Gluon

The Strong Force for Beginners

Image credit: Hyak / Martin Savage, eScience Institute, University of Washington.

“I found I could say things with color and shapes that I couldn’t say any other way — things I had no words for.” –Georgia O’Keeffe

When it comes to the Universe, it isn’t just the stuff that’s in it that’s important.

Image credit: 2MASS Extended Source Catalog (XSC).

It’s also how all that stuff interacts with itself and everything else. To the best of our knowledge, there are four fundamental forces in the Universe, and they’re all essential to our existence.

Image credit: Stichting Maharishi University of Management, the Netherlands.

Some of them are familiar, like gravitation. On the largest scales in the Universe, gravitation is not only the most important force, but arguably the only important force in play. The amount of mass and energy inherent to objects determines how spacetime itself is curved, and this curvature of spacetime in turn determines how objects move and accelerate.

Image credit: Mark Garlick / SPL.

There’s no “anti-mass” or “anti-energy” that causes some objects to be gravitationally repelled while others are gravitationally attracted. Gravity is always attractive, and we can interpret mass/energy as the lone type of gravitational charge, if we want to.

But other forces and interactions can be more complicated than gravity in this regard. Take the electromagnetic force, for example, just by looking at charged particles.

Image credit: http://Maxwells-Equations.com, copyright 2012.

Instead of one type of charge where like-attracts-like, we have two types of electric charge: positive and negative, where like charges repel and unlike charges attract. It’s very different than gravity and somewhat more complicated, but there are some things that are familiar to us.

Image credit: Science Photo Library.

A neutral atom, for instance, is a good example of electromagnetism, where the positively charged nucleus is orbited by a swarm of negatively charged electrons. The electrons repel each other, but they’re all attracted to the nucleus even moreso. As long as the total charge of the atom is zero and there is no sufficiently strong external radiation, the atom will remain stable and neutral.

We even understand, at a fundamental quantum level, how this works. The attraction and repulsion between all charged particles is mediated by the same particle: the photon.

Image credit: Ask a Mathematician / Ask a Physicist.

It just takes one particle to take care of both attraction and repulsion, because of the relatively simple structure — two charges, like-repels-like and opposites attract — of electromagnetism. But things get a lot more complicated if we go inside the nucleus, and ask just how it is, at a fundamental level, that these tiny, charged structures hold themselves together.

Image credit: Hyak / Martin Savage, eScience Institute, University of Washington.

An atomic nucleus, of course, is made up of protons and neutrons, except for hydrogen, which is just a proton by itself. But seeing as how protons have a positive electric charge and neutrons have no electric charge at all, there must be some sort of extra force — a force even stronger than the electromagnetic force — to hold these nuclei together.

In fact, the creatively-named strong force is required to hold even the individual protons and neutrons themselves together. Because a proton and neutron themselves are not fundamental, but composed of even smaller, fractionally-charged particles known as quarks.

Image credit: Learn EveryWare, © 2009 Alberta Education, error caught by Rich and Mike, edited by me.

The electric forces inside of a proton, for instance, would cause the nucleus itself to fly apart if there weren’t another type of charge attached to each of these quarks: in addition to electric charge, they also possess color charge, which comes in not one type (like gravity), nor two (like electromagnetism), but three.

Only unlike gravitation and electromagnetism, you can’t just have a color charge off by itself: you need a red, green, and blue together, to add up to “colorless,” just like red, green, and blue light together add up to white. (Any Americans who want the three colors to be “red, white, and blue” can leave now, as can any Frenchman who thinks they should be “blue, white, and red.” I’m making an analogy, and your flag does not trump physics.)

Image credit: Focusbox.net, retrieved from Nuno Canaveira at nColour.

Just like there’s matter and antimatter, there are quarks and anti-quarks, and so there are colors (red, green and blue) and anti-colors: cyan is the anti-red, magenta is the anti-green, and yellow is the anti-blue. So to add up to “colorless,” you either need three quarks (or three anti-quarks), or one quark and one anti-quark.

Image credit: McLean County Unit District Number 5, http://www.unit5.org/.

It’s a little bit weird: if red + green + blue makes white, but red + anti-red also makes white, does that mean that green+blue is the same as anti-red? Yes, yes it does, at least in terms of color. Which means you can pair a quark with either two other quarks, with an antiquark, or possibly even with three other quarks and one antiquark. As long as the color comes out white (or colorless), you’re in business.

And that’s why you can have combinations of three quarks, like protons and neutrons, or combinations of one quark and one anti-quark, like mesons. But unlike gravity, which bends spacetime, or electromagnetism, where photons (with no charge) are exchanged, the strong force works by exchanging a new type of particle — the gluon — which carries both a color and an anti-color!

These gluons are responsible for holding both individual particles — like protons, neutrons, and pions — together, as well as for binding larger atomic nuclei together.

Image credit: CERN / European Organization for Nuclear Research, http://www.physik.uzh.ch/.

How does this work? With three colors (red, green, and blue) and three anti-colors (anti-red = cyan, anti-green = magenta, and anti-blue = yellow), you might think there are nine types of gluons that you can get from matching each color with each anti-color. That’s a good first thought, and it’s almost right.

Imagine you’re a red quark, and you emit a red/anti-green gluon. You’re going to turn the red quark into a green quark, because color is conserved like that, and then that gluon is going to find a green quark, and turn it red. In this fashion, colors get exchanged.

Image credit: Wikipedia / Wikimedia Commons user Qashqaiilove.

That turns out to be a good explanation for six of the gluons: red/anti-green, red/anti-blue, green/anti-red, green/anti-blue, blue/anti-red, and blue/anti-green.

But what about the others: there should also be red/anti-red, green/anti-green, and blue/anti-blue, right?

Almost, it turns out. Because each of those has no inherent color, those quantum states are allowed to mix together. In quantum physics, whenever mixing isn’t forbidden, it happens. So you get mixtures of red/anti-red, green/anti-green, and blue/anti-blue states.

Image credit: Me, your gluon hero.

But one of them — the one that’s an equal mixture of all three color/anti-color pairs — is truly colorless, and doesn’t physically exist. So there are only eight physical gluons.

And it’s the exchange of these gluons between the quarks and antiquarks that keep protons, neutrons, mesons, baryons, and all other atomic nuclei together. There is much more to the strong interactions than what I’ve described here, and if you want to go deeper, I recommend this jaunt by Nobel Laureate Frank Wilczek. Whether you do or not, the strong force is what holds every atomic nucleus together; without it, we’d simply be a lifeless sea of fundamental particles, too repulsive to hold together in any meaningful fashion.

Image credit: Roy Uyematsu.

And yet, here we are, so much more than a cosmic soup, with galaxies, stars, planets, heavy elements, molecules, life, and you and me. It’s the strongest force in the Universe, and without it, none of this would be possible. Enjoy!

Quantum Chromodynamics

There are four forces in Nature: "gravity" as seen in celestial motion and described by the general theory of relativity; the "electromagnetic force" as seen in the interaction between the nucleus and electrons; the "weak force" which describes the beta decay of a nucleus (the "electromagnetic force" and the "weak force" are unified in the electro weak theory); and the "strong force" that acts between the quarks and gluons that form protons and neutrons.

Our research effort at the RIKEN BNL Research Center (RBRC) focuses on a wide variety of phenomena caused by the “Strong Force." These various and complex phenomena include the matter-creating process after the Big Bang. This “Strong Force" is described through a theory called QCD (Quantum Chromodynamics). We at RBRC are pursuing our research projects in order to elucidate various physical phenomena brought by the “Strong Force" from the principles of QCD.

The nucleus is at the center of the atoms that form all matter.  The nucleons (protons and neutrons) are the components of the nucleus.  This nucleon consists of  quarks and gluons, the function of  the gluons is to be the glue that binds the quarks.  In a normal vacuum, quarks and gluons are bound by the Strong Force and "confined" in the nucleon.

This Strong Force has many very interesting characters; property called “asymptotic freedom” is that the force is relatively weak when quarks are close together, and becomes stronger and stronger when they are further and further apart, found by D.J. Gross, H.D. Politzer, and F. Wilczek (Nobel Prize in 2004). This property along with the prediction of “parity symmetry breaking” (Nobel Prize in 2008) by T.D. Lee, the Director Emeritus of RBRC, with C.N. Yang, are two of the foundations of contemporary particle and nuclear physics.

Other prominent properties of Strong Force include “Spontaneous symmetry breaking” (Nobel Prize in 2008) founded by Y. Nambu, as well as M. Kobayashi and T. Maskawa‘s “Charge-Parity symmetry breaking” (Nobel Prize in 2008), which plays an essential role in the search for the ultimate law of physics comprising gravity.  All of these "symmetry breakings" in QCD (Quantum Chromodynamics) continue to be the grand theme of research conducted at RBRC.

Κβαντική Χρωμοδυναμική

(Quantum Chromodynamics-QCD)

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Η αλληλεπίδραση η οποία ευθύνεται για τον σχηματισμό των πρωτονίων και των νετρονίων, συγκρατώντας τα κουάρκ, είναι η Κβαντική Χρωμοδυναμική (ΚΧΔ). Η ισχυρή αλληλεπίδρασηείναι η έκφανση της ΚΧΔ στο επίπεδο των πρωτονίων και νετρονίων. 
 

Στα πλαίσια της κβαντικής θεωρίας πεδίου, κάθε αλληλεπίδραση ισοδυναμεί με ανταλλαγή σωματιδίου-φορέα ο οποίος ακριβώς διαδίδει την αλληλεπίδραση. Τα σωματίδια-φορείς, τα υπεύθυνα για την ΚΧΔ, είναι τα γκλουόνια (gluons). Το μαθηματικό υπόβαθρο της ΚΧΔ προβλέπει ότι:
 

1. Κάθε είδος (άνω-u , κάτω-d , παράξενο-s κλπ) κουάρκ παρουσιάζεται σε 3 διαφορετικές μορφές ("χρώματα"): κόκκινο, πράσινο, μπλε. Τα αντικουάρκ έχουν το αντίθετο χρώμα: γαλάζιο (αντι-κόκκινο), μωβ (αντι-πράσινο) και κίτρινο (αντι-μπλέ). Φυσικά τα ονόματα αυτά δεν έχουν καμιά σχέση με πραγματικά χρώματα. Απλά, η επίκληση των ονομάτων αυτών βοηθά στην κατανόηση της λειτουργίας της μαθηματικής ομάδας SU(3), η οποία περιγράφει την ΚΧΔ.

2. Χρειάζονται 8 ειδών γκλουόνια για τη σωστή περιγραφή της ΚΧΔ. Τα έξι από αυτά φτιάχνονται από ένα χρώμα και ένα αντι-χρώμα (όχι όμως του αντίστοιχου χρώματος). Τα άλλα δύο φτιάχνονται με κατάλληλους συνδυασμούς όλων των χρωμάτων και αντι-χρωμάτων,  τους οποίους ορίζει επακριβώς το μαθηματικό υπόβαθρο.

  
 

Η ανταλλαγή των γκλουονίων αλλάζει το "χρώμα" των κουάρκ, αλλά όχι το είδος τους (άνω-u , κάτω-d, παράξενο-s κλπ).

 

Σε κάθε αδρόνιο, δηλαδή σωματίδιο το οποίο αποτελείται από κουάρκ και παρατηρείται ελεύθερο στη φύση (για παράδειγμα πρωτόνιο, νετρόνιο, σωματίδιο π, σωματίδιο Λ κλπ), ο συνδυασμός των "χρωμάτων" των κουάρκ είναι τέτοιος ώστε το αδρόνιο να εμφανίζεται "άχρωμο". Τον κατάλληλο συνδυασμό των "χρωμάτων" υποδεικνύει και πάλι το μαθηματικό υπόβαθρο της ΚΧΔ.

  
 

Ελεύθερα κουάρκ δεν παρατηρούνται. Αν σ' ένα ζευγάρι κουάρκ που σχηματίζει ένα αδρόνιο (για παράδειγμα το σωματίδιο π που συγκροτείται από ζευγάρια χρώμα - αντιχρώμα) προσπαθήσουμε να απομακρύνουμε το ένα κουάρκ από το άλλο, η ενέργεια που απαιτείται είναι τόσο μεγάλη ώστε η αρχική αλυσίδα των γκλουονίων (η οποία τα συγκρατεί) σπάει, γεννώντας δυο νέα κουάρκ τα οποία ξανασχηματίζουν αδρόνια.

Όταν τα πρωτόνια συγκρούονται

Ο ισχυρός επιταχυντής LHC επιταχύνει και καθοδηγεί δισεκατομμύρια πρωτόνια σε σύγκρουση με άλλα δισεκατομμύρια πρωτόνια. Σκοπός είναι οι απαντήσεις σε θεμελιακά ερωτήματα που οδηγούν στην κατανόηση της Φύσης. Αλλά τι ακριβώς συμβαίνει όταν τα πρωτόνια συγκρούονται;

Τα πρωτόνια αποτελούνται από κουάρκ που συγκρατιούνται από γκλουόνια και στις μετωπικές συγκρούσεις πρωτονίων αυτά ακριβώς που συγκρούονται είναι τα κουάρκ και τα γκλουόνια. 

Η εικόνα παρακάτω δείχνει δύο πρωτόνια λίγο πριν τη σύγκρουση. Μέσα στο πρωτόνιο βλέπεις την λεγόμενη "θάλασσα" των κουάρκ και των γκλουονίων. Γιατί τόσα πολλά; Δεν έμαθες ότι το πρωτόνιο αποτελείται από 3 κουάρκ; Σωστά, λέμε ότι το πρωτόνιο αποτελείται από 3 κουάρκ "σθένους", αλλά επιπλέον υπάρχει μια ολόκληρη "θάλασσα" από εν δυνάμει (δυνητικά) ζεύγη κουάρκ-αντικουάρκ που προέρχονται από τα γκλουόνια. 

Ποιο λοιπόν είναι το αποτέλεσμα μιας τέτοιας σύγκρουσης; 

Όταν τα πρωτόνια συγκρούονται σε τόσες μεγάλες ενέργειες σαν αυτές του LHC, προκαλείται ένας καταιγισμός από πολλά σωματίδια, όπως αυτά που συγκροτούν την συνηθισμένη ύλη αλλά και άλλα που εμφανίστηκαν λίγο μετά την Μεγάλη Έκρηξη (Big Bang). 

Τα νέα σωματίδια είναι συνήθως πολύ βαρύτερα από τα αρχικά συγκρουόμενα σωματίδια, χάρη στην σχέση E=mc². Για να το πούμε απλούστερα: όλη η ενέργεια της σύγκρουσης μπορεί να εμφανιστεί ως μάζα! Δηλαδή, στις συγκρούσεις πρωτονίων όλα μπορούν να συμβούν, όπου βέβαια ορισμένες θεμελιώδεις αρχές γίνονται σεβαστές, όπως η διατήρηση ενέργειας και ορμής.