arXiv subjects

Francesca Borzumati

Publications and source records attributed to Francesca Borzumati.

17 recordsLinked to original sources

arXiv2023

Long-lived Higgsino pairs decaying within the LHC detectors

Pair-produced long-lived lightest Higgsinos decaying within a few meters in the LHC detectors are considered. The relatively light nearly-pure Higgsino states object of this study are those of a supersymmetric minimal model with gauge-mediated supersymmetry breaking and heavy sfermions and gauginos. The heavier Higgsinos decay instantly into the lightest one, with very little activities. Hence, all pair-produced Higgsinos end up as a pair of the lightest neutral Higgsinos, possibly long lived and decaying into the gravitino and either the Higgs or the Z boson. If they decay inside the detectors, these bosons deposit huge energies in the calorimeters. The cross section $σ_2$ ($σ_1$) for the events with two (at least one) decaying-in-flight lightest Higgsinos is examined. The combination $(σ_1)^2/σ_2$ is found to be almost insensitive to the lightest Higgsino lifetime, thereby providing a good variable to measure its mass. The ratio $σ_1/σ_2$, instead, mildly dependent on its mass, can give good information on its lifetime. It is argued that the lightest Higgsinos in the events are rather slow, as the very relativistic ones escape away. Thus, the initial state radiations tend to be soft and a trigger system appropriate for such events is necessary to detect them.

hep-ph↗

arXiv2014

The Higgs boson and the International Linear Collider

The Higgs boson will be subject of intense experimental searches in future high-energy experiments. In addition to the effort made at the Large Hadron Collider, where it was discovered, it will be the major subject of study at the International Linear Collider. We review here the reasons for that and some of the issues to be tackled at this future accelerator, in particular that of the precision of the Higgs-boson couplings.

physics.pop-ph↗

arXiv2010

Minimal supersymmetric SU(5) model with nonrenormalizable operators: Seesaw mechanism and violation of flavour and CP

Flavour and CP violations that the neutrino-seesaw couplings of types I, II, and III induce radiatively in the soft massive parameters of the minimal supersymmetric SU(5) model, made realistic by nonrenormalizable operators, are analyzed. Effective couplings are used to parametrize the couplings of renormalizable operators and of the corrections that nonrenormalizable ones provide at the tree level. It is found that for a limited, but sufficient accuracy in the calculations of such violations, it is possible to extend the picture of effective couplings to the quantum level, all the way to the cutoff scale. The arbitrariness introduced by nonrenormalizable operators is analyzed in detail. It is shown that it can be drastically reduced in the Yukawa sector if the effective Yukawa couplings involving colored triplet Higgs bosons are tuned to suppress the decay rate of the proton. In the supersymmetry-breaking sector, the usual requirement of independence of flavour and field type for the mechanism of mediation of supersymmetry breaking is not sufficient to forbid arbitrary flavour and CP violations at the tree level. Special conditions to be added to this requirement, under which such violations can be avoided, are identified. Depending on how and whether these conditions are implemented, different phenomenological scenarios emerge. Flavour and CP violations of soft massive parameters induced by neutrino-seesaw couplings are discussed explicitly for the simplest scenario, in which no such violations are present at the tree level. Guidelines for studying them in other, less simple scenarios are given. Lists of all renormalization group equations needed for their calculations are provided for each of the three types of seesaw mechanism, at all energies between the TeV scale and Planck scale.

hep-ph↗

arXiv2009

Testing supersymmetry at the LHC through gluon-fusion production of a slepton pair

Renormalizable quartic couplings among new particles are typical of supersymmetric models. Their detection could provide a test for supersymmetry, discriminating it from other extensions of the Standard Model. Quartic couplings among squarks and sleptons, together with the SU(3) gauge couplings for squarks, allow a new realization of the gluon-fusion mechanism for pair-production of sleptons at the one-loop level. The corresponding production cross section, however, is at most of ${\cal O}(1) $fb for slepton and squark masses of ${\cal O}(100) $GeV. We then extend our investigation to the gluon-fusion production of sleptons through the exchange of Higgs bosons. The cross section is even smaller, of ${\cal O}(0.1) $fb, if the exchanged Higgs boson is considerably below the slepton-pair threshold, but it is enhanced when it is resonant. It can reach ${\cal O}(10) $fb for the production of sleptons of same-chirality, exceeding these values for $\widetildeτ$'s of opposite-chirality, even when chirality-mixing terms in the squark sector are vanishing. The cross section can be further enhanced if these mixing terms are nonnegligible, providing a potentially interesting probe of the Higgs sector, in particular of parameters such as $A$, $μ$, and $\tanβ$.

hep-ph↗

arXiv2008

Dark matter from late decays and the small-scale structure problems

The generation of dark matter in late decays of quasi-stable massive particles has been proposed as a viable framework to address the excess of power found in numerical N-body simulations for cold dark matter cosmologies. We identify a convenient set of variables to illustrate which requirements need to be satisfied in any generic particle physics model to address the small scale problems and to fulfill other astrophysical constraints. As a result of this model-independent analysis, we point out that meeting these requirements in a completely natural way is inherently difficult. In particular, we re-examine the role of gravitinos and Kaluza-Klein gravitons in this context and find them disfavoured as a solution to the small-scale problems in case they are DM candidates generated in the decay of thermally produced WIMPs. We propose right-handed sneutrinos and right-handed Kaluza-Klein neutrinos as alternatives. We find that they are viable dark matter candidates, but that they can contribute to a solution of the small scale problems only in case the associated Dirac neutrino mass term appears as a subdominant contribution in the neutrino mass matrix.

hep-ph↗

arXiv2007

Non-CKM induced flavor violation in "minimal" SUSY SU(5) models

Patterns of flavor violation induced by neutrino Yukawa couplings are discussed in realistic ``minimal'' SUSY SU(5) models, obtained by adding nonrenormalizable operators to the minimal one, in order to fix the fermion spectrum and suppress proton decay. Results are presented for the three possible implementations of the seesaw mechanisms, i.e. of Type I, II and III.

hep-ph↗

arXiv2006

Addendum to ``Threshold corrections to $m_b$ and the $\bar{b}b \to H^0_i$ production in CP-violating SUSY scenarios''

In hep-ph/0401024: ``Threshold corrections to $m_b$ and the $\bar{b}b\to H^0_i$ production in CP-violating SUSY scenarios'', we have pointed out that the production cross sections of the three neutral Higgs bosons through $\bar{b}b$ fusion can deviate substantially from those obtained in CP conserving scenarios, thanks to the nontrivial role played by the threshold corrections to $m_b$ combined with the CP-violating mixing in the neutral-Higgs-boson sector. The deviations are largest for values of the CP-violating phases that maximize the mixing among at least two of the three neutral Higgs bosons. We complement our previous work focussing explicitly on the values of masses and widths of the three neutral Higgs bosons in this region of parameter space. We then address the issue of whether the three different peaks in the invariant mass distribution of the Higgs-decay products can be experimentally disentangled at the LHC.

hep-ph↗

arXiv2004

Threshold corrections to m_b and the b\bar{b} -> H^0_i production in CP-violating SUSY scenarios

The inclusion of supersymmetric threshold corrections to the $b$-quark mass has dramatic consequences in scenarios with large CP-mixing effects in the Higgs sector. In particular, when the phase of the combination $M_{\tilde{g}}μ$ is $\sim 180^{\rm o} \pm 30^{\rm o}$, the lightest-sbottom squark becomes tachyonic and, possibly, the $b$-quark Yukawa coupling nonperturbative for values of $\tan β$ ranging from intermediate up to large or very large, depending on the size of $arg(A_tμ)$, $arg(A_bμ)$, and the details of the spectrum. For these scenarios, when allowed, as well as scenarios with different values of $arg(M_{\tilde{g}}μ)$, the cross sections for the production of the three neutral-Higgs bosons through $b$-quark have rather interesting dependences on $arg(A_tμ)$ and $arg(A_bμ)$, and the deviations induced by the $m_b$ corrections are rather large. In general, such production channel cannot be neglected with respect to the production through gluon fusion. For large CP-mixing effects, the lightest neutral-Higgs boson can be mainly CP odd and the $b$-quark fusion becomes its main production mechanism. Searches at the Tevatron and the LHC can easily detect such a Higgs boson, or constrain the CP-violating scenarios that allow it.

hep-ph↗

arXiv2004

Beyond leading-order corrections to bar{B} -> X_s gamma at large tan beta: the charged-Higgs-boson contribution

Among the O(alpha_s tan beta) contributions to the Wilson coefficients C_7 and C_8, relevant for the decay bar{B} -> X_s gamma, those induced by two-loop diagrams with charged-Higgs-boson exchange and squark-gluino corrections are calculated in supersymmetric models at large tan beta. The calculation of the corresponding Feynman integrals is exact, unlike in previous studies that are valid when the typical supersymmetric scale M_SUSY is sufficiently larger than the electroweak scale m_weak (\sim m_W, m_t) and the mass of the charged Higgs boson m_{H^\pm}. Therefore, the results presented here can be used for any value of the various supersymmetric masses. These results are compared with those of an approximate calculation, already existing in the literature, that is at the zeroth order in the expansion parameter (m_weak^2,m_H^2)/M_SUSY^2, and with the results of two new approximate calculations in which the first and second order in the same expansion parameter are retained, respectively. This comparison allows us to assess whether the results of these three approximate calculations can be extended beyond the range of validity for which they were derived, i.e., whether they can be used for m_H^2 \gtap M_SUSY^2 and/or M_SUSY^2 \sim m_weak^2. It is found that the zeroth-order approximation works well even for m_H \gtap M_SUSY, provided M_SUSY^2 >> m_weak^2. The inclusion of the higher-order terms improves the zeroth-order approximation for m_H^2 << M_SUSY^2, but it worsens it for m_H \gtap M_SUSY$.

hep-ph↗

arXiv2003

Towards an exact evaluation of the supersymmetric O(alpha_s tan beta) corrections to \bar{B} -> X_s gamma

The charged-Higgs contributions to the decay \bar{B} -> X_s gamma are discussed in the minimal supersymmetric standard model at large tan beta. These contributions receive two-loop O(alpha_s tan beta) corrections by squark-gluino subloops, which are nondecoupling in the limit of heavy superpartners and possibly large. Their leading parts are already known and were evaluated by using an effective two-Higgs-doublet Lagrangian. Subleading corrections coming from higher-dimension operators in the effective Lagrangian were ignored, although this is not, a priori, justified when m_{H^\pm} is not much smaller than the typical supersymmetric mass M_{SUSY}. Here, we calculate all subleading terms of the O(alpha_s tan beta) corrections up to O((m_t^2,m_{H^\pm}^2/M_{SUSY}^2)^2), as well as all the exact two-loop diagrams with squark-gluino subloops, beyond the effective-Lagrangian approximation. Comments are made on the size of these corrections.

hep-ph↗

arXiv2002

Novel constraints on $ΔL=1$ interactions from neutrino masses

We reanalyze the constraints imposed on lepton-number violating interactions by radiative contributions to neutrino masses at the one- and two-loop level in supersymmetric models without R-parity. The interactions considered are the $ΔL=1$ superpotential operators $λ_{ijk} L_i L_j E^c_k$ and $λ^\prime_{ijk} L_i Q_j D^c_k$, and the $ΔL=1$ soft terms $A_{ijk} \tilde{L}_i \tilde{L}_j \tilde{E}^c_k$ and $A_{ijk}^\prime \tilde{L}_i \tilde{Q}_j \tilde{D}^c_k$. It is shown how the constraints on the couplings $λ_{ijk}$ and $λ^\prime_{ijk}$ coming from the one-loop analysis can be evaded. In such a case, the two-loop contributions to neutrino masses become important. The combined one- and two-loop analysis yields constraints on the couplings $λ_{i33}$ and $λ^\prime_{i33}$ that are rather difficult to escape. The two-loop analysis yields also new constraints on $A_{i33}$ and $A^\prime_{i33}$, which are not bounded at the one-loop level. More freedom remains for couplings $λ_{ijk}$, $λ^\prime_{ijk}$, and $A_{ijk}$, $A^\prime_{ijk}$, when $j$ and $k$ are first- or second-generation indices.

hep-ph↗

arXiv2002

3- and 2-body supersymmetric processes: predictions, challenges and prospectives

The processes subject of this talk are: 1) the 2 --> 3 processes contributing, together with the 2 --> 2 one to the hadron-collider production of a charged Higgs boson in association with a t-quark; 2) the 2 --> 2 and 2 --> 3 processes giving rise to the hadron-collider strahlung of a charged slepton from a t-quark in R_p-violating models in which lepton-number-violating trilinear couplings largely dominate over bilinear ones; 3) 3-body neutralino and chargino decays in similar R_p-violating scenarios. The significance of the R_p-violating processes is critically assessed against implications from neutrino physics. Contributions to neutrino masses arising at the tree level, at the one- and two-loop levels are reviewed. Comments are also made on the production of sneutrinos via gluon fusion and on the decay of sneutrinos into photons pairs, and on the influence that constraints from neutrino physics have on these processes.

hep-ph↗

arXiv2001

Low-Scale See-Saw Mechanisms for Light Neutrinos

Alternatives to the see-saw mechanism are explored in supersymmetric models with three right-handed or sterile neutrinos. Tree-level Yukawa couplings can be drastically suppressed in a natural way to give sub-eV Dirac neutrino masses. If, in addition, a B-L gauge symmetry broken at a large scale M_G is introduced, a wider range of possibilities opens up. The value of the right-handed neutrino mass M_R can be easily disentangled from that of M_G. Dirac and Majorana neutrino masses at the eV scale can be generated radiatively through the exchange of sneutrinos and neutralinos. Dirac masses m_D owe their smallness to the pattern of light-heavy scales in the neutralino mass matrix. The smallness of the Majorana masses m_L is linked to a similar see-saw pattern in the sneutrino mass matrix. Two distinct scenarios emerge. In the first, with very small or vanishing M_R, the physical neutrino eigenstates are, for each generation, either two light Majorana states with mixing angle ranging from very small to maximal, depending on the ratio m_D/M_R, or one light Dirac state. In the second scenario, with a large value of M_R, the physical eigenstates are two nearly unmixed Majorana states with masses \sim m_L and \sim M_R. In both cases, the (B-L)-breaking scale M_G is, in general, much smaller than that in the traditional see-saw mechanism.

hep-ph↗

arXiv2001

Gluino Contribution to Radiative B Decays: New Operators, Organization of QCD Corrections and Leading Order Results

The gluino-induced contribution to the decay $b \to s γ$ is investigated in supersymmetric frameworks with generic sources of flavour violation. It is emphasized that the operator basis of the standard model effective Hamiltonian is enlarged and that a suitable redefinition allows to realize the usual scheme of leading and next-to-leading logarithmic contributions when QCD corrections are included. The effects of the leading order QCD corrections on the inclusive branching ratio for $b \to s γ$ are shown. Constraints on supersymmetric sources of flavour violation are derived.

hep-ph↗

arXiv2000

Charged Higgs- and R-Parity-Violating Slepton-Strahlung at Hadron Colliders

It is shown that the radiation of a charged Higgs boson off a third-generation quark (charged-Higgs-strahlung) provides an important channel for the discovery of the charged Higgs at hadron colliders. Equivalently, in supersymmetric models with explicit lepton-number (R-parity) violation, sleptons may also be produced in association with quarks (slepton-strahlung). Higgs- and slepton-strahlung production cross sections are given for both the Tevatron and the LHC. The LHC cross sections imply that heavy ${\cal{O}}$(TeV) charged Higgs bosons can be produced via charged-Higgs-strahlung and that strahlung production of charged sleptons is possible even for small R-parity violating couplings. The possible discovery of sleptons through this channel offers a surprising handle on models of neutrino masses.

hep-ph↗

arXiv2000

Soft Yukawa couplings in supersymmetric theories

The possibility of radiatively generated fermion masses arising from chiral flavor violation in soft supersymmetry-breaking terms is explored. Vacuum stability constraints are considered in various classes of models, and allow in principle all of the first- and second-generation quarks and leptons and the $b$-quark to obtain masses radiatively. Radiatively induced Higgs-fermion couplings have non-trivial momentum-dependent form factors, which at low momentum are enhanced with respect to the case of tree-level Yukawa couplings. These form factors may be probed by various sum rules and relations among Higgs boson decay widths and branching ratios to fermion final states. An apparent, large, hard violation of supersymmetry also results for Higgsino couplings. Mixing between left- and right-handed scalar superpartners is enhanced. A radiative muon mass is shown to lead to a relatively large and potentially measurable contribution to the muon anomalous magnetic moment. If the light-quark masses arise radiatively, the neutron electric dipole moment is suppressed by a natural phase alignment between the masses and dipole moment, and is below the current experimental bound. The possibility of neutrino masses arising from softly broken lepton number, and concomitant enhanced sneutrino-antisneutrino oscillations, is briefly discussed.

hep-ph↗

arXiv1998

Fermion Masses without Yukawa Couplings

Radiatively generated fermion masses without tree level Yukawa couplings are re-analyzed within supersymmetric models. Special emphasis is given to the possible appearance of color and charge breaking vacua. Several scenarios in which the radiative mechanism can be accomodated for the first, second, and third generation fermion masses are presented. Some of these require a low scale of supersymmetry breaking.

hep-ph↗