The Physics of Colliding Particles

Will 2011 be the big year for the biggest particle accelerator?

7 June 2011

About 100 meters beneath the border of France and Switzerland, scientists have been smashing protons together at the Large Hadron Collider, the world's largest particle accelerator, hoping to reveal the secrets of our universe. Built by CERN, the European Organization for Nuclear Research, the LHC studies the smallest known particles—the fundamental building blocks of all things. After being turned on in 2008, it soon began recording its first proton-to-proton collisions. But the LHC ran into technical problems that required it to be shut down and repaired before it was turned back on in 2009.

Now, following 10 weeks of downtime for maintenance, the collider has been up and running again since late February. And LHC researchers say they hope to make some major discoveries this year.

One of those researchers is IEEE Member Fernando Lucas Rodríguez, control system coordinator of an LHC experiment called TOTEM (shorthand for total cross section, elastic scattering, and diffraction dissociation measurement).

"Everyone at CERN is very excited about this year," Lucas says. "The LHC is fully stable now and will be running continuously."

Lucas is in charge of the control system of TOTEM [shown above]. "The TOTEM experiment detector-control system runs 24 hours a day, 7 days a week," Lucas notes.

sub map Photo: Maximilien Brice

NUTS, BOLTS, AND PROTONS
The LHC sits in a circular tunnel 27 kilometers in circumference buried 50 to 175 meters underground. Inside, two beams of hadrons—either protons or lead ions—are accelerated in opposite directions at a velocity approaching the speed of light. The beams are steered into head-on collisions by powerful magnets at several points around the ring. The subatomic debris shot out of the collisions is recorded and analyzed by researchers.

The goal of the LHC is to better understand particle physics, validating or refuting theoretical models, finding new particles in collision debris, and revealing fundamental insights into the nature of the universe. This year researchers have a full wish list. They hope to find signs of dark matter, additional dimensions of space, microscopic black holes that evaporate and disappear, and the Higgs boson—the much sought-after particle, also known as the "God particle," whose existence was proposed by physicist Peter Higgs and others in 1964. It is thought to be the key to why some particles have mass and others do not.

The Higgs boson is the only standard-model particle that has never been observed in particle physics experiments. The physics standard model, which is composed of 16 particles, is the framework devised in the 1970s to explain how subatomic particles interact. The model has worked well for physicists thus far in understanding the laws of nature, but it cannot explain the best known of the four fundamental forces: gravity. It also describes only ordinary matter, which makes up a small part of the total universe; dark matter makes up a quarter of the mass in the observable universe.

TOTEM CONTROL
The LHC has four large detectors and two smaller ones, which gather collision data and conduct experiments. TOTEM, one of those smaller detectors, measures the size (total cross section) of the protons and how they scatter after a collision, as well as the LHC's luminosity, or the number of collisions in an area in a certain instant. Luminosity is a means of evaluating the accelerator's efficiency.

TOTEM measures particles circulating in main beam lines using specially designed detectors housed in vacuum chambers known as Roman pots. TOTEM is one of the smallest experiments at the LHC, Lucas says, but its detectors are innovative in how they analyze proton collisions. "The exceptional characteristic of TOTEM is that the Roman pots are movable, and we can get them very close to the particles accelerating inside," he says.

In addition, they can measure proton collisions with very small angles over a long distance in the LHC. "Sometimes particles don't disintegrate during a collision, but their trajectories change, and we want to study the angle of those collisions," he adds. Getting such precise measurements is important because it allows the researchers who process the LHC's data to better analyze the theoretical model of a proton.

To keep TOTEM running, Lucas's group has to keep an eye on several key areas, like temperature and pressure probes, voltages and currents, and accumulated radiation doses. "Over time, in the radiation environment of the collider, the sensors used in the detectors to gather data degrade and become less sensitive," he says. "In our control system, we have a set of dedicated sensors to measure the accumulated dose. When radiation increases, the current and voltage needed for each sensor change and finally, at a certain point, we may need to replace those sensors." All those parameters are constantly monitored—alarms go off if something is out of its expected range.

One of the biggest challenges is making sure all parts of the control system are interoperating as expected. That means overseeing the high voltage applied to the sensors, the low voltage applied to the electronics, the cooling system, cybersecurity, and the user interface. "But also, from a project management point of view," Lucas says, "having a solid management to understand what the configuration of the system is at each moment is a challenge.

"We're using a combination of commercial products, as well as custom-made technologies. But the main challenge of the control system is not just to provide a fast and real-time systems—it's making sure all the parts are well integrated and working together."

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