Chapter 32

The Giancoli onLine Tutor
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Elementary particles

Note: References to the Giancoli text are in the form G:32-6, where the 6 means the sixth section.
The number 2000000 or 2.0x106 is written as 2.0e6

I. Elementary Particles and their Decays

This chapter deals with elementary particles and their detection. Elementary particles are grouped in several categories: Choose the upper left most find field in the Particle Prospector below to familiarize yourself with the different type particles in each listed group and read about their significance in Giancoli 32-6. The way it works is: if you change the field to 'Bosons' a table will appear listing all Bosons, similarly, choosing 'Leptons' will produce a table of leptons. For the time being ignore the other choice fields. After completing this exercise go to The Interactive Example below. (Note only the best know particles are included, and + or - means a charge of + or - 1.6e-19 C)

Particle Prospector

Category Name Spin Rest Mass (MeV/c2) B
Le Lm Lt S Lifetime (s)


Besides the spin quantum number, there are several new quantum numbers each can have the value 0 or 1 (The antiparticles for each particle, also listed in the results of 'prospecting', have the corresponding values 0 and -1). They are All elementary particle reactions and decays conserve these numbers. The total of each of these numbers on both sides of any reaction must be the same. Use the Prospector to investigate each of these quantum numbers and then answer the questions below.


Interactive Example

What are the two heaviest particles? Enter your answer below and click outside.
.
The decay of these particles are an exception to the following rule. Most decays are caused by the weak interaction and have a relatively long half-life ( >1x10-13 s). These particles do decay by the weak interaction but their decay times are uncharacteristically short. This has to do with their function as exchange particles.

The speed of a particle's decay is determine by the force that causes it. The three forces that produce decay are the nuclear force, the electric force and the weak interaction force. The first of these is the strongest and produces the quickest decays, the last is the weakest and produces the longest decay times. No particle which decays by the nuclear force are listed here, but three that decay by the electromagnetic interaction (force) can be found.

Interactive Example

Use the Prospector to determine the three shortest lived particles (other than the two found above) and enter the one thing their decay products have in common. Enter your answer below and click outside.


What else do they have in common? Enter your answer below and click outside.


The three particles above decay electromagnetically. They all involve the emission of photons and all have zero charge. Two of the particles are mesons, yet there is another zero charge meson, the Ko, that does not decay electromagnetically. That is because there is yet another quantum number called Strangeness. Strangeness is conserved in Nuclear and Electromagnetic decays but not in Weak Interaction Decays. The Ko meson has Strangeness equal to +1 and since there is no lighter particle with Strangeness +1, this meson, unlike the other charge zero ones, must decay via the Weak Interaction.

Interactive Example

Only one of the listed strange particles decays in such a way that Strangness is conserved , which is it? Enter your answer below and click outside.
Hopefully you discoved above that the one strange particle that decays and conserves strangeness is the So particle. The third particle, in addition to the two 0 charge mesons, that decays electromagnetically.

II. Particle Reactions

In all particle reactions B, Le, Ln, Lt, S and Q(charge) must be conserved as in the following examples.
reaction e- + p+ -> n + ne
Q -1+1 = 0+0
B 0+1 = 1+0
Le 1+0 = 0+1
Lm 0+0 = 0+0
Lt 0+0 = 0+0
S 0+0 = 0+0


reaction p- + p -> K0+ L0
Q -1+1 = 0+0
B 0+1 = 0+1
Le 0+0 = 0+0
Lm 0+0 = 0+0
Lt 0+0 = 0+0
S 0+0 = 1+ -1


(Keep Table 32-2 in Giancoli handy when working on the following.)

Interactive Examples

Now you try (select the two particles you think will result from the reaction and click the box below to get your result. Repeat as often as you like.).

L0 + p0 -> ? + ?
p+ p- K- K+ e+ p+ Select two and click feedback


p+ + K- -> ? + ?
p0 p- K0 L0 e+ p+ Select two and click feedback


p+ + nmbar -> ? + ?
p0 n m+ L0 e+ p+ Select two and click feedback

III.Quarks


The hadrons are now thought to be made out of a new fundamental set of particles called quarks. (See G32-9). The three primary quarks are given below. All have a Baryon number of 1/3 so a combination of three produces the proper Baryon Number of 1. Two Baryons, S0 and L0, are made of the same three quarks. S0 is viewed as an excited state of L0. Note that S0 decays into L0 plus a photon just like an excited state of hydrogen decays into the ground state plus an emitted photon.

What you need to do is choose a particular baryon. In the adjoining window the charge and strangeness will be shown. Next figure out what combination of quarks will produce these numbers. Check your results by using the quark drop down lists to produce this combination and the true result will be shown in the results field. repeat for the other Baryons.

NameSymbolBaryon NumberChargeStrangeness
Up u 1/3 2e/3 0
Down d 1/3 -1e/3 0
Strange s 1/3 -1e/3 -1



Hadron Construction Kits

Baryons

Interactive Activity

Choose BaryonQ= , S= UpDownStrangeQ= , S=

Mesons


Mesons are composed of a quark and an antiquark. The antiquarks have a Baryon number of -1/3 so the Baryon Number of a meson is 0. Proceed as you did above and see if you can find the quark antiquark combination for each meson.

NameSymbolBaryon NumberChargeStrangeness
antiUp ubar -1/3 -2e/3 0
antiDown dbar -1/3 1e/3 0
antiStrange sbar -1/3 1e/3 1


Interactive Activity

Choose MesonQ= , S= QuarkAntiQuarkQ= , S=


IV. Particle Accelerators


Particle accelerators are used to study elementary particles. See G:32-2 . The following movie goes with the text below. It is meant to illustrate the cyclotron accelerator.
If you don't have the Quicktime plugin you will get a broken image . You can view a GIF animation version of the movie here.
The cyclotron is made of two metal "Ds" separated by a gap. Think of cutting a 'tuna can' in half through one of its diameters and pulling the two halves slightly apart and you pretty much have the picture. The Ds are horizontally placed in a vertical magnetic field, B. Any moving charge in such a field will negotiate a circular path with radius R,
R=mv/qB.
The metal Ds are hooked to a source of alternating voltage such that a positively charged particle is accelerated across the gap. The increase in speed causes an increase in R. While inside a D the particle can not detect any Electric Field so that the voltage polarity on the Ds can be switched (see movie) so that when the particle next encounters a gap it is again accelerated. This inturn increases R again. A circle with an increasing radius is a Spiral. The particle follow a spiral as it undergoes repeated accelerations.

Interactive Example

In the movie, if the acclerating voltage is 50000 V, what is the final energy of the particle in MeV? Enter your answer below and click outside.

Fortunately the increase in distance travelled as the particle spirals outward is matched by its increase in speed in such a way that the frequency it circulates with, and hence the frequency of the alternating voltage, is fixed.
f = qB/2pm
The final particle velocity and, hence, energy depends on the radius of the cyclotron and not the magnitude of the accelerating voltage. The latter determines the number of accelerations only. The exit energy is given by
KE =q2B2R2/2m

Interactive Example

Assuming the particle is a proton and that B=1.7 T, what must be the radius to the nearest cm of the cyclotron in the movie? Enter your answer below and click outside.

Now you can operate your own cyclotron.

Interactive Activity

You control the accelerating voltage, magnetic field strength, and type of particle, proton(Z=1, A=1), deuteron(Z=1, A=2) or alpha(Z=2, A=4). The size of this cyclotron is .25 m.
Choose VoltageChoose BChoose ParticleAccelerations (#)Radius (m)Energy (MeV)

© 1999 Carl Adler mailto:Carl@Image-ination.com