The status of the Standard Model (SM) is reviewed. We emphazize the fact that in spite of the success of the SM for the descrition of the fermionic sector, the status of the bosonic sector (gauge and scalar) suffers from many theoretical deficiencies and f
a r X i v :h e p -p h /9501305v 1 16 J a n 1995PM/95-01
January 1995
The Bosonic Sector of the Electroweak Interactions,Status and Tests at Present and Future Colliders F.M.Renard Physique Math´e matique et Th´e orique,CNRS-URA 768Universit´e Montpellier II,F-34095Montpellier Cedex 5.Lectures given at Regensburg University,January 1995Extended version of a talk given at the Festkolloqium Dieter Schildknecht,Bielefeld,Oct.14th 1994Abstract The status of the Standard Model (SM)is reviewed.We emphazize the fact that in spite
of the success of the SM for the descrition of the fermionic sector,the status of the bosonic sector (gauge and scalar)su?ers from many theoretical de?ciencies and from the lack of empirical support.This situation,which leaves room for several types of extensions or alternatives to SM,strongly motivates the pursue of intense e?orts for ?nding hints of New Physics (NP)e?ects.We present a phenomenological description valid for energies lying below the NP scale.We discuss the indirect constraints established from high precision tests at LEP1,as well as the direct tests that could be performed at future machines.
The status of the Standard Model (SM) is reviewed. We emphazize the fact that in spite of the success of the SM for the descrition of the fermionic sector, the status of the bosonic sector (gauge and scalar) suffers from many theoretical deficiencies and f
1Introduction,the status of the Standard Model
It is a common leitmotiv to say that the Standard Model(SM)is largely successful.On the one hand it is already remarkable that this model is able to make de?nite and unambiguous preditions for all processes involving usual particles.This property is the consequence of the gauge principle which allows to predict the dynamics once a classi?cation group has been chosen.The simpler QED case with the U(1)EM has been extended to the non-abelian cases of QCD with SU(3)colour and to the electroweak interactions with SU(2)×U(1).However the speci?c feature of electroweak interactions is the fact that W,Z bosons are massive.The gauge principle has to be completed with a mass generation mechanism. In the Standard Model it is chosen as the Higgs mechanism of spontaneous symmetry breaking(SSB).It is this last property that makes the SM a renormalizable theory which allows to compute high order e?ects and to make the accurate predictions mentioned above.These predictions practically agree with all available experimental results.In spite of this success many questions arise.
Let us?rst quickly review the status of the SM by clearly separating the caracteristics of its three sectors:
a)The fermionic sector
consists in theγ,W±,Z and the8gluons,as generated by the SU(3)×SU(2)×U(1)gauge group.
At this stage both fermions and bosons are massless states.They are coupled through gauge interactions.Self-boson interactions appear through the non-abelian Yang-Mills kinetic terms of the W±,3gauge bosons.
c)The scalar sector
The status of the Standard Model (SM) is reviewed. We emphazize the fact that in spite of the success of the SM for the descrition of the fermionic sector, the status of the bosonic sector (gauge and scalar) suffers from many theoretical deficiencies and f
the SM(Technicolour mechanism(TC),Grand Uni?ed Theories(GUT),substructures). New concepts like those introduced with superstrings may also be necessary.In any case it is not obvious at what energy scale(between the TeV range and the Planck mass)these features may originate.
The status of the bosonic sector is not yet empirically established because it is still not possible to perform signi?cant direct tests.The agreement of the SM predictions with experiments in fermionic processes(LEP1,low energy experiments)is often taken as a sign of general validity of the SM including the bosonic sector,because high order terms indirectly involve gauge boson and also higgs boson self-interactions.However as we will see in Sect.4,these indirect tests?rst su?er from a lack of accuracy,but also from many ambiguities which prevent to give well-de?ned model-independent statements. Many extensions of,or alternatives to,SM are also consistent with the fermionic results.
On another hand the bosonic sectors su?er from much more serious questions and de?ciencies.They concern the origin of three a priori independent gauge couplings(that one would like to unify),the origin of SSB,i.e.the origin of the scalar potential(not generated by the gauge principle but put by hand),and of the Fermi scale v(the basic mass scale of the electroweak interactions),the restricted choice of Higgs doublets,as well as the unpredicted value of the Higgs mass(seen either as an unpredicted coupling constant for theφ4term or as a new mass scale).