Brian Northcutt
Paul Nystrom
Weak Force

	The weak nuclear force is one of the four fundamental forces, but probably the least understood.  The weak force explains many radioactive decays, unstable particles, and neutrino interactions. The weak force acts only on particles that have a weak charge    ( as electromagnetism acts on particles with electric charge ). The weak charge is assigned, however, on the basis of handedness. Handedness is determined by the orientation of the particles intrinsic spin angular momentum. Only left-handed particles and right-handed anti-particles bear a weak charge, the rest are not involved in weak interactions. However, handedness can change if the observer were, to say, move ahead of the particle and thus the particle would be moving the other direction relative to the observer,  and handedness would change. Thus the weak charge is not conserved. Compared to the strong force, the weak force is approx. 10-13 times as strong although 1028 times stronger than gravity.  The weak force was first theorized to explain nuclear beta decay. In beta decay, a neutron emits an electron and an antineutrino while converting itself into a proton.  This interaction is unexplainable by any other force.
	The particles mediating the weak force are the W+, W-, and Z0. The range of the weak force is on the order of 10-18m, which is three times smaller than the range of the strong nuclear force. The weak force range is finite because the mediating particles have mass. The weak force, like the other forces, exchange virtual particles, particles that are not observable because they travel incredibly  short distances near the speed of light. When the W+,W-, and the Z0 are exchanged between leptons and quarks they are able to come into existence because the uncertainty principle allows, for a short time, violation of the conservation of energy. E * T  h/2 ...where E is the energy needed to create the particle and T is the time it exists. Planck’s constant, h, is 6.62610-34 J*s. The mass for the W- is 80.22 Gev/c2.   E = mc2 and T= d/c since the velocity of the particle is close to c.  Solving for d, the range of the exchange particle and thus the range of the force, d = h*c/(2*mc2). 1ev = 1.6*10-19J. so d  2.4* 10-18m. About 1/1000th the size of a nucleon. The very short range and relative weakness of the weak force make it difficult to study in the lab.
	Beta decay is the most convincing evidence of the weak nuclear force.  As mentioned before, a neutron within an unstable nucleus can mutate itself into a proton releasing a right handed antineutrino and a left handed electron.  The electron released is not an orbital electron, although it is indistinguishable from orbital electrons.  The neutron is composed of three quarks with order udd.  The decay begins when the left handed d quarks emits a W- and becomes a  left handed u quark. The resulting order is uud, a proton.  In turn the W- decays into an electron and an antineutrino. What is so interesting about this reaction is that it takes a long time (on a nuclear level) to happen.  Weak interactions last about 10-10s which is up to a trillion times longer than processes governed exclusively by the strong nuclear force.  The decay of a neutral pion (which is governed by the strong force) takes considerably less time to occur.
	Weak interactions involve a change of flavor ( i.e. weak charge), when the electron turns into a electron neutrino and when a u quark turns into a  d quark.  A quantum is being exchanged in the process of beta decay. These exchanges can be illustrated using a Feynman diagram ( and thus the meaning of the virtual particle will become more apparent):

                            dR  /                               \  Ve
                                /                                   \
\ 		      /			          \			
  \  Ve                    ~  /				\~	          dR   /	
    \                ~          \                     or                        /      ~            	 /			
      \	 ~  W-	        \			            /		~          /		
        \~		          \			           /	           W+     ~ /		
        /	                  uR     \                                      /  e                        \
      /							           \	
     /							       uR  \	
    /  e								 \	
   /											                   
	The  Z0 particle mediates interactions where the particle is transformed into itself, so that its identity is unchanged.  This is similar to the photon which mediates electromagnetic interactions without changing the particles identity. This is part of the theory that led to the unification of the two forces into one, the electroweak force.

Sources:

Pais, Abraham. Inward Bound: of Matter and Forces in the Physical World. New York: 	Oxford University Press, 1986.

Nuclear Physics. volume of “Physics through the 1990s.” Washington D.C.: National 	Academy Press, 1986.

Fishbane, Paul M. et all. Physics for Scientists and Engineers. Englewood Cliffs: Prentice, 	1993.

Davies, Paul  editor. The New Physics. Cambridge: Cambridge University Press, 1989.

Gell-Mann, Murray. The Quark and the Jaguar. New York: Freeman, 1994.