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Description of particles

Each row in the Particles Table describes a particle - anti-particle pair. The rows consist of 10 fields:

1.

Full name for a full name of particle. Just for clear orientation, not processed anywhere;

2&3.

"A" and "A+" containing designations of the particle and anti-particle, respectively. Any character is allowed. The name may contain one symbol, two symbols or three symbols started from '$\tilde{\;}$'. For a completely neutral particle the "A" and "A+" fields must be identical;

4.

2*Spin for a doubled particle spin: 0 for scalar, 1 for spinor and 2 for vector particles. Neutral spinor particle is teated as a Majorana one.

5.

Mass for a mass identifier or symbol '0'. In the first case its value must be defined in the Parameters or Constraints table. If this field contains zero, then CompHEP  considers this particle as massless;

6.

Width for a particle decay width. It must contain an identifier defined in the first two tables or '0';

7.

Color for a dimension of the color SU(3) group representation. You have to choose among 1, 3, 8. Unity corresponds to a colorless particle. Three corresponds to a color triplet (fundamental representation). In this case the anti-particle "A+" is transformed by conjugated $\bar{3}$ representation. Eight corresponds to a color octet (adjoint representation);

8.

Aux for an auxiliary field which allows to modify particle propagators. If the Aux field is empty the standard expressions for propagators are substituted:

(a)

spin 0 case:

$$<0\vert T[A(p_1) , A^+(p_2)]\vert> = ScPr(p_1,p_2,M) = \frac{\delta(p_1 + p_2)} {(2\pi)^4i(M^2 - p_1^2)}\;\;;$$

(b)

spin 1/2 case:

$$<0\vert T[A(p_1) , A^+(p_2) \; \gamma_0 ]\vert> = (\not p_1 + M) \, ScPr(p_1,p_2,M)\;\;,$$

where

$$\not p = p^{\mu}\gamma_{\mu}\;\;.$$

Here "$A^+$" is the Hermitian conjugation of "$A$"; thus the Dirac $\gamma_0$ matrix is needed to make the Dirac conjugate field;

(c)

spin 1 case:

$$<0\vert T[A^{m_1}(p_1), \; (A^{m_2})^+(p_2)]0> = -(g^{m_1 m_2} + p_1^{m_1} \; p_2^{m_2} / M^2)$$

$$\times ScPr(p_1,p_2,M)\;\;.$$ (6)

Zero mass vector particle must be marked as a gauge one using the Aux field (see below).

Possible objects for the Aux field are:

'l','L'

is permitted for massless fermions ( 2*spin = 1 ) only. The propagator is changed to

$$\frac{ \not p_1 (1 + \gamma_5)}{2} \; ScPr(p_1,p_2,M)\;\;.$$

This is a way to introduce a left-handed fermion as a neutrino or a massless polarized fermion. When CompHEP  performs the averaging over incoming particle polarizations it takes into account that there is only one polarization state;

'r','R'

is permitted for massless fermion ( 2*spin=1 ) particle only. They change the propagator for

$$\frac{\not p_1 (1 - \gamma_5)}{2} \; ScPr(p_1,p_2,M)\;\;.$$

This is a way to introduce right-handed fermions;

'*'

is permitted for massive particle only. In this case $ScPr(p_1,p_2,M)$ is replaced to

$$\frac{\delta(p_1+p_2)}{ (2 \pi)^4 i \;M^2}\;.$$

In the case of vector particle we also remove $p_1^{m1}p_2^{m2}$ term in the propagator numerator (6).

Such particles cannot appear as incoming or outgoing ones. They are used to describe a point-like interaction as one has in the electroweak 4-fermion interaction model;

'G','g'

is permitted for vector ( 2*spin=2 ) particle. In this case the propagator of vector particle accepts the Feynman form

$$-g^{m_1m_2} \, ScPr(p_1,p_2,M)\;\;;$$

9&10.

LaTeX(A) and LaTeX(A+) for particle and anti-particle designations in the L^ATEX  format. They are substituted in the L^ATEX  image of Feynman diagrams generated by CompHEP. The names are used in the mathematical mode.