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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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A beam of your own

Thursday, January 16th, 2014

As part of its 60th anniversary celebration and to help keep us young at heart, CERN has launched a special competition for students called: Beam line for schools.

CERN is inviting students aged 16 and upward from anywhere in the world to submit a proposal to do an experiment with a beam of particles from the Proton Synchrotron beam line. Each team can be composed of up to 30 students with at least one adult supervisor. This summer, up to nine students of the selected team will be invited to CERN to run the team’s experiment. Travelling and living expenses for the selected group will be covered by CERN.

PSA view of the Proton Synchrotron beam line.

The proposals will be pre-selected by a group of CERN scientists, and will then be reviewed by the same committee that validates all proposals for experiments at the laboratory’s Super Proton Synchrotron and Proton Synchrotron accelerators.

So what could you be doing? Essentially, you can investigate how beams of particles interact with matter. For example, you could study what happens when beams containing different types of particles hit targets made of various materials. The proposals will be judged on creativity, motivation, feasibility and adherence to the scientific method.

To help you understand what can be done, we have put together a short presentation that explains the basics about particles and beams. These short talks are available in English, French, Italian, Spanish and German and are part of a YouTube playlist that includes recordings of Google hangouts in English, French, Italian, Spanish and German, in which CERN scientists answer questions.

Here is your chance to come to run your own experiment at CERN. This will last about a week and take place in July, August or September. CERN physicists will be helping you to refine your idea before and during your stay at CERN.

Interested? Then you can stay up-to-date via the CERN website, #bl4s on Twitter, Facebook, Google+ or YouTube.

Don’t hesitate and fill out the registration form before 31 January 2014. All you need to do at this point is send us the name of the school and of the participants as well as a tweet-of-intent stating why you think you should win this competition. You will still have until 31 March to prepare your full project, including a 1-minute video giving the highlights. Here is your chance!

Pauline Gagnon

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A whole Universe to be discovered

Wednesday, January 15th, 2014

The past two years have been rather exceptional for CERN: first in 2012, the CMS and ATLAS experiments discovered the Higgs boson, confirming the mechanism elaborated 48 years earlier by Robert Brout, François Englert and Peter Higgs. Then in 2013, Englert and Higgs received the Nobel Prize for Physics for their theory.

2014 is also going to be special year since CERN is going to turn 60. But beyond this anniversary, CERN is preparing the Large Hadron Collider (LHC) to explore new territories.

With the Higgs boson discovery, we have completed the Standard Model, the current theory that explains what makes all visible matter around us. But that is just a mere 5% of the total content of the Universe. The existence of dark matter tells us our current model is incomplete. So far, the various analyses of the data taken at 8 TeV has not yet revealed traces of dark matter or any new particles. To push all our searches further and faster, we need to increase the reach of the LHC by going to higher energies.

This is why since February last year all accelerators and experiments at CERN began a long shutdown for maintenance and consolidation. This will continue in 2014 for the LHC but many accelerators of CERN complex will be coming back to life starting this summer.

H-bottle

The starting point of the chain of accelerators is a simple hydrogen bottle. The electrons are stripped from the hydrogen atoms using an electric field to leave single protons. These are then accelerated in a small linear accelerator (LINAC 2 at the bottom centre of the diagram below). The Low Energy Ion Ring (LEIR) plays a similar role but with heavy ions.

Accelerators

The protons get an extra kick in the Booster before being injected into what is CERN’s oldest circular accelerator still in operation, the Proton Synchrotron (PS). Then the protons head for the Super Proton Synchrotron (SPS), where they reach 450 GeV in energy (that is 450 billion electronvolts). This is the final stage before injection into the LHC where the energy will get nearly thirty times larger, namely 13 TeV.

The beams from the accelerator chain are also delivered to various other experimental areas, such as ISOLDE and n-TOF where a huge number of experiments involving nuclei are conducted. Other protons hit a target to produce antiprotons for the Antiproton Decelerator (AD), a facility dedicated to antimatter studies. These experiments will all resume their activities in 2014.

LS1-schedule-2014

All consolidation work for the LHC and its experiments will take place in parallel. ATLAS and CMS plan to complete all repairs and upgrades to their detector by November, ALICE at the beginning of December and LHCb in early January 2015.

Meanwhile, all physicists not involved with hardware are either completing the many ongoing analyses of all data taken up to 2013, preparing new simulations at higher energies, improving the data reconstruction algorithms or designing the new trigger selection criteria. Everybody is preparing to meet the challenge of dealing with more data at higher energy. All in the hope that we might be rewarded once more with new discoveries since there is still a whole new world to explore out there.

Pauline Gagnon

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One giant leap for the Higgs boson

Friday, December 6th, 2013

Both the ATLAS and CMS collaborations at CERN have now shown solid evidence that the new particle discovered in July 2012 behaves even more like the Higgs boson, by establishing that it also decays into particles known as tau leptons, a very heavy version of electrons.

Why is this so important? CMS and ATLAS had already established that the new boson was indeed one type of a Higgs boson. In that case, theory predicted it should decay into several types of particles. So far, decays into W and Z bosons as well as photons were well established. Now, for the first time, both experiments have evidence that it also decays into tau leptons.

The decay of a particle is very much like making change for a coin. If the Higgs boson were a one euro coin, there would be several ways to break it up into smaller coins, but, so far, the change machine seemed to only make change in some particular ways. Now, additional evidence for one more way has been shown.

There are two classes of fundamental particles, called fermions and bosons depending on their spin, their value of angular momentum. Particles of matter (like taus, electrons and quarks) belong to the fermion family. On the other hand, the particles associated with the various forces acting upon these fermions are bosons (like the photons and the W and Z bosons.).

The CMS experiment had already shown evidence for Higgs boson decays into fermions last summer with a signal of 3.4 sigma when combining the tau and b quark channels. A sigma corresponds to one standard deviation, the size of potential statistical fluctuations.  Three sigma is needed to claim evidence while five sigma is usually required for a discovery.

For the first time, there is now solid evidence from a single channel – and two experiments have independently produced it. ATLAS collaboration showed evidence for the tau channel alone with a signal of 4.1 sigma, while CMS obtained 3.4 sigma, both bringing strong evidence that this particular type of decays occurs.

Combining their most recent results for taus and b quarks, CMS now has evidence for decays into fermions at the 4.0 sigma level.

 ATLAS-H-tautau

The data collected by the ATLAS experiment (black dots) are consistent with coming from the sum of all backgrounds (colour histograms) plus contributions from a Higgs boson going into a pair of tau leptons (red curve). Below, the background is subtracted from the data to reveal the most likely mass of the Higgs boson, namely 125 GeV (red curve).

CMS is also starting to see decays into pairs of b quarks at the 2.0 sigma-level. While this is still not very significant, it is the first indication for this decay so far at the LHC. The Tevatron experiments have reported seeing it at the 2.8 sigma-level. Although the Higgs boson decays into b quarks about 60% of the time, it comes with so much background that it makes it extremely difficult to measure this particular decay at the LHC.

Not only did the experiments report evidence that the Higgs boson decays into tau leptons, but they also measured how often this occurs. The Standard Model, the theory that describes just about everything observed so far in particle physics, states that a Higgs boson should decay into a pair of tau leptons about 8% of the time. CMS measured a value corresponding to 0.87 ± 0.29 times this rate, i.e. a value compatible with 1.0 as expected for the Standard Model Higgs boson. ATLAS obtained 1.4 +0.5 -0.4, which is also consistent within errors with the predicted value of 1.0.

 CMS-Htautau1

One of the events collected by the CMS collaboration having the characteristics expected from the decay of the Standard Model Higgs boson to a pair of tau leptons. One of the taus decays to a muon (red line) and neutrinos (not visible in the detector), while the other tau decays into a charged hadron (blue towers) and a neutrino. There are also two forward-going particle jets (green towers).

With these new results, the experiments established one more property that was expected for the Standard Model Higgs boson. What remains to be clarified is the exact type of Higgs boson we are dealing with. Is this indeed the simplest one associated with the Standard Model? Or have we uncovered another type of Higgs boson, the lightest one of the five types of Higgs bosons predicted by another theory called supersymmetry.

It is still too early to dismiss the second hypothesis. While the Higgs boson is behaving so far exactly like what is expected for the Standard Model Higgs boson, the measurements lack the precision needed to rule out that it cannot be a supersymmetric type of Higgs boson. Getting a definite answer on this will require more data. This will happen once the Large Hadron Collider (LHC) resumes operation at nearly twice the current energy in 2015 after the current shutdown needed for maintenance and upgrade.

Meanwhile, these new results will be refined and finalised. But already they represent one small step for the experiments, a giant leap for the Higgs boson.

For all the details, see:

Presentation given by the ATLAS Collaboration on 28 November 2013

Presentation given by the CMS Collaboration on 3 December 2013

Pauline Gagnon

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