LEP Collaborations Research Articles

Exploring the mass radius of He4 and implications for nuclear structure

In this study, we determine the mass radius of He4, a very light nucleus, by examining the near-threshold ϕ-meson photoproduction data of the LEPS Collaboration. To assess the gravitational form factor of He4, we employ multiple models for the mass distribution, including Yukawa-type, exponential, Gaussian, and uniform functions. The mass radius of He4 is measured to be 1.70±0.14 fm, which is approximately equal to the charge radius of He4. Surprisingly, in contrast to the findings of the proton, no noticeable discrepancy between the charge radius and the mass radius is noted for the He4 nucleus. The proton and neutron distributions within He4 are likely to be identical, confirming its regular tetrahedral structure in a new way. We propose exploring the difference between charge and mass radii as a new approach to examine the nuclear structure. Published by the American Physical Society 2024

Extraction of ωn , ωp , and ϕN scattering lengths from ω and ϕ differential photoproduction cross sections on a deuterium target

In this study, we try to extract $\ensuremath{\omega}n$, $\ensuremath{\omega}p$, and $\ensuremath{\phi}N$ scattering lengths from the differential cross-section data of near-threshold $\ensuremath{\omega}$ and $\ensuremath{\phi}$ photoproductions at threshold ${t}_{\mathrm{thr}}$. The incoherent data of $\ensuremath{\omega}$ and $\ensuremath{\phi}$ photoproductions on deuterium target for scattering length extractions are provided by the CBELSA/TAPS Collaboration and LEPS Collaboration, respectively, where the deuteron is usually approximated as a weakly bound state of a quasifree neutron plus a quasifree proton. Under the vector meson dominance model and the assumption of energy independence of the differential cross section, we obtained the $|{\ensuremath{\alpha}}_{\ensuremath{\omega}{n}^{*}}|$ and $|{\ensuremath{\alpha}}_{\ensuremath{\omega}{p}^{*}}|$ of loosely bound neutron and proton in the deuteron to be 0.709 $\ifmmode\pm\else\textpm\fi{}$ 0.078 fm and 0.621 $\ifmmode\pm\else\textpm\fi{}$ 0.022 fm, respectively, at threshold ${t}_{\mathrm{thr}}$ for the first time by fitting the near-threshold data of $\ensuremath{\gamma}d$ $\ensuremath{\rightarrow}$ $\ensuremath{\omega}n(p)$ and $\ensuremath{\gamma}d$ $\ensuremath{\rightarrow}$ $\ensuremath{\omega}p(n)$. For a comparison study, we also extracted the scattering length $|{\ensuremath{\alpha}}_{\ensuremath{\omega}p}|$ of the free proton from the $\ensuremath{\omega}$ photoproduction on the hydrogen target by the CBELSA/TAPS Collaboration. With the near-threshold and incoherent $\ensuremath{\phi}$ photoproduction data of $\ensuremath{\gamma}d$ $\ensuremath{\rightarrow}$ $\ensuremath{\phi}pn$ by the LEPS Collaboration, we obtained scattering length $|{\ensuremath{\alpha}}_{\ensuremath{\phi}N}|$ of the bound nucleon inside the deuteron to be 0.109 $\ifmmode\pm\else\textpm\fi{}$ 0.008 fm at threshold ${t}_{\mathrm{thr}}$. The obtained $|{\ensuremath{\alpha}}_{\ensuremath{\omega}p}|$ and $|{\ensuremath{\alpha}}_{\ensuremath{\phi}N}|$ are basically in agreement with the experimental indications and the theoretical predictions within the current uncertainties.

The neutron and proton mass radii from the vector meson photoproduction data on the deuterium target

In this study, we try to extract the mass radii of the neutron and the proton from the differential cross section data of near-threshold $$\omega $$ and $$\phi $$ photoproductions on deuterium target, which is often approximated as a quasi-free neutron plus a quasi-free proton. The incoherent data of $$\omega $$ and $$\phi $$ photoproductions are provided by CBELSA/TAPS collaboration and LEPS collaboration respectively, where the deuteron is disintegrated in the experiments to measure the properties of individual nucleons. Under the VMD model and the assumption of dipole gravitational form factor, we determined the loosely bound neutron and proton mass radii to be $$0.795\pm 0.092\mathrm (stat.)\pm 0.073\mathrm (syst.)$$ fm and $$0.744\pm 0.029\mathrm (stat.)\pm 0.042\mathrm (syst.)$$ fm respectively from the near-threshold data of $$\gamma d \rightarrow \omega n (p)$$ and $$\gamma d \rightarrow \omega p (n)$$ , for the first time. With the near-threshold and incoherent $$\phi $$ photoproduction data of $$\gamma d \rightarrow \phi p n$$ , we determined the average mass radius of the bound nucleon (neutron or proton) inside the deuteron to be $$0.755\pm 0.039\mathrm (stat.)\pm 0.039\mathrm (syst.)$$ fm, for the first time. For a comparison study, we also extracted the mass radius of the free proton from the $$\omega $$ photoproduction on the hydrogen target by CBELSA/TAPS collaboration. Based on our analysis results under the assumptions of VMD model and a low energy QCD theorem, we find that the neutron mass radius is consistent with the proton mass radius within the current statistical uncertainties, and that the nuclear modification on the nucleon mass radius is small inside the deuteron.

Nucleon and Δ resonances in γn→K+Σ− photoproduction

The most recent data on beam asymmetries $\mathrm{\ensuremath{\Sigma}}$ and beam-target asymmetries $E$ from the CLAS Collaboration, together with the previous data on differential cross sections and beam asymmetries from the CLAS and LEPS Collaborations, for the $\ensuremath{\gamma}n\ensuremath{\rightarrow}{K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ reaction are studied based on an effective Lagrangian approach in the tree-level Born approximation. The $t$-channel $K$ and ${K}^{*}(892)$ exchanges, the $u$-channel $\mathrm{\ensuremath{\Sigma}}$ exchange, the interaction current, and the exchanges of $N$, $\mathrm{\ensuremath{\Delta}}$, and their excited states in the $s$ channel are considered in constructing the reaction amplitudes to describe the available experimental data. The reaction mechanisms of $\ensuremath{\gamma}n\ensuremath{\rightarrow}{K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ are analyzed, and the associated resonances' parameters are extracted. The numerical results show that the $\mathrm{\ensuremath{\Delta}}$ exchange and the $N(1710)1/{2}^{+}$, $N(1880)1/{2}^{+}$, $N(1900)3/{2}^{+}$, and $\mathrm{\ensuremath{\Delta}}(1920)3/{2}^{+}$ resonance exchanges in the $s$ channel dominate the $\ensuremath{\gamma}n\ensuremath{\rightarrow}{K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ reaction in the lower energy region, and the $t$-channel ${K}^{*}(892)$ exchange plays a crucial role at forward angles in the higher energy region.

K+Σ− photoproduction within an isobar model

We used an isobar model to study the K+Σ− photoproduction reaction on a neutron target with focus on the resonance region. In order to achieve a reasonable agreement with the data, we included spin-3/2 and spin-5/2 nucleon resonances in a consistent formalism together with a Δ resonance and two kaon resonances on top of the Born terms. The free parameters of the model were adjusted to the data from the CLAS and LEPS Collaborations on differential cross sections and photon beam asymmetry. The cornerstone of this analysis was an upgrade of the fitting method. Previously, we used only the plain χ2 minimization, which could not prevent us from overfitting the data. We, therefore, introduced a regularization method, the least absolute selection shrinkage operator (LASSO), which, together with information criteria, restricts the number of nonzero parameters and prevents us from overfitting the data. In our analysis, we arrived at two models, fit M, whose parameters were fitted with the MINUIT code only, and fit L, where we used the more advanced LASSO. Both models describe the data in a similar way and we observe only slight differences in the dσ/dΩ data description at very forward angles where the fit M is flat whereas the fit L produces two broad peaks, and in the photon beam asymmetries above 2 GeV at backward kaon angles where the fit M produces a bump. Surprisingly, no hyperon resonances are needed for the correct data description in these models. On the other hand, the N(1720)3/2+ nucleon resonance was found to be very important in both models.

Model selection for K+Σ− photoproduction within an isobar model

We utilize an isobar model to investigate the ${K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ photoproduction off a neutron in the resonance region. Except for the Born terms, we include high-spin (spin-3/2 and spin-5/2) nucleon resonances in the consistent formalism together with a few $\mathrm{\ensuremath{\Delta}}$ and kaon resonances to achieve an acceptable agreement with data. Interestingly, we reveal that no hyperon resonances are needed to achieve a reasonable description of data. On the other hand the $N(1720)3/{2}^{+}$ resonance was found to be very important for correct description of data. The free parameters of the model were fitted to experimental data from the LEPS and CLAS Collaborations on either differential cross sections or photon beam asymmetry. The novel feature of the fitting procedure is the use of a regularization method, the least absolute shrinkage selection operator, and information criteria in order to choose the best fit.

Proton and deuteron mass radii from near-threshold ϕ -meson photoproduction

We analyze the exclusive $\phi$-meson photoproduction on both hydrogen and deuterium targets based on the published data of CLAS, SAPHIR and LEPS collaborations. A dipole-form scalar gravitational form factor is applied to describe the $|t|$-dependence of the differential cross section. Based on the average of all the near-threshold data from CLAS, SAPHIR and LEPS, we find that the proton and deuteron mass radii are $0.75 \pm 0.02$ fm and $1.95 \pm 0.19$ fm respectively. The coherent and near-threshold quarkonium photoproduction seems to be sensitive to the radius of the hadronic system. The vector-meson-dominance model together with a low-energy QCD theorem well describe the data of near-threshold $\phi$ photoproduction on the hadronic systems.

Dynamical model of ϕ meson photoproduction on the nucleon and He4

We investigate $\phi$ meson photoproduction on the nucleon and the \nuclide[4]{He} targets within a dynamical model approach based on a Hamiltonian which describes the production mechanisms by the Pomeron-exchange, meson-exchanges, $\phi$ radiations, and nucleon resonance excitations mechanisms. The final $\phi N$ interactions are included being described by the gluon-exchange, direct $\phi N$ couplings, and the box-diagrams arising from the couplings with $\pi N$, $\rho N$, $K\Lambda$, and $K\Sigma$ channels. The parameters of the Hamiltonian are determined by the experimental data of $\gamma p \to \phi p$ from the CLAS Collaboration. The resulting Hamiltonian is then used to predict the coherent $\phi$-meson production on the \nuclide[4]{He} targets by using the distorted-wave impulse approximation. For the proton target, the final $\phi N$ rescattering effects, as required by the unitarity condition, are found to be very weak, which supports the earlier calculations in the literature. For the \nuclide[4]{He} targets, the predicted differential cross sections are in good agreement with the data obtained by the LEPS Collaboration. The role of each mechanism in this reaction is discussed and predictions for a wide range of scattering angles are presented, which can be tested in future experiments.

Analysis of recent CLAS data onΣ*(1385) photoproduction off a neutron target

Based on recent experimental data obtained by the CLAS Collaboration, the $\Sigma(1385)$ photoproduction off a neutron target at laboratory photon energies $E_\gamma$ up to 2.5\,GeV is investigated in an effective Lagrangian approach including $s$-, $u$-, and $t$-channel Born-term contributions. The present calculation does not take into account any explicit $s$-channel baryon-resonance contributions, however, in the spirit of duality, we include $t$-channel exchanges of mesonic Regge trajectories. The onset of the Regge regime is controlled by smoothly interpolating between Feynman-type single-meson exchanges and full-fledged Regge-trajectory exchanges. Gauge invariance broken by the Regge treatment is fully restored by introducing contact-type interaction currents that result from the implementation of \textit{local} gauge invariance in terms of generalized Ward-Takahashi identities. The cross sections for the $\gamma n\to K^+\Sigma^*(1385)^-$ reaction are calculated and compared with experimental results from the CLAS and LEPS collaborations. Despite its simplicity, the present theoretical approach provides a good description of the main features of the data. However, the parameters fitted to the data show that the gauge-invariance-restoring contact term plays a large role which may point to large contributions from final-state interactions.

Photoproduction ofKΛon the proton

Kaon photoproduction on the proton is studied in the resonance region using an isobar model. The higher-spin nucleon (3/2 and 5/2) and hyperon (3/2) resonances were included in the model utilizing the consistent formalism by Pascalutsa and they were found to play an important role in data description. The spin-1/2 and spin-3/2 hyperon resonances in combination with the Born terms contribute significantly to the background part of the amplitude. Various forms of the hadron form factor were considered in the construction and the dipole and multidipole forms were selected as those most suitable for the data description. Model parameters were fitted to new experimental data from CLAS, LEPS, and GRAAL collaborations and two versions of the model, BS1 and BS2, were chosen. Both models provide a good overall description of the data for the c.m. energies from the threshold up to 2.4 GeV. Predicted cross sections of the models at very small kaon angles being consistent with results of the Saclay-Lyon model indicate that the models could be also successful in predicting the hypernucleus production cross sections. Although kaon photoproduction takes place in the third-resonance region with many resonant states, the total number of included resonances, 15 and 16, is quite moderate and it is comparable with numbers of resonances in other models. The set of chosen nucleon resonances overlaps well with the set of the most probable contributing states determined in the Bayesian analysis with the Regge-plus-resonance model. Particularly, we confirm that the missing resonances $P_{13}(1900)$ and $D_{13}(1875)$ do play important role in the description of data. However, the spin-1/2 state $P_{11}(1880)$ included in the Bayesian analysis was replaced in our analysis with the near-mass spin-5/2 state $N^*(1860)$, recently considered by the Particle Data Group.

Kaon photoproduction off proton

We have recently constructed our version of the Regge-plus-resonance (RPR) model and two variants of an isobar model for photoproduction of kaons on the proton, utilizing new experimental data from CLAS, LEPS, and GRAAL collaborations for adjusting free parameters of the models. Higher-spin nucleon (3/2 and 5/2) and hyperon (3/2) resonances were included using the consistent formalism by Pascalutsa and found to play an important role in data description. The set of chosen nucleon resonances in our new isobar models agrees well with the set of the most probable contributing states determined in the Bayesian analysis with the RPR model whilst only 6 out of 10 N *’s selected in the RPR fit of ours overlap with the nucleon resonant states in the Bayesian analysis. Results of two versions of the isobar model are compared to the new version of the RPR model and experimental data in the third-resonance region and their properties are discussed. We place an emphasis on the choice of resonances, the predictions in the forward- and backward-angle region as well as the choice of the hadron form factor.

Fragmentation fractions of two-body b-baryon decays

We study the fragmentation fractions (fBb) of the b-quark to b-baryons (Bb). By the assumption of fΛb/(fu+fd)=0.25±0.15 in accordance with the measurements by LEP, CDF and LHCb Collaborations, we estimate that fΛb=0.175±0.106 and fΞb−,0=0.019±0.013. From these fragmentation fractions, we derive B(Λb→J/ψΛ)=(3.3±2.1)×10−4, B(Ξb−→J/ψΞ−)=(5.3±3.9)×10−4 and B(Ωb−→J/ψΩ−)>1.9×10−5. The predictions of B(Λb→J/ψΛ) and B(Ξb−→J/ψΞ−) clearly enable us to test the theoretical models, such as the QCD factorization approach in the b-baryon decays.

Internal structures of the nucleon resonancesN(1875)andN(2120)

A nucleon resonance with spin-parity $J^P=3/2^-$ and mass about 2.1 GeV is essential to reproduce the photoproduction cross sections for $\Lambda(1520)$ released by the LEPS and CLAS Collaborations. It can be explained as the third nucleon resonance state $[3/2^-]_3$ in the constituent quark model so that there is no position to settle the $N(1875)$ which is listed in the PDG as the third $N3/2^-$ nucleon resonance. An interpretation is proposed that the $N(1875)$ is from the interaction of a decuplet baryon $\Sigma(1385)$ and a octet meson $K$, which is favored by a calculation of binding energy and decay pattern in a Bethe-Salpeter approach.

Search for charged Higgs bosons: combined results using LEP data

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for pair-produced charged Higgs bosons in the framework of Two Higgs Doublet Models (2HDMs). The data of the four experiments have been statistically combined. The results are interpreted within the 2HDM for Type I and Type II benchmark scenarios. No statistically significant excess has been observed when compared to the Standard Model background prediction, and the combined LEP data exclude large regions of the model parameter space. Charged Higgs bosons with mass below 80 \({\rm GeV}/c^{2}\) (Type II scenario) or 72.5 \({\rm GeV}/c^{2}\) (Type I scenario, for pseudo-scalar masses above 12 \({\rm GeV}/c^{2}\)) are excluded at the 95 % confidence level.

Faddeev-chiral unitary approach to theK−dscattering length

Our earlier Faddeev three-body study in the $K^-$-deuteron scattering length, $A_{K^-d}$, is revisited here in the light of the recent developments in two fronts: {\it (i)} the improved chiral unitary approach to the theoretical description of the coupled $\Kbar N$ related channels at low energies, and {\it (ii)} the new and improved measurement from SIDDHARTA Collaboration of the strong interaction energy shift and width in the lowest $K^-$-hydrogen atomic level. Those two, in combination, have allowed us to produced a reliable two-body input to the three-body calculation. All available low-energy $K^-p$ observables are well reproduced and predictions for the $\Kbar N$ scattering lengths and amplitudes, $(\pi \Sigma)^\circ$ invariant-mass spectra, as well as for $A_{K^-d}$ are put forward and compared with results from other sources. The findings of the present work are expected to be useful in interpreting the forthcoming data from CLAS, HADES, LEPS and SIDDHARTA Collaborations.

New Result on Θ+ from LEPS

We report new results of the Θ+ search by the LEPS collaboration. First, we applied exactly the same inclusive analysis as previous one to new data, where the proton and neutron contributions were not separated. The inclusive analysis does not show a strong narrow peak which was observed in the previous analysis. Thus, two results are inconsistent. In order to investigate the reason for the inconsistency, we have carried out a new analysis, where the proton contributions are identified by using the energy loss information of a trigger plastic scintillator called Start Counter. When proton events were selected, a peak structure was seen in the previous data. This indicates that a part of the peak structure in the previous data comes from statistical fluctuation. Furthermore, a clear signal enhancement was observed in the proton rejected sample. This enhancement was confirmed by subtracting the proton contribution estimated by a Monte-Carlo based simulation.

K* 0 Σ+ Photoproduction with Evidence for the Kappa Meson Exchange at SPring-8/LEPS

Recently, LEPS collaboration reported the parity spin asymmetry for the γp → K*0Σ+ reaction using linearly polarized photons at beam energies from 1.85 to 2.96 GeV. The parity spin asymmetry (\({P_{\sigma} = 2\rho^{1}_{1-1} - \rho^{1}_{00}}\)) was obtained, with large positive asymmetry at forward angles. This supports that natural-parity exchange is dominant at forward angles for K*0Σ+ photoproduction, which is consistent with κ meson-exchange. The differential cross sections for the γp → K*0Σ+ reaction are presented at forward angles averaged over all beam energies. There is good agreement with the differential cross sections from the CLAS and the CBELSA/TAPS collaborations.

Beam polarization asymmetry and the electromagnetic production of kaons from protons

The beam polarization asymmetry in the reaction $ep\ensuremath{\rightarrow}{e}^{\ensuremath{'}}{K}^{+}\ensuremath{\Lambda}$ has been investigated in a tree-level effective Lagrangian model. The model incorporates most of the well-established baryon resonances with spins up to $\frac{5}{2}$, four less well-established nucleon resonances with larger mass, and the two kaon resonances ${K}^{\ensuremath{\star}}(892)$ and $K1(1270)$. The off-shell structure of the electromagnetic vertices was accounted for by the inclusion of electromagnetic form factors at those vertices. The free parameters of the model were fitted in a previous study to a large pool of photoproduction data from the CLAS, GRAAL, SAPHIR, and LEPS collaborations and to CLAS data for the virtual photoproduction structure functions ${\ensuremath{\sigma}}_{U}$, ${\ensuremath{\sigma}}_{T}$, ${\ensuremath{\sigma}}_{L}$, ${\ensuremath{\sigma}}_{TT}$, and ${\ensuremath{\sigma}}_{LT}$. Using this model, results were obtained for the beam polarization asymmetry structure function ${\ensuremath{\sigma}}_{L{T}^{\ensuremath{'}}}$ and compared with CLAS data. Two new fits to the combined photoproduction and electroproduction data with the ${\ensuremath{\sigma}}_{L{T}^{\ensuremath{'}}}$ data included were then generated. The first of these includes contributions from all of the resonances included in the previous study; the second excludes contributions from the $N(2080)$ and $N(2200)$ resonances. The results of both fits for both photoproduction and electroproduction observables are compared with the results of the previous fit and the data.

Role of theN*(2080)resonance in theγ⃗p→K+Λ(1520)reaction

We investigate the $\ensuremath{\Lambda}(1520)$ photoproduction in the $\mathrm{\ensuremath{\gamma}P\vec}p\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}(1520)$ reaction within the effective Lagrangian method near threshold. In addition to the ``background'' contributions from the contact, $t$-channel $K$-exchange, and $s$-channel nucleon pole terms, which were already considered in previous studies, the contribution from the nucleon resonance ${N}^{*}(2080)$ (spin-parity ${J}^{P}=3/{2}^{\ensuremath{-}}$) is also considered. We show that the inclusion of the nucleon resonance ${N}^{*}(2080)$ leads to a fairly good description of the new LEPS differential cross-section data, and that these measurements can be used to determine some of the properties of this latter resonance. However, serious discrepancies appear when the predictions of the model are compared to the photon-beam asymmetry,which was also measured by the LEPS Collaboration.

Is the nonmonotonic behavior in the cross section of ϕ photoproduction near threshold a signature of a resonance?

We study whether the nonmonotonic behavior found in the differential cross section of the ϕ-meson photoproduction near threshold can be described by a resonance. Namely, we add a resonance to a model consisting of Pomeron and (π,η) exchange by fiat and see if, with a suitable assignment of spin and parity, mass and width, as well as the coupling constants, one would be able to obtain a good description to all the data reported by the LEPS Collaboration in the low-energy region. The resonant contribution is evaluated by using an effective Lagrangian approach. We find that, with the assumption of a JP=3/2− resonance with mass of 2.10±0.03 GeV and width of 0.465±0.141 GeV, LEPS data can indeed be well described. The ratio of the helicity amplitudes A12/A32 calculated from the resulting coupling constants differs in sign from that of the known D13(2080). We further find that the addition of this postulated resonance can substantially improve the agreement between the existing theoretical predictions and the recent ω photoproduction data if a large value of the OZI evading parameter xOZI=12 is assumed for the resonance.