R-parity

R-parity is a concept in particle physics. In the Minimal Supersymmetric Standard Model, baryon number and lepton number are no longer conserved by all of the renormalizable couplings in the theory. Since baryon number and lepton number conservation have been tested very precisely, these couplings need to be very small in order not to be in conflict with experimental data. R-parity is a

Z

2

{\displaystyle \mathbb {Z} _{2}}

symmetry acting on the Minimal Supersymmetric Standard Model (MSSM) fields that forbids these couplings and can be defined as

P

R

=

(

−

1

)

3

B

+

L

+

2

s

{\displaystyle P_{\mathrm {R} }=(-1)^{3B+L+2s}}

or, equivalently, as

P

R

=

(

−

1

)

3

(

B

−

L

)

+

2

s

{\displaystyle P_{\mathrm {R} }=(-1)^{3(B-L)+2s}}

where s is spin, B is baryon number, and L is lepton number. All Standard Model particles have R-parity of +1 while supersymmetric particles have R-parity of −1.

Note that there are different forms of parity with different effects and principles, one should not confuse this parity with any other parity.

Dark matter candidate

With R-parity being preserved, the lightest supersymmetric particle (LSP) cannot decay. This lightest particle (if it exists) may therefore account for the observed missing mass of the universe that is generally called dark matter. In order to fit observations, it is assumed that this particle has a mass of 100 GeV/c2 to 1 TeV/c2, is neutral and only interacts through weak interactions and gravitational interactions. It is often called a weakly interacting massive particle or WIMP.

Typically the dark matter candidate of the MSSM is a mixture of the electroweak gauginos and Higgsinos and is called a neutralino.