arXiv subjects

Richard Ruiz

Publications and source records attributed to Richard Ruiz.

44 records · Page 3Linked to original sources

arXiv2017

A Comprehensive Framework for Studying $W'$ and $Z'$ Bosons at Hadron Colliders with Automated Jet Veto Resummation

The production of high-mass, color-singlet particles in hadron colliders is universally accompanied by initial state QCD radiation that is predominantly soft with respect to the hard process scale $Q$ and/or collinear with respect to the beam axis. At TeV-scale colliders, this is in contrast to top quark and multijet processes, which, by definition, are hard and central. Consequently, vetoing events with jets possessing transverse momenta above $p_T^{\rm Veto}$ in searches for new color-singlet states can efficiently reduce non-singlet backgrounds, thereby increasing experimental sensitivity. To quantify this generic observation, we investigate the production and leptonic decay of a Sequential Standard Model $W'$ boson at the 13 TeV LHC. We systematically consider signal and background processes at next-to-leading-order (NLO) in QCD with parton shower (PS) matching; for color-singlet channels, we resum Sudakov logarithms of the form $α_s^j(p_T^{\rm Veto})\log^k(Q/p_T^{\rm Veto})$ up to next-to-next-to-leading logarithmic accuracy (NNLL) with NLO matching. We obtain our results using the MadGraph5aMC@NLO and MadGraph5aMC@NLO-SCET frameworks, respectively. Associated FeynRules model files capable of handling NLO+PS- and NLO+NNLL-accurate computations are publicly available. We find that within their given uncertainties, both the NLO+PS and NLO+NNLL(veto) calculations give accurate and consistent predictions. Consequently, jet vetoes applied to color-singlet processes can be reliably modeled at the NLO+PS level. With respect to a $b$-jet veto of $p_{T}^{\rm Veto} = 30$ GeV, flavor-agnostic vetoes of $p_{T}^{\rm Veto} = 30-40$ GeV can further reduce single top and $t\overline{t}$ rates by a factor of 2-50 at a mild cost of the signal rate. Jet vetoes can increase signal-to-noise ratios by roughly 10\% for light $W'$ boson masses of 30-50 GeV and 25\%-250\% for masses of 300-800 GeV.

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arXiv2017

Neutrino Jets from High-Mass $W_R$ Gauge Bosons in TeV-Scale Left-Right Symmetric Models

We re-examine the discovery potential at hadron colliders of high-mass right-handed (RH) gauge bosons $W_R$ - an inherent ingredient of Left-Right Symmetric Models (LRSM). We focus on the regime where the $W_R$ is very heavy compared to the heavy Majorana neutrino $N$, and investigate an alternative signature for $W_R \rightarrow N$ decays. The produced neutrinos are highly boosted in this mass regime. Subsequently, their decays via off-shell $W_R$ bosons to jets, i.e., $N \rightarrow \ell^\pm j j$ are highly collimated, forming a single neutrino jet $(j_N)$. The final-state collider signature is then $\ell^\pm j_N$, instead of the widely studied $\ell^\pm\ell^\pm jj$. Present search strategies are not sensitive to this hierarchical mass regime due to the breakdown of the collider signature definition. We take into account QCD corrections beyond next-to-leading order (NLO) that are important for high-mass Drell-Yan processes at the 13 TeV Large Hadron Collider (LHC). For the first time, we evaluate $W_R$ production at NLO with threshold resummation at next-to-next-to-leading logarithm (NNLL) matched to the threshold-improved parton distributions. With these improvements, we find that a ${W_R}$ of mass $M_{W_R} = 3~(4)~[5]$ TeV and mass ratio of $(m_N/M_{W_R})<0.1$ can be discovered with a $5-6 σ$ statistical significance at 13 TeV after ${10~(100)~[2000]~\text{fb}^{-1}}$ of data. Extending the analysis to the hypothetical 100 TeV Very Large Hadron Collider (VLHC), $5σ$ can be obtained for $W_R$ masses up to $M_{W_R}=15~(30)$ with approximately $100~\text{fb}^{-1}~(10~\text{ab}^{-1})$.

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arXiv2016

Automated Neutrino Jet and Top Jet Predictions at Next-to-Leading-Order with Parton Shower Matching in Effective Left-Right Symmetric Models

Hadronic decays of boosted resonances, e.g., top quark jets, at hadronic super colliders are frequent predictions in TeV-scale extensions of the Standard Model of Particle Physics. In such scenarios, accurate modeling of QCD radiation is necessary for trustworthy predictions. We present the automation of fully differential, next-to-leading-order (NLO) in QCD corrections with parton shower (PS) matching for an effective Left-Right Symmetric Model (LRSM) that features $W_R^\pm, Z_R$ gauge bosons and heavy Majorana neutrinos $N$. Publicly available universal model files require remarkably fewer user inputs for predicting benchmark collider processes than leading order LRSM constructions. We present predictions for inclusive $W_R^\pm, Z_R$ production at the $\sqrt{s} = 13$ TeV Large Hadron Collider (LHC) and a hypothetical future 100 TeV Very Large Hadron Collider (VLHC), as well as inclusive $N$ production for a hypothetical Large Hadron Electron Collider (LHeC). As a case study, we investigate at NLO+PS accuracy the properties of heavy neutrino (color-singlet) jets and top quark (color-triplet) jets from decays of high-mass $W_R$ bosons at the LHC. Contrary to top jets, we find that the kinematic properties of heavy neutrinos jets, and in particular jet mass, are resilient against the effects of parton showers and hard QCD radiation. This suggests that in searches for neutrino jets, aggressive selection cuts that would otherwise be inappropriate for top jets can be imposed with minimal signal loss.

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arXiv2016

Fully-Automated Precision Predictions for Heavy Neutrino Production Mechanisms at Hadron Colliders

Motivated by TeV-scale neutrino mass models, we propose a systematic treatment of heavy neutrino $(N)$ production at hadron colliders. Our simple and efficient modeling of the vector boson fusion (VBF) $W^\pmγ\rightarrow N\ell^\pm$ and $N\ell^\pm+nj$ signal definitions resolve collinear and soft divergences that have plagued past studies, and is applicable to other color-singlet processes, e.g., associated Higgs $(W^\pm h)$, sparticle $(\tilde{\ell}^\pm\tilde{ν_\ell})$, and charged Higgs $(h^{\pm\pm}h^{\mp})$ production. We present, for the first time, a comparison of all leading $N$ production modes, including both gluon fusion (GF) $gg\rightarrow Z^*/h^*\rightarrow N\overset{(-)}{ν_\ell}$ and VBF. We obtain fully differential results up to next-to-leading order (NLO) in QCD accuracy using a Monte Carlo tool chain linking FeynRules, N{\small LO}CT, and MadGraph5\_aMC@NLO. Associated model files are publicly available. At the 14 TeV LHC, the leading order GF rate is small and comparable to the NLO $N\ell^\pm+1j$ rate; at a future 100 TeV Very Large Hadron Collider, GF dominates for $m_N={300-1500}$ GeV, beyond which VBF takes lead.

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arXiv2016

QCD Corrections to Pair Production of Type III Seesaw Leptons at Hadron Colliders

If kinematically accessible, hadron collider experiments provide an ideal laboratory for the direct production of heavy lepton partners in Seesaw models. In the context of the Type III Seesaw Mechanism, the $\mathcal{O}(α_s)$ rate and shape corrections are presented for the pair production of hypothetical, heavy $SU(2)_L$ triplet leptons in $pp$ collisions at $\sqrt{s}=$ 13, 14, and 100 TeV. The next-to-leading order (NLO) $K$-factors span, approximately, $K^{NLO}=1.1 - 1.4$ for both charged current and neutral current processes over a triplet mass range $m_T = 100~\text{GeV}-2~\text{TeV}$. Total production cross sections exhibit a $^{+5\%}_{-6\%}$ scale dependence at 14 TeV and $\pm1\%$ at 100 TeV. The NLO differential $K$-factors for heavy lepton kinematics are largely flat, suggesting that naïve scaling by the total $K^{NLO}$ is reasonably justified. The resummed transverse momentum distribution of the dilepton system is presented at leading logarithmic (LL) accuracy. The effects of resummation are large in TeV-scale dilepton systems. Discovery potential to heavy lepton pairs at 14 and 100 TeV is briefly explored: At the High-Luminosity LHC, we estimate a $4.8-6.3σ$ discovery potential maximally for $m_T = 1.5-1.6~\text{TeV}$ after 3000 fb$^{-1}$. With 300 (3000) fb$^{-1}$, there is $2σ$ sensitivity up to $m_T = 1.3-1.4~\text{TeV}~(1.7-1.8~\text{TeV})$ in the individual channels. At 100 TeV and with 10 fb$^{-1}$, a $5σ$ discovery can be achieved for $m_T=1.4-1.6~\text{TeV}$. Due to the factorization properties of Drell-Yan-type systems, the fixed order and resummed calculations reduce to convolutions over tree-level quantities.

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arXiv2015

Heavy Majorana Neutrinos from $Wγ$ Fusion at Hadron Colliders

Vector boson fusion processes become increasingly more important at higher collider energies and for probing larger mass scales due to collinear logarithmic enhancements of the cross section. In this context, we revisit the production of a hypothetic heavy Majorana neutrino $(N)$ at hadron colliders. Particular attention is paid to the fusion process $Wγ\rightarrow N\ell^{\pm}$. We systematically categorize the contributions from an initial state photon in the elastic, inelastic, and deeply inelastic channels. Comparing with the leading channel via the Drell-Yan production $q \bar{q}'\rightarrow W^{*}\rightarrow N\ell^{\pm}$ at NNLO in QCD, we find that the $Wγ$ fusion process becomes relatively more important at higher scales, surpassing the DY mechanism at $m_{N} \sim 1 \text{TeV} \ (770 \text{GeV})$ at the 14 TeV LHC (100 TeV VLHC). We investigate the inclusive heavy Majorana neutrino signal, including QCD corrections, and quantify the Standard Model backgrounds at future hadron colliders. We conclude that, with the currently allowed mixing $| V_{μN}| ^2<6\times 10^{-3}$, a $5σ$ discovery can be made via the same-sign dimuon channel for $m_N = 530 (1070)$ GeV at the 14 TeV LHC (100 TeV VLHC) after 1 ab$^{-1}$. Reversely, for $m_N = 500$ GeV and the same integrated luminosity, a mixing $| V_{μN}|^2$ of the order $1.1\times10^{-3} (2.5\times10^{-4})$ may be probed.

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arXiv2014

Higgs Bosons from Top Quark Decays

In light of the discovery of a Standard Model (SM)-like Higgs boson ($h$) at the LHC, we investigate the top quark to Higgs boson transition $t\rightarrow W^{*}bh$, which is the leading $t\to h$ decay mode in the SM. We find the decay branching fraction to be $1.80\times 10^{-9}$. In comparison, the two-body, loop-induced $t\rightarrow ch$ transition occurs at $\sim10^{-14}$ in the SM. We consider the consequences of gauge invariant dimension-6 operators affecting the $t\bar{t}h$ interaction and find that the decay branching fraction may be increased by a factor of two within current constraints on the coupling parameters from collider experiments. We also extend the calculation to the CP-conserving Type I and Type II Two Higgs Doublet Models (2HDM), including both CP-even and CP-odd Higgs bosons. For neutral scalar masses at about $100$ GeV, the decay rates can be several times larger than the SM result in the allowed range of model parameters. Observation prospects at present and future colliders are briefly addressed.

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arXiv2013

Lepton Number Violation and W' Chiral Couplings at the LHC

We study the observability for a heavy Majorana neutrino N along with a new charged gauge boson W' at the LHC. We emphasize the complementarity of these two particles in their production and decay to unambiguously determine their properties. We show that the Majorana nature of N can be verified by the lepton-number violating like-sign dilepton process, and by polar and azimuthal angular distributions. The chirality of the W' coupling to leptons and to quarks can be determined by a polar angle distribution in the reconstructed frame and an azimuthal angle distribution.

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