The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.
The production of J/ψ mesons in proton-proton collisions at √ s = 7 TeV is studied with the LHCb detector at the LHC. The differential cross-section for prompt J/ψ production is measured as a function of the J/ψ transverse momentum p T and rapidity y in the fiducial region p T ∈ [0; 14] GeV/c and y ∈ [2.0; 4 cross-section and fraction of J/ψ from b-hadron decays are also measured in the same p T and y ranges. The analysis is based on a data sample corresponding to an integrated luminosity of 5.2 pb −1 . The measured cross-sections integrated over the fiducial region are 10.52 ± 0.04 ± 1.40−2.20 µb for prompt J/ψ production and 1.14 ± 0.01 ± 0.16 µb for J/ψ from b-hadron decays, where the first uncertainty is statistical and the second systematic. The prompt J/ψ production cross-section is obtained assuming no J/ψ polarisation and the third error indicates the acceptance uncertainty due to this assumption.
We report on a new measurement of the neutron β-asymmetry parameter A with the instrument Perkeo II. Main advancements are the high neutron polarization of P = 99.7(1)% from a novel arrangement of super mirror polarizers and reduced background from improvements in beam line and shielding. Leading corrections were thus reduced by a factor of 4, pushing them below the level of statistical error and resulting in a significant reduction of systematic uncertainty compared to our previous experiments. From the result A0 = −0.11996(58), we derive the ratio of the axial-vector to the vector coupling constant λ = gA/gV = −1.2767(16).PACS numbers: 12.15. Ji,13.30.Ce,14.20.Dh,23.40.Bw The Standard Model of weak V − A interactions describes the β − decay of the free neutron n → p + e + ν e implementing the following parameters: The vector coupling constant g V is defined by the product, where g w is the electroweak coupling constant and M W is the W -boson mass, and the matrix element V ud of the quark mixing CabbiboKobayashi-Maskawa (CKM) matrix. The axial current is renormalized by the strong interaction at low energy. λ = g A /g V is defined as the ratio of the axial vector and vector coupling constants. λ is real, if the weak interaction is invariant under time reversal. Searches for time reversal violation can be found in [1,2].λ, V ud , and neutron's lifetime τ are interconnected by the following equation,whereR is the phase space factor [3,4] (including the model independent radiative correction) adjusted for the current value of the neutron-proton transition energy. ∆ R [5] is the model dependent radiative correction to the neutron decay rate. Thus λ serves as input for a determination of either the CKM matrix-element V ud or the lifetime τ . The Standard Model requests that the CKM matrix is unitary, a condition which is experimentally tested at the 10 −4 level for the first row [6], and unitary tests are sensitive tools for searches for physics beyond the Standard Model. Previous determinations of V ud and V us raised questions about the unitarity of the CKM-matrix as discussed in [7][8][9]. Refs. [10][11][12] list several other motivations for a determination of λ and searches for new symmetry concepts in neutron beta decay. In principle, the ratio λ can be determined from QCD lattice gauge theory calculations, but the results of the best calculations vary by up to 30% [10]. The most precise experimental determination is from the β-asymmetry in neutron decay but previous experimental results are not consistent within their uncertainties [13].In neutron decay, the probability that an electron is emitted with angle ϑ with respect to the neutron spin polarization P = σ z is [14]where v is the electron velocity. A is the parity violating β-asymmetry parameter which depends on λ. Accounting for order 1% corrections for weak magnetism A µm , g V − g A interference, and nucleon recoil, A in Eq. (2) reads [3]with total electron energy W = E e /m e c 2 + 1 (endpoint W 0 ). The coefficients A µm , A 1 , A 2 , A 3 are fr...
A new measurement of the neutrino asymmetry parameter B in neutron decay, the angular correlation between neutron spin and antineutrino momentum, is presented. The result, B=0.9802(50), confirms earlier measurements but features considerably smaller corrections. It agrees with the standard model expectation and permits updated tests on "new physics" in neutron decay.
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