Quark Radius Research Articles

First extraction of the proton mass radius and scattering length $$\vert\alpha_{\rho^{0}p}\vert$$ from ρ0 photoproduction

As it involves the lightest physical states excited from the vacuum by the vector quark current, near-threshold ρ0 photoproduction is considered a possible way to research the proton radius and the absolute value of the scattering lengths of the ρ0-proton interaction. In this study, under the assumption of a scalar form factor of dipole form, the value of the proton mass radius is calculated as (0.85 ± 0.06) fm by fitting the differential cross section of the γp → ρ0p reaction at near-threshold energy. For light vector meson photoproduction, because the exchange of a scalar quark-antiquark pair is not suppressed and should dominate the scalar gluon exchange, the radius we extract from ρ0 photoproduction is likely to represent the quark radius of the proton. This fact may explain why the value obtained in this study is very near the proton charge radius. Moreover, the absolute value of the ρ0-proton scattering length $$\vert\alpha_{\rho^{0}p}\vert=(0.31 \pm 0.06)$$ fm is obtained for the first time within the vector meson dominance model. This result disobeys the rule that the absolute value of the vector meson and proton scattering length $$\vert\alpha_{V_{p}}\vert$$ increases with the meson’s mass, which can be attributed to treating the ρ0 meson as a point in the analysis. These results provide useful theoretical information for an in-depth understanding of proton structure and the proton-vector meson interaction.

Limits on the effective quark radius from inclusive ep scattering at HERA

The high-precision HERA data allows searches up to TeV scales for Beyond the Standard Model contributions to electron-quark scattering. Combined measurements of the inclusive deep inelastic cross sections in neutral and charged current $ep$ scattering corresponding to a luminosity of around 1 fb$^{-1}$ have been used in this analysis. A new approach to the beyond the Standard Model analysis of the inclusive $ep$ data is presented; simultaneous fits of parton distribution functions together with contributions of "new physics" processes were performed. Results are presented considering a finite radius of quarks within the quark form-factor model. The resulting 95% C.L. upper limit on the effective quark radius is $0.43\cdot 10^{-16}$ cm.

High-[formula omitted] NC and CC cross sections at HERA and proton structure

A review of the recent HERA-II measurements from the H1 and ZEUS detectors on neutral-current and charged-current deep inelastic scattering of longitudinally polarized lepton beams with a proton up to a center-of-mass energy of 318 GeV are presented. Dependence of the cross sections on the polarization of the lepton beam is shown. The lepton charge asymmetry of the neutral current cross section is used to extract the structure function x F ˜ 3 . Electroweak couplings of the u and d quarks are determined from a combined QCD and electroweak analysis of the data. Results on searches for physics beyond the Standard Model are presented. No deviations from the theory is seen, from which limits are derived on the presence of four fermion contact interactions, heavy leptoquarks and on the quark radius.

Search for contact interactions in [formula omitted] collisions at HERA

A search for physics beyond the Standard Model in neutral current deep inelastic scattering at high negative four-momentum transfer squared Q2 is performed in e±p collisions at HERA. The differential cross section dσ/dQ2, measured using the full H1 data sample corresponding to an integrated luminosity of 446 pb−1, is compared to the Standard Model prediction. No significant deviation is observed. Limits on various models predicting new phenomena at high Q2 are derived. For general four-fermion eeqq contact interaction models, lower limits on the compositeness scale Λ are set in the range 3.6 TeV to 7.2 TeV. Leptoquarks with masses MLQ and couplings λ are constrained to MLQ/λ>0.41–1.86 TeV and limits on squarks in R-parity violating supersymmetric models are derived. A lower limit on the gravitational scale in 4+n dimensions of MS>0.9 TeV is established for low-scale quantum gravity effects in models with large extra dimensions. For the light quark radius an upper bound of Rq<0.65⋅10−18 m is determined.

Physics at Very High Energies and Search for New Physics at HERA

Results of searches for physics beyond the Standard Model (SM) from the H1 and ZEUS experiments at the e ± p collider HERA are reviewed. Limits obtained for contact interaction models and finite quark radius are presented. The results of a combined measurement of isolated leptons with high transverse momentum are summarised and the related search for anomalous top production is also reviewed. Finally, searches by H1 for leptoquarks and the results of the H1 general search are presented.

Proton and Valence Quark Charge Radii for a Quasi-potential Model

The problem of calculating the root-mean-square radius of proton charge distribution is studied for a relativistic quasi-potential quark model in the approximation of the SU(6) symmetry. It is demonstrated that in the case of point quarks, the proton radius is f(m/λ)/M2, where m is the quark mass, λ is the binding energy scale parameter, M is the proton mass, and f is a dimensionless function independent of M. It is demonstrated that the root-mean-square radius of the coupled system can take a negative value in the ultrarelativistic range. For a joint description of the nucleon magnetic moments and of the proton radius, a negative root-mean-square quark radius, whose absolute value depends on the quark mass as 〈r 1q 2 〉 = −1.875 m2, must be introduced into the model with oscillator forces.

Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA

A search for physics beyond the Standard Model has been performed with high-Q2 neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e+p→e+X and e−p→e−X, with respective integrated luminosities of 112 pb−1 and 16 pb−1, were analyzed. The data reach Q2 values as high as 40000 GeV2. No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions, the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85×10−16 cm.

Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA

A search for physics beyond the Standard Model has been performed with high- Q 2 neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e + p → e + X and e − p → e − X , with respective integrated luminosities of 112 pb −1 and 16 pb −1 , were analyzed. The data reach Q 2 values as high as 40 000 GeV 2 . No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions, the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85×10 −16 cm.

Short-range Correlation Effect on p?p Elastic Scattering with the Composite Model

Using a representation of the quark distribution inside the proton, where a short-range correlation between quarks is considered, we can describe the p-p elastic scattering differential cross section with experimental agreement in the range 0 :::; q2 :::; 10 (GeV/ c)2. Evaluations of the proton core and quark radii are obtained. From our analysis, we see that the suggested configuration of quarks inside the proton can be considered as a nearly realistic distribution even with a non-relativistic wavefunction.

The importance of nucleon substructure in nuclear ground states

Using a potential model, constrained by the hadron spectrum, for the confinement of relativistic quarks we explore the consequences of the substructure of nucleons for the binding energy and ground state wavefunction of 4He. In its simplest form, this model gives a binding energy of 19 MeV. Quark wavefunctions differ from those associated with free nucleons by less than 10%, the rms quark radius is 1.34 fm and the resulting structure differs considerably from that of an expansion beginning from the (0s) 4 shell model. Considerable contributions to the binding energy, attractive from quark delocalization and repulsive from the quark hyperfine interaction, appear unavoidable. We conclude that these effects cannot be excluded from a detailed understanding of the properties of nuclear ground states.

Nucleon core size and nucleon-nucleon spin-orbit forces

Information on the rms radius of quarks in nucleons is deduced from the nucleon-nucleon spin-orbit forces when the latter are derived from a simple quark model of nucleons in (ls) 3 spatial configurations with one-gluon-exchange and/or one-meson-exchange quark-quark interactions. Values of 0.3–0.5 fm appear to be favored. Some of the uncertainties and limitations of these estimates are briefly discussed.