Elsevier

Physics Letters B

Volume 477, Issues 1–3, 23 March 2000, Pages 233-241
Physics Letters B

Alternative supersymmetric spectra

https://doi.org/10.1016/S0370-2693(00)00187-8Get rights and content

Abstract

We describe the features of supersymmetric spectra, alternative to and qualitatively different from that of most versions of the MSSM. The spectra are motivated by extensions of the MSSM with an extra U(1)′ gauge symmetry, expected in many grand unified and superstring models, which provide a plausible solution to the μ problem, both for models with supergravity and for gauge-mediated supersymmetry breaking. Typically, many or all of the squarks are rather heavy (larger than one TeV), especially for the first two families, as are the sleptons in the supergravity models. However, there is a richer spectrum of Higgs particles, neutralinos, and (possibly) charginos. Concrete examples of such spectra are presented, and the phenomenological implications are briefly discussed.

Introduction

The minimal supersymmetric standard model (MSSM) and its simple extensions contain many free parameters associated with supersymmetry breaking. Most analyses have been based on two generic classes of models of soft supersymmetry breaking: (1) supergravity, in which supersymmetry breaking in a hidden sector is transmitted to the observable sector via supergravity. One usually assumes universality or at least a comparable scale for the soft parameters at the Planck scale. (2) Gauge-mediated models, with the breaking transmitted via messenger fields at relatively low energy, such as 105 GeV. In both cases the scale of the soft supersymmetry breaking parameters ultimately sets the electroweak scale via radiative electroweak breaking, provided that the supersymmetric μ parameter is of a comparable magnitude. Once universality is relaxed there are many free parameters in supergravity, and there are many versions of gauge-mediation. However, in both cases a typical spectrum involves sparticle masses in the several hundred GeV range due to naturalness arguments; i.e. the mass scale of the superpartners should be in this range (and at most O(1TeV)) for SUSY to explain the origin of the electroweak scale without excessive fine-tuning. Most studies of the implications for current and future colliders and precision measurements have been based on such a spectrum.

However, it is well known that naturalness does not necessarily require that all sparticles have masses below the TeV scale [1]. In the scalar sector, naturalness only constrains the masses of the third generation sfermions and the electroweak Higgs doublets, as these are the fields which have large Yukawa couplings and thus play dominant roles in radiative electroweak symmetry breaking. Therefore, the sparticle masses of the first and second generations can be significantly larger than the other sparticle (and particle) masses without violating naturalness criteria. Recent work has demonstrated that this hierarchy can be generated dynamically via renormalization group evolution (first pointed out in Ref. [2]and investigated in the context of grand unified models in Ref. [3]). In this scenario, the soft supersymmetry breaking scalar mass-squared parameters can be multi-TeV (∼4 TeV) at the high scale (while the gaugino masses and scalar trilinear couplings are ∼MW; the Higgs and third generation masses are driven to smaller values due to their large Yukawa couplings, while the first and second generations remain heavy. The results of a recent extension of this framework including the possibility of multi-TeV A parameters [4]indicate that such inverted hierarchies can be generated with the first two generations up to ∼20 TeV. This scenario has distinctive implications (such as in collider searches for superpartners; see, e.g. 2, 5), and can be advantageous phenomenologically, as stringent laboratory constraints on the SUSY parameter space from flavor-changing neutral currents (FCNC) and CP-violation can be considerably weakened [1]. Another possibility in supergravity models pointed out in Ref. [6]is that since the Higgs soft mass-squared parameter at the electroweak scale can be quite insensitive to the initial values of the scalar masses due to “focus-point” behavior of the RGE's, scalar masses for all three generations of squarks and sleptons of order 2−3 TeV can be consistent with naturalness (see Ref. [7]for a discussion in the context of gauge-mediated models).

The purpose of this Letter is to point out that there is another class of (string-motivated) models based on gauge extensions of the MSSM with an additional U(1)′ gauge group in which this type of spectrum is naturally achieved. In these models, it has been shown that the U(1)′ may be broken at the TeV scale by a radiative mechanism analogous to that for electroweak breaking provided there are sufficiently large Yukawa couplings of a standard model singlet S which carries U(1)′ charge [8]. Such extended gauge groups, exotic particle content, and large Yukawa couplings are generically present in classes of quasi-realistic perturbative superstring constructions. These models also provide an alternative resolution to the μ problem of the MSSM, since gauge invariance can forbid the elementary μ term while an effective μ term can be generated via a trilinear coupling of the SM singlet to the two electroweak Higgs doublets 8, 9. Furthermore, the enhanced symmetry avoids the problems of domain walls, which are common to models involving an effective μ generated by the VEV of a scalar but not associated with the breaking of an extra gauge symmetry [10].

In this framework the VEV of the singlet field sets the scale of the Z′ mass. This VEV is generally of order several TeV since the nonobservation of an additional Z′ boson and the stringent constraints on the ZZ′ mixing angle αZZ typically require that the mass of the Z′ is significantly heavier than the Z mass (the lower bounds on MZ are model dependent, but are in the range of 500 GeV to 1 TeV or so). Since the singlet VEV is achieved radiatively, its value generally sets the scale of the required initial values of the soft breaking parameters. Typical supersymmetry breaking parameters are at the TeV scale, at least for the first two generations. However, there is typically a much richer spectrum of Higgs particles and neutralinos, as well as the Z′ and (usually) exotic fermions and their partners. Specific models based on perturbative heterotic string constructions also involve extended chargino sectors [11].

Since the electroweak and U(1)′ symmetry breaking are coupled in these models, the large ratio of the Z′ and Z masses requires a certain amount of tuning of the parameters (cancellations are needed for the expectation values of the Higgs doublets to be sufficiently small). Nevertheless, such models are worth exploring as viable alternatives to the MSSM which are well motivated theoretically both within quasi-realistic string constructions and GUT models. An additional motivation to consider such models seriously arises from recent precision electroweak data. The Z lineshape and atomic parity data hint at the existence of an extra Z′ at a scale around 1 TeV [12]1. In this Letter, we illustrate typical spectra from several concrete models, some with supergravity-mediated SUSY soft breaking parameters and another with gauge-mediated supersymmetry breaking, and comment briefly on phenomenological implications.

Section snippets

Results: alternative supersymmetric spectra

The models we consider are extensions of the MSSM with an additional nonanomalous U(1)′ gauge symmetry and additional matter fields, typically including both SM singlets (with U(1)′ charges) and SM exotics. Such models are motivated from a class of quasi-realistic (perturbative heterotic) superstring models [11]. It was shown in Ref. [11]that after vacuum restabilization this class of string models generically contain extended Abelian gauge structures and additional matter content at the string

Concluding remarks

The purpose of this Letter has been to emphasize two main points: (i) supersymmetric models with an additional U(1)′ gauge symmetry broken at the TeV scale are well motivated extensions of the MSSM both theoretically and phenomenologically, and (ii) the characteristic low energy mass spectra of this class of models exhibit patterns which have distinctive features compared to that of the MSSM. In particular, the strong phenomenological constraints on the Z′ mass and mixing with the ordinary Z

Acknowledgements

We thank M. Cvetič, J. Erler, and J.R. Espinosa for helpful discussions and suggestions. L.E. also thanks G. Kribs for helpful discussions and comments. This work is supported in part by the U.S. Department of Energy Grants No. EY-76-02-3071 (P.L.,M.P.), DE-FG02-95ER40899 (L.E.), and DE-AC02-76CH03000 (J.W.), and in part by the Feodor Lynen Program of the Alexander von Humboldt Foundation (M.P.).

References (23)

  • J.L. Feng et al.

    Nucl. Phys. B

    (1999)
  • F. Gabbiani et al.

    Nucl. Phys. B

    (1996)
  • B. de Carlos et al.

    Phys. Lett. B

    (1997)
  • M. Drees, Phys. Rev. D 33 (1986) 1468; S. Dimopoulos, G.F. Giudice, Phys. Lett. B 357 (1995) 573; A. Pomarol, D....
  • J. Bagger, J.L. Feng, N. Polonsky,...
  • J. Bagger, J.L. Feng, N. Polonsky, R.-J. Zhang,...
  • V. Barger, C. Kao, R.-J. Zhang,...
  • J.L. Feng, K.T. Matchev, T. Moroi, hep-ph/9908309,...
  • K. Agashe,...
  • M. Cvetič, P. Langacker, Phys. Rev. D 54 (1996) 3570; Mod. Phys. Lett. A 11 (1996)...
  • D. Suematsu et al.

    Int. J. Mod. Phys. A

    (1995)
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