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CERN through the eyes of a young scientist

Tuesday, July 29th, 2014

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

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Two anomalies worth noticing

Monday, July 14th, 2014

The 37th International Conference on High Energy Physics just finished in Valencia, Spain. This year, no big surprises were announced: no new boson, no signs from new particles or clear phenomena revealing the nature of dark matter or new theories such as Supersymmetry. But as always, a few small anomalies were reported.

Looking for deviations from the theoretical predictions is precisely how experimentalists are trying to find a way to reveal “new physics”. It would help discover a more encompassing theory since everybody realises the current theoretical model, the Standard Model, has its limits and must be superseded by something else. However, all physicists know that small deviations often come and go. All measurements made in physics follow statistical laws. Therefore deviations from the expected value by one standard deviation occur in three measurements out of ten. Larger deviations are less common but still possible. A two standard deviation happens 5% of the time. Then there are systematic uncertainties that relate to the experimental equipment. These are not purely statistical, but can be improved with a better understanding of our detectors. The total experimental uncertainty quoted with each result corresponds to one standard variation. Here are two small anomalies reported at this conference that attracted attention this year.

The ATLAS Collaboration showed its preliminary result on the production of a pair of W bosons. Measuring this rate provides excellent checks of the Standard Model since theorists can predict how often pairs of W bosons are produced when protons collide in the Large Hadron Collider (LHC). The production rate depends on the energy released during these collisions. So far, two measurements can be made since the LHC operated at two different energies, namely 7 TeV and 8 TeV.

CMS and ATLAS had already released their results on their 7 TeV data. The measured rates exceeded slightly the theoretical prediction but were both well within their experimental error with a deviation of 1.0 and 1.4 standard deviation, respectively. CMS had also published results based on about 20% of all data collected at 8 TeV. It exceeded slightly the theoretical prediction by 1.7 standard deviation. The latest ATLAS result adds one more element to the picture. It is based on the full 8 TeV data sample. Now ATLAS reports a slightly stronger deviation for this rate at 8 TeV with 2.1 standard deviations from the theoretical prediction.

WWResults

The four experimental measurements for the WW production rate (black dots) with the experimental uncertainty (horizontal bar) as well as the current theoretical prediction (blue triangle) with its own uncertainty (blue strip). One can see that all measurements are higher than the current prediction, indicating that the theoretical calculation fails to include everything.

The four individual measurements are each reasonably consistent with expectation, but the fact that all four measurements lie above the predictions becomes intriguing. Most likely, this means that theorists have not yet taken into account all the small corrections required by the Standard Model to precisely determine this rate. This would be like having forgotten a few small expenses in one’s budget, leading to an unexplained deficit at the end of the month. Moreover, there could be common factors in the experimental uncertainties, which would lower the overall significance of this anomaly. But if the theoretical predictions remain what they are even when adding all possible little corrections, it could indicate the existence of new phenomena, which would be exciting. It would then be something to watch for when the LHC resumes operation in 2015 at 13 TeV.

The CMS Collaboration presented another intriguing result. They found some events consistent with coming from a decay of a Higgs boson into a tau and a muon. Such decays are prohibited in the Standard Model since they violate lepton flavour conservation. There are three “flavours” or types of charged leptons (a category of fundamental particles): the electron, the muon and the tau. Each one comes with its own type of neutrinos. According to all observations made so far, leptons are always produced either with their own neutrino or with their antiparticle. Hence, the decay of a Higgs boson in leptons should always produce a charged lepton and its antiparticle, but never two charged leptons of different flavour. Violating a conservation laws in particle physics is simply not allowed.

This needs to be scrutinised with more data, which will be possible when the LHC resumes next year. Lepton flavour violation is allowed outside the Standard Model in various models such as models with more than one Higgs doublet or composite Higgs models or Randall-Sundrum models of extra dimensions for example. So if both ATLAS and CMS confirm this trend as a real effect, it would be a small revolution.

HtomutauThe results obtained by the CMS Collaboration showing that six different channels all give a non-zero value for the decay rate of Higgs boson into pairs of tau and muon.

Pauline Gagnon

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Happy birthday, dear boson!

Friday, July 11th, 2014

Singing happy birthday slightly off-key but in good spirit. This is how several hundred physicists gathered for the 37th International Conference on High Energy Physics in Valencia, Spain closed the day on July 4th. 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.

gateau

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”.

ATLAS-Higgs-couplingsSeveral 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.

 CMS-mu

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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Major harvest of four-leaf clover

Wednesday, April 9th, 2014

The LHCb Collaboration at CERN has just confirmed the unambiguous observation of a very exotic state, something that looks strangely like a particle being made of four quarks. As exotic as it might be, this particle is sternly called Z(4430)-, which gives its mass at 4430 MeV, roughly four times heavier than a proton, and indicates it is has a negative electric charge. The letter Z shows that it belongs to a strange series of particles that are referred to as XYZ states.

So what’s so special about this state? The conventional and simple quark model states that there are six different quarks, each quark coming with its antiparticle.  All these particles form bound states by either combining two or three of them. Protons and neutrons for example are made of three quarks. All states made of three quarks are called baryons. Other particles like pions and kaons, which are often found in the decays of heavier particles, are made of one quark and one antiquark. These form the mesons category. Until 2003, the hundreds of particles observed were classified either as mesons or baryons.

And then came the big surprise: in 2003, the BELLE experiment found a state that looked like a bound state of four quarks. Many other exotic states have been observed since. These states often look like charmonium or bottomonium states, which contain a charm quark and a charm antiquark, or a bottom and antibottom quarks. Last spring, the BESIII collaboration from Beijing confirmed the observation of the Zc(3900)+ state also seen by BELLE.

On April 8, the LHCb collaboration reported having found the Z(4430)- with ten times more events than all other groups before. The data sample is so large that it enabled LHCb to measure some of its properties unambiguously. Determining the exact quantum numbers of a particle is like getting its fingerprints: it allows physicists to find out exactly what kind of particle it is. Hence, the Z(4430)- state appears to be made of a charm, an anti-charm, a down and an anti up quarks. Their measurement rules out several other possibilities.

LHCb-Z(4430)

The squared mass distribution for the 25,200 B meson decays to ψ’ π- found by LHCb in their entire data set. The black points represent the data, the red curve the result of the simulation when including the presence of the Z(4430)- state. The dashed light brown curve below shows that the simulation fails to reproduce the data if no contribution from Z(4430)- is included, establishing the clear presence of this particle with 13.9σ (that is, the signal is 13.9 times stronger than all possible combined statistical fluctuations. These are the error bars represented by the small vertical line attached to each point).

Theorists are hard at work now trying to come up with a model to describe these new states. Is this a completely new tetraquark, a bound state of four quarks, or some strange combination of two charmed mesons (mesons containing at least one charm quark)? The question is still open.

Pauline Gagnon

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For more information, see the LHCb website

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Moriond 2014: New Results, New Explorations… but no New Physics

Friday, April 4th, 2014

Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

ot0172hThis graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen

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Has anybody seen my supersymmetric particles?

Friday, March 21st, 2014

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

If Supersymmetry (or SUSY for short) is as good as it looks, why has none of the new SUSY particles been found yet? There could be many reasons, the simplest being that this theory is wrong and supersymmetric particles do not exist. If that were the case, one would still need another way to fix the Standard Model.

SUSY can still be the right solution if supersymmetric particles have eluded us for some reasons: we might have been looking in the wrong place, or in the wrong way or they could still be out of the reach of current accelerators.

So how does one go looking for supersymmetric particles? One good place to start is at CERN with the Large Hadron Collider or LHC. The 27-km long accelerator is the most powerful in the world. It brings protons into collisions at nearly the speed of light, generating huge amounts of energy in the tiniest points in space.  Since energy and matter are two forms of the same essence, like water and ice, the released energy materializes in the form of fundamental particles. The hope is to create some of the SUSY particles.

One major problem is that nobody knows the mass of all these new particles. And without the mass, it is very much like looking for someone in a large city without knowing the person’s address. All one can do then is comb the city trying to spot that person. But imagine the task if you don’t even know what the person looks like, how she behaves or even in which city, let alone which country she lives in.

Supersymmetry is in fact a very loosely defined theory with a huge number of free parameters. These free parameters are quantities like the masses of the supersymmetric particles, or their couplings, i.e. quantities defining how often they will decay into other particles. Supersymmetry does not specify which value all these quantities can take.

Hence, theorists have to make educated guesses to reduce the zone where one should search for SUSY particles. This is how various models of supersymmetry have appeared. Each one is an attempt at circumscribing the search zone based on different assumptions.

One common starting point is to assume that a certain property called R-parity is conserved. This leads to a model called Minimal SUSY but this model still has 105 free parameters. But with this simple assumption, one SUSY particle ends up having the characteristics of dark matter. Here is how it works: R-parity conservation states that all supersymmetric particles must decay into other, lighter supersymmetric particles. Therefore, the lightest supersymmetric particle or LSP cannot decay into anything else. It remains stable and lives forever, just like dark matter particles do. Hence the LSP could be the much sought-after dark matter particle.

SUSY-cascade-Fermilab Credit: Fermilab

How can the Large Hadron Collider help? Around the accelerator, large detectors act like giant cameras, recording how the newly created and highly unstable particles break apart like miniature fireworks. By taking a snapshot of it, one can record the origin, direction and energy of each fragment and reconstruct the initial particle.

Heavy and unstable SUSY particles would decay in cascade, producing various Standard Model particles along the way. The LSP would be the last possible step for any decay chain. Generally, the LSP is one of the mixed SUSY states with no electric charge called neutralino. Hence, each of these events contains a particle that is stable but does not interact with our detectors. In the end, there would be a certain amount of energy imbalance in all these events, indicating that a particle has escaped the detector without leaving any signal.

At the LHC, both the CMS and ATLAS experiments have searched billions of events looking for such events but to no avail. Dozens of different approaches have been tested and new possibilities are constantly being explored. Each one corresponds to a different hypothesis, but nothing has been found so far.

dijet-monjet Two events with jets as seen in the ATLAS detector. (Left) A very common event containing two jets of particles. The event is balanced, all fragments were recorded, no energy is missing. (Right) A simulation of a mono-jet event where a single jet recoils against something unrecorded by the detector. The imbalance in energy could be the signature of a dark matter particle like the lightest supersymmetric particle (LSP), something that carries energy away but does not interact with the detector, i.e. something we would not see.

One reason might be that all supersymmetric particles are too heavy to have been produced by the LHC. A particle can be created only if enough energy is available. You cannot buy something that costs more money than you have in your pocket. To create heavy particles, one needs more energy. It is still possible all SUSY particles exist but were out of the current accelerator reach. This point will be settled in 2015 when the LHC resumes operation at higher energy, going from 8 TeV to at least 13 TeV.

If the SUSY particles are light enough to be created at 13 TeV, the chances of producing them will also be decupled, making them even easier to find. And if we still do not find them, new limits will be reached, which will also greatly help focus on the remaining possible models.

SUSY has not said its last word yet. The chances are good supersymmetric particles will show up when the LHC resumes. And that would be like discovering a whole new continent.

Pauline Gagnon

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Supersymmetry: a tantalising theory

Wednesday, March 19th, 2014

This is the second part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explained what is the Standard Model and showed its limitations. I now 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 (LHC) at CERN.

Theorists often have to wait for decades to see their ideas confirmed by experimental findings. This was the case for François Englert, Robert Brout and Peter Higgs whose theory, elaborated in 1964, only got confirmed in 2012 with the discovery of the Higgs boson by the LHC experiments.

Today, many theorists who participated in the elaboration of what is now known as supersymmetry, are waiting to see what the LHC will reveal.

Supersymmetry is a theory that first appeared as a mathematical symmetry in string theory in early 1970s. Over time, several people contributed new elements that eventually led to a theory that is now one of the most promising successors to the Standard Model. Among the pioneers, the names of two Russian theorists, D. V. Volkov and V. P Akulov, stand out. In 1973, Julius Wess and Bruno Zumino wrote the first supersymmetric model in four dimensions, paving the way to future developments. The following year, Pierre Fayet generalized the Brout-Englert-Higgs mechanism to supersymmetry and introduced superpartners of Standard Model particles for the first time.

All this work would have remained a pure mathematical exercise unless people had noticed that supersymmetry could help fix some of the flaws of the Standard Model.

As we saw, the Standard Model has two types of fundamental particles: the grains of matter, the fermions with spin ½, and the force carriers, the bosons with integer values of spin.

The mere fact that bosons and fermions have different values of spin makes them behave differently. Each class follows different statistical laws. For example, two identical fermions cannot exist in the same quantum state, that is, something -one of their quantum numbers – must be different. Quantum numbers refer to various properties: their position, their charge, their spin or their “colour” charge for quarks. Since everything else is identical, two electrons orbiting on the same atomic shell must have different direction for their spin. One must point up, the other down. This means at most two electrons can cohabit on an atomic shell since there are only two possible orientations for their spins. Hence, atoms have several atomic shells to accommodate all their electrons.

On the contrary, there are no limitations on the number of bosons allowed in the same state. This property is behind the phenomenon called superconductivity. A pair of electrons forms a boson since adding two half spins gives a combined state with a spin of 0 or 1, depending if they are aligned or not. In a superconductor, all pairs of electrons can be identical, with exactly the same quantum numbers since this is allowed for combined spin values of 0 or 1. Hence, one can interchange two pairs freely, just like two grains of sand of identical size can swap position in quick sand, which makes it so unstable. Likewise, in a superconductor, all pairs of electrons can swap position with others, leaving no friction. An electric current can then flow without encountering any resistance.

Supersymmetry builds on the Standard Model and associates a “superpartner” to each fundamental particle. Fermions get bosons as superpartners, and bosons get associated with fermions. This unifies the building blocks of matter with the force carriers. Everything becomes more harmonious and symmetric.

SUSY-diagram-Particle-Fever

Supersymmetry builds on the Standard Model and comes with many new supersymmetric particles, represented here with a tilde (~) on them. (Diagram taken from the movie “Particle fever” reproduced with permission from Mark Levinson)

But there are other important consequences. The number of existing fundamental particles doubles. Supersymmetry gives a superpartner to each Standard Model particle. In addition, many of these partners can mix, giving combined states such as charginos and neutralinos

This fact has many implications. First major consequence: the two superpartners to the top quark, called the stops, can cancel out the large contribution from the top quark to the mass of the Higgs boson. Second implication: the lightest supersymmetric particle (in general one of the mixed states with no electric charge called neutralino) has just the properties one thinks dark matter should have.

Not only supersymmetry would fix the flaws of the Standard Model, but it would also solve the dark matter problem. Killing two huge birds with one simple stone. There is just one tiny problem: if these supersymmetric particles exist, why have we not found any yet? I will address this question in the next part in this series.

Pauline Gagnon

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The Standard Model: a beautiful but flawed theory

Friday, March 14th, 2014

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.

2000px-Standard_Model_of_Elementary_Particles.svg

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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Maria and Giuseppe: lives intertwined with CERN’s history

Wednesday, February 5th, 2014

CERN will be celebrating its 60th anniversary this year. That means 60 years of pioneering scientific research and exciting discoveries. Two Italian physicists, Maria and Giuseppe Fidecaro, remember nearly all of it since they arrived in 1956. Most impressively, they are still hard at work, every day!

The couple is easy to spot, even in the cafeteria during busy lunchtimes, where they usually engage in the liveliest discussions. “We argue quite a lot,” Maria tells me with a big smile. “We have very different styles.” “But in general, in the end, we agree,” completes Giuseppe.

Fidecaro-3-smallPhoto credit: Anna Pantelia, CERN

In October 1954, Giuseppe went to the University of Liverpool as a CERN Fellow to do research with their brand new synchrocyclotron. Maria also joined, having obtained a fellowship from the International Federation of University Women. After getting married in July 1955, they carried out experiments on pions, Giuseppe with a lead glass Cerenkov counter, Maria with a diffusion chamber.

In summer 1956, both moved to Geneva, and Maria got a CERN fellowship. “There were only about 300, maybe 400 people at CERN then”, explains Maria. A beautiful mansion called “Villa de Cointrin” housed the administrative offices on the airport premises, while physicists had their offices in nearby barracks.

Giuseppe was assigned to the Synchrocyclotron Division.  This was the first accelerator built at CERN and was operated from 1957 until 1990. Giuseppe set up a group and prepared the basic equipment for experiments  that was used in 1958 for a successful search for pions decaying into an electron and a neutrino. This was a hot topic at the time and was the first experiment involving a CERN accelerator. “The news went all over the world overnight”, recalls Giuseppe. Recently refurbished, the synchrocyclotron will soon become a permanent exhibit at CERN.

Meanwhile, Maria worked on a novel method to provide polarised proton beams.  As she recalls: “It was just a mere 10 years after the end of the war. The war feelings were still very much there”. “But it was really easy to work with each other,” Giuseppe adds, “everybody got along; we all had a common goal.”

Although there were very few women when she started, Maria feels she was respected by her peers. “In my group, I was simply one of them”, she comments.

Today, long after most have retired, they have both chosen to remain active and are still doing research but of another style.  Giuseppe delves in the history of physics while Maria is happy to revisit some of her past work, making sure she did not overlook any important detail. “In the heat of the moment, with the beams on and everything, there was no time to have a broad view”, she explains. “It’s a pleasure to go back and gain a deeper insight, and put our work in perspective with respect to what was going on at CERN and elsewhere”.

Both agree: every moment was good. “Having gone through all of it for 60 years is what has been best”, Maria says. “It was great to be able to pioneer so many different experiments”, adds Giuseppe, “and to share work with so many interesting people”.   Maria confirms “Life has been kind to us”.

Pauline Gagnon

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Anti-beam me up, Scotty!

Tuesday, January 28th, 2014

While the CERN accelerator complex was being revamped in 2013, the ASACUSA experiment took time to carefully review the data taken in 2012 at the Antiproton Decelerator (AD) facility. This painstaking work paid off and they just announced in Nature having produced the first ever beam of antihydrogen atoms.

In laboratory experiments like the ones conducted at CERN, matter and antimatter are always created in equal amounts. The Big Bang theory predicts that the same quantities of matter and antimatter were also created at the origin of the Universe. However, nowadays, one sees absolutely no trace of this “primordial” antimatter. So what happened to all the antimatter that once was in the Universe?

To answer this question, CERN has a full antimatter program underway at the AD to check if antimatter has the same properties as matter. One of the best ways to do so is to compare antihydrogen atoms with hydrogen atoms. This is the simplest of all atoms, having only one electron orbiting around one single proton.

Antihydrogen atoms are replica of hydrogen atoms but with an anti-electron – called positron – and an antiproton replacing the electron and proton of regular atoms.

All matter emits light when excited just as a piece of metal shines when heated up. The light emitted gives a unique signature for each atom. For example, hydrogen emits and absorbs light of specific frequency when an electron jumps from one energy level to another. It also has a “hyperfine structure” corresponding to magnetic interactions between the nucleus and the electron.

The ASACUSA experiment aims to check the hyperfine structure of antihydrogen. This can be done by observing which frequencies antihydrogen atoms absorb.

asacusa-realThe ASACUSA experiment at CERN (Image: Yasunori Yamakazi )

So here is what ASACUSA did: they produced antihydrogen atoms by first decelerating and cooling antiprotons down to very low temperature. Then they mixed antiprotons with positrons and combined them in a strong non-uniform magnetic field. These strong magnetic fields are necessary to keep antiprotons and positrons from touching any matter. That would cause their immediate annihilation and prevent the formation of antihydrogen atoms.

The next problem was to move the antihydrogen atoms away from this field to be able to study their hyperfine structure. Otherwise, the strong non-uniform magnetic field would mask the tiny effects generated by the magnetic interaction between the antiproton and the positron responsible for the hyperfine structure.

But atoms are neutral and cannot be controlled by electric fields. However, antihydrogen atoms are like tiny magnets. So by using non-uniform magnetic fields, the scientists were able to manipulate these tiny magnets and create a beam of antihydrogen atoms. It was directed towards a small detector located after a microwave cavity and a sextupole magnet.

The sextupole magnet focuses or defocuses antihydrogen atoms on the detector depending on the direction of the antihydrogen tiny magnets.

ASACUSA

The ASACUSA setup. From left to right: the magnets (grey) used to produce antihydrogen atoms, the microwave cavity (green) to induce hyperfine transitions, the sextupole magnet (red and grey) and the antihydrogen detector (gold). Credit: Stefan Meyer Institute.

The detector reveals the number of antihydrogen atoms passing through the device after they go through a microwave cavity. It was turned off in 2012 but will be on in the future.  The antihydrogen atoms will only absorb microwave photons having exactly the energy corresponding to its hyperfine transitions. This process will alter the trajectory of antihydrogen atom in the sextupole magnet, and eventually the number of antihydrogen atoms reaching the detector will be reduced.

By counting how many antihydrogen atoms reach the target when the microwave cavity is tuned to specific frequencies, the scientists will determine the frequencies of the hyperfine structure.

ASACUSA now has the proof that 80 antihydrogen atoms made it to their detector. The next step is to see if fewer are observed when the microwave cavity is turned on at the right frequency.

And then we will know if antihydrogen is the exact mirror image of hydrogen. This may reveal if antimatter differs from matter and explain why it has all vanished.

Pauline Gagnon

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