Author Archives: Alice, Content Developer

Happy birthday, Z boson

Alice Lighton, content developer for our Collider exhibition, writes about the history of quantum physics. Collider: step inside the world’s greatest experiment opens in November 2013 with a behind-the-scenes look at the famous CERN particle physics laboratory. 

The air brimmed with excitement on this momentous day. The discovery of the particle confirmed a theory that had taken years to devise, and justified the toil of hundreds of scientists.

You might think I’m referring to the Higgs boson – the particle that explains mass, discovered at the LHC last year. But thirty years ago this month, another event shaped modern physics – the discovery of the Z boson.

In the 1960s, physicists predicted the Z and W bosons, as a way to link the electromagnetic and weak forces. There was plenty of evidence the theory was correct, but the lynchpin would be the discovery of the Z boson.

A section of the 4.3 mile-round Super Proton Synchrotron, at CERN near Geneva. Image: CERN

To make a Z boson, two particles are smashed together. The energy of the crash creates new, heavy particles. If a Z boson is produced, it sticks around for only a fraction of a second before it decays into other particles. To claim the prize of discovering the Z boson, physicists would need to be able to forensically reconstruct what happens in a collision, never seeing the Z directly.

Europe and America built machines to discover the Z, including the Super Proton Synchrotron (SPS) at CERN. “The idea of creating this massive object (the Z) and letting it decay…was a riveting idea (well at least for me in the late 1970s),” said Crispin Williams, a physicist who now works on the ALICE experiment at the LHC.

Two CERN physicists, effusive Italian Carlo Rubbia and Dutchman Simon Van der Meer, realised that to beat the firepower of the newly-opened Tevatron in Chicago, the SPS had to take risks. The pair devised an audacious plan; rather than fire beams onto a fixed object, they would collide two opposing beams, each only a hair’s width across and both travelling at almost the speed of light.

What’s more, one of the beams would be made of antimatter, which destroys ordinary matter. Creating and manipulating a beam of antimatter was a revolutionary concept.

Williams remembers when Rubbia and Van der Meer announced their plan to collide two beams. “This was to a packed auditorium at CERN and I suspect that most people thought he was out of his mind,” said Williams.

Rubbia and Van der Meer celebrate receiving a Nobel prize for their efforts. Image: CERN

Despite the technical challenge, the new collider worked. One visitor to CERN in 1982 described the intense excitement the new development created. “I went to the CERN cafeteria for a coffee and there I saw something that I had not noticed before. There was a monitor on the wall and people were watching the screen with great interest. The monitor was showing the rate of proton–antiproton collisions in CERN’s latest challenge – a bold venture designed to produce the intermediate bosons, W and Z.”

In January 1983, the risk-takers received their reward, when the W boson was discovered.  On 1st June 1983, scientists at CERN announced they had seen five Z bosons in their detectors.

The tracks left by the decay of the Z boson in a detector. Image: UA1/CERN

The route to the discovery had revolutionised particle physics, with more intricate detectors and the ability to manipulate antimatter. For Williams, the discovery of the Higgs boson was much less elegant. “In comparison the Higgs at the LHC is just brute force,” he said.  “Maybe I am just getting old and cynical: and I look back at the Z discovery through rose tinted glasses.”

A hundred years of the quantum atom

Alice Lighton, content developer for our Collider exhibition, writes about the history of quantum physics. Colider: step inside the world’s greatest experiment opens in November 2013 with a behind-the-scenes look at the famous CERN particle physics laboratory. 

A few years ago, a friend asked a question that took me somewhat by surprise. “Alice,” he said, “is quantum physics right, or is it just a theory?”

At the time I was in the midst of a physics degree, so my initial response was “I hope so!” Quantum physics matches up to experiment extraordinarily well – it is often called the most accurate theory ever. But the question, and subsequent conversation, made me realise how little many people know about the subject, despite its profound impact on modern life and the way we think about the universe.

This year is the centenary of the publication of one of the theories that laid the foundation for our understanding of matter in terms of quanta – packets of energy. According to quantum mechanics, light is not a wave, but lump of energy called photons. Max Planck came up with the idea at the end of the 19th Century, though he considered his light ‘quanta’ a useful model, rather than reality.

Niels Bohr

Niels Bohr, one of the founders of modern physics.

One hundred years ago, in 1913, the young Danish researcher Niels Bohr sent a paper to the Philosophical Magazine in London that used these quanta to solve a serious problem with theories about the atom. At the time, scientists thought the atom was like a solar systems; electrons orbit a nucleus of protons and neutrons. But anything that moves in a circle gradually slowly radiates energy, and so moves closer to the centre of orbit. Eventually, electrons should fall into the nucleus of the atom.

But they blatantly don’t, otherwise everything in the Universe would collapse, and we wouldn’t exist. Bohr proposed that electrons could only sit in discrete orbits or distances from the nucleus – and therefore when electrons change orbit transitions between orbits emit only emit energy in discrete packets (quanta), not gradually. The electrons therefore stay put in their orbits, and don’t fall into the nucleus of the atom.

A hydrogen atom is made from one electron orbiting a proton. Photo credit: flickr/Ludie Cochrane

Bohr was the first to show that packets of energy could successfully explain and predict the behaviour of atoms, the stuff that makes up you and me. His results were only approximately correct, but a big improvement of previous theories.

Generations of scientists have built on Bohr’s insight to understand and create the modern world. When my friend asked whether quantum physics worked, I pointed at his laptop. Computers, nanotechnology, and the Large Hadron Collider owe their existence to the physics that began with Bohr’s generation.

The CMS experiment at the Large Hadron Collider tries to work out the rules governing the sub-atomic world. Photo credit: CERN

Bohr’s original papers are clear and comprehensible, a beautiful read for physicists. The mathematics involves nothing more difficult than multiplication and division, yet the philosophical implications are immense. Max Planck never fully accepted quantum physics; neither did Albert Einstein, despite winning a Nobel Prize for his work on the subject.

Bohr also won a Nobel Prize for his quantum theory, but his work did not stop. He founded the Niels Bohr Institute, a centre of theoretical physics in Copenhagen, worked on the Manhattan Project developing the atomic bomb, and continued to make contributions to quantum mechanics.

And he has a lovely link to the exhibition I’m currently working on, about the Large Hadron Collider. Bohr was influential in the founding of CERN, the Geneva laboratory that is home to the LHC. If he had his way, the LHC would be in Denmark, but other scientists objected – Northern Europe was too cloudy, and had too few ski resorts, for Italian tastes.

LHC home screen Jan 3rd

The LHC’s Christmas Holiday

Over the past three weeks, deep under the Jura Mountains on the Swiss-French border, a monster has been sleeping. Over Christmas, the Large Hadron Collider, the world’s largest experiment, takes a break from colliding protons together in an underground tunnel. The machine normally runs for 24 hour-a-day, seven days a week, but for four weeks in January and December, it is switched off.

LHC home screen Jan 3rd

So long, and thanks for all the fish! The LHC operators look forward to their Christmas holiday.

There are several reasons for the extended break. The physicists, engineers and support staff who operate the machine and experiments are human. Yes, they are devoted to the search for the fundamental laws that govern the Universe, but they also like to indulge on Christmas pudding and see their families.

That explains why the LHC doesn’t run on Christmas day, but why does it shut down for three weeks?

Because it’s cold outside.

The cold doesn’t affect how the LHC works – far from it, as the machine is cooled to -271ºC. But it does affect the power supply.

One of the most intriguing facts I’ve learned over the course of working on the Science Museum’s upcoming LHC exhibition is that even though the LHC does an extremely specialised and power-consuming task – accelerating protons so they have the energy of a high speed train and are travelling at nearly the speed of light – the machine takes its power from the French grid. The same nuclear, coal and hydro-electricity plants that provide the energy to light the Mona Lisa and charge your mobile on holiday also power the LHC.

When it’s cold outside French electricity consumption spikes. In December, France uses about 50 percent more electricity than it does in August, heating, cooking and lighting dark days. When all systems are go, CERN can use as much as a third of Geneva’s power, or the same as a large town. So during darkest depths of winter, when the French grid is being stretched the most, the LHC powers down.

The time off isn’t wasted. Repairs and upgrades are always needed, so engineers have been busy tweaking to ensure the LHC is in tip-top condition for its run in 2013. From next week LHC will fire protons into lead nuclei for a month. After that short run, the machine shuts for two years for a serious upgrade.

A Higgs boson is produced in the ATLAS detector

The boring boson?

Last week scientists working on the Large Hadron Collider in Geneva updated their colleagues on the newly-discovered Higgs boson. They revealed what they now know about the particle – and so far, it is behaving exactly as they expected. While this might seem like good news, for some people it is the opposite, because a well-behaved Higgs might rule out some intriguing new physics theories.

A Higgs boson is produced in the ATLAS detector

A Higgs boson is produced in the ATLAS detector

The Higgs – the particle which explains why others have mass – is incredibly unstable and only exists for a fraction of a second before decaying into other, more common particles. Any information about it comes second-hand from these other particles, and working out the properties is rather like putting together clues in a Sherlock Holmes tale, only with more mathematics.

Finding the Higgs in July was a wonderful coup for the LHC, but there now follows years of painstaking work to determine its precise properties. If the Higgs behaves even a smidgen differently from predictions, then it might point scientists in the direction of a new theory.

One particularly popular idea has the rather grand name of “supersymmetry”, which as we wrote on this blog last week, is looking less likely to be true.

There are lots of problems with current theories about the Universe – they don’t explain dark matter, and particle physics is completely incompatible with Einstein’s theories of gravity. Supersymmetry solves some of these issues in a whizz of complicated mathematics, but requires the existence of a whole family of new particles. If they exist, the Higgs’ properties should reveal them.

The results announced on Wednseday in Japan don’t lend the under-fire supersymmetry any more support. They suggest that so far, the Higgs behaves just as our current theory predicts it should. Specifically, when it decays, it turns into different types of particles at the rates we expect.

To some in the community, the Higgs’ conformity is rather disappointing.  But not all of the analysis was ready for the Japan conference and there is still uncertainty around the results that were announced, and supersymmetry still could work.

Even though the LHC has already analysed more data in two years than its predecessor managed in twenty, the measurements are not yet particularly precise, and the Higgs may still harbour surprises. The LHC still has not detected a Higgs decaying into quarks (the smallest unit of matter), for example – we just know that since we haven’t seen it yet, it can’t happen often. In other words: watch this space.

Visitors to the Science Museum will have a chance to get up close and personal with the LHC at a new exhibition opening in November 2013.

LHCb VELO Modules

LHC Exhibition: From Liverpool to the LHC, via easyJet

In autumn 2013 an exhibition about the LHC will open in the Science Museum, and we’re currently scouting out objects and stories for the show. This post is the first in a series about the exhibition. Myself and Harry Cliff from the LHC exhibition team ventured to Liverpool to take a closer look at the detector that sits at the heart of the LHCb experiment.

LHCb VELO Modules

The Oliver Lodge building, home to the Universityof Liverpool particle physics department, is a typically plain post-war block. But inside, technicians and researchers constructed one of the most beautiful parts of the Large Hadron Collider (LHC): the LHCb Vertex Locator or “VELO”.

The VELO is a precision engineered piece of equipment, and we had to put on teletubby-style outfits to enter the clean room where the modules were painstakingly put together. A peek through a microscope at a spare module revealed the intricate detail in each board; hundreds of perfectly aligned connections, delicate strips of silicon and tiny computer chips.

But once assembled, the modules are surprisingly hardy. Some were taken to the LHC in Geneva in hand baggage on an easyJet flight; brave researchers drove the rest through the Channel tunnel in a hire car. Once they arrived, this incredibly intricate device was carefully put in position. It sits just millimetres from awesome power of the LHC’s proton beams, enduring high levels of radiation for years on end without missing a beat.

Most of media flurry about the LHC has concentrated on the hunt for the Higgs boson. LHCb has a different mission. As Dr Tara Shears explained, our universe is made of normal matter, not its mirror image, antimatter, and at LHCb scientists are attempting to find out where the antimatter has gone.

The LHC collides protons at near light speed. The energy of the crash creates new particles that spray out in all directions. Our host at Liverpool, Dr Girish Patel, explained that the VELO comprises 42 modules, which are lined up in pairs to form circular detectors – the proton beams travel through the hole in the centre of each pair. The pairs are lined up along the beam to record the trajectory of the new particles.

The VELO allows scientists to work out precisely where particles were created, to within a hundredth of a millimetre. It is surrounded by much larger detectors that identify what types of particle were made in each collision. LHCb is looking for a type of particle known as a bottom quark. It doesn’t detect the bottom particles directly, because they decay into other particles before they reach VELO. LHCb tracks these other particles, looking for the fingerprint of the bottom quark among the mass of data.

Thanks to everyone at Liverpool for a fascinating day, particularly Girish, Tara and Themis. For more info on the VELO, take a look at the LHCb website.

Image courtesy of CERN