Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session B4: Electroweak Interactions |
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Sponsoring Units: DNP Chair: Xiaochao Zheng, University of Virginia Room: Holiday 3 |
Saturday, April 11, 2015 10:45AM - 10:57AM |
B4.00001: Measurement of Transverse Single-Spin Asymmetries from Elastic Electron Scattering on $^{27}$Al and $^{12}$C Kurtis Bartlett The Qweak experiment will determine the weak charge of the proton through a measurement of the parity-violating electron-proton helicity asymmetry by scattering electrons with longitudinal polarization from a liquid hydrogen target. During the running of the experiment additional data was recorded from $^{27}$Al and $^{12}$C targets for the purpose of determining background contributions. A small subsection of this aluminum and carbon data were taken with transversely polarized electrons for determining the effect of any remaining small transverse polarization component contributing to the parity-violating longitudinal asymmetry in the main experiment. This transverse aluminum data would lead to the first determination of the beam-normal single-spin asymmetry (BNSSA) on $^{27}$Al, which would help understand an observed anomaly of a near zero BNSSA for $^{208}$Pb by the PREX experiment [S. Abrahamyan, et al., Phys. Rev. Lett. 109(2012)192501]. I will present the status of our effort toward extracting the BNSSA from Qweak's $^{27}$Al and $^{12}$C data as well as the possible impact it will have on the current explanation of the $^{208}$Pb result from the PREX experiment. [Preview Abstract] |
Saturday, April 11, 2015 10:57AM - 11:09AM |
B4.00002: Analysis status of the Q-weak experiment Emmanouil Kargiantoulakis The Q-weak Collaboration completed a challenging measurement of the parity-violating asymmetry in elastic electron-proton scattering at the Thomas Jefferson National Accelerator Facility (JLab). The initial result reported here is extracted from the commissioning part of the experiment, constituting about 4\% of the full data set, and allowed for the first determination of the weak charge of the proton. The analysis of the full Q-weak data set is ongoing and expected to yield a high precision value for the weak charge, which will place significant constraints to models of physics beyond the standard model. The current status will be presented, with emphasis on analysis efforts to reduce the most important systematic uncertainties of the preliminary result. [Preview Abstract] |
Saturday, April 11, 2015 11:09AM - 11:21AM |
B4.00003: 1 Higgs search result in the tau-pair decay mode with ATLAS Xin Chen The search for the fermionic decay modes of Higgs boson is being extensively pursued at LHC, especially in the tau-tau mode. This decay mode is one of the most sensitive channels for a SM Higgs, as well as for the MSSM Higgs signature. The direct observation of the fermionic mode is a confirmation of the Higgs mechanism that also gives mass to fermions. We review the recent search result of SM H-\textgreater tau tau in different tau decay sub-channels with the LHC Run 1 data collected by ATLAS at $\backslash $sqrt\textbraceleft s\textbraceright $=$7 and 8 TeV p-p collisions, with focus on the analysis strategy, background estimation methods and result interpretation. We also give the challenges and outlook of Run 2 data analysis in the tau pair decay mode. [Preview Abstract] |
Saturday, April 11, 2015 11:21AM - 11:33AM |
B4.00004: Vector and Axial Vector Pion Form Factors Michael Vitz Radiative pion decay $\pi^+\rightarrow e^+\nu\gamma$ (RPD) provides critical input to chiral perturbation theory ($\chi$PT). Aside from the uninteresting ``inner bremsstrahlung'' contribution from QED, the RPD rate contains ``structure dependent'' terms given by $F_V$ and $F_A$, the vector and axial-vector pion form factors, respectively. The two appear in the decay rate in combinations $F_V-F_A$ and $F_V + F_A$, i.e., in the so-called $SD^-$ and $SD^+$ terms, respectively. The latter has been measured to high precision by the PIBETA collaboration.\footnote{M. Bychkov et al, Phys. Rev. Lett., \textbf{103}, 051802 (2009), http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.103.051802} We report on the analysis of new data, measured by the PEN collaboration\footnote{http://pen.phys.virginia.edu} in runs between 2008 and 2010 at the Paul Scherrer Institute, Switzerland. We particularly focus on the possibility of improvement in the determination of the $SD^-$ term. Precise determinations of $F_V$ and $F_A$ test the validity of the CVC hypothesis, provide numerical input for the $l_9+l_{10}$ terms in the $\chi$PT lagrangian, and constrain potential non-$(V-A)$ terms, such as a possible tensor term $F_T$. [Preview Abstract] |
Saturday, April 11, 2015 11:33AM - 11:45AM |
B4.00005: Realistic Monte Carlo Simulation of PEN Apparatus Charles Glaser The PEN collaboration undertook to measure the $\pi^+\rightarrow e^+\nu_e(\gamma)$ branching ratio with a relative uncertainty of $5\times10^{-4}$ or less at the Paul Scherrer Institute. This observable is highly susceptible to small non $V-A$ contributions, i.e, non-Standard Model physics. The detector system included a beam counter, mini TPC for beam tracking, an active degrader and stopping target, MWPCs and a plastic scintillator hodoscope for particle tracking and identification, and a spherical CsI EM calorimeter. GEANT 4 Monte Carlo simulation is integral to the analysis as it is used to generate fully realistic events for all pion and muon decay channels. The simulated events are constructed so as to match the pion beam profiles, divergence, and momentum distribution. Ensuring the placement of individual detector components at the sub-millimeter level and proper construction of active target waveforms and associated noise, enables us to more fully understand temporal and geometrical acceptances as well as energy, time, and positional resolutions and calibrations in the detector system. This ultimately leads to reliable discrimination of background events, thereby improving cut based or multivariate branching ratio extraction. [Preview Abstract] |
Saturday, April 11, 2015 11:45AM - 11:57AM |
B4.00006: Constraints on the electroweak tensor interaction from exclusive and semi-inclusive hadron production Simonetta Liuti, Aurore Courtoy, Martin Gonzalez-Alonso We evaluate the impact of recent developments in hadron phenomenology on constraining the electroweak effective theory Lagrangian beyond the standard model. We focus, in particular, on the scalar and tensor components which can be measured in precision neutron beta decay. We show how a class of new observables, the chiral-odd generalized parton distributions, along with the extraction of the transversity structure function from dihadron electroproduction, can provide for the first time experimental constraints on the tensor charge. Direct experimental extractions if sufficiently precise, provide a more stringent constraint than lattice QCD calculations. [Preview Abstract] |
Saturday, April 11, 2015 11:57AM - 12:09PM |
B4.00007: The Muon Capture Rate on the Deuteron in the MuSun Experiment Michael Murray Basic few-body nuclear systems are increasingly understood in terms of QCD-based effective field theories (EFTs) that establish rigorous relations between muon capture and processes such as p-p fusion and neutrino breakup of the deuteron. Experimentally, the muon capture rate on the deuteron tests this modern EFT description and determines the single, poorly-known low-energy constant appearing in the two-nucleon sector. The MuSun experiment will determine the capture rate via a precise measurement of the lifetime of negative muons stopped in a cryogenic deuterium TPC. Muon tracking must be done in a way that is independent of the decay time, but charged particles near the muon stop location such as the decay electron and products of muon-catalyzed fusion can lead to erroneous tracking. This talk will present an overview of the current status of the MuSun experiment and discuss the analysis of recent data. [Preview Abstract] |
Saturday, April 11, 2015 12:09PM - 12:21PM |
B4.00008: Measurement of Fusion Neutrons and Capture Neutrons From Muon Stops in Deuterium for MuSun R. Kreswell Neely The MuSun experiment at PSI will determine the $\mu^-$ capture rate in ultra-pure deuterium gas, $\Lambda_d$, to a precision of 1.5\% by measuring the time distribution of decay electrons. MuSun uses an array of eight liquid scintillator detectors to detect neutrons. Muons in deuterium produce neutrons by two processes: muon capture, $\mu + d \rightarrow n + n + \nu_\mu$, and muon catalyzed fusion, $dd \mu \rightarrow {}^{3}He + n + \mu$. Furthermore, neutrons from high-Z captures indicate the occurrence of muon stops in either gas impurities or chamber walls and may be used as a quality check. I will discuss particle identification and discrimination, as well as what may be inferred about $\mu d$ atomic kinetic parameters. [Preview Abstract] |
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