DØ experiment

DØ under construction, the installation of the central tracking system

DØ's control room

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,^[1] 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

DØ Detector with large liquid argon calorimeter

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. The 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.^[2]

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.^[3]

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

B mesons

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.

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

Proton-antiproton collisions happen inside the detector 2.5 million times every second. Not all of those events can be recorded; at most, perhaps 20 events per second can be stored on computer tape. The trigger is the system of fast electronics and computers that has to decide, in real time, whether an event is interesting enough to be worth keeping.

References

1. ^ after the Large Hadron Collider
2. ^ T. Aaltonen et al. (CDF and DØ Collaborations) (2010). "Combination of Tevatron searches for the standard model Higgs boson in the W⁺W⁻ decay mode". Physical Review Letters 104 (6). arXiv:1001.4162. Bibcode 2010PhRvL.104f1802A. doi:10.1103/PhysRevLett.104.061802. 
3. ^ V.M. Abazov et al. (DØ Collaboration) (2009). "Observation of Single Top Quark Production". Physical Review Letters 103 (9): 092001. arXiv:0903.0850. Bibcode 2009PhRvL.103i2001A. doi:10.1103/PhysRevLett.103.092001. 

External links

• The DØ Experiment