arXiv subjects

Cedric Weiland

Publications and source records attributed to Cedric Weiland.

7 recordsLinked to original sources

arXiv2019

Heavy Neutrinos with Dynamic Jet Vetoes: Multilepton Searches at $\sqrt{s} = 14,~27,$ and $100$ TeV

Heavy neutrinos $(N)$ remain one of most promising explanations for the origin of neutrinos' tiny masses and large mixing angles. In light of broad advances in understanding and modeling of hadron collisions at large momentum transfer, we revisit the long-standard search strategy for heavy $N$ decaying to multiple charged leptons $(\ell)$, $pp \to N\ell X \to 3\ell νX$. For electroweak and TeV-scale $N$, we propose a qualitatively new collider analysis premised on a dynamic jet veto and discriminating, on an event-by-event basis, according to the relative amount of hadronic and leptonic activity. We report that the sensitivity to $N$ at the Large Hadron Collider (LHC) can be improved by roughly an order of magnitude over the collider's lifetime. At $\sqrt{s}=14$ TeV with $\mathcal{L}=3~{\rm ab}^{-1}$, active-sterile mixing as small as $\vert V_{\ell N}\vert^2 = 10^{-2} ~(10^{-3})~[5\times10^{-4}]$ can be probed at $95\%$ CL for Dirac neutrinos masses $m_N \lesssim 1200~(300)~[200]$ GeV, well beyond present constraints for such heavy states. The improvement holds also for Majorana $N$, and is largely independent of whether charged lepton flavor is conserved or violated. The analysis, built almost entirely from inclusive, transverse observables, is designed to be robust across increasing collider energies, and hence serves as a basis for searches at future colliders: With $\mathcal{L}=15~{\rm ab}^{-1}$ at $\sqrt{s}=27$ TeV, one can probe mixing below $\vert V_{\ell N}\vert^2 = 10^{-2} ~(10^{-3})~[2\times10^{-4}]$ for $m_N \lesssim 3500~(700)~[200]$ GeV. At a hypothetical 100 TeV $pp$ collider with $\mathcal{L}=30~{\rm ab}^{-1}$, one can probe below $\vert V_{\ell N}\vert^2 = 9\times10^{-5}~(10^{-3})~[10^{-2}]$ for $m_N \lesssim 200$ GeV ($4$ TeV) [$15$ TeV]. We anticipate these results can be further improved with detector-specific tuning and application of machines learning techniques.

hep-ph↗

arXiv2018

$W^+ W^- H$ Production at Lepton Colliders: A New Hope for Heavy Neutral Leptons

We present the first study of the production of a Standard Model Higgs boson at a lepton collider in association with a pair of W bosons, $e^+_{} e^-_{} \to W^+_{} W^-_{} H$, in the inverse seesaw model. Taking into account all relevant experimental and theoretical constraints, we find sizable effects due to the additional heavy neutrinos up to -38% on the total cross-section at a center-of-mass energy of 3 TeV, and even up to -66% with suitable cuts. This motivates a detailed sensitivity analysis of the process $e^+_{} e^-_{} \to W^+_{} W^-_{} H$ as it could provide a new, very competitive experimental probe of low-scale neutrino mass models.

hep-ph↗

arXiv2018

Safe Jet Vetoes

Central jet vetoes are powerful tools for reducing QCD background in measurements and searches for electroweak and colorless, new physics processes in hadron collisions. In this letter, we report the key findings of a new philosophy to designing searches for such phenomena at hadron colliders, one designed and centered around a dynamical jet veto instead a static veto applied independently of other selection criteria. Specifically, we investigate the theoretical and phenomenological consequences of setting the jet veto scale to the transverse momentum $(p_T)$ of the leading charged lepton $\ell$ in multi-lepton processes on an event-by-event basis. We consider the case of a TeV-scale heavy neutrino $N$ decaying to the trilepton final state and find the following: (i) Perturbative uncertainties associated with the veto greatly reduce due to tying the veto scale to the hard process scale. (ii) The signal efficiency for passing the veto jumps to $\gtrsim95\%$ and exhibits little-to-no dependence on the neutrino mass scale. (iii) Top quark and `fake' lepton rejection capabilities also improve compared to only vetoing heavy flavor-tagged jets above a fixed $p_T$. This results in an increased sensitivity to active-sterile neutrino mixing by approximately an order of magnitude over the LHC's lifetime. For a Dirac neutrino with mass $m_N = 150-1000$ GeV and the representative active-sterile mixing hypothesis $\vert V_{e4}\vert = \vert V_{τ4}\vert$ with $\vert V_{μ4}\vert=0$, we find that LHC experiments can probe $\vert V_{e4}\vert^2, \vert V_{τ4}\vert^2 \lesssim 6\times10^{-4} - 8\times10^{-3}$, surpassing the global upper limit for $m_N < 450$ GeV, with $\mathcal{L}=3$ ab$^{-1}$ of data at $\sqrt{s}=14$ TeV. Due to the color structures of the heavy $N$ production mechanisms considered, we argue that our results hold broadly for other color-singlet processes.

hep-ph↗

arXiv2017

Impact of heavy sterile neutrinos on the triple Higgs coupling

New physics beyond the Standard Model is required to give mass to the light neutrinos. One of the simplest ideas is to introduce new heavy, gauge singlet fermions that play the role of right-handed neutrinos in a seesaw mechanism. They could have large Yukawa couplings to the Higgs boson, affecting the Higgs couplings and in particular the triple Higgs coupling $λ_{HHH}^{}$, the measure of which is one of the major goals of the LHC and of future colliders. We present a study of the impact of these heavy neutrinos on $λ_{HHH}^{}$ at the one-loop level, first in a simplified 3+1 model with one heavy Dirac neutrino and then in the inverse seesaw model. Taking into account all possible experimental constraints, we find that sizeable deviations of the order of 35% are possible, large enough to be detected at future colliders, making the triple Higgs coupling a new, viable observable to constrain neutrino mass models. The effects are generic and are expected in any new physics model including TeV-scale fermions with large Yukawa couplings to the Higgs boson, such as those using the neutrino portal.

hep-ph↗

arXiv2017

Sterile neutrinos facing kaon physics experiments

We discuss weak kaon decays in a scenario in which the Standard Model is extended by massive sterile fermions. After revisiting the analytical expressions for leptonic and semileptonic decays we derive the expressions for decay rates with two neutrinos in the final state. By using a simple effective model with only one sterile neutrino, compatible with all current experimental bounds and general theoretical constraints, we conduct a thorough numerical analysis which reveals that the impact of the presence of massive sterile neutrinos on kaon weak decays is very small, less than $1\%$ on decay rates. The only exception is $\mathcal{B} (K_L\to νν)$, which can go up to $\mathcal{O}( 10^{-10})$, thus possibly within the reach of the KOTO experiment. In other words, if all the future measurements of weak kaon decays turn out to be compatible with the Standard Model predictions, this would not rule out the existence of massive light sterile neutrinos with non-negligible active-sterile mixing. Instead, for a sterile neutrino of mass below $m_K$, one might obtain a huge enhancement of $\mathcal{B} (K_L\to νν)$, otherwise negligibly small in the Standard Model.

hep-ph↗

arXiv2013

Lepton Universality in Kaon Decays

In the Standard Model extended by sterile neutrinos, modified W-l-nu couplings arise, which are able to induce a tree-level enhancement to lepton flavour universality violation in kaon decays. The additional mixing between the active neutrinos and the sterile ones can generate deviations from unitarity in the leptonic mixing matrix for charged currents. We reconsidered this idea in the context of the inverse seesaw and evaluated its impact on the well measured ratio $R_K$. We show that the current experimental bound can be saturated in agreement with the different experimental and observational constraints. Similar results can be obtained when considering the ratio $R_π$.

hep-ph↗

arXiv2013

Enhanced lepton flavour violation in the supersymmetric inverse seesaw

In minimal supersymmetric seesaw models, the contribution to lepton flavour violation from Z-penguins is usually negligible. In this study, we consider the supersymmetric inverse seesaw and show that, in this case, the Z-penguin contribution dominates in several lepton flavour violating observables due to the low scale of the inverse seesaw mechanism. Among the observables considered, we find that the most constraining one is the muon to electron conversion rate which is already restricting the otherwise allowed parameter space of the model. Moreover, in this framework, the Z-penguins exhibit a non-decoupling behaviour, which has previously been noticed in lepton flavour violating Higgs decays.

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