Search Results (45 total)

We report on a precision measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_PV=[-131±14(stat)±10(syst)]×10^-9, leading to the determination of the weak mixing angle sin⁡²θ_W^eff=0.2397±0.0010(stat)±0.0008(syst), evaluated at Q²=0.026  GeV². Combining this result with the measurements of sin⁡²θ_W^eff at the Z⁰ pole, the running of the weak mixing angle is observed with over 6σ significance. The measurement sets constraints on new physics effects at the TeV scale.

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_PV=[-175±30(stat)±20(syst)]×10^-9. This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron’s weak charge at low energy Q_W^e=-0.053±0.011. This is consistent with the standard model expectation at the current level of precision: sin⁡²θ_W(M_Z)_MS̅ =0.2293±0.0024(stat)±0.0016(syst)±0.0006(theory).

Measurements are reported of the proton and deuteron spin structure functions g₁^p and g₁^d at beam energies of 29.1, 16.2, and 9.7 GeV, and g₂^p and g₂^d at a beam energy of 29.1 GeV. The integrals Γ_p=∫₀¹g₁^p(x,Q²)dx and Γ_d=∫₀¹g₁^d(x,Q²)dx were evaluated at fixed Q²=3 (GeV/c)² using the full data set to yield Γ_p=0.132±0.003(stat)±0.009(syst) and Γ_d=0.047±0.003±0.006. The Q² dependence of the ratio g₁/F₁ was studied and found to be small for Q²>1 (GeV/c)². Within experimental precision the g₂ data are well described by the twist-2 contribution, g₂^WW. Twist-3 matrix elements were extracted and compared to theoretical predictions. The asymmetry A₂ was measured and found to be significantly smaller than the positivity limit sqrt[R] for both proton and deuteron targets. A₂^p is found to be positive and inconsistent with zero. Measurements of g₁ in the resonance region show strong variations with x and Q², consistent with resonant amplitudes extracted from unpolarized data. These data allow us to study the Q² dependence of the integrals Γ_p and Γ_n below the scaling region.

Measurements of inclusive electron-scattering cross sections using hydrogen and deuterium targets in the region of the Δ(1232) resonance are reported. A global fit to these new data and previous data in the resonance region is also reported for the proton. Transition form factors have been extracted from the proton cross sections for this experiment over the four-momentum transfer squared range 1.64<Q²<6.75 (GeV/c)² and from previous data over the range 2.41<Q²<9.82 (GeV/c)². The results confirm previous reports that the Δ(1232) transition form factor decreases more rapidly with Q² than expected from perturbative QCD. The ratio of σ_n/σ_p in the Δ(1232) resonance region has been extracted from the deuteron data for this experiment in the range 1.64<Q²<3.75 (GeV/c)² and for a previous experiment in the range 2.4<Q²<7.9 (GeV/c)². A study has been made of the model dependence of these results. This ratio σ_n/σ_p for Δ(1232) production is slightly less than unity, while σ_n/σ_p for the nonresonant cross sections is approximately 0.5, which is consistent with deep inelastic scattering results.

We report on a precision measurement of the neutron spin structure function g₁ⁿ using deep inelastic scattering of polarized electrons by polarized ³He. For the kinematic range 0.014<x<0.7 and 1<Q²<17 (GeV/c)², we obtain ∫0.014^0.7g₁ⁿ(x) dx  =  -0.036±0.004(stat)±0.005(syst) at an average Q²  =  5 (GeV/c)². We find relatively large negative values for g₁ⁿ at low x. The results call into question the usual Regge theory method for extrapolating to x  =  0 to find the full neutron integral ∫1g₁ⁿ(x) dx, needed for testing the quark-parton model and QCD sum rules.

We have measured the proton and deuteron spin structure functions g₁^p and g₁^d in the region of the nucleon resonances for W²<5 GeV² and Q²≃0.5 and Q²≃1.2 GeV² by inelastically scattering 9.7 GeV polarized electrons off polarized ¹⁵NH₃ and ¹⁵ND₃ targets. We observe significant structure in g₁^p in the resonance region. We have used the present results, together with the deep-inelastic data at higher W², to extract γ(Q²)≡∫1 g₁(x,Q²)dx. This is the first information on the low- Q² evolution of γ toward the Gerasimov-Drell-Hearn limit at Q²  =  0.

The neutron longitudinal and transverse asymmetries A₁ⁿ and A₂ⁿ have been extracted from deep inelastic scattering of polarized electrons by a polarized ³He target at incident energies of 19.42, 22.66, and 25.51 GeV. The measurement allows for the determination of the neutron spin structure functions g₁ⁿ(x, Q²) and g₂ⁿ(x, Q²) over the range 0.03<x<0.6 at an average Q² of 2 (GeV/c)². The data are used for the evaluation of the Ellis-Jaffe and Bjorken sum rules. The neutron spin structure function g₁ⁿ(x, Q²) is small and negative within the range of our measurement, yielding an integral ∫0.03^0.6g₁ⁿ(x)dx=-0.028±0.006 (stat)±0.006 (syst). Assuming Regge behavior at low x, we extract Γ₁ⁿ=∫0¹g₁ⁿ(x)dx=-0.031±0.006 (stat)±0.009 (syst). Combined with previous proton integral results from SLAC experiment E143, we find Γ₁^p-Γ₁ⁿ=0.160±0.015 in agreement with the Bjorken sum rule prediction Γ₁^p-Γ₁ⁿ=0.176±0.008 at a Q² value of 3 (GeV/c)² evaluated using α_s=0.32±0.05.

The inclusive A(e,e^′) cross section for x≃1 was measured on ²H, C, Fe, and Au for momentum transfers Q² from 1 to 6.8 (GeV/c)². The scaling behavior of the data was examined in the region of transition from y scaling to x scaling. Throughout this transitional region, the data exhibit ξ scaling, reminiscent of the Bloom-Gilman duality seen in free nucleon scattering. © 1996 The American Physical Society.

We have measured proton and deuteron virtual photon-nucleon asymmetries A₂^p and A₂^d and structure functions g₂^p and g₂^d over the range 0.03<x<0.8 and 1.3<Q²<10 (GeV/c)² by inelastically scattering polarized electrons off polarized ammonia targets. Results for A₂ are significantly smaller than the positivity limit sqrt[R] for both targets. Within experimental precision the g₂ data are well described by the twist-2 contribution, g₂^WW. Twist-3 matrix elements have been extracted and are compared to theoretical predictions.

In virtual Compton scattering an electron is scattered off a nucleon such that the nucleon emits a photon. We show that these events can be selected experimentally, and present the first evidence for virtual Compton scattering from the proton in data obtained at the Stanford Linear Accelerator Center. The angular and energy dependence of the data is well described by a calculation that includes the coherent sum of electron and proton radiation.

We report on a high-statistics measurement of the deuteron spin structure function g₁^d at a beam energy of 29 GeV in the kinematic range 0.029<x<0.8 and 1<Q²<10 (GeV /c)². The integral γ₁^d  =  ∫1g₁^d dx evaluated at fixed Q²  =  3 (GeV /c)² gives 0.042±0.003(stat)±0.004(syst). Combining this result with our earlier measurement of g₁^p, we find γ₁^p-γ₁ⁿ  =  0.163±0.010(stat)±0.016(syst), which agrees with the prediction of the Bjorken sum rule with O(α_s³) corrections, γ₁^p-γ₁ⁿ  =  0.171±0.008. We find the quark contribution to the proton helicity to be Δq  =  0.30±0.06.

Cross sections are presented for the reaction ²H(e,e^′p)n for momentum transfers in the range 1.2≤Q²≤6.8 (GeV/c)² and for missing momenta from 0 to 250 MeV/c. The longitudinal-transverse interference structure function has been separated at Q²  =  1.2 (GeV/c)². The observables are compared to calculations performed in nonrelativistic and relativistic frameworks. The data are best described by a fully relativistic calculation.

Measurements were performed for the photodisintegration cross section of the deuteron for photon energies from 1.6 to 2.8 GeV and center-of-mass angles from 37° to 90°. The measured energy dependence of the cross section at θ_c.m.=90° is in agreement with the constituent counting rules.

We have measured the ratio g₁^p / F₁^p over the range 0.029<x<0.8 and 1.3<Q²<10 (GeV/c)² using deep-inelastic scattering of polarized electrons from polarized ammonia. An evaluation of the integral ∫0¹g₁^p(x, Q²) dx at fixed Q²=3 (GeV/c)² yields 0.127±0.004(stat)±0.010(syst), in agreement with previous experiments, but well below the Ellis-Jaffe sum rule prediction of 0.160±0.006. In the quark-parton model, this implies Δq=0.27±0.10.

The proton elastic form factors G_Ep(Q²) and G_Mp(Q²) have been extracted for Q²=1.75 to 8.83 (GeV/c)² via a Rosenbluth separation to ep elastic cross section measurements in the angular range 13°≤θ≤90°. The Q² range covered more than doubles that of the existing data. For Q²<4 (GeV/c)², where the data overlap with previous measurements, the total uncertainties have been reduced to < 14% in G_Ep and < 1.5% in G_Mp. Results for G_Ep(Q²) are consistent with the dipole fit G_D(Q²)=(1+Q²/0.71)^-2, while those for G_Mp(Q²)/μ_pG_D(Q²) decrease smoothly from 1.05 to 0.92. Deviations from form factor scaling are observed up to 20%. The ratio Q²F₂/F₁ is observed to approach a constant value for Q²>3 (GeV/c)². Comparisons are made to vector meson dominance, dimensional scaling, QCD sum rule, diquark, and constituent quark models, none of which fully characterize all the new data.

Cross sections for deep-inelastic electron scattering from liquid deuterium, gaseous ⁴He, and solid Be, C, Al, Ca, Fe, Ag, and Au targets were measured at the Stanford Linear Accelerator Center using electrons with energies ranging from 8 to 24.5 GeV. These data cover a range in the Bjorken variable x from 0.089 to 0.8, and in momentum transfer Q² from 2 to 15 (GeV/c)². The ratios of cross sections per nucleon (σ^A / σ^d)_is for isoscalar nuclei have been extracted from the data. These ratios are greater than unity in the range 0.1<x<0.3, while for 0.3<x<0.8 they are less than unity and decrease logarithmically with atomic weight A, or linearly with average nuclear density. No Q² dependence in the ratios was observed over the kinematic range of the data. These results are compared to various theoretical predictions.

Cross sections for threshold electron scattering from the proton have been measured in the missing mass squared region M²<W²<2 (GeV)² and the four-momentum transfer squared region 6<Q²<30 (GeV/c)². Scaling of the extracted values of the structure function F₂=νW₂ is examined in the variables x, ξ, and W². The best scaling is found for the quantity Q⁶F₂, which is found to be linearly proportional to (W²-W_th²), where W_th=M+M_π.

The cross section for quasielastic ¹²C(e,e’p) scattering has been measured at momentum transfer Q²=1, 3, 5, and 6.8 (GeV/c)². The results are consistent with scattering from a single nucleon as the dominant process. The nuclear transparency is obtained and compared with theoretical calculations that incorporate color transparency effects. No significant rise of the transparency with Q² is observed.

The spin structure function of the neutron g₁ⁿ has been determined over the range 0.03<x<0.6 at an average Q² of 2 (GeV/c)² by measuring the asymmetry in deep inelastic scattering of polarized electrons from a polarized ³He target at energies between 19 and 26 GeV. The integral of the neutron spin structure function is found to be F₀¹g₁ⁿ(x)dx=-0.022±0.011. Earlier reported proton results together with the Bjorken sum rule predict F₀¹g₁ⁿ(x)dx=-0.059±0.019.

Measurements of the forward-angle differential cross section for elastic electron-proton scattering were made in the range of momentum transfer from Q²=2.9 to 31.3 (GeV/c)² using an electron beam at the Stanford Linear Accelerator Center. The data span six orders of magnitude in cross section. Combinded statistical and systematic uncertainties in the cross section measurements ranged from 3.6% at low Q² to 19% at high Q². These data have been used to extract the proton magnetic form factor G_M^p(Q²) and Dirac form factor F₁^p(Q²) by using form factor scaling. The logarithmic falloff of Q⁴F₁^p expected from leading twist predictions of perturbative quantum chromodynamics is consistent with the new data at high Q². Some nonperturbative and hybrid calculations also agree with our results.

Deuteron electrodisintegration cross sections near 180° have been measured near breakup threshold for the four-momentum transfer squared Q² range 1.21–2.76 (GeV/c)². Evidence for a change of slope in the cross section near Q²=1 (GeV/c)² has been obtained. The data are compared to nonrelativistic calculations, which predict a strong influence of meson-exchange currents. The data are also compared to a hybrid quark-hadron model. None of these calculations agrees with the data over the entire measured range of Q². The ratio of inelastic structure functions W₁(Q²,E_np)/W₂(Q²,E_np) is extracted from the present results and previous forward angle data. No prediction is in good agreement with the deduced ratios at small relative energy E_np.

Quasielastic e-d cross sections have been measured at forward and backward angles. Rosenbluth separations were done to obtain R_L and R_T at Q²=1.75, 2.50, 3.25, and 4.00 (GeV/c)². The neutron form factors G_En and G_Mn have been extracted using a nonrelativistic model. The sensitivity to deuteron wave function, relativistic corrections, and models of the inelastic background are reported. The results for G_Mn are consistent with the dipole form, while G_En is consistent with zero. Comparisons are made to theoretical models based on vector meson dominance, perturbative QCD, and QCD sum rules, as well as constituent quarks.

Cross sections for inclusive electron scattering from aluminum have been measured at both forward and backward angles in the kinematic region near x=1 and 1.75<Q²<7 (GeV/c)². The forward-angle data are in good agreement with a recent calculation. Both the data and the calculation show scaling behavior for F₂(ξ,Q²)=νW₂(ξ,Q²) at high Q² for fixed values of the Nachtmann variable ξ. The ratio R=σ_L/σ_T has been extracted for 1.75<Q²<5 (GeV/c)² and is found to decrease with Q² to a value consistent with zero at the highest Q².

We have measured inelastic electron-deuteron, electron-proton, and electron-aluminum cross sections at 10° in the kinematic region between elastic deuteron scattering and the second resonance region at six beam energies between 9.8 and 21 GeV. The elastic electron-neutron cross section was extracted from the quasielastic data at Q²=2.5,4.0,6.0,8.0, and 10.0 (GeV/c)². The ratio of elastic cross sections σ_n / σ_p falls with increasing Q² above 6 (GeV/c)². The inelastic data are compatible either with y scaling (scattering from a single nucleon) or with ξ scaling (scattering from quarks).

The proton elastic electric and magnetic form factors, G_Ep(Q²) and G_Mp(Q²), have been separately measured in the range Q²=1.75 to 8.83 (GeV/c)², more than doubling the Q² range of previous data. Scaled by the dipole fit, G_D(Q²), the results for G_Mp(Q²)/μ_pG_D(Q²) decrease smoothly from 1.05 to 0.91, while G_Ep(Q²)/G_D(Q²) is consistent with unity. Comparisons are made to QCD sum rule, diquark, constitutent quark, and vector meson dominance models, none of which agree with all of the new data. The ratio Q²F₂/F₁ approaches a constant value for Q²>3 (GeV/c)².