arXiv subjects

Bogdan A. Dobrescu

Publications and source records attributed to Bogdan A. Dobrescu.

At least 37 records · Page 2Linked to original sources

arXiv2018

Collider Tests of the Renormalizable Coloron Model

The coloron, a massive version of the gluon present in gauge extensions of QCD, has been searched for at the LHC as a dijet or top quark pair resonance. We point out that in the Renormalizable Coloron Model (ReCoM) with a minimal field content to break the gauge symmetry, a color-octet scalar and a singlet scalar are naturally lighter than the coloron because they are pseudo Nambu-Goldstone bosons. Consequently, the coloron may predominantly decay into scalar pairs, leading to novel signatures at the LHC. When the color-octet scalar is lighter than the singlet, or when the singlet mass is above roughly 1 TeV, the signatures consist of multi-jet resonances of multiplicity up to 12, including topologies with multi-prong jet substructure, slightly displaced vertices, and sometimes a top quark pair. When the singlet is the lightest ReCoM boson and lighter than about 1 TeV, its main decays ($W^+W^-$, $γZ$, $ZZ$) arise at three loops. The LHC signatures then involve two or four boosted electroweak bosons, often originating from highly displaced vertices, plus one or two pairs of prompt jets or top quarks.

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arXiv2018

Exotic Signals of Vectorlike Quarks

Vectorlike fermions are an important target for hadron collider searches. We show that the vectorlike quarks may predominantly decay via higher-dimensional operators into a quark plus a couple of other Standard Model fermions. Pair production of vectorlike quarks of charge 2/3 at the LHC would then lead to a variety of possible final states, including $t\bar t + 4τ$, $t\bar bν+ 3τ$, $t\bar t + 4μ$, $t\bar bν+ 3μ$ or $t\bar t + 4b$. Additional channels ($b\bar b + 4τ$, $6b$, etc.) arise in the case of a vectorlike quark of charge $-1/3$. If the vectorlike quark decays into three light quarks, then the $(3j)(3j)$ signal is more difficult to observe, and the vectorlike quark mass is almost unconstrained by current searches.

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arXiv2017

Minimal $SU(3)\times SU(3)$ symmetry breaking patterns

We study the vacua of an $SU(3)\times SU(3)$-symmetric model with a bifundamental scalar. Structures of this type appear in various gauge theories such as the Renormalizable Coloron Model, which is an extension of QCD, or the Trinification extension of the electroweak group. In other contexts, such as chiral symmetry, $SU(3)\times SU(3)$ is a global symmetry. As opposed to more general $SU(N)\times SU(N)$ symmetric models, the $N=3$ case is special due to the presence of a trilinear scalar term in the potential. We find that the most general tree-level potential has only three types of minima: one that preserves the diagonal $SU(3)$ subgroup, one that is $SU(2)\times SU(2)\times U(1)$ symmetric, and a trivial one where the full symmetry remains unbroken. The phase diagram is complicated, with some regions where there is a unique minimum, and other regions where two minima coexist.

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arXiv2017

Higgs-photon resonances

We study models that produce a Higgs boson plus photon ($h^0 γ$) resonance at the LHC. When the resonance is a $Z'$ boson, decays to $h^0 γ$ occur at one loop. If the $Z'$ boson couples at tree-level to quarks, then the $h^0 γ$ branching fraction is typically of order $10^{-5}$ or smaller. Nevertheless, there are models that would allow the observation of $Z' \to h^0 γ$ at $\sqrt{s} = 13$ TeV with a cross section times branching fraction larger than 1 fb for a $Z'$ mass in the 200--450 GeV range, and larger than 0.1 fb for a mass up to 800 GeV. The 1-loop decay of the $Z'$ into lepton pairs competes with $h^0 γ$, even if the $Z'$ couplings to leptons vanish at tree level. We also present a model in which a $Z'$ boson decays into a Higgs boson and a pair of collimated photons, mimicking an $h^0 γ$ resonance. In this model, the $h^0 γ$ resonance search would be the discovery mode for a $Z'$ as heavy as 2 TeV. When the resonance is a scalar, although decay to $h^0 γ$ is forbidden by angular momentum conservation, the $h^0$ plus collimated photons channel is allowed. We comment on prospects of observing an $h^0 γ$ resonance through different Higgs decays, on constraints from related searches, and on models where $h^0$ is replaced by a nonstandard Higgs boson.

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arXiv2016

Multi-step production of a diphoton resonance

Among the questions that would be raised by the observation of a new resonance at the LHC, particularly pressing are those concerning the production mechanism: What is the initial state? Is the resonance produced independently or in association with other particles? Here we present two weakly-coupled renormalizable models for production of a diphoton resonance that differ in both their initial and final states. In one model, a scalar particle produced through gluon fusion decays into a diphoton particle and a light, long-lived pseudoscalar. In another model, a $Z'$ boson produced from the annihilation of a strange-antistrange quark pair undergoes a cascade decay that leads to a diphoton particle and two sterile neutrinos. Various kinematic distributions may differentiate these models from the canonical model where a diphoton particle is directly produced in gluon fusion.

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arXiv2016

Signals of a 2 TeV $W'$ boson and a heavier $Z'$ boson

We construct an $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model with a Higgs sector that consists of a bidoublet and a doublet, and with a right-handed neutrino sector that includes one Dirac fermion and one Majorana fermion. This model explains the Run 1 CMS and ATLAS excess events in the $e^+e^-jj$, $jj$, $Wh^0$ and $WZ$ channels in terms of a $W'$ boson of mass near 1.9 TeV and of coupling $g_R$ in the 0.4--0.5 range, with the lower half preferred by limits on $t \bar b$ resonances and Run 2 results. The production cross section of this $W'$ boson at the 13 TeV LHC is in the 700--900 fb range, allowing sensitivity in more than 17 final states. We determine that the $Z'$ boson has a mass in the 3.4--4.5 TeV range and several decay channels that can be probed in Run 2 of the LHC, including cascade decays via heavy Higgs bosons.

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arXiv2015

Right-handed neutrinos and the 2 TeV $W'$ boson

The CMS $e^+ e^- jj$ events of invariant mass near 2 TeV are consistent with a $W'$ boson decaying into an electron and a right-handed neutrino whose TeV-scale mass is of the Dirac type. We show that the Dirac partner of the right-handed electron-neutrino can be the right-handed tau-neutrino. A prediction of this model is that the sum of the $τ^+ e^+ jj$ and $τ^- e^- jj$ signal cross sections equals twice that for $e^+ e^- jj$. The Standard Model neutrinos acquire Majorana masses and mixings compatible with neutrino oscillation data.

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arXiv2015

Dark matter beams at LBNF

High-intensity neutrino beam facilities may produce a beam of light dark matter when protons strike the target. Searches for such a dark matter beam using its scattering in a nearby detector must overcome the large neutrino background. We characterize the spatial and energy distributions of the dark matter and neutrino beams, focusing on their differences to enhance the sensitivity to dark matter. We find that a dark matter beam produced by a $Z'$ boson in the GeV mass range is both broader and more energetic than the neutrino beam. The reach for dark matter is maximized for a detector sensitive to hard neutral-current scatterings, placed at a sizable angle off the neutrino beam axis. In the case of the Long-Baseline Neutrino Facility (LBNF), a detector placed at roughly 6 degrees off axis and at a distance of about 200 m from the target would be sensitive to $Z'$ couplings as low as 0.05. This search can proceed symbiotically with neutrino measurements. We also show that the MiniBooNE and MicroBooNE detectors, which are on Fermilab's Booster beamline, happen to be at an optimal angle from the NuMI beam and could perform searches with existing data. This illustrates potential synergies between LBNF and the short-baseline neutrino program if the detectors are positioned appropriately.

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arXiv2015

A W' boson near 2 TeV: predictions for Run 2 of the LHC

We present a renormalizable theory that includes a $W'$ boson of mass in the 1.8-2 TeV range, which may explain the excess events reported by the ATLAS Collaboration in a $WZ$ final state, and by the CMS Collaboration in $e^+\!e^- jj$, $Wh^0$ and $jj$ final states. The $W'$ boson couples to right-handed quarks and leptons, including Dirac neutrinos with TeV-scale masses. This theory predicts a $Z'$ boson of mass in the 3.4-4.5 TeV range. The cross section times branching fractions for the narrow $Z'$ dijet and dilepton peaks at the 13 TeV LHC are 10 fb and 0.6 fb, respectively, for $M_{Z'}= 3.4$ TeV, and an order of magnitude smaller for $M_{Z'}= 4.5$ TeV.

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arXiv2015

Heavy Higgs bosons and the 2 TeV $W'$ boson

The hints from the LHC for the existence of a $W'$ boson of mass around 1.9 TeV point towards a certain $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ gauge theory with an extended Higgs sector. We show that the decays of the $W'$ boson into heavy Higgs bosons have sizable branching fractions. Interpreting the ATLAS excess events in the search for same-sign lepton pairs plus $b$ jets as arising from $W'$ cascade decays, we estimate that the masses of the heavy Higgs bosons are in the 400--700 GeV range.

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arXiv2015

Leptophobic boson signals with leptons, jets and missing energy

Color-singlet gauge bosons with renormalizable couplings to quarks but not to leptons must interact with additional fermions ("anomalons") required to cancel the gauge anomalies. Analyzing the decays of such leptophobic bosons into anomalons, I show that they produce final states involving leptons at the LHC. Resonant production of a flavor-universal leptophobic $Z'$ boson leads to cascade decays via anomalons, whose signatures include a leptonically decaying $Z$, missing energy and several jets. A $Z'$ boson that couples to the right-handed quarks of the first and second generations undergoes cascade decays that violate lepton universality and include signals with two leptons and jets, or with a Higgs boson, a lepton, a $W$ and missing energy.

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arXiv2014

GeV-scale dark matter: production at the Main Injector

Assuming that dark matter particles interact with quarks via a GeV-scale mediator, we study dark matter production in fixed target collisions. The ensuing signal in a neutrino near detector consists of neutral-current events with an energy distribution peaked at higher values than the neutrino background. We find that for a $Z'$ boson of mass around a few GeV that decays to dark matter particles, the dark matter beam produced by the Main Injector at Fermilab allows the exploration of a range of values for the gauge coupling that currently satisfy all experimental constraints. The NO$ν$A detector is well positioned for probing the presence of a dark matter beam, while future LBNF near-detectors would provide more sensitive probes.

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arXiv2014

Coupling--mass mapping of di-jet peak searches

We study hypothetical gauge bosons that may produce dijet resonances at the LHC. Simple renormalizable models include leptophobic Z' bosons or colorons that have flavor-independent couplings and decay into a color-singlet or -octet quark-antiquark pair, respectively. We present the experimental results on dijet resonances at hadron colliders as limits in the coupling-versus-mass plane of a gauge boson associated with baryon number. This theoretical framework facilitates a direct comparison of dijet resonance searches performed at different center-of-mass energies or at different colliders.

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arXiv2014

Interpretations of anomalous LHC events with electrons and jets

The CMS Collaboration has recently reported some excess events in final states with electrons and jets, in searches for leptoquarks and $W'$ bosons. Although these excesses may be due to some yet-to-be-understood background mismodeling, it is useful to seek realistic interpretations involving new particles that could generate such events. We show that resonant pair production of vector-like leptons that decay to an electron and two jets leads to kinematic distributions consistent with the CMS data.

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arXiv2014

Composite Vectorlike Fermions

We study a dynamical mechanism that generates a composite vectorlike fermion, formed by the binding of an $N$-tuplet of elementary chiral fermions to an $N$-tuplet of scalars. Deriving the properties of the composite fermion in the large $N$ limit, we show that its mass is much smaller than the compositeness scale when the binding coupling is near a critical value. We compute the contact interactions involving four composite fermions, and find that their coefficients scale as $1/N$. Physics beyond the Standard Model may include composite vectorlike fermions arising from this mechanism.

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arXiv2014

Higgs mass from compositeness at a multi-TeV scale

Within composite Higgs models based on the top seesaw mechanism, we show that the Higgs field can arise as the pseudo Nambu-Goldstone boson of the broken U(3)_L chiral symmetry associated with a vector-like quark and the t-b doublet. As a result, the lightest CP-even neutral state of the composite scalar sector is lighter than the top quark, and can be identified as the newly discovered Higgs boson. Constraints on weak isospin violation push the chiral symmetry breaking scale above a few TeV, implying that other composite scalars are probably too heavy to be probed at the LHC, but may be within reach at a future hadron collider with center-of-mass energy of about 100 TeV.

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arXiv2014

Hidden GeV-scale interactions of quarks

We explore quark interactions mediated by new gauge bosons of masses in the 0.3 - 50 GeV range. A tight upper limit on the gauge coupling of light Z' bosons is imposed by the anomaly cancellation conditions in conjunction with collider bounds on new charged fermions. Limits from quarkonium decays are model dependent, while electroweak constraints are mild. We derive the limits for a Z' boson coupled to baryon number, and then construct a Z' model with relaxed constraints, allowing quark couplings as large as 0.2 for a mass of a few GeV.

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arXiv2014

W' signatures with odd Higgs particles

We point out that W' bosons may decay predominantly into Higgs particles associated with their broken gauge symmetry. We demonstrate this in a renormalizable model where the W' and W couplings to fermions differ only by an overall normalization. This "meta-sequential" W' boson decays into a scalar pair, with the charged one subsequently decaying into a W boson and a neutral scalar. These scalars are odd under a parity of the Higgs sector, which consists of a complex bidoublet and a doublet. The W' and Z' bosons have the same mass and branching fractions into scalars, and may show up at the LHC in final states involving one or two electroweak bosons and missing transverse energy.

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