Physics:D0 experiment

DØ under construction, the installation of the central tracking system

DØ's control room

DØ Detector with large liquid argon calorimeter

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 is the CDF experiment) located at the world's second highest-energy accelerator, the Tevatron Collider at the Fermilab in Batavia, Illinois, USA.

The research is focused on precise studies of interactions of protons and antiprotons at the highest available energies. It involves an intense search for subatomic clues that reveal the character of the building blocks of the universe.

Overview

The DØ experiment is located at one of the interaction regions, where proton and antiproton beams intersect, on the Tevatron synchrotron ring, labelled 'DØ'. It is expected to record data until the end of 2011. DØ is an international collaboration of about 550 physicists from 89 universities and national laboratories from 18 countries.

The experiment is a test of the Standard Model of particle physics. It is sensitive in a general way to the effects of high energy collisions and so is meant to be a highly model-independent probe of the theory. This is accomplished by constructing and upgrading a large volume elementary particle detector.

The detector is designed to stop as many as possible of the subatomic particles created from energy released by colliding proton/antiproton beams. The interaction region where the matter–antimatter annihilation takes place is close to the geometric center of the detector. The beam collision area is surrounded by tracking chambers in a strong magnetic field parallel to the direction of the beam(s). Outside the tracking chamber are the pre-shower detectors and the calorimeter. Muon chambers form the last layer in the detector. The whole detector is encased in concrete blocks which act as radiation shields. About 1.7 million collisions of the proton and antiproton beams are inspected every second and about 100 collisions per second are recorded for further studies.

Physics research

Higgs boson

One of the main physics goal of the DØ experiment is the search for the Higgs boson predicted by the Standard Model of particle physics. The LEP experiments at CERN have excluded the existence of such a Higgs boson with a mass smaller than 114.4 GeV/c². The combined measurements of the DØ and CDF experiments reported in January 2010 exclude a Higgs boson with a mass between 162 and 166 [[GeV/c²]].

On December 22, 2011, The DØ collaboration reported about the most stringent constraints on MSSM Higgs boson production in p-p collisions at sqrt(s)=1.96 TeV: "Upper limits on MSSM Higgs boson production are set for Higgs boson masses ranging from 90 to 300 GeV, and excludes tanβ>20-30 for Higgs boson masses below 180 GeV."

Top quark

On March 4, 2009, the DØ and CDF collaborations both announced the discovery of the production of single top quarks in proton-antiproton collisions. 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 other processes that happen at much higher rate. The observation of single top quarks is used to measure the element V_tb of the CKM matrix.

• DZero's top-quark physics group's home page

New particle

From a press release dated June 13, 2007:

Physicists of the DZero experiment at the Department of Energy's Fermi National Accelerator Laboratory have discovered a new heavy particle, the Ξ_b (pronounced "zigh sub b") baryon, with a mass of 5.774±0.019 GeV/c², approximately six times the proton mass. The newly discovered electrically charged Ξ_b baryon, also known as the "cascade b," is made of a down, a strange and a bottom quark. It is the first observed baryon formed of quarks from all three families of matter. Its discovery and the measurement of its mass provide new understanding of how the strong nuclear force acts upon the quarks, the basic building blocks of matter.

B mesons

The DØ collaboration has published results which may explain the matter-antimatter asymmetry responsible for the abundance of matter in the universe. B mesons, which oscillate between their matter and antimatter state trillions of times each second, may take longer to decay into antimatter than matter. This would eventually lead to a slightly greater abundance of matter than antimatter, explaining why some matter remains after annihilation in the early universe. Experimental results from physicists at the Large Hadron Collider, however, have suggested that "the difference from the Standard Model is insignificant."

Detector

Silicon Microstrip Tracker

The point where the beams collide is surrounded by "tracking detectors" to record the tracks (trajectories) of the high energy particles produced in the collision. The measurements closest to the collision are made using silicon detectors. These are flat wafers of silicon chip material. They give very precise information, but they are expensive, so they are concentrated closest to the beam where they do not have to cover as much area. The information from the silicon detector can be used to identify b-quarks (like the ones produced from the decay of a Higgs particle).

Central Fiber Tracker

Outside the silicon, DØ has an outer tracker made using scintillating fibers, which produce photons of light when a particle passes through. The whole tracker is immersed in a powerful magnetic field so the particle tracks are curved; from the curvature, the momentum can be deduced.

Calorimeter

Outside the tracker is a dense absorber to capture particles and measure their energies. This is called a calorimeter. It uses uranium metal bathed in liquefied argon; the uranium causes particles to interact and lose energy, and the argon detects the interactions and gives an electrical signal that can be measured.

Muon Detector

The outermost layer of the detector detects 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 don't get absorbed in the calorimeter so by putting particle detectors outside it, muons can 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 that you see when looking at DØ.

Trigger and DAQ

2.5 million proton-antiproton collisions happen every second in the detector. Because this exceeds current computing capabilities, only 20-50 events can be stored on tape per second. Therefore, an intricate Data Acquisition (DAQ) system is implemented at D0 that determines which events are "interesting" enough to be written to tape and which can be thrown out. DAQ takes place in three stages, somewhat analogous to a digital camera. The stages are set up such that the first is the fastest, but least exclusive and the third is slowest, but most exclusive. The first stage is a hardware stage and operates at 2.5 MHz. It is like the CMOS sensor in a digital camera. It detects the events and converts raw data into something useful. It then very quickly determines if the event is worth keeping and if it is, it sends it to the second stage. The second stage is both hardware- and software-based, and operates at about 1000 Hz. It further determines whether the event is "interesting". It is similar to the RAM storage in a digital camera, temporarily storing the data until it can be sent to the third stage. Finally, the third stage is entirely software based. It reads through each event to see if it is worth storing and writes those worthy to tape. It is similar to the SD card in a digital camera, writing the events to permanent storage.

External links

• The DØ Experiment