The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (16 page)

A cross-section of an experiment, showing the behavior of different particles. Neutral particles like photons and neutral hadrons are invisible to the inner detector, but charged particles leave curved tracks. Photons and electrons are captured by the electromagnetic calorimeter, while hadrons are captured by the hadron calorimeter. Muons make it to the outer detector, and neutrinos escape detection entirely. In the CMS experiment, muons curve in the opposite direction in the outer detector because the magnetic field points the opposite way.

The final layers of the experiments are the muon detectors. Muons have enough momentum to punch through the calorimeters, but can be precisely measured by the giant magnetic chambers that surround them. This is important because muons are not created by the strong interactions (since they are leptons, not quarks), and only rarely by the electromagnetic interactions (because they are so heavy and it’s easier just to make electrons). Therefore, muons generally come about from the weak interactions, or something brand-new. Either alternative is interesting, and muons play an important role in the search for the Higgs.

We now see why the design of the ATLAS and CMS experiments takes the form that it does. The inner detectors provide precision information about the trajectories of all charged particles leaving the collisions. Electrons and photons are captured, and their energies measured, by the electromagnetic calorimeter, while strongly interacting particles suffer the same fate in the hadronic calorimeter. Muons escape the calorimeters but are carefully studied in the muon detector. Among the known particles, only neutrinos escape undetected, and we can infer their existence by looking for missing momentum. All in all, an ingenious scheme to squeeze out all the information we can from the colliding protons produced by the LHC.

Information overload

At the LHC, bunches of protons come into collision 20 million times per second. Every crossing produces dozens of collisions, so we have hundreds of millions of collisions a second. Every collision is like fireworks going off inside the detector, creating multiple particles, up to a hundred or more. And the finely calibrated instruments inside the experiments collect precise information about what every one of those particles does.

That’s a lot of information. A single collision event at the LHC results in about one megabyte of data. (The raw data is more than twenty megabytes, but clever compression brings it close to a single megabyte.) That’s the size of the text of a large book, or the total amount of RAM in a space shuttle’s operating system. Decent home-computer hard drives these days can store a terabyte of data, or a million megabytes, which is huge—all the text in all the books of the Library of Congress amounts to only about twenty terabytes. You could store a million LHC events on one of these ordinary hard drives, which sounds good—except when you remember that there are hundreds of millions of events per second. So you would be filling up a thousand hard drives per second. Not really feasible, even given that CERN can afford better hard drives than you have on your laptop.

Outside the LHC, the largest single database in the world belongs to the World Data Center for Climate in Germany. It contains about six petabytes of climate data, or six thousand terabytes. If we recorded all the data created at the LHC, we would overflow a database of that size in a couple of seconds. Welcome to the world of Big Data.

Clearly, data storage (and transmission and analysis) at the LHC is a major challenge, one that is met by a combination of many different techniques. The most important one, however, is the most basic: not recording the data in the first place. This is worth emphasizing:
The overwhelming majority of data collected by the LHC is instantly thrown away
. We have no choice; there is no feasible way to record it all.

You might think that a more cost-effective strategy would simply be to not produce so much data in the first place, for example by lowering the luminosity of the machine. But particle physics doesn’t work that way—every collision is important, even if we don’t record its data to disk. That’s because quantum mechanics, which is ultimately responsible for the interactions that create these particles, only predicts the probability of certain outcomes. We can’t pick and choose what comes out when we collide two protons; we have to take what nature gives us. A large majority of the time, what nature gives us is pretty boring, at least in the sense that it’s stuff we already understand. To create a small number of interesting events, we have to produce an enormous number of pedestrian events, and swiftly pick out the interesting nuggets.

This raises a different problem, of course: how to figure out whether an event is “interesting,” and to do so extremely quickly, so that we can decide whether this is data worth keeping. That’s the job of the trigger, one of the most crucial aspects of an LHC experiment.

The trigger itself is a combination of hardware and software. The first-level trigger brings the output of all the instruments in the experiment into an electronic buffer and performs an ultrarapid scan (in about a microsecond) to see if anything potentially interesting is going on. About ten thousand events out of a billion get a stamp of approval and move on. The second-level trigger is a sophisticated piece of software that looks at more precise characterizations of the events (much like an ER doctor making a preliminary rapid diagnosis, then homing in with more delicate tests) to get you down to the events that are actually recorded for later analysis. We end up keeping only several hundred events out of the many millions produced per second—but they’re the most interesting ones.

As you might guess, a lot of hard thought and spirited disagreement go into deciding which events to keep and which to toss. It’s natural to worry that some real gems are being thrown away in all that discarded data, so the physicists at CMS and ATLAS are constantly working to refine their triggers in response to both improved experimental know-how and novel ideas from the theorists.

Sharing data

Even after running everything through the trigger, we’re still left with a hundred events per second, each characterized by about a megabyte of data. Now we have to analyze it. And by “we,” I mean “the thousands of members of the ATLAS and CMS experiments, working at institutions all over the world” (which don’t actually include me). For the physicists to analyze the data, they need access to it, which means a challenge in information transmission. Fortunately, this issue was anticipated for years, and physicists and computer scientists have worked hard to construct a Worldwide LHC Computing Grid that connects computing centers in thirty-five different countries, using a combination of the public Internet and private optical cables. In 2003, a new land speed record for data was set when more than a terabyte of information traveled more than five thousand miles from CERN to Caltech in under thirty minutes. That’s like downloading a full-length feature film in seven seconds.

This kind of crazy speed is necessary; in 2010, the four main experiments at the LHC produced more than thirteen petabytes of data. The Grid, as it is affectionately known, takes this data and parcels it out to different computing centers around the world, arranged in a series of tiers. Tier 0 is CERN itself. There are eleven Tier 1 sites, which play an important role in sifting through and classifying the data, and 140 Tier 2 sites, where specific analysis tasks are performed. This way every physicist in the world who wants to analyze LHC data doesn’t have to connect directly to CERN, running the risk of breaking the Internet for good.

Necessity is the mother of invention. It should come as no surprise that the unique data challenges presented by particle physics have led to unique solutions. One of those solutions, from many years ago now, has changed the way we all live: the World Wide Web. The Web originated in a 1989 proposal from Tim Berners-Lee, who at the time was working at CERN, and is currently director of the World Wide Web Consortium. Berners-Lee thought it would be useful for physicists at the lab to have access to different kinds of information, stored on distributed computers, through a hypertext system based on Web documents and links between them. The WWW is this system of interlinked files, built on top of the data-sharing network we call the Internet (for which we can’t give CERN any credit). The Web as we currently know it, and all the effects it has had on our lives, are spinoffs from basic research in particle physics.

Fabiola Gianotti, the Italian physicist who is the current leader of ATLAS, told me that the most pleasant surprise when the LHC first turned on wasn’t the performance of her experiment, although that was quite impressive—it was that the data transmission system functioned flawlessly right from the start. Not that the process has been entirely without challenges. In September 2008, soon after the first particles had circulated in the LHC, the computing system at CMS was hacked by a group billing itself as the “Greek Security Team.” They did no real damage, and in fact claimed to be performing a public service, as they replaced a Web page with a warning in Greek that said, “We’re pulling your pants down because we don’t want to see you running around naked looking to hide yourselves when the panic comes.” Order was quickly restored, and the disturbance didn’t delay the experiment in any way—although it maybe prompted a closer look at Internet security throughout CERN.

With the LHC itself humming along, CMS and ATLAS running at the peak of their capabilities, and data being rapidly shared and analyzed around the globe, all the pieces are in place for a full-on assault on the important questions in particle physics. One new particle is in the bag, and we’re looking for more.

SEVEN

PARTICLES IN THE WAVES

In which we suggest that everything in the universe is made out of fields: force fields that push and pull, and matter fields whose vibrations are particles.

T
he Insane Clown Posse, a hip-hop duo known for their provocative lyrics and scary clown makeup, caused a stir in 2010 with their single “Miracles.” At this point in their career, Violent J and Shaggy 2 Dope (not the names they were born with) were no strangers to controversy. They had engaged in a feud with Eminem, explored an unsuccessful stint as professional wrestlers, and once gave a brief concert to a bewildered audience only to find out that they were in the wrong building. Their songs tell stories of necrophilia and cannibalism, and in one case said mean things about Santa Claus. Also, Violent J was arrested after a show for hitting an audience member thirty times with his microphone.

But the “Miracles” controversy was something different. The lads weren’t aiming to shock but to share their wonder at the world around us. It came out like this:

Stop and look around, it’s all astounding

Water, fire, air and dirt

F***ing magnets,

How do they work?

Through the magic of the Internet, this little snippet gained quite a bit of notoriety, especially from scientifically minded types who were eager to point out that we actually have a pretty good idea of how magnets work.

I would like to stand up just a tiny bit for Insane Clown Posse. Yes, we’ve understood magnetism for quite some time, and scientific investigation generally enhances our appreciation of natural phenomena rather than draining the magic out of everything. However, they have put their fingers on an important fact we may be too quick to overlook: Magnets are actually pretty astounding.

What’s amazing about magnets is not that they stick to metal—lots of things stick to lots of other things, from geckos to pieces of chewing gum. What’s amazing is that, when you bring a magnet close to a piece of metal, you can feel it being attracted
before
they’re actually touching. Magnets aren’t like adhesive tape or glue, which must be in contact with something before sticking. Magnets reach out, across apparently empty space, to pull things toward them. Kind of freaky, when you think about it.

Physicists call this type of thing “action at a distance,” and it used to bother the world’s greatest minds as much as it bothers Violent J and Shaggy 2 Dope. These days we are less bothered, because we’ve figured out that the space across which the magnet is apparently reaching isn’t really “empty” at all. It’s filled with a magnetic field—invisible lines of force that reach out from the magnet—ready to grab ahold of any susceptible object that might come their way. We can make these lines of force seem more tangible by putting the magnet in the presence of some small iron filings, which line up with the magnetic field in beautiful patterns.

The important point is that the magnetic field is there whether or not it’s grabbing on to anything. If there is a magnet, there is a magnetic field that surrounds it, even though we can’t see it. The field is strong when we’re close to the magnet, and weaker far away. In fact, there’s a magnetic field at absolutely every point in space, regardless of whether there are any magnets nearby. The field might be quite small—or even precisely zero—but at every point there is some answer to the question “What’s the value of the magnetic field here?” (It really is “the” magnetic field, not a separate one for every magnet; put two magnets near each other and their fields just add together.)