Vepsäläinen, Mikko

Helsingin yliopiston digitaalinen arkisto

Faculty of Science Department of Physical SciencesMikko VepsäläinenLarge Extra Dimensions and Power Law UnicationTheoretical PhysicsMaster's ThesisAugust 200140Extra dimension, Grand Unied TheoryKumpula Science Library, University of HelsinkiThe Standard Model of particle physics, which describes subatomic phenomena with un-precedented accuracy, is nowadays generally understood to only be an eective low energy limitof a more fundamental theory. The most promising candidates for the underlying theory aresuperstring theory and its extension, M-theory, which both require the existence of multipleextra dimensions. Usually these dimensions are assumed to be compactied into an extremelysmall manifold of Planckean size, which would explain their absence in the current experi-ments. However, recently much greater compactication volumes have been proposed as analternative solution to the hierarchy problem of the Standard Model. If the extra dimensionsare large enough, they could be detected already in the near future colliders either directly orindirectly by the eects of virtual higher dimensional excitations of the Standard Model elds.In this thesis the eects of large extra dimensions on the low energy phenomenology arestudied. It is shown that from the four-dimensional point of view the higher dimensional eldscan be decomposed into innite towers of massive KaluzaKlein excitations, whose spectracontain information on the size and the topology of the extra dimensions. Including theseexcitations into the radiative corrections of gauge boson propagators leads to a modicationof the conventional logarithmic running of couplings, replacing it by a power law. The gaugecouplings then unify at much lower scales than in four-dimensional theories. Various modelswhere the unication actually takes place are discussed, as well as restrictions for buildinghigher dimensional models consistent with the grand unication scenario.Although the possibility of being able to probe GUT physics already in the LHC is excit-ing, there are several problems related to the low unication scale that strongly reduce theattractiveness of these models. One is the proton decay, which is no longer suppressed bythe enormous value of the GUT scale. Moreover, the grand unication seems less natural, forit has to inserted by hand into the higher dimensional extensions of the Standard Model orthe MSSM. The most important defect, however, is that although the extra dimensions aretheoretically well motivated, there is no solid reason for the compactication scale to lie rightbeyond the reach of current experiments, instead of the Planck scale.Tiedekunta/Osasto  Fakultet/Sektion  Faculty Laitos  Institution  DepartmentTekijä  Författare  AuthorTyön nimi  Arbetets titel  TitleOppiaine  Läroämne  SubjectTyön laji  Arbetets art  Level Aika  Datum  Month and year Sivumäärä  Sidoantal  Number of pagesTiivistelmä  Referat  AbstractAvainsanat  Nyckelord  KeywordsSäilytyspaikka  Förvaringsställe  Where depositedMuita tietoja  Övriga uppgifter  Additional informationHELSINGIN YLIOPISTO  HELSINGFORS UNIVERSITET  UNIVERSITY OF HELSINKI

Large Extra Dimensions and Power Law Unification

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Helsingin yliopiston digitaalinen arkisto