This is a typical candidate event in which a possible Higgs decays into two photons. The two high-energy photons (depicted by red towers) are measured in the CMS electromagnetic calorimeter. The yellow lines are the measured tracks of other particles produced in the collision. Events like this are part of the data set that, as it grows, will help physicists to discover or to rule out the Standard Model Higgs.
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Collisions of lead ions at the LHC have resumed and will continue until the winter shut shutdown. This lead ion collision event is from the ALICE detector. A 3-D view is on the left. On the right is an end-on view where only 1/5 of the volume of the detector is projected (to be able to see single particles). Why are there so many tracks? The super hot quark-gluon plasmas created in these high-energy density collisions produce multitudes of particles.
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The LHCb investigations of strange B mesons (B-zero-s) have yielded more and important results. Measurements of the difference in B-zero-s and anti-B-zero-s rates of decay (in this case to J/Psi and Phi particles) give a result that is slightly more positive, favoring matter over antimatter, than the Standard Model prediction. The graph above shows the LHCb result for this asymmetry (phi-s) plotted versus the difference in width difference between matter and antimatter signals. Abstract stuff: but two items stand out. First, compared to the CDF and D0 results from Fermilab the average is pretty much smack on the Standard Model. Second, the experimental errors are still large and more precision in the LHCb measurements will eventually tells us if the Standard Model stands up or there is New Physics in these decays.
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Upsilon particles come in three distinct "states" which depend on how tightly the quarks and antiquarks that make them are bound. In the lead collision program last autumn, upsilons were produced in a super-hot quark-gluon plasma. Less-bound states were often torn apart by the high energy, reducing the number detected in CMS. Physicists recently presented this phenomenon of upsilon "melting." The image shows a cacaphony of signals from particles produced in a lead-lead collision plus two distinct muon tracks (red lines). The sum of the energies of these muons (near 10 GeV) tags them as coming from an upsilon candidate. Learn more.Credit: Copyright CERN on behalf of the CMS Collaboration.
LHCb continues its investigations into the mysteries of B mesons. Here, a particular kind of B meson, called the Bs, decays into a J/Psi particle and a pi+pi- pair. The reconstructed pion pair mass distribution shows that they mostly came from an f0 particle that has a mass of 980 MeV and some from another possible state near 1420 MeV. The dashed curve shows backgrounds, while the dotted curve indicates the sum of the interfering signal shapes.
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This image is a screen shot of the the event display from one of the first events in ALICE of the 2011 run of the LHC. You can see all the different controls available to manipulate and analyze the image. This is what physicists in the ALICE control room see and use every day.
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CMS has an important first: detection and measurement of decays that appear to be from Z bosons inside the primordial fireballs that result from lead ion collisions. Here, two electrons emerge from the quark-gluon plasma and deposit their energy in the electromagnetic calorimeter. The tall red tower in the lower left of the image and the shorter red double tower on the upper right indicate an electron-positron pair that can be reconstructed to have come from a single Z-boson candidate.
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This event in the first lead-ion run of the LHC has two nearly back-to-back jets of particles from a single event. Their momenta should have about the same magnitude (conservation laws!) but the jet at the top right falls well short of the jet at the bottom left. The jet on the right seems to have interacted with the quark-gluon plasma and transfered some of its initial momentum to the particles which make up the plasma, resulting in a lower momentum measured in the calorimeter. This is "jet-quenching." The plasma is like that which existed in the very beginning of our universe: could there have been similar effects as particles emerged from that hot primordial soup?
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The lead ion program continues. ALICE is giving us a window to the first moments of the universe by better understanding plasmas. The beam is horizontal in this side view. The tracks on the right, the detector's main section, show particles from the hot dense plasma created in the heavy ion collision. The curving tracks going to the the left are muons created in the same interaction.
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ust a few days ago the LHC began colliding lead ions for the first time. When lead ions collide at high energies, they briefly form a plasma of quarks and gluons, the same condition that existed in the first nanoseconds of the universe. What comes from such a collision? Particles everywhere! The LHC is running at 3.5 TeV energy in each direction, the same as it did in the proton run just concluded. Lead ions have 207 times the mass but also have 82 times the charge of protons; thus a lead ion in the LHC has more inertia but feels much more of the electromagnetic force. The resulting collision energy of 2.76 TeV (as seen in the event image) is about a third that of a proton-proton collision.
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CMS found something interesting in high multiplicity events—those with many pairs of particles produced in proton-proton collisions—like this one. In these pairs, it appears that when one particle emerges at quite a different angle from the beamline than the other, they often have about the same angle from an axis which points straight up from the beamline. This suggests that the particles were associated when they were created. A similar effect was found in heavy ion collisions at Brookhaven Lab, but the explanation is not yet clear.
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The Zero Degree Calorimeter (ZDC) is part of the CMS detector. The ZDC system sits 140m down the beam line and is able to catch particles very close to the beam axis. This event display shows an 80 GeV photon candidate from a glancing blow between two colliding protons. The energy activated the electromagetic section of the ZDC which recorded its absorption as a deposit of a small amount of charge. Note the angle (theta-x): it is expressed in microradians, which is pretty close to zero degrees.
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Once again, the "event" is really a summation of events. These graphs show the consistency of the performance of the LHC and ALICE. The first graph shows the vertices of many events over several months—standard model particle decays, background events, etc.—in the plane perpendicular to the beam: they are all concentrated in a tight bluish spot. Each of the next three sum up the events in one dimension: Z (beamline), X, or Y. All three are narrow peaks, as they should be. Collsions in ALICE the very opposite of "all over the place".
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This "event" is really a summation of many events. The "Integrated Luminosity" is the total number of collisions per unit area since the start of the 7 TeV LHC run. Looking at the graph, we see that this number is now doubling every week or two. More data to handle, more data from which we can learn.
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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.
Copyright 2010 The ATLAS Experiment at CERN
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."
Copyright 2010 The ATLAS Experiment at CERN
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.
Copyright 2010 The ATLAS Experiment at CERN
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.
Copyright 2010 The ATLAS Experiment at CERN
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.
Copyright 2010 The ATLAS Experiment at CERN Collaboration
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.
Copyright 2010 The ATLAS Experiment at CERN
