• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • USLHC
  • USLHC
  • USA

  • James
  • Doherty
  • Open University
  • United Kingdom

Latest Posts

  • Andrea
  • Signori
  • Nikhef
  • Netherlands

Latest Posts

  • CERN
  • Geneva
  • Switzerland

Latest Posts

  • Aidan
  • Randle-Conde
  • Université Libre de Bruxelles
  • Belgium

Latest Posts

  • TRIUMF
  • Vancouver, BC
  • Canada

Latest Posts

  • Laura
  • Gladstone
  • MIT
  • USA

Latest Posts

  • Steven
  • Goldfarb
  • University of Michigan

Latest Posts

  • Fermilab
  • Batavia, IL
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Nhan
  • Tran
  • Fermilab
  • USA

Latest Posts

  • Alex
  • Millar
  • University of Melbourne
  • Australia

Latest Posts

  • Ken
  • Bloom
  • USLHC
  • USA

Latest Posts

Posts Tagged ‘spin’

In the late 1980s, as particle colliders probed deeper into the building blocks of nature, there were hints of a strange and paradoxical behaviour in the heart of atoms. Fundamental particles have a curious quantum mechanical property known as “spin”, which the electron carries in magnitude ½. While the description of electron’s spin is fairly simple, protons are made up of many particles whose “spins” can add together in complicated ways and yet remarkably, its total spin turns out to be the same as the electron: ½. This led to one of the great mysteries of modern physics: how do all the particles inside the proton conspire together to give it a ½ spin? And what might this mean for our understanding of hadrons, the particles that make up most of the visible universe?

[This article is largely intended for a lay-audience and contains an introduction to foundational ideas such as spin. If you’ve had a basic introduction to Quantum Mechanics before, you may wish to skip to section marked —— ]

We’ve known about the proton’s existence for nearly a hundred years, so you’d be forgiven for thinking that we knew all there was to know about it. For many of us, our last exposure to the word “proton” was in high school chemistry, where they were described as a little sphere of positive charge that clumps with neutrons to make atomic nuclei, around which negatively charged electrons orbit to create all the atoms, which make up Life, the Universe and Everything1.

2000px-Proton.svg

The simple, three-quark model of a proton (each coloured circle is a type of “quark”).

Like many ideas in science, this is a simplified model that serves as a good introduction to a topic, but skips over the gory details and the bizarre, underlying reality of nature. In this article, we’ll focus on one particular aspect, the quantum mechanical “spin” of the proton. The quest to measure its origin has sparked discovery, controversy and speculation that has lasted 30 years, the answer to which is currently being sought at a unique particle collider in New York.

The first thing to note is that protons, unlike electrons2, are composite particles, made up from lots of other particles. The usual description is that the proton is made up of three smaller “quarks” which, as far as we know, can’t be broken down any further. This picture works remarkably well at low energies but it turns out at very high energies, like those being reached at the at the LHC, this description turns out to be inadequate. At that point, we have to get into the nitty-gritty and consider things like quark-antiquark pairs that live inside the proton interacting dynamically with other quarks without changing its overall charge. Furthermore, there are particles called gluons that are exchanged between quarks, making them “stick” together in the proton and playing a crucial role in providing an accurate description for particle physics experiments.

So on closer inspection, our little sphere of positive charge turns out to be a buzzing hive of activity, with quarks and gluons all shuffling about, conspiring to create what we call the proton. It is by inferring the nature of these particles within the proton that a successful model of the strong nuclear force, known as Quantum Chromodynamics (QCD), was developed. The gluons were predicted and verfied to be the carriers of this force between quarks. More on them later.

Proton structure

A more detailed model of the proton. The golden chains between the quarks (the coloured spheres) are representations of gluons, transferred between them. Quark anti-quark pairs are also visible with arrows representing spins.

That’s the proton, but what exactly is spin? It’s often compared to angular momentum, like the objects in our everyday experience might have. Everyone who’s ever messed around on an office chair knows that once you get spun around in one, it often takes you a bit of effort to stop because the angular momentum you’ve built up keeps you going. If you did this a lot, you might have noticed that if you started spinning with your legs/arms outstretched and brought them inwards while you were spinning, you’d begin to spin faster! This is because angular momentum (L) is proportional to the radial (r) distribution of matter (i.e. how far out things are from the axis of rotation) multiplied by the speed of rotation3 (v). To put it mathematically L = m × v × r where m is just your constant mass. Since L is constant, as you decrease r (by bringing your arms/legs inwards), v (the speed at which you’re spinning) increases to compensate. All fairly simple stuff.

So clearly, for something to have angular momentum it needs to be distributed radially. Surely r has to be greater than 0 for L to be greater than 0. This is true, but it turns out that’s not all there is to the story. A full description of angular momentum at the quantum (atomic) level is given by something we denote as “J”. I’ll skip the details, but it turns out J = L + S, where L is orbital angular momentum, in a fashion similar to what we’ve discussed, and S? S is a slightly different beast.

Both L and S can only take on discrete values at the microscopic level, that is, they have quantised values. But whereas a point-like particle cannot have L > 0 in its rest frame (since if it isn’t moving around and v = 0, then L = 0), S will have a non-zero value even when the particle isn’t moving. S is what we call Spin. For the electron and quarks, it takes on the value of ½ in natural units.

Spin has a lot of very strange properties. You can think of it like a little arrow pointing in a direction in space but it’s not something we can truly visualise. One is tempted to think of the electron like the Earth, a sphere spinning about some kind of axis, but the electron is not a sphere, it’s a point-like particle with no “structure” in space. While an electron can have many different values of L depending on its energy (and atomic structure depends on these values), it only has one intrinsic magnitude of spin: ½. However, since spin can be thought of as an arrow, we have some flexibility. Loosely speaking, spin can point in many different directions but we’ll consider it as pointing “up” (+½) or “down” (- ½). If we try to measure it along a particular axis, we’re bound to find it in one of these states relative to our direction of measurement.

Spin250

Focus on one of the red faces. When the cube rotates every 360 degrees, the red ribbon appears to go above and below the cube alternatively! Because the cube is coupled to its environment, it takes 720 degrees to return it to it’s original orientation.


One of the peculiar things about spin-½ is that it causes the wave-function of the electron to exhibit some mind bending properties. For example, you’d think rotating any object by 360 degrees would put it back into exactly the same state as it was, but it turns out that doesn’t hold true for electrons. For electrons, rotating them by 360 degrees introduces a negative sign into their wave-function! You have to spin it another 360 degrees to get it back into the same state! There are ways to visualise systems with similar behaviour (see right) but that’s just a sort of “metaphor” for what really happens to the electron. This links into the famous conclusion of Pauli’s that no two identical particles with spin-½ (or any other half-integer spin) can share the same quantum mechanical state.

——

Spin is an important property of matter that only really manifests on the quantum scale, and while we can’t visualise it, it ends up being important for the structure of atoms and how all solid objects obtain the properties they do. The other important property it has is that the spin of a free particle likes to align with magnetic fields4 (and the bigger the spin, the greater the magnetic coupling to the field). By using this property, it was discovered that the proton also had angular momentum J = ½. Since the proton is a stable particle, it was modelled to be in a low energy state with L = 0 and hence J = S = ½ (that is to say, the orbital angular momentum is assumed to be zero and hence we may simply call J, the “spin”). The fact the proton has spin and that spin aligns with magnetic fields, is a crucial element to what makes MRI machines work.

Once we got a firm handle on quarks in the late 1960s, the spin structure of the proton was thought to be fairly simple. The proton has spin-½. Quarks, from scattering experiments and symmetry considerations, were also inferred to have spin-½. Therefore, if the three quarks that make up the proton were in an “up-down-up” configuration, the spin of the proton naturally comes out as ½ – ½ + ½ = ½. Not only does this add up to the measured spin, but it also gives a pleasant symmetry to the quantum description of the proton, consistent with the Pauli exclusion principle (it doesn’t matter which of the three quarks is the “down” quark). But hang on, didn’t I say that the three-quarks story was incomplete? At high energies, there should be a lot more quark-antiquark pairs (sea quarks) involved, messing everything up! Even so, theorists predicted that these quark-antiquark pairs would tend not to be polarised, that is, have a preferred direction, and hence would not contribute to the total spin of the proton.

If you can get the entirety of the proton spinning in a particular direction (i.e. polarising it), it turns out the scattering of an electron against its constituent quarks should be sensitive to their spin! Thus, by scattering electrons at high energy, one could check the predictions of theorists about how the quarks’ spin contributes to the proton.

In a series of perfectly conducted experiments, the theory was found to be absolutely spot on with no discrepancy whatsoever. Several Nobel prizes were handed out and the entire incident was considered resolved, now just a footnote in history. OK, not really.

In truth, the total opposite happened. Although the experiments had a reasonable amount of uncertainty due to the inherent difficulty of polarising protons, a landmark paper by the European Muon Collaboration found results consistent with the quarks contributing absolutely no overall spin to the proton whatsoever! The measurements could be interpreted with the overall spin from the quarks being zero5. This was a complete shock to most physicists who were expecting verification from what was supposed to be a fairly straightforward measurement. Credit where it is due, there were theorists who had predicted that the assumption about orbital angular momentum (L = 0) had been rather ad-hoc and that L > 0 could account for some of the missing spin. Scarcely anyone would have expected, however, that the quarks would carry so little of the spin. Although the nuclear strong force, which governs how quarks and gluons combine to form the proton, has been tested to remarkable accuracy, the nature of its self-interaction makes it incredibly difficult to draw predictions from.

The feynman diagram for Deep Inelastic Scattering (electron line at the top, proton on the bottom). This type of scattering is sensitive to quark spin.

The Feynman diagram for Deep Inelastic Scattering (electron line at the top, proton on the bottom, with a photon exchanged between them). This type of scattering is sensitive to quark spin.

Future experiments (led by father and son rivals, Vernon and Emlyn Hughes6 of CERN and SLAC respectively) managed to bring this to a marginally less shocking proposal. The greater accuracy of the measurements from these collaborations had found that the total spin contributions from the quarks was actually closer to ~30%. An important discovery was that the sea quarks, thought not to be important, were actually found to have measurable polarisation. Although it cleared up some of the discrepancy, it still left 60-70% of spin unaccounted for. Today, following much more experimental activity in Deep Inelastic Scattering and precision low-energy elastic scattering, the situation has not changed in terms of the raw numbers. The best estimates still peg the quarks’ spin as constituting only about 30% of the total.

Remarkably, there are theoretical proposals to resolve the problem that were hinted at long before experiments were even conducted. As mentioned previously, although currently impossible to test experimentally, the quarks may carry orbital angular momentum (L) that could compensate for some of the missing spin. Furthermore, we have failed to mention the contribution of gluons to the proton spin. Gluons are spin-1 particles, and were thought to arrange themselves such that their total contribution to the proton spin was nearly non-existent.

BNL AERIALS

The Brookhaven National Laboratory where RHIC is based (seen as the circle, top right).


The Relativistic Heavy Ion Collider (RHIC) in New York is currently the only spin-polarised proton collider in the world. This gives it a unique sensitivity to the spin structure of the proton. In 2014, an analysis of the data collected at RHIC indicated that the gluons (whose spin contribution can be inferred from polarised proton-proton collisions) could potentially account for up to 30 of the missing 70% of proton spin! About the same as the quarks. This would bring the “missing” amount down to about 40%, which could be accounted for by the unmeasurable orbital angular momentum of both quarks and gluons.

As 2016 kicks into gear, RHIC will be collecting data at a much faster rate than ever after a recent technical upgrade that should double it’s luminosity (loosely speaking, the rate at which proton collisions occur). With the increased statistics, we should be able to get an even greater handle on the exact origin of proton spin. 


The astute reader, provided they have not already wandered off, dizzy from all this talk of spinning protons, may be tempted to ask “Why on earth does it matter where the total spin comes from? Isn’t this just abstract accountancy?” This is a fair question and I think the answer is a good one. Protons, like all other hadrons (similar, composite particles made of quarks and gluons) are not very well understood at all. A peculiar feature of QCD called confinement binds individual quarks together so that they are never observed in isolation, only bound up in particles such as the proton. Understanding the spin structure of the proton can inform our theoretical models for understanding this phenomenon.

This has important implications, one being that 98% of the mass of all visible matter does not come from the Higgs Boson. It comes from the binding energy of protons! And the exact nature of confinement and precise properties of QCD have implications for the cosmology of the early universe. Finally, scattering experiments with protons have already revealed so much to fundamental physics, such as the comprehension of one of the fundamental forces of nature. As one of our most reliable probes of nature, currently in use at the LHC, understanding them better will almost certainly aid our attempts to unearth future discoveries.

Kind regards to Sebastian Bending (UCL) for several suggestions (all mistakes are unreservedly my own).

 

[1] …excluding dark matter and dark energy which constitute the dark ~95% of the universe.

[2] To the best of our knowledge.

[3] Strictly speaking the component of velocity perpendicular to the radial direction.

[4] Sometimes, spins in a medium like water like to align against magnetic fields, causing an opposite magnetic moment (known as diamagnetism). Since frogs are mostly water, this effect can and has been used to levitate frogs.

[5] A lot of the information here has been summarised from this excellent article by Robert Jaffe, whose collaboration with John Ellis on the Ellis-Jaffe rule led to many of the predictions discussed here.

[6] Emlyn was actually the spokesperson for SLAC, though he is listed as one of the primary authors on the SLAC papers regarding the spin structure of the proton.

Share

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

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

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

Share

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)

Share

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!

A typical Higgs boson candidate in the "golden mode" (ATLAS Collaboration)

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 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

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

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

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

Possible decays of a spin 1 particle

Uh-oh, it looks like we can’t make a spin 1 particle from photons! To align the spins correctly the photons must be in an antisymmetric state, which is absolutely forbidden by Bose-Einstein statistics. (Incidentally the term “boson” comes from the name Bose.) That means that this new boson is definitely not spin 1, because we see it decay to two photons.

So that means we have to do things the hard way to measure the spin of this new particle. For those who are interested, one of the main challenges presented here comes from the “acceptance” of the detectors- the kinematics of the final states we observe are significantly biased by the geometry of the detector. Even for a spin-0 boson, which decays isotropically, the distributions of the final decay products in the detector will not be isotropic, because the detectors do not have completely hermetic coverage. Fortunately since this post was first written we’ve gathered more data, and detailed studies have been performed eliminating all but the spin 0 hypothesis with a positive parity, indicating that what we have seen is most likely the long sought Standard Model Higgs boson after all.

Errata: In the original post I incorrectly made an argument stating that the decay of a spin 2 boson to a pair of quarks would be significantly more probable than the decay to a pair of leprons. Following discussions with Frank Close and Bob Cousins it was pointed out that well established graviton models would give a tensor interaction that would decay to leptons roughly 2% of the time per lepton flavour, making these final states accessible to the LHC experiments, and likely before the dijet final states would be accessible. My thanks go out to Close and Cousins for their correction.

Share

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…)

Share

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

Share