The G0 Experiment: Apparatus for Parity-Violating Electron Scattering Measurements at Forward and Backward Angles

In the G0 experiment, performed at Jefferson Lab, the parity-violating elastic scattering of electrons from protons and quasi-elastic scattering from deuterons is measured in order to determine the neutral weak currents of the nucleon. Asymmetries as small as 1 part per million in the scattering of a polarized electron beam are determined using a dedicated apparatus. It consists of specialized beam-monitoring and control systems, a cryogenic hydrogen (or deuterium) target, and a superconducting, toroidal magnetic spectrometer equipped with plastic scintillation and aerogel Cerenkov detectors, as well as fast readout electronics for the measurement of individual events. The overall design and performance of this experimental system is discussed.Comment: Submitted to Nuclear Instruments and Method

Q

The Qweak experiment, which took data at Jefferson Lab in the period 2010 - 2012, will precisely determine the weak charge of the proton by measuring the parity-violating asymmetry in elastic e-p scattering at 1.1 GeV using a longitudinally polarized electron beam and a liquid hydrogen target at a low momentum transfer of Q2 = 0.025 (GeV/c)2. The weak charge of the proton is predicted by the Standard Model and any significant deviation would indicate physics beyond the Standard Model. The technical challenges and experimental apparatus for measuring the weak charge of the proton will be discussed, as well as the method of extracting the weak charge of the proton. The results from a small subset of the data, that has been published, will also be presented. Furthermore an update will be given of the current status of the data analysis

Early Results from the Qweak Experiment

A subset of results from the recently completed Jefferson Lab Qweak experiment are reported. This experiment, sensitive to physics beyond the Standard Model, exploits the small parity-violating asymmetry in elastic e→p$\vec e{\rm{p}}$ scattering to provide the first determination of the proton’s weak charge Qwp$Q_w^p$. The experiment employed a 180 μA longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° < θ < 12° corresponding to Q2 = 0.025 GeV2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of e→p$\vec e{\rm{p}}$ to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin2(θw) at the level of 0.3%, and to help constrain the vector weak quark charges C1u and C1d. The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH2 target. The new constraints on C1u and C1d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron

Kinematic Evidence For Top-quark Pair Production In W Plus Multijet Events In P(p)over-bar Collisions At Root-s=1.8 Tev

We present a study of W+multijet events that compares the kinematics of the observed events with expectations from direct QCD W+jet production and from production and decay of top quark pairs. The data were collected in the 1992-93 run with the Collider Detector at Fermilab (CDF) from 19.3 pb-1 of proton-antiproton collisions at s =1.8 TeV. A W+2 jet sample and a W+3 jet sample are selected with the requirement that at least the two or three jets have energy transverse with respect to the beam axis in excess of 20 GeV. The jet energy distributions for the W+2 jet sample agree well with the predictions of direct QCD W production. From the W+3 jet events, a "signal sample" with an improved ratio of tt̄ to QCD produced W events is selected by requiring each jet to be emitted centrally in the event center of mass frame. This sample contains 14 events with unusually hard jet ET distributions not well described by expectations for jets from direct QCD W production and other background processes. Using expected jet ET distributions, a relative likelihood is defined and used to determine if an event is more consistent with the decay of tt̄ pairs, with Mtop=170 GeV/c2, than with direct QCD W production. Eight of the 14 signal sample events are found to be more consistent with top-quark than direct QCD W production, while only 1.7 such top-quark-like events are expected in the absence of tt̄. The probability that the observation is due to an upward fluctuation of the number of background events is found to be 0.8%. The robustness of the result was tested by varying the cuts defining the signal sample, and the largest probability for such a fluctuation found was 1.9%. Good agreement in the jet spectra is obtained if jet production from tt̄ pair decays is included. For those events kinematically more consistent with tt̄ we find evidence for a b-quark content in their jets to the extent expected from top quark decay, and larger than expected for background processes. For events with four or more jets, the discrepancy with the predicted jet distributions from direct QCD W production, and the associated excess of b-quark content, is more pronounced. © 1995 The American Physical Societ

Properties of jets in Z boson events from 1.8 TeV p\u304p collisions

We present a study of events with Z bosons and hadronic jets produced in p\uafp collisions at a center-of-mass energy of 1.8 TeV. The data consist of 6708 Z\u2192e+e 12 decays from 106pb 121 of integrated luminosity collected using the CDF detector at the Fermilab Tevatron Collider. The Z+ 65n jet cross sections and jet production properties have been measured for n=1 to 4. The data are compared to predictions of leading-order QCD matrix element calculations with added gluon radiation and simulated parton fragmentation

Inclusive jet cross section in p\u304p collisions at 1as = 1.8 TeV

The inclusive jet differential cross section has been measured for jet transverse energies, ET, from 15 to 440 GeV, in the pseudorapidity region 0.1 64|\u3b7| 640.7. The results are based on 19.5pb 121 of data collected by the CDF Collaboration at the Fermilab Tevatron collider. The data are compared with QCD predictions for various sets of parton distribution functions. The cross section for jets with ET>200GeV is significantly higher than current predictions based on O(\u3b13s) perturbative QCD calculations. Various possible explanations for the high- ET excess are discussed

Measurement of the \u3b3 + D*\ub1 cross section in p\u304p collisions at 1as = 1.8 TeV

We have measured the cross section of gamma + D-*+/- production in &lt;(p)over bar p&gt; collisions at root s = 1.8 TeV using the Collider Detector at Fermilab. In this kinematic region, the Compton scattering process (g(c) --&gt; gamma(c)) is expected to dominate and thus provide a direct link to the charm quark density in the proton. From the 45 +/- 18 gamma + D-*+/- candidates in a 16.4 pb(-1) data sample, we have determined the production cross section to be 0.38 +/- 0.15(stat) +/- 0.11(syst) nb for the rapidity range y(D-*+/-) &lt; 1.2 and y(gamma) &lt; 0.9, and for the transverse momentum range p(T)(D-*+/-) &gt; 6 GeV/c and 16 &lt; p(T)(gamma) &lt; 40 GeV/c. The measured cross section is compared to a theoretical prediction