Read and complete the text using the words below.

STANDARD MODEL

[1] All matter around us is made of elementary particles, the building blocks of matter. These particles occur in two basic types: quarks and leptons. Each group consists of six particles, which are related in pairs, or 1)…. The lightest and most stable particles make up the first generation, whereas the heavier and less stable particles belong to the second and third generations. All stable matter in the universe is made from particles that belong to the first generation, and heavier particles quickly decay to the next most stable level. The six quarks are paired in the three generations-the up quark and bottom (or beauty) quark; next come ‘strange’ and ‘charm’ quarks; finally, the ‘top and bottom’ quarks are the heaviest pair. Quarks also come in three different “colors”: red, blue and green. Just as electrons and protons carry “color charge”, which is 2)… when quarks change from one type to another. Color charge has nothing to do with the visible colors of light- it is just an arbitrary way of naming the quantum properties of quarks. Just as electric charges produce a force, so color charges (quarks) can exert forces on one another. Color force gets 3)… the further the quarks are apart, so they stick together as if held by an invisible elastic band. Because the color force field tie is so strong, quarks cannot exist on their own and must always be locked together in combinations that are color neutral overall (exhibiting no color charge).

[2] The second class of particles, the leptons, are related to and include 4)…. Again there are three generations with increasing masses: electrons, muons and taus. 5)… are 200 times heavier than an electron and taus 3700 times. (A) Leptons all have single negative charge. (B) They also have an associated particle called a neutrino (electron,muon and tau-neutrino) that has no charge. (C) Neutrinos have almost no mass and do not interact much with anything. (D)They can travel right through the Earth without noticing, so are difficult to catch. All leptons have antiparticles.

[3] Fundamental forces are mediated by the exchange of particles. There are four fundamental forces at work in universe: the strong force, the weak force, the electromagnetic force, the gravitation force. They work over different ranges and have different strengths. Gravity is the weakest but it has an infinite range. The electromagnetic force also has 7)… range but it is many times stronger than gravity. The weak and strong forces are effective only over a short range and dominate only at the level of 8)… particles. Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three. The strong force is the strongest of all four fundamental interactions.

[4] Three of the fundamental forces result from the exchange of force-carrier particles, which belong to a broader group called 9)…. Particles of matter transfer discrete amounts of energy by exchanging bosons with each other. Each fundamental force has its own corresponding boson- the strong force is carried by the 10)…, the electromagnetic force is carried by the photon and the W and the Z bosons are responsible for the weak force.

[5] The Standard model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains well how these forces act on all of the matter particles. However, the most familiar force in our everyday lives, 11)…, is not part of the Standard model, as fitting gravity comfortably into this framework has proved to be a difficult challenge.

[6] How do we know about these subatomic particles? Particle accelerators use giant magnets to accelerate particles to extremely high speeds and smash those particle beams either into a target or into another oppositely directed beam. At 12)… speeds, the particles break apart a little and the lightest generations of particles are released. Because mass means energy, you need a higher particle beam to release the heavier generations of particles.

[7] 1__ In the magnetic field, positive charged particles swerve one way and negative ones the other. 2__ The particles produced in the atom smashers then need to be identified and particle physicists do this by photographing their 13)… as they pass through a magnetic field. 3__ By mapping their characteristics in the detector, and comparing them with what they expect from their theories, particle physicists can tell what each particle is. 4__ The mass of the particle also dictates how fast it shoots through the detector and how much its path is curved by the magnetic field. 5__ So light particles barely curve and heavier particles may even spiral into loops.