Tag Archives: LHCb

Standard Model Stands Firm

Dr. Harry Cliff, a Physicist working on the LHCb experiment and the first Science Museum Fellow of Modern Science, writes about a recent discovery at CERN. A new Collider exhibition opens in November 2013, taking a behind-the-scenes look at the famous particle physics laboratory. 

On Friday afternoon, at the EPS conference in Stockholm, two colleagues of mine from CERN stood up to announce that the search for one of the rarest processes in fundamental physics is over. The result is a stunning success for the Standard Model, our current best theory of particles and forces, and yet another blow for those hoping for signs of new physics from CERN’s Large Hadron Collider (LHC).

The Compact Muon Spectrometer, an experiment at CERN. Image credit: CERN.

The Compact Muon Spectrometer, an experiment at CERN. Image credit: CERN.

The LHCb and CMS experiments at the LHC have made the first definitive observation of a particle called a Bs meson decaying into two muons, confirming a tentative sighting at LHCb (my experiment) last autumn. The discovery has far-reaching implications for the search for new particles and forces of nature.

Beyond the Standard Model

There are a lot of reasons to suspect that the current Standard Model isn’t the end of the story when it comes to the building blocks of our Universe. Despite agreeing with almost every experimental measurement to date, it has several gaping holes. It completely leaves out the force of gravity and has no explanation for the enigmatic dark matter and dark energy that are thought to make up 95% of the Universe. The theory also requires a large amount of “fine-tuning” to match experimental observations, leaving it looking suspiciously like the laws of physics have been tinkered with in a very unnatural way to produce the Universe we live in.

In the last few decades a number of theories have been put forward that attempt to solve some of the Standard Model’s problems. One particularly popular idea is supersymmetry (SUSY for short), which proposes a slew of new fundamental particles, each one a mirror image of the particles of the Standard Model.

The Large Hadron Collider beauty (LHCb) experiment at CERN. Image credit: CERN.

The Large Hadron Collider beauty (LHCb) experiment at CERN. Image credit: CERN.

SUSY has many attractive features: it provides a neat explanation for dark matter and unifies the strengths of the three forces of the Standard Model (this suggests that they could all be aspects of one unified force, which should definitely be referred to as The Force, if it turns out to exist someday). It would also keep my colleagues in work for decades to come, thanks to a whole new load of super-particles (or sparticles) to discover and study.

However, physicists were first attracted to it because the theory is aesthetically pleasing. Unlike the Standard Model, SUSY doesn’t require any awkward fine-tuning to produce laws of physics that match our experience. This is not a very scientific argument, more a desire amongst physicists for theories to be elegant, but historically it has often been the case that the most beautiful theory turns out to be right one.

On the hunt

The decay observed at LHCb and CMS is predicted to be extremely rare in the Standard Model, with a Bs meson only decaying into two muons about 3 times in every billion. However, if ideas like SUSY are correct than the chances of the decay can be significantly boosted.

Finding particle decays this rare makes hunting for a needle in a haystack seem like a doddle. Hundreds of millions of collisions take place every second at the LHC, each one producing hundreds of new particles that leave electrical signals in the giant detectors. Physicists from LHCb and CMS trawled through two years worth of data, searching untold trillions of collisions for signs of two muons coming from a Bs meson. The pressure to be the first to find evidence of this rare process was intense, as Dr. Marc-Olivier Bettler, a colleague of mine from Cambridge and member of the LHCb team told me.

“It is a very strange type of race. To avoid bias, we don’t allow ourselves to look at the data until the last minute. So it’s a bit like running blindfolded – you can’t see the landscape around you or your competitors, even though you know that they’re there, so you have no idea if you are doing well or not! You only find out after you cross the finish line.”

However, ultimately the race ended in a draw. Neither LHCb nor CMS alone had enough data to announce a formal discovery, each turning up just a handful of likely candidates. But when their results are formally combined next week it is expected that the number of observed decays will pass the all-important “five sigma” level, above which a discovery can be declared.

Standard Model Stands Firm

In a blow for supporters of SUSY, LHCb and CMS observed the decay occurring at exactly the rate predicted by the Standard Model – approximately 3 times in a billion. This is yet another triumph for the Standard Model and kills off a number of the most popular SUSY theories.

Professor Val Gibson, leader of the Cambridge particle physics group and member of the LHCb experiment explained that, Measurements of this very rare decay significantly squeeze the places new physics can hide. We are now looking forward to the LHC returning at even higher energy and to an upgrade of the experiment so that we can investigate why new physics is so shy.”

This result is certainly not the end of the road for ideas like supersymmetry, which has many different versions. However, combined with the recent discovery of the Higgs boson (whose mass is larger than predicted by many SUSY theories) this new result may only leave us with versions of SUSY that are somewhat inelegant, meaning that the original motivation – a natural description of nature – is lost.

This new result from CERN is yet another demonstration of the fantastic (and somewhat annoying) accuracy of the Standard Model. Incredible precision is now being achieved by experiments at the LHC, allowing physicists to uncover ever-rarer particles and phenomena. If ideas like supersymmetry are to survive the onslaught of high precision tests made by the LHC experiments, we may have to accept that we live in a spookily fine-tuned Universe.

View of the LHCb cavern

X-citing news from CERN

Dr. Harry Cliff, a Physicist working on the LHCb experiment and the first Science Museum Fellow of Modern Science, writes about a new discovery at CERN for our blog. A new Science Museum exhibition about the Large Hadron Collider will open in November 2013, showcasing particle detectors and the stories of scientific discoveries.

In 2003 physicists at the Belle experiment in Japan reported they had discovered a brand new particle.

Adding a new entry to the big book of particle physics is certainly satisfying, but not usually cause for much excitement. The discovery of the Higgs-like boson last year was an exception. After all, hundreds of particles have shown up in experiments over the last century. So many in fact, that they were often referred to, rather derisively, as a “zoo”.

The Large Hadron Collider at CERN. Image Credit: CERN

But the particle found at Belle was different.

It didn’t fit neatly into the picture painted by theory and there was no clear explanation for its origin. It was a bit of an enigma, and earned a suitably enigmatic name: the X particle.

Professor Val Gibson from the University of Cambridge told me that she and her colleagues “have been mesmerized” about the identify this mysterious particle for the last ten years.

The Particle Zoo

The vast majority of the particles that make up the particle zoo are not fundamental; in other words they are made up of smaller things and these things are fundamental particles called quarks. Six different types of quark have been discovered and they can form a large number of different combinations, explaining the particle zoo.

However, quarks only bind together in very specific ways. Two ways in fact. One option is a ménage à trois known as a baryon. Baryons include the proton and the neutron, the building blocks of the atomic nucleus. The other option is where a quark and an antiquark couple up to form a meson.

The X didn’t fit easily into either of these pictures. This generated a lot of excitement and there was speculation as to whether it could be an ordinary meson, or some new exotic combination involving four quarks, a tetraquark, or a “molecule” of two mesons stuck together.

If this were true it would be the first time such an exotic state had been definitively seen in nature.

The only way to tell would be to measure the quantum numbers of the X, three properties that give a clue to its internal structure. This hadn’t been possible, until now.

Exciting, Exotic X

Amid the hundreds of trillions of collisions generated by the Large Hadron Collider over the past three years physicists at the LHCb experiment (the experiment I work on) managed to pick out about 300 X particles.

View of the LHCb cavern

View of the LHCb cavern. Image credit: CERN

This week, they presented the first full measurement of the quantum numbers of the X, at a conference at La Thuile in Italy. The result was emphatic – the X is not a meson, it is something altogether more exotic.

LHCb physicist Dr Matt Needham told me that “this measurement is a great step forward in understanding this mysterious X” and a “very exciting result”. However, there is still work to be done.

“The real nature (of the X) is still unclear”. Whether it’s a tetraquark, meson molecule or something else entirely must now be determined.

His colleagues at LHCb will now search for signs of the X decaying in new ways to try to separate out the various different options. Although the Large Hadron Collider has now shut down for two years physicists at LHCb will have no shortage of data to work with. An unprecedented sample was collected during 2012, corresponding to 180 trillion collisions, each one producing hundreds of particles.

The true nature of this enigmatic particle may soon be known. Whatever the result, we have now had our first glimpse of an altogether new state of matter. Finding out exactly what the X is will bring us deeper understanding of nature’s fundamental building blocks and the forces that bind them together.

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. The exhibition will showcase real pieces of the LHC, including an intricate particle detector from the heart of the LHCb experiment.

View of the LHCb cavern

Supersymmetry in a spin

Dr. Harry Cliff, a Physicist working on the LHCb experiment and the first Science Museum Fellow of Modern Science, writes about a new discovery at CERN for our blog. A new Science Museum exhibition about the Large Hadron Collider will open in November 2013, showcasing particle detectors and the stories of scientific discoveries.

There were high hopes that the world’s most powerful particle collider would find evidence for the theory of supersymmetry, which postulates that every member of the known bestiary of sub-atomic particles has a related but much more massive “super-partner”. The theory is considered more elegant than the current Standard Model of particles and forces and is particularly appealing as some of these supersymmetric particles, or “sparticles”, could account for the “dark matter” that sculpts the structure of the visible universe.

But the experiment I work on at the Large Hadron Collider (LHC) has spotted of one of the rarest particle decays ever seen in nature, a result that poses a serious challenge to supporters of “new physics” theories like supersymmetry.

View of the LHCb cavern

View of the LHCb cavern. Image credit: CERN

Results presented at the Hadron Collider Physics conference in Kyoto early this morning show the first convincing evidence for a particle called a Bs meson decaying into two muons. The decay was seen by my colleagues at the LHC beauty (LHCb) experiment, a gigantic particle detector on the 27km LHC ring at CERN, near Geneva.

This process is predicted to be very rare in the Standard Model, but if ideas like supersymmetry are correct then it could be much more common. However, the decay seems to be just as rare as the Standard Model predicted.

As we sat sharing a coffee at the Cavendish lab in Cambridge, Dr Marc-Olivier Bettler, a member of the international team who produced the result, told me it puts “strong constraints” on supersymmetry.

Rarer than winning the lottery
The LHC has been smashing protons into each other at close to the speed of light almost non-stop since November 2009. Each collision creates a shower of new particles, and occasionally a Bs meson is produced. The LHCb detector was built to study exotic these exotic particles.

Dr Bettler and his colleagues churned through hundreds of trillions of collisions produced by the LHC in search of the decay. The huge amount of data recorded by the LHCb experiment was processed using a world-wide network of computer processors known as the Grid. In the end they turned up just a handful of likely candidates.

Their results show that the chance of a Bs meson converting to two muons is about one in 300 million. That’s thirty times less likely than winning the jackpot on the lottery with a single ticket.

New physics hiding
Finding evidence of the decay is a triumph for LHCb, but will be a big disappointment for theorists who have spent many years working on supersymmetry. Prof. Val Gibson, leader of the LHCb group at the University of Cambridge said “this key result is putting our supersymmetry theory colleagues in a spin”. The result also makes it much less likely that the other main LHC experiments, ATLAS and CMS, will discover signs of supersymmetric particles any time soon. “If new physics is present then it is hiding very well behind the Standard Model” said Dr Bettler.

Even though it may be less thrilling than discovering new particles or forces of nature, these extremely precise measurements are crucial to improving our understanding of the Universe. “This result is important because it tells us what new physics isn’t.” Dr Bettler certainly didn’t find the outcome disappointing, describing his reaction at seeing the results for the first time two weeks ago as “wow! I was very excited. It has been a very exciting two weeks, that’s for sure.”

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. The exhibition will showcase real pieces of the LHC, including an intricate particle detector from the heart of the LHCb experiment.