Mark Williams

The forward muon scintillation counters formed a crucial part of the DZero detector in the Tevatron Run II era (2001-2011). Their characteristic overlapping plates earned them the “fish scales” nickname. Their operation and performance are described in detail in a recent publication. Disponible en español The DZero experiment, like all such general-purpose particle detectors, is built from a number of distinct subsystems, each designed to perform a specific task. One such component is the muon system, which forms the outermost…

A new publication by the DZero collaboration measures the production of pairs of J/ψ mesons. There are two possible mechanisms for this process, depending on the number of parton interactions in the proton-antiproton collision. The contributions from single- and double-parton interactions are separated by examining the angular separation of the two mesons, which is quite different for the two cases. Disponible en español Most of the time, a proton-antiproton “collision” is actually the interaction of one parton (quark or gluon)…

A new publication by the DZero collaboration measures the production of both single- and double-b quarks in conjunction with a photon, probing the gluon and b quark content of the proton. Among the findings is the first observation that pairs of b quarks are more likely to be produced at higher energies, as predicted by the theory of quantum chromodynamics. Disponible en español We traditionally consider the proton as comprising three quarks, two up and one down. In fact, we…

Disponible en espaƱol For some time, we’ve known that the apparently fundamental forces of electromagnetism and the weak interaction are actually just the low-energy manifestations of a single unified electroweak interaction. This beautiful concept was developed by theorists in the 1960s and has been confirmed resoundingly by years of rigorous experimentation, including the discovery of the W, Z and Higgs bosons. This theory has only a handful of parameters; measuring them with the highest possible precision is crucial to test…

The DZero experiment this week released the single most precise measurement of the top quark mass ever made. This plot shows the favored region for the top mass (the x-axis) and an important experimental parameter kJES (the y-axis) which is extracted simultaneously to improve the overall precision. Disponible en español Just four weeks ago in this column we featured the world combination of top quark mass measurements, which used as ingredients the set of best measurements from both Tevatron and…

The Standard Model predicts that the forward-backward asymmetry of top-antitop events changes depending on the mass of the top-antitop system. For alternative models, this dependence can be quite different. A recent DZero paper on the lepton-plus-jets channel finds good agreement between the data (the points in the above plot) and various calculations of Standard Model predictions (the horizontal lines). Disponible en español Of the hundreds of publications released by the Tevatron experiments in recent years, few have provoked as much…

This week saw a historic collaboration between scientists from CERN and Fermilab, as the world combination of the top quark mass was reported, using inputs from the ATLAS, CDF, CMS and DZero collaborations. Disponible en español The top quark has only ever been produced and detected in two places on Earth: at Fermilab, where the DZero and CDF collaborations first discovered this most massive particle in 1995, and at CERN, where the Large Hadron Collider generates so many top quarks…

Identifying the specific types of particles in the DZero detector is absolutely crucial for performing measurements and searches. Different particles have characteristic signatures when interacting with the detector material, which allow them to be differentiated, as described in two recent DZero papers. Disponible en español What does an electron look like in the DZero detector? Or what about a bottom quark? Or even a photon? Answering such questions is a crucial part of any particle physics measurement. For a given…

Protons and antiprotons are composite objects, formed from a constantly changing mixture of quarks and gluons (partons). In a small fraction of collisions there can be more than one parton-parton interaction. Measuring the rate of such double interactions provides important information about the structure of the proton. Disponible en español What happens when protons and antiprotons collide? To answer this question, we first need to consider what we mean by a particle collision. The proton is not fundamental: It contains…

This analysis selects double top quark events that decay into four quark jets, an electron or muon, and a neutrino. After collecting a pure signal sample (the red part of the plot), the production properties of these events are extracted, including the cross section versus the top-antitop mass. Disponible en español It is now almost 20 years since the top quark was discovered in 1995 through the joint efforts of the CDF and DZero collaborations. As more data have been…