Knocking on Heaven's Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World - Lisa Randall (2011)
Part III. MACHINERY, MEASUREMENTS, AND PROBABILITY
Chapter 13. THE CMS AND ATLAS EXPERIMENTS
In August 2007, the Spanish physicist and CERN theory group leader Luis Álvarez-Gaumé enthusiastically encouraged me to join a tour of the ATLAS experiment that the experimental physicists Peter Jenni and Fabiola Gianotti were planning for the visiting Nobel Prize winner T. D. Lee and a few others. It was impossible to resist the infectious enthusiasm of Peter and Fabiola, who at the time were spokesperson and deputy spokesperson of the experiment, and who generously shared an expertise and familiarity with all the details of the experiment that suffused all of their words.
[ FIGURE 29 ] Looking down from the platform above into the ATLAS pit, with the tubes that transported materials down in view.
My fellow visitors and I donned our helmets and entered the LHC tunnel. Our first stop was a landing where we could stare down at the gaping pit beneath, as is shown in the photo in Figure 29. Witnessing the gargantuan cavern with its vertical tubes that would transport pieces of the detector from the place where we stood to the floor 100 meters below got me hooked. My fellow ATLAS tourists and I eagerly anticipated the experience we had in store.
After the first stop, we proceeded to the floor down below that housed the not-yet-completed ATLAS detector. The nice thing about the unfinished state was that you could see the detector’s innards, which would eventually be closed up and shielded from view—at least until the LHC turns off for an extended period of time for maintenance and repairs. So we had the opportunity to stare directly at the elaborate construction, which was impressively colorful and big—larger even than the nave of the Cathedral of Notre Dame.
But the size was not in itself the most magnificent aspect. Those of us who grew up in New York or any other big city are not necessarily overly impressed by enormous construction projects. What makes the ATLAS experiment so imposing is that this huge detector is composed of many small detection elements—some designed to measure distances with a precision at the level of microns. The irony of the LHC detectors is that you need such big experiments to accurately measure the smallest distances. When I now show an image of the detector in public lectures, I feel compelled to emphasize that ATLAS is not only big, but it is also precise. This is what makes it so amazing.
A year later, in 2008, I returned to CERN and saw the construction progress ATLAS had made. The ends of the detector that had been open the previous year were now closed up. I also took a spectacular tour of CMS, the LHC’s second general-purpose detector, along with the physicist Cinzia da Via and my collaborator, Gilad Perez, who appears in Figure 30.
[ FIGURE 30 ] My colleague, Gilad Perez, in front of part of the layered CMS muon detector/magnet return yoke.
Gilad hadn’t yet visited an LHC experiment, so I had the opportunity to relive my first experience through his excitement. We took advantage of the lax supervision to clamber around and even look down a beam pipe. (See Figure 31.) Gilad noted this could be the place where extra-dimensional particles get created and provide evidence for a theory I had proposed. But whether it will be evidence for this model or some other one, it was nice to be reminded that this beam pipe was where insight into new elements of reality would soon emerge.
Chapter 8 introduced the LHC machine that accelerates protons and collides them together. This chapter focuses on the two general-purpose LHC detectors—CMS and ATLAS—that will identify what comes out of the collisions. The remaining LHC experiments—ALICE, LHCb, TOTEM, ALFA, and LHCf—are designed for more specialized purposes, including better understanding the strong nuclear force and making precise measurements of bottom quarks. These other experiments will most likely study Standard Model elements in detail, but they are unlikely to discover the new high energy beyond the Standard Model physics that is the LHC’s primary goal. CMS and ATLAS are the chief detectors that will make the measurements that will, we hope, reveal new phenomena and matter.
[ FIGURE 31 ] Cinzia da Via (left) walking past the location where we could stare down the beam pipe and see inside (right).
This chapter contains a good amount of technical detail. Even theorists like me don’t need to know all these facts. Those of you interested only in the new physics that we might discover or the LHC concepts in general might choose to jump ahead. Still, the LHC experiments are clever and impressive. Omitting these details wouldn’t do justice to the enterprise.
In some sense, the ATLAS and CMS detectors are the logical evolution of the transformation Galileo and others instigated several centuries ago. Since the invention of the microscope at that time, successively advanced technology has allowed physicists to indirectly study increasingly remote distances. The study of small sizes has repeatedly revealed underlying structure of matter that can only be observed with very tiny probes.
Experiments at the LHC are designed to study substructure and interactions with a range a hundred thousand trillion times smaller than a centimeter. This is about a factor of ten smaller in size than anything any experiment has ever looked at before. Although previous high-energy collider experiments, such as those running at the Tevatron at Fermilab in Batavia, Illinois, were based on similar principles to these LHC detectors, the record energy and collision rate that the new detectors faced posed many novel challenges that forced their unprecedented size and complexity.
Like telescopes in space, the detectors, once built, are essentially inaccessible. They are enclosed deep underground and subject to large amounts of radiation. No one can access the detector while the machine is running. Even when it is not, reaching any particular detector element is extremely difficult and time-consuming. For this reason, the detectors were built to last at least a decade, even with no maintenance. However, long shut-down periods are planned for every two years of LHC running, during which time physicists and engineers will have access to many of the detector components.
In one important respect, however, particle experiments are very different from telescopes. Particle detectors don’t need to point in a particular direction. In some sense they look in all directions at once. Collisions happen and particles emerge. The detectors record any event that has the potential to be interesting. ATLAS and CMS are general-purpose detectors. They don’t record just one type of particle or event or focus on particular processes. These experimental apparatuses are designed to absorb the data from the broadest possible range of interactions and energies. Experimenters with enormous computational power at their disposal try to unambiguously extricate information about such particles and their decay products from the “pictures” experiments record.
More than 3,000 people from 183 scientific institutes, representing 38 countries, participate in the CMS experiment—building and operating the detector and analyzing the data. The Italian physicist Guido Tonelli—originally deputy spokesperson—now heads the collaboration.
In a break from CERN’s legacy of male physicists presiding, the impressive Italian donna Fabiola Gianotti also transitioned from deputy to spokesperson, this time for ATLAS, the other general-purpose experiment. She is well deserving of the role. She has a mild-mannered, friendly, and polite demeanor—yet her physics and organizational contributions have been tremendous. What makes me really jealous, however, is that she is also an excellent chef—maybe forgivable for an Italian with enormous attention to detail.
ATLAS too involves a gigantic collaboration. More than 3,000 scientists from 174 institutes in 38 countries participated in the ATLAS experiment (December 2009). The collaboration was initially formed in 1992 when two proposed experiments—EAGLE (Experiment for Ac-curate Gamma, Lepton, and Energy Measurements) and ASCOT (Apparatus with Super Conducting Toroids) joined together with a design combining features of both with some aspects of proposed SSC detectors. The final proposal was presented in 1994, and it was funded two years later.
The two experiments are similar in basic outline, but different in their detailed configurations and implementations, as is illustrated in some detail in Figure 32. This complementarity gives each experiment slightly different strengths so that physicists can cross-check the two experiments’ results. With the extreme challenges involved in particle physics discoveries, two experiments with common search targets will have much more credibility when they confirm the findings of each other. If they both come to the same conclusion, everyone will be much more confident.
The presence of two experiments also introduces a strong element of competition—something my experimenter colleagues frequently remind me about. The competition pushes them to get results more quickly and more thoroughly. The members of the two experiments also learn from each other. A good idea will find its way to both experiments, even if implemented somewhat differently in each. This competition and collaboration, coupled with the redundancy of having two independent searches relying on somewhat different configurations and technology, underlies the decision to have two experiments with common goals.
[ FIGURE 32 ] Cross sections of the ATLAS and CMS detectors. Note the overall sizes have been rescaled.
I am often asked when the LHC will run my experiments and search for the particular models that my collaborators and I have proposed. The answer is right away—but they are looking for everyone else’s proposals too. Theorists help by introducing new search targets and new strategies for finding stuff. Our research aims to identify ways to find whatever new physical elements or forces are present at higher energies, so that physicists will be able to find, measure, and interpret the results and thereby gain new insights into underlying reality—whatever it might be. Only after data is recorded do the thousands of experimenters, who are split up into analysis teams, study whether the information fits or rules out my models or any others that are potentially interesting.
Theorists and experimenters then examine the data that gets recorded to see whether they conform to any particular type of hypothesis. Even though many particles last only a fraction of a second and even though we don’t witness them directly, experimental physicists use the digital data that compose these “pictures” to establish which particles form the core of matter and how they interact. Given the complexity of the detectors and data, experimenters will have a lot of information to contend with. The rest of this chapter gives a sense of what, exactly, that information will be.
THE ATLAS AND CMS DETECTORS
So far we have followed LHC protons from their removal from hydro-gen atoms to their acceleration to high energy in the 27 km ring. Two completely parallel beams will never intersect, and neither will the two beams of protons traveling in opposite directions within them. So at several locations along the ring, dipole magnets divert them from their path while quadrupole magnets focus them so that the protons in the two beams meet and interact within a region less than 30 microns across. The points at the center of each detector where proton-proton collisions occur are known as the interaction points.
Experiments are set up concentrically around each of these interaction points to absorb and record the many particles that are emitted by the frequent proton collisions. (See Figure 33 for a graphic of the CMS detector.) The detectors are cylindrically shaped because even though the proton beams travel in opposite directions at the same speed, the collisions tend to contain a lot of forward motion in both directions. In fact, because individual protons are much smaller than the beam size, most of the protons don’t collide at all but continue straight down the beam pipe with only mild deflection. Only the rare event where individual protons collide head-on are of interest.
[ FIGURE 33 ] Computer image of CMS broken up to reveal individual detector components. (Graphic courtesy of CERN and CMS)
That means that although most particles continue to travel along the beam direction, the potentially interesting events contain a spray of particles that travel significantly transversely to the beam. The cylindrical detectors are designed to detect as much of these interaction products as possible, taking into account the large spread of particles along the beam direction. The CMS detector is located around one proton collision point below ground at Cessy in France, close to the Geneva border, while the ATLAS interaction region is under the Swiss town of Meyrin, very near the main CERN complex. (See Figure 34 for a simulation of particles coming out of a collision and emanating through a cross section of the ATLAS detector.)
Standard Model particles are characterized by their mass, spin, and the forces through which they interact. No matter what is ultimately created, both experiments rely on detecting it through known Standard Model forces and interactions. That’s all that’s possible. Particles with no such charges would leave the interaction region without a trace.
But when experiments measure Standard Model interactions, they can identify what passed through. So that’s what the detectors are designed to do. Both CMS and ATLAS measure the energy and momentum of photons, electrons, muons, taus, and strongly interacting particles, which get subsumed into jets of closely aligned particles traveling in the same direction. Detectors emanating from the proton collision region are designed to measure energy or charge in order to identify particles, and they contain sophisticated computer hardware, software, and electronics to deal with the overwhelming abundance of data. Experimenters identify charged particles since they interact with other charged stuff that we know how to find. They also find anything that interacts via the strong force.
The detector components all ultimately rely on wires and electrons produced through interactions with the material in the detector to record what passed through. Sometimes charged particle showers occur because many electrons and photons are produced and sometimes material is simply ionized with charges recorded. But either way wires record the signal and send it along for it to be processed and analyzed by physicists at their computers.
[ FIGURE 34 ] Simulation of an event in the ATLAS detector showing the transverse spray of particles though the detector layers. (Note that the person gives a sense of scale, but collisions don’t happen when people are in the cavern.) The distinctive toroidal magnets are clearly visible. (Courtesy of CERN and ATLAS)
Magnets are also critical to both detectors. They are essential to mea-suring both the sign of the charges and the momenta of charged particles. Electromagnetically charged particles bend in a magnetic field according to how fast they are moving. Particles with bigger momenta tend to go straighter, and particles with opposite charges bend in opposite directions. Because particles at the LHC have such large energies (and momenta), the experiments need very strong magnets to have a chance of measuring the small curvature of the energetic charged particle tracks.
The Compact Muon Solenoid (CMS) apparatus is the smaller in size of the two large general-purpose detectors, but it is heavier, weighing in at a whopping 12,500 metric tons. Its “compact” size is 21 meters long by 15 meters in diameter—smaller than ATLAS but still big enough to cover the area of a tennis court.
The distinguishing element in CMS is its strong magnetic field of 4 tesla, which the “solenoid” piece of the name refers to. The solenoid in the inner part of the detector consists of a cylindrical coil six meters in diameter made up of superconducting cable. The magnetic return yoke that runs through the outer part of the detector is also impressive and contributes most of the huge weight. It contains more iron than Paris’s Eiffel Tower.
You might also wonder about the word “muon” in the name CMS (I did too when I first heard it). Rapidly identifying energetic electrons and muons, which are heavier counterparts of electrons that penetrate to the outer reaches of the detector, can be important for new particle detection—since these energetic particles are sometimes produced when heavy objects decay. Since they don’t interact via the strong nuclear force, they are more likely to be something new—since protons won’t automatically make them. These readily identifiable particles could therefore indicate the presence of an interesting decaying particle that has emerged from the collision. The magnetic field in CMS was initially designed with special attention paid to energetic muons so that it could trigger on them. This means it will record the data from any event involving them, even when it is forced to throw a lot of other data out.
ATLAS, like CMS, features its magnet in its name since a big magnetic field is also critical to its operation. As noted earlier, ATLAS is the acronym for A Toroidal LHC ApparatuS. The word “toroid” refers to the magnets, whose field is less strong than that of CMS but extends over an enormous region. The huge magnetic toroids help make ATLAS the larger of the two general-purpose detectors and in fact the largest experimental apparatus ever constructed. It is 46 meters long and 25 meters in diameter and fits rather snugly into its 55-meter-long, 40-meter-high cavern. At 7,000 metric tons, ATLAS is a little more than half the weight of CMS.
To measure all the particle properties, increasingly large cylindrical detector components emanate from the region where collisions occur. The CMS and ATLAS detectors both contain several embedded pieces designed to measure the trajectory and charges of the particles as they pass through. Particles emerging from the collision first encounter the inner trackers that precisely measure the paths of charged particles close to the interaction point, next the calorimeters that measure energy deposited by readily stopped particles, and finally the muon detectors that are at the outer edges and measure the energy of highly penetrating muons. Each of these detector elements has multiple layers to increase the precision for each measurement. We’ll now tour the experiments from the innermost detectors to the outermost as measured radially from the beams and explain how the spray of particles leaving a collision turns into recorded identifiable information.
The innermost portions of the apparatuses are the trackers that record the positions of charged particles as they leave the interaction region so that their paths can be reconstructed and their momenta measured. In both ATLAS and CMS, the tracker consists of several concentric components.
The layers closest to the beams and interaction points are the most finely segmented and generate the most data. Silicon pixels, with extremely tiny detector elements, sit in this innermost region, starting at a few centimeters from the beam pipe. They are designed for extremely precise tracking very close to the interaction point where the particle density is highest. Silicon is used in modern electronics because of the fine detail that can be etched into each tiny piece, and particle detectors use it for the same reason. Pixel elements at ATLAS and CMS are designed to detect charged particles with extremely high resolution. By connecting the dots to one another and to the interaction points from which they emerged, experimenters find the paths the particles followed in the innermost region very near to the beam.
The first three layers of the CMS detector—out to 11 centimeter radius—consist of 100 by 150 micrometer pixels, 66 million in total. ATLAS’s inner pixel detector is similarly precise. The smallest unit that can be read out in the ATLAS innermost detector is a pixel of size 50 by 400 micrometers. The total number of ATLAS pixels is about 82 million, a little more than the number in CMS.
The pixel detectors, with their tens of millions of elements, require elaborate electronic readouts. The extent and speed required for the readout systems, as well as the huge radiation the inner detectors will be subjected to, were two of the major challenges for both of the detectors. (See Figure 35.)
Because there are three layers in these inner trackers, they record three hits for any long-lasting enough charged particle that passes through. These tracks will generally continue to an outer tracker beyond the pixel layers to create a robust signal that can be definitively associated with a particle.
My collaborator Matthew Buckley and I paid a good deal of attention to the geometry of the inner trackers. We realized that by sheer coincidence, some conjectured new charged particles that decay via the weak force into a neutral partner would leave a track that’s only a few centimeters long. That means that in these special cases, tracks might extend only through the inner tracker so that the information read out here would be all there is. We considered the additional challenges faced by experimenters who had only the pixels—the innermost layers of the inner detector—to rely on.
[ FIGURE 35 ] Cinzia da Via and an engineer, Domenico Dattola, standing on scaffolding in front of one of the bulkheads of the CMS silicon tracker, to which the cables are connected.
Most charged particles, however, live long enough to make it to the next tracker component, so detectors record a much greater length path. Therefore, outside the inner pixel detectors with fine resolution in two directions are silicon strips with asymmetric size in the two directions, much coarser in one of the two. The longer strips are consistent with the cylindrical shape of the experiment and make covering a larger area (remember the area gets far bigger with bigger radius) feasible.
The CMS silicon tracker consists of a total of 13 layers in the central region and 14 layers in the forward and backward regions. After the first three finely pixilated layers we just described, the next four layers, consisting of silicon strips, extend to 55 centimeters radius. The detector elements here are 10-centimeter-long, 180-micrometer-wide strips. The remaining six layers are even less precise in the coarser orientation, consisting of strips up to 20 centimeters long and varying in width between 80 and 205 micrometers, with the strips extending out to a radius of 1.1 meters. The total number of strips in the CMS inner detector is 9.6 million. These strips are essential to reconstructing the tracks of most charged particles that pass through. In total, CMS has silicon covering essentially the area of a tennis court—a significant advance over the previous largest silicon detector of only two square meters.
The ATLAS inner detector extends to a slightly smaller radius of one meter and is seven meters long longitudinally. As with CMS, outside the three inner silicon pixel layers, the Semiconductor Tracker (SCT) consists of four layers of silicon strips. In ATLAS’s case, they are 12.6 centimeters by 80 micrometers in size. The total area of the SCT is also enormous, covering 61 square meters. Whereas the pixel detectors are useful for reconstructing fine measurements near the interaction points, the SCT is most critical to overall track reconstruction because of the large region it covers with high precision (albeit in one direction).
Unlike CMS, the outer detector of the ATLAS apparatus is not made of silicon. The transition radiation tracker (TRT), the outermost component of the inner detector, consists of tubes filled with gas and acts as both a tracking device and a transition radiation detector. Charged particle tracks are measured when they ionize the gas in the straws, which are 144 centimeters by 4 millimeters in size, with wires down the center to detect the ionization. Here again there is highest resolution in the transverse direction. The straws measure the tracks with a precision of 200 micrometers, which is less precise than with the innermost tracker but covers a far greater region. The detectors also discriminate among particles moving very close to the speed of light that produce so-called transition radiation. This discriminates among particles of different mass, since lighter particles will generally be moving faster. This helps identify electrons.
If you’re finding all these details a bit overwhelming, keep in mind that this is more information than even most physicists need to know. They give a sense of the magnitude and precision, and are of course important to anyone working on a particular detector component. But even those who have extreme familiarity with one component don’t necessarily keep track of all the others, as I accidentally learned when trying to track down some detector photos and make sure some diagrams were precise. So don’t feel too badly if you don’t get it all the first time. Though some experts coordinate the overall operation, even many experimenters don’t necessarily have every detail at their fingertips.
THE ELECTROMAGNETIC CALORIMETER (ECAL)
Once through the three types of trackers, the next section of detector a particle encounters on its outward radial journey is the electromagnetic calorimeter (ECAL), which records the energy deposited by charged and neutral particles that stop there—electrons and photons in particular—and the position where they left it. The detection mechanism looks for the spray of particles that incident electrons or photons produce when they interact with the detector material. This piece of the detector yields both precise energy and position tracking information for these particles.
The material used for the ECAL in the CMS experiment is a wonder to behold. It is made of lead tungstate crystals, chosen because they are dense but optically clear—exactly what you want for stopping and detecting electrons and photons as they arrive. You can perhaps get a sense of this from my photograph in Figure 36. The reason they are fascinating is their incredible clarity. You’ve never seen anything this dense and this transparent. The reason they are useful is that they measure electromagnetic energy incredibly precisely, which could turn out to be critical to finding the elusive Higgs particle as Chapter 16 will describe.
The ATLAS detector uses lead to stop electrons and photons. Interactions in this absorbing material transform the energy from the initial charged track into a shower of particles whose energy will then be detected. Liquid argon, which is a noble gas that doesn’t chemically interact with other elements and is very resistant to radiation, is then used to sample the energy of the shower to deduce the incident particle energy.
[ FIGURE 36 ] Photograph of the lead tungstate crystal that is used in CMS’s electromagnetic calorimeter.
Despite my theoretical inclinations, I was fascinated to see this detector element at ATLAS on my tour. Fabiola participated in the pioneering development and construction of this calorimeter’s novel geometry with radial layers of accordion-shaped lead plates separated by thin layers of liquid argon and electrodes. She described how this geometry makes readout of the electronics much faster, since the electronics is much closer to the detector elements. (See Figure 37.)
[ FIGURE 37 ] The accordion-like structure of ATLAS’s electromagnetic calorimeter.
THE HADRONIC CALORIMETER (HCAL)
Next in line along our radial outward journey from the beam pipe is the hadronic calorimeter (HCAL). The HCAL measures the energy and positions of hadronic particles—those particles that interact through the strong force—though it does so less precisely than the electron and photon energy measurements made by the ECAL. That’s by necessity. The HCAL is huge. In ATLAS, for example, the HCAL is eight meters in diameter and 12 meters long. It would be prohibitively expensive to segment the HCAL with the precision of the ECAL, so the precision of the track measurement is necessarily degraded. On top of that, energy measurements are simply harder for strongly interacting particles, independent of segmentation, since the energy in hadronic showers fluctuates more.
The HCAL in CMS contains layers of dense material—brass or steel—alternating with plastic scintillator tiles that record the energy and position of the hadrons that pass through, based on the intensity of the scintillating light. The absorber material in the central region of ATLAS is iron, but the HCAL there works pretty much the same way.
The outermost elements in any general-purpose detector are the muon chambers. Muons, you will remember, are charged particles like electrons, but they are 200 times heavier. They don’t stop in the electromagnetic or hadronic calorimeters but instead barrel straight through the thick outer region of the detector. (See Figure 38.)
Energetic muons are very useful when looking for new particles because, unlike hadrons, they are sufficiently isolated that they are relatively clean to detect and measure. Experimenters want to record all events with energetic muons in the transverse direction because muons are likely to be associated with the more interesting collisions. Muon detectors could also prove useful for any heavy stable charged particle that makes it to the outer reaches of the detector.
[ FIGURE 38 ] CMS’s magnetic return coil interlaced with its muon detector—all under construction.
Muon chambers record the signals left by the muons that reach these outermost detectors. They are similar in some respects to the inner detector with its trackers and magnetic fields bending the muon tracks so their trajectories and momenta can be measured. However, in the muon chambers, the magnetic field is different, and the thickness of the detector is much bigger, permitting measurements of smaller curvatures and hence higher-momentum particles (high-momentum particles bend less in a magnetic field). In CMS, the muon chambers extend from about three meters to the outer radius of the detector at about 7.5 meters, while in ATLAS they extend from four meters to the outer reaches of that detector at 11 meters. These huge structures permit 50-micrometer particle track measurements.
The last detector elements to describe are the endcaps, the detectors at the forward and backward ends of the experiments. (See Figure 39 to get a sense of the overall structure.) We are no longer working our way radially outward from the beam—the muon detectors were the last step in that direction—but rather we now are proceeding along the axis of the cylindrical detectors to the two ends that cap them off. The cylindrical portions of the detectors are “capped” off there with detectors covering the end regions that ensure that as many particles as possible get recorded. Since the endcaps were the last components of the detector to be moved to their final positions, I could readily see the multiple layers that sit inside the detectors when I visited in 2009.
[ FIGURE 39 ] Computer image of ATLAS showing its many layers and the endcaps separated. (Courtesy of CERN and ATLAS)
Detectors are placed in these end regions to ensure that LHC experiments measure all the particles’ momenta. The goal is to make the experimental apparatuses hermetic, meaning there is coverage in all directions with no holes or missing regions. Hermetic measurements ensre that even noninteracting or very weakly interacting particles can be discovered. If “missing” transverse momentum is observed, one or more particles with no directly detectable interactions must have been produced. Such particles carry momentum, and the momentum they take away makes experimenters aware of their existence.
If you know the detector is measuring all the transverse momentum, and the momentum perpendicular to the beam doesn’t appear to be conserved after a collision, then something must have disappeared undetected and carried away momentum. Detectors, as we have seen, measure momentum in the perpendicular directions very carefully. The calorimeters in the forward and backward regions ensure hermeticity by guaranteeing that very little energy or momentum perpendicular to the beam can escape unnoticed.
The CMS apparatus has steel absorbers and quartz fibers in the end regions, which separate the particle tracks better because they are denser. The brass in the endcaps is recycled material—it was originally used in Russian artillery shells. The ATLAS apparatus uses liquid-argon calorimeters in the forward region to detect not only electrons and photons but also hadrons.
The remaining pieces of both detectors that remain to be described in more detail are the magnets that give both experiments their names. A magnet is not a detector element in that it doesn’t record particle properties. But magnets are essential to particle detection because they help determine momentum and charge, properties that are critical to identifying and characterizing particle tracks. Particles bend in magnetic fields, so their tracks appear to be curved rather than straight. How much and in which direction they bend depends on their energies and charges.
CMS’s enormous solenoidal magnet made of refrigerated superconducting niobium-titanium coils is 12.5 meters long and six meters in diameter. This magnet is the defining feature of the detector and is the largest magnet of its type ever made. The solenoid has coils of wire surrounding a metal core, generating a magnetic field when electricity is applied. The energy stored in this magnet is the same as that generated by a half-metric ton of TNT. Needless to say, precautions have been taken in case the magnet quenches and suddenly loses superconductivity. The solenoid’s successful 4-tesla test was completed in September 2006, but it will be run at a slightly lower field—3.8 tesla—to ensure greater longevity.
The solenoid is sufficiently big to enclose the tracking and calorimeter layers. The muon detectors, on the other hand, are on the outer perimeter of the detector, outside the solenoid. However, the four layers of muon detector are interlaced with a huge iron structure surrounding the magnetic coils that contains and guides the field, ensuring uniformity and stability. This magnetic return yoke, 21 meters long and 14 meters in diameter, reaches to the full seven-meter radius of the detector. In effect, it also forms part of the muon system since the muons should be the only known charged particles to penetrate the 10,000 metric tons of iron and cross the muon chambers (though in reality energetic hadrons will sometimes also get in, creating some headaches for the experimenters). The magnetic field from the yoke bends the muons in the outer detector. Since the amount muons bend in the field depends on their momenta, the yoke is vital to measuring muons’ momenta and energy. The structurally stable enormous magnet plays another role as well. It supports the experiment and protects it from the giant forces exerted by its own magnetic field.
The ATLAS magnet configuration is entirely different. In ATLAS, two different systems of magnets are used: a 2-tesla solenoid enclosing the tracking systems and huge toroidal magnets in the outer regions interleaved with the muon chambers. When you look at pictures of ATLAS (or the experiment itself), the most notable elements are these eight huge toroidal structures (seen in Figure 34) and the two additional toroids that cap the ends. The magnetic field they create stretches 26 meters along the beam axis and extends from the start of the muon spectrometer 11 meters in the radial direction.
Among the many interesting stories I heard when visiting the ATLAS experiment was how when the magnets were originally lowered by the construction crews, they started off in a more oval configuration (when viewed from the side). The engineers had factored in gravity before installing them so they correctly anticipated that after some time, due to their own weight, the magnets would become more round.
Another story that impressed me was about how ATLAS engineers factored in a slight rise of the cavern floor of about one millimeter per year caused by the hydrostatic pressure from the cavern excavation. They designed the experiment so that the small motion would put the machine in optimal position in 2010, when the initial plan was to have the first run at full capacity. With the LHC delays, that hasn’t been the case. But by now, the ground under the experiment has settled to the point that the experiment has stopped moving, so it will remain in the correct position throughout operation. Despite Yogi Berra’s admonition that it’s “tough to make predictions, especially about the future,”52 the ATLAS engineers got it right.
No description of the LHC is complete without describing its enormous computational power. In addition to the remarkable hardware that goes into the trackers, calorimeters, muon systems, and magnets we just considered, coordinated computation around the world is essential to dealing with the overwhelming amount of data the many collisions will generate.
Not only is the LHC seven times higher in energy than the Tevatron—the highest-energy collider before—but it also generates events at a rate 50 times faster. The LHC needs to handle what are essentially extremely high resolution pictures of events that are happening at a rate of up to about a billion collisions per second. The “picture” of each event contains about a megabyte of information.
This would be way too much data for any computing system to deal with. So trigger systems make decisions on the fly about which data to keep and which to throw away. By far the most frequent collisions are just ordinary proton interactions that occur via the strong force. No one cares about most of these collisions, which represent known physical processes but nothing new.
The collisions of protons are analogous in some respects to two beanbags colliding. Because beanbags are soft, most of the time they wilt and hang and don’t do anything interesting during the collision. But occasionally when beanbags bang together, individual beans hit each other with great force—maybe even so much so that individual beans collide and the bags themselves break. In that case, individual colliding beans will fly off dramatically since they are hard and collide with more localized energy, while the rest of the beans will fly along in the direction in which they started.
Similarly, when protons in the beam hit each other, the individual subunits collide and create the interesting event, whereas the rest of the ingredients of the proton just continue in the same direction down the beampipe.
However unlike bean collisions, in which the beans simply collide and change directions, when protons bang into each other, the ingredients inside—quarks, antiquarks, and gluons—collide together—and when they do the original particles can convert into energy or other types of matter. And, whereas at lower energies, collisions involve primarily the three quarks that carry the proton charge, at higher energies virtual effects due to quantum mechanics create significant gluon and antiquark content, as we saw earlier in Chapter 6. The interesting collisions are those in which any of these subcomponents of the protons hit each other.
When the protons have high energy, so do the quarks, antiquarks, and gluons inside them. Nonetheless, that energy is never the entire energy of the proton. In general, it is a mere fraction of the total. So more often than not, quarks and gluons collide with too small a fraction of the proton’s energy to make heavy particles. Due possibly to a smaller interaction strength or to the heavier mass expected for new particles, interesting collisions involving as-yet-unseen particles or forces occur at a much lower rate than “boring” Standard Model collisions.
As with the beanbags, most of the collisions therefore are uninteresting. They involve either protons just glancing off each other or protons colliding to produce Standard Model events that we already know should be there and that won’t teach us much. On the other hand, predictions tell us that roughly one-billionth as often as that the LHC might produce a new exciting particle such as the Higgs boson.
The upshot is that only in a small but lucky fraction of the time does the good stuff get made. That’s why we need so many collisions in the first place. Most of the events are nothing new. But a few rare events could be very special and informative.
It’s up to the triggers—the hardware and software designed to identify potentially interesting events—to ferret these out. One way to understand the enormity of this task (once you account for different possible channels) is as if you had a 150-megapixel (the amount of data from each bunch crossing) camera that can snap pictures at a rate of 40 million per second (the bunch crossing rate). This amounts to about a billion physics events per second, when you account for the 20 to 25 events expected to occur during each bunch crossing. The trigger would be the analog of the device responsible for keeping only the few interesting pictures. You might also think of the triggers as spam filters. Their job is to make sure that only interesting data make it to the experimenters’ computers.
The triggers need to identify the potentially interesting collisions and discard the ones that won’t contain anything new. The events themselves—what leaves the interaction point and gets recorded in the detectors—must be sufficiently distinguishable from usual Standard Model processes. Knowing when the events look special tells us which events to keep. This makes the rate for readily recognizable new events even smaller still. The triggers have a formidable task. They are responsible for winnowing down the billion events per second to the few hundred that have a chance of being interesting.
A combination of hardware and software “gates” accomplishes this mission. Each successive trigger level rejects most of the events it receives as uninteresting, leaving a far more manageable amount of data. These data in turn get analyzed by the computer systems at 160 academic institutions around the globe.
The first-level trigger is hardware based—built into the detectors—and does a gross pass at identifying distinctive features, such as selecting events containing energetic muons or large transverse energy depositions in the calorimeters. While waiting a few microseconds for the result of the level-one trigger, the data from each bunch crossing are held in buffer. The higher-level triggers are software based. The selection algorithms run on a large computer cluster near the detector. The first-level trigger reduces the billion per second event rate to about 100,000 events per second, which the software triggers further reduced by a factor of about a thousand to a few hundred events.
Each event that passes the trigger carries a huge amount of information—the readouts of the detector elements we just discussed—of more than a megabyte. With a few hundred events per second, the experiments keep well over 100 megabytes of disk space per second, which amounts to over a petabyte, which is 1015 bytes, or one quadrillion bytes (how often do you get to use that word?), the equivalent of hundreds of thousands of DVDs worth of information, each year.
Tim Berners-Lee first developed the World Wide Web to deal with CERN data and let experimenters around the world share information on a computer in real time. The LHC Computing Grid is CERN’s next major computational advance. The Grid was launched late in 2008—after extensive software development—to help handle the enormous amounts of data that the experimenters intend to process. The CERN Grid uses both private fiber-optic cables and high-speed portions of the public Internet. It is so named because data aren’t associated with any single location but are instead distributed in computers around the world—much as the electricity in an urban area isn’t associated with one particular power plant.
Once the trigger-happy events that made it through are stored, they are distributed via the Grid all over the globe. With the Grid, computer networks all over the globe have ready access to the redundantly stored data. Whereas the web shares information, the Grid shares computational power and data storage among the many participating computers.
With the Grid, tiered computing centers process the data. Tier 0 is CERN’s central facility where the data get recorded and reprocessed from their raw form to one more suitable for physics analyses. High-bandwidth connections send the data to the dozen large national computing centers constituting Tier 1. Analysis groups can access these data if they choose to do so. Fiber-optic cables connect Tier 1 to the roughly 50 Tier 2 analysis centers located at universities, which have enough computing power to simulate physics processes and do some specific analyses. Finally, any university group can do Tier 3 analyses, where most of the real physics will ultimately be extracted.
At this point, experimenters anywhere can go through their data to sleuth out what the high-energy proton collisions might reveal. This can be something new and exciting. But in order to establish whether or not this is the case, the first task for the experiments—which we’ll explore further in the following chapter—is deducing what was there.