This is a pile-up event, in which four separate collisions occurred (vertices at the red dots) when two bunches of LHC protons crossed each other inside ATLAS. There are about 100 billion protons in a bunch, so four collisions is not all that many--except that protons are incredibly small. In fact, most protons miss each other in a bunch crossing. Currently, there are about four million bunch crossings per second and this is being increased the LHC ramps up. Even four collisions per crossing are quite enough for now.
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LHCb was built for b-physics but this event display shows that it can do more. This event is a candidate for the decay of a Z-boson into two muons. The muon tracks are thick white lines which point to the hits in the muon system, revealed as prominent green dots.
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A muon (red track) emerges from a collision and penetrates all the way out of the detector; we use conservation of momentum to calculate the trajectory of an unseen neutrino (green arrow) that rebounds off the muon. Combining these two gives a net energy of 75.3 GeV - close to the mass of the W boson. This is a good candidate for the decay of a W into a muon and a neutrino.
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Top quarks from proton collisions are usually found in pairs: a top quark and its antimatter partner (antitop). Each of these most likely decays into a W boson and a b quark. In this top-antitop candidate from ATLAS, one W seems to have decayed into an electron and a neutrino, the other W and the b quarks into jets. The "lego plot" at the bottom right tells most of the story: four jets show up as yellow towers and the electron as a lone green tower. The neutrino is counted as "missing energy."
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The yellow cones mark a set of six "jets" from a proton-proton collision. The jets are streams of particles created by quarks and/or gluons produced in the collision through strong interactions. The energy from the colliding protons is enough to make the new particles.
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A 7 TeV collision at the point PV (Primary Vertex) results in a B meson which consists of a b quark and a anti-strange quark. The B meson decays at SV (Secondary Vertex) to a Ds meson made of a charm and the anti-strange quark along with a muon and an unseen neutrino. The Ds meson itself decays at the TV (Tertiary Vertex). This sort of interaction allows us to measure interesting properties of the Bs mesons, for example how often
they decay into Ds and a muon versus some other way.
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To make new discoveries in an accelerator, scientists can increase the energy and/or the luminosity (related to collision rate). Increased energy is pretty clear: the harder protons hit protons in the LHC, the more likely interesting things will come out. Luminosity is different. The greater the rate of collisions (luminosity), the more chances we have to create something new - and the more complicated the analysis. This CMS event has four different collisions at pretty much the same time. Count them!
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Compare this event to the one from last week. Both are candidates for the decay of a Z-boson into a muon pair (red tracks). The calculated masses are a little different but both are near the Z mass. This is not unusual; it is part of the nature of experimental particle physics. This view shows part of the ATLAS detector, including its framework and the huge toroidal magnets used to nudge the paths of the muons. This deflection gives us the data we need to calculate the mass of the Z-boson from which they came.
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Two muons (red tracks) depart from a single vertex, seen in two views. Measurement of the energies and momenta of the muons indicates that they came from the decay of a particle of mass 85.5 GeV. This is close to the mass of a Z boson; what's more, we know from previous experiments that the Z can decay into a positive and a negative muon, making this a good candidate for just such an event.
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Like many events, this ATLAS event is unclear. The two towers in the lego plot (top right) and the yellow splotches in the green ring (main image) both indicate energy deposited by particles into the electromagnetic calorimeter. Which particles? This could be a low-energy electron-positron event, such as the decay of a J/psi particle. Not all events are easily understood, which is one reason particle physics is interesting, challenging but fun.
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The two opposing bundles of thick green lines are "jets." When protons collide, their constituent quarks can be knocked out in opposite directions. The strong force between them increases in energy as they separate. According to Einstein, energy converts to matter — more quarks bind together to form hadrons. The two jets are sprays of hadrons from this process. Note the curved red track: it is likely a lone muon formed from a B-quark decay in one of the jets.
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In this ATLAS 7 TeV event display, two straight tracks in the tracker (prominent yellow lines) indicate two energetic particles coming from the decay of an object made in the proton-proton collision. These tracks end as yellow splotches where the particles deposit all their energy in the electromagnetic calorimeter (green area); these deposits are seen as yellow towers in the "lego plot" at the top right. The reconstructed mass of 89 GeV is close enough to the accepted mass of the Z boson to declare the "object" to be a good candidate for a Z, which has decayed into an electron and a positron.
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The LHCb experiment is designed to measure B mesons...but here is a W boson candidate event. Before we see it, the W decays into a muon and a neutrino. LHCb cannot detect the neutrino, but the muon is very visible: in the end view to the left, the muon shows up as a thick white track pointing downward from the center (ending in green dots that indicate "muon hits" in the muon detection system). Based on conservation of momentum, what do you think is the direction of the unseen neutrino? In the 3D view to the right, the muon track is red and the blue dots are muon hits.
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CMS stands for "Compact Muon Solenoid". The solenoid is a powerful magnet system that bends the paths of muons, particles that penetrate from the collision at the center of the detector all the way out. The red track emerging at about 10 o'clock in the large image is a muon candidate. The reddish panes that it intersects are parts of the detector activating as the muon passes through. In the frame on the lower right the same track points upward. Note the curved path, the work of the solenoid magnet. This curvature gives us a very good read on the momentum of the muon. Where do you think all those other tracks (in yellow) come from?
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ALICE is designed to study collisions of heavy ions. These collisions, which ought to begin at LHC toward the end of 2010, will give physicists the chance to briefly reproduce conditions of the early universe. In the meantime, ALICE is studying proton-proton events like this one, which shows particles emerging from the collision at low momentum; we can tell this by the curvature of their tracks in the magnetic field (the tighter the turn, the less the momentum). Also, a look at the side view on the lower right reveals that the endcap detectors are off. We can tell because we see no tracks less than about 45 degrees from the beam line, which is not shown but runs horizontally across the middle of the image.
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The LHCb experiment measures B mesons. Their decays might yield clues about what happened after the Big Bang that allowed antimatter to all but disappear and matter to build our universe. In this first reconstructed LHCb B event, the collision of two protons at the primary vertex yields many particles (in black), including a B^+ meson (in yellow), which travels a distance--you can measure it!--before it decays into other particles at the B decay vertex. The tracks have been adjusted to make the Primary vertex clearer to the observer.
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The single straight track in yellow on the right ends in a shower of energy in the electromagnetic calorimeter: this is a positron. To balance momentum, there must be an unseen partner particle--a ghostly neutrino. Its projected path is shown as a red dashed line. A positive W-boson can decay into a positron and a neutrino--and that is what the ATLAS people tell us this most likely is, a good candidate for a W decay.
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