Quantum Diaries

Inspired by the event at the UNESCO headquarters in Paris that celebrated the anniversary of the signature of the CERN convention, Sophie Redford wrote about her impressions on joining CERN as a young researcher. A CERN fellow designing detectors for the future CLIC accelerator, she did her PhD at the University of Oxford, observing rare B decays with the LHCb experiment.

The “60 years of CERN” celebrations give us all the chance to reflect on the history of our organization. As a young scientist, the early years of CERN might seem remote. However, the continuity of CERN and its values connects this distant past to the present day. At CERN, the past isn’t so far away.

Of course, no matter when you arrive at CERN for the first time, it doesn’t take long to realize that you are in a place with a special history. On the surface, CERN can appear scruffy. Haphazard buildings produce a maze of long corridors, labelled with seemingly random numbers to test the navigation of newcomers. Auditoriums retain original artefacts: ashtrays and blackboards unchanged since the beginning, alongside the modern-day gadgetry of projectors and video-conferencing systems.

The theme of re-use continues underground, where older machines form the injection chain for new. It is here, in the tunnels and caverns buried below the French and Swiss countryside, where CERN spends its money. Accelerators and detectors, their immense size juxtaposed with their minute detail, constitute an unparalleled scientific experiment gone global. As a young scientist this is the stuff of dreams, and you can’t help but feel lucky to be a part of it.

If the physical situation of CERN seems unique, so is the sociological. The row of flags flying outside the main entrance is a colourful red herring, for aside from our diverse allegiances during international sporting events, nationality is meaningless inside CERN. Despite its location straddling international borders, despite our wallets containing two currencies and our heads many languages, scientific excellence is the only thing that matters here. This is a community driven by curiosity, where coffee and cooperation result in particle beams. At CERN we question the laws of our universe. Many answers are as yet unknown but our shared goal of discovery bonds us irrespective of age or nationality.

As a young scientist at CERN I feel welcome and valued; this is an environment where reason and logic rule. I feel privileged to profit from the past endeavour of others, and great pride to contribute to the future of that which others have started. I have learnt that together we can achieve extraordinary things, and that seemingly insurmountable problems can be overcome.

In many ways, the second 60 years of CERN will be nothing like the first. But by continuing to build on our past we can carry the founding values of CERN into the future, allowing the next generation of young scientists to pursue knowledge without borders.

By Sophie Redford

Singing happy birthday slightly off-key but in good spirit. This is how several hundred physicists gathered for the 37^th International Conference on High Energy Physics in Valencia, Spain closed the day on July 4^th. Two years before, the CMS and ATLAS experiments had announced the discovery of the Higgs boson on the eve of the same conference that was then held in Melbourne, Australia. Lots of people reminisced about the day of the announcement, where they were when they heard the news since many were traveling.

Two years later, the two experiments have now gathered an impressive amount of knowledge on the Higgs boson. Both groups have measured with high precision the Higgs boson mass, how it is produced and how it decays. ATLAS presented its published Higgs boson mass combination, namely 125.36 ± 0.41 GeV also in perfect agreement with the  CMS measurement, presented for the first time at this conference, of 125.03 ± 0.30 GeV.

By presenting its final Higgs boson decay to two photons results, the CMS Collaboration has now completed its analysis of all the data taken so far. The obtained value for the combined signal strength, which is how many Higgs bosons are observed compared to the number predicted by the theory, is 1.00 ± 0.13. ATLAS measured 1.3 ± 0.18. Both results indicate that, within errors, this boson is compatible with what the Standard Model predicts.

Its spin and parity, two properties of fundamental particles, are also known. These are like fingerprints. Knowing them reveals the identity of a particle and that is how we know the boson discovered two years ago is really a Higgs boson.

The question is still open though to see if this is the unique Higgs boson that was predicted by Robert Brout, François Englert and Peter Higgs in 1964 in the framework of the current theory, the Standard Model. But it could also be the lightest of the five Higgs bosons predicted by a more encompassing theory like Supersymmetry that would fix some problems of the Standard Model and open the door to the so-called “new physics”.

Several measurements from ATLAS on the signal strength, i.e. how often Higgs bosons are produced in different ways, and decay into different types of particles, compared to the theoretical predictions. The result should therefore be equal to 1.0 if the theoretical predictions are right. The black “+”symbol indicates the predicted value while the various circles give the zone where the experiment expects the real value to be with 68% or 95% confidence level.

Nearly all the data collected up to the end of 2012 – before the Large Hadron Collider (LHC) was shutdown to undergo a massive consolidation and maintenance program – were used for the many analyses presented at the conference. Everything measured so far agrees within experimental uncertainties with the predictions of the Standard Model. Not only did the experiments improve the precision on most measurements, but they are also looking at new aspects all the time. For example, CMS and ATLAS also showed the distribution of the momentum of the Higgs boson and its decay products afterwards. All these measurements test the Standard Model with increasing precision . Experimentalists are looking for any deviation from the theoretical predictions in the hope of finding the key to reveal what is the more encompassing theory lying beyond the Standard Model.

A series of results by the CMS Collaboration on the signal strength. With the current level of precision, all these measurements agree with a value of 1.0, as predicted by the Standard Model. A deviation would suggest the manifestation of something beyond the Standard Model.

But none of the numerous direct attempts to find particles related to this new physics has proved successful yet. Despite having looked at hundreds of different possibilities, each one corresponding to a particular scenario involving one of the hypothetical particles of Supersymmetry, no sign of their presence has been discovered yet.

However, this is quite similar to doing archaeology: one needs to shovel a lot of dirt before extracting something meaningful. Each analysis is like one bucket of dirt removed. And each small piece of information found helps get the bigger picture. Today, with the wealth of new results, theorists are in a much better position to draw general conclusions, eliminate wrong models and zoom in on the right solution.

The whole community is eagerly awaiting the restart of the LHC in early 2015 to collect more data at higher energy to open up a new world of opportunities. All hopes to discover this new physics will then be renewed.

Pauline Gagnon

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This is the first part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explain what is the Standard Model and show its limitations. Then I introduce supersymmetry and explain how it would fix the main flaws of the Standard Model. Finally, I will review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider at CERN.

The Standard Model describes what matter is made of and how it holds together. It rests on two basic ideas: all matter is made of particles, and these particles interact with each other by exchanging other particles associated with the fundamental forces.

The basic grains of matter are fermions and the force carriers are bosons. The names of these two classes refer to their spin – or angular momentum. Fermions have half-integer values of spin whereas bosons have integer values as shown in the diagram below.

Fermions come in two families. The leptons family has six members, with the electron being the best known of them. The quarks family contains six quarks. The up and down quarks are found inside protons and neutrons. The twelve fermions are the building blocks of matter and each one has a spin value of ½.

These particles interact with each other through fundamental forces. Each force comes with one or more force carriers. The nuclear force comes with the gluon and binds the quarks within the proton and neutrons. The photon is associated with the electromagnetic force. The weak interaction is responsible for radioactivity. It comes with the Z and W bosons. All have a spin of 1.

The main point is: there are grains of matter, the fermions with spin ½, and force carriers, the bosons with integer values of spin.

The Standard Model is both remarkably simple and very powerful. There are complex equations expressing all this in a mathematical way. These equations allow theorists to make very precise predictions. Nearly every quantity that has been measured in particle physics laboratories over the past five decades falls right on the predicted value, within experimental error margins.

So what’s wrong with the Standard Model? Essentially, one could say that the whole model lacks robustness at higher energy. As long as we observe various phenomena at low energy, as we have done so far, things behave properly. But as accelerators are getting more and more powerful, we are about to reach a level of energy which existed only shortly after the Big Bang where the equations of the Standard Model start getting shaky.

This is a bit like with the laws of physics at low and high speed. A particle moving at near the speed of light cannot be described with the simple laws of mechanics derived by Newton. One needs special relativity to describe its motion.

One major problem of the Standard Model is that it does not include gravity, one of the four fundamental forces. The model also fails to explain why gravity is so much weaker than the electromagnetic or nuclear forces. For example, a simple fridge magnet can counteract the gravitational attraction of a whole planet on a small object.

This huge difference in the strength of fundamental forces is one aspect of the “hierarchy problem”. It also refers to the wide range in mass for the elementary particles. In the table shown above, we see the electron is about 200 times lighter than the muon and 3500 times lighter than the tau. Same thing for the quarks: the top quark is 75 000 times heavier than the up quark. Why is there such a wide spectrum of masses among the building blocks of matter? Imagine having a Lego set containing bricks as disparate in size as that!

The hierarchy problem is also related to the Higgs boson mass. The equations of the Standard Model establish relations between the fundamental particles. For example, in the equations, the Higgs boson has a basic mass to which theorists add a correction for each particle that interact with it. The heavier the particle, the larger the correction. The top quark being the heaviest particle, it adds such a large correction to the theoretical Higgs boson mass that theorists wonder how the measured Higgs boson mass can be as small as it was found.

This seems to indicate that other yet undiscovered particles exist and change the picture. In that case, the corrections to the Higgs mass from the top quark could be cancelled out by some other hypothetical particle and lead to the observed low Higgs boson mass. Supersymmetry just happens to predict the existence of such particles, hence its appeal.

Last but not least, the Standard Model only describes visible matter, that is all matter we see around us on Earth as well as in stars and galaxies. But proofs abound telling us the Universe contains about five times more “dark matter”, a type of matter completely different from the one we know, than ordinary matter. Dark matter does not emit any light but manifests itself through its gravitational effects.  Among all the particles contained in the Standard Model, none has the properties of dark matter. Hence it is clear the Standard Model gives an incomplete picture of the content of the Universe but supersymmetry could solve this problem.

Pauline Gagnon

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