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F. M. Renard

Publications and source records attributed to F. M. Renard.

At least 37 records · Page 2Linked to original sources
arXiv2009

One-loop electroweak effects on stop-chargino production at LHC

The process of stop-chargino production at LHC has been calculated in the Minimal Supersymmetric Standard Model at the complete electroweak one-loop level, assuming a mSUGRA symmetry breaking scheme. Several properties of the angular and invariant mass distributions of the basic bottom-gluon to stop-chargino amplitudes have been derived. For a meaningful collection of different benchmark points the overall electroweak one-loop effects are at most of the order of a few percent. At the realistically expected LHC accuracy, the main supersymmetric electroweak features of the process can be therefore essentially derived in this theoretical scheme from the simple Born level expressions.

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arXiv2009

A new class of SUSY signatures in the processes $gg \to HH', ~VH$

Within the MSSM and SM frameworks, we analyze the 1loop electroweak (EW) predictions for the helicity amplitudes describing the 17 processes $gg\to HH'$, and the 9 processes $gg\to VH$; where $H,H'$ denote Higgs or Goldstone bosons, while $V= Z, ~W^\pm$. Concentrating on MSSM, we then investigate how the asymptotic helicity conservation (HCns) property of SUSY, affects the amplitudes at the LHC energy range; and what is the corresponding situation in SM, where no HCns theorem exists. HCns is subsequently used to construct many relations among the cross sections of the above MSSM processes, depending only on the angles $α$ and $β$. These relations should be asymptotically exact, but with mass-depending deviations appearing, as the energy decreases towards the LHC range. Provided the SUSY scale is not too high, they may remain roughly correct, even at the LHC energy range.

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arXiv2008

A complete one-loop calculation of electroweak supersymmetric effects in $t$-channel single top production at LHC

We have computed the complete one-loop electroweak effects in the MSSM for single top (and single antitop) production in the $t$-channel at hadron colliders, generalizing a previous analysis performed for the dominant $dt$ final state and fully including QED effects. The results are quite similar for all processes. The overall Standard Model one-loop effect is small, of the few percent size. This is due to a compensation of weak and QED contributions that are of opposite sign. The genuine SUSY contribution is generally quite modest in the mSUGRA scenario. The experimental observables would therefore only practically depend, in this framework, on the CKM $Wtb$ coupling.

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arXiv2008

Stop-antistop and sbottom-antisbottom production at LHC: a one-loop search for model parameters dependence

We have computed the one-loop electroweak expression of diagonal and non diagonal stop-antistop and sbottom-antisbottom production from initial state gluons at LHC. We have investigated the possibility that the one-loop effects exhibit a dependence on "extra" supersymmetric parameters different from the final squark masses. Our results, given for a choice of twelve SUSY benchmark points in the MSSM with mSUGRA symmetry breaking, show that in some cases a mild dependence might arise, at the percent relative level, of not simple experimental detection.

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arXiv2008

Subleading terms in asymptotic Passarino-Veltman functions

We write explicit and self-contained asymptotic expressions for the tensorial B, C and D Passarino-Veltman functions. These include quadratic and linear logarithmic terms, as well as subleading constant terms. Only mass-suppressed O(m^2/s) contributions are neglected. We discuss the usefulness of such expressions, particularly for studying one-loop effects in 2-to-2 body processes at high energy.

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arXiv2008

Genuine SUSY signatures from $ug\to \tilde d \tchi_i^+$ and $ug \to d W^+$, at high energies

We analyze the quark-gluon induced process $u g\to \tilde d\tchi_i^+$, including the one loop electroweak (EW) effects in the minimal supersymmetric model (MSSM). This process is determined by four helicity amplitudes, three of which are violating helicity conservation, and another one which respects it and is logarithmically enhanced at high energy. Combining this $u g\to \tilde d\tchi_i^+$ analysis with a corresponding one for $u g \to d W^+$, we obtain simple approximate relations between the two processes, testing the MSSM structure at the amplitude and the cross section levels. These relations become exact at asymptotic energies and, provided the SUSY scale is not too heavy, they may be approximately correct at LHC energies also. In addition to these, we study the 1loop EW corrections to the inclusive $\tilde d\tchi_i^+$ production at LHC, presenting as examples, the $p_T$ and angular distributions. Comparing these to the corresponding predictions for $W$+jet production derived earlier, provides an accurate test of the supersymmetric assignments.

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arXiv2008

Remarkable virtual SUSY effects in $W^{\pm}$ production at high energy hadron colliders

We present a complete 1-loop study of the electroweak corrections to the process $ug\to dW^+$ in MSSM and SM. The occurrence of a number of remarkable properties in the behavior of the helicity amplitudes at high energies is stressed, and the crucial role of the virtual SUSY contributions in establishing them, is emphasized. The approach to asymptopia of these amplitudes is discussed, comparing the effects of the logarithmic and constant contributions to the mass suppressed ones, which are relevant at lower energies. Applying crossing to $ug\to d W^+$, we obtain all subprocesses needed for the 1-loop electroweak corrections to $W^\pm$-production at LHC. The SUSY model dependence of such a production is then studied, and illustrations are given for the transverse $W^{\pm}$ momentum distribution, as well as the angular distribution in the subprocess center of mass.

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arXiv2008

A complete one-loop description of associated tW production at LHC and an estimate of possible genuine supersymmetric effects

We compute, in the MSSM framework, the sum of the one-loop electroweak and of the total QED radiation effects for the process $pp \to t W+X$, initiated by the parton process $bg\to tW$. Combining these terms with the existing NLO calculations of SM and SUSY QCD corrections, we analyze the overall one-loop supersymmetric effects on the partial rates of the process, obtained by integrating the differential cross section up to a final variable invariant mass. We conclude that, for some choices of the SUSY parameters and for relatively small final invariant masses, they could reach the relative ten percent level, possibly relevant for a dedicated experimental effort at LHC.

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arXiv2008

Collider aspects of flavour physics at high Q

This review presents flavour related issues in the production and decays of heavy states at LHC, both from the experimental side and from the theoretical side. We review top quark physics and discuss flavour aspects of several extensions of the Standard Model, such as supersymmetry, little Higgs model or models with extra dimensions. This includes discovery aspects as well as measurement of several properties of these heavy states. We also present public available computational tools related to this topic.

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arXiv2006

Neutralino pair production at CERN LHC

We consider the production of neutralino pairs $\tchi^0_i\tchi^0_j$ at a high energy hadron collider, putting a special emphasis on the case where one of them is the lightest neutralino $\tchi^0_1$, possibly constituting the main Dark Matter component. At tree level, the only relevant subprocess is $q\bar q\to \tchi^0_i\tchi^0_j$, while the subprocess $gg\to \tchi^0_i\tchi^0_j$ first appears at the one loop level. Explicit expressions for the $q\bar q$-helicity amplitudes are presented, including the tree level contributions and the leading-log one loop radiative corrections. For the one-loop $ gg\to \tchi^0_i\tchi^0_j$ process, a numerical code named PLATONggnn is released, allowing the computation of $dσ/d \that$ in any MSSM model with real soft breaking parameters. It turns out that acceptable MSSM benchmark models exist for which the $q\bar q $ and the gluonic contributions may give comparable effects at LHC, due to the enhanced gluonic structure functions at low fractional momenta. Depending on the values of the MSSM parameters, we find that the LHC neutralino pair production may provide sensitive tests of SUSY models generating neutralino Dark Matter.

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arXiv2006

Supersymmetry spectroscopy in stop-chargino production at LHC

We consider the process of associated stop-chargino production in the MSSM at LHC and show that, at the simplest Born level, the production rate is dramatically sensitive to the choice of the benchmark points, oscillating from potentially "visible" maxima of the picobarn size to much smaller, hardly "visible", values. Adopting a canonical choice of SM type CKM matrices, we also show that in some "visible" cases the total rate exhibits a possibly relevant dependence on tan(beta).

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arXiv2006

The relevance of virtual electroweak effects in the overall $t$-channel single top production at LHC

We compute the complete one loop electroweak effects in the MSSM for the eight processes of single top (and single antitop) production in the $t$-channel at hadron colliders, generalizing a previous analysis performed for the dominant $dt$ final state. The results are quite similar for all processes, showing an impressively large Standard Model effect and a generally modest genuine SUSY contribution in the mSUGRA scenario. The one loop effect on the total rate is shown and the possibility of measuring it at LHC is discussed.

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arXiv2006

Neutralino pair production in a $γγ$ Collider

We present a complete 1-loop study of the process $γγ\to \tchi^0_i \tchi^0_j$ and the predicted cross section in a $γγ$ Linear Collider. A suitable numerical code PLATONlc, valid for any set of real MSSM parameters, is released. This study and code are complementary to those suitable for Dark Matter detection through the inverse process $\tchi^0_i \tchi^0_j \to γγ$ describing neutralino-neutralino annihilation at rest, which were presented previously. If SUSY is realized in Nature, both codes should be very helpful in future astrophysical and collider studies of the neutralino sector.

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arXiv2006

Description of $e^-e^+\toγγ, Z γ, ZZ$ in SM and MSSM

We present a complete analysis of the one loop electroweak corrections to $e^-e^+\toγγ, ~Zγ, ~ZZ$ in the Standard (SM) and the Minimal Supersymmetric Standard Model (MSSM). Analytic expressions are written for the contributions to the helicity amplitudes. Several observables accessible for polarized or unpolarized beams and transverse, longitudinal or unpolarized final states are computed. We show that in the few hundred GeV region, these observables provide a test of the various SM or MSSM components. For the dominant TT amplitude at high energy, the sensitivity to the details of the various sectors disappears, but the energy dependence fixed by leading logarithmic contributions, provides a model independent signature discriminating SM from MSSM. Subdominant TL or LL amplitudes though, remain sensitive to the details of the SM or MSSM sectors. Numerical illustrations are given for energies up to several TeV. The analysis may also be used to search for new physics characterized by anomalously strong interactions among the neutral gauge bosons.

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arXiv2006

Single top production in the $t$-channel at LHC: a realistic test of electroweak models

We compute the complete electroweak one-loop effect on the process of $t$-channel single top production at LHC in the Standard Model and in the MSSM within the mSUGRA symmetry breaking scheme. We find that the one-loop electroweak SM effect is large, and decreases the cross section of an amount that is of the same size as that of the NLO QCD one. The genuine SUSY effect in the mSUGRA scheme, for a general choice of benchmark points, is rather small. It might become large and visible in more general scenarios around thresholds involving light stop and neutralino mass values.

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arXiv2006

Addendum to "Helicity conservation in gauge boson scattering at high energy"

In a previous paper we have established that for any two-body process involving even numbers of transverse vector bosons and gauginos, the dominant asymptotic amplitudes obey helicity conservation (HC); i.e. the sum of the helicities of the two incoming particles equals to the sum of the helicities of the two outgoing ones. This HC has been proved to all orders in MSSM, provided $(s, |t|,|u|)$ are much larger than all masses in the model; but only to leading 1-loop logarithmic order in SM. In the present addendum, the validity of HC is extended to all two-body processes. Renormalizability is crucial for the HC validity, while all known anomalous couplings violate it.

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arXiv2006

Neutralino-neutralino annihilation to gammaZ in MSSM

The 1-loop computation of the processes $\tchi_i \tchi_j \to γZ$ has been performed at an arbitrary c.m. energy for any pair of MSSM neutralinos. As an application suitable for Dark Matter (DM) searches, the neutralino-neutralino annihilation is studied at the limiting case of vanishing relative velocity, describing the present DM distribution in the galactic halo; and at a relative velocity of about 0.5, determining the neutralino relic density contributions. The most useful situation is obviously for $i=j=1$, but the case of non-identical neutralinos may also be useful in some corners of the parameter space. Our results are contained in the FORTRAN code PLATONdmgZ, applying to any set of real MSSM parameters. Numerical results are also presented for a sample of 6 MSSM models, describing the various possible neutralino properties. A comparison with other existing works is also made.

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