We present a precision measurement of the axial-vector coupling constant gA in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain λ = gA/gV = −1.27641(45)stat(33)sys which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of A0 = −0.11985(17)stat(12)sys. We discuss implications on the CKM matrix element V ud and derive a limit on left-handed tensor interaction.
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...
We describe the beam characteristics of the first ballistic supermirror neutron guide H113 that feeds the neutron user facility for particle physics PF1B of the Institute Laue-Langevin, Grenoble (ILL). At present, the neutron capture flux density of H113 at its 20×6cm 2 exit window is Φ C =1.35⋅10 10 cm −2 s −1 , and will soon be raised to above 2⋅10 10 cm −2 s −1 . Beam divergence is no larger than beam divergence from a conventional Ni coated guide. A model is developed that permits rapid calculation of beam profiles and absolute event rates from such a beam. We propose a procedure that permits intercomparability of the main features of beams emitted from ballistic or conventional neutron guides.
We present a case study on a new type of cold neutron beam station for the investigation of angular correlations in the beta-decay of free neutrons. With this beam station, called PERC, the 'active decay volume' lies inside the neutron guide, and the charged neutron decay products are magnetically guided towards the end of the neutron guide. Hence, the guide delivers at its exit a beam of decay electrons and protons, under well-defined and precisely variable conditions, which can be well separated from the cold neutron beam. In this way a general-purpose source of neutron decay products is obtained which can be used for various different experiments in neutron decay correlation spectroscopy. A gain in phase space density of several orders of magnitude can be achieved with PERC, as compared to existing neutron decay spectrometers. Neutron beam related background is separately measurable in PERC, and magnetic mirror effects on the charged neutron decay products and edge effects in the active neutron beam volume are both strongly suppressed. Therefore the spectra and angular distributions of the emerging decay particles will be distortion-free on the level of 10^-4, more than 10 times better than achieved today.Comment: 20 pages, 6 figure
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