![]() Howe ver, this is particula rly difficul t for a variety of reasons. If we follow the intuition that the fundame ntal theory should describe within the same quantum frame work all forces of nature, we need to extend the Standard Model in such a way that it consistently includes this force. Here, we describe some of these argu ments in more detail, starting from gravit y. It does not explain the observed neutrino masses and oscillations and does not predict any gauge coupling unification, suggested by experiments. In additi on the SM does not describe the dark matter or the dark ener gy of the universe. Also, the SM does not explain why the charges of elementar y parti cles are quantiz ed. The way to cure this discrepanc y within the SM require s an incre dible fine tuning of parameter s. Thi s is seve ral order s of magnit ude higher than the ele ctr ow eak scale implie d by exper iment. W ith in the SM, the mas s of the higgs partic le is extre mel y sensitive to any new physics at higher energies and its natural value is of order of the Planck mass, if the SM is va lid up to that sca le. Another is the popu lar ‘mass hiera rch y’ pro ble m. It says absolute ly nothing about one of the four fundamental forces of nature. Firstl y, the Standa rd Model does not include gra vity. W e briefly mention some of the most important argumen ts. ![]() Actually there is a variety of such arguments, both theoretical and experimental, that lead to the undoubtable conclusion that the Standard Model should be only an effective theory of a more fundamental one. W e shall start by briefl y exami ning the reaso ns for leaving behind such a successful SM. Thus it is important to know if this quest is justifi ed or not, give n that the SM is a very successful theory that has offered to us some of the most striking agreements betwe en experiment al data and theoretica l predi ctions. Almost all research in theoretical high energy physics of the last thirty years has been concentrated on the quest for the theory that will replace the Standard Model (SM) as the proper description of nature at the energ ies beyond the TeV sca le.
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