Knocking on Heaven's Door (38 page)

At the LHC, the ATLAS and CMS experiments are designed to detect and identify Standard Model particles. The real goal, of course, is to go beyond what we already know—to find new ingredients or forces that address outstanding mysteries. But to do so, physicists need to be able to distinguish Standard Model background events and identify the Standard Model particles into which any exotic new particles might decay. Experimenters at the LHC are like detectives who analyze data to piece together clues and ascertain what was there. They will be able to deduce the existence of something new only after they have ruled out everything that is familiar.

Having toured the general-purpose experiments, we will now revisit them in this chapter to better understand how LHC physicists identify individual particles. A bit more familiarity with the particle physics status quo and how Standard Model particles are found will help when we discuss the discovery potential of the LHC in Part IV.

HANDEDNESS

Particles are left-handed or right-handed according to which way they appear to spin about the axis of their direction of motion.

[
FIGURE 40
]
The elements of the Standard Model of particle physics, with masses shown. Also shown are separate left- and right-handed particles. The weak force that changes particle type acts only on the left-handed ones.

FINDING LEPTONS

Particle physicists divide the elementary matter particles of the Standard Model into two categories. One type is called leptons, which includes particles such as the electron that don’t experience the strong nuclear force. The Standard Model also includes two heavier versions of the electron, which have the same charge but much bigger masses, and which are called the
muon
and the
tau
. It turns out that every Standard Model matter particle has three versions, all with the same charge but with each successive
generation
heavier than the next. We don’t know why there should be three versions of these particles, all with the same charges. The Nobel Prize—winning physicist Isidor Isaac Rabi, on hearing of the muon’s existence, notably expressed his bafflement with the exclamation, “Who ordered that?”

The lighter leptons are the easiest to find. Although both electrons and photons deposit energy in the electromagnetic calorimeter, because the electron is charged and the photon is not, the electron is readily distinguished from a photon. Only an electron leaves a a track in the inner detector before depositing energy in the ECAL.

Muons too are relatively straightforward to identify. Like all the other heavier Standard Model particles, muons decay so quickly that they aren’t found in ordinary matter, so we rarely find them on Earth. However, muons live long enough to travel to the outer reaches of the detectors before they decay. They therefore leave long clearly visible tracks throughout that experimenters can match up from the inner detector to the outer muon chambers. Because muons are the only Standard Model particle to reach these outer detectors and leave a visible signal, they are easy to pick out.

Though visible, taus are not quite so simple to find. The tau is a charged lepton like the electrons and the muon, but it is even heavier. Like most heavy particles, it too is unstable, which is to say it decays—leaving only other particles in its wake. A tau rapidly decays into a lighter charged lepton and two particles called neutrinos or into a single neutrino along with a particle called a pion that experiences the strong force. Experimenters study these decay products—the particles the initial particle decayed into—to figure out whether a heavy decaying particle was responsible for their presence and if so, what its properties are. Even though the tau doesn’t directly leave a track, all the information the experiments record about the decay products helps identify it and its properties.

The electron, muon, and the even heavier tau lepton have charge—1, the opposite charge of a positively charged proton. Colliders also produce the antiparticles associated with these charged leptons—the positron, antimuon, and antitau. These antiparticles carry charge +1, and leave similar-looking tracks in the detectors. However, because of their opposite charges, they curve in the opposite direction in the presence of a magnetic field.

In addition to the three types of charged leptons just described, the Standard Model also includes neutrinos, which are leptons that don’t carry electric charge at all. Whereas the three charged leptons experience both the force of electromagnetism and the weak nuclear force, neutrinos have zero charge and are therefore impervious to the electric force. Until the 1990s, experimental results indicated that neutrinos had zero mass. One of the most interesting discoveries in that decade was the extremely tiny but nonvanishing masses of neutrinos, which provided important information about the structure of the Standard Model.

Although neutrinos are very light and therefore well within the energy reach of colliders, they are impossible to directly detect at the LHC because they have no electric charge and therefore interact only weakly—so weakly that although more than 50 trillion neutrinos from the Sun pass through you every second, you really have no idea until someone tells you.

In spite of their invisibility, the physicist Wolfgang Pauli conjectured neutrinos existed as a “desperate way out” to explain where the energy went when neutrons decay. Without the neutrino carrying off some of the energy, it appeared that energy conservation was violated by this process, since the proton and the electron that were detected after the decay didn’t add up to the same energy as the neutron that went in. Even well-established physicists such as Niels Bohr were willing at the time to sacrifice their principles and accept that energy could be lost. Pauli was more faithful to known physical premises and conjectured instead that energy is indeed conserved, but experimenters just couldn’t see the charge-neutral particle that carried the remaining energy off. He turned out to be right.

Pauli named his then-hypothetical particle the neutron, but the name has since been used for other purposes—namely, the neutral partner of the proton that sits inside a nucleus. So Enrico Fermi, the Italian physicist who developed the theory of the weak interactions but is perhaps best known for helping develop the first nuclear reactor, gave it the cutish name neutrino, which in Italian means “little neutron.” It’s of course not a little neutron, but—like a neutron—it carries no electric charge. And a neutrino is indeed much lighter than a neutron.

As with all the other types of Standard Model particles, three types of neutrinos exist. Each charged lepton—the electron, muon, and tau—has an associated neutrino that it interacts with via the weak nuclear force”
⁵⁴

We have already seen how to find electrons, muons, and taus. So the remaining experimental question about leptons is how experimenters find neutrinos. Because neutrinos have no electric charge and interact so weakly, they escape the detector without leaving any trace at all. How does anyone at the LHC tell they were there?

Momentum (which is velocity times mass when particles move slowly but is more like energy moving in a particular direction when the particle travels near the speed of light) is conserved in all directions. As with energy, we have never found any evidence that momentum can be lost. So if the momentum of the particles measured in the detector is less than the momentum that went in, some other particle (or particles) must have escaped, carrying away the missing momentum in the process. This type of logic led Pauli to deduce the existence of neutrinos in the first place (in his case in nuclear beta decay), and to this day it’s how we learn of the existence of weakly interacting particles that seem to be invisible.

At hadron colliders, experimenters measure all the momentum transverse to the beam and calculate if something is missing. They focus on momentum transverse to the beam since a lot of momentum is carried away by particles that head down the beam pipe and is therefore too difficult to keep track of. The momentum perpendicular to the initial protons is much simpler to measure and account for.

Since the initial collision has essentially zero total momentum transverse to the beam, so too should the final state. So if measurements don’t agree with expectations, experimenters can “detect” that something is missing. The only remaining question is how to distinguish which of the many potential noninteracting particles it was. For Standard Model processes, we know neutrinos will be among the undetected elements. Based on the neutrino’s known weak force interactions that we will get to shortly, physicists calculate and predict the rate at which neutrinos should be produced. In addition, physicists already know what the decay of a
W
boson should look like—for example, an isolated electron or muon whose transverse momentum carries energy comparable to half the
W
mass is fairly unique. So using momentum conservation and theoretical input, neutrinos can be “found.” Clearly, there are fewer identifying tags on these particles than ones we see directly. Only a combination of theoretical considerations and missing energy measurements can tell us what was there.

It’s important to keep such ideas in mind when we consider new discoveries. Similar considerations apply for other novel particles without any charges, or with charges so weak that they can’t be directly detected. Only a combination of missing energy and theoretical input can be used in those cases to deduce what was there. That’s why hermeticity—detecting as much momentum as possible—is so important.

FINDING HADRONS

We’ve now considered leptons (electrons, muons, taus, and their associated neutrinos). The remaining category of particles in the Standard Model have the name
hadrons
—particles that interact through the strong nuclear force. This category includes all particles made from quarks and gluons, such as protons and neutrons and other particles called
pions
. Hadrons have internal structure—they are bound states of quarks and gluons held together by the strong nuclear force.

However, the Standard Model doesn’t list the many possible bound states. It lists the more fundamental particles that get bound together into hadronic states—namely, the quarks and gluons. In addition to the up and down quarks that sit inside protons and neutrons, heavier quarks called
charm
and
strange
and
top
and
bottom
exist as well. As with the charged and neutral leptons, the heavier quarks have charges identical to their lighter counterparts—the up and down quarks. The heavier quarks are also not readily found in nature. Colliders are needed to study them too.

Hadrons (which interact via the strong force) look very different from leptons (which don’t) in particle collisions. That is primarily because quarks and gluons have such strong interactions that they never appear in isolation. They are always in the middle of a jet that might contain the original particle, but will always include a bunch of others that also experience the strong force. Jets don’t contain single particles, but a spray of strongly interacting particles “protecting” the initial one, as can be seen in Figure 41. Even if not present in the initial event, the strong interactions will create many new quarks and gluons from the quark or gluon that initiated the jet in the first place. Proton colliders produce a lot of jets since protons themselves are made of strongly interacting particles. Such particles produce sprays of many additional strongly interacting particles that travel alongside them. They also sometimes create quarks and gluons that go off in different directions and form their own independent jets.

The quote I used in
Warped Passages
from the “Jet Song” in
West Side Story
actually describes hadronic jets quite well:

You’re never alone,

You’re never disconnected!

You’re home with your own:

When company’s expected,

You’re well protected.