high-lying $$N^*$$ states preferentially decay through the $$\pi \pi N$$ channel instead of the $$\pi N$$ channel. Therefore, data from the KY channels already measured with CLAS will be crucial to provide an independent analysis to compare the extracted electrocouplings for the high-lying $$N^*$$ states against those determined from the $$\pi N$$ and $$\pi \pi N$$ channels. These comparisons await the development of suitable reaction models. Starting in 2018, a program to study the structure of $$N^*$$ states in various exclusive electroproduction channels using the new CLAS12 spectrometer will get underway. These studies will probe the structure of these states for masses W up to 3 GeV and $$Q^2$$ up to 12 GeV $$^2$$ , thus providing a means to access $$N^*$$ structure information spanning a broad regime encompassing both low and high energy degrees of freedom.

Studying the interplay of strong and electromagnetic forces in heavy-ion collisions with NICA

In the following we stress the advantages of the NICA research programme in the context of studying the spectator-induced electromagnetic phenomena present in heavy-ion collisions. We point at the specific interest of using these phenomena as a new, independent source of information on the space-time evolution of the reaction and of the non-perturbative process of particle production. We propose an extended series of measurements of well-defined observables to be performed in different types of nuclear reactions and in the whole range of collision energies available to NICA. We expect these measurements to bring very valuable new insight into the mechanism of non-perturbative strong interactions, complementary to the studies made at the SPS at CERN, RHIC at BNL, and the LHC.

Updates on the Studies of $${\varvec{N}}^\mathbf{*}$$ Structure with CLAS and the Prospects with CLAS12

The recent results on $\gamma_vpN^*$ electrocouplings from analyses of the data on exclusive meson electroproduction off protons measured with the CLAS detector at Jefferson Lab are presented. The impact of these results on the exploration of the excited nucleon state structure and non-perturbative strong interaction dynamics behind its formation is outlined. The future extension of these studies in the experiments with the CLAS12 detector in the upgraded Hall-B at JLab will provide for the first time $\gamma_vpN^*$ electrocouplings of all prominent resonances at the still unexplored distance scales that correspond to extremely low (0.05 GeV$^2 < Q^2 <$ 0.5 GeV$^2$) and the highest photon virtualities (5.0 GeV$^2 < Q^2 <$ 12.0 GeV$^2$) ever achieved in the exclusive electroproduction measurements. The expected results will address the most important open problems of the Standard Model: on the nature of more than 98\% of hadron mass, quark-gluon confinement and emergence of the excited nucleon state structure from the QCD Lagrangian, as well as allowing a search for the new states of hadron matter predicted from the first principles of QCD, the so-called hybrid baryons.

Nucleon Resonance Structure Studies via Exclusive KY Electroproduction

Studying the structure of excited nucleon states employing the electroproduction of exclusive reactions is an important avenue for exploring the nature of the non-perturbative strong interaction. The electrocouplings of $N^*$ states in the mass range below 1.8~GeV have been determined from analyses of CLAS $\pi N$, $\eta N$, and $\pi \pi N$ data. This work has made it clear that consistent results from independent analyses of several exclusive channels with different couplings and non-resonant backgrounds but the same $N^*$ electro-excitation amplitudes, is essential to have confidence in the extracted results. In terms of hadronic coupling, many high-lying $N^*$ states preferentially decay through the $\pi \pi N$ channel instead of $\pi N$. Data from the $KY$ channels will therefore be critical to provide an independent analysis to compare the extracted electrocouplings for the high-lying $N^*$ states against those determined from the $\pi N$ and $\pi \pi N$ channels. A program to study excited $N^*$ state structure in both non-strange and strange exclusive electroproduction channels using CLAS12 will measure differential cross sections and polarization observables to be used as input to extract the $\gamma_vNN^*$ electrocoupling amplitudes for the most prominent $N^*$ states in the range of invariant energy $W$ up 3~GeV in the virtually unexplored domain of momentum transfers $Q^2$ up to 12~GeV$^2$.

New results from the studies of theN(1440)1/2+, N(1520)3/2−, andΔ(1620)1/2−resonances in exclusiveep→e′p′π+π−electroproduction with the CLAS detector

The transition helicity amplitudes from the proton ground state to the $N(1440)1/2^+$, $N(1520)3/2^-$, and $\Delta(1620)1/2^-$ resonances ($\gamma_vpN^*$ electrocouplings) were determined from the analysis of nine independent one-fold differential $\pi^+ \pi^- p$ electroproduction cross sections off a proton target, taken with CLAS at photon virtualities 0.5 GeV$^2$ $< Q^2 <$ 1.5 GeV$^2$. The phenomenological reaction model employed for separation of the resonant and non-resonant contributions to this exclusive channel was further developed. The $N(1440)1/2^+$, $N(1520)3/2^-$, and $\Delta(1620)1/2^-$ electrocouplings were obtained from the resonant amplitudes of charged double-pion electroproduction off the proton in the aforementioned area of photon virtualities for the first time. Consistent results on $\gamma_vpN^*$ electrocouplings available from independent analyses of several $W$-intervals with different non-resonant contributions offer clear evidence for the reliable extraction of these fundamental quantities. These studies also improved the knowledge on hadronic branching ratios for the $N(1440)1/2^+$, $N(1520)3/2^-$, and $\Delta(1620)1/2^-$ decays to the $\pi \Delta$ and $\rho N$ final states. These new results provide a substantial impact on the QCD-based approaches that describe the $N^*$ structure and demonstrate the capability to explore fundamental ingredients of the non-perturbative strong interaction that are behind the excited nucleon state formation.

Status of Theory and Experiment in Hadronic Parity Violation

Hadronic parity violation uses quark-quark weak interactions to probe nonperturbative strong interaction dynamics through two nonperturbative QCD scales: [Formula: see text] and the fine-tuned MeV scales of NN bound states in low energy nuclear physics. The current and projected availability of high-intensity neutron and photon sources coupled with ongoing experiments and continuing developments in theoretical methods provide the opportunity to greatly expand our understanding of hadronic parity violation in few-nucleon systems. The current status of these efforts and future plans are discussed.

STUDIES OF N* STRUCTURE FROM THE CLAS MESON ELECTROPRODUCTION DATA

The transition γvpN* amplitudes (electrocouplings) for prominent excited nucleon states obtained in a wide area of photon virtualities offer valuable information for the exploration of the N* structure at different distances and allow us to access the complex dynamics of non-perturbative strong interaction. The current status in the studies of γvpN* electrocouplings from the data on exclusive meson electroproduction off protons measured with the CLAS detector at Jefferson Lab is presented. The impact of these results on exploration of the N* structure is discussed.

STUDIES OF NUCLEON RESONANCE STRUCTURE IN EXCLUSIVE MESON ELECTROPRODUCTION

Studies of the structure of excited baryons are key factors to the N* program at Jefferson Lab (JLab). Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever achieved, with a kinematic reach up to Q2 = 12 GeV 2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with Quantum Chromodynamics (QCD)-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the nonperturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

Recent development in SU(3) covariant baryon chiral perturbation theory

Baryon chiral perturbation theory (BChPT), as an effective field theory of low-energy quantum chromodynamics (QCD), has played and is still playing an important role in our understanding of non-perturbative strong interaction phenomena. In the past two decades, inspired by the rapid progress in lattice QCD simulations and the new experimental campaign to study the strangeness sector of low-energy QCD, many efforts have been made to develop a fully covariant BChPT and to test its validity in all scenarios. These new endeavours have not only deepened our understanding of some long-standing problems, such as the power-counting-breaking problem and the convergence problem, but also resulted in theoretical tools that can be confidently applied to make robust predictions. Particularly, the manifestly covariant BChPT supplemented with the extended-on-mass-shell (EOMS) renormalization scheme has been shown to satisfy all analyticity and symmetry constraints and converge relatively faster compared to its non-relativistic and infrared counterparts. In this article, we provide a brief review of the fully covariant BChPT and its latest applications in the $u$, $d$, and $s$ three-flavor sector.