We have studied D* production mechanisms using data from a photoproduction experiment at the Fermilab Tagged Photon Spectrometer. A large sample of charged D*’s was selected via the clean signature of the cascade decay D*→D0π+ and subsequently D0→K−π+ or D0→K−π+π0. The cross section for the process γp→(D*++anything)p at an average energy of 105 GeV was measured to be 88±32 nb. Only (11±7)% of D*’s were found to be consistent with being accompanied solely by a D¯* or a D¯; the remaining events contain additional particles. The distribution of the production angle of the D* in the photon-fragmentation-system center of mass is strongly anisotropic and consistent with the form f(θ*)=cos4θ*. We set a limit on the associated-production-process cross section σ(γp→(D¯*−+anything)Λc) x)<60 nb (90% C.L.).
Inelastic and elastic $J/\psi$ photoproduction on hydrogen are investigated at a mean energy of 105 GeV. The inelastic cross section with $E_{\psi} / E_{\gamma}$ < 0.9 is significantly lower than the corresponding result for muoproduction on iron targets, but is consistent with a second-order perturbative QCD calculation.
The elastic photoproduction cross sections for ρ and ϕ mesons from protons have been measured from 30 to 180 GeV. The energy dependences agree well with predictions made by using vector-meson dominance and an additive quark model. The ρ cross section is approximately constant with energy while the ϕ cross section rises from 0.5 to 0.7 μb with increasing energy.
Elastic ω-meson photoproduction on protons has been measured from 46 to 180 GeV. The cross section is approximately constant with photon energy and averages 1.10 ± 0.08 μb. The t dependence of the differential cross section is consistent with A exp(bt), where b=8.4±0.7 GeV−2. The photon-omega coupling constant, obtained from a normalization of hadron elastic-scattering cross sections to the photoproduction data of this experiment (with use of vector-meson dominance and an additive quark model), is γω24π=5.4±0.4.
We report the result of a brief experiment to measure the cross section for photoproduction of Jψ(3100). At a mean energy of 55 GeV we find this cross section per nucleon to be 37.5 ± 8.2 (statistical) ± 4 (systematic) nb. The result establishes the previously indicated rise in Jψ photoproduction on protons above 20 GeV and suggests that the rise has occurred by 55 GeV.
The photon total cross section on protons has been measured with high precision in the Fermilab tagged-photon beam for photon energies from 18 to 185 GeV. The cross section decreases to a broad minimum near 40 GeV, and then rises by about 4 μb over the remainder of the range. A ρ+ω+ϕ vector-dominance model (normalized to low-energy data) falls below the high-energy results by 2 to 6 μb, suggesting a contribution from charm-anticharm states.
We have measured total hadronic photoproduction cross sections on carbon, copper, and lead. Tagged-photon energies ranged from 20 to 185 GeV for copper and from 45 to 82 GeV for carbon and lead. The energy and A dependence of shadowing were computed by comparing these results to the hydrogen cross section as measured nearly simultaneously with the same apparatus. We observed somewhat more shadowing than did most experiments at lower photon energies.
A very narrow resonance with a mass of 3.105 GeV/c2 is observed in the reaction γ+Be→μ++μ−+X. The total cross section for this process, as well as its t distribution, is given.
Quasireal photons exchanged in relativistic heavy ion interactions are powerful probes of the gluonic structure of nuclei. The coherent J/$\psi$ photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon (W$^\text{Pb}_{\gamma\text{N}}$), over a wide range of 40 $\lt$ W$^\text{Pb}_{\gamma\text{N}}$$\lt$ 400 GeV. Results are obtained using data at the nucleon-nucleon center-of-mass energy of 5.02 TeV collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 1.52 nb$^{-1}$. The cross section is observed to rise rapidly at low W$^\text{Pb}_{\gamma\text{N}}$, and plateau above W$^\text{Pb}_{\gamma\text{N}}$$\approx$ 40 GeV, up to 400 GeV, a new regime of small Bjorken-$x$ ($\approx$ 6 $\times$ 10$^{-5}$) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.
We report on the study of charm baryons decaying to Λ c + : Λ c ★+ (2625) → Λ c + π + π − , Λ c ★+ (2593) → Λ c + π + π − , Σ c 0 → Λ c + π − and Σ c ++ → Λ c + π + . We present a confirmation of the state Λ c ∗+ (2593) and determine its mass difference to be M ( Λ c ★+ (2593)) − M ( Λ c + ) = 309.2 ± 0.7 ± 0.3 MeV/ c 2 . We determine the lower limit on the resonant branching ratio to be BR (Λ c ★+ (2593) → Σ c π ± Λ c ★+ (2593) → Λ c + π + π − ) > 0.51 (90% c.l.). We also measure the mass differences M ( Σ c 0 ) − M ( Λ c + ) = 166.6±0.5±0.6 MeV/ c 2 and M ( Σ c ++ ) − M ( Λ c + ) = 167.6±0.6±0.6 MeV/ c 2 . Finally, we measure the relative photoproduction cross sections for Λ c ★+ and Σ c with respect to the (inclusive) photoproduction cross section for Λ c + .
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