Session X08: Top Physics, and Lepton Interactions

Top pair production, spin correlations and polarized gluon distributions

Top-antitop pairs are produced prolifically at the LHC, primarily by gluon fusion. The spin dependences of gluon distributions leave an imprint on the spin correlations of the top pairs. Decays of top pairs through dilepton, single lepton and pure jet channels produce a variety of correlations among the decay products - particles and jets. Combinations of the gluon distributions, either polarized or unpolarized, can be accessed experimentally through angular dependences of decay products.   

Measurement of the cross section of top quark pairs in association with a photon in lepton+jets events at sqrt(s) = 13 TeV

The production cross section of a top quark pair plus a radiated photon is measured during proton-proton collisions at the centre of mass energy of 13 TeV corresponding to an integrated luminosity of 35.86 fb$^{-1}$ at the LHC, at CERN. The data was recorded by the Compact Muon Solenoid experiment. The signal region is defined by top quark pairs, an isolated lepton, photon, jets from the hadronization of quarks, and missing transverse energy. The photons may be emitted directly from initial state radiation, top quarks as well as from its decay products. An important part of the analysis is calculation of photon purity and photon identification efficiency, which are done using data-driven methods and MC simulation.   

Design and performance of the calorimeter system for the Muon g-2 experiment at Fermilab

The Muon g-2 experiment at Fermilab is now running. The goal is to achieve a 140 ppb measurement of the muon's anomalous magnetic moment, $a_\mu$. This will require a dataset 21 times larger than the one used by the Brookhaven experiment E821, whose measurement of $a_\mu$ exceeds the standard model prediction by more than 3 standard deviations. Central to realizing the new goal is development of a calorimeter system able to handle high data rates, maintain 0.04\% gain stability, resolve pulses separated by 5 ns or more, and operate near the 1.45 T storage region without perturbing the field. Our design employs a suite of 24 calorimeters, each comprising 54 lead fluoride Čerenkov crystals read out individually by large-area silicon photomultipliers (SiPMs). In this talk I will discuss the final design of the calorimeters and their performance in the first year of data taking, emphasizing the gain stability and energy calibration. SiPM gains are monitored and correction algorithms are developed based on an extensive laser system. Absolute energy calibration is achieved by matching the end-point of the decay positron energy spectra and by fitting the minimum ionizing particle peaks. Finally, I will present examples of the precession frequency acquired using the calorimeters.   

Beam measurements using the Straw Tracking Detectors of the Fermilab Muon g-2 Experiment

The Fermilab Muon g-2 experiment will measure the anomalous magnetic moment of the muon to a precision of 140 parts per billion, which is a factor of four improvement over the previous E821 measurement at Brookhaven. The experiment will also extend the search for the muon's electric dipole moment (EDM) by approximately two orders of magnitude with a sensitivity down to 10$^{-21}$ e.cm. Both of these measurements are made by an analysis of the modulation of the decay rate of the higher-energy positrons from the (anti-)muon decays recorded by 24 calorimeters and 2 straw tracking detectors. The straw tracking detectors will be used to cross-calibrate the calorimeter, identify pileup and muons lost from the storage region, and to measure the beam-profile. A tracker measurement of the up-down modulation of positrons will be used in the EDM analysis. The performance of the tracking detectors in beam-tests, simulation and the first data from the g-2 experiment will be described and the expected performance in the physics data-taking in 2018/19 will be presented, particularly in the context of the experiment's sensitivity to a muon EDM.   

Electric field effects on the muon anomalous precession frequency in the Fermilab Muon g-2 Experiment

The muon anomalous magnetic moment (g-2) has played an important role in constraining physics beyond the Standard Model for many years. The Fermilab Muon g-2 Experiment has a goal to measure it to unprecedented precision: 0.14 ppm. To achieve this goal, we need to understand the systematic uncertainties associated with beam dynamics. We will present a study of the electric field correction to the muon anomalous precession frequency based on fast rotation analysis, which uses the evolution of beam bunch structure to determine the muon momentum distribution.   

Search for Higgs boson decays to beyond-the-Standard-Model light bosons in four-lepton events with the ATLAS detector at $\sqrt{s}$ = 13 TeV

A search is conducted for a new beyond-the-Standard-Model boson using events where a Higgs boson with mass 125 GeV decays to four leptons (l = e or $\mu$). This decay is presumed to occur via an intermediate state which contains one or two on-shell, promptly decaying bosons: H $\rightarrow$ ZX/XX $\rightarrow$ 4l, where X is a new vector boson Zd or pseudoscalar a with mass between 1 and 60 GeV. The search uses proton-proton collision data collected with the ATLAS detector at the LHC during 2015 and 2016, with an integrated luminosity of 36.1 $fb^{-1}$ at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. No significant excess of events above Standard Model background predictions was observed; therefore, upper limits at 95 \% confidence level are set on model-independent fiducial cross-sections, and on the Higgs boson decay branching ratios to vector and pseudoscalar bosons in two benchmark models.   

Branching Fraction for Z decays to four leptons and constraints on new physics

The LHC experiments have measured the branching fraction for Z decays to four leptons (electrons or muons). We have combined these measurements for a precise comparison to the standard model value. This comparison leads to an upper limit on anomalous contributions to this decay, and limits on new physics.   

Search for new physics in the four lepton final state in CMS

Many recent studies by BaBar, LHCb, Belle, ATLAS and CMS have published intriguing signs of new physics physics known as ``B anomalies''. We are interested here in the anomaly related to the process b -\textgreater sll. Many flavour changing neutral currents involve the existence of new vector bosons often named Z'. On the other hand, a link between the Higgs sector and the dark matter represents an interesting scenario to be probed at the LHC. Such models predict the existence of so-called dark photons that can decay into two leptons. We explore the possible implications of these two sets of models in the four lepton final state. This topology constitutes a very clean signature. The analysis techniques will be developed with the estimation of the irreducible and reducible background contributions. The pairing algorithms will be presented as well as the statistical procedure to put limits on the models.