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Marco Fabbrichesi

Publications and source records attributed to Marco Fabbrichesi.

33 records · Page 2Linked to original sources
arXiv2015

Natural minimal dark matter

We show how the Higgs boson mass is protected from the potentially large corrections due to the introduction of minimal dark matter if the new physics sector is made supersymmetric. The fermionic dark matter candidate (a 5-plet of $SU(2)_L$) is accompanied by a scalar state. The weak gauge sector is made supersymmetric and the Higgs boson is embedded in a supersymmetric multiplet. The remaining standard model states are non-supersymmetric. Non vanishing corrections to the Higgs boson mass only appear at three-loop level and the model is natural for dark matter masses up to 15 TeV--a value larger than the one required by the cosmological relic density. The construction presented stands as an example of a general approach to naturalness that solves the little hierarchy problem which arises when new physics is added beyond the standard model at an energy scale around 10 TeV.

hep-ph↗
arXiv2015

Vector boson scattering at the LHC. A study of the WW -> WW channels with the Warsaw cut

We study W boson scattering in the same- and opposite-sign channels under the assumption that no resonances are present in the collider processes $pp\rightarrow l^{\pm}ν_{l}l^{\pm}ν_{l}jj$ and $pp\rightarrow l^{\pm}ν_{l}l^{\mp}ν_{l}jj$, respectively. Basic selection cuts together with a restriction on the combination of the final lepton and jet momenta (the Warsaw cut) makes it possible to argue that at the LHC a luminosity of 100 fb$^{-1}$ and a center-of-mass energy of $\sqrt{s}= 13$ TeV will allow to constrain the leading effective lagrangian coefficients at the permil level. We also discuss limits on the other coefficients of the effective lagrangian as well as stronger constraints provided by higher energy and luminosity. We show that the same-sign $WW \rightarrow WW$ channel suffices in providing the most stringent constraints.

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arXiv2015

Naturalness redux

The idea of naturalness, as originally conceived, refers only to the finite renormalization of the Higgs boson mass induced by the introduction of heavier states. In this respect, naturalness is still a powerful heuristic principle in model building beyond the standard model whenever new massive states are coupled to the Higgs field. The most compelling case is provided by the generation of neutrino masses. In this paper we confront this problem from a new perspective. The right-handed sector responsible for the seesaw mechanism---which introduces a large energy threshold above the electroweak scale---is made supersymmetric to comply with naturalness while the standard model is left unchanged and non-supersymmetric. Cancellations necessary to the naturalness requirement break down only at two loops, thus offering the possibility to increase the right-handed neutrino mass scale up to one order of magnitude above the usual values allowed by naturalness. If also the weak boson sector of the standard model is made supersymmetric, cancellations break down at three loops and the scale of new physics can be further raised. In the type-I seesaw, this implementation provides right-handed neutrino masses that are natural and at the same time large enough to give rise to baryogenesis (via leptogenesis). The model contains a dark matter candidate and distinctive new physics in the leptonic sector.

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arXiv2008

Dynamical electro-weak symmetry breaking from deformed AdS: vector mesons and effective couplings

We study a modification of the five-dimensional description of dynamical electro-weak symmetry breaking inspired by the AdS/CFT correspondence. Conformal symmetry is broken in the low-energy region near the IR brane by a power-law departure from the pure AdS background. Such a modification--while not spoiling the identification of the IR brane with the scale of confinement-- has a dramatic effect on both the coupling of the first composite states to the standard model currents and their self-couplings. Chiral symmetry breaking can take place at a scale larger than the IR cut-off. This study shows that observables, such as the precision parameter $\hat{S}$, which depend on the couplings of the lightest composite states to the currents are very sensitive to the details of the dynamics in the low energy region where conformal symmetry is lost and electro-weak symmetry is broken just above the scale of confinement. Therefore results of calculations of these observables in AdS/CFT inspired scenarios should be interpreted conservatively. The most important phenomenological consequence for physics at the LHC is that the bound on the mass scale of the heavy excitations (technirho mesons) in a realistic model is in general lower than in the pure AdS background with a simple hard-wall cut off in the IR.

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arXiv2004

A heavy Higgs boson from flavor and electroweak symmetry breaking unification

We present a unified picture of flavor and electroweak symmetry breaking based on a nonlinear sigma model spontaneously broken at the TeV scale. Flavor and Higgs bosons arise as pseudo-Goldstone modes. Explicit collective symmetry breaking yields stable vacuum expectation values and masses protected at one loop by the little-Higgs mechanism. The coupling to the fermions generates well-definite mass textures--according to a U(1) global flavor symmetry--that correctly reproduce the mass hierarchies and mixings of quarks and leptons. The model is more constrained than usual little-Higgs models because of bounds on weak and flavor physics. The main experimental signatures testable at the LHC are a rather large mass m_{h^0} = 317\pm 80 GeV for the (lightest) Higgs boson and a characteristic spectrum of new bosons and fermions at the TeV scale.

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arXiv2000

An updated analysis of eps'/eps in the standard model with hadronic matrix elements from the chiral quark model

We discuss the theoretical and experimental status of the CP violating ratio eps'/eps. We revise our 1997 standard-model estimate-based on hadronic matrix elements computed in the chiral quark model up to O(p^4) in the chiral expansion-by including an improved statistical analysis of the uncertainties and updated determination of the Cabibbo-Kobayashi-Maskawa elements and other short-distance parameters. Using normal distributions for the experimental input data we find Re eps'/eps = (2.2 \pm 0.8) x 10^{-3}, whereas a flat scanning gives 0.9 x 10^{-3} < Re eps'/eps < 4.8 x 10^{-3}. Both results are in agreement with the current experimental data. The key element in our estimate is, as before, the fit of the Delta I=1/2 rule, which allows us to absorb most of the theoretical uncertainties in the determination of the model-dependent parameters in the hadronic matrix elements. Our semi-phenomenological approach leads to numerical stability against variations of the renormalization scale and scheme dependence of the short- and long-distance components. The same dynamical mechanism at work in the selection rule also explains the larger value obtained for \ratio with respect to other estimates. A coherent picture of K -> pi pi decays is thus provided.

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arXiv2000

eps'/eps in the standard model

I discuss the estimate of the CP-violating ratio eps'/eps by stressing the role played by the chiral quark model in predicting the experiment and in showing that the same dynamical mechanism at work in the Delta I=1/2 rule also explains the larger value obtained for eps'/eps in this model with respect to other estimates.

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arXiv2000

Estimating eps'/eps in the standard model independently of Im λ_t

The CP-violating parameter eps'/eps is estimated in a novel way by including the explicit computation of eps in the ratio as opposed to the usual procedure of taking its value from the experiments. This approach has the advantage of being independent from the determination of the CKM parameters Im λ_t and of showing more directly the dependence on the long-distance parameters. The matrix elements are taken from the chiral quark model approach, the parameters of which have been recently determined by an updated fit of the CP-conserving amplitudes entering the ΔI=1/2 rule. By taking a normal distribution for the experimental inputs and a flat one for the theoretical uncertainties, it is found that eps '/eps = (2.5 \pm 0.9) \times 10^{-3} in agreement with the current experimental data. A more conservative estimate 0.9 x 10^{-3} < eps'/eps < 5.8 x 10^{-3} is found by considering the smallest and the largest values obtained by taking a flat distribution for all inputs.

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arXiv1999

Estimating eps'/eps . A user's manual

I review the current theoretical estimates of the CP-violating parameter eps'/eps, compare them to the experimental result and suggest a few guidelines in using the theoretical results.

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arXiv1999

Lattice matrix elements confronting the experimental value of eps'/eps

A new lattice estimate of K -> 2 pi transitions claims, contrary to all other computations, that the hadronic matrix element for the gluon penguin operator Q_6 has opposite sign and, in addition, is much larger than the vacuum saturation approximation. We comment under what conditions (if any) it is possible to reconcile this lattice result with the experimental value of eps'/eps. The dramatic impact of new physics in the kaon system that seems to be required is not easily accommodated within our present theoretical understanding.

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arXiv1999

D-particles in the space-time of the shock wave

We study the interaction of two D-particles in the space-time of the shock wave. We first write the amplitude in string theory and find that, at large distances from the shock-wave source, the O(v^4) term in the relative velocity v is an α'-independent function of the interbrane separation b. The amplitude is therefore that of supergravity--for large b, only closed-string massless modes contribute. We then show how the same result is obtained in the matrix model (at small b) by setting up the formulation of the dimensionally reduced super Yang-Mills theory in the curved background of the shock wave.

hep-th↗
arXiv1998

Supergravity and matrix theory do not disagree on multi-graviton scattering

We compare the amplitudes for the long-distance scattering of three gravitons in eleven dimensional supergravity and matrix theory at finite N. We show that the leading supergravity term arises from loop contributions to the matrix theory effective action that are not required to vanish by supersymmetry. We evaluate in detail one type of diagram---the setting sun with only massive propagators---reproducing the supergravity behavior.

hep-th↗
arXiv1997

Four-Fermion Effective Interactions and Recent Data at HERA

We discuss the possibility of explaining the excess in the cross section at high-Q^2 recently found at HERA in terms of four-fermion effective interactions. To avoid the constraints from low-energy data, we select three special cases in which the contact interaction is a product of vector-vector, axial-axial and vector-axial currents. For these a satisfactory fit of HERA data is possible, while keeping agreement with LEPII and CDF data, for an interaction scale $Λ=3.5$ TeV. As the statistics of the experiments improves-and if the effect persists-it will be soon possible to discriminate between such contact terms and the alternative explanation in terms of leptoquark exchange.

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arXiv1996

The supersymmetric prediction for the muon transverse polarization in the $K^+ \leftarrow π^0 μ^+ ν_μ$ decay

The muon transverse polarization in the $K^+ \leftarrow π^0 μ^+ ν_μ$ decay will be measured at the $10^{-4}$ level in forthcoming experiments. We compare the phenomenological perspectives with the theoretical predictions in supersymmetric extensions of the standard model. In the minimal extension, CP-violating phases lead to a non-zero transverse polarization, that however is too small to account for a positive experimental signal. The problems that one encounters when departing from minimal assumptions are discussed. An observable effect is possible if the hypothesis of R-parity conservation is relaxed, but only at the price of assuming a very special pattern for the R-parity breaking couplings.

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