93 The scalar and fermion lines in the loop are messenger fields Recall that

93 the scalar and fermion lines in the loop are

This preview shows page 221 - 224 out of 272 pages.

loop graph shown in Fig. 9.3. The scalar and fermion lines in the loop are messenger fields. Recall that the interaction vertices in Fig. 9.3 are of gauge coupling strength even though they do not involve gauge bosons; compare Fig. 6.3g. In this way, gauge-mediation provides that q, q messenger loops give masses to the gluino and the bino, and , messenger loops give masses to the wino and bino fields. By computing the 1-loop diagrams one finds that the resulting MSSM gaugino masses are given by M a = α a 4 π Λ , ( a = 1 , 2 , 3) , (9.40) (in the normalization discussed in section 8.4) where we have introduced a mass parameter Λ F S / S . (9.41)
Image of page 221
222 9 Origins of supersymmetry breaking Fig. 9.4. Contributions to MSSM scalar squared masses in gauge-mediated supersymmetry breaking models arise in leading order from these two-loop Feynman graphs. (Note that if F S were 0, then Λ = 0 and the messenger scalars would be degenerate with their fermionic superpartners and there would be no contribution to the MSSM gaugino masses.) In contrast, the corresponding MSSM gauge bosons cannot get a corresponding mass shift, since they are protected by gauge invariance. So supersymmetry breaking has been successfully communicated to the MSSM (“visible sector”). To a good approximation, eq. (9.40) holds for the running gaugino masses at an RG scale Q 0 corresponding to the average characteristic mass of the heavy messenger particles, roughly of order M mess y i S . The running mass parameters can then be RG-evolved down to the electroweak scale to predict the physical masses to be measured by future experiments. The scalars of the MSSM do not get any radiative corrections to their masses at one-loop order. The leading contribution to their masses comes from the two-loop graphs shown in Fig. 9.4, with the messenger fermions (heavy solid lines) and messenger scalars (heavy dashed lines) and ordinary gauge bosons and gauginos running around the loops. By computing these graphs, one finds that each MSSM scalar φ gets a (mass) 2 given by: m 2 φ = 2Λ 2 α 3 4 π 2 C φ 3 + α 2 4 π 2 C φ 2 + α 1 4 π 2 C φ 1 . (9.42) Here C φ a are the quadratic Casimir group theory invariants for the scalar φ for each gauge group. They are defined by C φ a δ j i = ( T a T a ) j i where the T a are the group generators which act on the scalar φ . Explicitly, they
Image of page 222
9.4 Gauge-mediated supersymmetry breaking models 223 are: C φ 3 = 4 / 3 for φ = Q i , u i , d i ; 0 for φ = L i , e i , H u , H d (9.43) C φ 2 = 3 / 4 for φ = Q i , L i , H u , H d ; 0 for φ = u i , d i , e i (9.44) C φ 1 = 3 Y 2 φ / 5 for each φ with weak hypercharge Y φ . (9.45) The squared masses in eq. (9.42) are positive (fortunately!). The terms a u , a d , a e arise first at two-loop order, and are suppressed by an extra factor of α a / (4 π ) compared to the gaugino masses. So, to a very good approximation one has, at the messenger scale, a u = a d = a e = 0 , (9.46) a significantly stronger condition than eq. (8.16). Again, eqs. (9.42) and (9.46) should be applied at an RG scale equal to the average mass of the messenger fields running in the loops. However, after evolving the
Image of page 223
Image of page 224

You've reached the end of your free preview.

Want to read all 272 pages?

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture