Seraphwolf wrote:Also Auto, I still don't understand what your siggy is about and it's bugging me. Eta's usually used for efficiency, so is it some kind of energy acceptance thing? I could cheat but I refuse to xD.
You could cheat, but I had to go through a few presentations to find the graph I wanted so it might take a while, so I'll tell you, but you won't get any cookies...and I'll have to think of something else to have there instead lol.
Eta in this case isn't efficiency, but is pseudorapidity. In the graph, different experiments are listed, with the bottom four being the main LHC experiments. The pseudorapidity is a measurement of angular acceptance by the detector. From the formula for it, a value of 0 corresponds to an angle of 90 degrees and infinity corresponds to an angle of 0. This angle is the angle between the direction of travel of a particle and the beamline (I presume measured from the interaction point of the detector (the interaction point is where the beams crossover and collide)).
It is interesting because both ATLAS and CMS are designed as hermetic detectors which means the effectively seal the interaction point within the detector. They are interested in looking for the high energy collisions which will hopefully throw up signals of the Higgs boson and other exotic particles. In order to access the highest energy collisions possible, one needs to consider that the momenta carried by protons is actually subdivided between its constituent quarks. If both beams of protons are circulating with the same energies (which they do at the LHC) it means the highest energy collisions between two protons will happen when the interacting quarks are both carrying a large amount of that energy. Therefore, it is unlikely that a large energy collision will result in daughter particle being emitted in the forwards or backwards directions (as that would require a net gain in momenta parallel to the beam axis) and instead they will be emitted with very large angles to the beam line, ie transverse to it. This is why ATLAS and CMS do not go very far into the forward region when we examine the graph's range of pseudorapidity.
The LHCb experiment, however, is not as interested in detecting as high energy as possible particles, but is more interested in hadron decays involving b and c quarks. At the LHC b quarks are formed in pairs and due to the QCD effects (which I do not really know about) the predominant process is gg->b b-bar (where g are gluons). This process happens to produce the b quark pairs very close to the beam line in the forwards and backwards direction. Therefore to access as much of the B-physics data as possible, the LHCb has been designed as a "forward arm spectrometer" which means that it has a large acceptance for particles produced close to the beam line in the forwards direction, which is why the LHCb has a much larger pseudorapidity acceptance level in the forwards region than any of the other experiments.
Tahdah
