A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on 6.8×106 positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to ∼250 GeV, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena
A strong signal for double parton (DP) scattering is observed in a 16 pb(-1) sample of <(p)over bar p> --> gamma/pi(0) + 3 jets + X data from the CDF experiment at the Fermilab Tevatron. In DP events, two separate hard scatterings take place in a single <(p)over bar p> collision. We isolate a large sample of data (similar to 14 000 events) of which 53% are found to be DP. The process-independent parameter of double parton scattering, sigma(eff), is obtained without reference to theoretical calculations by comparing observed DP events to events with hard scatterings in separate <(p)over bar p> collisions. The result sigma(eff) = (14.5 +/- 1.7(-2.3)(+1.7)) mb represents a significant improvement over previous measurements, and is used to constrain simple models of parton spatial density. The Feynman x dependence of sigma(eff) is investigated and none is apparent. Further, no evidence is found for kinematic correlations between the two scatterings in DP events
We present a new measurement of the inclusive forward-backward tt production asymmetry and its rapidity and mass dependence. The measurements are performed with data corresponding to an integrated luminosity of 5.3 fb −1 of pp collisions at √ s = 1.96 TeV, recorded with the CDF II Detector at the Fermilab Tevatron. Significant inclusive asymmetries are observed in both the laboratory frame and the tt rest frame, and in both cases are found to be consistent with CP conservation under interchange of t andt. In the tt rest frame, the asymmetry is observed to increase with the tt rapidity difference, ∆y, and with the invariant mass M tt of the tt system. Fully corrected parton-level asymmetries are derived in two regions of each variable, and the asymmetry is found to be most significant at large ∆y and M tt . For M tt ≥ 450 GeV/c 2 , the parton-level asymmetry in the tt rest frame is A tt = 0.475 ± 0.114 compared to a next-to-leading order QCD prediction of 0.088 ± 0.013.
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