Cross Section Dσ Research Articles

Analytical Expressions for the Hard-Scattering Production of Massive Partons

We obtain explicit expressions for the two-particle differential cross section EcEKdσ(AB→cKX)/dcdk and the two-particle angular correlation function dσ(AB→cKX)/d∆ϕd∆y in the hard-scattering production of massive partons in order to exhibit the “ridge” structure on the away side in the hard-scattering process. The single-particle production cross section dσ(AB→cX)/dyccTdcT is also obtained and compared with the ALICE experimental data for charm production in pp collisions at 7 TeV at LHC.

Microscopic optical potential with two and three body forces for nucleon–nucleus scattering

The proton - nucleus optical potentials generated by folding the calculated complex, density and energy dependent g- matrices (with and without three-body forces (TBF): Urbana IX (UVIX) and TNI) over the target nucleon density distributions obtained from the relativistic mean field theory, are used for the calculation of the differential cross section dσ / dθ , polarization Ay , spin rotation function (Q). for 65 and 200 MeV polarized proton incident on 40 Ca and 208 Pb . The agreement with the experiment is rather impressive. It is found that the inclusion of TBF (Urbana IX )UVIX) and TNI) reduces the strength of the central part of the optical potential in the nuclear interior and affects the calculated spin-orbit potential only marginally and leads to an improvement in the agreement with the corresponding experimental results.

Measurement of the differential cross sectiondσ/dtin elasticpp¯scattering ats=1.96 TeV

We present a measurement of the elastic differential cross section dσ(pp¯→pp¯)/dt as a function of the four-momentum-transfer squared t. The data sample corresponds to an integrated luminosity of ≈31 nb−1 collected with the D0 detector using dedicated Tevatron pp¯ Collider operating conditions at s=1.96 TeV and covers the range 0.26<|t|<1.2 GeV2. For |t|<0.6 GeV2, dσ/dt is described by an exponential function of the form Ae−b|t| with a slope parameter b=16.86±0.10(stat)±0.20(syst) GeV−2. A change in slope is observed at |t|≈0.6 GeV2, followed by a more gradual |t| dependence with increasing values of |t|.5 MoreReceived 5 June 2012DOI:https://doi.org/10.1103/PhysRevD.86.012009© 2012 American Physical Society

Electroproduction of ρ 0 mesons on protons in quasielastic kinematics at intermediate energies and spin-flip mechanism of direct meson knockout

It is shown that the amplitude for the direct knockout of ρ 0 mesons plays an important role at energies W above the resonance region, W ≳ 2 GeV, and rather high values of Q 2 ≳ 1.5−2 GeV2/c 2 and that it corresponds to the t pole in the channel of the virtual decay p → p + π 0 accompanied by quark-spin flip upon momentum transfer to the meson, π 0 + γ* T → ρ 0. The contributions of several scalar mesons (p → p + f 0 channel), the contribution of the tensor meson f 2, and effects of the interference between different contributions were taken into account in addition to the contribution of the π 0 meson. The vectorand tensor-meson-dominance models were used to estimate the respective vertex constants. Within the proposed mechanism, the differential cross sections dσ L /dt and dσ T /dt were calculated for several W and Q 2 values at which the respective total cross sections were measured by the CLAS Collaboration. Agreement with data on the transverse part of the total cross section, σ T , was attained, whereby the assumption of a dominant role of magnetic spin transitions in the meson skin of the nucleon under conditions of quasielastic-knockout kinematics is confirmed. At the same time, the contribution of spin-flip transitions is suppressed in the longitudinal part of the cross section, σ L , and this is confirmed by the results of our calculations. The behavior of the differential cross section dσ T /dt is predicted with an eye to future experiments.

Scaling condition for multiple scattering in fractal aggregates

It is well known that under the first Born approximation, the differential cross-section dσ/ dΩ for light scattered by a fractal of dimension d f scales with wave vector Q as Q − d f . The use of this formula to interpret experiments performed at frequencies close to the resonances of the system for which multiple scattering is not negligible is questionable. We study the scaling properties of the light scattered by colloidal aggregates employing a novel multi-resolution hierarchical algorithm which allows the study of large systems taking fully into account the long range of the interactions in multiple scattering calculations. We obtain the conditions under which scaling may be present and find that even under resonant conditions, the scattering cross-section may scale with the fractal dimension d f, but only if the aggregate is larger than a dissipation and frequency-dependent length-scale L h, confirming within a full vectorial calculation a result previously found for scalar interactions. For smaller aggregates, dσ/ dΩ might scale with a different exponent or it might not scale at all, depending on the frequency of light employed. This could explain the discrepancies between experiments performed on similar systems within the strong scattering regime.

Diffractive photoproduction of J/ψ mesons with large momentum transfer at HERA

The diffractive photoproduction of J/ψ mesons is measured with the H1 detector at the ep collider HERA using an integrated luminosity of 78 pb−1. The differential cross section dσ(γp→J/ψY)/dt is studied in the range 2<|t|<30 GeV2, where t is the square of the four-momentum transferred at the proton vertex. The cross section is also presented as a function of the photon–proton centre-of-mass energy Wγp in three t intervals, spanning the range 50<Wγp<200 GeV. A fast rise of the cross section with Wγp is observed for each t range and the slope for the effective linear Pomeron trajectory is measured to be α′=−0.0135±0.0074(stat.)±0.0051(syst.) GeV−2. The measurements are compared with perturbative QCD models based on BFKL and DGLAP evolution. The data are found to be compatible with s-channel helicity conservation.

Small-angle proton elastic scattering from the neutron-rich isotopes 6He and 8He, and from 4He, at 0.7 GeV in inverse kinematics

Absolute differential cross sections for elastic p 4He, p 6He and p 8He small-angle scattering were measured in inverse kinematics with secondary 4,6,8He-beams at an energy near 0.7 GeV /u. The experiment was performed using beams from the heavy-ion synchrotron SIS and the fragment separator FRS of GSI Darmstadt. The hydrogen-filled ionization chamber IKAR served simultaneously as a gas target and a detector for the recoil protons. Projectile scattering angles were measured with multi-wire tracking detectors. For proton scattering from the neutron-rich isotopes 6He and 8He, differential elastic-scattering cross sections dσ/ dt were deduced in the range 0.002⩽|t|⩽0.05 (GeV /c) 2 of the four-momentum transfer squared t. For elastic p 4He scattering, the data obtained in the t-range 0.002⩽|t|⩽0.02 (GeV /c) 2 supplement the results from an earlier work performed in direct kinematics. From the differential cross sections the integral elastic-scattering cross sections σ el , the total cross sections σ tot , and the total reaction cross sections σ r for the proton–nucleus strong interaction were evaluated. The data obtained on dσ/ dt allow the radial matter distributions in the 6He and 8He nuclei to be determined and the corresponding root-mean-square matter radii to be deduced.

Measurement of high- Q2 charged current cross sections in e−p deep inelastic scattering at HERA

Cross sections for e − p charged current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV with an integrated luminosity of 16.4 pb −1 using the ZEUS detector at HERA. Differential cross sections dσ/ dQ 2, dσ/ dx and dσ/ dy are presented for Q 2>200 GeV 2. In addition, d 2σ/(dx dQ 2) was measured in the kinematic range 280 GeV 2<Q 2<30 000 GeV 2 and 0.015< x<0.42. The predictions of the Standard Model agree well with the measured cross sections. The mass of the W boson, determined from a fit to dσ/ dQ 2, is M W =80.3±2.1(stat.)±1.2(syst.)±1.0(PDF) GeV.

Next-to-next-to-leading logarithmic threshold resummation for deep-inelastic scattering and the Drell–Yan process

The soft-gluon resummation exponents G N in moment space are investigated for the quark coefficient functions in deep-inelastic structure functions and the quark–antiquark contribution to the Drell–Yan cross section dσ/ dM. Employing results from two- and three-loop calculations we obtain the next-to-next-to-leading logarithmic terms α s ( α s ln N) n of G N to all orders in the strong coupling constant α s . These new contributions facilitate a reliable assessment of the numerical effect and the stability of the large- N expansion.

Elastic photoproduction of J/ ψ and ϒ mesons at HERA

Cross sections for elastic photoproduction of J/ ψ and ϒ mesons are presented. For J/ ψ mesons the dependence on the photon-proton centre-of-mass energy W γp is analysed in an extended range with respect to previous measurements of 26≤W γp≤285 GeV . The measured energy dependence is parameterized as σ γp ∝ W γp δ with δ=0.83±0.07. The differential cross section dσ/ dt for J/ ψ mesons is derived, its dependence on W γp and on t is analysed and the effective trajectory (in terms of Regge theory) is determined to be α( t)=(1.27±0.05)+(0.08±0.17)· t/GeV 2. Models based on perturbative QCD and on pomeron exchange are compared to the data.

Inclusive neutral current and charged current cross sections at high Q2

The latest measurements of the HERA neutral and charged current cross sections of deep inelastic (DIS) ep scattering are presented and confronted with SM expectations. The positron proton results from the years 1994–1997 as well as preliminary electron proton measurements from the 1998–1999 period are summarized. No deviations from the SM predictions are seen in the single and double differential cross sections dσ/ dx, dσ/ dQ 2, and d 2 σ/ dxdQ 2. In the neutral current channel, the effect of the Z 0 exchange in visible for the first time in DIS ep scattering. In the charged current channel, the propagator mass of the W boson is shown to be consistent with the world average.

Production of D ∗ mesons in photon-photon collisions at √ see = 183 GeV and 189 GeV using the OPAL detector at LEP

The inclusive production of D ∗± mesons in photon-photon collisions has been measured using the OPAL detector at LEP at e +e − centre-of-mass energies √ s ee of 183 and 189 GeV. The D ∗+ mesons are reconstructed in their decay to D 0 π + with the D 0 observed in the two decay modes K − π + and K − π + π − π +. After background subtraction, 121 ± 14 (stat.) D ∗± events have been selected. Jets are reconstructed using a cone jet finding algorithm to separate direct and single-resolved events. Differential cross-sections dσ/ dp T D ∗ and dσ/ dvbη D ∗ vb as functions of the D ∗± transverse momentum p T D ∗ and pseudorapidity η D ∗ are presented in the kinematic region 2 < p T D ∗ < 12 GeV and vbη D ∗ vb < 1.5 . They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process e +e − → e +e −c c in the anti-tagged mode is measured to be σ (e +e − → e +e −c c ) = (945 ± 239) pb with a direct contribution of σ (e +e − → e +e −c c ) dir = (392 ± 117) pb and a single-resolved contribution of σ (e +e − → e +e −c c ) res = (553 ± 184) pb.

Search for contact interactions and leptoquarks at HERA

The measured single differential cross section dσ dQ 2 is compared to the Standard Model expectation and analyzed to search for ( ee) ( qq) contact interactions. No significant deviation is observed and limits on leptonquark compositeness scales and virtual leptoquark exchange are presented.

Diffractive cross sections at low Q2 from ZEUS

The diffractive cross section dσ γ ∗p→XY diff dM X in the mass range 3.0 < M X < 12.2 GeV and M Y < 6.0 GeV has been measured at low Q 2, 0.22 < Q 2 < 0.70 GeV 2, by detecting the scatterd electron with the ZEUS Beam Pipe Calorimeter. From the W dependence of the diffractive cross section, the effective intercept of Pomeron trajectory is extracted and compared with the values measured in other Q 2 ranges. It is also compared with the value obtained from the W dependence of the γ ∗p total cross section in the same Q 2 range.

The QED structure of the photon

Measurementsof the QED structure of the photon based on the reaction ee → eeγ (∗) (P 2)γ ∗ (Q 2) → eeμμ are discussed. This review is an update of the discussion of the results on the QED structure of the photon presented in Refs. [1], and covers the published measurements of the photon structure functions F 2 γ , F A γ and F B γ and of the differential cross-section dσ dx for the exchange of two virtual photons.

Disintegration of the deuteron by linearly polarized photons at low energies

The cross section and the Σ-asymmetry of the reaction d(γ→,p) were measured in the energy interval Eγ = 5–10 MeV. Cross sections dσ and dσ⊥ were derived from the measured quantities dσ0 and Σ-asymmetry. The results are compared with theoretical calculations available for this energy region.

HERA prospects on compositeness and new vector bosons

The absence of deviations from the Standard Model for the differential cross section dσ dQ 2 at HERA is used to set limits on electron quark compositeness scale and on new vector bosons, especially the hadrophilic one recently introduced as a possible explanation for LEP/SLC and CDF anomalies.

Measurement of elastic φ photoproduction at HERA

The production of φ mesons in the reaction e + p → e + φp ( gf → K + K −) at a median Q 2 of 10 −4 GeV 2 h been studied with the ZEUS detector at HERA. The differential φ photoproduction cross section dσ dt has an exponential shape and has been determined in the kinematic range 0.1 < | t| < 0.5 GeV 2 and 60 < W < 80 GeV. An integrated cross sect σ γp→ φp = 0.96±0.19 −0.18 +0.21 μb has been obtained by extrapolating to t = 0. When compared to lower energy data the results show a weak energy dependence of both σ γp→ φp and the slope of the t distribution. The φ decay angular distributions are consistent with s-channel helicity conservation. From lower energies to HERA energies, the features of φ photoproduction are compatible with those of a soft diffractive process.

Signatures from polarization observables for photon absorption on a [formula omitted] proton pair

Photoabsorption on a bound 1S 0 proton pair has been investigated with respect to possible specific signatures from polarization observables. Assuming the dominance of any two of the three lowest possible multipoles (E1,E2,M2) we obtain simple forms for the differential cross section dσ dΩ , the photon polarization asymmetry Σ and the proton polarization P y . Specifically we find distinct relations between dσ dΩ and Σ. They may be applied to test the assumption of a dominance of the two-body process in (γ, pp) reactions in nuclei.

Muon pair and vector meson cross-sections in p- W and S U collisions at 200 GeV/nucleon

Muon pair production is studied in p- W and S U collisions at 200 GeV per nucleon, as a function of transverse momentum P Tμμ . The inclusive ϱ+ ω and Φ differential cross-sections dσ dP T are measured in the dimuon decay channel, for P T ≥ 0.6 GeV/c, in the central rapidity region, 3.0≤ y ≤ 4.0. Assuming the power law A-dependence σ = σ 0( A beam· A target) α , the study of the integrated cross-sections for p- W and S U collisions leads to α ϱ+ ω = 1.00±0.02±0.07 and α Φ = 1.23±0.03±0.05, showing clear evidence of Φ enhancement in S U interactions as compared to p- W collisions.