Session Y08: Electroweak Physics, and Lepton Non-Universality

Generation of magnetic field in a first-order electroweak phase transition

We study the generation of magnetic field during a first-order electroweak phase transition by evolving the classical equations of motion. The bubble production is done by a simple random nucleation scheme, controlled by a nucleation probability $p_b$, related to the quantum tunneling rate. The Higgs orientations of different bubbles are randomly and uniformly distributed on a 3-sphere. A damping term is introduced to damp the physical Higgs degree of freedom. We find magnetic field is generated during the phase transition, the strength of which is related to $p_b$ and the strength of the Higgs damping. Furthermore, we discuss the factors controlling the shape of the spectrum of the generated magnetic field.   

Measurement of the forward-backward asymmetry of high mass Drell-Yan lepton pairs at 13 TeV

A measurement of the forward-backward asymmetry ($A_{FB}$) of lepton pairs (dielectron and dimuon) produced via the Drell-Yan process in pp collisions at $\sqrt{s} = 13$ TeV is presented. The data sample corresponds to an integrated luminosity of 36 fb$^{-1}$ collected with the CMS detector at the LHC during 2016. The measurement of $A_{FB}$ is performed for dilepton masses above 150 GeV. Rather than traditional counting methods, the $A_{FB}$ is measured by fitting the dilepton angular distribution to reweigthed Monte Carlo samples. The $A_{FB}$ measurements as a function of dilepton mass are compared with the Standard Model predictions.   

Preliminary Results from the AlCap Experiment

Observation of neutrinoless muon-to-electron conversion in the presence of a nucleus would be unambiguous evidence of physics Beyond the Standard Model. Two experiments, COMET at J-PARC and Mu2e at Fermilab, will search for this process in the next few years. These experiments will provide upper-limits on this branching ratio up to 10,000 times better than previously published. COMET/Mu2e developed a joint venture, the AlCap Experiment, to measure particle emission spectra from muonic interactions in a number of materials. As a major source of background hits in COMET/Mu2e detectors, AlCap measured the charged particle and neutron spectra following nuclear capture on the candidate target materials aluminum and titanium. Additionally, COMET/Mu2e are exploring schemes for determining the number of muon stops via AlCap’s measurement of the photon spectra following both atomic and nuclear capture. In late 2015, AlCap collected data in its third run at the Paul Scherrer Institut in Switzerland. Preliminary results will be presented of the proton emission spectrum.   

A Search for CPT-Violation in Ortho-Positronium

We present the preliminary results from a search for $CPT$-violating correlations in the decay of ortho-positronium (o-Ps) with CALIOPE, or CP(T) Aberrant Leptons in o-Ps Experiment. Using a tagged source flush against a cylindrical piece of aerogel, we generate positronium at the center of an annular array of 24 NaI(Tl) bars. PMTs are optically coupled to each end of a bar. Signals from the PMT feed into the DAQ, which uses QDCs and TDCs to record the charge amplitude of pulses and timing information. We isolate o-Ps events with various analysis cuts such as a timing cut and a cut on the number of bars hit. We also include a study of systematics resulting from detailed Monte Carlo simulations.   

Update on PEN: A Precision Measurement of $\pi\rightarrow $e$\nu (\gamma)$ Branching Ratio

The PEN collaboration performed a precision measurement of the $\pi^+\rightarrow \text{e}\nu_\text{e}(\gamma)$ branching ratio at the Paul Scherrer Institute with the goal of obtaining a relative uncertainty of $5×10^{−4}$ or better. Measurement of the branching ratio $\Gamma(\pi\rightarrow e\bar{\nu}(\gamma))/\Gamma(\pi\rightarrow \mu \bar{\nu}(\gamma))$ provides the most sensitive test of lepton universality. Deviations from Standard Model predictions would signify ``new'' or non V−A interactions, physics not currently in the Standard Model. The PEN detector consists of active target and beam counters, a mini time projection chamber, two cylindrical multi-wire proportional chambers, a plastic scintillating hodoscope, and a spherical 240-module pure CsI electromagnetic calorimeter. Discrimination between the main background and signal is facilitated by using predicted timings and energies from multiple detector elements. Proper accounting for decays in flight, detector efficiencies, and detector response, specifically the low energy tail response of the CsI calorimeter, is required for branching ratio extraction. We will update the status of the PEN project.   

Probing New Physics with Experiment E36 at J-PARC

The Standard Model (SM) has far reaching success in describing the building blocks of matter, nonetheless it is incomplete: falling short to explain dark matter, baryogenesis, neutrino masses and much more. Fueled by the LHCb measurements of $R(D)$ and $R(D^*)$ mesons, lepton non-universality has become the flagship for beyond SM searches. The TREK/E36 (E36) experiment conducted at J-PARC, Japan aims to test lepton universality in the $R_{K} = \Gamma(K_{e2})/\Gamma(K_{\mu2})$ ratio, utilizing an active fiber target to stop a beam of up to 1.2 million $K^+$ per spill. $K^+$ decay products were detected with MWPCs and a large-acceptance toroidal spectrometer capable of analyzing charged particles with high resolution, combined with a CsI(Tl) photon calorimeter having a solid angle covering $75\%$ of $4\pi$ and particle identification systems. Since the SM the ratio of leptonic $K^{+}$ decays is very precise, any observed deviation from the SM prediction would provide evidence of New Physics beyond the SM. Additionally, the E36 detector apparatus allows us to search for light $U(1)$ gauge bosons and sterile neutrinos below 300 MeV/$c^2$, which could be associated with dark matter or explain muon-related anomalies. The status of the data analysis will be presented.   

Constraining New Physics in Lepton Flavor Universality

Searching for new physics beyond the Standard Model at high energy colliders requires an enormous investment in resources and time. Frequently these searches are applicable to test a broader range of theoretical models than they were originally designed to. The reinterpretation of experimental results imparts a broader impact to a given search while not requiring the further reprocessing of data, nor the further investment of resources. Our research presents the implementation of a newly developed software framework for reinterpretation, ATOM, the Automated Testing of Models. ATOM was utilized to reinterpret Z', W' and leptoquarks searches from the ATLAS experiment. Because of recent anomalies in lepton flavor universality measurements in heavy flavor decays, an evaluation of theoretical models that seek to account for these anomalies is of interest to the particle physics community. ATOM was used to evaluate viable models that endeavor to explain these heavy flavor anomalies by testing their relative correlation with experimental data from ATLAS.   

Constraining anomalous gauge boson couplings in $e^+e^-\to W^+W^-$ using polarization asymmetries with polarized beams

We study the anomalous $W^+W^-V$ ($V=\gamma,Z$) couplings in $e^+e^-\to W^+W^-$ using the complete set of polarization observables of $W$ boson with longitudinally polarized beams. We use most general Lorentz invariant form factors parametrization as well as $SU(2)\times U(1)$ invariant dimension $6$ effective operators for the effective $W^+W^-V$ couplings. We estimate simultaneous limits on the anomalous couplings in both the parametrization using cross section, forward backward asymmetry and polarization observables of $W$ boson with different kinematical cuts using Markov-Chain--Monte-Carlo (MCMC) method for an $e^+e^-$ collider running at centre of mass energy of $\sqrt{s}=500$ GeV and ${\cal L}=100$ fb$^{-1}$. The best limits on form factors are obtained to be $1 \sim 5 \times 10^{-2}$ for $e^-$ and $e^+$ polarization being $(+0.4,-0.4)$. For operator's coefficients, the best limits are obtained to be $1\sim 16$ TeV$^{-2}$.