32 research outputs found
Mass splitting of the pseudoscalar and vector mesons induced by the homogeneous vacuum gluon field
No full textProduction of the Charmonium States and in Proton Nucleus Interactions at GeV.
No full textA measurement of the ratio R χ c = ( χ c → J / ψ + γ ) / J / ψ in p C , p Ti , and p W interactions at 920 GeV / c ( √ s = 41.6 GeV ) in the Feynman-x range − 0.35 < x J / ψ F < 0.15 is presented. Both μ + μ − and e + e − J / ψ decay channels are observed with an overall statistics of about 15 000 χ c events, which is by far the largest available sample in pA collisions. The result is R χ c = 0.188 ± 0.013 st + 0.024 − 0.022 sys averaged over the different materials, when no J / ψ and χ c polarizations are considered. The χ c 1 to χ c 2 production ratio R 12 = R χ c 1 / R χ c 2 is measured to be 1.02 ± 0.40 , leading to a cross section ratio σ ( χ c 1 ) σ ( χ c 2 ) = 0.57 ± 0.23 . The dependence of R χ c on the Feynman-x of the J / ψ , x J / ψ F , and its transverse momentum, p J / ψ T , is studied, as well as its dependence on the atomic number, A, of the target. For the first time, an extensive study of possible biases on R χ c and R 12 due to the dependence of acceptance on the polarization states of J / ψ and χ c is performed. By varying the polarization parameter, λ obs , of all produced J / ψ 's by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible χ c 1 and χ c 2 polarizations, it is shown that R χ c could change by a factor between 1.02 and 1.21 and R 12 by a factor between 0.89 and 1.16
Production in p+A Collisions at = 41.6 GeV.
No full textInclusive doubly differential cross sections d 2 σ pA /dx F dp T 2 as a function of Feynman-x (x F ) and transverse momentum (p T ) for the production of K S 0 , Λ and Λ¯ in proton-nucleus interactions at 920 GeV are presented. The measurements were performed by HERA-B in the negative x F range (−0.12<x F <0.0) and for transverse momenta up to p T =1.6 GeV/c. Results for three target materials: carbon, titanium and tungsten are given. The ratios of production cross sections are presented and discussed. The Cronin effect is clearly observed for all three V 0 species. The atomic number dependence is parameterized as σ pA =σ pN ⋅A α where σ pN is the proton-nucleon cross section. The measured values of α are all near one. The results are compared with EPOS 1.67 and PYTHIA 6.3. EPOS reproduces the data to within ≈20% except at very low transverse momentum
Angular Distributions of Leptons from J/psi's Produced in 920 GeV Fixed-Target Proton-Nucleus Collisions
No full textA study of the angular distributions of leptons from decays of J/ψ"s produced in p-C and p-W collisions at s√=41.6~GeV has been performed in the J/ψ Feynman-x region −0.341 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises
and Meson Production in Proton-Nucleus Interactions at ~GeV.
No full textThe inclusive production cross sections of the strange vector mesons K*0, K̄*0, and φ have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT = 3.5 GeV/c. The atomic number dependence is parametrised as σpA=σpN*Aα, where σpN is the proton–nucleon cross section. Within the phase space accessible, α(K*0)=0.86±0.03, α(K̄*0)=0.87±0.03, and α(φ)=0.96±0.02. The total proton–nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are σpN→K*0=(5.06±0.54) mb, σpN→K̄*0=(4.02±0.45) mb, and σpN→φ=(1.17±0.11) mb. For all resonances the Cronin effect is observed; compared to the measurements of Cronin et al. for K± mesons, the measured values of α for φ mesons coincide with those of K+ mesons for all transverse momenta, while the enhancement for K*0/K̄*0 mesons is smaller
Inclusive V 0 production cross sections from 920 GeV fixed target proton-nucleus collisions
No full textInclusive differential cross sections dσpA/dxF and dσpA/dp2t
for the production of K0S, Λ, and Λ particles are measured at HERA in proton-induced reactions on C, Al, Ti, and W targets. The incident beam energy is 920 GeV, corresponding to √s = 41.6 GeV in the proton-nucleon system. The ratios of differential cross sections dσpA(K0 S)/dσpA(Λ) and dσpA( ¯ Λ)/dσpA(Λ) are measured to be 6.2 ± 0.5 and 0.66 ± 0.07, respectively, for xF ≈−0.06. No significant dependence upon the target material is observed.
Within errors, the slopes of the transverse momentum distributions dσpA/dp2t also show no significant
dependence upon the target material. The dependence of the extrapolated total cross sections σpA on the atomic mass A of the target material is discussed, and the deduced cross sections per nucleon σpN are compared with results obtained at other energies
Measurement of the Upsilon production cross section in 920 GeV fixed-target proton-nucleus collisions
No full textThe cross section ratio RJ/ψ=Br(Υ→l+l−)dσ(Υ)/dy|y=0/σ(J/ψ) has been measured with the HERA-B spectrometer in fixed-target proton–nucleus collisions at 920 GeV proton beam energy corresponding to a proton–nucleon c.m.s. energy of √s=41.6 GeV. The combined results for the decay channels Υ→e+e− and Υ→μ+μ− yield a ratio RJ/ψ=(9.0±2.1)×10−6. The corresponding Υ production cross section per nucleon at mid-rapidity (y=0) has been determined to be Br(Υ→l+l−)dσ(Υ)/dy|y=0=4.5±1.1 pb/nucleon
Limits for the central production of Θ+ and Ξ-- pentaquarks in 920-GeV pA collisions
No full textWe have searched for Θ+(1540) and Ξ--(1862) pentaquark candidates in proton-inducedreactions on C, Ti, and W targets at midrapidity and s√=41.6 GeV. In 2×108 inelastic eventswe find no evidence for narrow (σ≈5 MeV) signals in the Θ+→pK0S and Ξ--→Ξ-π- channels; our 95% C.L. upper limits (UL) forthe inclusive production cross section times branching fraction B dσ/dy $y ≈0 are (4-16) μb/N for a Θ+ mass between 1521 and 1555 MeV,and 2.5μb/N for the Ξ--. The UL of the yield ratio of Θ+/Λ(1520)<(3-12)% is significantly lower than model predictions.Our UL of B Ξ--/Ξ(1530)0<4% is at variance with the results that have provided the first evidencefor the Ξ--
Kinematic Distributions and Nuclear Effects of Production in 920 GeV Fixed-Target Proton-Nucleus Collisions
No full textMeasurements of the kinematic distributions of mesons produced in C, Ti and W collisions at in the Feynman- region -0.34 < x_{F} < 0.14 and for transverse momentum up to are presented. The and dependencies of the nuclear suppression parameter, , are also given. The results are based on mesons in both the and decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of for the first time. The average value of in the measured region is . The data suggest that the strong nuclear suppression of production previously observed at high turns into an enhancement at negative
