• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • 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 ‘positron’

To celebrate the first five years of operation on board the International Space Station, Professor Sam Ting, the spokesperson for the Alpha Magnetic Spectrometer (AMS-02) Collaboration just presented their latest results at a recent seminar held at CERN. With a sample of 90 million events collected in cosmic rays, they now have the most precise data on a wide range of particles found in outer space.

ams-02

source: ©NASA

Many physicists wonder if the AMS Collaboration will resolve the enigma on the origin of the excess of positrons found in cosmic rays. Positrons are the antimatter of electrons. Given that we live in a world made almost uniquely of matter, scientists have been wondering for more than a decade where these positrons come from. It is well known that some positrons are produced when cosmic rays interact with the interstellar material. What is puzzling is that more positrons are observed than what is expected from this source alone.

Various hypotheses have been formulated to explain the origin of these extra positrons. One particularly exciting possibility is that these positrons could emanate from the annihilation of dark matter particles. Dark matter is some form of invisible matter that is observed in the Universe mostly through its gravitational effects. Regular matter, everything we know on Earth but also everything found in stars and galaxies, emits light when heated up, just like a piece of heated metal glows.

Dark matter emits no light, hence its name. It is five times more prevalent than regular matter. Although no one knows, we suspect dark matter, just like regular matter, is made of particles but no one has yet been able to capture a particle of dark matter. However, if dark matter particles exist, they could annihilate with each other and produce an electron and a positron, or a proton and antiproton pair. This would at long last establish that dark matter particles exist and reveal some clues on their characteristics.

An alternative but less exotic explanation would be that the observed excess of positrons comes from pulsars. Pulsars are neutron stars with a strong magnetic field that emit pulsed light. But light is made of photons and photons can also decay into an electron and a positron. So both the pulsar and the dark matter annihilation provide a plausible explanation on the source of these positrons.

To tell the difference, one must measure the energy of all positrons found in cosmic rays and see how many are found at high energy. This is what AMS has done and their data are shown on the left plot below, where we see the flux of positrons (vertical axis) found at different energies (horizontal axis). The flux combines the number of positrons found with their energy cube. The green curve gives how many positrons are expected from cosmic rays hitting the interstellar material (ISM).

If the excess of positrons were to come from dark matter annihilation, no positron would be found with an energy exceeding the mass of the dark matter particle. They would have an energy distribution similar to the brown curve on the plot below as expected for dark matter particles having a mass of 1 TeV, a thousand times heavier than a proton. In that case, the positrons energy distribution curve would drop off sharply. The red dots represent the AMS data with their experimental errors shown by the vertical bars. If, on the other end, the positrons came from pulsars, the drop at high energy would be less pronounced.

ams-2016

source: AMS Collaboration

The name of the game is therefore to figure out precisely what is happening at high energy. But there are much fewer positrons there, making it very difficult to see what is happening as indicated by the large error bars attached to the data points at higher energy. These indicate the size of the experimental errors.

But by looking at the fraction of positrons found in all data collected for electrons and positrons (right plot above), some of the experimental errors cancel out. AMS has collected over a million positrons and 16 million electrons. The red dots on the right plot show the fraction of positrons found in their sample as a function of energy. Given the actual precision of these measurements, it is still not completely clear if this fraction is really falling off at higher energy or not.

The AMS Collaboration hopes however to have enough data to distinguish the two hypotheses by 2024 when the ISS will cease operation. These projections are shown on the next two plots both for the positrons flux (left) and the positron fraction (right). As it stands today, both hypotheses are still possible given the size of the experimental errors.

ams-2024

source: AMS Collaboration

There is another way to test the dark matter hypothesis. By interacting with the interstellar material, cosmic rays produce not only positrons, but also antiprotons. And so would dark matter annihilations but pulsars cannot produce antiprotons. If there were also an excess of antiprotons in outer space that could not be accounted for by cosmic rays, it would reinforce the dark matter hypothesis. But this entails knowing precisely how cosmic rays propagate and interact with the interstellar medium.

Using the AMS large sample of antiprotons, Prof. Sam Ting claimed that such excess already exists. He showed the following plot giving the fraction of antiprotons found in the total sample of protons and antiprotons as a function of their energy. The red dots represent the AMS measurements, the brown band, some theoretical calculation for cosmic rays, and the blue band, what could be coming from dark matter.

antiproton-fraction

source: AMS Collaboration

This plot clearly suggests that more antiprotons are found than what is expected from cosmic rays interacting with the interstellar material (ISM). But both Dan Hooper and Ilias Cholis, two theorists and experts on this subject, strongly disagree, saying that the uncertainty on this calculation is much larger. They say that the following plot (from Cuoco et al.) is by far more realistic. The pink dots represent the AMS data for the antiproton fraction. The data seem in good agreement with the theoretical prediction given by the black line and grey bands. So there are no signs of a large excess of antiprotons here. We need to wait for a few more years before the AMS data and the theoretical estimates are precise enough to determine if there is an excess or not.

antiprotons-theorie

source: Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

The AMS Collaboration could have another huge surprise is stock: discovering the first antiatoms of helium in outer space. Given that anything more complex than an antiproton is much more difficult to produce, they will need to analyze huge amounts of data and further reduce all their experimental errors before such a discovery could be established.

Will AMS discover antihelium atoms in cosmic rays, establish the presence of an excess of antiprotons or even solve the positron enigma? AMS has lots of exciting work on its agenda. Well worth waiting for it!

Pauline Gagnon

To find out more about particle physics and dark matter, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN ».

To be notified of new blogs, follow me on Twitter : @GagnonPauline or sign up on this distribution list

 

Share

Pour célébrer les cinq premières années d’opération à bord de la Station spatiale internationale, le Professeur Sam Ting, porte-parole de la Collaboration Alpha Magnetic Spectrometer (AMS-02) vient de présenter leurs derniers résultats lors d’un récent séminaire tenu au CERN. Avec plus de 90 millions d’évènements recueillis dans les rayons cosmiques, ce groupe dispose des données les plus précises sur une vaste gamme de particules trouvées dans l’espace.

ams-02

source: ©NASA

La question qui intrigue de nombreux scientifiques est de savoir s’ils pourront résoudre l’énigme de l’origine de l’excès de positrons trouvés dans les rayons cosmiques. Les positrons sont l’antimatière des électrons. Étant donné que nous vivons dans un monde fait presque uniquement de matière, les scientifiques se demandent depuis plus d’une décennie d’où émanent ces positrons. Il est bien connu que des positrons sont produits lorsque les rayons cosmiques interagissent avec la matière interstellaire mais on en observe bien plus que ce à quoi on s’attendait de cette seule source.

Des hypothèses diverses ont été formulées pour expliquer l’origine de ces positrons excédentaires. Une des plus fascinantes suggère que ces positrons pourraient venir de l’annihilation de particules de matière sombre. La matière sombre est une nouvelle forme de matière invisible qu’on détecte dans l’Univers par ses effets gravitationnels. La matière régulière, tout ce que nous voyons sur la Terre, mais aussi dans les étoiles et les galaxies, émet de la lumière lorsque chauffée, tout comme une pièce métallique irradie à haute température.

La matière sombre n’émet aucune lumière, d’où son nom. Elle est cinq fois plus répandue que la matière régulière. Personne ne le sait encore mais on soupçonne que cette matière, tout comme la matière ordinaire, soit faite de particules, mais on n’a toujours pas capturé de particules de matière sombre. Mais si de telles particules existaient, elles pourraient s’annihiler entre elles, produisant des électrons et des positrons, ou des paires de protons et d’antiprotons. Si un tel processus était établi, cela confirmerait enfin l’existence de particules de matière sombre et révèlerait quelques indices sur leurs caractéristiques.

Une explication alternative mais moins exotique serait que l’excès observé de positrons provienne de pulsars. Les pulsars sont des étoiles à neutrons ayant un fort champ magnétique et qui émettent de la lumière pulsée. Mais la lumière est faite de photons et les photons peuvent eux aussi produire des paires d’électrons et de positrons. Donc, les pulsars tout comme l’annihilation de matière sombre, fournissent une explication plausible quant à la source de ces positrons.

Pour les distinguer, il faut mesurer l’énergie des positrons captés dans les rayons cosmiques et voir combien on en trouve à haute énergie. C’est ce que AMS a fait et leurs résultats sont visibles dans le graphe de gauche ci-dessous où nous voyons le flux de positrons (axe vertical) trouvé à une énergie particulière (axe horizontal). Le flux combine le nombre de positrons trouvés avec leur énergie au cube. La courbe en vert donne le nombre de positrons produits lorsque des rayons cosmiques frappent de la matière interstellaire (ISM).

Si l’excès de positrons devait venir de l’annihilation de matière sombre, on ne trouverait aucun positron au-delà de l’énergie correspondant à la masse des particules de matière sombre. Ils auraient une distribution d’énergie semblable à la courbe en brun sur le graphe ci-dessous tel que prédit pour des particules de matière sombre ayant une masse de 1 TeV, soit mille fois plus lourd qu’un proton. Dans ce cas, la courbe de distribution d’énergie des positrons chuterait rapidement. Les points en rouge représentent les données d’AMS avec leurs erreurs expérimentales indiquées par les barres verticales. Par contre, si les positrons venaient de pulsars, la chûte à haute énergie serait moins prononcée.

ams-2016

source: Collaboration AMS

Toute la difficulté consiste à comprendre précisément leur comportement à haute énergie. Mais comme on y trouve moins de positrons, il est beaucoup plus difficile de voir ce qu’il en est comme l’indiquent les larges marges d’erreur associées aux mesures faites à plus haute énergie.

Mais si on mesure plutôt la fraction de positrons trouvés dans les données en combinant positrons et électrons, certaines des erreurs expérimentales s’annulent. AMS a rassemblé plus d’un million de positrons et 16 millions d’électrons. Les points en rouge sur le graphe de droite ci-dessus montrent la fraction de positrons trouvée dans leur échantillon en fonction de leur énergie. Malgré les pas de géants accomplis, la précision actuelle de ces mesures ne permet toujours pas d’établir clairement si cette fraction tombe abruptement à haute énergie ou pas.

La Collaboration AMS espère toutefois avoir assez de données pour distinguer les deux hypothèses d’ici à 2024, date à laquelle la Station Spatiale Internationale cessera ses opérations. On peut voir ces projections sur les deux graphes suivants tant pour le flux de positrons (à gauche) que pour la fraction de positrons (à droite). À ce jour, les deux hypothèses sont toujours valides étant donné la taille des erreurs expérimentales.

ams-2024

source: Collaboration AMS

L’hypothèse de la matière sombre peut aussi être testée d’une autre façon. En interagissant avec la matière interstellaire, les rayons cosmiques produisent non seulement des positrons mais aussi des antiprotons. Les annihilations de matière sombre pourraient aussi en produire mais pas les pulsars. Il faut donc déterminer s’il y a ou pas plus d’antiprotons dans l’espace que ce que les rayons cosmiques peuvent produire. Si c’était établi, ce serait un argument de plus contre l’hypothèse des pulsars. Mais pour ce faire, il faut savoir précisément comment les rayons cosmiques se propagent et interagissent avec la matière interstellaire.

S’appuyant sur le vaste échantillon d’antiprotons recueillis par AMS, le Prof. Sam Ting a soutenu qu’un tel excès existe, présentant le graphe suivant à l’appui. On y voit la fraction d’antiprotons trouvés dans l’échantillon total de protons et des antiprotons en fonction de leur énergie. Les points en rouge représentent les mesures d’AMS, la bande brune, les calculs théoriques pour les rayons cosmiques et la bande bleue, ce qui pourrait venir de la matière sombre.

antiproton-fraction

source: Collaboration AMS

Ce graphe suggère fortement un surplus d’antiprotons par rapport à ce que l’on s’attend des rayons cosmiques interagissant avec la matière interstellaire (ISM). Mais tant Dan Hooper qu’Ilias Cholis, deux théoriciens experts en la matière, s’objectent carrément, disant que l’incertitude sur les prédictions théoriques sont beaucoup plus grandes que ce que ce graphe suggère. Ils soutiennent que le graphe suivant (de Cuoco etal.) est de loin plus réaliste. Les points en rose représentent les données d’AMS pour la fraction d’antiprotons et le trait en noir, les prédictions théoriques avec leur marge d’erreur. Les deux concordent ou presque, suggérant l’absence de tout excès. Nous devrons patienter encore quelques années avant que les données d’AMS et les prédictions théoriques soient assez précises pour savoir s’il y a excès ou pas.

antiprotons-theorie

            source : Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

La Collaboration AMS pourrait nous réserver une autre belle surprise : la découverte d’antiatomes d’hélium dans l’espace. Étant donné l’extrême difficulté à produire une particule d’antimatière plus complexe qu’un antiproton, les scientifiques d’AMS devront trier d’énormes quantités de données et réduire toutes les erreurs expérimentales encore davantage avant qu’une telle découverte ne puisse être établie.

La découverte d’antihélium, ou celle d’un excès d’antiprotons ou encore la résolution de l’énigme des positrons, tout cela vaut bien la peine d’attendre encore quelques années. AMS a du beau pain sur la planche!
Pauline Gagnon

Pour en savoir plus sur la physique des particules et la matière sombre, consultez mon livre “Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels“.

Pour être au courant des nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou inscrivez-vous sur cette liste de distribution

Share

The ILC site has been chosen. What does this mean for Japan?

Credit: linearcollider.org

The two ILC candidate sites: Sefuri in the South and Kitakami in the North. Credit: linearcollider.org

Hi Folks,

It is official [Japanese1,Japanese2]: the Linear Collider Collaboration and the Japanese physics community have selected the Kitakami mountain range in northern Japan as the site for the proposed International Linear Collider. Kitakami is a located in the Iwate Prefecture and is just north of the Miyagi prefecture, the epicenter of the 2011 Tohoku Earthquake. Having visited the site in June, I cannot aptly express how gorgeous the area is, but more importantly, how well-prepared Iwate City is for this responsibility.

Science is cumulative: new discoveries are used to make more discoveries about how nature works, and physics is no different. The discovery of the Higgs boson at the Large Hadron Collider was a momentous event. With its discovery, physicists proved how some particles have mass and why others have no mass at all. The Higgs boson plays a special role in this process, and after finally finding it, we are determined to learn more about the Higgs. The International Linear Collider (ILC) is a proposed Higgs boson factory that would allow us to intimately understand the Higgs. Spanning 19 miles (31 km) [310 football pitches/soccer fields], if constructed, the ILC will smash together electrons and their antimatter partners, positrons, to produce a Higgs boson (along with a Z boson). In such a clean environment (compared to proton colliders), ultra-precise measurements of the Higgs boson’s properties can be made, and thereby elucidate the nature of this shiny new particle.

credit: li

The general overview schematic of the International Linear Collider. Credit: linearcollider.org

However, the ILC is more than just a experiment. Designing, constructing, and operating the machine for 20 years will be a huge undertaking with lasting effects. For staters, the collider’s Technical Design Report (TDR), which contains every imaginable detail minus the actual blueprints, estimates the cost of the new accelerator to be 7.8 billion USD (2012 dollars). This is not a bad thing. Supposing 50% of the support came from Asia, 25% from the Americas, and 25% from Europe, that would be nearly 2 billion USD invested in new radio frequency technology in England, Germany, and Italy. In the US, it would be nearly 2 billion USD invested in coastal and Midwestern laboratories developing new cryogenic and superconducting technology. In Asia, this would be nearly 4 billion USD invested in these technologies as well as pure labor and construction. Just as the LHC was a boon on the European economy, a Japanese-based ILC will be a boon for an economy temporarily devastated  by an historic earthquake and tsunami. These are just hypothetical numbers; the real economic impact will be  larger.

I had the opportunity to visit Kitakami this past June as a part of a Higgs workshop hosted by Tohoku University. Many things are worth noting. The first is just how gorgeous the site is. Despite its lush appearance, the site offers several geological advantages, including stability against earthquakes of any size. Despite its proximity to the 2011 earthquake and the subsequent tsunami, this area was naturally protected by the mountains. Below is a photo of the Kitakami mountains that I took while visiting the site. Interestingly, I took the photo from the UNESCO World Heritage site Hiraizumi. The ILC is designed to sit between the two mountains in the picture.

ilcSite_Kitakami

The Kitamaki Mountain Range as seen from the UNESCO World Heritage Site in Hiraizumi, Japan. Credit: Mine

What I want to point out in the picture below is the futuristic-looking set of tracks running across the photo. That is the rail line for the JR East bullet train, aka the Tohoku Shinkansen. In other words, the ILC site neighbours a very major transportation line connecting the Japanese capital Tokyo to the northern coast. It takes the train just over 2 hours to traverse the 250 miles (406.3 km) from Tokyo station to the Ichinoseki station in Iwate. The nearest major city is Sendai, capital of Miyagi, home to the renown Tohoku University, and is only a 10 minute shinkansen ride from Ichinoseki station.

...

The Kitamaki Mountain Range as seen from the UNESCO World Heritage Site in Hiraizumi, Japan. Credit: Mine

What surprised me is how excited the local community is about the collider. After exiting the Ichinoseki station I discovered this subtle sign of support:

There is much community support for the ILC: The Ichinoseki Shinkansen Station in Iwate Prefecture, Japan. Credit: Mine

The residents of Iwate and Miyagi, independent of any official lobbying organization, have formed their own “ILC Support Committee.” They even have their own facebook page. Over the past year, the residents have invited local university physicists to give public lectures on what the ILC is; they have requested that more English, Chinese, Korean, and Tagalog language classes be offered at local community centers; that more Japanese language classes for foreigners are offered in these same facilities; and have even discussed with city officials how to prepare Iwate for the prospect of a rapid increase in population over the next 20 years.

Despite all this, the real surprises were the pamphlets. Iwate has seriously thought this through.

asdsad

Pamphlets showcasing the Kitakami Mountain Range in Iwate, Japan. Credit: Mine

The level of detail in the pamphlets is impressive. My favourite pamphlet has the phrase, “Ray of Hope: Tohoku Is Ready to Welcome the ILC” on the front cover. Inside is a list of ways to reach the ILC site and the time it takes. For example: it takes 12 hours 50 minutes to reach Tokyo from Rome and 9 hours 40 minutes from Sydney. The brochure elaborates that the Kitakami mountains maintain roughly the same temperature as Switzerland (except in August-September) but collects much more precipitation through the year. Considering that CERN is located in Geneva, Switzerland, and that many LHC experimentalists will likely become ILC experimentalists, the comparison is very helpful. The at-a-glance annual festival schedule is just icing on the cake.

asdd

“Ray of Hope” pamphlet describing how to each different ILC campuses by train.  Credit: Mine

Now that the ILC site has been selected, surveys of the land can be conducted so that blue prints and a finalized cost estimate can be established. From my discussions with people involved in the site selection process, the decision was very difficult. I have not visited the Fukuoka site, though I am told it is a comparably impressive location. It will be a while still before any decision to break ground is made. And until that happens, there is plenty of work to do.

Happy Colliding

– Richard (@bravelittlemuon)

 

Share

Fun post for everyone today. In response to last week’s post on describing KEK Laboratory’s discovery of additional exotic hadrons, I got an absolutely terrific question from a QD reader:

Surprisingly, the answer to “How does an electron-positron collider produce quarks if neither particle contains any?” all begins with the inconspicuous photon.

No Firefox, I Swear “Hadronization” is a Real Word.

As far as the history of quantum physics is concerned, the discovery that all light is fundamentally composed of very small particles called photons is a pretty big deal. The discovery allows us to have a very real and tangible description of how light and electrons actually interact, i.e., through the absorption or emission of photon by electrons.

Figure 1: Feynman diagrams demonstrating how electrons (denoted by e) can accelerate (change direction of motion) by (a) absorbing or (b) emitting a photon (denoted by the Greek letter gamma: γ).

The usefulness of recognizing light as being made up many, many photons is kicked up a few notches with the discovery of anti-particles during the 1930s, and in particular the anti-electron, or positron as it is popularly called. In summary, a particle’s anti-particle partner is an identical copy of the particle but all of its charges (like electric, weak, & color!) are the opposite. Consequentially, since positrons (e+) are so similar to electrons (e) their interactions with light are described just as easily.

Figure 2: Feynman diagrams demonstrating how positrons (e+) can accelerate (change direction of motion) by (a) absorbing or (b) emitting a photon (γ). Note: positrons are moving from left to right; the arrow’s direction simply implies that the positron is an anti-particle.

Then came Quantum Electrodynamics, a.k.a. QED, which gives us the rules for flipping, twisting, and combining these diagrams in order to describe all kinds of other real, physical phenomena. Instead of electrons interacting with photons (or positrons with photons), what if we wanted to describe electrons interacting with positrons? Well, one way is if an electron exchanges a photon with a positron.

Figure 3: A Feynman diagram demonstrating the exchange of a photon (γ) between an electrons (e)  and a positron (e+). Both the electron and positron are traveling from the left to the right. Additionally, not explicitly distinguishing between whether the electron is emitting or absorbing is intentional.

And now for the grand process that is the basis of all particle colliders throughout the entire brief* history of the Universe. According to electrodynamics, there is another way electrons and positrons can both interact with a photon. Namely, an electron and positron can annihilate into a photon and the photon can then pair-produce into a new electron and positron pair!

Figure 4: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that then produces an e+e pair. Note: All particles depicted travel from left to right.

However, electrons and positrons is not the only particle-anti-particle pair that can annihilate into photons, and hence be pair-produced by photons. You also have muons, which are identical to electrons in every way except that it is 200 times heavier than the electron. Given enough energy, a photon can pair-produce a muon and anti-muon just as easily as it can an electron and positron.

Figure 5: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that then produces a muon (μ) and anti-muon(μ+) pair.

But there is no reason why we need to limit ourselves only to particles that have no color charge, i.e., not charged under the Strong nuclear force. Take a bottom-type quark for example. A bottom quark has an electric charge of -1/3 elementary units; a weak (isospin) charge of -1/2; and its color charge can be red, blue, or green. The anti-bottom quark therefore has an electric charge of +1/3 elementary units; a weak (isospin) charge of +1/2; and its color charge can be anti-red, anti-blue, or anti-green. Since the two have non-zero electric charges, it can be pair-produced by a photon, too.

Figure 6: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that then produces a bottom quark (b) and anti-bottom quark (b) pair.

On top of that, since the Strong nuclear force is, well, really strong, either the bottom quark or the anti-bottom quark can very easily emit or absorb a gluon!

Figure 7: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that produces a bottom quark (b) and anti-bottom quark (b) pair, which then radiate gluons (blue).

In electrodynamics, photons (γ) are emitted or absorbed whenever an electrically charged particle changes it direction of motion. And since the gluon in chromodynamics plays the same role as the photon in electrodynamics, a gluon is emitted or absorbed whenever  a “colorfully” charged particle changes its direction of motion. We can absolutely take this analogy a step further: gluons are able to pair-produce, just like photons.

Figure 8: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that produces a bottom quark (b) and anti-bottom quark (b) pair. These quarks then radiate gluons (blue), which finally pair-produce into quarks.

At the end of the day, however, we have to include the effects of the Weak nuclear force. This is because electrons and quarks have what are called “weak (isospin) charges”. Firstly, there is the massive Z boson (Z), which acts and behaves much like the photon; that is to say, an electron and positron can annihilate into a Z boson. Secondly, there is the slightly lighter but still very massive W boson (W), which can be radiated from quarks much like gluons, just to a lesser extent. Phenomenally, both Weak bosons can decay into quarks and form semi-stable, multi-quark systems called hadrons. The formation of hadrons is, unsurprisingly, called hadronization. Two such examples are the the π meson (pronounced: pie mez-on)  or the J/ψ meson (pronounced: jay-sigh mezon). (See this other QD article for more about hadrons.)

Figure 9: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) or a Z boson (Z) that produces a bottom quark (b) and anti-bottom quark (b) pair. These quarks then radiate gluons (blue) and a W boson (W), both of which finally pair-produce into semi-stable multi-quark systems known as hadrons (J/ψ and π).

 

In summary, when electrons and positrons annihilate, they will produce a photon or a Z boson. In either case, the resultant particle is allowed to decay into quarks, which can radiate additional gluons and W bosons. The gluons and W boson will then form hadrons. My friend Geoffry, that is how how you can produce quarks and hadrons from electron-positron colliders.

 

Now go! Discuss and ask questions.

 

Happy Colliding

– richard (@bravelittlemuon)

 

* The Universe’s age is measured to be about 13.69 billion years. The mean life of a proton is longer than 2.1 x 1029 years, which is more than 15,000,000,000,000,000,000 times the age of the Universe. Yeah, I know it sounds absurd but it is true.

Share