Hadron Spectroscopy Research Articles

Baryon Spectroscopy with Polarization Observables from CLAS

The spectrum of nucleon excitations is dominated by broad and overlapping resonances. Polarization observables in photoproduction reactions are key in the study of these excitations. They give indispensable constraints to partial-wave analyses and help clarify the spectrum. A series of polarized photoproduction experiments have been performed at the Thomas Jefferson National Accelerator Facility with the CEBAF Large Acceptance Spectrometer (CLAS). These measurements include data with linearly and circularly polarized tagged-photon beams, longitudinally and transversely polarized proton and deuterium targets, and recoil polarizations through the observation of the weak decay of hyperons. An overview of these studies and recent results will be given.

Covariant Spectator Theory and Hadron Structure

We present the first results of a study on meson spectroscopy using a covariant formalism based on the Covariant Spectator Theory. Our approach is derived directly in Minkowski space and it approximates the Bethe–Salpeter equation by taking effectively into account the contributions from both ladder and crossed ladder diagrams in the \(q{\bar{q}}\) interaction kernel. A general Lorentz structure of the kernel is tested and chiral constraints on the kernel are discussed. Results for the pion form factor are also presented.

Hadron Spectroscopy at BESIII

The BESIII detector has collected the largest data samples around τ – charm energy region since the upgrade was completed in 2008. The hadron spectroscopy, as one of the main physics goals, was extensively studied and many important progresses were achieved these years. In this report the recent results on the study of hadron spectroscopy were presented, including the light hadron spectroscopy and the observations of the charmonium-like Zc particles.

Baryon spectroscopy in a three-quark model

In this paper, we present a three-body quark model for investigating the internal structure of baryons as well as baryon spectroscopy. In order to describe the SU(6) -invariant part of the spectrum, we assumed the spin-independent part of the interaction hypercentral, and treated using the hyperspherical formalism. For SU(6) -invariant potential, we used a generalized version of the popular “Coulomb-plus-linear” potential which contains “linear-plus-logarithmic” terms as confinement part and some inverse power terms. To obtain an analytical solution, we applied some approximations for dealing with problematic linear and logarithmic terms, leading to a qualitative reproducing of the spectrum. Then, to describe the hyperfine structure of the baryon and the splittings within the SU(6) -multiplets, we used the generalized Gursey-Radicati Mass Formula as a SU(6) breaking interaction. Our calculations lead to a generally fair description of the baryon spectrum.

Overview and future developments of the FPGA-based DAQ of COMPASS

COMPASS is a fixed-target experiment at the SPS at CERN dedicated to the study of hadron structure and spectroscopy. Since 2014, a hardware event builder consisting of nine custom designed FPGA-cards replaced the previous online computers increasing compactness and scalability of the DAQ. By buffering data, the system exploits the spill structure of the SPS and averages the maximum on-spill data rate over the whole SPS cycle. From 2016, a crosspoint switch connecting all involved high-speed links shall provide a fully programmable system topology and thus simplifies the compensation for hardware failure and improves load balancing.

The hybrid mesons quest: the MesonEx experiment at Jefferson Laboratory

The meson spectroscopy plays nowadays a central role in the investigation of hadron structure thanks to the possible existence of exotic hybrid mesons, quark-antiquark-gluon bound states. Their explicit gluonic degrees of freedom which should clearly emerge from a Partial Wave Analysis (PWA) of the corresponding Dalitz plot of the exotic particle decay, may result in final JPC configurations not allowed in the constituent quark model. Besides this clear signature, hybrid mesons are also expected to have a large particle multiplicity decays, requiring for their search an experimental apparatus with high performances in terms of rate capability, resolution and almost a full acceptance to apply PWA methods. New-generation experiments are planned at Thomas Jefferson National Laboratory (VA, USA) for which an unprecedented statistics of large multiplicity decay events with fully reconstructed kinematics will be available. In particular for the MesonEx (CLAS12) experiment in Hall B, a wide scientific program that will start in 2016 has been deployed to study the meson spectrum at energies up to 11 GeV. A key role in such program is played by the Forward Tagger apparatus of the experiment, which will allow to extend the study of meson electro-production to very low Q2 values, in a quasi-real photo production kinematical region, where the production of hybrid mesons is expected to be favorite. Currently a new analysis framework for the search of the hybrid mesons is being set up by the HASPECT network, an international structure which gather people involved into theoretical and experimental hadronic physics all over the world. The goals of the network is to develop new analysis models and statistical techniques to unfold the signal and background distributions in high-statistics datasets. In this work are briefly presented the first preliminary results from the application of a statistical technique, namely the sPlot, to the data already acquired by the CLAS experiment for the decay f1 (1285) → (η)π+ π-.

On the interface between perturbative and nonperturbative QCD

The QCD running coupling $\alpha_s(Q^2)$ sets the strength of the interactions of quarks and gluons as a function of the momentum transfer $Q$. The $Q^2$ dependence of the coupling is required to describe hadronic interactions at both large and short distances. In this article we adopt the light-front holographic approach to strongly-coupled QCD, a formalism which incorporates confinement, predicts the spectroscopy of hadrons composed of light quarks, and describes the low-$Q^2$ analytic behavior of the strong coupling $\alpha_s(Q^2)$. The high-$Q^2$ dependence of the coupling $\alpha_s(Q^2)$ is specified by perturbative QCD and its renormalization group equation. The matching of the high and low $Q^2$ regimes of $\alpha_s(Q^2)$ then determines the scale $Q_0$ which sets the interface between perturbative and nonperturbative hadron dynamics. The value of $Q_0$ can be used to set the factorization scale for DGLAP evolution of hadronic structure functions and the ERBL evolution of distribution amplitudes. We discuss the scheme-dependence of the value of $Q_0$ and the infrared fixed-point of the QCD coupling. Our analysis is carried out for the $\bar{MS}$, $g_1$, $MOM$ and $V$ renormalization schemes. Our results show that the discrepancies on the value of $\alpha_s$ at large distance seen in the literature can be explained by different choices of renormalization schemes. We also provide the formulae to compute $\alpha_s(Q^2)$ over the entire range of space-like momentum transfer for the different renormalization schemes discussed in this article.

Highlights from the COMPASS experiment at CERN. Hadron spectroscopy and excitations

The COMPASS experiment at the CERN-SPS studies the spectrum and the structure of hadrons by scattering high energy hadrons and polarised muons off various fixed targets. Recent results for the hadron programme comprise highlights from different topics. A selective overview is given and, among others, the following results are discussed. The precise determination of the pion polarisability, a long standing puzzle that has been solved now, is presented as well as measurements of radiative widths. The observation of a new narrow axial-vector state, the $a_1(1420)$, as well as deeper insights into the exotic $1^{-+}$-wave, which is under study since decades by several experiments, are discussed and further, the search for the charmonium-like exotic $Z_c(3900)$ state in the COMPASS data is covered.

Hadron spectroscopy and interactions from lattice QCD

Lattice QCD approach to study the hadronic resonances and exotic hadrons is described at an introductory level. The main challenge is that these states decay strongly via one or more decay channels, and they often lie near thersholds. Specific results for conventional and exotic hadrons are shown to illustrate the current status.

Hadron spectroscopy from lattice quantum chromodynamics

Lattice calculations of hadron spectroscopy are discussed. A brief introduction to numerical simulations of QCD, with a focus on spectroscopy is given. Results for spec- troscopy of low-lying, excited and exotic states including hybrids and the X, Y, Z states in charmonium are described. New developments that have allowed for determinations of scattering parameters, including in a coupled-channel analysis are briefly described providing a roadmap for future lattice calculations of excited and exotic states.

Hadron Spectroscopy at BESIII

The BESIII experiment, hosted at the IHEP of Beijing, has collected the world largest data sample in the charmonium energy region. One of the most important physics goals of BESIII is the investigation of the QCD prediction. QCD can be accessed in a unique way by means of hadron spectroscopy, which was extensively studied and many important progresses were achieved in the last years. Charmonium decays provide an excellent scenario for studying nucleons, hyperons and their excited states, XYZ resonances, as well as light hadrons. Some of the most recent results for hadron spectroscopy from BESIII will be reported.

D meson spectroscopy and their decay properties using Martin potential in a relativistic Dirac formalism

The mass spectra of $D$ meson states are calculated in the framework of a relativistic independent quark model. For the present study, we have used the martin like potential for the quark confinement. Our predicted states in S-wave, $2\ ^3S_1$ (2605.86 MeV) and $2\ ^1S_0$ (2521.72 MeV) are in very good agreement with experimental result of $2608\pm{2.4}\pm{2.5}$ MeV and $2539.4\pm{4.5}\pm{6.8}$ MeV respectively reported by BABAR Collaboration. The calculated P-wave $D$ meson states, $1^3P_2$ (2468.22 MeV), $1^3P_1$ (2404.94 MeV), $1^3P_0$ (2315.24 MeV) and $1^1P_1$ (2367.94 MeV) are in close agreement with experimental average (Particle Data Group) values of $2462.6 \pm 0.7 $ MeV, $2427 \pm 26 \pm 25$ MeV, $2318 \pm 29 $ MeV and $2421.3 \pm 0.6 $ MeV respectively. The pseudoscalar decay constant ($f_P$= 202.57 MeV) of $D$ meson obtained using this relativistic formalism is in very good agreement with the experiment as well as with the lattice and other available theoretical predictions. The Cabibbo favoured hadronic decay branching ratios, BR$(D^0\rightarrow K^- \pi^+)$ as $3.835 \%$ and BR $(D^0\rightarrow K^+ \pi^-)$ as $1.069 \times 10^{-4} $ are also in very good agreement with the respective experimental values of $ 3.91 \pm 0.08\%$ and $(1.48\pm 0.07) \times 10^{-4}$ reported by CLEO Collaboration. Our predicted results in leptonic decay widths of $D$ meson are also in better accord with experiment as well as other theoretical results. The mixing parameters of $D^0 - \bar{D}^0$ oscillation, $x_q$ (5.14 $\times 10^{-3}$), $y_q$ (6.02 $\times 10^{-3}$) and $R_M$ (3.13 $\times 10^{-5}$) are in very good agreement with BaBar and Belle Collaboration results.

Meson spectroscopy at COMPASS

The goal of the COMPASS experiment at CERN is to study the structure and dynamics of hadrons. The two-stage spectrometer used by the experiment has large acceptance and covers a wide kinematic range for charged as well as neutral particles and can therefore measure a wide range of reactions. The spectroscopy of light mesons is performed with negative (mostly π−) and positive (p, π+) hadron beams with a momentum of 190 GeV/c. The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer t to the target between 0.1 and 1.0 (GeV=c)2. The flagship channel is the π−π−π+ final state, for which COMPASS has recorded the currently world’s largest data sample. These data not only allow to measure the properties of known resonances with high precision, but also to observe new states. Among these is a new axial-vector signal, the a1(1420), with unusual properties. Novel analysis techniques have been developed to extract also the amplitude of the π−π+ subsystem as a function of 3π mass from the data. The findings are confirmed by the analysis of the π−π0π0 final state.

Double polarisation experiments in meson photoproduction

One of the remaining challenges within the standard model is to gain a good understanding of QCD in the non-perturbative regime. A key step towards this aim is baryon spectroscopy, investigating the spectrum and the properties of baryon resonances. To gain access to resonances with small $\pi N$ partial width, photoproduction experiments provide essential information. Partial wave analyses need to be performed to extract the contributing resonances. Here, a complete experiment is required to unambiguously determine the contributing amplitudes. This involves the measurement of carefully chosen single and double polarisation observables. In a joint endeavour by MAMI, ELSA, and Jefferson Laboratory, a new generation of experiments with polarised beams, polarised proton and neutron targets, and $4\pi$ particle detectors have been performed in recent years. Many results of unprecedented quality were recently published by all three experiments, and included by the various partial wave analysis groups in their analyses, leading to substantial improvements, e.g. a more precise determination of resonance parameters. An overview of recent results is given, with an emphasis on results from the CBELSA/TAPS experiment, and their impact on our understanding of the nucleon excitation spectrum is discussed.

Overview of LHCb results on beauty and charm spectroscopy

We present a summary of new experimental results from LHCb experiment on the status of the charm spectroscopy using inclusive approaches and Dalitz plot analyses of B and Bs decays. We also summarize latest results on the spectroscopy of heavy baryons.

Transverse Spin Azimuthal Asymmetries in SIDIS at COMPASS: Multidimensional Analysis

COMPASS is a high-energy physics experiment operating at the SPS at CERN. Wide physics program of the experiment comprises study of hadron structure and spectroscopy with high energy muon and hadrons beams. As for the muon-program, one of the important objectives of the COMPASS experiment is the exploration of the transverse spin structure of the nucleon via spin (in)dependent azimuthal asymmetries in single-hadron production in deep inelastic scattering of polarized leptons off transversely polarized target. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and transversely polarized [Formula: see text] (in 2002, 2003 and 2004) and [Formula: see text] (in 2007 and 2010) targets. The experimental results obtained by COMPASS for unpolarized target azimuthal asymmetries, Sivers and Collins effects and other azimuthal observables play an important role in the general understanding of the three-dimensional nature of the nucleon. Giving access to the entire twsit-2 set of transverse momentum dependent parton distribution functions and fragmentation functions COMPASS data triggers constant theoretical interest and is being widely used in phenomenological analyses and global data fits. In this review main focus is given to the very recent results obtained by the COMPASS collaboration from first ever multi-dimensional extraction of transverse spin asymmetries.

GPD study programme of COMPASS at CERN

COMPASS is a fixed target experiment at CERN dedicated to studies of the spin structure of the nucleon and of the spectroscopy of hadrons. High energy polarized muon beams available at CERN and the option of using either positive or negative ones, give an excellent opportunity for studying GPD, through the DVCS process and the HEMP reaction. The first result of COMPASS GPD programme came from transverse target-spin asymmetries measured in hard exclusive ρ0 production using a transversely polarized proton target. Recent results on transverse target-spin asymmetries for the exclusive ω production are presented as well. Projections of the achievable accuracies and preliminary results of 2012 DVCS pilot run measurements are discussed. Beginning of that run was devoted to commissioning of the long (2.5 m) liquid hydrogen target and new detectors, the large recoil proton detector and large angle electromagnetic calorimeter. These detectors will be a backbone for the future measurements of exclusive processes during the dedicated data taking in 2016–2017.

Exclusive Centralπ+π−Production in Proton Antiproton Collisions at the CDF

Exclusive π + π − production in proton-antiproton collisions at √ s = 0.9 and 1.96 TeV in the Collider Detector at Fermilab has been measured. We select events with two particles with opposite charge in pseudorapidity region −1.3 <η< 1.3 with no other particles detected in −5.9 <η< 5.9. Particles are assumed to be pions. The π + π − system is required to have rapidity −1.0 <y< 1.0. The data are expected to be dominated by the double pomeron exchange mechanism. Therefore, the quantum numbers of the cen- tral state are constrained. The data extend up to dipion mass M(π + π − ) = 5000 MeV/c 2 . Resonance structures consistent with f0 and f2(1270) mesons are visible. The results are valuable for light hadron spectroscopy and for providing information about the nature of the pomeron in a region between non-perturbative and perturbative quantum chromody- namics.

Soft-wall Modelling of Meson Spectra

The holographic methods inspired by the gauge/gravity correspondence from string theory have been actively applied to the hadron spectroscopy in the last eleven years. Within the phenomenological bottom-up approach, the linear Regge-like trajectories for light mesons are naturally reproduced in the so-called ”Soft-wall” holographic models. I will give a very short review of the underlying ideas and technical aspects related to the meson spectroscopy. A generalization of soft-wall description of Regge trajectories to arbitrary intercept is proposed. The problem of incorporation of the chiral symmetry breaking is discussed.

Richard Henry Dalitz. 28 February 1925 — 13 January 2006

Richard (Dick) Henry Dalitz was a theoretical physicist whose principal contributions were intimately connected to some of the major breakthroughs of the twentieth century in particle and nuclear physics. His formulation of the ‘τ–θ’ puzzle led to the discovery that parity is not a symmetry of nature—the first of the assumed space-time symmetries to fail. He pioneered the theoretical study of hypernuclei, of strange baryon resonances, and of baryon spectroscopy in the quark model (at a time when many considered it ‘naive’), to all of which he made lasting contributions. The ‘Dalitz plot’ and ‘Dalitz pairs’ are part of the vocabulary of particle physics. Throughout his career he remained in close touch with many experimentalists, and he had an encyclopaedic knowledge of the data. Many of his papers were stimulated by experimental results and were concerned with their analysis and interpretation, work that often required the forging of new phenomenological tools; many also indicated what new experiments needed to be done. As a consequence, he was a theorist exceptionally valued by experimentalists. He created and ran a strong particle theory group at Oxford, which attracted many talented students and researchers, and which has continued to thrive.