Explainer: How Does an Experiment At The Large Hadron Collider Work?

Explainer: How Does an Experiment At The Large Hadron Collider Work?

Gavin Hesketh, UCL

It’s not every day my Twitter feed is full of people talking about flat-tops, squeezing and injections, but then Wednesday 3 June was not an average day for the Large Hadron Collider.

The LHC is the world’s largest particle accelerator and lies in a tunnel below CERN, the European physics lab just outside Geneva. And on Wednesday it was restarted after two year break for repairs and upgrades, ready to push our understanding of the universe to new limits.

As my fellow physicists crowded into the control rooms and waited for things to get underway, I was at a workshop in France. But I was able to follow the switch-on online. Here’s how things went down.

8.09am. Injection: Billions of protons are loaded into the LHC.

The LHC is a ring roughly 28km around that accelerates protons almost to the speed of light before colliding them head on. Protons are particles found in the atomic nucleus, roughly one thousand-million-millionth of a metre in size.

They are easiest to get from hydrogen, the simplest atom with just one electron orbiting one proton. The LHC starts with a bottle of hydrogen gas, which is sent through an electric field to strip away the electrons, leaving just the protons. Electric and magnetic fields are the key to a particle accelerator: because protons are positively charged, they accelerate when in an electric field and bend in a circle in a magnetic field.

Big data

9.45am. Ramp: Once the LHC is fully loaded, its two proton beams are slowly accelerated up to collision energy, now a world-record 6.5TeV per beam.

Accelerating billions of protons to close to the speed of light, directing them all the way around the LHC, and then colliding them head-on, is a delicate balancing act performed by high voltage equipment and giant magnets. This is an amazing technical achievement. Indeed one of the main applications of particle physics research is in the industrial applications of the technology it develops along the way, from proton therapy cancer treatment to the world wide web.

But for me, the excitement is in the science: the LHC is exploring the universe at the smallest scales. Everything we have learned so far is formulated in the Standard Model, a theory which describes the universe made of tiny particles, and gives the rules for how these particles behave. By smashing some of these particles together at high energy, we are able to test these rules and make new discoveries.

The LHC “Run 1” (2010-2013) provided enough data to test the Standard Model to new levels of precision and discover the Higgs boson. This particle was predicted in the 1960s and plays a central role in the Standard Model. But it was almost 50 years before we had a machine powerful enough to discover it. As well as high energy, it needed lots of data: the Higgs boson is a rare thing, and fewer than one in a billion collisions at the LHC produce one.

Tense moments
Laurent Egli/CERN

10.12am. Flat top: Beam energy levels off after reaching the target.

These were tense moments for the CERN team on Wednesday. The LHC was operating at the highest energy ever achieved in a particle accelerator. “Run 2” will collide protons at 60% higher energies than Run 1 by pushing the magnets and accelerators to the limit. We hope this extra reach will allow us to tackle some of the big questions in particle physics.

One of the main topics is dark matter. This seems to be a new type of particle spread through the entire universe. And with the LHC Run 2 we hope to make it in the lab for the first time. But if the Higgs boson is rare, dark matter is even rarer, and we will need to sort through a lot of collisions before having a hope of finding it.

Worlds collide

10.17am. Squeeze: The beams are fine-tuned, and focused at the four points around the LHC where they cross, and the experiments will record the collisions

Almost there. The experiments now need to wait for the all-clear before they can start recording, and we begin studying things that have never been seen before. Still, many of the collisions will not be interesting, as the protons just smash apart without doing anything exciting.

To make matters worse, the rare new particles we are looking for also tend to be very unstable, and decay too quickly to be seen directly. So the job of the experiments is to measure whatever particles do come out of a collision and try to reconstruct what happened, looking for evidence of something unusual.

As well as dark matter, there are many other ideas to test, such as supersymmetry, new gauge bosons, quantum black holes and heavy neutrinos, all of which we could reconstruct from the LHC collisions. Part of the joy and pain of science is that a new discovery could come in a matter of days, or a matter of years.

Champagne flowing
Mike Struik/CERN

10.43am. Stable beams: The LHC is now running smoothly, the beams are behaving as expected, and the experiments can start recording data.

Run 2 has begun! Champagne is flowing at CERN. Now the attention moves to analysing the new data, and it’s time for the rest of us to get back to work.

The Conversation

Gavin Hesketh is Lecturer in Particle Physics at UCL.

This article was originally published on The Conversation.
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Physicists Closer to Understanding Balance of Matter, Antimatter in Universe

Physicists Closer to Understanding Balance of Matter, Antimatter in Universe


Physicists in the College of Arts and Sciences have made important discoveries regarding Bs meson particles—something that may explain why the universe contains more matter than antimatter.

Distinguished Professor Sheldon Stone and his colleagues recently announced their findings at a workshop at CERN in Geneva, Switzerland. Titled “Implications of LHCb Measurements and Their Future Prospects,” the workshop enabled him and other members of the Large Hadron Collider beauty (LHCb) Collaboration to share recent data results.

The LHCb Collaboration is a multinational experiment that seeks to explore what happened after the Big Bang, causing matter to survive and flourish in the Universe. LHCb is an international experiment, based at CERN, involving more than 800 scientists and engineers from all over the world. At CERN, Stone heads up a team of 15 physicists from Syracuse.

“Many international experiments are interested in the Bs meson because it oscillates between a matter particle and an antimatter particle,” says Stone, who heads up Syracuse’s High-Energy Physics Group. “Understanding its properties may shed light on charge-parity [CP] violation, which refers to the balance of matter and antimatter in the universe and is one of the biggest challenges of particle physics.”

Scientists believe that, 14 billion years ago, energy coalesced to form equal quantities of matter and antimatter. As the universe cooled and expanded, its composition changed. Antimatter all but disappeared after the Big Bang (approximately 3.8 billion years ago), leaving behind matter to create everything from stars and galaxies to life on Earth.

“Something must have happened to cause extra CP violation and, thus, form the universe as we know it,” Stone says.

He thinks part of the answer lies in the Bs meson, which contains an antiquark and a strange quark and is bound together by a strong interaction. (A quark is a hard, point-like object found inside a proton and neutron that forms the nucleus of an atom.)

Enter CERN, a European research organization that operates the world’s largest particle physics laboratory.

In Geneva, Stone and his research team—which includes Liming Zhang, a former Syracuse research associate who is now a professor at Tsinghua University in Beijing, China—have studied two landmark experiments that took place at Fermilab, a high-energy physics laboratory near Chicago, in 2009.

The experiments involved the Collider Detector at Fermilab (CDF) and the DZero (D0), four-story detectors that were part of Fermilab’s now-defunct Tevatron, then one of the world’s highest-energy particle accelerators.

“Results from D0 and CDF showed that the matter-antimatter oscillations of the Bs meson deviated from the standard model of physics, but the uncertainties of their results were too high to make any solid conclusions,” Stone says.

He and Zhang had no choice but to devise a technique allowing for more precise measurements of Bs mesons. Their new result shows that the difference in oscillations between the Bs and anti-Bs meson is just as the standard model has predicted.

Stone says the new measurement dramatically restricts the realms where new physics could be hiding, forcing physicists to expand their searches into other areas. “Everyone knows there is new physics. We just need to perform more sensitive analyses to sniff it out,” he adds.


Story Source:

The above story is based on materials provided by Syracuse University. Note: Materials may be edited for content and length.


ALPHA-2 arrives at CERN

ALPHA-2 arrives at CERN

While many experiments are methodically planning for intense works over the long shutdown, there is one experiment that is already working at full steam: ALPHA-2. Its final components arrived last month and will completely replace the previous ALPHA set-up. Unlike its predecessor, this next generation experiment has been specifically designed to measure the properties of antimatter.
Read more about ALPHA-2: http://cdsweb.cern.ch/journal/CERNBulletin/2012/47/News%20Articles/1493544?ln=en

The Large Hadron Collider and the Higgs boson: Latest news from the energy frontier

The Large Hadron Collider and the Higgs boson: Latest news from the energy frontier

Prof Jon Butterworth, Head of Department (Physics & Astronomy), UCL Mathematical and Physical Sciences Faculty

The Large Hadron Collider at CERN is using proton-proton collisions to explore physics at higher energies than ever before. This is the region where the weak and electromagnetic forces unify, where mass originates, and where a new particle — looking very much like the Higgs boson — has just been found. The talk will give an account of the discovery and an update on progress since.