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Register a new userProgress Toward Understanding Baryon Resonances
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The composite nature of baryons manifests itself in the existence of a rich spectrum of excited states, in particular in the important mass region 1-2 GeV for the light-flavoured baryons. The properties of these resonances can be identified by systematic investigations using electromagnetic and strong probes, primarily with beams of electrons, photons, and pions. After decades of research, the fundamental degrees of freedom underlying the baryon excitation spectrum are still poorly understood. The search for hitherto undiscovered but predicted resonances continues at many laboratories around the world. Recent results from photo- and electroproduction experiments provide intriguing indications for new states and shed light on the structure of some of the known nucleon excitations. The continuing study of available data sets with consideration of new observables and improved analysis tools have also called into question some of the earlier findings in baryon spectroscopy. Other breakthrough measurements have been performed in the heavy-baryon sector, which has seen a fruitful period in recent years, in particular at the B factories and the Tevatron. First results from the LHC indicate rapid progress in the field of bottom baryons. In this review, we discuss the recent experimental progress and give an overview of theoretical approaches.
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We report the latest results on the search for the QCD critical point in the QCD phase diagram through high energy heavy-ion collisions. The measurements discussed are based on the higher moments of the net-proton multiplicity distributions in heavy-ion collisions. A non-monotonic variation in the product of kurtosis times the variance of the net-proton distribution is observed as a function of the collision energy with 3$sigma$ significance. We also discuss the results of the thermal model in explaining the measured particle yield ratios in heavy-ion collisions and comparison of the different variants of hardon resonance gas model calculation to the data on higher moments of net-proton distributions. We end with a note that the upcoming programs in high baryon density regime at various experimental facilities will complete the search for the QCD critical point through heavy-ion collisions.
A model for the $bar K d to pi Y N$ reactions with $Y=Lambda, Sigma$ is developed, aiming at establishing the low-lying $Lambda$ and $Sigma$ hyperon resonances through analyzing the forthcoming data from the J-PARC E31 experiment. The off-shell amplitudes generated from the dynamical coupled-channels (DCC) model, which was developed in Kamano et al. [Phys. Rev. C 90, 065204 (2014)], are used as input to the calculations of the elementary $bar K N to bar K N$ and $bar K N to pi Y$ subprocesses in the $bar K d to pi Y N$ reactions. It is shown that the cross sections for the J-PARC E31 experiment with a rather high incoming-$bar{K}$ momentum, $|vec p_{bar K}|= 1$ GeV, can be predicted reliably only when the input $bar K N to bar K N$ amplitudes are generated from a $bar KN$ model, such as the DCC model used in this investigation, which describes the data of the $bar K N$ reactions at energies far beyond the $bar K N$ threshold. We find that the data of the threefold differential cross section $dsigma/(dM_{piSigma}dOmega_{p_n})$ for the $K^- d to pi Sigma n$ reaction below the $bar K N$ threshold can be used to test the predictions of the resonance poles associated with $Lambda(1405)$. We also find that the momentum dependence of the threefold differential cross sections for the $K^- d to pi^- Lambda p$ reaction can be used to examine the existence of a low-lying $J^P=1/2^+$ $Sigma$ resonance with a pole mass $M_R = 1457 -i39$ MeV, which was found from analyzing the $K^-p$ reaction data within the employed DCC model.
This White Paper presents the science case of an Electron-Ion Collider (EIC), focused on the structure and interactions of gluon-dominated matter, with the intent to articulate it to the broader nuclear science community. It was commissioned by the managements of Brookhaven National Laboratory (BNL) and Thomas Jefferson National Accelerator Facility (JLab) with the objective of presenting a summary of scientific opportunities and goals of the EIC as a follow-up to the 2007 NSAC Long Range plan. This document is a culmination of a community-wide effort in nuclear science following a series of workshops on EIC physics and, in particular, the focused ten-week program on Gluons and quark sea at high energies at the Institute for Nuclear Theory in Fall 2010. It contains a brief description of a few golden physics measurements along with accelerator and detector concepts required to achieve them, and it benefited from inputs from the users communities of BNL and JLab. This White Paper offers the promise to propel the QCD science program in the U.S., established with the CEBAF accelerator at JLab and the RHIC collider at BNL, to the next QCD frontier.
Observation of a narrow structure at $Wsim 1.68$ GeV in the excitation functions of some photon- and pion-induced reactions may signal a new narrow isospin-1/2 $N(1685)$ resonance. New data on the $gamma N to pi eta N$ reactions from GRAAL seems to reveal the signals of both $N^+(1685)$ and $N^0(1685)$ resonances.
The reaction p(e,e^{prime}pi^+)X^0 was studied with two high resolution magnetic spectrometers to search for narrow baryon resonances. A missing mass resolution of 2.0 MeV was achieved. A search for structures in the mass region of 0.97 < M_{X^0} < 1.06 GeV yielded no significant signal. The yield ratio of p(e,e^{prime}pi^+)X^0/p(e,e^{prime}pi^+)n was determined to be (-0.35 +/- 0.35) times 10^{-3} at 1.004 GeV and (0.34 +/- 0.42) times 10^{-3} at 1.044 GeV.
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