Browsing by Subject "Measurement"

The mass of the $W$ boson is one of the most important parameters in the

Standard Model. A precise measurement of the $W$ boson mass, together

with a precise measurement of the top quark mass, can constrain the

mass of the undiscovered Higgs boson within the Standard Model

framework or give a hint for physics beyond the Standard Model.

This dissertation describes a measurement of the $W$ boson mass

through its decay into a muon and a neutrino using

$\approx$ 2.2 fb$^{-1}$ of $\sqrt{s} = 1.96$ TeV $p\bar{p}$ data taken

with the CDF II detector at Fermilab. We measure the $W$ boson mass

to be ($80.374 \pm 0.015_{\rm stat.} \pm 0.016_{\rm syst.}$)

GeV/c$^2$. This result, when combined with the $W$ mass

measurement in the electron channel, leads to the single most

precise $m_W$ value and greatly constrains the possible mass

range of the undiscovered Higgs boson.

The three papers in this dissertation seek to measure more accurately critical concepts in the field of American political institutions so as to inform and enable theory-building. The first paper addresses committee prestige in the U.S. House of Representatives, arguing that the seniority of members transferring off of standing committees reveals important information about the relative prestige of those committees. In response, I import a measure called PageRank which enables me to exploit information on seniority while measuring committee prestige. I then demonstrate that the prestige of a legislator's committee portfolio predicts the political action committee (PAC) contributions she receives for the next campaign cycle. A second paper tackles the theoretical possibility that majority parties may manipulate their control over committee assignments for partisan goals, countering a vast literature which has generally failed to find evidence of partisan manipulation across stage legislatures. I argue that several theoretical and practical constraints render universal stacking impractical and introduce a new measure of partisan stacking called "seats above expectation" (SAE); I find little evidence of universal partisan (or ideological) stacking in state legislatures. I then argue that majority parties should selectively stack committees under two circumstances: (1) when the operations of committees affect the electoral prospects of all legislators, generating so-called "uniform electoral externalities;" and (2) when committees in a polarized setting are endowed with gatekeeping rights. Leveraging SAE, evidence from the states confirms these selective stacking hypotheses. The final paper examines political polarization, offering a more behavioral conception and a network-based measure, called modularity, applicable to collegial courts and legislatures alike. After demonstrating validity, I then measure polarization at the U.S. Supreme Court and in both houses of Congress using opinion-joining and cosponsorship networks. My primary contributions here are to argue that polarization need not be equated with partisan polarization and to develop a measure which permits such a distinction and decomposition. Indeed, I find that while polarization is tantamount to partisan polarization in the present-day Congress, as recently as four decades ago partisanship accounted for just over half of measured polarization.

A trapped Fermi gas near a collisional resonance provides a unique laboratory for testing many-body theories in a variety of fields. The ultracold Fermi gas produced in our lab is comprised of the lowest two spin states of $^6$Li. At 834 G there is a collisional or Feshbach resonance between the two spin states. The scattering length between trapped atoms of opposing spins far exceeds the interparticle spacing of the gas. On resonance, a strongly interacting, unitary, Fermi gas is created which exhibits universal behavior. The unitary Fermi gas is a prototype for other exotic systems in nature from nuclear matter to neutron stars and high temperature superconductors.

For magnetic fields less than 834 G the scattering length is positive, and pairs Fermi atoms can form molecular dimers. These dimers, comprised of two fermions, are bosons. At ultracold temperatures the molecular bosons populate the lowest energy level and form a Bose Einstein Condensate (BEC). For magnetic fields greater than 834G the scattering length between fermions in opposing spin states is negative, like Cooper pairs formed between electrons in a superconductor. The Bardeen, Cooper, and Shriefer (BCS) theory was developed to describe the pairing effect in the context of superconductors. In our experiment we produce an ultracold unitary gas. By tuning the magnetic field to either side of the Feshbach resonance we can transform the gas into a weakly interacting BEC or BCS superfluid. Therefore, the region near a Feshbach resonance is called the BEC-BCS crossover.

This dissertation presents a precision measurement of the hydrodynamic sound velocity in an ultracold Fermi gas near a Feshbach resonance. The sound velocity is measured at various magnetic fields both above and below resonance. Moreover, we are able compare our measurements to theoretical descriptions of hydrodynamic sound propagation. Further, our measurement of sound velocity exactly reproduces the non-perturbative case, eliminating the need to consider nonlinear effects. At resonance the sound velocity exhibits universal scaling with the Fermi velocity to within 1.8\% over a factor of 30 in density. In a near zero temperature unitary gas the average sound velocity at the axial center was measured, $c(0)/v_F$ = 0.364(0.005), as well as the universal constant, $\beta$ = -0.565(0.015). The measurement of sound velocity in an ultracold gas throughout the BEC-BCS crossover provides further evidence of the continuous connection between the physics of the BEC, unitary, and BCS systems.