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LHC's Compact Muon Solenoid detector.

LHC's Compact Muon Solenoid detector.


Five things scientists could learn with their new, improved particle accelerator

SAN JOSE, CALIFORNIA—The Large Hadron Collider (LHC) is back, and it’s better than ever. The particle accelerator, located at CERN, the European particle physics lab near Geneva, Switzerland, shut down in February 2013, and since then scientists have been upgrading and repairing it and its particle detectors. The LHC will be back up to full speed this May. Yesterday, scientists discussed the new prospects for the LHC at the annual meeting of AAAS (which publishes Science).

The LHC is the world’s most powerful particle accelerator. Protons blast along its 17-mile (27-kilometer) ring at nearly light speed, colliding at the sites of several particle detectors, which sift through the resulting particle debris. In 2012, LHC’s ATLAS and CMS experiments discovered the Higgs boson with data from the LHC’s first run, thereby explaining how particles get mass. The revamped LHC will run at a 60% higher energy, with more sensitive detectors, and a higher collision rate. What might we find with the new-and-improved machine? Here are five questions scientists hope to answer:

1. Does the Higgs boson hold any surprises?

Now that we’ve found the Higgs boson, there’s still a lot we can learn from it. Thanks to the LHC’s energy boost, it will produce Higgs bosons at a rate five times higher, and scientists will be using the resulting abundance of Higgs to understand the particle in detail. How does it decay? Does it match the theoretical predictions? Anything out of the ordinary would be a boon to physicists, who are looking for evidence of new phenomena that can explain some of the unsolved mysteries of physics.

2. What is “dark matter”?

Only 15% of the matter in the universe is the kind we are familiar with. The rest is dark matter, which is invisible to us except for subtle hints, like its gravitational effects on the cosmos. Physicists are clamoring to know what it is. One likely dark matter culprit is a WIMP, or weakly interacting massive particle, which could show up in the LHC. Dark matter’s fingerprints could even be found on the Higgs boson, which may sometimes decay to dark matter. You can bet that scientists will be sifting through their data for any trace.

3. Will we ever find supersymmetry?

Supersymmetry, or SUSY, is a hugely popular theory of particle physics that would solve many unanswered questions about physics, including why the mass of the Higgs boson is lighter than naively expectedif only it were true. This theory proposes a slew of exotic elementary particles that are heavier twins of known ones, but with different spin—a type of intrinsic rotational momentum. Higher energies at the new LHC could boost the production of hypothetical supersymmetric particles called gluinos by a factor of 60, increasing the odds of finding it.

4. Where did all the antimatter go?

Physicists don’t know why we exist. According to theory, after the big bang the universe was equal parts matter and antimatter, which annihilate one another when they meet. This should have eventually resulted in a lifeless universe devoid of matter. But instead, our universe is full of matter, and antimatter is rare—somehow, the balance between matter and antimatter tipped. With the upgraded LHC, experiments will be able to precisely test how matter might differ from antimatter, and how our universe came to be.

5. What was our infant universe like?

Just after the big bang, our universe was so hot and dense that protons and neutrons couldn’t form, and the particles that make them up—quarks and gluons—floated in a soup known as the quark-gluon plasma. To study this type of matter, the LHC produces extra-violent collisions using lead nuclei instead of protons, recreating the fireball of the primordial universe. Aided by the new LHC’s higher rate of collisions, scientists will be able to take more baby photos of our universe than ever before.

Check out our full coverage of the AAAS annual meeting.

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