Production
Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high energy collisions. These occur naturally in the Earth's upper atmosphere as cosmic rays collide with particles in the air, or can be created in a particle accelerator. As of 2011, the only operational accelerators that generate beams of sufficient energy to produce top quarks are the Tevatron at Fermilab, in which protons and antiprotons are collided at a center-of-mass energy of 1.96 TeV,[23] and the Large Hadron Collider at CERN, with a center-of-mass energy of 7 TeV.
There are multiple processes that can lead to the production of a top quark. The most common is production of a top-antitop pair via strong interactions. In a collision a highly energetic gluon is created which subsequently decays into a top and antitop. This process is responsible for the majority of the top events at Tevatron and is the process observed when the top was first discovered in 1995.[24] It is also possible to produce pairs of top-antitop through the decay of an intermediate photon or Z-boson. However, these processes are predicted to be much rarer and have a virtually identical experimental signature in a hadron collider like Tevatron.
A distinctly different process is the production of single tops via weak interaction. This can happen in two ways (called channels) either an intermediate W-boson decays into a top and antibottom quark ("s-channel") or a bottom quark (probably created in a pair through the decay of a gluon) transforms to top quark by exchanging a W-boson with an up or down quark ("t-channel"). The first evidence for these processes was published by the DØ collaboration in December 2006,[25] and in March 2009 the CDF[26] and DØ[24] collaborations released twin papers with the definitive observation of these processes. The main significance of measuring these production processes is that their frequency is directly proportional to the |Vtb|^2 component of the CKM matrix.
Decay
The only known way that a top quark can decay is through the weak interaction producing a W-boson and a down-type quark (down, strange, or bottom). Because of its enormous mass, the top quark is extremely short lived with a predicted lifetime of only 5×10^-25 s.[3] As a result top quarks do not have time to form hadrons before they decay, as other quarks do. This provides physicists with the unique opportunity to study the behavior of a "bare" quark.
In particular, it is possible to directly determine the branching ratio for the decay of a top quark into a bottom quark. The best current determination of this ratio is 0.99+0.09-0.08.[1] Since this ratio is equal to |Vtb|^2 according to the standard model, this gives another way of determining the CKM element |Vtb|, or in combination with the determination of |Vtb| from single top production provides tests for the assumption that the CKM matrix is unitary.[23]
The Standard Model also allows more exotic decays, but only at one loop level, meaning that they are extremely suppressed. In particular, it is possible for a top quark to decay into another up-type quark (an up or a charm) by emitting a photon or a Z-boson.[27] Searches for these exotic decay modes have provided no evidence for their existence in accordance with expectations from the standard model. The branching ratios for these decays have been determined to be less than 5.9 in 1,000 for photonic decay and less than 3.7 in 100 for Z-boson decay at 95% confidence.[1]
Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high energy collisions. These occur naturally in the Earth's upper atmosphere as cosmic rays collide with particles in the air, or can be created in a particle accelerator. As of 2011, the only operational accelerators that generate beams of sufficient energy to produce top quarks are the Tevatron at Fermilab, in which protons and antiprotons are collided at a center-of-mass energy of 1.96 TeV,[23] and the Large Hadron Collider at CERN, with a center-of-mass energy of 7 TeV.
There are multiple processes that can lead to the production of a top quark. The most common is production of a top-antitop pair via strong interactions. In a collision a highly energetic gluon is created which subsequently decays into a top and antitop. This process is responsible for the majority of the top events at Tevatron and is the process observed when the top was first discovered in 1995.[24] It is also possible to produce pairs of top-antitop through the decay of an intermediate photon or Z-boson. However, these processes are predicted to be much rarer and have a virtually identical experimental signature in a hadron collider like Tevatron.
A distinctly different process is the production of single tops via weak interaction. This can happen in two ways (called channels) either an intermediate W-boson decays into a top and antibottom quark ("s-channel") or a bottom quark (probably created in a pair through the decay of a gluon) transforms to top quark by exchanging a W-boson with an up or down quark ("t-channel"). The first evidence for these processes was published by the DØ collaboration in December 2006,[25] and in March 2009 the CDF[26] and DØ[24] collaborations released twin papers with the definitive observation of these processes. The main significance of measuring these production processes is that their frequency is directly proportional to the |Vtb|^2 component of the CKM matrix.
Decay
The only known way that a top quark can decay is through the weak interaction producing a W-boson and a down-type quark (down, strange, or bottom). Because of its enormous mass, the top quark is extremely short lived with a predicted lifetime of only 5×10^-25 s.[3] As a result top quarks do not have time to form hadrons before they decay, as other quarks do. This provides physicists with the unique opportunity to study the behavior of a "bare" quark.
In particular, it is possible to directly determine the branching ratio for the decay of a top quark into a bottom quark. The best current determination of this ratio is 0.99+0.09-0.08.[1] Since this ratio is equal to |Vtb|^2 according to the standard model, this gives another way of determining the CKM element |Vtb|, or in combination with the determination of |Vtb| from single top production provides tests for the assumption that the CKM matrix is unitary.[23]
The Standard Model also allows more exotic decays, but only at one loop level, meaning that they are extremely suppressed. In particular, it is possible for a top quark to decay into another up-type quark (an up or a charm) by emitting a photon or a Z-boson.[27] Searches for these exotic decay modes have provided no evidence for their existence in accordance with expectations from the standard model. The branching ratios for these decays have been determined to be less than 5.9 in 1,000 for photonic decay and less than 3.7 in 100 for Z-boson decay at 95% confidence.[1]
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