CLAS Data Research Articles

Polarized Proton Structure in the Resonance Region

In view of the precise data available on inclusive polarized electron scattering off polarized proton targets in the nucleon resonance excitation region, we compare these results with the coherent sum of resonant contributions to the polarized structure function g_1 and virtual photon asymmetry A_1. To this goal, we employ the nucleon resonance electroexcitation amplitudes determined for photon virtualities Q^2 < 5.0 GeV^2 from analyses of the CLAS data on exclusive electroproduction off protons in the resonance region. Most of the well established resonances of four star PDG status in the mass range up to 1.75 GeV are included. We find that the resonance-like structures observed in the inclusive g_1 data are related to the resonant contributions in the entire range of photon virtuality Q^2 where the data on g_1 are available. In the range of invariant mass of the final hadron system W > 1.5 GeV, the data on the asymmetry A_1 are well reproduced even when accounting for resonant contributions only, especially for the larger values of Q^2 and energies analysed. This observation offers an interesting hint to quark-hadron duality seen in polarized inclusive electron scattering observables.

Extraction of proton trace anomaly energy from near-threshold $$\phi $$ and $$J/\psi $$ photo-productions

The trace anomalous energy contribution to the proton mass is a very important topic in non-perturbative QCD and hadron physics. In experiments, it is under the hot discussions on how to measure the trace anomalous energy.The QCD interpretation of proton trace anomaly is still not clear.To connect the theory with the experiment,we extract the trace anomaly by analyzing the near-threshold photo-production data of $\phi$ and J/$\psi$ vector mesons. Based on the vector-meson-dominance model and QCD Van der Waals representation, we find that the percentage of trace anomaly in the proton mass ranges from $16\%$ to $24\%$, which is of similar order of magnitude as the 23$\%$ given by Lattice QCD. We also provide the approximate magnitudes of the systematic uncertainties of the extracted results from the model assumptions as well as the data fitting procedures. We give relative statistical uncertainties of 17.2$\%$, 17.7$\%$, 3.6$\%$, and 8.2$\%$, total relative systematic uncertainties of 21.4$\%$, 54.2$\%$, 37.2$\%$, and 25.7$\%$, for the analyses of the GlueX, LEPS, CLAS, and SAPHIR data, respectively. We argue that the near-threshold $\Upsilon$ photo-production experiments are more beneficial for the measurement of trace anomaly in proton mass in the future.

The sin 2phi _h azimuthal asymmetry of pion production in SIDIS within TMD factorization

We study the sin 2phi _h azimuthal asymmetry of charged and neutral pion productions off the longitudinally polarized nucleon targets in semi-inclusive deeply inelastic scattering (SIDIS) process within the transverse momentum dependent (TMD) factorization. The asymmetry is contributed by the convolution of the TMD distribution h_{1L}^{perp } and the Collins fragmentation function H_{1}^perp . We adopt the Wandzura–Wilczek-type (WW-type) approximation for h_{1L}^{perp } and two different parametrizations on the nonperturbative Sudakov form factor for h_{1L}^{perp } and H_{1}^perp . We estimate the sin 2phi _h asymmetry at HERMES and CLAS and compare them with the corresponding experimental data. We also provide the prediction at kinematical configuration of CLAS12. It is shown that our theoretical calculation can describe the HERMES and CLAS data, except the asymmetry of the pi ^- production off proton target at HERMES and CLAS. We also find that different choices of the nonperturbative part of TMD evolution lead qualitatively similar results.

Extraction of the higher-twist parton distribution e(x) from CLAS data

This manuscript presents the new analysis of recent CLAS and CLAS12 data aimed at the extraction of $e(x)$. This publication benefits from a proof-of-principle analysis of a smaller preliminary data set published in a preprint form in \url{1405.7659}. We present the first point-by-point extraction of a twist-3 PDF. The scalar PDF, $e(x)$, is accessed through the analysis of the data for the $\sin\phi_R$-moment of the beam-spin asymmetry for dihadron production in semi-inclusive DIS off proton target at CLAS and CLAS12. The dihadron formalism allows for use of collinear framework, hence calling for a minimal set of approximations and hypotheses. The extracted PDF $e(x)$ carries insights into the physics of the largely-unexplored quark-gluon correlations, and its first Mellin moment is related to the marginally-known scalar charge of the nucleon. We show that the proton flavor combination of the scalar PDF is non-zero at more than $74\%$ probability.

Test of the hyperon-nucleon interaction within leading order covariant chiral effective field theory

Motivated by the recent experimental measurements of differential cross sections of the $\Sigma^{-}p$ elastic scattering in the momentum range of $470$ to $850$ MeV$/c$ by the J-PARC E$40$ experiment, we extend our previous studies of $S=-1$ hyperon-nucleon interactions to relatively higher energies up to $900$ MeV$/c$ for both the coupled-channel $\Lambda p\rightarrow(\Lambda p, \Sigma^{+}n, \Sigma^{0}p)$, $\Sigma^{-}p\rightarrow(\Lambda n, \Sigma^{0}n, \Sigma^{-}p)$ and single-channel $\Sigma^{+}p\rightarrow\Sigma^{+}p$ reactions. We show that although the leading order covariant chiral effective field theory is only constrained by the low energy data, it can describe the high energy data reasonably well, in particular, the J-PARC E40 differential cross sections. The predicted cusp structure close to the $\Sigma N$ threshold in the $\Lambda p\to \Lambda p$ reaction agrees with the latest ALICE observation as well as with the results of the next-to-leading order heavy baryon chiral effective theory. On the other hand, the comparison with the latest CLAS data on the $\Lambda p$ cross sections between 0.9 and 2.0 GeV$/c$ clearly indicates the need of higher order chiral potentials for such high momenta. This is also the case for the latest J-PARC data on the $\Sigma p \rightarrow \Lambda n$ differential cross sections. Nevertheless, even for these cases, the predictions are in qualitative agreement with the data, albeit with large uncertainties, implying that the predicted total and differential cross sections are of relevance for ongoing and planned experiments.

Nuclear Fermi momenta of 2H, 27Al and 56Fe from an analysis of CLAS data

Nuclear Fermi momentum is a basic property of a nucleus where many nucleons dwell. However, in experiments only the nuclear Fermi momenta of just a few nuclei are measured using quasielastic electron scattering on the nuclear targets so far. Particularly, we still do not know experimentally the Fermi momentum of the lightest nucleon composite – the deuteron. In this paper, we apply both gaussian distribution and Cauchy distribution to describe the quasielastic peak in the cross section of electron-nucleus scattering. The dip of the cross-section ratio at about xB=1 is explained with the nuclear Fermi momentum. By performing the least-square fits to the published CLAS data in the narrow kinematic region of quasielastic scattering, we obtain the nuclear Fermi momenta of 2H, 27Al and 56Fe, which are 116±7 MeV/c, 232±27 MeV/c, and 244±28 MeV/c respectively. The extracted nuclear Fermi momenta are compared to the simple calculations based on Fermi gas model, and the consistencies are found.

Threshold Upsilon-meson photoproduction at the EIC and EicC

High-accuracy $\Upsilon$-meson photoproduction data from EIC and EicC experiments will allow the measurement of the near-threshold total cross section of the reaction $\gamma p\to\Upsilon p$, from which the absolute value of the $\Upsilon p$ scattering length, $|\alpha_{\Upsilon p}|$, can be extracted using a Vector-Meson Dominance model. For this evaluation, we used $\Upsilon$-meson photoproduction quasi-data from the QCD approach (the production amplitude can be factorized in terms of gluonic generalized parton distributions and the quarkonium distribution amplitude). A comparative analysis of $|\alpha_{\Upsilon p}|$ with the recently determined scattering lengths for $\omega p$, $\phi p$, and $J/\psi p$ using the A2, CLAS, and GlueX experimental data are performed. The role of the "young" vector-meson effect is evaluated.

Exclusive f1(1285) meson production for energy ranges available at the GSI-FAIR with HADES and PANDA

We evaluate the cross section for the $p p \to p p f_{1}(1285)$ and $p \bar{p} \to p \bar{p} f_{1}(1285)$ reactions at near threshold energies relevant for the HADES and PANDA experiments at GSI-FAIR. We assume that at energies close to the threshold the $\omega \omega \to f_{1}(1285)$ and $\rho^{0} \rho^{0} \to f_{1}(1285)$ fusion processes are the dominant production mechanisms. The vertex for the $VV \to f_{1}$ coupling is derived from an effective coupling Lagrangian. The $g_{\rho \rho f_1}$ coupling constant is extracted from the decay rate of $f_{1}(1285) \to \rho^{0} \gamma$ using the vector-meson-dominance ansatz. We assume $g_{\omega \omega f_1} = g_{\rho \rho f_1}$, equality of these two coupling constants, based on arguments from the naive quark model and vector-meson dominance. The amplitude for the $VV \to f_{1}$ fusion, supplemented by phenomenological vertex form factors for the process, is given. The differential cross sections at energies close to the threshold are calculated. In order to determine the parameters of the model the $\gamma p \to f_{1}(1285) p$ reaction is discussed in addition and results are compared with the CLAS data. The possibility of a measurement by HADES@GSI is presented and discussed. We performed a Monte Carlo feasibility simulations of the $p p \to p p f_1$ reaction for $\sqrt{s}$ = 3.46 GeV in the $\pi^+ \pi^- \pi^+ \pi^-$ (not shown explicitly) and $\pi^+ \pi^- \pi^+ \pi^- \pi^0$ final states using the PLUTO generator. The latter one is especially promising as a peak in the $\pi^+ \pi^- \eta$ should be observable by HADES.

Genuine empirical pressure within the proton

A phenomenological extraction of pressure within the proton has recently been performed using JLab CLAS data (arXiv:2104.02031 [nucl-ex]). The extraction used a 3-dimensional Breit frame description in which the initial and final proton states have different momenta. Instead, we obtain the two-dimensional transverse light front pressure densities that incorporate relativistic effects arising from the boosts that cause the initial and final states to differ. The mechanical radius is then determined to be $0.518~ \pm 0.062_{\mathrm{fit}} \pm 0.126_{\mathrm{sys}}~\mathrm{fm}$, which is smaller than the electric charge radius and larger than the light front momentum radius. The forces within the proton are shown to be predominantly repulsive at distances less than $0.43~\pm 0.12~\mathrm{fm}$ from the center, and predominantly attractive further out.

Photoproduction of Λ* and Σ* resonances with JP=1/2− off the proton

We study the photoproduction of the $\Lambda(1405)$ and $\Sigma(1400)$ hyperon resonances, the latter of which is not a well established state. We evaluate the $s$-, $t$- and $u$-channel diagrams in the Born approximation by employing the effective Lagrangians. A new ingredient is the inclusion of a nucleon resonance $N^*(1895)$ that is dynamically generated with predictions for its coupling to the $K\Lambda(1405)$ and $K\Sigma(1400)$ channels. To extend the applicability of the model to energies beyond the threshold region, we consider a Regge model for the $t$-channel $K$- and $K^*$-exchanges. Our results are in good agreement with the CLAS data available on $\Lambda(1405)$, while for $\Sigma(1400)$ we predict observables for its production. We also provide polarization observables for both hyperon productions, which can be useful in future experimental investigations. The present study provides new information on the nucleon resonance $N^*(1895)$ which can be an alternative source for generating the hyperon resonances $\Lambda(1405)$ and $\Sigma(1400)$.

Constituent counting rule and ω photoproduction

The constituent counting ruling (CCR) has been found to hold for numerous hard, exclusive processes. It predicts the differential cross section at high energies and fixed $cos{\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}$ should follow $\frac{d\ensuremath{\sigma}}{dt}\ensuremath{\sim}\frac{1}{{s}^{n\ensuremath{-}2}}$, where $n$ is the minimal number of constituents involved in the reaction. Conversely, there are hard, exclusive processes for which it has been found that the CCR does not work. The exact reasons for these have not been clearly established. One such example, for which the analysis of CLAS data deviates from the prediction of the CCR, is the omega photoproduction reaction. Here, we provide an in-depth analysis of the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\omega}p$ at ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\approx}{90}^{\ensuremath{\circ}}$ using CLAS data with an energy range of $s=5\text{--}8\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{2}$, where the CCR has been shown to work in other reactions. We argue for a stringent method to select data to test the CCR. Na\"{\i}vely, this reaction would have $n=9$ and we would expect a scaling of ${s}^{\ensuremath{-}7}$. Instead, a scaling of ${s}^{\ensuremath{-}(9.08\ifmmode\pm\else\textpm\fi{}0.11)}$ was observed. A careful analysis of conservation of angular momentum is proposed to explain the discrepancy, supporting the validity of the CCR when applied properly.

Photoproductions of f1(1285) and η′(958) from the analysis of CLAS data with the Primakoff effect at high energies

We investigate photoproduction of axial vector meson $f_1(1285)$ based on the CLAS experiment $\gamma p\to pf_1(1285)\to p\eta\pi^+\pi^-$ using the Regge model for $\rho^0+\omega$ exchanges. The combined analysis of $\gamma p\to p\eta'\to p\eta\pi^+\pi^-$ reaction including $\eta(1295)$ is accompanied to evaluate the decay mode $\eta(1295)\to \eta\pi^+\pi^-$ from the potential overlap with the $f_1$ reaction. The predominance of $\rho^0(770)$ exchange over the $\omega(782)$ with the coupling constant $\gamma\rho^0f_1$ extracted from the CLAS experiment leads to a good description of the measured cross sections for $f_1$, while the scale of the cross section is corrected by the branching fraction $\eta'\to\eta\pi^+\pi^-$, and $f_1\to\eta\pi^+\pi^-$, respectively. As an extension to study the nonmesonic production mechanism, the Primakoff effect by the virtual photon exchange is investigated in the exclusive reaction $\gamma p\to pf_1$ up to $\sqrt{s}\approx50$ GeV, finding that it plays a similar role to the Pomeron in the vector meson photoproduction at high energies. Such an unique role of the virtual photon exchange similar to the $f_1$ case is identified up to $\sqrt{s}\approx250$ GeV in the $\gamma p\to p\pi^0$ and $\gamma p\to p\eta$ as well as $\gamma p\to p\eta'$ reaction.

Can we fit nuclear PDFs with the high-x CLAS data?

Nuclear parton distribution functions (nuclear PDFs) are non-perturbative objects that encode the partonic behaviour of bound nucleons. To avoid potential higher-twist contributions, the data probing the high-x end of nuclear PDFs are sometimes left out from the global extractions despite their potential to constrain the fit parameters. In the present work we focus on the kinematic corner covered by the new high-x data measured by the CLAS/JLab collaboration. By using the Hessian re-weighting technique, we are able to quantitatively test the compatibility of these data with globally analyzed nuclear PDFs and explore the expected impact on the valence-quark distributions at high x. We find that the data are in a good agreement with the EPPS16 and nCTEQ15 nuclear PDFs whereas they disagree with TuJu19. The implications on flavour separation, higher-twist contributions and models of EMC effect are discussed.

Two Pion Photo- and Electroproduction with CLAS

Exclusiveπ+π-pphoto- and electroproduction data from CLAS have considerably extended the information on the spectrum and structure of nucleon resonances. The data from theπ+π-pandNπchannels have provided results on the electrocouplings of most resonances in the mass region up to 1.8 GeV and at photon virtualities up to 5.0 GeV2. The recent CLAS data onπ+π-pphotoproduction have improved knowledge on the photocouplings of nucleon resonances in the mass range of 1.6 GeV &lt;MN*&lt;2.0 GeV and on their decays to theπΔ andρpfinal hadron states. Analyses of the combinedπ+π-pphoto- and electroproduction data have revealed evidence for the candidate-stateN’(1720)3/2+. The new results on the nucleon resonance spectrum, electroexcitation amplitudes from analysis of the CLASπ+π-pphoto- and electroproduction data, and their impact on the exploration of strong QCD are presented.

Nucleon resonances in γp→ωp reaction

The most recent high-precision data on spin observables $\Sigma$, $T$, $P'$, $E$, $F$ and $H$ reported by the CLAS Collaboration together with the previous data on differential cross sections and spin-density-matrix elements reported by the CLAS, A2, GRAAL, SAPHIR and CBELSA/TAPS Collaborations for the reaction $\gamma p \to \omega p$ are analyzed within an effective Lagrangian approach. The reaction amplitude is constructed by considering the $t$-channel $\pi$ and $\eta$ exchanges, the $s$-channel nucleon and nucleon resonances exchanges, the $u$-channel nucleon exchange and the generalized contact current. The latter accounts effectively for the interaction current and ensures that the full photoproduction amplitude is gauge invariant. It is shown that all the available CLAS data can be satisfactorily described by considering the $N(1520)3/2^-$, $N(1700)3/2^-$, $N(1720)3/2^+$, $N(1860)5/2^+$, $N(1875)3/2^-$, $N(1895)1/2^-$ and $N(2060)5/2^-$ resonances in the $s$-channel. The parameters of these resonances are extracted and compared with those quoted by PDG.

Investigating the glue content of f1(1285)

The radiative decay of f1(1285) to vector mesons ρ and ϕ is calculated in a non-relativistic approach. The results obtained are compared to experimental data to estimate the glue content of f1. We find a small or nonexistent glue content when our results are compared to the PDG data. However a considerable glue content for f1(1285) was found when we compared our results to the CLAS data.

Coupled K+Λ and K0Λ photoproduction off the nucleon: Consequences from the recent CLAS and MAMI data and the N(1680)P11 narrow state

The new $\gamma n\to K^0\Lambda$ data obtained from the CLAS and MAMI collaborations are analyzed by employing an effective Lagrangian method. The constructed model can describe all available experimental data in both $\gamma p \to K^+\Lambda$ and $\gamma n\to K^0\Lambda$ channels, simultaneously. The background part of the model is built from the appropriate intermediate states involving the nucleon, kaon, and hyperon exchanges, whereas the resonance part is constructed from the consistent interaction Lagrangians and propagators. To check the performance of the model a detailed comparison between the calculated observables and experimental data in both isospin channels is presented, from which a nice agreement can be observed. The discrepancy between the CLAS and MAMI data in the $\gamma n\to K^0\Lambda$ channel is analyzed by utilizing three different models; M1, M2, and M3 that fit the CLAS, MAMI, and both CLAS and MAMI data sets, respectively. The effect of this discrepancy is studied by investigating the significance of individual nucleon resonances and the predicted beam-target helicity asymmetry $E$ that has been measured by the CLAS collaboration recently. It is found that the $N(1720)P_{13}$, $N(1900)P_{13}$, and $N(2060)D_{15}$ resonances are significant for improving the agreement between model calculation and data. This result is relatively stable to the choice of the model. The helicity asymmetry $E$ can be better explained by the models M1 and M3. Finally, the effect of the $N(1680)P_{11}$ narrow resonance on the cross section of both isospin channels is explored. It is found that the effect is more sensitive in the $\gamma n\to K^0\Lambda$ channel. In this case the model M3, that fits both CLAS and MAMI data, yields a more realistic effect.

K+Λ and K+Σ0 Photoproduction in View of the New CLAS Data

We have investigated the effect of the new CLAS data consisting of the recoiled Λ polarization P, photon asymmetry Σ, target asymmetry T, and the double polarizations Ox and Oz, on the latest isobar models constructed for the kaon photoproduction processes γp → K+Λ and γp → K+Σ0. We found that those data might have significant effect, especially in the K+Λ channel, where apparent changes are observed in the predicted differential cross sections after including the new CLAS data in the fitting database. The isobar model for the K+Λ photoproduction must be accordingly modified in order to improve the agreement between model calculation and previous experimental data.

Addendum to predicting the $K^0\Lambda$ photoproduction observables by using the multipoles approach

Abstract We present an addendum to our previous paper to improve the performance of the proposed isobar model in describing the $\gamma + n\to K^0+\Lambda$ reaction observables. Since two new data sets have become available for this reaction channel that show sizable discrepancies, i.e. the CLAS and MAMI data, we propose three different models, M1, M2, and M3, obtained from including the CLAS, MAMI, and both data sets, respectively, in the fitting database. The new models can nicely describe the $\gamma+p\to K^++\Lambda$ observables, as in the case of the previous model, but significantly improve the agreement with the new $\gamma + n\to K^0+\Lambda$ cross section data. The extracted resonance properties in the present models can be well constrained within the current Particle Data Group estimates. The different contributions of the background and resonance terms in the previous and present calculations are thoroughly discussed. A brief discussion on the problem of data discrepancy is also presented.

Search for the Σ⁎ state in [formula omitted] decay by triangle singularity

A Σ⁎ resonance with spin-parity JP=1/2− and mass in the vicinity of the K¯N threshold has been predicted in the unitary chiral approach and inferred from the analysis of CLAS data on the γp→K+π0Σ0 reaction. In this work, based on the dominant Cabibbo favored weak decay mechanism, we perform a study of Λc+→π+π0Σ⁎ with the possible Σ⁎ state decaying into π−Σ+ through a triangle diagram. This process is initiated by Λc+→π+K¯⁎N, then the K¯⁎ decays into K¯π and K¯N produce the Σ⁎ through a triangle loop containing K¯⁎NK¯ which develops a triangle singularity. We show that the π−Σ+ state is generated from final state interaction of K¯N in S-wave and isospin I=1, and the Λc+→π+π0π−Σ+ decay can be used to study the possible Σ⁎ state around the K¯N threshold. The proposed decay mechanism can provide valuable information on the nature of the Σ⁎ resonance and can in principle be tested by facilities such as LHCb, BelleII and BESIII.