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What will we discover when we switch the LHC back on?

What will we discover when we switch the LHC back on?
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Sarah Charley

Opinion – Sarah Charley, Fermilab/CERN science writer

Some of the greatest Explorers of our time spend their lives focused around a giant underground tunnel near Geneva. Particle physicists may not have the public image of the outdoor pioneer at the edge of the unknown, but their research is most definitely at the limit of what’s physically and intellectually possible here on planet Earth. In this article, Fermilab/CERN science writer Sarah Charley looks forward to the moment when the undisputed king of science experiments, the LHC, swings back into action next year

Physicists can truly only imagine what they might find when they switch back on the LHC, or Large Hadron Collider—the world’s largest and most powerful particle accelerator.

In 2012 it gave us the Higgs boson, a particle that helps explain why the universe contains solid, massive objects and not just a soup of unruly energy.

So, what about 2015? Will physicists find new fundamental particles? Or hints of dark matter? Or something completely unexpected? The truth is, nobody can be sure, and a true spirit of exploration reins over the tight-knit community at CERN on the outskirts of Geneva.

In the physics community, the LHC is called a “discovery machine” because of the huge potential it has to give us insights into the realm of the unknown. And this is only the beginning.

The Large Hadron Collider is buried in a tunnel 100 metres below ground, between Lake Geneva in Switzerland and the Jura mountains in France. At a startling 27 kilometers in circumference, the Large Hadron Collider is the world’s largest and most powerful particle accelerator.

But in order to reach energies previously unattainable on Earth, the LHC must operate under some of the most extreme conditions in the universe. Its superconducting magnets—which steer and focus particle beams—are colder than outer space. The vacuum inside the 27-kilometre long beam pipe, is almost as thin as the atmosphere of the Moon

Inside the LHC beam pipe, protons and other atomic nuclei travel at close to the speed of light and generate the highest-energy collisions on Earth. These collisions can reach temperatures 500,000 times hotter than the center of the Sun, generating conditions similar to those that existed immediately after the Big Bang.

Four huge and amazingly sophisticated detectors are situated around the LHC ring in order to record and track the particle shrapnel produced during these collisions.

The LHC operates under these extreme conditions with one goal in mind: to create elementary particles—the smallest bits of matter known to man—in order to learn more about matter at its most basic level.

As the home of the LHC, CERN has become a sort of Mecca for fundamental research. Its halls are continually buzzing with both staff and visiting scientists from all over the world. But the LHC is much bigger than CERN itself. The data from these high-energy collisions is farmed out to institutions all over the word—places like, Taipei, Vancouver and Illinois—for analysis. Only a fraction of the data analysis actually occurs at CERN.

The history of this field dates back to long before anyone could have even imagined a machine like the LHC. In the late 1890s scientists first started finding weird and unexplainable phenomenon like x-rays and radioactive decay. In the 1930s, they had their first glimpse of a muon—a particle with no apparent purpose that left them baffled because it didn’t fit the standard proton-neutron-electron model of matter.

Since that groundbreaking discovery, physicists have used particle accelerators to search for more of these basic constituents of matter—the fundamental particles with strange names like quarks, leptons and bosons.

Researchers try to understand how these particles behave and interact to gain insight into such mind-bendingly tricky questions as, “What is matter,” “Where did it come from,” and “Why does it behave the way it does?”

Just 50 years ago, scientists were probing the inner workings of protons. Now, collider experiments like the LHC have enabled physicists to delve deeper into the fundamental structure of matter, developing and expanding models that describe our universe.

Our best understanding of how matter behaves is summed up in the Standard Model of Particle Physics—an elegant set of equations and theories that describes subatomic particles and their interactions.

The Standard Model was developed over the course of several decades through a combination of theory and experiment. Based on the equations of the Standard Model physicists were able to predict the existence of particles such as the top quark and the Higgs boson long before they were able to experimentally show they existed.

But this model has cracks and corners that need to be filled.

When the LHC first operated —on 10 September 2008—theorists and experimentalists thought that it would immediately provide heaps of evidence to support theories that fill those cracks in the Standard Model.

Within three years, the machine confirmed the existence of the previously theoretical Higgs boson, which describes how fundamental particles gain mass. The discovery provided strong evidence for the veracity of the Standard Model, but left other popular theories out to dry.

Supersymmetry, for example, predicts a supersymmetric partner for each fundamental particle. Scientists are searching for evidence of this theory, but so far have not seen any hints of supersymmetric particles buried in the LHC data.

But a lack of evidence for one theory presents an exciting opportunity for others. As physicists learn more, their questions grow more complex: “How did the universe come into existence?,” “Why is there an imbalance of matter and antimatter?,” and “Are there extra dimensions?”

The Standard Model draws a blank on these questions and struggles to accommodate key concepts such as dark matter, gravity, and general relativity.

At the LHC, scientists search for hints of new physical phenomenon by looking at what their detectors record—or don’t record—during the high-energy collisions. When the protons collide, they produce hundreds of new particles, which deposit energy as they pass through the detectors.

Researchers use the timing and location of these deposits to work out the paths that the particles took through the detector. But if a large chunk of energy suddenly goes missing, it could also be the telltale sign of a new particle our detectors cannot perceive, such as dark matter.

Another way scientists search for new physics is by scrutinizing the properties of known particles. The Standard Model makes strong predictions about the properties of particles such as the top quark and Higgs boson. But if these predictions are proved false by experimental tests, it is an indication that some other type of physics might be at play.

Since the accelerator shut down for upgrades and repairs in early 2013, physicists have been busily combing through the data generated during this first round of high-energy collisions. They have made huge advances—precisely measuring characteristics of the Higgs boson, narrowing their search for supersymmetry and dark matter, and pinning down evidence for an exotic four-quark particle.

But physicists can only imagine what they might see during the next run of the LHC in early 2015. Will they find evidence for new particles or phenomena that fill the cracks and corners of the Standard Model? Or something completely unexpected? We can’t wait to find out.

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