arXiv subjects

Bogdan A. Dobrescu

Publications and source records attributed to Bogdan A. Dobrescu.

At least 19 recordsLinked to original sources

arXiv2026

Minimal cascade decay of the top quark through a new scalar

We explore a possible decay of the top quark into a new scalar $ϕ_t$ plus a charm quark. The decay is mediated by a dimension-5 operator that may be generated by the exchange of a heavy vectorlike quark, $χ$, and can have a branching fraction close to a percent. In a minimal model, $ϕ_t$ decays due to the same operator that allows the nonstandard top decay, leading to a final state involving a $c$ jet, a $b$ jet and a $W$ boson. Hence, the top quark would undergo a nonstandard cascade decay through $ϕ_t$ and then through an off-shell top quark. The main nonstandard collider signal is a $t \bar t \to c \, \bar c \, b \, \bar b \, W^+ W^-$ process. In addition, production of the vectorlike quark at the LHC would lead to a signal of the type $χϕ_t \, j \to 3c + b \, \bar b \, W W j$.

hep-ph↗

arXiv2026

Axion-like particle at the 10 GeV scale: Higgs decays to wide jets and photons

If axion-like particles (ALPs) exist, they may have renormalizable couplings only to the Higgs boson ($h^0$) or to fields beyond the Standard Model. An ALP $A_h$ lighter than about 60 GeV would allow the $h^0 \to A_h A_h$ decay, with a branching fraction determined by the explicit global symmetry breaking responsible for the main contributions to the $A_h$ mass ($M_A$). New heavy fields that carry color and electric charge, such as squarks, can mediate $A_h$ decays at one loop mostly into gluons, but also into photons. Exploring LHC sensitivity to $A_h$, we show that the $h^0 \to A_h A_h \to (gg)(γγ)$ channel leads to a diphoton resonance at $M_A$ whose production rate is consistent with a $3.5σ$ excess reported by a CMS search at $M_A \approx 13.6$ GeV. For $M_A$ of order 10 GeV, the $h^0 \to A_h A_h \to 4g$ cascade decay leads to two wide jets (each with 2-prong substructure) that form a resonance at 125 GeV.

hep-ph↗

arXiv2025

Multi-top signals of vectorlike quarks at the LHC

We point out that events with 6 or more top quarks may be observed at the LHC if certain particles exist at the TeV scale. In a model where a vectorlike quark of charge 2/3 decays into a top quark and a pseudoscalar particle, which subsequently decays into a top-antitop pair, the LHC production cross section for events with 6 top quarks may be above 10 fb. If the pseudoscalar is part of a complex scalar field, then longer cascade decays, involving the scalar partner, may lead to events with 8 or even 10 top quarks. We show that for a region of parameter space the dominant LHC signal in this model is 8 top quarks (i.e., four $t\bar t $ pairs). The ensuing signals would be spectacular, including many leptons and $b$ jets. A discovery in that case would allow several cross section measurements that may determine the masses of all three new particles.

hep-ph↗

arXiv2025

Octet scalars shaping LHC distributions in 4-jet final states

We study properties of a hypothetical scalar particle, $Θ$, which is a color octet and an electroweak singlet. At hadron colliders, $Θ$ is pair produced through its QCD coupling to gluons, so that its mass determines the cross section. It decays at tree level into $q\bar q$ through dimension-5 operators, and at one loop into gluons. Thus, the main LHC signature of $Θ$ is a pair of dijets of equal invariant mass. The CMS search in this channel shows a $3.6σ$ excess over the QCD background for a dijet mass $M_{jj} \approx 0.95$ TeV, which can be due to $Θ$: its production cross section (65 fb for a real scalar) and the acceptance of the CMS event selection applied to $p p \to ΘΘ\to \! (q \bar q)(q \bar q)$ yield a rate consistent with the excess. Furthermore, the shape of the $dσ/d M_{jj}$ signal is in agreement with the CMS result. Given the data-driven background fit performed by CMS, we find that a complex scalar fits better the data than a real scalar. Besides the pair of dijets, testable LHC signals include a trijet-dijet topology, a $t\bar t$ pair plus a dijet resonance, as well as final states involving a Higgs, $W$ or $Z$ boson plus jets.

hep-ph↗

arXiv2025

Ultraheavy diquark decaying into vectorlike quarks at the LHC

We explore the discovery potential of an ultraheavy $(7-8.5$ TeV) diquark scalar produced in the collisions of two up quarks at the LHC. Assuming that the diquark scalar decays into two vectorlike quarks of mass around 2 TeV, each of them decaying into a $W^{+}$ boson and a $b$ quark, we focus on the fully-hadronic final state. We present a signal-from-background separation study based on a discriminator built with Machine Learning techniques. For this six-jet final state and a luminosity of $3000 \ \text{fb}^{-1}$, we estimate that a diquark scalar of mass near 8 TeV may be discovered or ruled out even when its coupling to up quarks is as low as 0.2.

hep-ph↗

arXiv2025

Quark-universal $U(1)$ breaking scalar at the LHC

If the quarks or leptons are charged under a new $U(1)$ gauge symmetry, then besides a $Z'$ boson there must exist at least one new boson whose decay products include Standard Model particles. In the case of a minimal symmetry breaking sector, that new boson is a scalar $ϕ$ that couples to the $Z'$ boson as well as to the new fermions required to cancel the $U(1)$ gauge anomalies. The scalar may be produced at the LHC in association with a $Z'$ boson, or through $Z'$ boson fusion, while its decays are typically into four jets or two photons. We analyze in detail the case where the $Z'$ boson is leptophobic, and all the quarks have the same charge under the new $U(1)$. If $ϕ$ mixes with the Standard Model Higgs boson, then the new scalar can also be produced via gluon fusion, and the discovery mode is likely to be a diphoton resonance.

hep-ph↗

arXiv2025

Diophantine equations with sum of cubes and cube of sum

We solve Diophantine equations of the type $ a \, (x^3 \!+ \! y^3 \!+ \! z^3 ) = (x \! + \! y \! + \! z)^3$, where $x,y,z$ are integer variables, and the coefficient $a\neq 0$ is rational. We show that there are infinite families of such equations, including those where $a$ is any cube or certain rational fractions, that have nontrivial solutions. There are also infinite families of equations that do not have any nontrivial solution, including those where $1/a = 1- 24/m$ with restrictions on the integer $m$. The equations can be represented by elliptic curves unless $a = 9$ or 1, and any elliptic curve of nonzero $j$-invariant and torsion group $\mathbb{Z}/3k\mathbb{Z}$ for $k = 2,3,4$, or $\mathbb{Z}/2\mathbb{Z} \times \mathbb{Z}/6\mathbb{Z} $ corresponds to a particular $a$. We prove that for any $a$ the number of nontrivial solutions is at most 3 or is infinite, and for integer $a$ it is either 0 or $\infty$. For $a = 9$, we find the general solution, which depends on two integer parameters. These cubic equations are important in particle physics, because they determine the fermion charges under the $U(1)$ gauge group.

math.NT↗

arXiv2024

TeV-scale particles and LHC events with dijet pairs

Scalar particles that couple to two up quarks may be produced at the LHC even if they are ultraheavy, in the $8-10$ TeV mass range. A renormalizable theory that includes a diquark particle of this type ($S_{uu}$), two vectorlike quarks ($χ_1, χ_2$), and a gauge-singlet pseudoscalar, predicts LHC signals involving four or more jets of very high $p_T$. Two remarkable events observed by the CMS experiment, each involving four high $p_T$-jets, may be due to an $S_{uu}$ of mass near 8.5 TeV, and a $χ_2$ mass of 2.1 TeV. A separate excess reported by CMS in the nonresonant dijet pair search is consistent with a $χ_1$ mass of $0.95$ TeV. This hypothesis may be tested through CMS and ATLAS searches for signals with a pair of dijet resonances of masses clustered around 1 TeV, and separately around 2 TeV, which have $4j$ invariant masses in the $5-8$ TeV range. These additional signals would arise from cascade decays of $S_{uu} \to χ_2 χ_2 $, which lead to $5j$ and $6j$ events with invariant masses around 8 TeV. Depending on the couplings of the heavy colored particles, additional signals are possible, involving for example highly-boosted top quarks.

hep-ph↗

arXiv2024

Dijet and electroweak limits on a $Z'$ boson coupled to quarks

An insightful way of presenting the LHC limits on dijet resonances is the coupling-mass plot for a $Z'$ boson that has flavor-independent quark interactions. This also illustrates the comparison of low-mass LHC sensitivity with constraints on the flavor-independent $Z'$ boson from electroweak and quarkonium measurements. To derive these constraints, we compute the $Z'$ mixing with the $Z$, the photon, and the $Υ$ meson, emphasizing the logarithmic dependence on the masses of the new electroweak-charged fermions (``anomalons") required to cancel the gauge anomalies. We update the coupling-mass plot, extending it for $Z'$ masses from 5 GeV to 5 TeV.

hep-ph↗

arXiv2024

Mutually elusive: vectorlike antileptons and leptoquarks

We study the properties of vectorlike fermions that have the same gauge charges as the Standard Model lepton doublets, but opposite lepton number. These antileptons undergo decays mediated by heavier scalar leptoquarks, while the symmetries of this renormalizable model protect the vectorlike fermions and the leptoquarks from standard decays probed so far at colliders. If the new particles couple predominantly to second-generation quarks, then their collider probes involve multiple jets and two taus or neutrinos, and are hampered by large backgrounds. If couplings to third-generation quarks are large, then the collider signals involve top quarks, and can be probed more efficiently at the LHC. Even in that case, both the vectorlike fermion doublet and the leptoquarks remain more elusive than in models with standard decays.

hep-ph↗

arXiv2023

Vectorlike leptons and long-lived bosons at the LHC

We study a renormalizable theory that includes a heavy weak-singlet vectorlike lepton, $τ^\prime$, that decays into a pseudoscalar boson and a tau lepton. We show that this can be the dominant decay mode of $τ^\prime$ provided the pseudoscalar couplings deviate from the case of a Nambu-Goldstone boson. For a range of parameters, the pseudoscalar is long-lived, and mainly decays into a photon pair at one loop, or into a tau pair at tree level. Electroweak production of $τ^{\prime +} τ^{\prime -}$ leads to a rich phenomenology at colliders, including signals with many taus or photons. We analyze in detail the LHC signal involving two prompt taus and two displaced photon pairs. Particularly interesting is the case where the pseudoscalar has a decay length of a few meters and thus would typically deposit energy in the muon chambers of the CMS or ATLAS detectors.

hep-ph↗

arXiv2023

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

hep-ph↗

arXiv2022

Proton decay from quark and lepton compositeness

Within a chiral $SU(15)$ gauge theory in which the Standard Model fermions are bound states of massless preons, we show that proton-decay operators are likely induced at the compositeness scale, $Λ_{\rm pre}$. Our estimate of the limit imposed by searches for proton decays is $Λ_{\rm pre} > 10^4 \, {\rm TeV}^{1/2} \, C_8^{1/4} \, m_{\cal Q}^{1/2}$, where $C_8$ is a rescaled coefficient of an 8-prebaryon operator induced by $SU(15)$ dynamics, and $m_{\cal Q}$ is the mass of a composite vectorlike quark. The latter has a lower limit related to the mass of a composite vectorlike lepton, which in turn is required by LHC searches to be above 1 TeV. For $C_8$ in the $10^{-5} - 1$ range, the lower limit on $Λ_{\rm pre}$ varies between $3 \times 10^3$ TeV and $5 \times 10^4$ TeV. We point out that exotic proton decay modes, into a $π^+$ and a heavy right-handed neutrino, could be observed using the Super-Kamiokande or DUNE detectors.

hep-ph↗

arXiv2021

A model of quark and lepton compositeness

In the chiral $SU(15)$ gauge theory presented here, the quarks and leptons are bound states ("prebaryons") of massless preons. The Standard Model charges of the preons imply 3 generations of quarks and leptons, plus some vectorlike fermions lighter than the confining scale $Λ_{\rm pre}$. Under certain assumptions about the chiral dynamics, bound states of two prebaryons behave as Higgs fields. The QCD and electroweak groups may unify above $Λ_{\rm pre}$, while $SU(15)$ prevents rapid proton decay.

hep-ph↗

arXiv2020

Chiral Abelian gauge theories with few fermions

We construct chiral theories with the smallest number $n_χ$ of Weyl fermions that form an anomaly-free set under various Abelian gauge groups. For the $U(1)$ group, where $n_χ= 5$, we show that the general solution to the anomaly equations is a set of charges given by cubic polynomials in three integer parameters. For the $U(1) \times U(1)$ gauge group we find $n_χ= 6$, and derive the general solution to the anomaly equations, in terms of 6 parameters. For $U(1) \times U(1) \times U(1)$ we show that $n_χ= 8$, and present some families of solutions. These chiral gauge theories have potential applications to dark matter models, right-handed neutrino interactions, and other extensions of the Standard Model. As an example, we present a simple dark sector with a natural mass hierarchy between three dark matter components.

hep-ph↗

arXiv2019

LHC probes of the 10 TeV scale

The usual range of new particle masses, up to a few TeV, searched for at the LHC may be substantially extended if ultraheavy diquark particles exist. A diquark scalar, $S_{uu}$, that interacts perturbatively with two up quarks may be as heavy as 10 TeV and would still produce tens of spectacular events at the 14 TeV LHC. It is shown here that an ultraheavy $S_{uu}$ could be discovered through final states of very high energy in various channels, especially if the diquark can decay into other new heavy particles. Examples include cascade decays of $S_{uu}$ via a second scalar produced in pairs, which leads to two dijet resonances, or to more exotic signals with top quarks, Higgs bosons, electroweak bosons, and high-$p_T$ jets. Another possibility is that the diquark decays into a vectorlike quark of multi-TeV mass and a top or up quark. Signal events include one or two highly boosted top quarks and a Higgs boson or a $Z$, without counterparts containing top antiquarks. Similarly, direct decays of the diquark into $tj$ or $tt$ with leptonic top decays involve only positively charged leptons.

hep-ph↗

arXiv2019

General solution to the U(1) anomaly equations

The anomaly cancellation equations for the $U(1)$ gauge group can be written as a cubic equation in $n-1$ integer variables, where $n$ is the number of Weyl fermions carrying the $U(1)$ charge. We solve this Diophantine cubic equation by providing a parametrization of the charges in terms of $n-2$ integers, and prove that this is the most general solution.

hep-th↗

arXiv2018

Ultraheavy resonances at the LHC: beyond the QCD background

We study the theory and some experimental hints of ultraheavy resonances at the LHC. The production of an ultraheavy narrow particle may have a larger rate than the QCD background even when the final state includes only hadronic jets. We consider two classes of models that lead to 4-jet signals. In the first class a diquark scalar decays into two vectorlike quarks. In the second one a coloron decays into two color-octet scalars or into a pair of vectorlike quarks. We show that a diquark as heavy as 11.5 TeV, or a coloron as heavy as 8.5 TeV may be discovered at the LHC. We point out that a CMS 4-jet event may be due to an 8 TeV resonance decaying into two secondary particles, each with a mass of 1.8 TeV. We find that the QCD background with a 4-jet mass and dijet masses that equal or exceed those of the CMS event is approximately $5\times 10^{-5}$ events in 78 fb$^{-1}$ of data, while the diquark signal could have easily produced that event. The diquark also decays directly into two jets, which may be the origin of some of the three other events of mass near 8 TeV observed by ATLAS and CMS.

hep-ph↗