Composite Leptons Research Articles

Extending global fits of 4D Composite Higgs Models with partially composite leptons

We perform the first convergent Bayesian global fits of 4D Composite Higgs Models with partially-composite third generation quarks and leptons based on the minimal SO(5) → SO(4) symmetry breaking pattern. We consider two models with the τ lepton and its associated neutrino in different representations of SO(5). Fitting each model with a wide array of experimental constraints allows us to analyse the Bayesian evidence and currently-observed fine-tuning of each model by calculating the Kullback-Leibler divergence between their respective priors and posteriors. Notably both models are found to be capable of satisfying all constraints simultaneously at the 3σ level at scales of < 5 TeV. From a Bayesian viewpoint of naturalness the model with leptons in the 14 and 10 representations is preferred over those in the 5 representation due to its lower fine-tuning. Finally, we consider the experimental signatures for the preferred parameters in these models, including lepton partner decay signatures and gluon-fusion produced Higgs signal strengths, and discuss their potential phenomenology at future high-luminosity LHC runs.

Proton decay from quark and lepton compositeness

Within a chiral SU(15) gauge theory in which the Standard Model fermions are bound states of massless preons, we show that proton-decay operators are likely induced at the compositeness scale, Λpre. Our estimate of the limit imposed by searches for proton decays is Λpre> 104 TeV1/2 {C}_8^{1/4} {m}_{mathcal{Q}}^{1/2} , where C8 is a rescaled coefficient of an 8-prebaryon operator induced by SU(15) dynamics, and {m}_{mathcal{Q}} is the mass of a composite vectorlike quark. The latter has a lower limit related to the mass of a composite vectorlike lepton, which in turn is required by LHC searches to be above 1 TeV. For C8 in the 10−5 – 1 range, the lower limit on Λpre varies between 3 × 103 TeV and 5 × 104 TeV. We point out that exotic proton decay modes, into a π+ and a heavy right-handed neutrino, could be observed using the Super-Kamiokande or DUNE detectors.

Testing the mechanism of lepton compositeness

Strict gauge invariance requires that physical left-handed leptons are actually bound states of the elementary left-handed lepton doublet and the Higgs field within the standard model. That they nonetheless behave almost like pure elementary particles is explained by the Fr"ohlich-Morchio-Strocchi mechanism. Using lattice gauge theory, we test and confirm this mechanism for fermions. Though, due to the current inaccessibility of non-Abelian gauged Weyl fermions on the lattice, a model which contains vectorial leptons but which obeys all other relevant symmetries has been simulated.

The bearable compositeness of leptons

Partial compositeness as a theory of flavor in the lepton sector is assessed. We begin presenting the first systematic analysis of neutrino mass generation in this context, and identifying the distinctive mass textures. We then update the bounds from charged lepton flavor and CP violating observables. We put forward a U(1)3 × CP symmetry of the composite sector, in order to allow the new physics to be not far above the TeV scale. This hypothesis effectively suppresses the new contributions to the electron EDM and μ → eγ, by far the most constraining observables, and results in a novel pattern of flavor violation and neutrino masses. The CP violation in the elementary-composite mixing is shown to induce a CKM phase of the correct size, as well as order-one phases in the PMNS matrix. We compare with the alternative possibility of introducing multiple scales of compositeness for leptons, that also allow to evade flavor and CP constraints. Finally, we examine violations of lepton flavor universality in B-meson semi-leptonic decays. The neutral-current anomalies can be accommodated, predicting strong correlations among different lepton flavors, with a few channels close to the experimental sensitivity.

LHC signals for singlet neutrinos from a natural warped seesaw mechanism. II

A natural seesaw mechanism for obtaining the observed size of SM neutrino masses can arise in a warped extra dimensional/composite Higgs framework. In a previous paper, we initiated the study of signals at the LHC for the associated $\sim$ TeV mass SM singlet neutrinos, within a canonical model of $SU(2)_L \times SU(2)_R \times U(1)_{ B - L }$ (LR) symmetry in the composite sector, as motivated by consistency with the EW precision tests. Here, we investigate LHC signals in a different region of parameter space for the same model, where production of singlet neutrinos can occur from particles beyond those in usual LR models. Specifically, we assume that composite $(B - L)$ gauge boson is lighter than all the others in the EW sector. We show that the composite $(B - L)$ gauge boson can acquire a significant coupling to light quarks simply via mixing with elementary hypercharge gauge boson. Thus, the singlet neutrino can be pair-produced via decays of $(B - L)$ gauge boson, without a charged current counterpart. Furthermore, there is no decay for $(B - L)$ gauge boson directly into dibosons, unlike for the usual case of $W_R^{ \pm }$ and $Z^{ \prime }$. Independently of the above extension of the EW sector, we analyze production of singlet neutrinos in decays of composite partners of $SU(2)_L$ doublet leptons, which are absent in the usual LR models. In turn, these doublet leptons can be produced in composite $W_L$ decays. We show that $4 - 5 \sigma$ signal can be achieved for both cases described above for the following spectrum with 3000 fb$^{-1}$ luminosity: $2 - 2.5$ TeV composite gauge bosons, $1$ TeV composite doublet lepton (for the second case) and $500 - 750$ GeV singlet neutrino.

The Matter-Antimatter Asymmetry Problem

The matter-antimatter asymmetry problem, corresponding to the virtual nonexistence of antimatter in the universe, is one of the greatest mysteries of cosmology. According to the prevailing cosmological model, the universe was created in the so-called “Big Bang” from pure energy and it is generally considered that the Big Bang and its aftermath produced equal numbers of particles and antiparticles, although the universe today appears to consist almost entirely of matter rather than antimatter. This constitutes the matter-antimatter asymmetry problem: where have all the antiparticles gone? Within the framework of the Generation Model (GM) of particle physics, it is demonstrated that the asymmetry problem may be understood in terms of the composite leptons and quarks of the GM. It is concluded that there is essentially no matter-antimatter asymmetry in the present universe and that the observed hydrogen-antihydrogen asymmetry may be understood in terms of statistical fluctuations associated with the complex many-body processes involved in the formation of either a hydrogen atom or an antihydrogen atom.

Constraining fine tuning in composite Higgs models with partially composite leptons

Minimal Composite Higgs Models (MCHM) have long provided a solution to the hierarchy problem of the Standard Model, yet suffer from various sources of fine tuning that are becoming increasingly problematic with the lack of new physics observations at the LHC. We develop a new fine tuning measure that accurately counts each contribution to fine tuning (single, double, triple, etc) that can occur in a theory with np parameters, that must reproduce no observables. We then use a novel scanning procedure to perform a comprehensive study of three different two-site, 4D, SO(5) → SO(4) MCHMs with all third generation fermions included, distinguished by the choice of the lepton embeddings. These are the MCHM5 − 5 − 55 − 5 − 5, MCHM14 − 14 − 105 − 5 − 5 and MCHM14 − 1 − 105 − 5 − 5, where MCHMl − τ − νq − t − b has the lepton doublet partner in representation l, tau partner in representation τ, and so on. We find that embedding at least one massive lepton in the symmetric 14 of SO(5) moderately reduces the tuning for the case of low top partner masses (in line with previous results), but that this is balanced against the increased complexity of the model when one properly accounts for all sources of fine tuning. We study both the current relative fine-tuning of each scenario, and the future prospects. Noting that the different scenarios behave differently with respect to future improvements in collider measurements, we find that the MCHM14 − 1 − 105 − 5 − 5 enjoys a relatively low increase in fine tuning even for a future lower bound on the top partner masses of 3.4 TeV (or equivalently a maximum Higgs-fermion or Higgs-gluon coupling deviation of 2%).

Searching for spin- 3/2 leptons

A model of spin-$3/2$ vectorlike leptons is introduced, which serve as an alternative probe of possible lepton compositeness. Possible signatures at the Large Hadron Collider are explored, and current data are used to constrain the masses of the hypothetical new leptons.

Can galileons solve the muon problem?

The leptonic bound states positronium and muonium are used to constrain galileon contributions to the Lamb shift of muonic hydrogen. Through the application of a variety of bounds on lepton compositeness, it is shown that either the assumption of equating the charge radius of a particle with its galileon scale radius is incompatible with experiments or the scale of galileons must be $M>1.33$ GeV, too large to solve the muon problem. The possibility of stronger constraints in the future from true muonium are discussed.

Boundary conditions for a composite model of leptons and quarks

Because the existence of families of elements and hadrons was ultimately understood by the realization that atoms and hadrons are composite, an obvious approach to explaining the existence of lepton and quark families is to assume that the particles in these families are also composite. The mass and spin spectra of leptons and quarks suggest that if these particles are composite, they are most likely bound states of a scalar and spin-1/2 fermion interacting via electrodynamics. However, if they are composite, the bound states must be highly relativistic since in each family the least massive member has a small mass compared with the others. Also, composite leptons and quarks must be extremely tightly bound since no internal structure has ever been conclusively detected. Highly relativistic, bound-state, Bethe- Salpeter solutions of a scalar and a spin-1/2 fermion bound by minimal electrodynamics are discussed. These specific solutions cannot describe leptons or quarks as bound states because the magnitude of the charges of the constituents are an order of magnitude larger than e. The boundary conditions, however, allow solutions when the constituents have charges with magnitudes on the order of e.

Search for physics beyond the standard model in dilepton mass spectra in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

Dimuon and dielectron mass spectra, obtained from data resulting from proton-proton collisions at 8 TeV and recorded by the CMS experiment, are used to search for both narrow resonances and broad deviations from standard model predictions. The data correspond to an integrated luminosity of 20.6 (19.7) inverse femtobarns for the dimuon (dielectron) channel. No evidence for non-standard-model physics is observed and 95% confidence level limits are set on parameters from a number of new physics models. The narrow resonance analyses exclude a Sequential Standard Model Z'[SSM] resonance lighter than 2.90 TeV, a superstring-inspired Z'[psi] lighter than 2.57 TeV, and Randall-Sundrum Kaluza-Klein gravitons with masses below 2.73, 2.35, and 1.27 TeV for couplings of 0.10, 0.05, and 0.01, respectively. A notable feature is that the limits have been calculated in a model-independent way to enable straightforward reinterpretation in any model predicting a resonance structure. The observed events are also interpreted within the framework of two non-resonant analyses: one based on a large extra dimensions model and one based on a quark and lepton compositeness model with a left-left isoscalar contact interaction. Lower limits are established on M[S], the scale characterizing the onset of quantum gravity, which range from 4.9 to 3.3 TeV, where the number of additional spatial dimensions varies from 3 to 7. Similarly, lower limits on Lambda, the energy scale parameter for the contact interaction, are found to be 12.0 (15.2) TeV for destructive (constructive) interference in the dimuon channel and 13.5 (18.3) TeV in the dielectron channel.

Leptons in composite MFV

We study the lepton sector of composite Higgs models with partial compositeness. The standard anarchic scenario is in conflict with the absence of observable charged lepton flavor violation. This tension can be completely solved in MFV scenarios that require either left-handed or right-handed SM leptons to be equally composite. Constraints on this scenario are weak and the composite lepton partners could be as light as few hundreds GeVs with interesting LHC signatures. The contribution to the muon (g-2) in theories where the Higgs is a pseudo Nambu-Goldstone boson is also discussed.

Probing color octet electrons at the LHC

Models with quark and lepton compositeness predict the existence of colored partners of the Standard Model leptons. In this paper we study the LHC phenomenology of a charged colored lepton partner, namely the color octet electron, $e_8$ in an effective theory framework. We explore various mechanisms for resonant production of $e_8$'s. With the pair production channel the 14 TeV LHC can probe $e_8$'s with masses up to 2.5 TeV (2.8 TeV) with 100 fb$^{-1}$ (300 fb$^{-1}$) of integrated luminosity. A common feature in all the resonant production channels is the presence of two high $p_T$ electrons and at least one high $p_T$ jet in the final state. Using this feature, we implement a search method where the signal is a combination of pair and single production events. This method has potential to increase the LHC reach significantly. Using the combined signal we estimate the LHC discovery potential for the $e_8$'s. Our analysis shows that the LHC with 14 TeV center-of-mass energy and 100 fb$^{-1}$ (300 fb$^{-1}$) of integrated luminosity can probe $e_8$'s with masses up to 3.4 TeV (4 TeV) for the compositeness scale of 5 TeV.

Implications of Lepton Flavor Universality Violations inBDecays

Present measurements of b→cτν and b→uτν transitions differ from the standard model predictions of lepton flavor universality by almost 4σ. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super-)B factories are presented.

Phenomenology of excited doubly charged heavy leptons at the LHC

We consider the production at the LHC of exotic composite leptons of charge Q=+2e. Such states are allowed in composite models which contain extended isospin multiplets (Iw=1 and Iw=3/2). These doubly charged leptons couple with Standard Model [SM] fermions via gauge interactions, thereby delineating and restricting their possible decay channels. We discuss the production cross section at the LHC of L++ (p p --> L++, l-) and concentrate on the leptonic signature deriving from the cascade decays L++ --> W+, l+ --> l+, l+, \nu_l i.e. p p --> l-, l+, l+, \nu_l showing that the invariant mass distribution of the like-sign dilepton has a sharp end point corresponding to excited lepton mass m*. We find that the sqrt{s}=7 TeV run is sensitive at the 3-sigma (5-sigma) level to a mass of the order of 600 GeV if L=10 fb^-1 (L=20 fb^-1). The sqrt{s}=14 TeV run can reach a sensitivity at 3-sigma (5-sigma) level up to m*=1 TeV for L=20 fb^-1 (L=60 fb^-1).

A hybrid Higgs

We construct composite Higgs models admitting a weakly coupled Seiberg dual description. We focus on the possibility that only the up-type Higgs is an elementary field, while the down-type Higgs arises as a composite hadron. The model, based on a confining SQCD theory, breaks supersymmetry and electroweak symmetry dynamically and calculably. This simultaneously solves the \mu/B_\mu problem and explains the smallness of the bottom and tau masses compared to the top mass. The proposal is then applied to a class of models where the same confining dynamics is used to generate the Standard Model flavor hierarchy by quark and lepton compositeness. This provides a unified framework for flavor, supersymmetry breaking and electroweak physics. The weakly coupled dual is used to explicitly compute the MSSM parameters in terms of a few microscopic couplings, giving interesting relations between the electroweak and soft parameters. The RG evolution down to the TeV scale is obtained and salient phenomenological predictions of this class of "single-sector" models are discussed.

Dynamical supersymmetry breaking, with flavor

We explore calculable models with low-energy supersymmetry where the flavor hierarchy is generated by quark and lepton compositeness, and where the composites emerge from the same sector that dynamically breaks supersymmetry. The observed pattern of Standard Model fermion masses and mixings is obtained by identifying the various generations with composites of different dimension in the ultraviolet. These "single-sector" supersymmetry breaking models give rise to various spectra of soft masses which are, in many cases, quite distinct from what is commonly found in models of gauge or gravity mediation. In typical models which satisfy all flavor-changing neutral current constraints, both the first and second generation sparticles have masses of order 20 TeV, while the stop mass is near 1 TeV. In other cases, all sparticles obtain masses of order 1 TeV predominantly from gauge mediation, even though the first two generations are composite.

Status and commissioning of the ATLAS experiment at the LHC

The ATLAS experiment (A Toroidal LHC ApparatuS) is one of the two general purpose detectors (CMS is the other) at the Large Hadron Collider, at CERN (Geneva). It is designed to study p–p collision at a center of mass energy of s = 14 TeV and at a luminosity of 10 34 cm − 1 s − 1 . ATLAS with its dimensions, 46 m long, 25 m in diameter and a weight of about 7000 ton is the biggest collider experiment ever built. The project has involved roughly 2000 scientists and engineers of 165 institutions in 35 countries. The installation in the underground cavern is scheduled to be completed in July 2008 to be ready for the initial p–p collisions at a center of mass energy of s = 10 TeV by late summer 2008. The detector design has been optimized to cover the largest possible search of new physics at the LHC energy: Higgs bosons and alternative schemes for the spontaneous symmetry-breaking mechanism; supersymmetric particles, new gauge bosons, leptoquarks, quark and lepton compositeness, extra dimensions. At the same time high-precision measurements of Standard Model processes will be performed and this will allow more stringent tests of the present electroweak theory and an indirect search for physics beyond the Standard Model. This paper presents a comprehensive overview of the ATLAS detector prior to the first LHC collisions.

Supersymmetric composite models on intersecting D-branes

We construct supersymmetric composite models of quarks and leptons from type IIA T 6 / ( Z 2 × Z 2 ) orientifold with intersecting D6-branes. In case of T 6 = T 2 × T 2 × T 2 with no tilted T 2 , a composite model of supersymmetric SU ( 5 ) grand unified theory with four generations is constructed. In case of that one T 2 is tilted, a composite model with SU ( 3 ) c × SU ( 2 ) L × U ( 1 ) Y gauge symmetry with three generations of left-handed quarks and leptons is constructed. These models are not realistic, but contain relatively fewer additional exotic particles and U ( 1 ) gauge symmetries due to the introduction of the compositeness of quarks and leptons. The masses of some exotic particles are naturally generated through the Yukawa interactions among “preons”.

Composite quarks and leptons in higher space-time dimensions

A new approach towards the composite structure of quarks and leptons in the context of the higher dimensional unified theories is proposed. Because of certain strong dynamics much like ordinary QCD, every possible vectorlike set of composites appears in higher dimensional bulk space-time. However, through a proper Sherk-Schwarz compactification only chiral multiplets of composite quarks and leptons survive as the massless states in four dimensions. In this scenario restrictions related to 't Hooft's anomaly matching condition turn out to be avoided and, as a result, the composite models look rather simple and economical. We demonstrate our approach by an explicit construction of a model of preons and their composites unified in the supersymmetric $\mathrm{SU}(5)$ GUT in five space-time dimensions. The model predicts precisely three families of composite quarks and leptons being the triplets of the chiral horizontal symmetry ${\mathrm{SU}(3)}_{h}$ which automatically appears in the composite spectrum when going to ordinary four dimensions.