The Penn group participates in an experiment called A Toriodal LHC ApparatuS (ATLAS), which will detect the particles produced by colliding protons and protons at the highest energy in the world, that is at a center-of-mass energy of 14 trillion electron volts. A primary physics goal of ATLAS is to find or exclude the Higgs boson.
The ATLAS detector is currently under construction at CERN, near Geneva Switzerland. This web-cam shows part of the ATLAS cavern 100 metres underground. The ATLAS detector is about the size of a five-story building. The Penn group has contributed significantly to one of the charged particle detectors, the Transition Radiation Tracker. The purpose of this detector is to provide information on the trajectory of charged particles ("tracking") and for the identification of the type of charged particle. The Transition Radiation Tracker has approximately 400,000 straws, each with a central fine wire at high positive electric potential and a surrounding cylindrical cathode. The straws are filled with a gas mixture containing Xenon. The passage of a high energy charged particle through the gas leaves a trail of a few ion-electron pairs per centimetre. The ionised electrons drift towards the central anode wire with constant drift velocity, and in the high electric field region very close to the central anode wire cause an avalanche of ion-electron pairs that amplifies the signal by a factor of 10,000 and creates a detectable electrical signal on the wire. The TRT provides on average 35 two-dimensional measurement points with less than 0.150 mm resolution for charged particle tracks with pseudorapidity (-ln (tan(0.5*polar angle))) below 2.5 and transverse momentum above 0.5 GeV/c. For particle identification, the passage of a high energy charged particle with small mass through repeated boundaries between different materials (air and plastic foam in between the straws) produces Transition Radiation that is absorbed well by the Xenon gas, resulting in an increased number (x100) of ion-electron pairs per centimetre and a larger signal than for the passage of a much more massive high energy charged particle. This provides discrimination between electrons (the charged particle with the smallest mass) and charged pions (270 times more massive than the electron), which improves the ability of ATLAS to identify electrons.
The Penn group has played a major role in the design, construction, and commissioning of electronics to read the electrical signals on each wire. This includes both analog electronics (ASDBLR) to discriminate and shape the analog signal from each wire, and digital electronics (DTMROC) to record the signal every 3.125ns and interface with the detector read-out. A good overview of the TRT electronics is available in this recent talk by Mitch Newcomer (pdf file of 5.6 MB!). In 2006, the instrumented barrel section of the TRT detector was lowered into the ATLAS cavern. The instrumented end-cap sections will follow in March 2007.
Extensive commissioning has already been done with cosmic rays in test-beams in 2004, and on the surface at CERN in 2005 and 2006. The next stage is to commission the TRT with cosmic rays in the ATLAS cavern, and with first proton-proton collisions in summer 2008. We will contribute to the optimal use of TRT information in the trigger, charged particle reconstruction, and electron identification.
From 2008 on, ATLAS will be collecting data at the highest energy in the world. The ultimate Higgs hunting machine will finally be in operation! We look forward to discovery of the Higgs, or something unexpected...
