## Posts Tagged ‘spin’

### How to tell a Higgs from another boson?

Thursday, September 20th, 2012

On July 4, when CERN announced “the observation of a new particle” and not the discovery of the Higgs boson, many wondered why be so cautious. It was simply too early to tell what kind of boson we had found. The Higgs boson is the last missing piece of the Standard Model of particle physics, a model that has enabled theorists to make extremely precise predictions. But to fully trust this model, it should have all its pieces. Who would want to complete a 5000-piece puzzle with the wrong piece?

Both the CMS and ATLAS experiments have been conducting several checks since July:

1) Are all possible decay modes predicted by the Standard Model observed?

2) Is each observed decay happening at the right rate?

3) What are the fundamental properties of the new boson?

The first checks (based on half the data now available) indicate that the new boson is compatible with being the Higgs boson. But the precision is still too low to tell as shown on the plots below (the signal strength and σ/σSM H are the same quantity).

The Higgs boson can decay in many ways and the plot shows which decays have been observed and at what rates. A signal strength (of 1 means the signal corresponds exactly to what is expected for a Higgs boson.  Zero would mean there is no signal seen for this particular decay channel. The black points represent the measured values and the horizontal bar, the error margin.

At this point, we cannot tell unambiguously if the first two measurements are more compatible with 0 (the decay does not exist) or 1 (yes, it decays at the predicted rate).  Both CMS and ATLAS need to analyze more data to say if the new boson decays into two b quarks (H → bb) and two tau leptons (H → ττ).

The other three decay modes, namely WW, two photons (H → γγ) and ZZ occur at about the rate or slightly more often than expected by the Standard Model.

The decisive test will come by measuring its spin and parity, two “quantum numbers” or properties of fundamental particles. The spin is similar to the angular momentum of a spinning object. But for fundamental particles, only discrete values can be used. For bosons (the particles carrying the various forces), these values can be 0, ±1, ±2 and so on. For fermions, the building blocks of matter like quarks and leptons (electron, muon, tau and neutrinos), it can only be +½ or -½.

Aidan Randle-Conde has compiled all possibilities on his blog. A particle with spin 1 cannot decay into two photons. Since we have seen the new boson decaying into photons, spin 1 is already ruled out in the table below. Moreover, a spin 2 boson could not decay into two taus, which is why it is so important to look for this decay in the latest data.

(from Aidan Randle-Conde’s blog)

The Standard Model predicts that the spin and parity of the Higgs boson will be 0+. To distinguish between 0+ and 0-, as well as 2+ and 2-, the only way is to carefully measure the angles at which all the decay products fly apart. So if we observe the new boson decaying into photons, we must measure the angle between the photons and the beam axis. If it decays into two Z, each one going into two electrons or two muons, we must carefully measure the angles of these four particles and their combined mass. Here is what Sara Bolognesi and her colleagues predict for Higgs bosons decaying into ZZ, WW or two photons. We must measure specific quantities, namely the mass and angles of the decay products, to distinguish them. If they match the red curve, we will know it is the Higgs boson, but it they look like one of the other curves, it will mean the new boson corresponds to a different theoretical model.

Each experiment now has about 14 fb-1 of data on tape and expects about 25 fb-1 in total by the end of the year. With the 5 fb-1 collected last year, it should be sufficient to unmask the new comer. “All” we need to do is measure these extremely complex quantities.

Pauline Gagnon

For more info, see these two CERN news videos  on CERN YouTube (part 1 and part 2) on the Higgs boson spin.

### Comment distinguer le Higgs d’un autre boson?

Thursday, September 20th, 2012

Le 4 juillet, le CERN annonçait avoir «observé une nouvelle particule » et non « découvert le boson de Higgs. » Pourquoi faire preuve de tant de retenue? Simplement parce ce qu’il était trop tôt pour se prononcer. Le boson de Higgs est la dernière pièce manquante au Modèle Standard de la physique des particules, un modèle qui a permis aux théoriciennes et théoriciens de faire des prédictions d’une extrême précision. Mais qui voudrait compléter un casse-tête de 5000 morceaux en y insérant la mauvaise pièce?

Les expériences CMS et ATLAS ont déjà attaqué les questions suivantes:

1) Voit-on tous les modes de désintégration prédits par le Modèle Standard?

2) Est-ce que chacun se produit aussi souvent que prévu?

3) Quelles sont les propriétés fondamentales de ce nouveau boson?

Bien que les premières vérifications effectuées (basées sur la moitié des données disponibles aujourd’hui) indiquent que le nouveau boson aie tout l’air du Higgs, la précision actuelle est encore trop faible pour trancher comme le montre les graphes suivants. (signal strength et σ/σSM H représentent la même quantité).

Le boson de Higgs peut se désintégrer de plusieurs façons et le graphe montre les différents canaux observés ainsi que leur fréquence. Une « force de signal » (signal strength) de 1 implique que le signal correspond exactement à ce que prédit le modèle pour un boson de Higgs. Et zéro veut dire que ce canal de désintégration n’est pas observé. Les points en noir représentent les mesures faites et la barre horizontale, la marge d’erreur associée.

Comme on le voit bien, il est encore impossible de dire si les deux premiers canaux sont compatible avec 0 (non, ce canal n’est pas observé) ou 1 (oui, on le voit au taux prévu). ATLAS et CMS doivent analyser plus de données pour déterminer si ce boson se désintègre en deux quarks b (H → bb) et deux leptons tau (H → ττ). Les trois autres canaux sont bel et bien observés mais à des taux légèrement supérieurs à ceux prévus par le Modèle Standard.

Le test décisif viendra des mesures de son spin et de sa parité, deux « nombres quantiques » (ou particularités mesurables) attachés aux particules fondamentales. Le « spin » est semblable à la quantité de mouvement angulaire qu’on associe à un corps en rotation. Sauf que pour les particules fondamentales, cette quantité ne peut prendre que certaines valeurs bien précises. Pour les bosons, les particules associées aux champs de forces, la valeur doit être 0, ±1, ±2 etc. Pour les fermions, les grains de matière tels que les quarks et les leptons (électron, muon, tau and neutrinos), le spin est soit +½, soit -½.

Aidan Randle-Conde résume bien toutes les possibilités dans son blog (en anglais). Seule une particule de spin 0 ou 2 peut se désintégrer en deux photons. Puisqu’on a vu que le nouveau boson se désintègre en deux photons, il ne peut avoir qu’un spin 0 ou 2. De plus, un boson de spin 2 ne peut se désintégrer en deux taus. Il est donc crucial de mesurer si c’est le cas ou pas en utilisant toutes les données accumulées récemment.

(tiré du blog d’Aidan Randle-Conde)

Le Modèle Standard impose que le spin et la parité du boson de Higgs soit 0+. Reste donc à déterminer si le nouveau boson est de type 0+ ou encore 0-, 2+ ou 2-. Le seul moyen est de mesurer les angles auxquels les produits de désintégration s’échappent. Si on observe une désintégration en deux photons, on doit mesurer l’angle entre les photons et la direction des faisceaux du LHC. Lorsque le boson se brise en deux Z, chacun donnant  à son tour deux électrons ou deux muons, il faut mesurer les angles et la masse combinée des quatre particules finales.

Voici ce que Sara Bolognesi et ses collègues prédisent pour un boson de Higgs se désintégrant soit en ZZ, WW ou deux photons. En mesurant la masse et les angles des produits de désintégration, on pourra déterminer le spin et la parité du nouveau boson. Si leur distribution correspond aux courbes en rouge dans les diagrammes suivants, c’est qu’on a bel et bien trouvé le boson de Higgs. Si cela ressemble plutôt aux autres courbes, celles associées à d’autres modèles, c’est qu’il s’agit d’un autre type de boson.

Chaque expérience a maintenant en main 14 femtobarn inverse (fb-1) de données et on espère atteindre 25 fb-1 au total d’ici la fin de l’année. Avec les 5 fb-1 accumulés l’an dernier, ce devrait être suffisant pour arriver à démasquer le nouveau venu. Il ne reste « plus » qu’à mesurer toutes ces quantités assez complexes.

Pauline Gagnon

Pour être averti-e lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution

Pour plus d’info sur le spin du boson de Higgs, regardez ces deux récents vidéos sur CERN YouTube (première et seconde partie) (en anglais seulement)

### Spinning out of control!

Monday, July 16th, 2012

We’ve all heard the big news from CERN by now (if not then you might want to catch up on the latest gossip!) Right now most of the focus at ATLAS and CMS is on measuring the properties of the new boson we’ve found. The numbers of events are small, so studies are very difficult. One of the most important properties that we need to study is the particle’s spin, and luckily we can say something about that right now!

The big news: One of many Higgs boson candidates in the "golden mode" (ATLAS Collaboration)

There are two ways to study the spin of this boson, the hard way and the easy way. The hard way involved looking at angles between the final state particles and that’s tricky, but it can be done with the existing data. This method is hard because we have to model both signal and background to get it right. The easy way is to look at the decays of the boson and see which ones happen and which ones don’t. We need a little more data to do this, but we can perform this study by the end the data taking for the year. Richard has already discussed the “hard” method, so I’m going to show the “easy” method. It comes with nice pictures, but there are a few subtleties.

I want to consider four decays: a decay to two photons, a decay to two $$Z$$ bosons (the same applies to two $$W$$ bosons), a decay to two $$\tau$$ leptons, and a decay to two $$b$$ quarks. All of these decay modes should be seen by both experiments if what we have seen is the Standard Model Higgs boson.

We need to label our particles properly and describe them a little before we begin. We can never measure the spin of a particle exactly, and the best we can do is measure its total spin, and its projection along a certain axis. The spin along the other two axes remains a mystery, because as soon as we measure its spin along one axis, the other two components of spin become indeterminate. That’s quantum mechanics for you! A component of spin can be increased or decreased with “raising” and “lowering” operators, and the change is always in natural units of 1. (This is just a result of the universe having three spatial dimensions, so if the answer was any different then the universe would look very different!)

Let’s take the electron and work out what spin states it can have. The electron’s total spin has been measured to be 1/2, so we need to project this spin onto an axis and find out the allowed values. A little thought shows that there are only two states that can exist: spin +1/2 and spin -1/2 (which we call “spin up” and “spin down”.) The $$J/\psi$$ meson has spin 1, so it’s allowed states are +1, 0, -1. When the $$J/\psi$$ is in state spin 0 what really mean is that it has “hidden” its spin at 90 degrees to the axis, so it’s total spin is still 1 and its projection along our chosen axis is 0.

So let’s get on with the job of considering the spins of all these other particles. The photon is a massless boson with spin 1, and it can only arrange its spin transversely (for obscure reasons that Flip explains very well), so it can’t hide its spin when it projects along an axis. That means that it can only have spin of +1 and -1. (There’s one more particle we’re going to use in these arguments, and that’s the gluon. The gluon is the same as the photon, except it interacts with a different field, so like the photon it can only have spin states +1 and -1):

The spin projections of the photon

The $$Z$$ and $$W$$ bosons are similar, except they have mass, so they have the luxury of hiding their spin. This means that they can have spin -1, 0, and 1, just like the $$J/\psi$$ did:

Spin projections of the massive boson

Both the $$b$$ quark and $$\tau$$ lepton are fermions, which means that they have spin 1/2. We already know what spin states are allowed for fermions, spin up and spin down:

Spin projections of fermions

Now that we know the spin states of all these particles we can just add them up and confirm or refute which spin our new boson has. Let’s see how we can get spin 0:

Possible decays of a spin 0 particle

It looks like we can a spin 0 particle by combining any of our particles.

Let’s try spin 1:

Possible decays of a spin 1 particle

Uh-oh, it looks like we can’t make a spin 1 particle from photons! That means that this new boson is definitely not spin 1, because we see it decay to two photons.

Moving on to spin 2:

Possible decays of a spin 2 particle

Hmm, it looks like it can only decay to bosons (photons or $$W/Z$$ bosons) if it’s spin 2. Unfortunately this argument isn’t completely airtight, because when we produce $$b$$ quarks we also produce gluons, and there’s no way to produce one without the other. $$\tau$$ leptons are different though, because when they are produced they are usually produced without anything extra. Let’s revisit the fermions to see if we can make a spin 2 particle if we include a gluon:

Possible decays of a spin 2 particle, allowing a gluon in the final state

Well this is interesting! It looks like we can a spin 2 boson from a pair of $$b$$ quarks, but not from a pair of $$\tau$$ leptons. Let’s compile all this information into a table and see how to measure the spin:

Putting it all together

If we really have seen a spin 0 boson then we must see it decay to $$\gamma\gamma$$, $$ZZ^*$$, $$b\bar{b}$$ and $$\tau\tau$$. It cannot be a spin 1 boson, because we’ve already seen the two photon final state. We can exclude a spin 2 boson if we see $$b\bar{b}$$ as well as $$\tau\tau$$, and that’s why these two modes are so important to us! Before we shut down near the start of 2013 we must have enough statistics to measure these branching fractions.

It’s interesting to look at what CMS saw. They managed to place a limit of 1.06 times the Standard Model cross section for the decay to the $$\tau\tau$$, which is suspiciously low. If the limit gets lower as we add more data we might not have the Standard Model Higgs boson after all… Keep watching this space for more update.

The results of the search for ττ mode at CMS and the unexpected dip at 125GeV (CMS Collaboration)

### The Glue that Binds Us All

Wednesday, June 13th, 2012

RHIC, the Relativistic Heavy Ion Collider at Brookhaven Lab, found it first: a “perfect” liquid of strongly interacting quarks and gluons – a quark-gluon plasma (QGP) – produced by slamming heavy ions together at close to the speed of light. The fact that the QGP produced in these particle smashups was a liquid and not the expected gas, and that it flowed like a nearly frictionless fluid, took the physics world by surprise. These findings, now confirmed by heavy-ion experiments at the Large Hadron Collider (LHC) in Europe, have raised compelling new questions about the nature of matter and the strong force that holds the visible universe together.

Similarly, searches for the source of “missing” proton spin at RHIC have opened a deeper mystery: So far, it’s nowhere to be found.

To probe these and other puzzles, nuclear physicists would like to build a new machine: an electron-ion collider (EIC) designed to shine a very bright “light” on both protons and heavy ions to reveal their inner secrets. (more…)

### Pumping Up Proton Polarization

Thursday, April 7th, 2011

Brookhaven Lab’s oldest and most-trophied workhorse, the Alternating Gradient Synchrotron (AGS), has broke its own world record for producing intense beams of polarized protons – particles that “spin” in the same direction.

Spin, a quantum property that describes a particle’s intrinsic angular momentum,  is used in a wide range of fields, from astronomy to medical imaging. But where spin comes from is still unknown.

In this picture of a proton-proton collision, the spin of the particles is shown as arrows circling the spherical particles. The red and green particles represent reaction products from the collision that are "seen" and analyzed by RHIC detectors.

To explore the mystery of spin, Brookhaven’s Relativistic Heavy Ion Collider (RHIC) smashes beams of polarized protons at close to the speed of light. RHIC is the only machine in the world with this capability. But before reaching RHIC’s high-speed collision course, the protons travel about one million miles through a series of linear and circular accelerators, including the AGS, a 41-year old circular accelerator more than a half mile around. Home to three of BNL’s seven Nobel Prize-winning discoveries, the AGS is Brookhaven’s longest-running accelerator.

Now, with a new upgrade, the AGS can keep up to 75 percent of those particles in the beam polarized while they accelerate – a 5 to 8 percent increase over the previous record. This feat was accomplished with custom-built power supplies created from old inventory and two revamped 1960s quadrupole magnets pulled from storage.

The two refurbished quadrupole magnets before being installed at the AGS

As the particles race through the AGS, two of the customized power supplies quickly pulse, hold, and pull back surges of power for each of the quadrupoles in a matter of milliseconds. Forty-two times within half a second, these pulsed currents produce magnetic kicks that keep the particles spinning in the correct direction.

For more details, see this story.

-Kendra Snyder, BNL Media & Communications