Search for new neutral gauge bosons at LHC
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EMENTARY PARTICLES AND FIELDS Theory
Search for New Neutral Gauge Bosons at LHC I. A. Golutvin, V. V. Palichik, M. V. Savina* , and S. V. Shmatov** Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Received September 22, 2005; in final form, April 28, 2006
Abstract—New heavy gauge bosons belonging to an extended (in relation to the Standard Model) gauge sector are present in Grand Unified Theories, as well as in a number of models aimed at solving the hierarchy problem—in particular, in the technicolor model and in some versions of multidimensional field theories (for example, in the Little Higgs model). The LHC potential for seeking new neutral gauge bosons in decays to two muons is studied. PACS numbers : 12.60.-i, 12.10.Dm, 11.10.Kk, 04.50.+h DOI: 10.1134/S1063778807010085
INTRODUCTION
matrices in the Higgs sector, which describe the interaction of fermion fields with Higgs fields (in this way, matter fields acquire masses); the gauge coupling constants g, g , and gs , which describe electromagnetic, weak, and strong interactions; the Higgs coupling constant; and the mass parameter mH .
Despite the remarkable successes of the Standard Model in describing existing experimental data, it is obvious that the Standard Model is not an ultimate theory of elementary particles and interactions, but it is likely to be a low-energy limit of some future, more fundamental theory. The basic unresolved problems in the Standard Model, which require invoking wider symmetry groups or other totally new concepts (like string theory or string-theory-inspired models that involve extra spatial dimensions), are the following: (i) The Standard Model does not describe gravitational interactions. (ii) There is a hierarchy problem in the Standard Model. The essence of this problem is that the existence of two scales in the theory that differ dramatically in magnitude (the Higgs boson mass versus a scale that is on the order of the Grand Unification scale or even the Planck mass and which characterizes the limit of applicability of standard quantum field theory, [mH /M ]2 ∼ 10−28 −10−34 ) is a signal of a serious conceptual weakness in the theory. First, the origin of so small a number is a challenging problem in itself; second, the hierarchy is not safe against loop contributions, which push the vacuum expectation value for the Higgs boson, and all values of the matter fields together with it, to the Grand Unification scale or the Planck scale MPl . A fine tuning of the Higgs boson mass to the above enormous accuracy is required for compensating these contributions. (iii) The Standard Model Lagrangian involves a large number of free parameters that are not fixed within the model itself. These include the Yukawa * **
(iv) The number of fermion generations is not fixed in the Standard Model (according to modern concepts, it is assumed to be equal to three). (v) The mechanism of CP violation is still an open problem in the Standard Model. Of course, all of the aforementioned problems are of importance and must be solved in cons
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