arXiv subjects

Claudio O. Dib

Publications and source records attributed to Claudio O. Dib.

At least 19 recordsLinked to original sources

arXiv2025

Searching for long-lived light neutralinos from $B$-meson decays with baryonic R-parity violation at Belle II

In a supersymmetry scenario with R-parity violation (RPV), neutralinos with GeV-scale mass, which are necessarily bino-like, are allowed by all constraints and can be produced in association with a baryon in $B$-meson decays via certain $\bar U \bar D \bar D$ operators. In this work, we investigate this scenario with two non-vanishing RPV couplings at the low-energy scale. With one RPV coupling governing the neutralino production rate and another determining its lifetime, this scenario can lead to observable signals with displaced-vertex signatures in the tracking volume of $B$-factories. To maximize the sensitivity to such signals, we develop a new partial-reconstruction technique that yields high efficiency and utilizes most of the decays of relatively heavy, long-lived particles, achieving much better sensitivity than standard full reconstruction. We consider potential background sources and devise selection criteria to suppress their event yields to very low levels. Using a parameterized model of the detector, we estimate in detail the displaced-vertex reconstruction efficiency as a function of neutralino lifetime and mass. For squark masses beyond the LHC limits, we calculate the signal sensitivity of Belle~II, showing that the experiment can probe the RPV couplings well beyond the present bounds, obtained from searches for dinucleon decays, baryon-antibaryon oscillations, and $B^+\to p +\text{missing}$.

hep-ph↗

arXiv2023

Probing $R$-parity violation in $B$-meson decays to a baryon and a light neutralino

We propose a search for $B$ meson decays to a baryon plus missing energy at the Belle II experiment to probe supersymmetry with a GeV-scale lightest neutralino ${\tildeχ_1^0}$ and $R$-parity violation (RPV). We perform analytic computations of the signal branching fractions in the framework of effective field theory, with a single nonzero RPV operator $λ''_{ij3}\bar{U}_{i}^c\bar{D}_{j}^c\bar{D}_3^c$, where $i,j=1,2$. The hadronic form factors are calculated using an SU(3) phenomenological Lagrangian approach for the proton, as well as several hyperons and charmed baryons. Since the decay of the neutralino is kinematically and CKM suppressed in this theoretical scenario, it decays outside the detector and appears experimentally only as missing energy. We detail the analysis techniques at the experimental level and estimate the background in the ${B^+ \to p {\tildeχ_1^0}}$ search using published results for $B^+\to K^+ ν\barν$. Our final sensitivity plots are shown for both $λ''_{113}$ versus the squark mass $m_{\tilde{q}}$ and $λ''_{113}/m^2_{\tilde{q}}$ versus the neutralino mass $m_{\tildeχ_1^0}$. We find that the search at Belle II could probe $λ''_{113}/m^2_{\tilde{q}}$ down to the order of $10^{-8}$ GeV$^{-2}$ in the kinematically allowed $m_{\tildeχ_1^0}$ range. We also obtain current limits on $λ''_{123}$ by recasting an existing search interpreted as ${B^0 \to Λ^0 {\tildeχ_1^0}}$, and comment about searches for ${B^+ \to Σ^+ {\tildeχ_1^0}}$, ${B^0 \to Σ^0 {\tildeχ_1^0}}$, ${B^+ \to Λ_c^+ {\tildeχ_1^0}}$, and ${B^+ \to Ξ_c^+ {\tildeχ_1^0}}$. In closing, we briefly discuss potential searches at the LHCb and BESIII experiments.

hep-ph↗

arXiv2021

Long-lived light neutralinos at Belle II

We consider light neutralinos of mass about 1 GeV, produced from $τ$ lepton rare decays at Belle II, in the context of R-parity-violating (RPV) supersymmetry. With large and clean samples of $τ$ leptons produced at the Belle II experiment, excellent sensitivity to such light neutralinos with the exotic signatures of displaced vertices is expected. We focus on two benchmark scenarios of single RPV operators, $λ'_{311} L_3 Q_1 \bar{D}_1$ and $λ'_{312} L_3 Q_1 \bar{D}_2$, which induce both the production and decay of the lightest neutralino. For the reconstruction of a displaced vertex, we require at least two charged pions in the final states. We perform Monte-Carlo simulations for both signal and background events, and find that Belle II can explore regions in the parameter space competitive with other probes. In particular, for the $λ'_{311}$ scenario, it can put limits up to two orders of magnitude stronger than the current bounds.

hep-ph↗

arXiv2020

When $\tan β$ meets all the mixing angles

Models with two-Higgs-doublets and natural flavour conservation contain $\tan β= v_2 / v_1$ as a physical parameter. We offer here a generalization of a recently proposed idea where only the Cabibbo angle, $θ_\text{c} \simeq 0.22$, was related to $\tan β$ by virtue of the $\mathbb{D}_{4}$ dihedral symmetry group. The original proposal consisted of a massless first generation of quarks and no mixing with the third generation. In our case, through the addition of a third Higgs doublet with a small vacuum-expectation-value but very large masses, thus later decoupling, all quarks become massive and quark mixing is fully reproduced. In fact, all quark mixing angles are expressed in terms of $\tan β$ and one recovers trivial mixing in the limit $β\rightarrow 0$. We also explore the consequences in lepton mixing by adopting a type I seesaw mechanism with three heavy right-handed neutrinos.

hep-ph↗

arXiv2020

PCAC relation for Dstar -> D axial form factors

A relation among the form factors of $ $ at $q^2 \to 0$ is derived. It is verified to lowest order in both the $1/m_c$ expansion and the expansion in number of derivatives using the effective lagrangian that incorporates the heavy quark symmetries for the b and c quarks, and the chiral symmetry for the u, d, s quarks.

hep-ph↗

arXiv2020

Search for light sterile neutrinos from $W^\pm$ decays at the LHC

We study the feasibility to observe sterile neutrinos with masses in the range 5 GeV $< m_N < 20$ GeV at the LHC, using exclusive semileptonic modes involving pions, $W\to \ell N \to π\ell\ell, 2π\ell\ell$ and $3π\ell\ell$. We thus cover a mass window that is between what can be studied in meson factories and high energy colliders. We run simulations for these exclusive events, where pions should be distinguished from the background provided the neutrino decay exhibits a vertex displacement from its production point. In a previous work we have estimated the theoretical rates and here we analyze the observability of the processes at the LHC, given the fact that exclusive hadronic states may be difficult to identify. We study the sensitivity bounds for the observation and discovery of sterile neutrinos in the above mass range. By the end of Run-3, current bounds on heavy-to-light lepton mixings in the lower mass end ($\sim 5$ GeV) could be improved by about an order of magnitude to $|U_{\ell N}|^2\sim 5\times 10^{-6}$, and the High-Luminosity LHC could reach $|U_{\ell N}|^2 \lesssim 3\times 10^{-7}$ in the mass range below 11 GeV. Studying in addition equal sign and opposite sign dileptons, the Majorana or Dirac character of the sterile neutrino could be revealed.

hep-ph↗

arXiv2019

Bounds on lepton flavor violating physics and decays of neutral mesons from $τ(μ) \to 3 \ell, \ell γγ$-decays

We study two- and three-body lepton flavor violating (LFV) decays involving leptons and neutral vector bosons $V=ρ^0, ω, ϕ, J/ψ, Υ, Z^0$, as well as pseudo-scalar $P=π^0, η, η', η_c$ and scalar $S=f_0(500), f_0(980), a_0(980), χ_{c0}(1P)$ mesons, without referring to a specific mechanism of LFV realization. In particular, we relate the rates of the three-body LFV decays $τ(μ) \to 3 \ell$, where $\ell = μ$ or $e$, to the two-body LFV decays $(V,P) \to τμ(τe, μe)$, where $V$ and $P$ play the role of intermediate resonances in the decay process $τ(μ) \to 3 \ell$. From the experimental upper bounds for the branching ratios of $τ(μ) \to 3 \ell$ decays, we derive upper limits for the branching ratios of $(V,P) \to τμ(τe, μe)$. We compare our results to the available experimental data and known theoretical upper limits from previous studies of LFV processes, and find that some of our limits are several orders of magnitude more stringent. Using the idea of quark-hadron duality, we extract limits on various quark-lepton dimension-six LFV operators from data on lepton decays. Some of these limits are either new or stronger than those existing in the literature.

hep-ph↗

arXiv2018

Search for sterile neutrinos decaying into pions at the LHC

We study the possibility to observe sterile neutrinos with masses in the range between 5 GeV and 20 GeV at the LHC, using the exclusive semileptonic modes involving pions, namely W to lepton + N to n pions + lepton+lepton (n = 1, 2, 3). The two pion and three pion modes require extrapolations of form factors to large time-like $q^2$, which we do using vector dominance models as well as light front holographic QCD, with remarkable agreement. This mass region is difficult to explore with inclusive dilepton+dijet modes or trilepton modes and impossible to explore in rare meson decays. While particle identification is a real challenge in these modes, vertex displacement due to the long living neutrino in the above mass range can greatly help reduce backgrounds. Assuming a sample of $10^9$ W bosons at the end of the LHC Run 2, these modes could discover a sterile neutrino in the above mass range or improve the current bounds on the heavy-to-light lepton mixings by an order of magnitude, $U_{l N}^2 \sim 2 \times 10^{-6}$. Moreover, by studying the equal sign and opposite sign dileptons, the Majorana or Dirac character of the sterile neutrino may be revealed.

hep-ph↗

arXiv2017

Search for Heavy Sterile Neutrinos in Trileptons at the LHC

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of $W^\pm \to e^\pm e^\pm μ^\mp ν$ and $μ^\pm μ^\pm e^\mp ν$ at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count (CC) and Multi-Variate Analysis (MVA) methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass $m_N = 20$ GeV and 50 GeV are tested. For an integrated luminosity of 3000 ${\rm fb}^{-1}$, sterile neutrinos can be found with $5 σ$ significance if heavy-to-light neutrino mixings $|U_{Ne}|^2 \sim |U_{Nμ}|^2\sim 10^{-6}$, while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order $10^{-5}$.

hep-ph↗

arXiv2017

Signatures of Dirac and Majorana Sterile Neutrinos in Trilepton Events at the LHC

Heavy sterile neutrinos with masses below $M_W$ can induce trilepton events at the 14 TeV LHC through purely leptonic $W$ decays of $W^\pm \to e^\pm e^\pm μ^\mp ν$ and $μ^\pm μ^\pm e^\mp ν$ where the heavy neutrino will be in an intermediate state on its mass shell. Discovery and exclusion limits for the heavy neutrinos are found using both Cut-and-Count (CC) and a Multi-Variate Analysis (MVA) methods in this study. We also show that it is possible to discriminate between a Dirac and a Majorana heavy neutrino, even when lepton number conservation cannot be directly tested due to unobservability of the final state neutrino. This discrimination is done by exploiting a combined set of kinematic observables that differ between the Majorana vs. Dirac cases. We find that the MVA method can greatly enhance the discovering and discrimination limits in comparison with the CC method. For a 14-TeV $pp$ collider with integrated luminosity of 3000 ${\rm fb}^{-1}$, sterile neutrinos can be found with 5$σ$ significance if heavy-to-light neutrino mixings $|U_{Ne}|^2 \sim |U_{Nμ}|^2 \sim 10^{-6}$, while the Majorana vs. Dirac type can be distinguished if $|U_{Ne}|^2 \sim |U_{Nμ}|^2 \sim 10^{-5}$ or even $|U_{N\ell}|^2\sim 10^{-6}$ if one of the mixing elements is at least an order of magnitude smaller than the other.

hep-ph↗

arXiv2017

Constraints on vector resonances from a strong Higgs sector

We consider a scenario of a composite Higgs arising from a strong sector. We assume that the lowest lying composite states are the Higgs scalar doublet and a massive vector triplet, whose dynamics below the compositeness scale are described in terms of an effective Lagrangian. Electroweak symmetry breaking takes place through a vacuum expectation value just as in the Standard Model, but with the vector resonances strongly coupled to the Higgs field. We determine the constraints on this scenario imposed by (i) the Higgs diphoton decay rate, (ii) the electroweak precision tests and (iii) searches of heavy resonances at the LHC in the final states $l^+l^-$ and $lν_l$ ($l=e,μ$), $τ^+τ^-$, $jj$, $t\bar{t}$, $WZ$, $WW$, $WH$ and $ZH$. We find that the heavy vector resonances should have masses that are constrained to be in the range $2.1$ - $3$ TeV. On the other hand, the mixing of the heavy vectors with the Standard Model gauge bosons is constrained to be in the range $\tan\vartheta\sim 0.1 - 0.3$, which is consistent with the assumption that the Higgs couples weakly to the Standard sector, even though it couples strongly to the heavy vector resonances.

hep-ph↗

arXiv2016

Distinguishing Dirac/Majorana Sterile Neutrinos at the LHC

We study the purely leptonic decays of $W^\pm \to e^\pm e^\pm μ^\mp ν$ and $μ^\pm μ^\pm e^\mp ν$ produced at the LHC, induced by sterile neutrinos with mass $m_N$ below $M_W$ in the intermediate state. Since the final state neutrino escapes detection, one cannot tell whether this process violates lepton number, what would indicate a Majorana character for the intermediate sterile neutrino. Our study shows that when the sterile neutrino mixings with electrons and muons are different enough, one can still discriminate between the Dirac and Majorana character of this intermediate neutrino by simply counting and comparing the above decay rates. After performing collider simulations and statistical analysis, we find that at the $14~\text{TeV}$ LHC with an integrated luminosity of $3000~\text{fb}^{-1}$, for two benchmark scenarios $m_N$ = 20 GeV and 50 GeV, at least a $3σ$ level of exclusion on the Dirac case can be achieved for disparities as mild as e.g. $|U_{Ne}|^2 < 0.7~ |U_{Nμ}|^2$ or $|U_{Nμ}|^2 < 0.7~ |U_{N e}|^2$, provided that $|U_{Ne}|^2$, $|U_{Nμ}|^2$ are both above $\sim 2\times 10^{-6}$.

hep-ph↗

arXiv2015

Composite Resonances effects on EWPT and Higgs diphoton decay rate

In scenarios of strongly coupled electroweak symmetry breaking, heavy composite particles of different spin and parity may arise and cause observable effects on signals that appear at loop levels. The recently observed process of Higgs to $γγ$ at the LHC is one of such signals. We study the new constraints that are imposed on composite models from $H\to γγ$, together with the existing constraints from the high precision electroweak tests. We use an effective chiral Lagrangian to describe the effective theory that contains the Standard Model spectrum and the extra composites below the electroweak scale. Considering the effective theory cutoff at $Λ= 4πv \sim 3 $ TeV, consistency with the $T$ and $S$ parameters and the newly observed $H\to γγ$ can be found for a rather restricted range of masses of vector and axial-vector composites from $1.5$ TeV to $1.7$ TeV and $1.8$ TeV to $1.9$ TeV, respectively, and only provided a non-standard kinetic mixing between the $W^{3}$ and $B^{0}$ fields is included.

hep-ph↗

arXiv2015

Higgs boson coupling to a new strongly interacting sector

In the framework of strongly interacting dynamics for electroweak symmetry breaking, heavy composite particles may arise and cause observable effects, as they should couple strongly to the resulting Higgs boson and affect the signals that appear at one loop level. Here we study this expected behavior, contrasting it with current experimental knowledge. We work in a simple and generic scenario where the lowest lying composite states are the Higgs scalar doublet and a massive vector triplet. We use an effective chiral Lagrangian to describe the theory below the compositeness scale $Λ$, assumed to be $4πv \simeq~3$ TeV. The effective theory contains the Standard Model spectrum and the extra composites. We determine the constraints on this scenario imposed by our current knowledge of the $Zb\bar{b}$ vertex, the $T$ and $S$ oblique parameters, and the recently measured Higgs mass and its diphoton decay rate. We found that the $T$ and $S$ parameters as well as the Higgs diphoton decay do not provide important constraints on the model. In contrast, the constraints arising from the $Zb\bar{b}$ vertex and from the Higgs mass at $126$ GeV are fulfilled only if the heavy vector resonances do not couple strongly with quarks, and at the same time the Higgs boson has a moderate but not too strong coupling to the heavy composite resonances.

hep-ph↗

arXiv2015

Discovering sterile Neutrinos ligther than $M_W$ at the LHC

We study the purely leptonic $W$ decays $W^+ \to e^+ μ^- e^+ ν_e$ and $W^+ \to e^+ e^+ μ^- \bar ν_μ$ (or their charge conjugates) produced at the LHC, induced by sterile neutrinos with mass below $M_W$ in the intermediate state. While the first mode is induced by both Dirac or Majorana neutrinos, the second mode is induced only by Majorana neutrinos, as it violates lepton number. We find that, even when the final (anti-)neutrino goes undetected, one could distinguish between these two processes, thus distinguishing the Dirac or Majorana character of the sterile neutrinos, by studying the muon spectrum in the decays.

hep-ph↗

arXiv2015

Left-Right Symmetric Models at the High-Intensity Frontier

We study constraints on Left-Right Symmetric models from searches of semileptonic decays of $D$, $D_{s}$, $B$ mesons, mediated by heavy neutrinos $N$ with masses $m_N\sim $ GeV that go on their mass shell leading to a resonant enhancement of the rates. Using these processes we examine, as a function of $m_N$ and $M_{W_R}$, the physics reach of the recently proposed high-intensity beam dump experiment SHiP, which is expected to produce a large sample of $D_s$ mesons. We compare these results with the corresponding reach of neutrinoless double beta decay experiments, as well as like-sign dilepton searches with displaced vertices at the LHC. We conclude that the SHiP experiment has clear advantages in probing the Left-Right Symmetric models for heavy neutrinos in the GeV mass range.

hep-ph↗

arXiv2014

Mass as a form of Energy in a simple example

A major consequence of special relativity, expressed in the relation $E_0 = m c^2$, is that the total energy content of an object at rest, including its thermal motion and binding energy among its constituents, is a measure of its inertia, i.e. its mass. This relation was first stated by Einstein. He showed that, in order to be consistent with the principles of special relativity, there must be a loss of inertia in a block that emits two pulses of electromagnetic radiation. A pedagogical difficulty with this example is that radiation is a purely relativistic phenomenon, and so the connection with the examples one learns in introductory Mechanics courses is not simple. Here we use a more familiar example of masses and springs, where the non-relativistic limit can be easily found and where the potential energy is clearly shown to be part of the mass of the bound system.

physics.class-ph↗

arXiv2014

CP Violation with Majorana neutrinos in K Meson Decays

We study the possibility of having CP asymmetries in the decay Kaon(+/-) -> pion(-/+) lepton(+/-) lepton(+/-) (lepton = muon or electron). This decay violates Lepton Number by two units and occurs only if there are Majorana particles that mediate the transition. Even though the absolute rate is highly suppressed by current bounds, we search for Majorana neutrino scenarios where the CP asymmetry arising from the lepton sector could be sizeable. This is indeed the case if there are two or more Majorana neutrinos with similar masses in the range around 10^2 MeV. In particular, the asymmetry is potentially near unity if two neutrinos are nearly degenerate, in the sense that the mass difference is similar to the decay rate. The full decay, however, may be difficult to detect not only because of the suppression caused by the heavy-to-light lepton mixing, but also because of the long lifetime of the heavy neutrino, which would induce large space separation between the two vertices where the charge leptons are produced. This particular problem should be less serious in heavier meson decays, as they involve heavier neutrinos with shorter lifetimes.

hep-ph↗