arXiv subjects

Berthold Stech

Publications and source records attributed to Berthold Stech.

At least 19 recordsLinked to original sources

arXiv2015

Low-energy phenomenology of trinification: an effective left-right-symmetric model

The trinification model is an interesting extension of the Standard Model (SM) based on the gauge group $SU(3)_C\times SU(3)_L\times SU(3)_R$. We study its low-energy phenomenology by constructing a low-energy effective field theory, thereby reducing the number of particles and free parameters that need to be studied. The resulting model predicts that several new scalar particles have masses in the $\mathcal{O}\left(100\text{ GeV}\right)$ range. We study a few of the interesting phenomenological scenarios, such as the presence of a light fermiophobic scalar in addition to a SM-like Higgs, or a degenerate (twin) Higgs state at 126 GeV. We point out regions of the parameter space that lead to measurable deviations from SM predictions of the Higgs couplings. Hence the trinification model awaits crucial tests at the Large Hadron Collider in the coming years.

hep-ph↗

arXiv2014

Trinification Phenomenology and the structure of Higgs Bosons

The extension of the Standard Model to $SU(3)_L \times SU(3)_R \times SU(3)_C$ (the trinification group) augmented by the $ SO(3)_G $ flavor group is considered. In our phenomenological treatment partly known and partly proposed vacuum expectation values of the scalar Higgs fields play a dominant role. All Higgs fields are taken to be flavor singlets, all flavon fields trinification singlets. We need two flavor (generation) matrices. One determines the mass hierarchy of all fermions, the second one is responsible for all mixings including the CP-violating phase in the CKM matrix. The mixing with higher states contained in the group representation provides for an understanding of the difference between the up quark and the down quark spectrum. There is a close connection between charged and neutral fermions. An inverted neutrino hierarchy is predicted. Examples for the tree-level potential of the Higgs fields are given. To obtain an acceptable spectrum of scalar states, the construction of the potential requires the combination of matrix fields that differ with respect to fermion couplings and flavor-changing properties. As a consequence bosons with fermiophobic components or, alternatively, flavor-changing components are predicted in this model. Nevertheless, the Higgs boson at 125 GeV is very little different from the Standard Model Higgs boson in its couplings to fermions but may have self-coupling constants larger by a factor 2.

hep-ph↗

arXiv2013

Degenerate states in the scalar boson spectrum. Is the Higgs Boson a Twin ?

The extension of the standard model to $SU(3)_L\times SU(3)_R \times SU(3)_C$ is considered. Spontaneous symmetry breaking requires two $(3^*, 3, 1)$ Higgs field multiplets with a strong hierarchical structure of their vacuum expectation values. An invariant potential is constructed to provide for these vacuum expectation values. This potential gives masses to all scalar fields apart from the 15 Goldstone bosons. In case there exists a one-to-one correspondence between the vacuum expectation values of the two field multiplets, the scalar boson spectrum contains degenerate eigenstates. The lowest eigenstate has a mass near 123 GeV close to the Higgs-like particle discovered at the LHC. In one class of solutions this lowest state is a nearly degenerate twin state. Each member is a superposition of fields from both multiplets with about equal strength. The twins are non identical twins, namely different combinations of a conventional Higgs and a Higgs field which is not coupled to fermions, only to gauge bosons. A second class of solutions leads again to degenerate states but in this case the state near 123 GeV remains a single state even for identical low scale vacuum expectation values in both multiplets.

hep-ph↗

arXiv2012

Universal behaviour of the $γ^*γ\to (π^0,η,η')$ transition form factors

The photon transition form factors of $π$, $η$ and $η'$ are discussed in view of recent measurements. It is shown that the exact axial anomaly sum rule allows a precise comparison of all three form factors at high-$Q^2$ independent of the different structures and distribution amplitudes of the participating pseudoscalar mesons. We conclude: (i) The $πγ$ form factor reported by Belle is in excellent agreement with the non-strange I=0 component of the $η$ and $η'$ form factors obtained from the BaBar measurements. (ii) Within errors, the $πγ$ form factor from Belle is compatible with the asymptotic pQCD behavior, similar to the $η$ and $η'$ form factors from BaBar. Still, the best fits to the data sets of $πγ$, $ηγ$, and $η'γ$ form factors favor a universal small logarithmic rise $Q^2 F_{Pγ}(Q^2)\sim \log(Q^2)$.

hep-ph↗

arXiv2012

The mass of the Higgs boson in the trinification subgroup of E6

The extension of the standard model to SU(3)_L x SU(3)_R x SU(3)_C is considered. Spontaneous symmetry breaking requires two Higgs field multiplets with a strong hierarchical structure of vacuum expectation values. These vacuum expectation values, some of them known from experiment, are used to construct invariant potentials in form of a sum of individual potentials relevant at the weak scale. As in a previous suggestion one may normalize the most important individual potentials such that their mass eigenvalues agree with their very large vacuum expectation values. In this case (for a wide class of parameters) the scalar field corresponding to the standard model Higgs turns out to have the precise mass value m_Higgs = v/sqrt(2) = 123 GeV at the weak scale. The physical mass (pole mass) is larger and found to be 125 +/- 1.4 GeV.

hep-ph↗

arXiv2012

On the $γ^*γ\toπ(η,η')$ transition form factors

The surprising results by the BarBar collaboration on the $πγ$ transition form factor require new thoughts about the high-$Q^2$ dependence of the form factors with virtual photons. We make use of the anomaly sum rule [J. Horejsi and O. Teryaev, Z. Phys. C65, 691 (1995).] which relates the hadron spectral density to the axial anomaly [S. Adler, Phys. Rev. 177, 2426 (1969); J. S. Bell and R. Jackiw, Nuovo Cimento A 60, 47 (1969).]. We study the quark-hadron duality relation for this sum rule and find out that the increase of the rescaled form factor $Q^2F_{πγ}(Q^2)\sim\log(Q^2)$ suggested by the BaBar data requires the presence of a $1/s$-correction term in the relation between the one-loop spectral density and the hadron-continuum spectral density.

hep-ph↗

arXiv2011

Flavor Symmetry and Grand Unification

The combination of flavor symmetries with grand unification is considered: GUT $ \times$ flavor . To accommodate three generations the flavor group SO(3) is used. All fermions transform as 3-vectors under this group. The Yukawa couplings are obtained from vacuum expectation values of flavon fields. For the flavon fields (singlets with respect to the GUT group) and the Higgs fields (singlets with respect to the generation group) a simple form for the effective potentials is postulated. It automatically leads to spontaneous symmetry breaking for these scalar fields. Discrete S4 transformations relate the different locations of the minima of the potentials.These potentials can be used to describe the hierarchy of the well known up quark mass spectrum. Also the huge hierarchy of the masses of the Higgs fields in grand unified models can be parametrized in this way. It leads to a prediction of the mass of the lightest Higgs boson in terms of its vacuum expectation value $v_0$: $ m_{Higgs} = \frac{v_0}{\sqrt{2}} = 123 GeV$.

hep-ph↗

arXiv2010

Neutrino Properties from E_6 x SO(3) x Z(2)

The group E_6 for grand unification is combined with the generation symmetry group SO(3) x Z(2). The coupling matrices in the Yukawa interaction are identified with the vacuum expectation values of scalar flavons. The symmetric part of this 3x3 coupling matrix can be identified with the known up-quark hierarchy. The antisymmetric part is responsible for fermion mixings and CP violation. Numerical fits with only few parameters reproduce quantitatively all known fermion properties. The model predicts an inverted neutrino hierarchy, CP violation properties as well as the 0v2beta decay parameter. It also predicts that the masses of the two lightest of six `right handed' neutrinos lie in the low TeV region.

hep-ph↗

arXiv2008

Generation Symmetry and E_6 Unification

The group E_6 for grand unification is combined with the generation symmetry group SO(3)_g. The coupling matrices in the Yukawa interaction are identified with the vacuum expectation values of scalar fields which are representations of the generation symmetry. These values determine the hierarchy of the fermions as well as their mixings and CP-violation. This generation mixing appears in conjunction with the mixing of the standard model fermions with the heavy fermions present in the lowest representation of E_6. A close connection between charged and neutral fermions is observed relating for instance the CKM mixings with the mass splittings of the light neutrinos. Numerical fits with only few parameters reproduce quantitatively all known fermion properties. The model predicts an inverted neutrino hierarchy and gives rather strict values for the light and heavy neutrino masses as well as for the 0ν2βdecay parameter. It also predicts that the masses of the two lightest of six `right handed' neutrinos lie in the low TeV region.

hep-ph↗

arXiv2006

Fall-apart decays of polyquark hadrons

We analyse fall-apart decays of poliquark (tetra, penta and molecule type) hadrons within the constituent quark picture. For processes in which a poliquark hadron goes to final states containing a light pseudoscalar meson the constraints given by chiral symmetry are implemented. As an application of the approach developed, fall-apart decays of $a(980)$ and X(3872) are studied, assuming these mesons are poliquark hadrons. Two extreme options - confined diquark-diquark states and molecular states - are considered. For $a^0(980)$, the observed width can be obtained assuming that this meson is a diquark-diquark composite with a relatively large size of around $1÷1.5$ fm. The pure $K \bar K $ molecular-type state, however, can be excluded. For the X(3872), a sufficiently small width can be obtained if it is a dominantly isospin-0 diquark-diquark composite with a very large size of $\ge 2.5$ fm. The pure molecular option appears possible if the binding energy is tiny, $E_b\lesssim 0.2$ MeV, corresponding to a huge size.

hep-ph↗

arXiv2005

Can Chiral Symmetry Explain the Small Pentaquark Width?

It is shown that the decay amplitude for the Jaffe-Wilczek type pentaquarks is not suppressed by chiral symmetry. On the other hand, pentaquarks of positive or negative parity built up of an antiquark and two chirally different diquarks in an $S$-state are stable in the limit of a strictly unbroken chiral symmetry. These states can decay only via the spontaneous breaking of chiral symmetry. However, this breaking is strong because of the sizeable magnitude of the quark condensate. Thus, chiral symmetry cannot be the cause of a tiny decay amplitude, even for pentaquarks which are stable in the strict chiral symmetry limit.

hep-ph↗

arXiv2004

Pentaquarks in the Chiral Symmetry Limit

We demonstrate that a five quark state of positive parity with an internal P-wave structure - usually pictured as a composite of an antiquark and two diquarks in a P-wave state - can couple to nucleons and Goldstone particles in a chirally invariant way. The corresponding decay width is generally not suppressed. A pentaquark of positive or negative parity with an internal S-wave structure, which may be viewed as a composite of an antiquark and two chirally different diquarks in an S-state, does not couple to nucleons and light mesons in the limit of an unbroken chiral symmetry. It is stable in this limit. However, such states can decay via the effect of the spontaneous breaking of chiral symmetry. This breaking is strong because of the sizeable magnitude of the quark condensate. Thus, chiral symmetry cannot be the cause of a tiny decay amplitude, even for pentaquarks stable in a strict chiral symmetry limit.

hep-ph↗

arXiv2004

Fermion Masses and Coupling Unification in E6. Life in the Desert

We present an $E_6$ Grand Unified model with a realistic pattern of fermion masses. All standard model fermions are unified in three fundamental 27-plets (i.e. supersymmetry is not invoked), which involve in addition right handed neutrinos and three families of vector like heavy quarks and leptons. The lightest of those can lie in the low TeV range, being accessible to future collider experiments. Due to the high symmetry, the masses and mixings of all fermions are closely related. The new heavy fermions play a crucial role for the quark and lepton mass matrices and the bilarge neutrino oscillations. In all channels generation mixing and ${\cal CP}$ violation arise from a single antisymmetric matrix. The $E_6$ breaking proceeds via an intermediate energy region with $SU(3)_L\tm SU(3)_R\tm SU(3)_C$ gauge symmetry and a discrete left-right symmetry. This breaking pattern leads in a straightforward way to the unification of the three gauge coupling constants at high scales, providing for a long proton lifetime. The model also provides for the unification of the top, bottom and tau Yukawa couplings and for new interesting relations in flavor and generation space.

hep-ph↗

arXiv2000

Weak form factors for heavy meson decays

We calculate the form factors for weak decays of $B_{(s)}$ and $D_{(s)}$ mesons to light pseudoscalar and vector mesons within a relativistic dispersion approach based on the constituent quark picture. This approach gives the form factors as relativistic double spectral representations in terms of the wave functions of the initial and final mesons. The form factors have the correct analytic properties and satisfy general requirements of nonperturbative QCD in the heavy quark limit. The effective quark masses and meson wave functions are determined by fitting the quark model parameters to lattice QCD results for the $B\to ρ$ transition form factors at large momentum transfers and to the measured $D\to (K,K^*)lν$ decay rates. This allows us to predict numerous form factors for all kinematically accessible $q^2$ values.

hep-ph↗

arXiv2000

A Predictive Minimal Model for Neutrino Masses and Mixings

A model is considered in which the scale of the heavy singlet neutrinos is a few orders of magnitude below the grand unification scale and where right-handed vector bosons play still a negligible role. In a basis with diagonal up-quark and Dirac-neutrino mass matrices it is assumed that the heavy neutrino mass matrix has only zero elements in its diagonal, in analogy to the light neutrino mass matrix in the Zee model. Connecting then the remaining matrix elements with the small parameter describing the hierarchy of quark masses and mixings and by assuming commutativity of the charged lepton with the down-quark mass matrix, the calculation of all neutrino properties can be performed in terms of the two mass differences relevant for atmospheric and solar neutrino oscillations. CP-violation is directly related to CP-violation in the quark sector.

hep-ph↗

arXiv1998

Hints from the Standard Model for Particle Masses and Mixings

The standard model taken with a momentum space cut-off may be viewed as an effective low energy theory. The structure of it and its known parameters can give us hints for relations between these parameters. In the present investigation the Higgs problem will be discussed, the possible connection of the Higgs meson with the heavy top quark, and the geometric structure of the quark and lepton mass matrices.

hep-ph↗

arXiv1997

Exclusive Hadronic B-Decays

Exclusive non-leptonic two-body decays are discussed on the basis of a generalized factorization approach which also includes non-factorizeable contributions. Numerous decay processes can be described satisfactorily. The success of the method makes possible the determination of decay constants from non-leptonic decays. In particular, we obtain f_{D_s}=(234+-25) MeV and f_{D^*_s}=(271+-33) MeV. The observed constructive and destructive interference pattern in charged B- and D-decays, respectively, can be understood in terms of the different alpha_s-values governing the interaction among the quarks. The running of alpha_s is also the cause of the observed strong increase of the amplitude of lowest isospin when going to low energy transitions.

hep-ph↗

arXiv1997

Hadronic B-Decays

Non-leptonic two-body decays are discussed on the basis of a generalized factorization approach. It is shown that a satisfactory description of numerous decay processes can be given using the same two parameters a_1^{eff} and a_2^{eff}. Although in general process-dependent, these parameters are not expected to change markedly. In fact, within error limits, there is no evidence for a process dependence in energetic B-decays. The success of factorization allows the determination of decay constants from non-leptonic decays. For the D_s meson one obtains f_{D_s}=(234\pm25) MeV, for the D^*_s meson f_{D_s^*}=(271\pm33) MeV. The ratio a_2^{eff}/a_1^{eff} is positive in B-decays and negative in D-decays corresponding to constructive and destructive interference in B^- and D^+ decays, respectively. Qualitatively, this can be understood considering the different scales or α_s-values governing the interaction among the outgoing quarks. The running of α_s is also the cause of the observed strong increase of the amplitude of lowest isospin when going to low energy transitions.

hep-ph↗