Gilad Perez

CERN LHC signals from warped extra dimensions

We study production of Kaluza-Klein (KK) gluons at the Large Hadron Collider (LHC) in the framework of a warped extra dimension with the standard model fields propagating in the bulk. We show that the detection of the KK gluon is challenging since its production is suppressed by small couplings to the proton’s constituents. Moreover, the KK gluon decays mostly to top pairs due to an enhanced coupling and hence is broad. Nevertheless, we demonstrate that for MKKG?4??TeV, 100??fb-1 of data at the LHC can provide discovery of the KK gluon. We utilize a sizable left-right polarization asymmetry from the KK gluon resonance to maximize the signal significance, and we explore the novel feature of extremely highly energetic “top-jets.” We briefly discuss how the detection of electroweak gauge KK states (Z/W) faces a similar challenge since their leptonic decays (golden modes) are suppressed. Our analysis suggests that other frameworks, for example, little Higgs, which rely on UV completion via strong dynamics might face similar challenges, namely, (1) suppressed production rates for the new particles (such as Z?), due to their “light-fermion-phobic” nature, and (2) difficulties in detection since the new particles are broad and decay predominantly to third generation quarks and longitudinal gauge bosons

Flavor Anarchy in a Randall-Sundrum Model with 5D Minimal Flavor Violation and a Low Kaluza-Klein Scale

We present a variant of the warped extra dimension, Randall-Sundrum (RS), framework which is based on five dimensional (5D) minimal flavor violation (MFV), in which the only sources of flavor breaking are two 5D anarchic Yukawa matrices. The Yukawa matrices also control the bulk masses, which are responsible for the resulting flavor structure and mass hierarchy in the low energy theory. An interesting result of this set-up is that at low energies the theory flows to next to MFV model where flavor violation is dominantly coming from the third generation. Low energy flavor violation is further suppressed by a single parameter that dials the amount of violation in the up or down sector. There is therefore a sharp limit in which there is no flavor violation in the down type quark sector which, remarkably, is favored when we fit for the flavor parameters. This mechanism is used to eliminate the current RS flavor and CP problem even with a Kaluza-Klein scale as low as 2 TeV! Our construction also suggests that economic supersymmetric and non-supersymmetric, strong dynamic-based, flavor models may be built based on the same concepts. Introduction. The standard model (SM) agrees very well with data. However, it is widely perceived to be an incomplete theory. In particular, in the SM, the hierarchy between the Planck scale and the electroweak (EW) symmetry breaking (EWSB) scale is unnatural since the Higgs mass is ultra-violet (UV) sensitive. Solutions to the hierarchy problem therefore involve extending the SM at just above the EWSB scale which, in general, spoils the good agreement of the SM with data when trying to explain flavor as well. Given this inherent tension, it is important to identify new physics (NP) frameworks that preserve the approximate symmetries of the SM. In this letter we consider the Randall-Sundrum scenario (RS1) [1], which potentially provides an elegant solution to the hierarchy problem. In this framework, due to warped higher-dimensional spacetime, the mass scales in an effective 4D description depend on location in an extra dimension: the Higgs sector is localized at the “TeV” brane where it is protected by a low warped-down fundamental scale of order a TeV while 4D gravity is localized near the “Planck” brane which has a Planckian fundamental scale. In the original RS1 model, the entire SM was localized on the TeV brane. In this set-up, flavor issues are sensitive to the UV completion of the RS1 effective field theory: there is no understanding of the hierarchies in fermion masses or of smallness of flavor changing neutral currents (FCNCs) from higher-dimensional operators that would be too large if suppressed only by the warped-down cut-off ∼ TeV. Similar tension arises when considering the model predictions regarding EW precision tests. Allowing the SM fermions and gauge fields to propagate in the bulk gives an opportunity to explain flavor, and makes flavor issues UV-insensitive as follows. The light fermions can be localized near the Planck brane (using a 5D fermion mass parameter [2, 3]) where the effective cut-off is much higher than TeV so that FCNCs from higher-dimensional operators are suppressed [3, 4]. Moreover, this results in small 4D Yukawa couplings to the Higgs, even if there are no small 5D Yukawa couplings [3, 4]. The top quark can be localized near the TeV brane to obtain a large 4D top Yukawa coupling. Because the fermion profiles depend exponentially on the bulk masses, this provides an understanding of the hierarchy of fermion masses (and mixing) without hierarchies in fundamental (5D) parameters, solving the SM flavor puzzle. However, with bulk fermions and gauge fields, calculable FCNCs from exchange of gauge Kaluza-Klein (KK) modes are induced. The couplings of light fermions to gauge KK modes are non-universal which induce FCNCs. Unlike the flat case, there is a significant protection from a built-in RS1-GIM [5] due to the approximate flatness of the KK gauge boson wavefunctions in the UV and a hierarchy in the fermion wavefunctions in the IR; nevertheless, the resulting contributions to FCNCs are nonnegligible. In [6] it was shown that at low energies this class of models flows to next to MFV (NMFV); that is, flavor changing effects are generated primarily through mixing with the third generation. Generically, within the NMFV framework flavor violation occurs through NP sources, with a typical scale of ΛNMFV, which breaks the

Substructure of high- p T jets at the LHC

We study high-p{sub T} jets from QCD and from highly boosted massive particles such as tops, W, Z, and Higgs bosons, and argue that infrared-safe observables can help reduce QCD backgrounds. Jets from QCD are characterized by different patterns of energy flow compared to the products of highly boosted heavy particle decays, and we employ a variety of jet shapes, observables restricted to energy flow within a jet, to explore this difference. Results from Monte Carlo generators and arguments based on perturbation theory support the discriminating power of the shapes we refer to as planar flow and angularities. We emphasize that for massive jets, these and other observables can be analyzed perturbatively.

Natural Neutrino Masses and Mixings from Warped Geometry

We demonstrate that flavor symmetries in warped geometry can provide a natural explanation for large mixing angles and economically explain the distinction between the quark and lepton flavor sectors. We show how to naturally generate Majorana neutrino masses assuming a gauged a U(1)B−L symmetry broken in the UV that generates see-saw masses of the right size. This model requires lepton minimal flavor violation (LMFV) in which only Yukawa matrices (present on the IR brane) break the flavor symmetries. The symmetry-breaking is transmitted to charged lepton bulk mass parameters as well to generate the hierarchy of charged lepton masses. With LMFV, a GIM-like mechanism prevents dangerous flavor-changing processes for charged leptons and permits flavor-changing processes only in the presence of the neutrino Yukawa interaction and are therefore suppressed when the overall scale for the neutrino Yukawa matrix is slightly smaller than one in units of the curvature. In this case the theory can be consistent with a cutoff of 10 TeV and 3 TeV Kaluza-Klein masses.

Implications of the LHCb Evidence for Charm CP Violation

The LHCb collaboration recently announced preliminary evidence for CP violation in D meson decays. We discuss this result in the context of the standard model (SM), as well as its extensions. In the absence of reliable methods to evaluate the hadronic matrix elements involved, we can only estimate qualitatively the magnitude of the non-SM tree level operators required to generate the observed central value. In the context of an effective theory, we list the operators that can give rise to the measured CP violation and investigate constraints on them from other processes.

The little Randall–Sundrum model at the large hadron collider

Abstract We present a predictive warped model of flavor that is cut off at an ultraviolet scale O ( 10 3 ) TeV . This “Little Randall–Sundrum (LRS)” model is a volume-truncation, by a factor y ≈ 6 , of the RS scenario and is holographically dual to dynamics with number of colors larger by y. The LRS couplings between Kaluza–Klein states and the Standard Model fields, including the proton constituents, are explicitly calculable without ad hoc assumptions. Assuming separate gauge and flavor dynamics, a number of unwanted contributions to precision electroweak, Z b b ¯ and flavor observables are suppressed in the LRS framework, compared with the corresponding RS case. An important consequence of the LRS truncation, independent of precise details, is a significant enhancement of the clean (golden) di-lepton LHC signals, by O ( y 3 ) , due to a larger “ρ-photon” mixing and a smaller inter-composite coupling.

Exclusive Window onto Higgs Yukawa Couplings

We show that both flavor-conserving and flavor-violating Yukawa couplings of the Higgs boson to first- and second-generation quarks can be probed by measuring rare decays of the form h→MV, where M denotes a vector meson and V indicates either γ, W or Z. We calculate the branching ratios for these processes in both the standard model and its possible extensions. We discuss the experimental prospects for their observation. The possibility of accessing these Higgs couplings appears to be unique to the high-luminosity LHC and future hadron colliders, providing further motivation for those machines.

Modified Higgs physics from composite light flavors

A bstractWe point out that Higgs rates into gauge bosons can be significantly modified in composite pseudo Nambu-Goldstone boson (pNGB) Higgs models if quarks belonging to the first two generation are relatively composite objects as well. Although the lightness of the latter a priori screen them from the electroweak symmetry breaking sector, we show, in an effective two-site description, that their partners can lead to order one shifts in radiative Higgs couplings to gluons and photons. Moreover, due to the pseudo-Goldstone nature of the Higgs boson, the size of these corrections is completely controlled by the degree of compositeness of the individual light quarks. The current measurements of flavor-blind Higgs decay rates at the LHC thus provide an indirect probe of the flavor structure of the framework of pNGB Higgs compositeness.

Deciphering top flavor violation at the CERN LHC with B factories

The CERN LHC will have unprecedented sensitivity to flavor-changing neutral current (FCNC) top quark decays, whose observation would be a clear sign of physics beyond the standard model. Although many details of top flavor violation are model dependent, the standard model gauge symmetries relate top FCNCs to other processes, which are strongly constrained by existing data. We study these constraints in a model-independent way, using a low energy effective theory from which the new physics is integrated out. We consider the most important operators which contribute to top FCNCs and analyze the current constraints on them. We find that the data rule out top FCNCs at a level observable at the LHC due to most of the operators comprising left-handed first or second generation quark fields, while there remains a substantial window for top decays mediated by operators with right-handed charm or up quarks. If FCNC top decays are observed at the LHC, such an analysis may help decipher the underlying physics.

Charming CP violation and dipole operators from RS flavor anarchy

A bstractRecently the LHCb collaboration reported evidence for direct CP violation in charm decays. The value is sufficiently large that either substantially enhanced Standard Model contributions or non-Standard Model physics is required to explain it. In the latter case only a limited number of possibilities would be consistent with other existing flavor-changing constraints. We show that warped extra dimensional models that explain the quark spectrum through flavor anarchy can naturally give rise to contributions of the size required to explain the the LHCb result. The D meson asymmetry arises through a sizable CP-violating contribution to a chromomagnetic dipole operator. This happens naturally without introducing inconsistencies with existing constraints in the up quark sector. We discuss some subtleties in the loop calculation that are similar to those in Higgs to γγ. Loop-induced dipole operators in warped scenarios and their composite analogs exhibit non-trivial dependence on the Higgs profile, with the contributions monotonically decreasing when the Higgs is pushed away from the IR brane. We show that the size of the dipole operator quickly saturates as the Higgs profile approaches the IR brane, implying small dependence on the precise details of the Higgs profile when it is quasi IR localized. We also explain why the calculation of the coefficient of the lowest dimension 5D operator is guaranteed to be finite. This is true not only in the charm sector but also with other radiative processes such as electric dipole moments, b → sγ, ϵ′/ϵK and μ → eγ. We furthermore discuss the interpretation of this contribution within the framework of partial compositeness in four dimensions and highlight some qualitative differences between the generic result of composite models and that obtained for dynamics that reproduces the warped scenario.