Exotic Pentaquark Research Articles

Search for Θ+ in KLp→K+n reaction in KLF at JLab

The possibility of the existence of multiquark hadrons made of 4-quarks for mesons and 5-quarks for baryons was predicted by Gell-Mann [Phys. Lett. 8, 214 (1964)]. The renewed interest for the search for exotic pentaquark states was initiated by the paper by Diakonov, Petrov, and Polyakov [Z. Phys. A 359, 305 (1997)]. The 2003 experimental reports on the observation of [Formula: see text] pentaquark with [Formula: see text] quark content created a great excitement and many following experiments have reported its observation [K. H. Hicks, Eur. Phys. J. H 37, 1 (2012)]. After high-statistics experiments at JLab, which did not confirm previous claims by the CLAS collaboration, the community concluded that the [Formula: see text] pentaquark either does not exist at all or has an extremely small cross-section, making it currently unobserved. There were different review papers on this subject, questioning the existence of [Formula: see text] or trying to explain the reasons why reaching a conclusion based on production experiments is challenging [M. Amaryan, Eur. Phys. J. Plus 137, 684 (2022)]. To address the challenge of minimal 3-body final states, a formation experiment with a projectile-kaon beam is proposed. In the following, we discuss how the [Formula: see text] could be observed in the [Formula: see text] reaction in the KLF experiment at JLab [M. Amaryan et al. [KLF Collaboration], arXiv:2008.08215 [nucl-ex]].

A Theoretical Study on the Form Factors of Pentaquark States

The form factors of the exotic pentaquarks such as etc. have been investigated for the momentum transfers, Q 2 in the range of 0.01 < Q 2 < 1.0 GeV2. The wave function from the statistical model has been used to compute the form factors. The form factors are found to decrease sharply with the increase of Q 2 in the range of 0.01 < Q 2 < 0.1 GeV2 which indicates the scaling violation with the gluon contribution to the form factor and the structure function whereas the form factors show gradual decrease with a small scaling violation in the range of 0.1 < Q 2 < 1.0 GeV2. The results have been compared with the other available theoretical and experimental data.

Prediction of five-flavored pentaquarks

We analyze the possibility of the existence of a fully exotic pentaquark state udscb¯, which is made from the five different flavors participating in the strong interactions. We investigate the coupled channel effects of the B(⁎)Ξc(′) system through t-channel vector meson exchange to search for such states. A BΞc and a BΞc′ bound state are found to have binding energies of about 10−30 MeV with BsΛc and BcΛ being the possible decay channels. Similarly, a bound state is formed in the B⁎Ξc and B⁎Ξc′ channels, respectively. The properties of these states including pole positions, decay widths and couplings to other channels are extracted. These states could be searched for through the pp→BcΛX or pp→Bs(⁎)ΛcX processes by the LHCb collaboration.

Manifestly exotic pentaquarks with a single heavy quark

Inspired by the observed $X(2900)$, we study systematically the mass spectra of the ground pentaquark states with the $qqqq\bar{Q}$ ($Q=c,b$; $q=n,s$; $n=u,d$) configuration in the framework of the Chromomagnetic Interaction model. We present a detailed analysis of their stabilities and decay behaviors. Our results indicate that there may exist narrow states or even stable states. We hope that the present study may inspire experimentalist's interest in searching for such a type of the exotic pentaquark state.

Fully heavy pentaquarks

Very recently, the LHCb Collaboration reported a fully charmed tetraquark state $X(6900)$ in the invariant mass spectrum of $J/\psi$ pairs. If one $J/\psi$ meson is replaced with a fully charmed baryon, we obtain a fully charmed pentaquark candidate. In this work, we perform a systematic study on the mass spectra of the S-wave fully heavy pentaquark $QQQQ\bar{Q}$ in the framework of the chromomagnetic interaction model. Based on our results in two different schemes, we further investigate the decay behaviors for them. We hope that our study will be helpful in searching for such types of exotic pentaquark states in experiments in the future.

Exotic pentaquark states with the qqQQQ¯ configuration

In the framework of the color-magnetic interaction model, we have systematically calculated the mass splittings for the S-wave triply heavy pentaquark states with the configuration $qqQQ\bar{Q}$ $(Q=c,b;q=u,d,s)$. Their masses are estimated and their stabilities are discussed according to possible rearrangement decay patterns. Our results indicate that there may exist several stable or narrow such states. We hope the present study can help experimentalists to search for exotic pentaquarks.

Surveying exotic pentaquarks with the typical QQqqq¯ configuration

As a hot issue, exploring exotic pentaquarks is full of challenges and opportunities for both theorist and experimentalist. In this work, we focus on a type of pentaquark with the $QQqq\bar{q}$ ($Q=b,c$; $q=u,d,s$) configuration, where their mass spectrum is estimated systematically. Especially, our result indicates that there may exist some stable or narrow exotic pentaquark states. Obviously, our study may provides valuable information for further experimental search for the $QQqq\bar{q}$ pentaquarks. With the running of LHCb and forthcoming Belle II, we have a reason to believe that these predictions present here can be tested.

Possibility of the existence of charmed exotica

We employ the chiral quark-soliton model to describe excited baryons with one heavy quark. Identifying known charmed baryons with multiplets allowed by the model, we argue that apart from regular excitations of the ground state multiplets, some of recently reported by the LHCb collaboration narrow $\Omega^{0}_{c}$ states, may correspond to the exotic pentaquarks. This interpretation can be easily verified experimentally, since exotic $\Omega^{0}_{c}$ states -- contrary to the regular excitations -- form isospin triplets, rather than singlets.

Dependence of the Decay width of the Exotic Pentaquark theta;+ on Its Mass and the Mass of N* within the Framework of a Chiral Soliton Model

Dependence of the Decay width of the Exotic Pentaquark theta;+ on Its Mass and the Mass of N* within the Framework of a Chiral Soliton Model

Kinematic Selection Criteria in the Search for Resonances

In this paper, we discuss some general features of two-body decay kinematics and derive a new set of equations describing boundaries of the physical region in decays. These features via a graphical representation and implementation of selection criteria provide an efficient tool for the background suppression in experimental searches for conventional and exotic (pentaquark, etc.) resonance states.

PRODUCTION AND SEARCHES FOR CASCADE BARYONS WITH CLAS

We present the results of photoproduction cross sections of the ground state cascade Ξ- and the first excited state Ξ*-(1530) measured with the CLAS detector. The photoproduction of the cascade resonances has been investigated in the reactions γp → K+K+(X) and γp → K+K+π-(X). The differential and total cross sections of the Ξ- were determined for photon beam energies from 2.75 to 4.75 GeV and are consistent with a production mechanism of Y* → K+Ξ- through a t-channel process. The cross-section of the Ξ*-(1530) has been determined for photon beam energies from 3.35 to 4.75 GeV. The reaction γp → K+K+π-(Ξ0) has also been investigated in the search for excited cascade resonances decaying to π-Ξ0. No significant signal of excited cascade states other than the well-known Ξ*-(1530) is observed. We also present the latest results of a search for the Φ--(1862) exotic pentaquark state in a photoproduction experiment on a deuterium target. A high-statistics sample of π-Ξ- events have been collected and analyzed. A preliminary invariant mass spectrum of the π-Ξ- system is presented, which is used to set upper limits on the photoproduction of the Φ-- pentaquark state.

Sea quark flavor asymmetry of hadrons in statistical balance model

We suggested a Monte Carlo approach to simulate a kinetic equilibrium ensemble, and proved the equivalence to the linear equations method on equilibrium. With the convenience of the numerical method, we introduced variable splitting rates representing the details of the dynamics as model parameters which were not considered in previous works. The dependence on model parameters was studied, and it was found that the sea quark flavor asymmetry weakly depends on model parameters. It reflects the statistics principle, contributes the dominant part of the asymmetry, and the effect caused by details of the dynamics is small. We also applied the Monte Carlo approach of the statistical model to predict the theoretical sea quark asymmetries in kaons, octet baryons $\ensuremath{\Sigma}$, $\ensuremath{\Xi}$, and $\ensuremath{\Delta}$ baryons, even in exotic pentaquark states.

Exotic pentaquarks as Gamov–Teller resonances

If the number of colors Nc is taken large, baryons and their excitations can be considered in a mean-field approach. We argue that the mean field in baryons breaks spontaneously the spherical and SU(3) flavor symmetries, but retains the SU(2) symmetry of simultaneous rotations in space and isospace. The one-quark and quark-hole excitations in the mean field, together with the SU(3) rotational bands about them determine the spectrum of baryon resonances, which turns out to be in good accordance with reality when one puts Nc = 3. A by-product of this scheme is a confirmation of the light pentaquark Θ+ baryon uudds̄ as a typical Gamov–Teller resonance long known in nuclear physics. An extension of the same large-Nc logic to charmed baryons leads to a prediction of a anti-decapenta (1̄5̄)-plet of charmed pentaquarks, two of which, Bc++ = cuuds̄ and Bc+ = cudds̄, may be light and stable with respect to strong decays.

Theoretical Status of Pentaquarks

We review the current status of the theoretical pentaquark search from the direct QCD calculation. The works from the QCD sum rule and the lattice QCD in the literature are carefully examined. The importance of the framework which can distinguish the exotic pentaquark state (if any) from the NK scattering state is emphasized.

Statistical production of pentaquarks in high-energy nuclear collisions

The production yield of exotic pentaquarks in high-energy nuclear collisions is estimated on the basis of the standard statistical description of the hadronic freeze-out.

PentaquarkΘ+production viaγN→K¯*Θ+(3/2±)

We study the photoproduction of the exotic pentaquark ${\ensuremath{\Theta}}^{+}$ baryon with the vector kaon, assuming that the quantum numbers of the ${\ensuremath{\Theta}}^{+}$ to be ${J}^{P}=3/{2}^{\ifmmode\pm\else\textpm\fi{}}$ and ${J}^{P}=1/{2}^{+}$. Scalar meson $\ensuremath{\kappa}(800)$ exchange is also taken into account. In contrast with the $\ensuremath{\gamma}N\ensuremath{\rightarrow}\overline{K}{\ensuremath{\Theta}}^{+}(3/{2}^{\ifmmode\pm\else\textpm\fi{}})$ process, the large suppression from the proton target is not observed in the total cross sections. We also suggest a method to determine which meson exchange is the most dominant by analyzing the polarizations of incident photon and outgoing ${K}^{*}$. We find that \ensuremath{\kappa} exchange turns out to be prominent when the polarizations of the photon and ${K}^{*}$ are aligned to be parallel, whereas $K$ exchange predominates when they are perpendicular to each other.

Search for theΘ+Pentaquark in theγd→ΛnK+Reaction Measured with the CLAS Spectrometer

For the first time, the reaction gamma d -> Lambda n K+ has been analyzed in order to search for the exotic pentaquark baryon Theta+(1540). The data were taken at Jefferson Lab, using the Hall-B tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the nK+ invariant mass distribution. The upper limit on the gamma d -> Lambda Theta+ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported.

Prediction of the masses and decay processes of strange, charmed and bottomed pentaquarks from the linear-molecular crypto-heptaquark model

In this paper, the masses and decay processes of several new strange, charmed and bottomed exotic pentaquarks are predicted. Multiquarks are studied microscopically in a standard quark model. In pure ground-state pentaquarks, the short-range interaction is computed and it is shown to be repulsive. The long-range and medium-range interactions are not expected to provide sufficient attraction. An additional quark–antiquark pair is then considered, and this is suggested to produce a narrow linear-molecular system. The quarks assemble in three hadronic clusters, and the central hadron provides stability. The possible crypto-heptaquark hadrons with exotic pentaquark flavours, with any number of strange, charmed and bottomed quarks, are listed. Several new exotics may still be observed.

Structure and reactions of pentaquark baryons

We review the current status of the exotic pentaquark baryons. After a brief look at experiments of both positive and negative results, we discuss theoretical methods to study the structure and reactions for the pentaquarks. First we introduce the quark model and the chiral soliton model, where we discuss the relation of mass spectrum and parity with some emphasis on the role of chiral symmetry. It is always useful to picture the structure of the pentaquarks in terms of quarks. As for other methods, we discuss a model independent method, and briefly mention the results from the lattice and QCD sum rule. Decay properties are then studied in some detail, which is one of the important properties of Theta+. We investigate the relation between the decay width and the quark structure having certain spin-parity quantum numbers. Through these analyses, we consider as plausible quantum numbers of Theta+, JP = 3/2-. In the last part of this note, we discuss production reactions of Theta+ which provide links between the theoretical models and experimental information. We discuss photoproductions and hadron-induced reactions which are useful to explore the nature of Theta+

Observation of a narrow baryon resonance decaying to pK S 0 in proton-nucleus interactions at 70 GeV/c with the SVD-2 setup

SVD-2 experiment data have been analyzed to search for an exotic baryon state, the $\Theta^+$-baryon, in a $pK^0_s$ decay mode at $70 GeV/c$ on IHEP accelerator. The reaction $pA \to pK^0_s+X$ with a limited multiplicity was used in the analysis. The $pK^0_s$ invariant mass spectrum shows a resonant structure with $M=1526\pm3(stat.)\pm 3(syst.) MeV/c^2$ and $\Gamma < 24 MeV/c^2$. The statistical significance of this peak was estimated to be of $5.6 \sigma$. The mass and width of the resonance is compatible with the recently reported $\Theta^+$- baryon with positive strangeness which was predicted as an exotic pentaquark ($uudd\bar{s}$) baryon state. The total cross section for $\Theta^+$ production in pN-interactions for $X_F\ge 0$ was estimated to be $(30\div120) \mu b$ and no essential deviation from A-dependence for inelastic events $(\sim A^{0.7})$ was found.