The Strong Force. Announcements Exam#3 next Monday. (Bring your calculator) HW10 will be posted today. Solutions will be posted Thurs. afternoon (I’m not collecting HW#10) Q&A session on Sunday at 5 pm . Tentative course grades will be posted by Tuesday evening.
Why do protons & protons, protons & neutrons, andneutrons & neutrons all bind together in the nucleusof an atom?
Electromagnetic? No, this would cannot cause protons to bind to one another.Gravity ? NO, way too feeble (even weaker than EM force)
Need a force which: A) Can overcome the electrical repulsion between protons.
B) Is ‘blind’ to electric charge (i.e., neutrons bind to other neutrons!)
Quantum theory of EM Interactions is incredibly precise. That is, thetheoretical calculations agree with experimental observations toincredible accuracy.
Build a similar theory of the strong interaction, based on force carriers
Electric charge = +2/3
Electric charge = -1
Which of these might you expect experiences a larger electricalrepulsion?
What does it really mean for a particle to have electric charge ?
It means the particle has an attribute which allows it to talk to (or ‘couple to’) the photon, the mediator of the electromagnetic interaction.
The ‘strength’ of the interaction depends on the amount of charge.
uStrong Force & Color
We hypothesize that in addition to the attribute of ‘electric charge’,quarks have another attribute known as ‘color charge’, or just ‘color’for short. The attribute’s name, color, is just by convention. It’s easyto visualize this attribute and how colors combine…(coming up)
The property of color allows quarks to talk to the mediator of the strong interaction, the gluon (g).
Unlike electric charge, we find (experimentally) that there are 3 values for color: We assign these possible values of color to be: red, green, blue
Also, unlike Electromagnetism, we find that the carrier of the strongforce carries ‘color charge’. Recall the photon is electrically neutral!
Each of the 8 color combinationshave a “color” and an “anti-color”
This cannot happen, becausethe gluon does not interact withobjects unless they have color charge! Leptons do not have color charge !
e -Gluons – Important Points
Could also beQuark-antiquarkScatteringorAntiquark-antiquarkScattering
This can’t happen because the photononly interacts withelectrically chargedobjects !
gFlashback to EM Interactions
Recall that photons do not interact with each other.Why?
Because photons only interact with objects which have electric charge, and photons do not have electric charge !
Gluons carry the “charge” of the strong force, which is “color charge”, or just “color” !
~10-15 [m]Where do the gluons come from ?
In fact quarks are < 1/1000th of the size of the proton, so they are still too big in this picture !
Since the strong force increases as quarks move apart, they can only get so far…The quarks are confined together inside hadrons.Hadron jail !
dHadronization (process of forming hadrons)
As quarks move apart, the potentialenergy stored in the “spring”increases, until its large enough, to‘snap’ and convert its potentialenergy into mass energy (qq pairs)
In this way, you can see that quarksare always confined inside hadrons(that’s CONFINEMENT) !
The strong force !
Recall: Mass of Proton ~ 938 [MeV/c2]
2 up quarks: 2 * (5 [MeV/c2]) = 10 [MeV/c2]
1 down quark: 1 * 10 [MeV/c2] = 10 [MeV/c2]
Total quark mass in proton: ~ 20 [MeV/c2]
Where’s all the rest of the mass ?????
It’s incorporated in the binding energy associated with the gluons !
~98% of our mass comes from glue-ons !!!!