Talk given at International Conference on Particle Physics: In memoriam Engin Arik and her colleagues, Istanbul, Turkey, 27-31 Oct 2009. To appear in Proceedings
The necessity of the fourth family follows from the SM basics. According to flavor democracy the Dirac masses of the fourth SM family fermions are almost equal with preferable value 450 GeV, which corresponds to common (for all fundamental fermions) Yukawa coupling equal to SU(2) gauge coupling gW. In principle, one expect u4 a little bit lighter than d4, while nu4 could be essentially lighter than l4 due to Majorana mass terms for right-handed components of neutrinos. Obviously, the fourth family quarks will be copiously produced at the LHC. However, the first indication of the fourth SM family may be provided by early Higgs boson observation due to almost an order enhancement of the gluon fusion to Higgs cross-section. For the same reason the Tevatron still has a chance to observe the Higgs boson before the LHC. Concerning the fourth family leptons, in general, best place will be NLC/CLIC. However, for some mass regions and MNS matrix elements double discovery of both the nu4 and H could be possible at the LHC.
A class of non-supersymmetric extensions of the Standard Model is proposed in which there is a multiplicity of light scalar doublets in a multiplet of a non-abelian family group with the Standard Model Higgs doublet. Anthropic tuning makes the latter light, and consequently the other scalar doublets remain light because of the family symmetry. The family symmetry greatly constrains the pattern of FCNC and proton decay operators coming from scalar-exchange. Such models show that useful constraints on model-building can come from an extended naturalness principle when the electroweak scale is an…
Existence of the fourth family follows from the basics of the Standard Model and the actual mass spectrum of the third family fermions. We discuss possible manifestations of the fourth SM family at existing and future colliders. The LHC and Tevatron potentials to discover the fourth SM family have been compared. The scenario with dominance of the anomalous decay modes of the fourth family quarks has been considered in details.
We study the naturalness problem using a model independent bottom up approach by considering models where only the interaction terms needed to cancel the Higgs quadratic divergences are present. If quadratic divergences are canceled by terms linear in the Higgs field, then the collider phenomenology is well covered by current electroweakino and fourth generation searches. If quadratic divergences are canceled by terms bilinear in the Higgs field, then the signatures are highly dependent on the quantum numbers of the new particles. Precision Higgs measurements can reveal the presence of new par…
The potential of $μ^{+}μ^{-}$ colliders to investigate the fourth SM family fermions predicted by flavour democracy has been analyzed. It is shown that muon colliders are advantageous for both pair production of fourth family fermions and resonance production of fourth family quarkonia. Also isosinglet quarks production at $μ^{+}μ^{-}$ colliders has been investigated.
The resonant production of the fourth family slepton ~l_4 via R-parity violating interactions of supersymmetry at the Large Hadron Collider has been investigated. We study the decay mode of ~l_4 into the fourth family neutrino nu_4 and W boson. The signal will be a like-sign dimuon and dijet if the fourth family neutrino has Majorana nature. We discuss the constraints on the R-parity violating couplings lambda and lambda' of the fourth family charged slepton at the LHC with the center of mass energies of 7, 10 and 14 TeV.
It is shown that if the fourth SM fermion family exists then the Higgs boson could be observed at the LHC with an integrated luminosity of few fb-1. The Higgs discovery potential for different channels is discussed in the presence of the fourth SM family.
The latest electroweak precision data allow the existence of additional chiral generations in the standard model. We study prospects of search for the fourth standard model family fermions and quarkonia at $e^{+}e^{-}$ and $γγ$ options of CLIC. It is shown that CLIC will be powerfull machine for discovery and investigation of both fourth family leptons and quarkonia. Moreover, the formation of the fourth family quarkonia will give a new opportunity to investigate Higgs boson properties.
It is shown that the fourth standard model (SM) family quarks can be observed at the Fermilab Tevatron if their anomalous interactions with known quarks have sufficient strength.
Possible single productions of fourth family u_{4} and d_{4} quarks via anomalous q_{4}qV interactions at Tevatron are studied. Signature of such processes are discussed and compared with the recent results from Tevatron.
Superjet events observed by the CDF Collaboration are interpreted as anomalous single production of the fourth SM family $u_4$ quark, decaying into a new light scalar particle. The specific predictions of the proposed mechanism are discussed.
The properties of new heavy mesons containing new heavy quarks have been investigated. As an example, the fourth SM family quarks and weak isosinglet quarks predicted by E_{6} GUT are considered. Production of these hadrons at TeV energy lepton colliders have been analyzed.
We study the observability for a Higgs boson at upgraded Tevatron via the modes gg->h->ZZ->4l (l=e,mu). We find that the signal can be observed at an integrated luminosity of 30 fb^(-1) if the fourth SM family exists.
The heavy fourth generation of quarks that have sufficiently small mixing with the three known SM families form hadrons. In the present work, we calculate the masses and decay constants of mesons containing either both quarks from the fourth generation or one from fourth family and the other from known third family SM quarks in the framework of the QCD sum rules. In the calculations, we take into account two gluon condensate diagrams as nonperturbative contributions. The obtained results reduce to the known masses and decay constants of the $\bar b b$ and $\bar c c$ quarkonia when the fourth f…
Superjet events observed by the CDF Collaboration are interpreted as anomalous single production of the fourth SM family u_4 quark, decaying into a new light scalar particle. The specific predictions of the proposed mechanism are discussed.
We examined the single production of fourth family b' quarks at the Large Hadron electron Collider (LHeC).We have analyzed the background and the signal processes for the mass range 300-800 GeV. We find the discovery region for an optimal parametrization of the Vqb' matrix elements.
We investigate the production of fourth family up-type quarks using effective lagrangian approach at future lepton-hadron colliders and study the kinematical characteristics of the signal with an optimal set of cuts. We obtain the upper mass limits 500 GeV at THERA and one TeV at Linac $\otimes$ LHC.
We study the pair production of $\tilde{l}_{4}$, which is the supersymmetric partner of the fourth family charged lepton, at the $e^{+}e^{-}$ colliders. It is shown that the investigation of this process at ILC/CLIC will give opportunity to differentiate the MSSM with three and four families.
We evaluate the LHC discovery potential for the fourth family Standard Model neutrinos in the process $pp\to Z/h\toν_{4}{\barν_{4}}\to WμWμ$. We show that, depending on their masses, the simultaneous discovery of both the Higgs boson and the heavy neutrinos is probable at early stages of LHC operation. Results are presented for both Majorana and Dirac type fourth family neutrinos.
Imagine to discover a new fourth family of leptons at the Large Hadron Collider (LHC) but no signs of an associated fourth family of quarks. What would that imply? An intriguing possibility is that the new fermions needed to compensate for the new leptons gauge anomalies simultaneously address the big hierarchy problem of the Standard Model. A natural way to accomplish such a scenario is to have the Higgs itself be composite of these new fermions. This is the setup we are going to investigate in this paper using as a template Minimal Walking Technicolor. We analyze a general heavy neutrino mas…
The first evidence of new strong interactions may be a sufficiently massive fourth family observed at the LHC. The fourth family masses, of the leptons in particular, are constrained by the electroweak precision data, and this leads to signatures at the LHC that may imply early discovery. We study the implications of this discovery from a bottom-up perspective, where effective 4-fermion operators model the dominant effects of the new dynamics. We identify simple approximate symmetries of these operators that may be required for realistic masses of the third and fourth families. The large top m…
If fourth family condensates are responsible for electroweak symmetry breaking then they may also break approximate global symmetries. Among the resulting pseudo-Goldstone bosons are those that can have diquark quantum numbers. We describe the variety of diquarks and their decay modes, and we find aspects that are particular to the fourth family framework. Spectacular signatures at the LHC appear and are explored for color sextet diquarks with 600 GeV mass. We consider a simple search strategy which avoids diquark reconstruction. We also consider 350 GeV mass diquarks that are accessible at th…
Condensates of colored fermions driving electroweak symmetry breaking could give rise to a light pseudoscalar that would show up in the Higgs search at the LHC. The branching ratios of its decay will distinguish it from the Higgs and illuminate its origin. In particular the colored fermions may be the techniquarks of a one family technicolor theory or the quarks of a strongly interacting fourth family. The former has more difficulties in this context.
CKM mixing between third family quarks and a possible fourth family is constrained by global fits to the precision electroweak data. The dominant constraint is from nondecoupling oblique corrections rather than the Zbb vertex correction used in previous analyses. The possibility of large mixing suggested by some recent analyses of FCNC processes is excluded, but 3-4 mixing of the same order as the Cabbibo mixing of the first two families is allowed.
We study the single production of fourth-family $t'$ quarks via the process $ep\to t'ν$ at Large Hadron electron Collider (LHeC). We calculate the background and signal cross sections for the mass range 300-800 GeV. It is shown that the LHeC can discover single $t'$ quark up to the mass of 800 GeV for the optimized mixing parameters.
We investigate the possible enhancement to the discovery of the heavy Higgs boson through the possible fourth SM family heavy neutrino. Using the channel h-> v4 v4->mu W mu W-> mu j j mu j j, it is found that for certain ranges of Higgs boson and v4 masses LHC could discover both of them simultaneously with 1 fb^-1 integrated luminosity.
We investigate the forward-backward asymmetries in the pair production of fourth family fermions through the new Z' interactions in the e+e- collisions. The Z' boson having family universal couplings can contribute to the pair production of fourth family fermions via the s-channel exchange. The linear colliders will provide a clean environment for the physics of Z' boson to measure its couplings precisely. The effects of the Z' boson to the asymmetries are shown to be important in some parameter regions for different Z' models. Among these parameters, the invariant mass distribution m_FFbar wi…
We study the single production of the fourth family quarks through the process pp--> Q'jX at the Large Hadron Collider (LHC). We have calculated the decay widths and branching ratios of the fourth family quarks (b' and t') in the mass range 300-800 GeV. The cross sections of signal and background processes have been calculated in a Monte Carlo framework. It is shown that the LHC can discover single t' and b' quarks if the CKM matrix elements |V_{t'q}|,|V_{qb'}|>=0.01.
Considering the present limits on the masses of fourth family quarks from the Tevatron experiments, the fourth family quarks are expected to have mass larger than the top quark. Due to their expected large mass they could have different dynamics than the quarks of three families of the Standard Model. The resonant production of the fourth family up type quark t' has been studied via anomalous production subprocess gq_i-->t' (where q_i=u,c) at the LHC with the center of mass energy 10 TeV and 14 TeV. The signatures of such process are discussed within the SM decay modes. The sensitivity to anom…
It is known that Flavor Democracy favors the existence of the fourth standard model (SM) family. In order to give nonzero masses for the first three family fermions Flavor Democracy has to be slightly broken. A parametrization for democracy breaking, which gives the correct values for fundamental fermion masses and, at the same time, predicts quark and lepton CKM matrices in a good agreement with the experimental data, is proposed. The pair productions of the fourth SM family Dirac $(ν_{4})$ and Majorana $(N_{1})$ neutrinos at future linear colliders with $\sqrt{s}=500$ GeV, 1 TeV and 3 TeV ar…
For a model with a fourth family of quarks, new sources of flavor changing neutral currents are identified by confronting the unitary 4x4 quark mixing matrix with the experimental measured values of the familiar 3x3 quark mixing matrix. By imposing as experimental constraints the known bounds for the flavor changing neutral currents, the largest mixing of the known quarks with the fourth family ones is established. The predictions are: a value for |V_{tb}| significantly different from unity, large rates for rare top decays as t->cγand t->cZ, the last one reachable at the Large Hadrion Collider…
In models of dynamical electroweak symmetry breaking due to strongly coupled fourth-family quarks and leptons, their low-energy effective descriptions may involve multiple composite Higgs fields, leading to a possibility that the electroweak phase transition at finite temperature is first order due to the Coleman-Weinberg mechanism. We examine the behavior of the electroweak phase transition based on the effective renormalizable Yukawa theory which consists of the fourth-family quarks and two SU(2)-doublet Higgs fields corresponding to the bilinear operators of the fourth-family quarks with/wi…
The single production of fourth family sneutrino $\tildeν_{4}$ via R-parity violating interactions in electron-positron collisions has been investigated. We study the decays of $\tildeν_{4}$ into different flavor dilepton $e^{\pm}μ^{\mp}$ via R-parity violation. It is shown that R-parity violating couplings $(λ_{411},λ_{412})$ down to 0.001 will be reachable at future linear colliders which would provide better accuracy comparing to the indirect measurements as complementary to the LHC results.
The observed pattern of neutrino mass splittings and mixing angles indicates that their family structure is significantly different from that of the charged fermions. We investigate the implications of these data for the fermion mass matrices in grand unified theories with a type-I seesaw mechanism. We show that, with simple assumptions, naturalness leads to a strongly hierarchical Majorana mass matrix for heavy right-handed neutrinos and a partially cascade form for the Dirac neutrino matrix. We consider various model building scenarios which could alter this conclusion, and discuss their con…
From the present limits on the masses and mixings of fourth family quarks, they are expected to have mass larger than the top quark and allow a large range of mixing of the third family. They could also have different dynamics than the quarks of three families of the Standard Model. The single production of the fourth family up type quark t' has been studied via anomalous production process pp-> t'VX (where V=g,Z,γ) at the LHC with the center of mass energy of 7 and 14 TeV. The signatures of such process are discussed within both the SM decay modes and anomalous decay modes of t' quarks. The s…
A heavy fourth family is an example of new physics which is well defined and familiar in some respects, but which nevertheless has radical implications. In particular it eliminates a light Higgs description of electroweak symmetry breaking. We discuss an early signal for heavy quarks at the LHC in the form of an excess of "$W$-jets", and as well show how $W$-jets may be useful in the reconstruction of the heavy quark masses. We argue that fourth family quarks can be distinguished from vector-like quarks of a similar mass at roughly the same time that a same sign lepton signal becomes visible. …
We propose a dynamical model with a (2 + 1)-structure of composite Higgs doublets: two nearly degenerate composites of the fourth family quarks t' and b', $Φ_{t^{\prime}} \sim \bar{t^{\prime}}_{R}(t^{\prime},b^{\prime})_L$ and $Φ_{b^{\prime}} \sim \bar{b^{\prime}}_{R}(t^{\prime},b^{\prime})_L$, and a heavier top-Higgs resonance $Φ_t \sim \bar{t}_{R}(t,b)_L$. This model naturally describes both the top quark mass and the electroweak symmetry breaking. Also, a dynamical mechanism providing the quark mass hierarchy can be reflected in the model. The properties of these composites are analyzed in …
The possibility to violate baryon or lepton number without introducing any new flavor structures, beyond those needed to account for the known fermion masses and mixings, is analyzed. With four generations, but only three colors, this minimality requirement is shown to lead to baryon number conservation, up to negligible dimension-18 operators. In a supersymmetric context, this same minimality principle allows only superpotential terms with an even number of flavored superfields, hence effectively enforces R-parity both within the MSSM and in a GUT context.
The existence of fourth family follows from the basics of the Standard Model. Because of the high masses of the fourth family quarks, their anomalous decays could be dominant, if certain criteria are met. This will drastically change the search strategy at hadron colliders. We show that the fourth SM family down quarks with masses up to 400-450 GeV can be observed (or excluded) via anomalous decays by Tevatron before the LHC.
The existence of a fourth fermion generation has mostly been considered as a source of enhanced Higgs signals with respect to the 3 family Standard Model predictions. However, a fourth Standard Model family neutrino could cause the opposite situation. It is shown that relatively light fourth family neutrino (2m_(nu_(4))<m_(H)) could drastically change the interpretation of the search results for the Higgs boson, especially if m_(H)<170 GeV.
The fourth Standard Model (SM) family quarks and weak iso-singlet quarks The fourth Standard Model (SM) family quarks and weak iso-singlet quarks predicted by ${\rm E}_{6\text{}}$ GUT are considered. The spin-average of the pseudoscalar $η_{4}(n^{1}S_{0})$ and vector $ψ_{4}(n^{3}S_{1})$ quarkonium binding masses of the new mesons formed by the fourth Standard Model (SM) family and iso-singlet ${\rm E}_{6\text{}}$ with their mixings to ordinary quarks are investigated. Further, the fine and hyperfine mass splittings of the these states are also calculated. We solved the Schrödinger equation wit…
Resonant production of fourth chiral family up quark at the LHC via anomalous interactions have been analyzed. It is shown that search for resonances in W^{+}b final states could lead to discovery of the fourth chiral family and simultaneously determine scale of the new physics, presumabely related to the quark and lepton compositeness. Obtained results emphasize an importance of W-leading jet invariant mass analysis in search for W+jets final states at the LHC, both with and without b-tagging.
We revisit a three-family Pati-Salam model with a realistic phenomenology from intersecting D6-branes in Type IIA string theory compactified on a $\mathbf{T}^6/(\mathbb{Z}_2 \times \mathbb{Z}_2)$ orientifold, and study its naturalness in view of the current LHC and dark matter searches. We discuss spectrum and phenomenological features of this scenario demanding fine tuning better than 1\%. This requirement restricts the lightest neutralino to have mass less than about 600 GeV. We observe that the viable parameter space is tightly constrained by the requirements of naturalness and consistency …
In the spin-charge-family theory four massless families of quarks and leptons before the electroweak break are predicted. Mass matrices of all the family members demonstrate in this proposal the same symmetry, determined by the family groups. There are scalar fields - two $SU(2)$ triplets, the gauge fields of the family quantum numbers, and three singlets, the gauge fields of the three charges ( $Q, Q'$ and $Y'$)- all doublets with respect to the weak charge, which determine mass matrices on the tree level and, together with other contributions, also beyond the tree level. The symmetry of mass…
We provide a theoretical justification for the existence of fourth family fermions of the Standard Model by showing that in a novel form of spontaneous $CP$ breaking, fourth family is inevitable. This requires that fermions of third and fourth families must be mirror matter conjectured long back by Lee and Yang.
We discuss $Z$ mediated flavour changing neutral currents within a model where the hierarchical quark and lepton masses are explained via a fourth vector-like family, together with a scalar sector consisting of two Higgs doublets augmented by a gauge singlet scalar field that spontaneously breaks an extra global $U(1)^{\prime}$ symmetry. The $Z$ mediated flavor violating interactions arise from the mixings between the SM fermions and the vector-like fermions, where the mixing is discussed in an analytic approximation and also exactly numerically. We first discuss charged lepton flavor violatin…
The Standard Model (SM) is amended by one generation of quarks and leptons which are vector-like (VL) under the SM gauge group but chiral with respect to a new $\mathrm{U}(1)_{3-4}$ gauge symmetry. We show that this model can simultaneously explain the deviation of the muon $g-2$ as well as the observed anomalies in $b\rightarrow sμ^+μ^-$ transitions without conflicting with the data on Higgs decays, lepton flavor violation, or $B_s-\bar{B}_s$ mixing. The model is string theory motivated and GUT compatible, i.e. UV complete, and fits the data predicting VL quarks, leptons and a massive $Z'$ at…
I introduce a model to explain the recent anomalies in the muon anomalous magnetic moment and the rare semi-leptonic $B$ meson decays. The Standard Model is extended by a $U(1)^\prime$ gauge symmetry and a complete fourth family fermions which are vector-like under the gauge symmetry. We found parameter points which can explain the anomalies consistently with the other observables. We then propose an interesting possibility to have signals with four muons or more from vector-like lepton pair production.
We study the detection of the $t^\prime$ of a fourth family during the early running of LHC with 7 TeV collision energy and 1 fb$^{-1}$ integrated luminosity. By use of a neural network we show that it is feasible to search for the $t'$ even with a mass close to the unitarity upper bound, which is in the 500 to 600 GeV range. We also present results for the Tevatron with $10 \,\, \textrm{fb}^{-1}$. In both cases the search for a fourth family quark doublet can be significantly enhanced if one incorporates the contribution that the $b'$ can make to a $t'$-like signal. Thus the bound on the mass…
Limits on the Higgs boson mass restrict CKM mixing of a possible fourth family beyond the constraints previously obtained from precision electroweak data alone. Existing experimental and theoretical bounds on the Higgs mass already significantly restrict the allowed parameter space. Zero CKM mixing is excluded and mixing of order the Cabibbo angle is allowed. Upper and lower limits on 3-4 CKM mixing are exhibited as a function of the Higgs mass. We use the default inputs of the Electroweak Working Group and also explore the sensitivity of both the three and four family fits to alternative inpu…
Possible single productions of the fourth standard model family charged leptons via anomalous interactions at the future ep colliders are studied. Signatures of such anomalous processes and backgrounds are discussed in detail.
On-going Higgs searches in the light mass window are of vital importance for testing the Higgs mechanism and probing new physics beyond the standard model (SM). The latest ATLAS and CMS searches for the SM Higgs boson at the LHC (7TeV) found some intriguing excesses of events in the γγ/VV^* channels (V=Z,W) around the mass-range of 124-126 GeV. We explore a possible explanation of the γγand VV^* signals from the light CP-odd Higgs A^0 or CP-even Higgs h^0 from the general two-Higgs-doublet model with fourth-family fermions. We demonstrate that by including invisible decays of the Higgs boson A…
The flavor democracy favors the existence of the nearly degenerate fourth SM family, whereas the fifth SM family is disfavored both by the mass phenomenology and precision tests of the Standard Model. The multi-hundreds GeV fourth family quarks will be copiously produced at the LHC. However, the first indication of the fourth family may come from the Higgs search at the upgraded Tevatron.
We study the possibility of dark matter in the form of heavy neutrinos from a fourth lepton family with helicity suppressed couplings such that dark matter is produced thermally via annihilations in the early Universe. We present all possible constraints for this scenario coming from LHC and collider physics, underground direct detectors, neutrino telescopes, and indirect astrophysical searches. Although we embed the WIMP candidate within a model of composite dynamics, the majority of our results are model independent and applicable to all models where heavy neutrinos with suppressed couplings…
We perform an exploratory study of the allowed parameter range for the CKM-like mixing of hypothetical quarks of a fourth generation. As experimental constraints we use the tree-level determinations of the 3 $\times$ 3 CKM elements and FCNC processes ($K$-, $D$-, $B_d$-, $B_s$-mixing and the decay $b \to s γ$) under the assumption that the 4 $\times$ 4 CKM matrix is unitary. For the FCNCs we use some simplifying assumptions concerning the QCD corrections. Typically small mixing with the fourth family is favoured, but contrary to expectation we find that also a quite large mixing with the 4th f…
We show that a fourth family of quarks with $m_{t'}$ in the range of (400 - 600) GeV provides a rather simple explanation for the several indications of new physics that have been observed involving CP asymmetries of the b-quark. The built-in hierarchy of the 4$\times$4 mixing matrix is such that the $t'$ readily provides a needed {\it perturbation} ($\approx 15%$) to $\sin 2 β$ as measured in $B \to ψK_s$ and simultaneously is the dominant source of CP asymmetry in $B_s \to ψϕ$. The correlation between CP asymmetries in $B_s \toψϕ$ and $B_d\toϕK_s$ suggests $m_{t'} \approx$ (400 - 600) GeV. S…
We present Higgs boson mass bounds in a lattice regularization allowing thus for non-perturbative investigations. In particular, we employ a lattice modified chiral invariant Higgs-Yukawa model using the overlap operator. We show results for the upper and lower Higgs boson mass bounds in the presence of a heavy mass-degenerate quark doublet with masses ranging up to 700 GeV. We perform infinite volume extrapolations in most cases, and examine several values of the lattice cutoff. Furthermore, we argue that the lower Higgs boson mass bound is stable with respect to the addition of higher dimens…
We consider the possibility that a massive fourth family neutrino, predicted by a recently proposed minimal technicolor theory, could be the source of the dark matter in the universe. The model has two techniflavors in the adjoint representation of an SU(2) techicolor gauge group and its consistency requires the existence of a fourth family of leptons. By a suitable hypercharge assignement the techniquarks together with the new leptons look like a conventional fourth standard model family. We show that the new (Majorana) neutrino N can be the dark matter particle if $m_N \sim 100-500$ GeV and …
In this paper we present an in-depth analysis of a recently proposed Standard Model extension with a complete fourth generation of quarks and leptons, which are vector-like with respect to the Standard Model gauge group and charged under a new spontaneously broken vector-like $\mathrm{U(1)^\prime}$ gauge symmetry. The model is designed to explain the known muon anomalies, i.e. the observed deviations from Standard Model predictions in the anomalous magnetic moment of the muon, $Δa_μ$, and in $b \rightarrow s \ell^+ \ell^-$ processes. We perform a global $χ^2$ analysis of the data with $65$ mod…
