arXiv subjects
Publications and source records attributed to S. Raby.
This report, prepared for the Community Planning Study - Snowmass 2013 - summarizes the theoretical motivations and the experimental efforts to search for baryon number violation, focussing on nucleon decay and neutron-antineutron oscillations. Present and future nucleon decay search experiments using large underground detectors, as well as planned neutron-antineutron oscillation search experiments with free neutron beams are highlighted.
The NSF has chosen the site for the Deep Underground Science and Engineering Laboratory (DUSEL) to be in Lead, South Dakota. In fact, the state of South Dakota has already stepped up to the plate and contributed its own funding for the proposed lab, see http://www.sanfordlaboratoryathomestake.org/index.html. The final decision by NSF for funding the Initial Suite of Experiments for DUSEL will be made early in 2009. At that time the NSF Science Board must make a decision. Of order 200 experimentalists have already expressed an interest in performing experiments at DUSEL. In order to assess the interest of the theoretical community, the Center for Cosmology and Astro-Particle Physics (CCAPP) at The Ohio State University (OSU) organized a 3-day DUSEL Theory Workshop in Columbus, Ohio from April 4 - 6, 2008. The workshop focused on the scientific case for six proposed experiments for DUSEL: long baseline neutrino oscillations, proton decay, dark matter, astrophysical neutrinos, neutrinoless double beta decay and N-Nbar oscillations. The outcome of this workshop is the DUSEL Theory White paper addressing the scientific case at a level which may be useful in the decision making process for policy makers at the NSF and in the U.S. Congress. In order to assess the physics interest in the DUSEL project we have posted the DUSEL Theory White paper on the following CCAPP link http://ccapp.osu.edu/whitepaper.html . Please read the white paper and, if you are interested, use the link to show your support by co-signing the white paper.
We investigate whether the hypercharge assignments in the Standard Model can be interpreted as a hint at Grand Unification in the context of heterotic string theory. To this end, we introduce a general method to calculate U(1)_Y for any heterotic orbifold and compare our findings to the cases where hypercharge arises from a GUT. Surprisingly, in the overwhelming majority of 3-2 Standard Models, a non-anomalous hypercharge direction can be defined, for which the spectrum is vector-like. For these models, we calculate sin^2 theta to see how well it agrees with the standard GUT value. We find that 12% have sin^2 theta = 3/8, while all others have values which are less. Finally, 89% of the models with sin^2 theta = 3/8 have U(1)_Y in SU(5).
We discuss the results of a global $χ^2$ analysis of a simple SO(10) SUSY GUT with $D_3$ family symmetry and low energy R parity. The model describes fermion mass matrices with 14 parameters and gives excellent fits to 20 observable masses and mixing angles in both quark and lepton sectors, giving 6 predictions. Bi-large neutrino mixing is obtained with hierarchical quark and lepton Yukawa matrices; thus avoiding the possibility of large lepton flavor violation. The model naturally predicts small 1-3 neutrino mixing, with $\sin θ_{13} \simeq 0.05 - 0.06$. In this paper we evaluate the predictions for the lepton flavor violating processes, $μ\to e γ$, $τ\to μγ$ and $τ\to e γ$ and also the electric dipole moment of the electron, $d_e$, muon and tau, assuming universal squark and slepton masses, $m_{16}$, and a universal soft SUSY breaking A parameter, $A_0$, at the GUT scale. We find $Br(μ\to e γ)$ is naturally below present bounds, but may be observable by MEG. Similarly, $d_e$ is below present bounds; but is within the range of future experiments. We also give predictions for the light Higgs mass (using FeynHiggs). We find an upper bound given by $m_h \leq 127$ GeV, with an estimated $\pm 3$ GeV theoretical uncertainty. Finally we present predictions for SUSY particle masses in the favored region of parameter space.
We update and extend to larger masses our previous analysis of the MSSM with minimal $SO_{10}$ [MSO$_{10}$SM] soft SUSY breaking boundary conditions. We find a well--defined, narrow region of parameter space which provides the observed relic density of dark matter, in a domain selected to fit precision electroweak data, including top, bottom and tau masses. The model is highly constrained which allows us to make several predictions. We find the light Higgs mass $m_h \leq 121 \pm 3$ GeV and also upper bounds on the mass of the gluino $\mgluino\lsim3.1$ TeV and lightest neutralino $\mchi\lsim450$ GeV. As the CP odd Higgs mass $m_A$ increases, the region of parameter space consistent with WMAP data is forced to larger values of $M_{1/2}$ and smaller values of $m_h$. Hence, we find an upper bound $m_A \lsim 1.3$ TeV. This in turn leads to lower bounds on ${\rm BR}(B_s\to μ^+ μ^-) > 10^{-8}$ (assuming minimal flavor violation) and on the dark matter spin independent detection cross section $\sigsip > 10^{-9}$ pb. Finally, we extend our previous analysis to include WIMP signals in indirect detection and find prospects for WIMP detection generally much less promising than in direct WIMP searches.
CMSSM boundary conditions are usually used when calculating cosmological dark matter densities. In this paper we calculate the cosmological density of dark matter in the MSSM using minimal $SO_{10}$ soft SUSY breaking boundary conditions. These boundary conditions incorporate several attractive features: they are consistent with $SO_{10}$ Yukawa unification, they result in a "natural" inverted scalar mass hierarchy and they reduce the dimension 5 operator contribution to the proton decay rate. With regards to dark matter, on the other hand, this is to a large extent an unexplored territory with large squark and slepton masses $m_{16}$, large $A_0$ and small $ \{μ, M_{1/2} \} $. We find that in most regions of parameter space the cosmological density of dark matter is considerably less than required by the data. However there is a well--defined, narrow region of parameter space which provides the observed relic density of dark matter, as well as a good fit to precision electroweak data, including top, bottom and tau masses, and acceptable bounds on the branching fraction of $B_s \to μ^+ μ^-$. We present predictions for Higgs and SUSY spectra, the dark matter detection cross section and the branching ratio ${\rm BR}(B_s\to μ^+ μ^-)$ in this region of parameter space.
In simple SO(10) SUSY GUTs the top, bottom and tau Yukawa couplings unify at the GUT scale. A naive renormalization group analysis, neglecting weak scale threshold corrections, leads to moderate agreement with the low energy data. However it is known that intrinsically large threshold corrections proportional to $\tanβ\sim m_t(M_Z)/m_b(M_Z) \sim 50$ can nullify these $t, b$, $τ$ mass predictions. In this paper we turn the argument around. Instead of predicting fermion masses, we use the constraint of Yukawa unification and the observed values $M_t, m_b(m_b), M_τ$ to constrain SUSY parameter space. We find a narrow region survives for $μ> 0$ with $μ, M_{1/2} << m_{16}$, $A_0 \approx - 1.9 m_{16}$ and $m_{16} > 1200$ \gev. Demanding Yukawa unification thus makes definite predictions for Higgs and sparticle masses. In particular we find a light higgs with mass $m_h^0 = 114 \pm 5 \pm 3$ GeV and a light stop with $(m_{\tilde t_1})_{MIN} \sim 450$ GeV and $m_{\tilde t_1} << m_{\tilde b_1}$. In addition, we find a light chargino and a neutralino LSP. It is also significant that in this region of parameter space the SUSY contribution to the muon anomalous magnetic moment $a_μ^{SUSY} < 16 \times 10^{-10}$.
We use $t, b, τ$ Yukawa unification to constrain SUSY parameter space. We find a narrow region survives for $μ> 0$ (suggested by \bsgam and the anomalous magnetic moment of the muon) with $A_0 \sim - 1.9 m_{16}$, $m_{10} \sim 1.4 m_{16}$, $m_{16} \sim 1200 -3000$ \gev and $μ, M_{1/2} \sim 100 - 500$ \gev. Demanding Yukawa unification thus makes definite predictions for Higgs and sparticle masses.
Discrete nonabelian gauge symmetries appear to be the most advantageous candidates for a family symmetry. We present a predictive SO(10) SUSY GUT model with $D_3 \times U(1)$ family symmetry (D_3 is the dihedral group of order 6). The hierarchy in fermion masses is generated by the family symmetry breaking $D_3 \times U(1) \to Z_N \to $ nothing. This model fits the low energy data in the charged fermion sector quite well and naturally provides large angle $ν_μ$-$ν_τ$ mixing describing atmospheric neutrino oscillation data and small angle $ν_e$-$ν_s$ mixing consistent with the small mixing angle MSW solution to the solar neutrino data. In addition, the non-abelian family symmetry D_3 is sufficient to suppress large flavor violations.
In a previous paper we analyzed fermion masses (focusing on neutrino masses and mixing angles) in an SO(10) SUSY GUT with U(2)$\timesU(1)^n$ family symmetry. The model is "natural" containing all operators in the Lagrangian consistent with the states and their charges. With minimal family symmetry breaking vevs the model is also predictive giving a unique solution to atmospheric (with maximal $ν_μ\to ν_τ$ mixing) and solar (with SMA MSW $ν_e \to ν_s$ mixing) neutrino oscillations. In this paper we analyze the case of general family breaking vevs. We now find several new solutions for three, four and five neutrinos. For three neutrinos we now obtain SMA MSW, LMA MSW or vacuum oscillation solutions for solar neutrinos. In all three cases the atmospheric data is described by maximal $ν_μ\to ν_τ$ mixing. In the four and five neutrino cases, in addition to fitting atmospheric and solar data as before, we are now able to fit LSND data. All this is obtained with the additional parameters coming from the family symmetry breaking vevs; providing only minor changes in the charged fermion fits.
In this letter we present a predictive SO(10) SUSY GUT with flavor symmetry U(2) \times U(1) which has several nice features. We are able to fit fermion masses and mixing angles, including recent neutrino data, with 9 parameters in the charged fermion sector and 4 in the neutrino sector. The flavor symmetry plays a preeminent role -- (i) The model is "natural" -- we include all terms allowed by the symmetry. It restricts the number of arbitrary parameters and enforces many zeros in the effective mass matrices. (ii) Flavor symmetry breaking from U(2) \times U(1) \to U(1) \to nothing generates the family hierarchy. It also constrains squark and slepton mass matrices, thus ameliorating flavor violation resulting from squark and slepton loop contributions. (iii) Finally, it naturally gives large angle $ν_μ- ν_τ$ mixing describing atmospheric neutrino oscillation data and small angle $ν_e - ν_{s}$ mixing consistent with the small mixing angle MSW solution to solar neutrino data.
In gauge-mediated SUSY breaking models, messengers transmit SUSY breaking from a partially hidden sector to the standard model sector via common standard model gauge interactions. The minimal set of messengers has quantum numbers of a $5 + \bar 5$ of SU(5); identical to the quantum numbers of the minimal Higgs sector of an SU(5) GUT. We show in a simple model with messenger masses of order the GUT scale that Higgs - messenger mixing quite naturally leads to a low energy MSSM with gluinos as the lightest supersymmetric particles [LSP]. We study the phenomenological consequences of such a model.
It is shown that the doublet-triplet splitting problem can be solved in SO(10) using the Dimopoulos-Wilczek mechanism with a very economical Higgs content and simple structure. Only one adjoint Higgs field is required, together with spinor and vector fields. The successful SUSY GUT prediction of gauge coupling unification is preserved. Higgsino-mediated proton decay can be suppressed below (but not far below) present limits without fine-tuning.
We present a self-consistent $χ^2$ analysis of several supersymmetric (SUSY) grand unified theories recently discussed in the literature. We obtain global fits to low energy data, including gauge couplings, fermion masses and mixing angles, gauge boson masses and $BR(b\rightarrow sγ)$. One of the models studied provides an excellent fit to the low energy data with $χ^2\sim 1$ for 3 degrees of freedom, in a large region of the experimentally allowed SUSY parameter space. We also discuss the consequences of our work for a general MSSM analysis at the $Z$ scale.
A general operator expansion is presented for quark and lepton mass matrices in unified theories based on a U(2) flavor symmetry, with breaking parameter of order $V_{cb} \approx m_s/m_b \approx \sqrt{m_c/m_t}$. While solving the supersymmetric flavor-changing problem, a general form for the Yukawa couplings follows, leading to 9 relations among the fermion masses and mixings, 5 of which are precise. The combination of grand unified and U(2) symmetries provides a symmetry understanding for the anomalously small values of $m_u/m_c$ and $m_c/m_t$. A fit to the fermion mass data leads to a prediction for the angles of the CKM unitarity triangle, which will allow a significant test of these unified U(2) theories. A particular SO(10) model provides a simple realization of the general operator expansion. The lighter generation masses and the non-trivial structure of the CKM matrix are generated from the exchange of a single U(2) doublet of heavy vector generations. This model suggests that CP is spontaneously broken at the unification scale --- in which case there is a further reduction in the number of free parameters.
We reanalyse the problem of fermion masses in supersymmetric SO(10) grand unified models. In the minimal model, both low energy Higgs doublets belong to the same {\bf{10}} representation of SO(10) implying the unification not only of the gauge but also of the third generation Yukawa couplings. These models predict large values of $\tanβ\sim 50$. In this paper we study the effects of departing from the minimal conditions in order to see if we can find models with a reduced value of $\tanβ$. In order to maintain predictability, however, we try to do this with the addition of only one new parameter. We still assume that the fermion masses arise from interactions of the spinor representations with a single ${\bf 10}$ representation, but this ${\bf 10}$ now only contains a part of the two light Higgs doublets. This enables us to introduce one new parameter $ω=λ_b/λ_t$. For values of $ω\ll 1$ we can in principle reduce the value of $\tanβ$. In fact, $ω$ is an overall factor which multiplies the down quark and charged lepton Yukawa matrices. Thus the theory is still highly constrained. We show that the first generation quark masses and the CP-violation parameter $ε_K$ yield strong constraints on the phenomenologically allowed models. In the end, we find that large values of $\tanβ$ are still preferred.
A complete supersymmetric SO(10) model is constructed, which is the most general consistent with certain $R$, discrete, and $U(1)$ flavor symmetries. The desired vacuum of the theory has vevs which lie in particular directions of group space. This leads to both doublet triplet splitting and to the generation of just four operators for charged fermion masses. The model illustrates how many features of superunification become related in the context of a complete theory. The features discussed here include: the weak mixing angle prediction, the doublet-triplet splitting problem, proton decay, the generation of the $μ$ parameter, neutrino masses and the generation of the operators which lead to charged fermion mass predictions.
A new approach for deducing the theory of fermion masses at the scale of grand unification is proposed. Combining SO(10) grand unification, family symmetries and supersymmetry with a systematic operator analysis, the minimal set of fermion mass operators consistent with low energy data is determined. Exploiting the full power of SO(10) to relate up, down and charged lepton mass matrices, we obtain predictions for 7 of the mass and mixing parameters. The assumptions upon which the operator search and resulting predictions are based are stressed, together with a discussion of how the predictions are affected by a relaxation of some of the assumptions.The masses of the heaviest generation, $m_t,m_b$ and $m_τ$, are generated from a single renormalizable Yukawa interaction, while the lighter masses and the mixing angles are generated by non-renormalizable operators of the grand unified theory. The hierarchy of masses and mixing angles is thereby related to the ratio of grand to Planck scales, $M_G / M_P$. An explicit realization of the origin of such an economical pattern of operators is given in terms of a set of spontaneously broken family symmetries. In the preferred models the top quark is found to be heavy: $M_t = 180 \pm 15$ GeV, and $\tan β$ is predicted to be very large. Predictions are also given for $m_s, m_s/m_d , m_u/m_d, V_{cb}, V_{ub}/V_{cb}$ and the amount of CP violation. Stringent tests of these theories will be achieved by more precise measurements of $M_t, V_{cb}, α_s$ and $ V_{ub}/V_{cb}$ and by measurements of CP violation in neutral B meson decays.