Experiment VI-9

Standard Model

the search for the REAL fundamental particles


Enrico Fermi's explanation of β decay in 1933 was revolutionary.  The emission of α and γ radiation from radioactive atoms had been easier to understand.  It was understood that atomic nuclei contain protons and neutrons so the emission of a package of two protons and two neutrons, an α particle, from an unstable atom was plausible.  It also seemed reasonable that if after such an emission a nuclei had excess energy, the electrically charged particles could emit electromagnetic radiation in the form of a γ ray.  But what was revolutionary about β decay was that Fermi claimed nuclei contain neither electrons nor neutrinos.  He proposed that a transformation occurs in which a neutron disappears and in its place are formed a proton, an electron, and the electron's anti-neutrino, the latter two are and not bound by the nuclear force and so are ejected:

n ⇒ p+1 + e–1 + νe

This was the first theory for any process where one of the conserved elementary particles was required to change.

Yet Fermi's explanation allowed established conserved properties to remain conserved.

But Fermi's theory went further:  It had been observed that higher energy emissions generally occur within shorter half lives.  But for equal energies, α emissions occur in the shortest times, nuclei that emit γ last a little longer, and those nuclei the emit β linger the longest before emission.  Furthermore, it was understood that the attractive nuclear force which would be responsible for both holding the nucleus together and emitting α radiation had to be stronger inside a nucleus than the electromagnetic force responsible for repulsions between positively charged protons and for γ emission.  Apparently when other factors are equal, a stronger force results in quicker changes.  If that is true, then this slowest change, β decay, probably involves some sort of force weaker than either than either the strong nuclear force or the electromagnetic force.  Realizing this, Fermi proposed the existence of a weak nuclear force responsible for the relatively slow β decay.

Hideki Yukawa proposed in 1935 a new field theory which explained the strong nuclear force as due to virtual exchanges of a carrier boson particle.  After the π meson (pi meson = pion) predicted by Yukawa was finally observed in 1947, it was appreciated that a similar carrier would be needed for the weak nuclear force.  But that boson would be harder to detect.

Then just before Christmas 1947, Nature published evidence from cosmic ray photographs by G.D.Rochester and C.C.Butler of Manchester, England, of two additional new particles which appeared to be rapidly created in 10-23 sec but decayed much slower taking a long 10-10 sec.  That time difference suggested that both the strong and weak nuclear forces were at work.  It seemed plausible that hadron particles which experience the strong nuclear force might rapidly create other hadrons.  But there was considerable confusion why the decay of the new hadrons would take so long, and apparently only occur via the slower weak nuclear force.

In 1953 Murray Gell-Mann in the United States and Kazuhiko Nishijima in Japan independently proposed that these new hadrons possessed a previously unknown property which was conserved by the strong force but changed by the weak force.  In the first reaction the strong nuclear force rapidly created one hadron with a positive quanta of this strangeness and the other with negative strangeness (here colored purple and gold) keeping the total strangeness conserved.

π + p+ ⇒ Λo + Ko

But once these particles flew apart the strong nuclear force was forbidden from causing an decay which would remove any single particle's strangeness.  But the weak nuclear force could eventually do so in two separate reactions.

Λo  ⇒ p+ + π

Ko ⇒ π+ + π

Over the next four decades, hundreds of new particles were discovered as a result of collisions in particle accelerators.  The large number of particles confused many physicists who believed the universe should be understandable in terms of only a few truly elementary particles.  So many attempts were made to understand the zoo of diverse particles.  Luis Alvarez (1911-1988) was awarded the 1968 Nobel Prize in Physics for understanding that many of the particles with greater masses are excited resonance forms of more elementary particles.  Just as electrons can be excited to higher quantized energy levels inside atoms, it was realized other elementary particles could also be excited to higher quantized energy levels.  But unlike electron excitations which typically require only small amounts of energy, the excitation energy of these new heavier particles would be great enough so that the extra mass produced by the excitation energy, via E = mc2, is significant.  (That is, the excited particles were so much heavier, that they had been previously assigned to be entirely different particles.)  These excited resonances typically decay by the strong nuclear force in very short times.timeline of particle discoveries

Murray Gell-MannIn 1964 Murray Gell-Mann (1929- , see photo) and George Zweig (1937- ) proposed the major breakthrough by suggesting that hadrons have substructure that Gell-Mann called quarks.  At that time only three quarks were needed (called up, down, and strange) and anti-particles for each.  Each baryon is composed of three quarks.  Each meson is composed of a quark and an anti-quark.  More recently mesons have been discovered requiring the existence of three additional quarks called charm, top and bottom.  Gell-Mann obtained the rather strange name, quark, from James Joyce's novel, Finnegan's Wake, because as Joyce described, the creatures had been detected only by their palpitant piping, chirrup, croak and quark.  The theory of the six quarks (and their anti-particles) combined with what is known about the six leptons (and their anti-particles) has become known as the Standard Model.  (Note the chocolate, vanilla, strawberry flavor scheme shown below.)  The Standard Model is actually a theory written in mathematical language called Quantum Chromo Dynamics, QCD.  What is described here is a brief description of some of the key ideas.  There is much more to be understood by those who have found this interesting so far.


Leptons spin = 1/2       Quarks spin = 1/2
Flavor Mass
  Flavor Approx. Mass
νe electron
<1x10-8 0 u up 0.003 2/3
e electron 0.000511 –1 d down 0.006 –1/3
νμ muon
<0.0002 0 c charm 1.3 2/3
μ muon 0.106 –1 s strange 0.1 –1/3
ντ tau
<0.02 0 t top 178 2/3
τ tau 1.777 –1 b bottom 4.3 –1/3

The large and confusing collection of particles mostly discovered since mid-century could be accounted for as combinations of the 6 quarks and their anti-particles.
A proton is a fermion combination of u + u + d quarks.
A neutron is a fermion combination of u + d + d quarks.
A meson is a combination of a quark and an anti-quark.  For example the pion, π+, is a boson combination of u + d.

Only the chocolate flavored particles are stable in our current universe conditions.  The others flavors are unstable and decay with a flavor change to chocolate.  The lambda, Λ, described above is a combination of u + d + s quarks.  The Λ contains an s quark which decays by a flavor change mediated by the weak force.

The neutral kaon, Ko, described above is a combination of d + s.  The Ko contains an s anti-quark which also decays with a flavor change mediated by the weak force.

The meson exchange particles were originally proposed as elementary particles by Yukawa to mediate the nuclear force.  But in the Standard Model they are no longer considered the boson responsible for what is now called the strong force.  Recall that two identical fermions cannot occupy the same space.  So for example, the two up quarks in a proton must have some distinguishing property.  Each quark must possesses another property, a kind of nuclear charge called color charge which can be responsible for the strong nuclear force (just as electric charge is responsible for electromagnetic force.  But this property has no real connection with either electric charge nor the colors we see with our eyes).  Each baryon and each meson is combined in such a way that it is color neutral.  (In the same way that atoms contain equal numbers of electrically charged electrons and protons such that the total electric charge on the atom is neutral, the assigned colors of quarks—red, green, and blue—combine to make the neutral color white).  The actual boson, the exchange particle for the strong (color) force, is called a gluon.  (Note that these processes provide no sensual experiences for us and so we have inherited no useful vocabulary.  So terms such as flavor and color have been misappropriated since they have limited similarities.)


Unified Electro-weak spin = 1       Strong (color) spin = 1
Name Mass
  Name Mass
γ photon 0 0 g gluon 0 0
W 80.4 –1
W+ 80.4 +1
Zo 91.188 0

While the electromagnetic and weak nuclear forces are related, this strong nuclear force is distinct from electromagnetism and gravity.  (The boson for gravity, gravitons, unlike the other bosons, are exclusively attractive and should accordingly have a spin = 2.)  The weak nuclear force has three bosons, W-, W+, and Zo.


From the above descriptions of experiments and discoveries it should be clear that the notion of fundamental particles of matter has changed radically over the last century or two.  Chemists and physicists had come to believe that the indivisible atom was elementary.  For a while it was believed that matter was made exclusively of protons and electrons, but shortly after 1930 the neutron was recognized.  Physicists were resistant to the discovery of a long list of new elementary particles that followed.  Eventually efforts to find flaws in the conservation laws turned to efforts to organize the particles in hope of finding a unifying explanation.  The introduction of a new level of substructure of the Standard Model has turned out remarkably successful.  Glashow said in his Nobel acceptance speech in 1980 It is in a sense, a complete and apparently correct theory, offering a qualitative description of all particle phenomena and precise quantitative predictions in many instances.  There are no experimental data that contradict the theory...  The Standard Model does describe all experiments so far without contradiction.  But there remain some unanswered questions such as Why are there three flavors of Fermions? and Why particles have the masses they do?  And there are the new questions about the nature of dark matter and dark energy.  It might be wise to recall that physicists have a nearly perfect record of incorrectly predicting the future developments of physics.  Nature is more original and interesting than even the most original of our best thinkers!


Much of the evidence leading to development and clarification of the Standard Model has been obtained by particle detectors in high energy accelerators.  The earliest evidence was obtained from particle collisions resulting from naturally occurring particles ejected from radioactive material.  But physicists soon suspected that additional understanding was likely if electrically charged particles such as electrons and protons could be accelerated so that the energies involved in collisions could according the Einstein's E =  mc2 create new particles and interactions.  Bunches of accelerated particles were collided into fixed targets with the products sprayed into detector chambers.  To provide higher energy collisions, accelerators of larger size were built.  As the price for such apparatus grew large and technology improved, it was more economical to collide two beams of particles traveling opposite directions to maximize the energy available for producing new particles and information.  Today the highest energy collisions occur in such colliders, often producing short lived particles which decay in a spray of other particles detected in an onion-like arrangement of detectors.  By tuning the accelerator to produce collisions with energy matching the mass of a particular elementary particle, production of that particle can be optimized.  The procedure below looks at collisions tuned to about 91 GeV/c2 to produce Z0 particles.  Significant insight about the nature of elementary particles, the primary forces and the nature and limitations of such experimental evidence can be obtained by analyzing a series of collision events.

As mentioned above, the vanilla and strawberry flavors of quarks and particles which contain them decay by the weak interaction which changes their flavor.  The bosons which have mass also decay.  Generally a decaying particle can change to any other particle or combination of particles unless prohibited by some conservation rule.  So a given particle often may produces different sets of products such that a diagram of its possible decay is forked or branched.  By studying the distribution of products, or branching ratio, it is possible to learn more about the elementary particles and their interactions.  The procedure below studies a number of events where a Z0 boson decays.  Since such a boson has both zero lepton and baryon numbers, any produced lepton must be accompanied by an anti-lepton.  And any produced quark must be accompanied by an anti-quark.  And since quarks can't exist as lone particles, lighter bosons or baryons must be produced in pairs.

decay particle detectors

While earlier detectors often captured images of particle tracks on photographic film, current detectors send electronic signals to computers which reconstruct a three-dimensional record of the particle tracks, energy, and momenta.  For particle accelerators which operate as colliders, the detectors are layered concentric tubes built around the locations where collisions are planned to occur.

  1. The inner layers just outside the beam's vacuum pipe typically detect the passing of electrically charged particles.  A strong magnetic field slightly deflects the tracks of the particles revealing their momenta.
  2. A dense material is used in a calorimeter layer to stop electrons and photons, absorbing and measuring their energies.
  3. Additional dense materials are used in a larger calorimeter layer to stop hadrons composed of quarks such as mesons, protons and neutrons which are more penetrating because of their greater mass and lower charge to mass ratio.  They are stopped and their energies measured.
  4. Muons are much more penetrating than electrons because of their lower charge to mass ratio.  They continue through additional layers of dense material and tracking detectors, often continuing outside the detector.
  5. Neutrinos have no charge so leave no detectable tracks.  And with very little interaction with ordinary matter, they escape direct detection.  But by summing the energies of the other particles, the neutrino energies are obtained as the amount of missing energy when compared to that of the original colliding particles.
CMS detector

View (in another browser tab or window ) a flash animation of the Large Hadron Collider (LHC) at CERN and the largest of its detectors (ATLAS).  The largest of the accelerator rings is 100 m under ground with a circumference of 27 kilometers.  The ATLAS detector is 22 meters high and 44 meters long.

View (in another browser tab or window) a flash animation of different particles traveling through or being stopped in the slightly smaller CMS (Compact Muon Solenoid) detector at CERN.  Click on each kind of particle to view in slowed motion what happens to each inside the detector.  For example, the dotted path for the photon means that it passes through without leaving a detectable track.

Possible Decay Branches for Z0 Boson
type fermion flavor typical track record features to look for
Lepton &
  e & e+
electrons   (1) charged particle tracks in inner track detector
  (2) 2 simple, opposite tracks
  (3) lose ALL energy in inner calorimeter
  μ & μ+
muons   (1) charged particle tracks in inner track detector
  (2) typically aren't stopped, escape out of detector
      note symbol for escaping particle:
      leaving little energy in either calorimeter
        redder boxes indicate lower energy
        (white is greatest energy).
  τ & τ+
taus   (1) a tau is short lived;
      tau pairs immediately decay, each producing...
          ½ leave 3 hadron tracks
          1/6 leave 1 hadron track
          recall neutral particles leave no tracks
  (2) significant energy deposited in outer calorimeter
  (3) neutrinos carry away missing energy
quark &
baryons &
quarks   charged particles leave tracks & energy (boxes)
  neutral particles only leave energy in calorimeters

  (1) lots of tracks; dozen or more
  (2) significant energy deposited in outer calorimeter
  (3) neutrinos carry away missing energy;
      (i.e., total energy is short of collision energy)


  1. Study the images above to develop the ability to distinguish the different branch possibilities.

  2. Look at Z0 decay events recorded at CERN.  To cause each recorded event, the accelerator was operated so that there was about 91 GeV of energy available for each collision.  Select from one of these two libraries of events:
    1. Z0 particle data from the DELPHI detector at CERN.
    2. Z0 particle data from the OPAL detector at CERN.
    3. When the LHC begins to provide collisions for the ATLAS and CMS detectors, the plan is to provide links to even the most recently detected events.  The LHC will typically operate at much higher energies and so may need more background information.

  3. For each of at least 100 events, determine whether the primary products were
  4. Record the identification number of each event, and the type of event.

  5. Determine the branching ratios for each kind of event.  For example, is one type occurs ½ of the total, the branch ratio would be 0.50

You may find some events which are hard to classify.  (Welcome to the world of particle physics!!!)  Study such events more carefully.  The title bar often gives the number of tracks the computer identified, the total energy of all the detected particles, the collision energy, if the computer believes there were likely an neutrinos, and if any tracks seem to escape beyond the detector (presumed to be muons).  Discuss any remaining difficult events with your colleagues.  And keep in mind that occasionally the detector works less than perfectly, that particles sometimes with trajectories along the beam pipe go undetected, that cosmic rays from elsewhere in the universe can cause events in the detector, and best of all, on rare occasions, a discrepant event leads to the discovery of a previously unknown particle!  At minimum you should gain an understanding of how particle physicists expand their understanding, and a bit of the tedium and uncertainty that go with it.  The branching ratios give clues whether there are more elementary particles yet to be discovered.

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.



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created 30 November 2004
revised 29 March 2007
next link revised 20 May 2008
by D Trapp
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