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Register a new userRosenbluth separation of the $pi^0$ electroproduction cross section
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We present deeply virtual $pi^0$ electroproduction cross-section measurements at $x_B$=0.36 and three different $Q^2$--values ranging from 1.5 to 2 GeV$^2$, obtained from experiment E07-007 that ran in the Hall A at Jefferson Lab. The Rosenbluth technique was used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component, and thus is far from the asymptotic limit predicted by perturbative Quantum Chromodynamics. An indication of a non-zero longitudinal contribution is provided by the interference term $sigma_{LT}$ also measured. Results are compared with several models based on the leading twist approach of Generalized Parton Distributions (GPDs). In particular, a fair agreement is obtained with models where the scattering amplitude is described by a convolution of chiral-odd (transversity) GPDs of the nucleon with the twist-3 pion distribution amplitude. Therefore, neutral pion electroproduction may offer the exciting possibility of accessing transversity GPDs through experiment.
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We report the first longitudinal/transverse separation of the deeply virtual exclusive $pi^0$ electroproduction cross section off the neutron and coherent deuteron. The corresponding four structure functions $dsigma_L/dt$, $dsigma_T/dt$, $dsigma_{LT}/dt$ and $dsigma_{TT}/dt$ are extracted as a function of the momentum transfer to the recoil system at $Q^2$=1.75 GeV$^2$ and $x_B$=0.36. The $ed to edpi^0$ cross sections are found compatible with the small values expected from theoretical models. The $en to enpi^0$ cross sections show a dominance from the response to transversely polarized photons, and are in good agreement with calculations based on the transversity GPDs of the nucleon. By combining these results with previous measurements of $pi^0$ electroproduction off the proton, we present a flavor decomposition of the $u$ and $d$ quark contributions to the cross section.
We report on a Rosenbluth separation using previously published data by the CLAS collaboration in Hall B, Jefferson Lab for exclusive $pi^{0}$ deeply virtual electroproduction (DVEP) from the proton at a mean $Q^{2}$ of $approx$ 2 (GeV/c)$^{2}$. The central question we address is the applicability of factorization in $pi^0$ DVEP at these kinematics. The results of our Rosenbluth separation clearly demonstrate the dominance of the longitudinal contribution to the cross section. The extracted longitudinal and transverse contributions are in agreement with previous data from Hall A at Jefferson Lab, but over a much wider $-t$ range (0.12 - 1.8 (GeV/c)$^{2}$). The measured dominance of the longitudinal contribution at $Q^{2} approx$ 2 (GeV/c)$^{2}$ is consistent with the expectation of the handbag factorization theorem. We find that $sigma_L(t) sim 1/(-t)$ for $-t >$ 0.5 (GeV/c)$^2$. Determination of both longitudinal and transverse contributions to the deeply virtual $pi^{0}$ electroproduction cross section allows extraction of additional GPDs.
We report measurements of the exclusive neutral pion electroproduction cross section off protons at large values of $x_B$ (0.36, 0.48 and 0.60) and $Q^2$ (3.1 to 8.4 GeV$^2$) obtained from Jefferson Lab Hall A experiment E12-06-014. The corresponding structure functions $dsigma_L/dt+epsilon dsigma_T/dt$, $dsigma_{TT}/dt$, $dsigma_{LT}/dt$ and $dsigma_{LT}/dt$ are extracted as a function of the proton momentum transfer $t-t_{min}$. The results suggest the amplitude for transversely polarized virtual photons continues to dominate the cross-section throughout this kinematic range. The data are well described by calculations based on transversity Generalized Parton Distributions coupled to a helicity flip Distribution Amplitude of the pion, thus providing a unique way to probe the structure of the nucleon.
We report on a measurement of hard exclusive $pi^0$ muoproduction on the proton by COMPASS using 160 GeV/$c$ polarised $mu^+$ and $mu^-$ beams of the CERN SPS impinging on a liquid hydrogen target. From the average of the measured $mu^+$ and $mu^-$ cross sections, the virtual-photon proton cross section is determined as a function of the squared four-momentum transfer between initial and final proton in the range $0.08,(text{GeV/}c)^2 < |t| < 0.64,(text{GeV/}c)^2$. The average kinematics of the measurement are $langle Q^2 rangle =2.0; {(text{GeV}/c)^2}$, $langle u rangle = 12.8; {text{GeV}}$, $langle x_{Bj} rangle = 0.093 $ and $langle -t rangle = 0.256; {(text{GeV}/c)^2} $. Fitting the azimuthal dependence reveals a combined contribution by transversely and longitudinally polarised photons of $(8.1 pm 0.9_{text{stat}}{}_{- 1.0}^{+ 1.1}bigrvert_{text{sys}}),{text{nb}}/{(text{GeV}/c)^{2}}$, as well as transverse-transverse and longitudinal-transverse interference contributions of $(-6.0 pm 1.3_{text{stat}}{}_{- 0.7}^{+ 0.7}bigrvert_{text{sys}}),{text{nb}}/{(text{GeV}/c)^{2}}$ and $(1.4 pm 0.5_{text{stat}}{}_{- 0.2}^{+ 0.3}bigrvert_{text{sys}}),{text{nb}}/{(text{GeV}/c)^{2}}$, respectively. Our results provide important input for modelling Generalised Parton Distributions. In the context of the phenomenological Goloskokov-Kroll model, the statistically significant transverse-transverse interference contribution constitutes clear experimental evidence for the chiral-odd GPD $overline{E}_T$.
The process $e^+e^- to K^+K^-pi^+pi^-$ has been studied in the center-of-mass energy range from 1500 to 2000,MeV using a data sample of 23 pb$^{-1}$ collected with the CMD-3 detector at the VEPP-2000 $e^+e^-$ collider. Using about 24000 selected events, the $e^+e^- to K^+K^-pi^+pi^-$ cross section has been measured with a systematic uncertainty decreasing from 11.7% at 1500-1600,MeV to 6.1% above 1800,MeV. A preliminary study of $K^+K^-pi^+pi^-$ production dynamics has been performed.
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