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Abdelhak Djouadi

Publications and source records attributed to Abdelhak Djouadi.

At least 73 records · Page 4Linked to original sources
arXiv2006

Resolving the A_{FB}^b puzzle in an extra dimensional model with an extended gauge structure

It is notorious that, contrary to all other precision electroweak data, the forward-backward asymmetry for b quarks $A_{FB}^b$ measured in Z decays at LEP1 is nearly three standard deviations away from the predicted value in the Standard Model; significant deviations also occur in measurements of the asymmetry off the Z pole. We show that these discrepancies can be resolved in a variant of the Randall-Sundrum extra-dimensional model in which the gauge structure is extended to $SU(2)_L \times SU(2)_R \times U(1)_X$ to allow for relatively light Kaluza-Klein excitations of the gauge bosons. In this scenario, the fermions are localized differently along the extra dimension, in order to generate the fermion mass hierarchies, so that the electroweak interactions for the heavy third generation fermions are naturally different from the light fermion ones. We show that the mixing between the Z boson with the Kaluza-Klein excitations allows to explain the $A_{FB}^b$ anomaly without affecting (and even improving) the agreement of the other precision observables, including the $Z \to bb$ partial decay width, with experimental data. Some implications of this scenario for the ILC are summarized.

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arXiv2006

The Higgs intense--coupling regime in constrained SUSY models and its astrophysical implications

We analyze the Higgs intense--coupling regime, in which all Higgs particles of the Minimal Supersymmetric Standard Model are light with masses of the same order and the value of $\tb$ the ratio of vacuum expectation values of the two Higgs fields is large, in the framework of Supergravity scenarios with non--universal soft Supersymmetry breaking scalar masses in the Higgs sector. In particular, we calculate the relic density abundance of the lightest neutralino candidate for cold dark matter and the rates in direct and indirect detection at present and future experiments. We first show that while in the mSUGRA model this regime is disfavored by present data, there are regions in the parameter space of models with non--universal Higgs masses where it can occur. We then show that because of the large value of $\tanβ$ and the relatively low values of the neutral Higgs boson masses, the cross section for neutralino--nucleon scattering is strongly enhanced in this regime and would allow for the observation of a signal in direct detection experiments such as CDMS--Soudan. The expected sensitivity of gamma--ray detectors like GLAST might be also sufficient to observe the annihilation of neutralinos in such a regime.

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arXiv2006

Dark Matter and the ILC

We discuss the solution to the Dark Matter problem provided by the lightest neutralino of the Minimal Supersymmetric Standard Model (MSSM) and highlight the role of the International Linear Collider (ILC) in determining its cosmological relic density.

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arXiv2006

Updated Constraints on the Minimal Supergravity Model

We provide an up-to-date analysis of the parameter space of the minimal supergravity model (mSUGRA). Novel features include the new central value of the top quark mass, an improved calculation of the masses of the supersymmetric particles and the neutral Higgs bosons, constraints from b to s \ell^+ \ell^- decays, and a careful treatment of the most important experimental and theoretical uncertainties. In addition to the by now traditional plots of the allowed region in the (m_0, m_{1/2}) plane, we show allowed regions in the planes spanned by pairs of {\em physical} sparticle or Higgs boson masses. Moreover, we search for the minimal allowed masses of new particles for various sets of constraints. We find that in many cases the direct experimental limits from collider and Dark Matter searches can be saturated even in this minimal model, and even after including the by now quite restrictive constraint on the Dark Matter relic density.

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arXiv2005

SuSpect: a Fortran Code for the Supersymmetric and Higgs Particle Spectrum in the MSSM

We present the Fortran code SuSpect version 2.3, which calculates the Supersymmetric and Higgs particle spectrum in the Minimal Supersymmetric Standard Model (MSSM). The calculation can be performed in constrained models with universal boundary conditions at high scales such as the gravity (mSUGRA), anomaly (AMSB) or gauge (GMSB) mediated breaking models, but also in the non-universal MSSM case with R-parity and CP conservation. Care has been taken to treat important features such as the renormalization group evolution of parameters between low and high energy scales, the consistent implementation of radiative electroweak symmetry breaking and the calculation of the physical masses of the Higgs bosons and supersymmetric particles taking into account the dominant radiative corrections. Some checks of important theoretical and experimental features, such as the absence of non desired minima, large fine-tuning in the electroweak symmetry breaking condition, as well as agreement with precision measurements can be performed. The program is user friendly, simple to use, self-contained and can easily be linked with other codes; it is rather fast and flexible, thus allowing scans of the parameter space with several possible options and choices for model assumptions and approximations.

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arXiv2005

Neutralino Dark Matter in mSUGRA: reopening the light Higgs pole window

The requirement that the lightest neutralino $\tildeχ_1^0$ has the right thermal relic density to explain all Dark Matter in the universe strongly constrains the parameter space of supersymmetric models in general, and of the mSUGRA model in particular. Recently improved calculations of the mass of the light CP-even Higgs boson $h$ present in this model, and the increased central value of the mass of the top quark, have re--opened the possibility that $2 \mlsp \lsim m_h$. In this ``$h-$pole region'' the LSP annihilation cross section is enhanced by near-resonant $h$ exchange in the $s-$channel, reducing the relic density to acceptable values. We delineate the corresponding region of mSUGRA parameter space, and explore its phenomenology. In particular, we find strong upper bounds on the masses of the gluino, lighter chargino and LSP.

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arXiv2004

Phenomenology of SM and SUSY Higgs bosons at the LHC

I briefly review the physics of the Higgs sector in the Standard Model (SM) and its supersymmetric extension, in particular the MSSM, and discuss the prospects for discovering the Higgs particles at the Large Hadron Collider. Some emphasis will be put on the theoretical developments which occurred in the last few years.

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arXiv2003

PDF uncertainties in Higgs production at hadron colliders

Using the new schemes provided by the CTEQ and MRST collaborations and by Alekhin, we analyse the uncertainties due to the parton distribution functions (PDFs) on the next-to-leading-order cross sections of the four main production processes of the Standard Model Higgs boson at the LHC and the Tevatron. In the Higgs mass range where the production rates are large enough, the spread in the uncertainties when the three sets of PDFs are compared is of about 15% in all processes and at both colliders. However, within one given set of PDFs, the deviations from the values obtained with the reference sets are much smaller, being of ${\cal O}(5$%), except in the gluon--gluon fusion mechanism at relatively large Higgs boson masses, where they can reach the level of 10% (15%) at the LHC (Tevatron).

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arXiv2003

NNLO QCD corrections to the Higgs-strahlung processes at hadron colliders

We implement, at next-to-next-to-leading order, the QCD corrections to Standard Model Higgs boson production in association with vector bosons at hadron colliders, $q\bar{q} \to HV$ with $V=W,Z$. They consist of the two--loop corrections to the Drell--Yan process for the production of off-shell vector bosons, $q\bar{q} \to V^*$, and in the case of $Z$ final states, of the additional contribution from heavy-quark loop mediated processes, in particular $gg \to HZ$. For the Higgs boson masses relevant at the Tevatron and the LHC, $M_H \lsim 200$--300 GeV, the two-loop corrections are small, increasing the production cross sections by less than 5% and 10%, respectively; the scale dependence is reduced to a level of less than a few per cent. This places these processes among the most theoretically clean Higgs boson production channels at hadron colliders.

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arXiv2003

Detection of the neutral MSSM Higgs bosons in the intense-coupling regime at the LHC

We analyse the prospects to detect at the LHC the neutral Higgs particles of the Minimal Supersymmetric Standard Model, when the masses of the two CP-even $h,H$ and of the CP-odd $A$ boson are close to one another, and the value of $\tb$ is large. In this "intense-coupling regime", the Higgs bosons have strongly enhanced couplings to isospin down-type fermions and large total decay widths, so that the $γγ, WW^*$ and $ZZ^*$ decay modes of the three Higgs bosons are strongly suppressed. We advocate the use of the decays into muon pairs, $h,H,A \to μ^+ μ^-$, to resolve the three Higgs boson peaks: although the branching ratios are small, ${\cal O}(10^{4})$, the resolution on muons is good enough to allow for their detection, if the mass splitting is large enough. Using an event generator analysis and a fast detector simulation, we show that only the process $pp \to b\bar{b} μ^+ μ^-$, when at least one of the $b$-quarks is detected, is viable.

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arXiv2003

Z' studies at the LHC: an update

We reanalyse the potential of the LHC to discover new $Z'$ gauge bosons and to discriminate between various theoretical models. Using a fast LHC detector simulation, we have investigated how well the characteristics of $Z'$ bosons from different models can be measured. For this analysis we have combined the information coming from the cross section measurement, which provides also the $Z'$ mass and total width, the forward-backward charge asymmetries on- and off-peak, and the $Z'$ rapidity distribution, which is sensitive to its $u \bar{u}$ and $d \bar{d}$ couplings. We confirm that new $Z'$ bosons can be observed in the process $pp \to Z' \to l^+ l^-$, up to masses of about 5 TeV for an integrated luminosity of 100 fb$^{-1}$. The off- and on-resonance peak forward-backward charge asymmetries $A_{\rm FB}^{l}$ show that interesting statistical accuracies can be obtained up to $Z'$ masses of the order of 2 TeV. We then show how the different experimental observables allow for a diagnosis of the $Z'$ boson and the distinction between the various considered models.

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arXiv2003

Higgs Physics at Future Colliders: recent theoretical developments

I review the physics of the Higgs sector in the Standard Model and its minimal supersymmetric extension, the MSSM. I will discuss the prospects for discovering the Higgs particles at the upgraded Tevatron, at the Large Hadron Collider, and at a future high--energy $e^+e^-$ linear collider with centre--of--mass energy in the 350--800 GeV range, as well as the possibilities for studying their fundamental properties. Some emphasis will be put on the theoretical developments which occurred in the last two years.

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arXiv2003

Detection of MSSM Higgs bosons from supersymmetric particle cascade decays at the LHC

In the context of the Minimal Supersymmetric extension of the Standard Model, we study the production of Higgs bosons at the Large Hadron Collider via cascade decays of scalar quarks and gluinos. We focus on the cascades involving heavier charginos and neutralinos, which decay into the neutral $h,A,H$ and charged $H^\pm$ bosons and lighter charginos and neutralinos, but we will also discuss direct decays of third--generation squarks into their lighter partners and Higgs bosons as well as top quark decays into $H^\pm$ bosons. We show that the production rates of relatively light Higgs bosons, $M_Φ\lsim 250$ GeV, via these mechanisms can be rather large in some areas of the parameter space. Performing a fast detector simulation analysis that takes into account the signals and the various backgrounds, we show that the detection of the neutral Higgs bosons through their decays into $b\bar{b}$ pairs, and of the charged Higgs particles through the $τ^\pm ν$ signature, is possible at the LHC.

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arXiv2002

SUSY Calculation Tools

I discuss the various available tools for the study of the properties of the new particles predicted in the Minimal Supersymmetric extension of the Standard Model. Emphasis will be put on the codes for the determination of the sparticle and Higgs boson spectrum. Codes for the calculation of production cross sections, decay widths and branching ratios, Dark Matter relic density and detection rates, as well as codes for automatic analytical calculations and Monte-Carlo event generators for Supersymmetric processes will be briefly discussed.

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arXiv2002

Associated production of sfermions and gauginos at high-energy e+e- colliders: the case of selectrons and electronic sneutrinos

We analyze the associated production at future high--energy $e^+e^-$ colliders, of first generation sleptons with neutralinos and charginos in the modes $e^+ e^-$ and $e γ$, in the framework of the Minimal Supersymmetric extension of the Standard Model. We show that the production rates, in particular for associated production of right--handed selectrons and the lightest neutralino which in general is the first accessible kinematically, can be much larger than the corresponding ones for second and third generation scalar leptons and for scalar quarks. With the high--luminosities expected at these colliders, the detection of first generation sleptons with masses significantly above the kinematical two--body threshold, $\sqrt{s}= 2m_{\tilde{l}}$, is thus possible in favourable regions of the parameter space.

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