Violation Experiment Research Articles

The beam and detector for the NA48 neutral kaon CP violation experiment at CERN

The beam and detector, used for the NA48 experiment, devoted to the measurement of Re ( ε ′ / ε ) , and for the NA48/1 experiment on rare K S and neutral hyperon decays, are described.

A luminosity monitor for the A4 parity violation experiment at MAMI

A water Cherenkov luminosity monitor system with associated electronics has been developed for the A4 parity violation experiment at MAMI. The detector system measures the luminosity of the hydrogen target hit by the MAMI electron beam and monitors the stability of the liquid hydrogen target. Both are required for the precise study of the count rate asymmetries in the scattering of longitudinally polarized electrons on unpolarized protons. Any helicity correlated fluctuation of the target density leads to false asymmetries. The performance of the luminosity monitor, investigated in about 2000 h with electron beam, and the results of its application in the A4 experiment are presented.

A high power liquid hydrogen target for the Mainz A4 parity violation experiment

We present a new powerful liquid hydrogen target developed for the precise study of parity violating electron scattering on hydrogen and deuterium. This target has been designed to have minimal target density fluctuations under the heat load of a 20 μ A CW 854.3 MeV electron beam without rastering the electron beam. The target cell has a wide aperture for scattered electrons and is axially symmetric around the beam axis. The construction is optimized to intensify heat exchange by a transverse turbulent mixing in the hydrogen stream, which is directed along the electron beam. The target is constructed as a closed loop circulating system cooled by a helium refrigerator. It is operated by a tangential mechanical pump with an optional natural convection mode. The cooling system supports removal of up to 250 W of beam heating removal. Deeply subcooled liquid hydrogen is used to keep the in-beam temperature below the boiling point. The target density fluctuations are found to be at the level of 10 - 3 at a beam current of 20 μ A .

VELO vertexing and tracking algorithms of the LHCb trigger system

LHCb is a dedicated CP violation experiment situated at the Large Hadron Collider (LHC) at CERN. It is designed to detect and reconstruct events in which B hadrons are produced. Its silicon vertex detector, the Vertex Locator (VELO), will provide precise topological information for use in LHCb's second level trigger with the aim of selecting events with high sensitivity to CP violation parameters while reducing the data rates from 40 MHz to 40 kHz. Therefore the VELO must provide fast, robust and precise vertexing and tracking for the LHCb trigger system and is a crucial component of the experiment's trigger strategy. The second level trigger vertexing and tracking algorithms are presented in the context of the LHCb trigger strategy, and results presented in terms of efficiency, background suppression and timing.

The cryogenic target for the [formula omitted] experiment at Jefferson lab

A cryogenic horizontal single loop target has been designed, built, tested and operated for the G 0 experiment in Hall C at Jefferson Lab. The target cell is 20 cm long, the loop volume is 6.5 l and the target operates with the cryogenic pump fully immersed in the fluid. The target has been designed to operate at 30 Hz rotational pump speed with either liquid hydrogen or liquid deuterium. The high-power heat exchanger is able to remove 1000 W of heat from the liquid hydrogen, while the nominal electron beam with current of 40 μ A and energy of 3 GeV deposits about 320 W of heat into the liquid. The increase in the systematic uncertainty due to the liquid hydrogen target is negligible on the scale of a parity violation experiment. The global normalized yield reduction for 40 μ A beam is about 1.5% and the target density fluctuations contribute less than 238 ppm (parts per million) to the total asymmetry width, typically about 1200 ppm, in a Q 2 bin.

A Ramsey's method with pulsed neutrons for a T-violation experiment

A Ramsey’s method with pulsed neutrons is discussed for neutron spin manipulation in a time reversal (T) symmetry violation experiment. The neutron spin (sn) is aligned to the direction of a vector product of the nuclear spin (I) and the neutron momentum (kn) for the measurement of a T-odd correlation term, which is represented as sn · (kn × I), during propagation through a polarized nuclear target. The phase control and amplitude modulation of separated oscillatory fields are discussed for the measurement of the T-odd correlation term.

The Mainz A4 parity violation experiment (single spin asymmetries in elastic scattering)

The A4 experiment at the MAMI facility in Mainz measures a parity violating single spin asymmetry in the scattering of longitudinally polarized electrons off unpolarized protons. The longitudinal single spin asymmetry is parity violating and arises from the interference of the weak and electromagnetic one-boson exchange amplitude. It is sensitive to the strangeness contribution to the electromagnetic form factors of the nucleon. Recently the transverse single spin asymmetry has gained much interest. It is purely electromagnetic in origin and arises from the interference of the one-photon with the two-photon exchange amplitude. It is a time-odd quantity and sensitive to the imaginary part of the two-photon amplitude only. The A4 collaboration has measured both asymmetries at two different momentum transfer values. The physical background, the experimental apparatus, the experimental results and the implications are discussed.

The transmission Compton polarimeter of the A4 experiment

A transmission Compton polarimeter as a simultaneous, relative monitor for the longitudinal polarisation of a stabilised, polarised electron beam (polarisation degree ∼ 80 %) has been constructed in a new design. It is located in the vacuum between the target and the beam dump of the A4 parity violation experiment at MAMI. The analysing power is ∼ 80 ppm at 854 MeV and ∼ 115 ppm at 570 MeV. The measurement precision for the polarimeter asymmetry (which is proportional to the longitudinal beam polarisation degree) is ∼ 3 ppm within 5 min. at 854 MeV and ∼ 7 ppm within 5 min. at 570 MeV and the systematical error is about 0.5 ppm + 1 – 2 %. This simple polarimeter consists mainly of two graphite scatterers, a water-cooled Samarium-Cobalt permanent magnet, an aluminum secondary electron emission monitor, an understructure and electronics.

YO!—a time-of-arrival receiver for removal of helicity-correlated beam effects

A parity violation experiment, G 0, at Jefferson Lab is sensitive to arrival time differences, at the target, of electron beams in the two helicity states. Instead of the Jefferson Lab standard 499 MHz beam structure, G 0 uses a 31.1875 MHz structure where only 1 out of 16 microbunches contains electrons. Photon counters triggered by time-of-arrival at the target mandate that timing must be independent of delays associated with different orbits taken by the electrons in two helicity states. Corrections to the parity violating asymmetries due to any arrival time differences require the generation of a clean 31.1875 MHz trigger signal and phase matching this signal to the beam's arrival at the target. The time of arrival receiver, named the YO! receiver, has 10 kHz output bandwidth which is sufficiently larger than the settling time (500 μs) of the ≈30 Hz helicity flip. This enables the correction of each helicity bin for any orbit-induced timing inequalities. The device combines conventional receiver and DSP techniques for maximum sensitivity, bandwidth and flexibility and eliminates the 2 π/ n phase shifts associated with frequency dividers by means of a sampling phase detection scheme. This paper describes the performance of this device during bench testing, commissioning and in data taking phase of the experiment.

Measurement of strange-quark contributions to the nucleon's form factors at Q(2) = 0.230 (GeV/c)(2).

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.

Cylindrical symmetry discrimination of magnetoelectric optical systematic effects in a pump-probe atomic parity violation experiment

A pump-probe atomic parity violation (APV) experiment performed in a longitudinal electric field, provides a signal breaking mirror symmetry while preserving the cylindrical symmetry of the set-up. The excited vapor acts on the probe beam as a linear dichroic amplifier, imprinting a very specific signature on the detected signal. Our differential polarimeter is oriented to yield a null result unless the excited atoms are endowed with a chirality of some kind. Ideally, only the APV (E-odd) and the calibration (E-even) signals contribute to a chiral atomic response, a situation highly favourable to the detection of a tiny effect. In the present work we give a thorough analysis of possible unwanted defects like stray transverse fields or misalignments which would spoil the ideal set-up and lead to chiral systematics. A possible way to suppress such effects is to perform global rotations of the experiment by incremental steps of 45 degrees, leaving both stray fields and misalignments unaltered. The conspiration of at least two defects is necessary to affect the E-odd polarimeter signal. The transverse nature of the defects manifests itself by an azimuthal cosine square modulation. The harmful systematics are those which survive the averaging over four successive configurations. They require the presence of a stray transverse electric field, which can be determined and eventually minimized by auxiliary measurements of the systematic effects, amplified by applying a known external magnetic field. Transverse stray magnetic fields must be compensated by a similar procedure. We also propose statistical correlation tests as diagnoses of the aforementioned systematic effects.

G 0 experiment status

The G 0 project is a parity violation experiment dedicated to the measurement of the proton weak and axial form factors by means of elastic electron scattering. Combining these weak form factors with the known electromagnetic ones makes possible the extraction of the contribution of strange quarks to the charge and magnetization distributions in the nucleon. After introducing the physics case, this paper describes the G 0 apparatus and the measurements planned. An engineering run of this experiment took place recently ; first results are presented.

The LHCb VErtex LOcator

The dedicated CP violation experiment at the Large Hadron Collider (LHC), LHCb, will be equipped with a novel silicon detector (Vertex Locator—VELO). The VELO will provide precise measurements of tracks from displaced b-vertices and will allow to trigger on them. The entire detector will be housed in a mobile secondary vacuum system, and after the injection and stabilisation of the beams each fill, the silicon detectors will move inwards and approach to within 7 mm of the beams. In order to fulfill the trigger requirements, the VELO must combine in an unprecedented way the use of high resolution silicon detectors and large CPU farms. The extreme, non-uniform radiation environment puts additional constraints on the sensor design. The design of the VELO is described, along with the R&D of the silicon sensors and its production status.

SLAC's polarized electron source laser system and minimization of electron beam helicity correlations for the E-158 parity violation experiment

SLAC E-158 is an experiment designed to make the first measurement of parity violation in M øller scattering. E-158 will measure the right-left cross-section asymmetry, A LR M øller , in the elastic scattering of a 45-GeV polarized electron beam from unpolarized electrons in a liquid hydrogen target. E-158 plans to measure the expected Standard Model asymmetry of ∼10 −7 to an accuracy of better than 10 −8. To make this measurement, the photoemission-based polarized electron source requires an intense circularly polarized laser beam and the ability to quickly switch between right- and left-helicity polarization states with minimal right-left helicity-correlated asymmetries in the resulting beam parameters (intensity, position, angle, spot size, and energy), beam A LR's. This laser beam is produced by a unique SLAC-designed flashlamp-pumped Ti:Sapphire laser and is directed through a carefully designed set of polarization optics. We analyze the transport of nearly circularly polarized light through the optical system and identify several mechanisms that generate beam A LR's. We show that the dominant effects depend linearly on particular polarization phase shifts in the optical system. We present the laser system design and a discussion of the suppression and control of beam A LR's. We also present results on beam performance from engineering and physics runs for E-158.

Measurements and simulations of Cherenkov light in lead fluoride crystals

The anticipated use of more than 1000 lead fluoride (PbF 2) crystals as a fast and compact Cherenkov calorimeter material in a parity violation experiment at MAMI stimulated the investigation of the light yield (LY) of these crystals. The number of photoelectrons (p.e.) per MeV deposited energy has been determined with a hybrid photomultiplier tube (HPMT). In response to radioactive sources a LY between 1.7 and 1.9 p.e./MeV was measured with 4% statistical and 5% systematic error. The LY optimization with appropriate wrappings and couplings was investigated by means of the HPMT. Furthermore, a fast Monte Carlo simulation based on the GEANT code was employed to calculate the characteristics of Cherenkov light in the PbF 2 crystals. The computing time was reduced by a factor of 50 compared to the regular photon tracking method by implementing detection probabilities as a three-dimensional look-up table. For a single crystal a LY of 2.1 p.e./MeV was calculated. The corresponding detector response to electrons between 10 and 1000 MeV was highly linear with a variation smaller than 1%.

New measurement of parity violation in elastic electron–proton scattering and implications for strange form factors

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The result is A = -15.05 +- 0.98(stat) +- 0.56(syst) ppm at the kinematic point theta_lab = 12.3 degrees and Q^2 = 0.477 (GeV/c)^2. The measurement implies that the value for the strange form factor (G_E^s + 0.392 G_M^s) = 0.025 +- 0.020 +- 0.014, where the first error is experimental and the second arises from the uncertainties in electromagnetic form factors. This measurement is the first fixed-target parity violation experiment that used either a `strained' GaAs photocathode to produce highly polarized electrons or a Compton polarimeter to continuously monitor the electron beam polarization.

Implementation of a sapphire cell with external electrodes for laser excitation of a forbidden atomic transition in a pulsed E-field

We demonstrate the production of an electric field inside a high temperature cesium vapor cell with external electrodes. This external control of the electric field, which is not possible with a glass cell in presence of a cesium vapor, is achieved using a cell made of sapphire, and is of particular interest for our ongoing Parity Violation experiment. We describe the main components and the implementation on the set-up, including the pulsed high voltage generator. With pulse duration not exceeding 200 ns the system provides a reversible longitudinal E-field of up to 2 kV/cm in the vapor at a density of ∼ 2×1014 at/cm3 without discharge. Atomic signals attest the application of the electric field in the cell, with the predicted value. Further improvements obtained with sapphire cells are also presented.

NUCLEON-NUCLEON PARITY VIOLATION EXPERIMENTS

Measurements of parity-violating longitudinal analyzing powers Az (normalized asymmetries) in polarized proton-proton scattering and in polarized neutron capture on the proton (n-p→d - γ) provide a unique window on the interplay between the weak and strong interactions between and within hadrons. Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future: at TRIUMF at 221 MeV and 450 MeV and at COSY (Forschungszentrum Jülich) in the multi-GeV range. A new measurement of the parity-violating γ ray asymmetry with a ten-fold improvement in the accuracy over previous measurements is being developed at LANSCE. These experiments are intended to provide stringent constraints on the set of six effective weak meson-nucleon coupling constants, which characterize the weak interaction between hadrons in the energy domain where meson exchange models provide an appropriate description. The 221 MeV p - p experiment is unique in that it selects a single transition amplitude (3P2-1D2) and consequently constrains the weak meson-nucleon coupling constant [Formula: see text]. The n-p→d-γ experiment is mainly sensitive to the weak pion-nucleon coupling constant fπ. Together with the existing p-p parity violation experimental results one may be able to delineate the various weak meson-nucleon coupling constants. The TRIUMF 221 MeV p-p parity violation experiment will be described in some detail. Other parity violation nucleon-nucleon and nucleon-very-light-nucleus experiments are commented on. The anomalous result obtained at 6 GeV/c on a water target requires that a new multi-GeV p-p parity violation experiment be performed.

The NA48 data acquisition system

NA48 is a CP violation experiment that is starting its period of data taking at the CERN SPS accelerator. The expected average data rate of a few thousand events/s (with the possibility to go up to 10 kHz), a data volume up to 100 MB/s sustained over the beam burst, and the necessity to handle different subsystems with very wide variation in the number of channels have lead to the design of a modular pipelined data acquisition system with a sampling rate of 25 ns.

Phenomenological implications of the topflavour model

We explore phenomenologies of the topflavour model for the LEP experiment at $m_{_Z}$ scale and the atomic parity violation (APV) experiment in the $C_s$ atoms at low energies. Implications of the model on the $Z$ peak data are studied in terms of the precision variables $\epsilon_i$'s. We find that the LEP data give more stringent constraints on the model parameters than the APV data.