Knocking on Heaven's Door (35 page)

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 TRACKERS

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.