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Posts Tagged ‘Supersymmetry’

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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Zooming in on new particles

Friday, September 20th, 2013

The Large Hadron Collider (LHC) at CERN has stopped in the spring to undergo a major consolidation program but this has not stopped the search for new physics. On the contrary, physicists are taking advantage of the interruption to finalise all analyses with the whole data collected so far.

Dozens of new results have been presented by the four LHC experiments at several conferences since the end of operation. While only a handful of these results have made the headlines, a wealth of new information is now available, allowing theorists to refine their models.

Even with the discovery of a Higgs boson, physicists know that the Standard Model of particle physics cannot be the final answer since it has known shortcomings. For example, it fails to provide an explanation for dark matter or why the masses of fundamental particles such as electrons and muons are so different. Another theory called supersymmetry (or SUSY for short) is one of the most popular and most promising ways to extend the Standard Model, but it has yet to manifest itself.

One major difficulty when testing this new theory is the large number of parameters it introduces. To find the new particles predicted by SUSY, we must explore a vast territory spanned by 105 dimensions, corresponding to its 105 free parameters. Finding these new particles is like trying to spot a stranger in a crowd of millions.

Fortunately, theorists have attempted to give us experimentalists some guidance to constrain these parameters using theoretical or experimental considerations. One model that has gained popularity lately is called the phenomenological Minimal Supersymmetric Model or pMSSM and uses only 19 parameters. It takes into account information from all aspects of particle physics, incorporating constraints from the measured characteristics of the Z and Higgs bosons, b-quark physics, astrophysics as well as direct searches for dark matter at underground facilities and supersymmetric particles at the LHC.

Several groups of theorists and experimentalists have combined all these recent results to see which areas of the reduced but still huge parameter space of the pMSSM model are still allowed.

Their approach consists in generating millions of possible values corresponding to the masses and couplings of the hypothesised SUSY particles. The couplings are quantities related to the probability to produce these particles at the LHC.

Then they impose various constraints obtained from the many quantities measured by past and current experiments to see which points among all possibilities are still allowed.

Two theorists, Alex Arbey and Nazila Mahmoudi, and experimentalist Marco Battaglia, contrary to their earlier work, performed their latest scan assuming the four positive results reported by direct dark matter experiments were true dark matter signals to see if these results could be explained within SUSY.

While attempts by other groups were not able to find SUSY scenarios in agreement with the parameters of the possible dark matter signal, their results were rather surprising: they found surviving scenarios pointing to a light neutralino, with a mass of only 10 GeV, twelve times lighter than the Higgs boson. The second lightest particle is the super partner of the bottom quark, called sbottom, at around 20 GeV.

SUSY-predictions

The mass ranges predicted for different SUSY particles coming out of this study. The Higgs boson discovered last summer, h0, is assumed to be the lightest of the five Higgs bosons predicted by SUSY and the lightest SUSY particle is the neutralino, χ0.

If this scenario were correct, why would such a light particle have escaped detection? The reason is that most searches led by the CMS and ATLAS experiments have focused so far on events where a large amount of energy is missing.

This would be the case when some heavy but invisible SUSY particle escapes from our detectors. Such criteria are needed to reduce the overwhelming background and isolate the few events containing traces of SUSY particles. But a light neutralino would only carry a small quantity of energy and would have gone undetected.

While theorists are assessing which corners of the parameter space are still allowed, experimentalists are evaluating the impact of their selection criteria on detecting particles having the characteristics of the remaining allowed regions. New strategies are now being sought to explore this possibility.

Operating the LHC at higher energy and collecting larger datasets starting in 2015 should give definite answers to these questions. These combined efforts may soon pave the way to new discoveries.

Pauline Gagnon

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Après les résultats spectaculaires annoncés hier au CERN sur la découverte d’un nouveau boson, la plus grande conférence en physique des particules de l’année a débuté aujourd’hui à Melbourne. Mais cette première présentation sera dure à battre.

Comme plusieurs personnes l’ont mentionné, il est encore tôt pour dire si ce boson est bien le boson de Higgs bien que toutes les chances soient de ce côté. Il faut d’abord établir s’il se comporte exactement comme le boson de Higgs du Modèle Standard. Se désintègre-t-il dans les proportions prescrites par la théorie? Il nous faut donc vérifier tout ça avec la plus grande précision possible, pas que nous soyons compulsifs mais la moindre petite variation pourrait révéler l’entrée du « passage secret ».

Des théoriciens comme Peter Higgs, François Englert et Robert Brout, ont permis cette avancée en postulant en 1964 l’existence du mécanisme de Higgs et du boson de Higgs. Encore aujourd’hui, ce sont souvent les théoriciennes et théoriciens qui nous orientent dans la bonne direction.

Tous et toutes s’entendent à dire que le modèle théorique actuel a ses limites. Le Modèle Standard serait à la physique des particules ce que les quatre opérations de base (addition, soustraction, multiplication et division) sont aux mathématiques. Bien qu’elles suffisent à accomplir la plupart des tâches quotidiennes, on doit à l’occasion faire appel à la géométrie ou au calcul différentiel.

Tout ça pour dire qu’il existe des signes indiquant que le Modèle Standard n’est que la première couche d’une théorie plus complexe. Plusieurs pensent que la couche supérieure est une théorie appelée supersymétrie ou SUSY.

Une des difficultés majeures de cette théorie, c’est qu’elle comporte une centaine de paramètres non définis, ce qui la rend incapable de faire des prédictions concrètes. Sauf si on fixe la valeur de plusieurs de ces paramètres. On a alors des modèles plus gérables, comme par exemple le CMSSM ou Constrained Minimal Supersymmetric Model.

Aujourd’hui, à la Conférence Internationale de Physiques des Hautes Énergies, plusieurs théoricien-ne-s ont discuté de l’impact sur ces modèles de savoir maintenant que la masse du Higgs est 126 GeV. Par exemple, Dmitri Kanikov a montré qu’on peut mettre à profit les différentes interconnections au sein de la théorie pour voir comment les plus récentes limites établies expérimentalement peuvent substantiellement contraindre les paramètres du CMSSM.

Nazila Mahmoudi a quant à elle pousser cette approche un peu plus loin en démontrant qu’on peut non seulement circonscrire les paramètres de modèles tels que ceux du CMSSM, mais aussi ceux de SUSY. Ceci l’a conduite avec ses collègues à réaliser que la toute nouvelle valeur de la masse du boson de Higgs permet déjà d’éliminer certains de ces modèles réduits.

L’axe vertical montre la valeur de la masse du boson de Higgs et les deux traits horizontaux, la marge d’erreur sur cette valeur. Tous les modèles qui tombent en dehors de cette marge comme le « minimal Gauge Mediated SUSY Breaking Model » et le « no-scale » (en gris et en rose sur le graphe) sont éliminés.

Elle s’est montrée très optimiste même si les recherches actuelles au LHC n’ont toujours pas révélé la présence de particules supersymétriques. Elle a démontré qu’en fait il reste encre bien des valeurs permises pour les paramètres de SUSY. Si on ne les a toujours pas observées, ce n’est pas parce qu’elles n’existent pas mais peut-être simplement parce qu’elle sont plus lourdes ou appartiennent à des configurations plus complexes, les rendant plus difficiles à débusquer. En éliminant un à un les modèles erronés, on progresse dans la bonne direction.

Rien de tel qu’une note d’optimisme pour clore cette première journée d’une conférence qui promet.

Pauline Gagnon

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After the spectacular results reported yesterday at CERN on the discovery of a new boson, the largest particle physics conference of the year started today in Melbourne. Such announcement put the bar high for all the speakers.

As many people have pointed out already, it is still early to call the new boson a “Higgs boson” although the odds are really high. First we must check that it behaves exactly like the Higgs boson. Is it produced as often as the Standard Model predicts, and does it decay in the same proportions as expected? Verifying these properties with the highest accuracy will be the main task in the coming months and years. It’s not that physicists are compulsive about precision, but this is exactly where we might find the opening to the “secret passage”.

Theorists like Peter Higgs, François Englert and Robert Brout in 1964 showed us the way when they postulated the existence of the Higgs boson and Higgs mechanism. Today still, theorists are trying to guide the experimentalists in the right direction.

All theorists today agree that our current theoretical model has its limits. The Standard Model appears to be to the world of particle physics what the four basic operations are to mathematics. Most daily tasks are achieved using only additions, subtractions, multiplications and divisions. But we all know that there is more to mathematics: geometry and trigonometry for example are needed to solve more complex problems.

All this to say that there are clear signs that the Standard Model is only the first layer of a more complex theory. Many believe the next layer is a theory called supersymmetry or SUSY.

One major difficulty with this theory is that is has more than 100 free parameters, making it impossible to obtain predictions without assigning fixed values to some of these parameters. This lead to more manageable models, like the Constrained Minimal Supersymmetric Model or CMSSM.

Today, at the International Conference on High Energy Physics, several theorists discussed the impact of the recently revealed mass of the Higgs boson on the CMSSM model. For example, Dmitri Kanikov showed that one can use the intrinsic interconnections within the theory to see how the current limits obtained from the most recent experiments substantially constrain the parameters of the CMSSM.

Nazila Mahmoudi took this approach one step further by imposing constraints not to the CMSSM model but rather to the whole set of free SUSY parameters. This lead her and her colleagues to realize that with the actual searches and mostly, the stringent constraint coming from the Higgs mass at about 126 GeV, many of the constrained models are nearly ruled out.

The vertical axis shows the Higgs boson mass. If one assumes a Higgs mass between 123-129 GeV, scenarios such a minimal Gauge Mediated SUSY Breaking Model and no-scale (shown in gray and magenta) are excluded.

She was very optimistic even though the current searches at the LHC have not yet revealed any new SUSY particles. She showed that in fact there are plenty of values still allowed for the many parameters of SUSY. As she stated, if we have not found SUSY particles yet, it does not mean they are not there but simply that they must be much heavier or belong to more complex configurations, making them harder to find. By eliminating models like that, it helps zoom on the right one.

Nice optimistic way to close this first day of the conference.

Pauline Gagnon

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