DØ Central Calorimeter under construction at Fermilab
The DØ Collaboration in February 1992.
DØ under construction, the installation of the central tracking system


The DØ experiment (sometimes written D0 experiment, or DZero experiment) consists of a worldwide collaboration of scientists conducting research on the fundamental nature of matter. DØ was one of two major experiments (the other was the CDF experiment) located at the Tevatron Collider at Fermilab in Batavia, Illinois, USA. The Tevatron was the world's highest-energy accelerator from 1983 until 2009, when its energy was surpassed by the Large Hadron Collider.[1] The DØ experiment stopped taking data in 2011, when the Tevatron shut down,[2] but data analysis is still ongoing. The DØ detector is preserved in Fermilab's DØ Assembly Building as part of a historical exhibit for public tours.[3]

DØ research is focused on precise studies of interactions of protons and antiprotons at the highest available energies. These collisions result in "events" containing many new particles created through the transformation of energy into mass according to the relation E=mc2. The research involves an intense search for subatomic clues that reveal the character of the building blocks of the universe.[4]

Contents

OverviewEdit

In 1981, Fermilab director Leon M. Lederman asked for preliminary proposals for a "modest detector built by a modestly sized group" that would be located at the 'DØ' interaction region in the Tevatron ring and complement the planned Collider Detector at Fermilab.[5] More than fifteen groups submitted proposals. Three of these proposals were merged into one effort under the leadership of Paul Grannis, which officially began on September 1, 1983. The group produced a design report in November 1984. The detector was completed in 1991, it was placed in the Tevatron in February 1992, and observed its first collision in May 1992.[6][7] It recorded data from 1992 until 1996, when it was shut down for major upgrades. Its second run began in 2001 and lasted until September 2011. As of 2019, data analysis is still ongoing. [8]

The DØ experiment is an international collaboration that, at its peak, included about 650 physicists from 88 universities and national laboratories from 19 countries.[9] It studied the collisions between the protons and antiprotons circulating in the Tevatron to test many aspects of the Standard Model of particle physics.

The DØ detector consisted of several nested subdetector groups surrounding the region where the beam protons and antiprotons collided. The subdetectors provided over a million channels of electronics[10] that were collected, digitized and logged for off-line analyses. About 1.7 million collisions of the proton and antiproton beams were inspected every second, and about 100 collisions per second were recorded for further studies.[11]

Physics researchEdit

DØ conducted its scientific studies within six physics groups: Higgs, Top, Electroweak, New Phenomena, QCD, and B Physics. Significant advances were made in each of them.[12]

 
DØ's control room
 
DØ Detector with large liquid argon calorimeter

Top quarkEdit

One of the early goals of the DØ experiment was to discover the top quark,[13] the last of the six constituents of matter predicted by the Standard Model of particle physics. The DØ and CDF experiments both collected data for the search, but they used different observation and analysis techniques that allowed independent confirmation of one another's findings.

On February 24, 1995, DØ and CDF submitted research papers to Physical Review Letters describing the observation of top and antitop quark pairs produced via the strong interaction.[14] On March 2, 1995, the two collaborations jointly reported the discovery of the top quark at a mass of about 175 GeV/c2 (nearly that of a gold nucleus). [15][16][17]

On March 4, 2009, the DØ and CDF collaborations both announced the discovery of the production of single top quarks via the weak interaction. This process occurs at about half the rate as the production of top quark pairs but is much more difficult to observe since it is more difficult to distinguish from background processes that can create false signals. The single top quark studies were used to measure the top quark lifetime of about 5 × 10-25 seconds, measure the last unknown element of the CKM matrix of quark inter-generational mixing, and to search for new physics beyond the Standard Model.[18]

Precision measurements of top quark properties such as mass, charge, decay modes, production characteristics, and polarization were reported in over one hundred publications.

The European Physical Society awarded the 2019 European Physical Society High Energy and Particle Physics Prize to the DØ and CDF collaborations "for the discovery of the top quark and the detailed measurement of its properties."[19]

Higgs bosonEdit

In later years, one of the main physics goals of the DØ experiment was the search for the Higgs boson, which was predicted to exist by the Standard Model, but with an unknown mass.[20] Before they concluded in 2000, the LEP experiments at CERN had ruled out the existence of such a Higgs boson with a mass smaller than 114.4 GeV/c2.[21] In 2010 DØ and CDF extended the forbidden region to include a window around 160 GeV/c2.[22]

On July 2, 2012, anticipating an announcement from CERN of the discovery of the Higgs boson, the DØ and CDF collaborations announced their evidence (at about three standard deviations) for Higgs bosons decaying into the dominant b quark final states, which indicated that the particle had a mass between 115 and 135 GeV/c2.[23] On July 4, 2012, CERN's ATLAS and CMS experiments announced their discovery of the Higgs boson with a mass of 125 GeV/c2.[24]

The techniques developed at the Tevatron for the Higgs boson searches served as a springboard for subsequent LHC analyses.[25]

W and Z bosonsEdit

The properties of the W and Z bosons that transmit the weak nuclear force are sensitive indicators of the internal consistency of the Standard Model. In 2012, DØ measured the W boson mass to a relative precision of better than 0.03%, ruling out many potential models of new physics.[26]

The DØ and CDF experiments combined to measure the forward-backward asymmetry in the decays of Z bosons (the tendency of positive decay leptons to emerge closer to the incoming proton direction more often than negative decay leptons). From these asymmetry measurements, the weak mixing angle governing the breaking of the electroweak symmetry into distinct electromagnetic and weak forces was measured to a precision of better than 0.15%. This result has comparable precision to electron positron collider experiments at CERN and SLAC and helps to resolve a long-standing disagreement between those measurements.[27]

Bottom and charm quarksEdit

Although the B-factory experiments at KEK, SLAC and IHEP in Beijing and the LHCb experiment at CERN have dominated many aspects of the study of hadrons containing b- or c-quarks, DØ has made notable contributions using large samples containing all heavy flavor hadrons that can be seen through their decays to muons.

In July 2006, the DØ collaboration published the first evidence for the transformation of the Bs meson (containing an anti-b quark and a strange quark) into its antiparticle. The transition occurs about 20 trillion times per second. If there were new particles beyond those in the Standard Model, this rate would have been modified.[28]

On May 14, 2010, the DØ collaboration announced a tendency for b and anti-b quarks produced in proton-antiproton collisions to lead to a pair of positively charged muons more frequently than a negatively charged pair.[29] This tendency, together with measurements of single muon asymmetries, could help explain the matter-antimatter asymmetry responsible for the dominance of matter in the universe.[30] Experimental results from physicists at the Large Hadron Collider, however, have suggested that "the difference from the Standard Model is insignificant."[31]

On June 12, 2007, the DØ collaboration submitted a paper to Physical Review Letters announcing the discovery of a new particle called the Ξb (pronounced "zigh sub b") with a mass of 5.774±0.019 GeV/c2, approximately six times the mass of a proton. The Ξb baryon is made of a down, a strange and a bottom quark, making it the first observed baryon formed of quarks from all three generations of matter.[32]

The original quark hypotheses by Murray Gell-Mann and George Zweig noted that exotic mesons containing two quarks and two antiquarks (instead of just a quark and antiquark) are possible. Examples were finally observed 40 years later in cases where the exotic meson contains the more distinctive heavy b- and c-quarks. DØ has contributed new understanding of these heavy flavor exotic states.[33]

Strong forceEdit

Quantum chromodynamics (QCD) is the theory of the strong interaction, in which quarks and gluons interact through a quantum property, analogous to electric charge for electromagnetism, called "color." QCD makes quantitative predictions for the production of jets (collimated sprays of particles evolved from scattered quarks or gluons), photons and W or Z bosons. A noteworthy result in 2012 from DØ was the measurement of very high energy jets produced at large scattering angles. This occurs when single quarks carry more than half of the energy of their parent proton or antiproton, despite the fact that the proton and antiproton are typically built from dozens of quarks and gluons. The measurement was in excellent agreement with the prediction. In a series of publications in which two pairs of jets or photons stemming from two independent scatterings of quarks and gluons within a single proton-antiproton encounter were observed, the pattern of these rates indicated that the spatial extent of gluons within the proton is smaller than that for quarks.[34]

DetectorEdit

The DØ detector consisted of several "sub-detectors," which were grouped into three layers or shells surrounding the collision point. The innermost shell was the Central Tracking System consisting of tracking detectors enclosed in a superconducting magnet. These were surrounded by the second shell consisting of calorimeters that measured the energy of electrons, photons, and hadrons and identified "jets" of particles arising from scattered quarks and gluons. The third shell, the muon system, had tracking chambers and scintillator panels before and after magnetized solid iron magnets to identify muons. The whole detector was encased in concrete blocks which acted as radiation shields. The detector measured about 10m × 10m × 20m and weighed about 5,500 tons. It is preserved in Fermilab's DØ Assembly Building as part of a public historical exhibit.[35]

Silicon Microstrip TrackerEdit

The Silicon Microstrip Tracker ("SMT") was installed in the detector for the Tevatron Run II collider program, which began in 2001.[36] The SMT was fully functional by April 2002.[37] The point in the detector where the Tevatron's proton and antiproton beams collided was surrounded by "tracking detectors" to record the tracks (trajectories) of the high-energy particles produced in the collision. The measurements closest to the collision were made using silicon detectors. These are flat wafers of silicon chip material. They give very precise information, but they are expensive, so they were concentrated closest to the beam where they did not have to cover as much area. The information from the silicon detector could be used to identify b-quarks (like the ones produced from the decay of a Higgs particle).[38]

Central Fiber TrackerEdit

Outside the silicon, DØ had an outer tracker made using scintillating fibers, which produce photons of light when a particle passes through them. The light generated by these fibers then propagated to Visible Light Photon Counters (VLPCs). These signals were then amplified and digitized by Analog Front End (AFE) boards. The whole tracker was immersed in a powerful magnetic field so the particle tracks were curved; from the curvature, the particle momentum could be deduced.[39]

CalorimeterEdit

The calorimeter system consisted of three sampling calorimeters (a cylindrical Central Calorimeter and two End Calorimeters), an intercryostat detector, and a preshower detector. This calorimeter system was a dense absorber outside the Central Tracking System that captured particles and measured their energies. It used uranium metal bathed in liquefied argon; the uranium caused particles to interact and lose energy, and the argon detected the interactions and gave an electrical signal that could be measured.[40]

Muon DetectorEdit

The outermost layer of the detector detected muons. Muons are unstable particles but they live long enough to leave the detector. High energy muons are quite rare and a good sign of interesting collisions. Unlike most common particles, they didn't get absorbed in the calorimeter, so by putting particle detectors outside it, muons could be identified. The muon system is very large because it has to surround all of the rest of the detector, and it is the first thing you see when looking at the DØ detector.[41]

Trigger and DAQEdit

2.5 million proton-antiproton collisions happened every second in the detector. Because this exceeded computing capabilities, only 20–50 events could be stored on tape per second. Therefore, an intricate Data Acquisition (DAQ) system was implemented at D0 that determined which events were "interesting" enough to be written to tape and which could be thrown out. DAQ took place in three stages, somewhat analogous to a digital camera. The stages were set up such that the first was the fastest, but least exclusive, and the third was slowest, but most exclusive. The first stage was a hardware stage and operated at 2.5 MHz. It was like the CMOS sensor in a digital camera. It detected the events and converted raw data into something useful. It then very quickly determined if the event was worth keeping and if it was, it sent it to the second stage. The second stage was both hardware- and software-based, and operated at about 1000 Hz. It further determined whether the event was "interesting." It was similar to the RAM storage in a digital camera, temporarily storing the data until it could be sent to the third stage. Finally, the third stage was entirely software based. It read through each event to see if it was worth storing and wrote those worthy of saving to tape. It was similar to the SD card in a digital camera, writing the events to permanent storage.[42][43]

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External linksEdit