The role of triangle singularity in isospin breaking process J/ψ→ΛΛˉπJ/\psi \to \Lambda \bar{\Lambda} \pi and the possible evidence of Σ∗(12−)\Sigma^*(\frac{1}{2}^-) states

In this study, the impact of triangle singularity is investigated in the isospin-breaking process $J/\psi \to \Lambda \bar{\Lambda} \pi$. The triangle singularity is found to play a significant role in the process, resulting in the creation of a resonance-like structure around 1.4 GeV in the $\Lambda\pi(\bar{\Lambda}\pi)$ invariant mass spectrum. To amplify the impact of this triangle singularity, the presence of two $\Sigma^*(\frac{1}{2}^-)$ states around 1.4 GeV and 1.6 GeV is essential, yet these states have not been definitively identified in the current baryon spectrum. We recommend that experiments, particularly the Beijing Spectrometer (BESIII) and the future Super Tau-Charm Factory (STCF), to investigate the process $J/\psi \to \Lambda \bar{\Lambda} \pi$ to offer direct evidences for our predicted triangle singularity and additional evidence regarding the $\Sigma(\frac{1}{2}^-)$ states

The hidden-charm pentaquark and tetraquark states

In the past decade many charmonium-like states were observed experimentally. Especially those charged charmonium-like $Z_c$ states and bottomonium-like $Z_b$ states can not be accommodated within the naive quark model. These charged $Z_c$ states are good candidates of either the hidden-charm tetraquark states or molecules composed of a pair of charmed mesons. Recently, the LHCb Collaboration discovered two hidden-charm pentaquark states, which are also beyond the quark model. In this work, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark states. We list the puzzles and theoretical challenges of these models when confronted with the experimental data. We also discuss possible future measurements which may distinguish the theoretical schemes on the underlying structures of the hidden-charm multiquark states.Comment: Review accepted by Physics Reports, 152 pages, 66 figures, and 29 table

Dynamical Hadrons: Case Studies of Meson-Meson and Meson-Baryon Molecules and Triangle Singularities

In this thesis we use various methods to study the interaction of hadrons. We focus on topics related to exotic hadrons, such as tetraquarks from meson-meson interaction, pentaquarks from meson-baryon, and also triangular singularities. We show how experimental data can be explained with our theoretical models, and make predictions that can be compared with future experiments. In chapter 1 we show our method to describe meson-meson interactions, known as chiral unitary approach, showing how the interaction of pseudoscalars generates the f0(500), f0(980) and a0(980), which appear in the two articles discussed in this chapter: that of the eta_c -> eta pi+ pi- decay, and that of the a0(980)-f0(980) mixing in chi_c1 -> pi0 pi0 eta and chi_c1 -> pi0 pi+ pi-. Both works share common features of a method used in an earlier study of the chi_c1 -> eta pi+ pi- decay, which consists in using SU(3) symmetry to see the weight of different trios of pseudoscalars produced in the charmonium decay c cbar -> 123. In chapter 2 we show a method for studying meson-baryon interactions and looking for new states as poles in the scattering amplitude. We discuss how to extend the local hidden gauge approach to the charm sector, through the exchange of light vectors with SU(3) symmetry and the heavy quark as spectator. We present three papers: the one on the five Omega_c states recently discovered by the LHCb collaboration, three of them in remarkable agreement with our calculations; then the follow-up article on how to observe these states in the weak decay Omega_b -> Xi_c+ K- pi- and a third one with predictions for Omega_b molecular states. In chapter 3 we see how triangular singularities can be formed in the decay A -> 1+R, followed by R -> 2+3 and the rescattering 1+2 -> 1'+2'. We see that in the case when there is a resonance from the rescattering, at the same energy corresponding to the singularity, its effect can be seen in the experiment and misinterpreted as a new state. In our first work we study the production and decay of the f1(1285) in pi a0(980) and K* Kbar. We find an enhancement tied to a triangular singularity concluding that the f1(1420) is not a genuine resonance, but the manifestation of these decay modes at higher energies. Next, we study the reaction gamma p -> p pi0 eta paying attention to the two main mechanisms at low energies, gamma p -> Delta(1700) -> eta Delta(1232) or pi N(1535), where the second involves a triangular singularity. Finally, we investigate the Schmid theorem that states the possible triangle singularity developed by the elastic reescattering does not change the cross section provided by the tree level. We investigate the process in terms of the width of the unstable particle produced and determine the violation and the limits for validity of the theorem. Overall we have shown the importance of the dynamically generated states and how this description should be part of our understanding of the fundamental properties of matter

Advances in Precision Calculations of Higgs Boson and Single Top Quark Production at the Large Hadron Collider

Since the discovery of the Higgs boson in 2012, particle physics has entered an era of precision. With the upcoming increase in luminosity of the Large Hadron Collider (LHC), we will gain access to deep and detailed insights into the behaviour of fundamental particles. On the theoretical side, the description of partonic cross sections with next-to-next-to-leading order (NNLO) accuracy in Quantum Chromodynamics (QCD) is becoming a standard. Despite the astonishing agreement between the experiments and theoretical predictions, it is clear that the Standard Model (SM) of particle physics is incomplete. One way to search for New Physics is to push the accuracy of the theoretical predictions and experimental measurements further. In this thesis, we study three problems related to precision description of Higgs boson and top quark production at the LHC. In the first part, we investigate the interference contribution between two Higgs production mechanisms in the pp → H + jetc process. This process can be used to study the Yukawa coupling of the charm quark. The interference studied in this thesis requires a helicity flip on the charm-quark line, forcing us to treat the charm quarks as massive. This requirement leads to unconventional QCD phenomena, such as the importance of soft quarks and unusual collinear factorisation. In the second part, we calculate the so-called non-factorisable corrections to t-channel single top production. These corrections arise from the crosstalk between the two fermion lines present in this process. Until now, the non-factorisable contributions to single top production have been neglected because they do not appear at next-to-leading order (NLO) and they are colour-suppressed compared to the factorisable ones. However, recent studies indicate that the factorisable corrections are relatively small at NNLO and that the non-factorisable ones can be dynamically enhanced. We compute the non-factorisable corrections and discuss their numerical impact on t-channel single top production at the LHC and the Future Circular Collider (FCC). In the third part, we consider the same type of corrections to Higgs production in weak boson fusion (WBF). Contrary to the case of t-channel single top production, an exact computation of these corrections is currently impossible. We construct an expansion of the double-virtual contribution around the forward limit of the tagging jets. It turns out that the expression of the double-virtual contribution at the next-to-leading order in the eikonal approximation can be expressed in a quite compact form

Quantum phase transitions in transverse field spin models: from statistical physics to quantum information

We review quantum phase transitions of spin systems in transverse magnetic fields taking the examples of the spin-1/2 Ising and XY models in a transverse field. Beginning with an overview of quantum phase transitions, we introduce a number of model Hamiltonians. We provide exact solutions in one spatial dimension connecting them to conformal field theoretical studies. We also discuss Kitaev models and some other exactly solvable spin systems. Studies of quantum phase transitions in the presence of quenched randomness and with frustrating interactions are presented in detail. We discuss novel phenomena like Griffiths-McCoy singularities. We then turn to more recent topics like information theoretic measures of the quantum phase transitions in these models such as concurrence, entanglement entropy, quantum discord and quantum fidelity. We then focus on non-equilibrium dynamics of a variety of transverse field systems across quantum critical points and lines. After mentioning rapid quenching studies, we dwell on slow dynamics and discuss the Kibble-Zurek scaling for the defect density following a quench across critical points and its modifications for quenching across critical lines, gapless regions and multicritical points. Topics like the role of different quenching schemes, local quenching, quenching of models with random interactions and quenching of a spin chain coupled to a heat bath are touched upon. The connection between non-equilibrium dynamics and quantum information theoretic measures is presented at some length. We indicate the connection between Kibble-Zurek scaling and adiabatic evolution of a state as well as the application of adiabatic dynamics as a tool of a quantum optimization technique known as quantum annealing. The final section is dedicated to a detailed discussion on recent experimental studies of transverse Ising-like systems.Comment: 106 pages, 38 figures; an expanded version has been published as a book (330 pages, 72 figures, 874 references) as A. Dutta, G. Aeppli, B. K. Chakrabarti, U. Divakaran, T. F. Rosenbaum and D. Sen, Quantum Phase Transitions in Transverse Field Spin Models: From Statistical Physics to Quantum Information (Cambridge University Press, Cambridge, 2015