The ATLAS (A Toroidal LHC ApparatuS) detector is one of the experiments utilizing the LHC (Large Hadron Collider) at the European Organization for Nuclear Research (CERN) in Switzerland. The ATLAS Collaboration consists of 2,000 scientists and engineers from 165 institutions in 35 countries. The detector is one of two multipurpose detectors (the other is CMS) designed to search for the Higgs and physics beyond the Standard Model. Two additional detectors (ALICE and LHCb) are designed for more specialized measurements.
A schematic of the LHC located at CERN outside Geneva, Switzerland. PS and SPS are smaller accelerators feeding particles to the LHC. The circumference of the LHC is about 27 km (16 miles). Two proton beams circulate in opposite directions and collide in the centers of the four detectors shown.
Schematic of the ATLAS detector. It is 46m long, 25m high and 25m wide with a weight of 7000 tonnes.
The main ATLAS detector components are the Inner Detector, Electron and Hadron Calorimeters and the Muon Detector. The Inner Detector consists of the Pixel and SCT systems based on silicon sensor technology, and the TRT. Its main purpose is to precisely track charged particles and measure their momenta. The Electro-Magnetic and Hadron Calorimeters measure the position and energy of electrons, photons and hadrons (such as pions, protons, and neutrons) as well as helping to identify the different particle types. The Muon Detector detects muons which are highly penetrating.
The Duke ATLAS group made a key contribution in one of the Inner Tracking components, called the Barrel TRT (Transition Radiation Tracker). The TRT effort led by Prof. S. Oh was deeply involved from the design, prototyping, construction, testing and installation, which took close to 10 years. The TRT provides two important functions. One is to track charged particles in a magnetic field and the other is to identify electrons using transition radiation. The basic detector component is a 4 mm diameter straw tube with a 30 micron diameter anode wire inside the tube similar to a Geiger Tube detector. As a charged track traverses a tube, it ionizes gas molecules. The ionized electrons drift to the sense wire where the signal is amplified and detected. For extremely relativistic particles with speeds greater than ~99.9999999% of the speed of light, they produce photons having a few keV (103 eV) energy as they traverse a fibrous material surrounding the straws. The photons are captured inside a tube (photoelectric effect) and used to identify relativistic particles. Because the electron mass is low, they become very relativistic even at relatively low momentum. This is how electrons from a few GeV (106 eV) to a few hundred GeV can be discriminated against other particles which are typically a few hundred to several thousand times heavier in mass.
A 3D view of the Inner Tracker. The Barrel TRT is the outermost component. There are three types of modules (trapezoid shape). Many straw tubes inside modules are visible, and there are about 50,000 1.5 meter long straw tubes.
art of the Duke TRT construction team. In the front, from the left, Greta Toncheva (supervisor), and two technicians, Samantha Ridgeway and Lee Ballenger. Behind, left to right, Dr. Bill Ebenstein, Mr. Jack Fowler (Engineer), Dr. ByeongRok Ko (now in Korea Univ.), Prof. Seog Oh, Dr. Chiho Wang. Missing in this photo: Dr. Doug Benjamin, Dr. Vassilis Vassilakopoulos (now in Hampton Univ.). Check http://picasaweb.google.com/chihosheu/TRT# for construction photos.
The Barrel TRT (inside the orange cage) is being inserted into the center of the ATLAS detector.
Constructing a detector is just the beginning. Good physics depends on understanding the detector. Duke Research Associates (Andrea Bocci and Esben Klinkby working with Prof. Mark Kruse) resident at CERN and students, including several undergraduate students have been working hard to calibrate and align the TRT over the last few years with test beam and cosmic ray data. With real collision data, this work is now being refined. Thanks to this effort, the TRT performance now meets the design expectation and will likely exceed it in the near future. Other members of the Duke HEP group have more recently joined this effort (Prof. Ashutosh Kotwal, Prof. Al Goshaw), and everyone has been preparing for analysis of the much anticipated collision data, including the newest member of the HEP faculty, Prof. Ayana Arce.
So, what is all the hype about ?
The Standard Model has been very successful since its inception in the early 1970s. Even with its success however, there are a few shortcomings. One difficulty is caused by the recently discovered neutrino oscillations implying neutrinos have mass. Although the Standard Model can be modified to accommodate this mass, the fix is not "elegant". Another is the so called hierarchy problem where the calculated Higgs mass becomes infinite at very high energy without extremely fine tuning. And there is no room for dark matter and dark energy. With these problems, it is a common belief that some new physics has to appear as the energy increases. We do not know what the new physics will be. There are several candidates such as SUSY (theory based on Super Symmetric particles) and extra dimensions (nature could have more than four space-time dimensions). The prospects for discovering what new physics lies beyond the Standard Model is one of the main reasons why the LHC and its experiments were constructed. With the higher center of mass energy and intensity delivered by the LHC, the particles related to whatever new physics might exist could be produced and detected.
Another goal for the experiments is to search for the Higgs particle. This is the last undiscovered Standard Model particle, and is a manifestation of the mechanism that provides mass to other particles. There has not been a particle like this. There are particles which make up the universe, like quarks, electrons and neutrinos, and there are particles which mediate the forces of nature, such as the W and Z particles, photons, and gluons. The Higgs particle is a product of the so-called symmetry breaking in the electroweak sector of the Standard Model. If the Higgs exists and is discovered, it would be the final triumph of the Standard Model.
Since its inauguration on March 30, 2010, the LHC has been performing well. In a matter of two weeks, we have accumulated well over 10 million events. However because the beam intensity has been low, it is unlikely that new physics will be discovered from these events. CERN's plan is to increase the beam intensity by the order of 100,000 this year such that rare events can be produced and recorded. The experiments will take data continuously until the end of 2011. With the high beam intensity, we hope to see new physics as well as discover the Higgs particle, or whatever counterpart it has in theories beyond the Standard Model. We believe that high energy physics is on the threshold of many exciting discoveries that could revolutionize the way we understand the universe. So….stay tuned, the best is yet to come
First pp → W → eV (electron+ neutrino) candidate. This is a cross sectional view of the inner detector. Proton beams are traveling perpendicular with respect to the page in opposite directions and collide at the center. The dots are hits registered in detectors and lines are reconstructed tracks from hits. The yellow line is the electron candidate. The long dashed red line is the expected neutrino direction which does not leave hits in the detector.