Exotic Quantum Numbers Research Articles

Hybrid exotic mesons in soft-wall AdS/QCD

Hybrid mesons with exotic quantum numbers $J^{PC}=1^{-+}$ are examined in soft-wall AdS/QCD. The predicted mass spectrum is compared to the measured values of the candidates $\pi_1(1400)$, $\pi_1(1600)$ and $\pi_1(2015)$. Thermal effects are analysed through the spectral function in the AdS-Black Hole model, and the differences with the Hawking-Page description are discussed.

Aspects of Anomalous Glue

Non-perturbative glue associated with gluon topology plays a vital role in determining the mass of the etaprime meson. We give an introduction to axial U(1) physics and explain how this non-perturbative glue also contributes to etaprime interactions with other hadrons. We concentrate on resonant etaprime pi production in partial waves with exotic quantum numbers not compatible with a quark-antiquark state and on the etaprime in nuclear media where there are new results from the COMPASS and CBELSA/TAPS experiments respectively.

Gluonic excitations in the hadronic spectrum

QCD at low energy features a gluonic field that is strongly coupled to itself and to quarks. I will present a summary of what we know about the role that excitations of the gluonic field play in determining the spectrum of meson resonances. Recent studies using lattice techniques have suggested a phenomenology of gluonic excitations within QCD that leads to hybrid mesons with both exotic and non-exotic quantum numbers. I will discuss these calculations and describe their relationship to current experimental knowledge and to forthcoming experiments at Jefferson Lab and elsewhere.

Mass spectrum of heavy quarkonium hybrids

We have extended the calculation of the correlation functions of heavy quarkonium hybrid operators with various $J^{PC}$ quantum numbers to include QCD condensates up to dimension six. In contrast to previous analyses which were unable to optimize the QCD sum-rules for certain $J^{PC}$, recent work has shown that inclusion of dimension six condensates stabilizes the hybrid sum-rules and permits reliable mass predictions. In this work we have investigated the effects of the dimension six condensates on the remaining channels. After performing the QCD sum-rule analysis, we update the mass spectra of charmonium and bottomonium hybrids with exotic and non-exotic quantum numbers. We identify that the negative-parity states with $J^{PC}=(0, 1, 2)^{-+}, 1^{--}$ form the lightest hybrid supermultiplet while the positive-parity states with $J^{PC}=(0, 1)^{+-}, (0, 1, 2)^{++}$ belong to a heavier hybrid supermultiplet, confirming the supermultiplet structure found in other approaches. The hybrid with $J^{PC}=0^{--}$ has a much higher mass which may suggest a different excitation of the gluonic field compared to other channels. In agreement with previous results, we find that the $J^{PC}=1^{++}$ charmonium hybrid is substantially heavier than the X(3872), which seems to preclude a pure charmonium hybrid interpretation for this state.

Meson Spectroscopy at JLab@12 GeV

Meson, being the simplest hadronic bound system, is the ideal "laboratory" to study the interaction between quarks, to understand the role of the gluons inside hadrons and to investigate the origin of color confinement. To perform such studies it is important to measure the meson spectrum, with precise determination of resonance masses and properties, looking for rare qq̄ states and for unconventional mesons with exotic quantum numbers (i.e. mesons with quantum numbers that are not compatible with a qq̄ structure).With the imminent advent of the 12 GeV upgrade of Jefferson Lab a new generation of meson spectroscopy experiments will start: "Meson-Ex" in Hall B and "GLUEX" in Hall D. Both will use photo-production to explore the spectrum of mesons in the light-quark sector, in the energy range of few GeVs.

Exotic Mesons with Hidden Bottom Near Thresholds

We discuss exotic meson spectroscopy near open bottom thresholds. Assuming the exotic mesons as \({B^{(\ast)}\bar{B}^{(\ast)}}\) molecular states, we study the interaction among two heavy mesons in terms of the one boson exchange potential model. It is shown that masses of Z b (10610) and Z b (10650) are reproduced as \({B^{(\ast)}\bar{B}^{(\ast)}}\) bound and resonance states. Besides, we also show that \({B^{(\ast)}\bar{B}^{(\ast)}}\) molecular states having various exotic quantum numbers can exist around the thresholds. By contrast, there are no \({D^{(\ast)}\bar{D}^{(\ast)}}\) molecular states having exotic quantum numbers.

Staggered Spin-Orbit Order: A New Paradigm of Broken Symmetry Phase of Matter

We propose a novel spin-orbit density wave order, which can arise in a variety of materials classes. In systems where the noninteracting wavefunctions are defined by an exotic quantum number such as total angular momentum, pseudospin, or helical quantum number owing to spin-orbit coupling of various natures, interaction can induce an emergent spin-orbit density wave even when time-reversal symmetry is intact. This density wave order is different from standard time-reversal breaking spin or orbital density wave. We apply this idea to explain the enigmatic “hidden order” phase in heavy fermion URu2Si2 as well as an unknown gapped quasiparticle state observed in two-dimensional electron gases, such as the surface state of BiAg2.

QCD quark cyclobutadiene and light tetraquark spectra

The QCD quark cyclobutadiene (ringlike), a new color structure of a tetraquark system, is proposed and studied in the flux-tube model with a multibody confinement interaction. Numerical calculations show that the light tetraquark systems ($u$, $d$, $s$ only) with cyclobutadiene, diquark-antidiquark flux-tube structures have similar energies and they can be regarded as QCD isomeric compounds. The energies of some tetraquark states are close to the energies of some excited mesons, and so in the study of these mesons, the tetraquark components should be taken into account. There are also some meson states, $\ensuremath{\sigma}$, $\ensuremath{\kappa}(800)$, ${f}_{0}(980)$, ${f}_{0}(1500)$, ${\ensuremath{\pi}}_{1}(1400)$, ${\ensuremath{\pi}}_{1}(1600)$, ${f}_{2}(1430)$ and ${K}^{*}(1410)$, where tetraquark components might be dominant. The meson states with exotic quantum numbers are studied as the tetraquark states. The multibody confinement interaction reduces the energy of the tetraquark state in comparison with the usual additive two-body confinement interaction model.

Is the1−+meson a hybrid?

We calculate the vacuum to meson matrix elements of the dimension-4 operator $\overline{\ensuremath{\psi}}{\ensuremath{\gamma}}_{4}{\stackrel{\ensuremath{\leftrightarrow}}{D}}_{i}\ensuremath{\psi}$ and dimension-5 operator $\overline{\ensuremath{\psi}}{\ensuremath{\epsilon}}_{ijk}{\ensuremath{\gamma}}_{j}\ensuremath{\psi}{B}_{k}$ of the ${1}^{\ensuremath{-}+}$ meson on the lattice and compare them to the corresponding matrix elements of the ordinary mesons to discern if it is a hybrid. For the charmoniums and strange quarkoniums, we find that the matrix elements of ${1}^{\ensuremath{-}+}$ are comparable in size as compared to other known $q\overline{q}$ mesons. They are particularly similar to those of the ${2}^{++}$ meson, since their dimension-4 operators are in the same Lorentz multiplet. Based on these observations, we find no evidence to support the notion that the lowest ${1}^{\ensuremath{-}+}$ mesons in the $c\overline{c}$ and $s\overline{s}$ regions are hybrids. As far as the exotic quantum number is concerned, the nonrelativistic reduction reveals that the leading terms in the dimension-4 and dimension-5 operators of ${1}^{\ensuremath{-}+}$ are identical up to a proportional constant and it involves a center-of-mass momentum operator of the quark-antiquark pair. This explains why ${1}^{\ensuremath{-}+}$ is an exotic quantum number in the constituent quark model where the center of mass of the $q\overline{q}$ is not a dynamical degree of freedom. Since QCD has gluon fields in the context of the flux tube which is appropriate for heavy quarkoniums to allow the valence $q\overline{q}$ to recoil against them, it can accommodate such states as ${1}^{\ensuremath{-}+}$. By the same token, hadronic models with additional constituents besides the quarks can also accommodate the $q\overline{q}$ center-of-mass motion. To account for the quantum numbers of these $q\overline{q}$ mesons in QCD and hadron models in the nonrelativistic case, the parity and total angular momentum should be modified to $P=(\ensuremath{-}{)}^{L+l+1}$ and $\stackrel{\ensuremath{\rightarrow}}{J}=\stackrel{\ensuremath{\rightarrow}}{L}+\stackrel{\ensuremath{\rightarrow}}{l}+\stackrel{\ensuremath{\rightarrow}}{S}$, where $L$ is the orbital angular momentum of the $q\overline{q}$ pair in the meson.

Excited and exotic charmonium spectroscopy from lattice QCD

We present a spectrum of highly excited charmonium mesons up to around 4.5 GeV calculated using dynamical lattice QCD. Employing novel computational techniques and the variational method with a large basis of carefully constructed operators, we extract and reliably identify the continuum spin of an extensive set of excited states, states with exotic quantum numbers (0+-, 1-+, 2+-) and states with high spin. Calculations are performed on two lattice volumes with pion mass approximately 400 MeV and the mass determinations have high statistical precision even for excited states. We discuss the results in light of experimental observations, identify the lightest 'supermultiplet' of hybrid mesons and comment on the phenomenological implications of the spectrum of exotic mesons.

Exotic baryons from a heavy meson and a nucleon: Positive parity states

We study heavy baryons with exotic flavor quantum numbers formed by a heavy meson and a nucleon ($\overline{D}N$ and $BN$) with positive parity. One-pion-exchange interaction, providing a tensor force, dominates as a long-range force to bind the $\overline{D}N$ and $BN$ systems. In the heavy-quark mass limit, pseudoscalar meson and vector meson are degenerate and the binding mechanism by the tensor force analogous to that in the nuclear systems becomes important. As a result, we obtain the $\overline{D}N$ and $BN$ resonant states in the ${J}^{P}=1/{2}^{+}$, $3/{2}^{+}$, and $5/{2}^{+}$ channels with $I=0$.

Wave-function-based characteristics of hybrid mesons

We propose some extensions of the quark potential model to hybrids, fit them to the lattice data and use them for the purpose of calculating the masses, root mean square radii and wave functions at the origin of the conventional and hybrid charmonium mesons. We treat the ground and excited gluonic field between a quark and an antiquark as in the Born-Oppenheimer expansion, and use the shooting method to numerically solve the required Schr$\ddot{\textrm{o}}$dinger equation for the radial wave functions; from these wave functions we calculate the mesonic properties. For masses we also check through a Crank Nichelson discretization. For hybrid charmonium mesons, we consider the exotic quantum number states with $ J^{PC} = 0^{+ -}, 1^{- +}$ and $2^{+ -}$. We also compare our results with the experimentally observed masses and theoretically predicted results of the other models. Our results have implications for scalar form factors, energy shifts, magnetic polarizabilities, decay constants, decay widths and differential cross sections of conventional and hybrid mesons.

Exotic baryons from a heavy meson and a nucleon: Negative parity states

We study heavy baryons with an exotic flavor quantum number formed by a heavy meson and a nucleon (Dbar-N and B-N) through a long range one pion exchange interaction. The bound state found previously in the (I,JP) =(0,1/2-) channel survives when short range interaction is included. In addition, we find a resonant state with (I,JP)=(0,3/2-) as a Feshbach resonance predominated by a heavy vector meson and a nucleon (Dbar*-N and B*-N). We find that these exotic states exist for the charm and heavier flavor region.

Composite GUTs: models and expectations at the LHC

We investigate grand unified theories (GUTs) in scenarios where electroweak (EW) symmetry breaking is triggered by a light composite Higgs, arising as a Nambu-Goldstone boson from a strongly interacting sector. The evolution of the standard model (SM) gauge couplings can be predicted at leading order, if the global symmetry of the composite sector is a simple group G that contains the SM gauge group. It was noticed that, if the right-handed top quark is also composite, precision gauge unification can be achieved. We build minimal consistent models for a composite sector with these properties, thus demonstrating how composite GUTs may represent an alternative to supersymmetric GUTs. Taking into account the new contributions to the EW precision parameters, we compute the Higgs effective potential and prove that it realizes consistently EW symmetry breaking with little fine-tuning. The G group structure and the requirement of proton stability determine the nature of the light composite states accompanying the Higgs and the top quark: a coloured triplet scalar and several vector-like fermions with exotic quantum numbers. We analyse the signatures of these composite partners at hadron colliders: distinctive final states contain multiple top and bottom quarks, either alone or accompanied by a heavy stable charged particle, or by missing transverse energy.

Identifying Multiquark Hadrons from Heavy Ion Collisions

Identifying hadronic molecular states and/or hadrons with multiquark components either with or without exotic quantum numbers is a long-standing challenge in hadronic physics. We suggest that studying the production of these hadrons in relativistic heavy ion collisions offers a promising resolution to this problem as yields of exotic hadrons are expected to be strongly affected by their structures. Using the coalescence model for hadron production, we find that, compared to the case of a nonexotic hadron with normal quark numbers, the yield of an exotic hadron is typically an order of magnitude smaller when it is a compact multiquark state and a factor of 2 or more larger when it is a loosely bound hadronic molecule. We further find that some of the newly proposed heavy exotic states could be produced and realistically measured in these experiments.

Tetra-Quark Mesons with Exotic Quantum Numbers

Tetra-quark mesons with exotic quantum numbers, their production rates and decay properties are studied, because they are useful to establish the existence of tetra-quark mesons.

Higher extended technicolor representations and fermion generations

In usual extended technicolor (ETC) theories based on the group ${\rm{SU}(N_{ETC}})_{ETC}$, the quarks of charge 2/3 and -1/3 and the charged leptons of all generations arise from ETC fermion multiplets transforming according to the fundamental representation. Here we investigate a different idea for the origin of SM fermion generations, in which quarks and charged leptons of different generations arise from ETC fermions transforming according to different representations of ${\rm{SU}(N_{ETC}})_{ETC}$. Although this mechanism would have the potential, {\it a priori}, to allow a reduction in the value of $N_{ETC}$ relative to conventional ETC models, we show that, at least in simple models, it is excluded by the fact that the technicolor sector is not asymptotically free or by the appearance of fermions with exotic quantum numbers which are not observed.

Non-Abelian Dynamics and Heavy Multiquarks

A brief review is first presented of attempts to predict stable multiquark states within current models of hadron spectroscopy. Then a model combining flip–flop and connected Steiner trees is introduced and shown to lead to stable multiquarks, in particular for some configurations involving several heavy quarks and bearing exotic quantum numbers.

Exotic and excited-state meson spectroscopy and radiative transitions from lattice QCD

We discuss recent progress in extracting the excited meson spectrum and radiative transition form factors from lattice QCD. We mention results in the charmonium sector, including the first lattice QCD calculation of radiative transition rates involving excited charmonium states, highlighting results for high spin and exotic states. We present recent results on a highly excited isovector meson spectrum from dynamical anisotropic lattices. Using carefully constructed operators we show how the continuum spin of extracted states can be reliably identified and confidently extract excited states, states with exotic quantum numbers and states of high spin. This spectrum includes the first spin-four state extracted from lattice QCD. We conclude with some comments on future prospects.

Status of exotic-quantum-number mesons

The search for mesons with non-quark-antiquark (exotic) quantum numbers has gone on for nearly thirty years. There currently is experimental evidence of three isospin one states, the $\pi_{1}(1400)$, the $\pi_{1}(1600)$ and the $\pi_{1}(2015)$. For all of these states, there are questions about the identification of these state, and even if some of them exist. In this article, we will review both the theoretical work and the experimental evidence associated with these exotic quantum number states. We find that the $\pi_{1}(1600)$ could be the lightest exotic quantum number hybrid meson, but observations of other members of the nonet would be useful.