arXiv subjects

Ezequiel Alvarez

Publications and source records attributed to Ezequiel Alvarez.

At least 37 records · Page 2Linked to original sources

arXiv2019

Four-top as probe of light top-philic New Physics

We study the four-top final state at the LHC as a probe for New Physics (NP) effects due to new particles that couple predominantly to the top quark and whose masses are below the top-quark-pair production threshold. We consider simple NP models containing a new particle with either spin 0, spin 1, or spin 2, and find benchmark points compatible with current experimental results. We find that interference effects between NP and QED amplitudes can be large, pointing out the necessity of NLO contributions to be explicitly computed and taken into account when NP is present. We examine kinematic differences between these models and the Standard Model (SM) at the parton level and the reconstructed level. In the latter case, we focus on events selected requiring two same-sign leptons and multiple jets. We investigate how the different Lorentz structure of the light NP affects the kinematic hardness, the polarization, the spin correlations, and the angular distributions of the parton-level and/or final-state particles. We find that spin-2 light NP would be identified by harder kinematics than the SM. We also show that the angular separation between the same-sign leptons is a sensitive observable for spin-0 NP. The spin-0 and spin-2 NP cases would also yield a signal in $t\bar t γγ$ with the invariant mass of the photons indicating the mass of the new particle. The spin-1 NP would be identified through an excess in four-top signal and slight or not modification in other observables, as for instance the lack of signal in $t\bar t γγ$ due to the Landau-Yang theorem. We comment on the opportunities that would open from the kinematic reconstruction of some of the top quarks in the four-top state. Our results provide new handles to probe for light top-philic NP as part of the ongoing experimental program of searches for four-top production at the LHC Run 2 and beyond.

hep-ph↗

arXiv2019

Non-resonant Leptoquark with multigeneration couplings to $μμjj$ and $μνjj$ at LHC

CMS has recently reported a moderate excess in the $μνjj$ final state in a second generation Leptoquark search, but they have disregarded it because the excess is not present in the $μμjj$ final state and because they do not observe the expected resonant peak in the distributions. As a proof of concept we show that a simple Leptoquark model including second and third generation couplings with non-negligible single- and non-resonant production in addition to usual pair production could explain the data: excess ($μνjj$), lack of excess ($μμjj$) and missing peak in the distributions; while being in agreement with collider constraints. We take this result and analysis as a starting point of a reconsideration of the ATLAS and CMS second generation Leptoquark searches. We also discuss which would be the consequences and modifications that should be performed in the searches to test if this deviation would correspond to a New Physics signal. We observe that low-energy flavor constraints can be avoided by adding heavier particles to the model.

hep-ph↗

arXiv2018

A composite pNGB leptoquark at the LHC

The measurements of $R_K^{(*)}$ and $R_{D}^{(*)}$ by BaBar, Belle and the LHCb collaborations could be showing a hint of lepton flavor universality violation that can be accommodated by the presence of suitable leptoquarks at the TeV scale. We consider an effective description, with leptoquarks arising as composite pseudo Nambu-Goldstone bosons, as well as anarchic partial compositeness of the SM fermions. Considering the $R_K^{(*)}$ anomaly within this framework, we study pair production of $S_3\sim(\bar 3,3)_{1/3}$ at the LHC. We focus on the component $S_3^{1/3}$ of the triplet, which decays predominantly into $tτ$ and $bν$, and study the bounds from existing searches at $\sqrt{s}=13$ TeV at the LHC. We find that sbottom searches in the $b\bar{b}$+MET final state best explore the region in parameter space preferred by our model and currently exclude $S_3^{1/3}$ masses up to $\sim$1 TeV. Additional searches, considering the $tτ$ and $tμ$ decay modes, are required to probe the full physical parameter space. In this paper we also recast existing studies on direct leptoquark searches in the $tτtτ$ channel and SM $t\bar{t}t\bar{t}$ searches, and obtain the regions in parameter space currently excluded. Practically the whole physical parameter space is currently excluded for masses up to $\sim$0.8 TeV, which could be extended up to $\sim$1 TeV with the full Run 3 dataset. We conclude that pair production searches for this leptoquark can benefit from considering the final state $t τb$ +MET, where the largest branching ratio is expected. We appraise that future explorations of leptoquarks explaining the B-anomalies with masses beyond the TeV should also consider single and non-resonant production in order to extend the mass reach.

hep-ph↗

arXiv2018

Exercise: Dark Matter as fields that evolve backward in time

We do the exercise of thinking the qualitative features of a new model in which the problems of Dark Matter (DM) and the Cosmic Microwave Background (CMB) homogeneity are apparently simultaneously solved. We consider that DM consists of fields that evolve backward in time and have a positive defined energy to provide the correct observed curvature of space. We postulate that these fields do not interact with SM fields other than by sharing the same metric, therefore physics in small curvature regimes is causal, without contradictions with available experiments. In strongly curved space causality is lost and solutions must fulfill given equations. The homogeneity problem is naturally solved by these DM fields since, evolving from our future to our past, they bring patches that are currently thought to be causally disconnected into thermal equilibrium for $t\gtrsim 0$ by the Big Bang time. This thermal equilibrium of DM fields in a strongly curved regime must be shared by the metric and our fields, therefore we see these patches with the same CMB temperature. Although we find unlikely that Nature is described by this theory, we find interesting to conclude that there are not obvious logical nor observational inconsistencies and we explore its features. Hopefully some thoughts and results in this work may be useful for other ideas.

gr-qc↗

arXiv2017

Measuring $|V_{td}|$ at LHC

We propose a direct measurement of the CKM element $V_{td}$ at the LHC. Taking profit of the imbalance between $d$ and $\bar d$ quark content in the proton, we show that a non-zero $V_{td}$ induces a charge asymmetry in the $tW$ associated production. The main backgrounds to this process, $t\bar t$ production, and $tW$ associated production mediated by $V_{tb}$, give charge symmetric contributions at leading order in QCD. Therefore, using specific kinematic features of the signal, we construct a charge asymmetry in the di-lepton final state which, due also to a reduction of systematic uncertainties in the asymmetry, is potentially sensitive to $V_{td}$ suppressed effects. In particular, using signal and background simulations up to detector level, we show that this new observable could improve the current direct upper bound on $|V_{td}|$ already with existing LHC data. We also project that $|V_{td}|$ values down to $\sim 10$ times the Standard Model prediction could be probed in the high luminosity phase of the LHC.

hep-ph↗

arXiv2017

Four Tops for LHC

We design a search strategy for the Standard Model $t\bar t t \bar t$ production at the LHC in the same-sign dilepton and trilepton channels. We study different signal features and, given the small expected number of signal events, we scrutinize in detail all reducible and irreducible backgrounds. Our analysis shows that by imposing a set of basic jet and lepton selection criteria, the SM $pp \to t\bar t t \bar t$ process could be evidenced in the near future, within Run-II, when combining both multi-lepton search channels. We argue that this search strategy should also be used as a guideline to test New Physics coupling predominantly to top-quarks. In particular, we show that a non-resonant New Physics enhancement in the four-top final state would be detectable through this search strategy. We study two {\it top-philic} simplified models of this kind, a neutral scalar boson and a $Z^\prime$, and present current and future exclusion limits on their mass and couplings.

hep-ph↗

arXiv2017

ttbb as a probe of New Physics at the LHC

We study the $t\bar t b \bar b$ final state at the LHC as a probe of New Physics that couples mainly to third generation of quarks. We analyze New Physics simplified models with resonances of spin 0, 1 and 2. The sensitivity of the final states $t\bar t b\bar b$, $t\bar t t \bar t$, $b \bar b b \bar b$ and $t \bar t$ on each of these models is used to identify an important region in parameter space that is still not excluded and where $t\bar t b \bar b$ is the most sensitive final state. We indicate possible experimental hints and discuss potential issues of observables that rely mainly in Montecarlo predictions. A new observable is proposed that, at the price of requiring more statistics, reduces the impact of Montecarlo predictions. We use preliminary 13 TeV results to give a raw estimate on the discovery reach and propose simple improvements on the observables.

hep-ph↗

arXiv2016

Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data

The astrophysical sources responsible for ultra high-energy cosmic rays (UHECRs) continue to be one of the most intriguing mysteries in astrophysics. We present a comprehensive search for correlations between high-energy ($\gtrapprox 1$ GeV) gamma-ray events from the Fermi Large Area Telescope (LAT) and UHECRs ($\gtrapprox 60$ EeV) detected by the Telescope Array and the Pierre Auger Observatory. We perform two separate searches. First, we conduct a standard cross-correlation analysis between the arrival directions of 148 UHECRs and 360 gamma-ray sources in the Second Catalog of Hard Fermi-LAT sources (2FHL). Second, we search for a possible correlation between UHECR directions and unresolved Fermi-LAT gamma-ray emission. For the latter, we use three different methods: a stacking technique with both a model-dependent and model-independent background estimate, and a cross-correlation function analysis. We also test for statistically significant excesses in gamma rays from signal regions centered on Cen A and the Telescope Array hotspot. No significant correlation is found in any of the analyses performed, except a weak ($\lessapprox 2σ$) hint of signal with the correlation function method on scales $\sim 1^\circ$. Upper limits on the flux of possible power-law gamma-ray sources of UHECRs are derived.

astro-ph.HE↗

arXiv2016

Graviton resonance phenomenology and a pNGB Higgs at the LHC

We present an effective description of a spin two massive state and a pseudo Nambu-Goldstone boson Higgs in a two site model. Using this framework we model the spin two state as a massive graviton and we study its phenomenology at the LHC. We find that a reduced set of parameters can describe the most important features of this scenario. We address the question of which channel is the most sensitive to detect this graviton. Instead of designing search strategies to estimate the significance in each channel, we compare the ratio of our theoretical predictions to the limits set by available experimental searches for all the decay channels and as a function of the free parameters in the model. We discuss the phenomenological details contained in the outcome of this simple procedure. The results indicate that, for the studied masses between 0.5 and 3 TeV, the channels to look for such a graviton resonance are mainly $ZZ$, $WW$ and $γγ$. This is the case even though top and bottom quarks dominate the branching ratios, since their experimental sensitivity is not as good as the one of the electro-weak gauge bosons. We find that as the graviton mass increases, the $ZZ$ and $WW$ channels become more important because of its relatively better enhancement over background, mainly due to fat jet techniques. We determine the region of the parameter space that has already been excluded and the reach for the LHC next stages. We also estimate the size of the loop-induced contributions to the production and decay of the graviton, and show in which region of the parameter space their effects are relevant for our analysis.

hep-ph↗

arXiv2016

A 750 GeV graviton and the Higgs as a pNGB

We study the diphoton excess at 750 GeV reported by ATLAS and CMS, by assuming that it corresponds to a new spin-two resonant state. We model this state as a massive graviton in a two-site model. We show that the very stringent bounds from $VV$ final states can be evaded naturally by considering that the Higgs is a pseudo Nambu-Goldstone boson. In this case the couplings of the graviton to the longitudinal electroweak gauge bosons can be parametrically suppressed. On the other hand, partial compositeness allows to suppress the leptonic channels. We compute loop-induced contributions to the graviton couplings by the presence of the SM third generation of quarks and composite partners of the SM fermions and obtain that they are not important. We find that the diphoton signal and the experimental constraints from other decay channels can be reproduced in a large and natural region of the parameter space of the theory.

hep-ph↗

arXiv2014

Deepening of the ocean mixed layer at the northern Patagonian continental shelf: a numerical study

A possible deepening of the ocean mixed layer was investigated at a selected point of the Patagonian continental shelf where a significant positive wind speed trend was estimated. Using a 1-dimensional vertical numerical model (S2P3) forced by atmospheric data from NCEP/NCAR I reanalysis and tidal constituents from TPXO 7.2 global model on a long term simulation (1979-2011), it was found that the mixed layer thickness presents a significant and positive trend of 10.1 +/- 1.4 cm/yr. Several numerical experiments were carried out in order to evaluate the impact of the different atmospheric variables (surface zonal and latitudinal wind components, air temperature, atmospheric pressure, specific humidity and cloud coverage) considered in this study. As a result it was found that an increase in the wind speed can be considered as the main responsible of the ocean mixed layer deepening at the selected location of the Patagonian continental shelf. A possible increasing in the mixed layer thickness could be directly impacting on the sea surface temperature. Preliminary results obtained in this paper show that a slight but significant cooling in the temperature of the sea upper layer (of the order of 1 C every 50 years) could be happening since some decades ago at the northern Patagonian continental shelf waters.

physics.ao-ph↗

arXiv2013

Single production of an exotic bottom partner at LHC

We study single production and detection at the LHC run II of exotic partners of the bottom quark. For masses larger than 1 TeV single production can dominate over pair production that is suppressed due to phase space. The presence of exotic partners of the bottom is motivated in models aiming to solve the $A_{FB}^b$ anomaly measured at LEP and SLC. Minimal models of this type with partial compositeness predict, as the lightest bottom partner, a new fermion V of electric charge -4/3, also called mirror. The relevant coupling for our study is a WVb vertex, which yields a signal that corresponds to a hard W, a hard b-jet and a forward light jet. We design a search strategy for the leptonic decay of the W, which avoids the large QCD multijet background and its large uncertainties. We find that the main backgrounds are W+jets and ttbar, and the key variables to enhance the signal over them are a hard b-jet and the rapidity of the light jet. We determine the discovery reach for the LHC run II, in particular we predict that, for couplings of order ~g/10, this signal could be detected at a 95% confidence level with a mass up to 2.4 TeV using the first 100 fb-1.

hep-ph↗

arXiv2013

Leptonic Monotops at LHC

We study the possibility of detecting New Physics (NP) phenomena at the LHC through a new search strategy looking at the monotop (top plus missing energy) signature which is common to a variety of NP models. We focus on the leptonic top decay mode and study the discovery or exclusion reach of the 2012 LHC data for three example models. Contrary to the hadronic mode, in this case the problematic QCD multijet background can be safely neglected. We find that the key kinematic variable to suppress most of the remaining SM backgrounds is the transverse mass of the charged lepton and missing energy. In fact, one could expect that the single-top production measurements already address the monotop signature in this mode. This is however not the case because in the SM single-top production the transverse mass has an end point determined by the W mass, while the NP signals typically have an additional source of missing energy. We compare, under the same conditions, our monotop search strategy with existing single-top measurements and find a considerable improvement in the monotop signature reach.

hep-ph↗

arXiv2012

Top-antitop resonance searches beyond 1 TeV

We perform a general parton level analysis for the search of heavy resonant states in the production of top-antitop pairs at the LHC with an integrated luminosity of 30 fb^-1. We assume the existence of resonances that only couple to quarks and propose kinematic cuts in order to increase the amount of events produced through quark-annihilation. We study the interplay between different variables and their impact on the purity of the selected sample. We make focus on the longitudinal ($β$) and transverse ($p_T$) momentum of the top-antitop pair, and the scattering angle ($θ$) in the center of mass reference frame. We observe that $β$ is replaced by $θ$ as a suitable discriminating variable of quark-annihilation processes for invariant masses above 1 TeV. Finally, we illustrate the analysis with a gluon resonance of 1.5 TeV and show the improvement in the sensitivity of the signal when cuts on $θ$ are imposed.

hep-ph↗

arXiv2012

Enhancing the sensitivity to New Physics in the top-antitop invariant mass distribution

We propose selection cuts on the LHC top-antitop production sample which should enhance the sensitivity to New Physics signals in the study of the top-antitop invariant mass distribution. We show that selecting events in which the top-antitop object has little transverse and large longitudinal momentum enlarges the quark-fusion fraction of the sample and therefore increases its sensitivity to New Physics which couples to quarks and not to gluons. We find that systematic error bars play a fundamental role and assume a simple model for them. We check how a non-visible new particle would become visible after the selection cuts enhance its resonance bump. A final realistic analysis should be done by the experimental groups with a correct evaluation of the systematic error bars.

hep-ph↗

arXiv2012

A charged Z' to conciliate the apparent disagreement between top-antitop Tevatron forward-backward asymmetry and LHC charge asymmetry

We propose a charged, electrically neutral, and flavour changing Z' model to conciliate the apparent disagreement between the important excess found in the top-antitop Tevatron forward-backward asymmetry and the null -compatible with negative- results found in the LHC charge asymmetry. We show that this model contributes positively to the forward-backward asymmetry, whereas naturally a new cancellation is turned on at the LHC, yielding a null, or even negative, charge asymmetry. We found the region in parameter space that is simultaneously allowed by the stringent Tevatron and LHC observables. We show that the model is safe to atomic parity violation constraints and to tj resonance searches, and propose a possible increase in the Z' width to avoid ttj constraints. We evaluate the constraints to the model, as well as distinctive features in the fore-coming experimental results.

hep-ph↗

arXiv2012

Improving top quark induced charge asymmetries at the LHC using ttbar transverse momentum

We study how the significance of top quark induced charge asymmetries at the LHC may be enhanced exploiting the $t\bar t$ total transverse momentum to enrich the fraction of quark fusion events in the sample. We combine this variable with previous variables related to the boost and the invariant mass of the $t\bar t$ pair to find an optimum cut which maximizes the significance of the asymmetry when systematic and statistic errors are taken into account. We find that including the $t\bar t$ transverse momentum in the analysis of the expected 2012 LHC data provides a considerable enhancement in the significance of the asymmetry.

hep-ph↗

arXiv2012

Reach the Bottom Line of the Sbottom Search

We propose a new search strategy for directly-produced sbottoms at the LHC with a small mass splitting between the sbottom and its decayed stable neutralino. Our search strategy is based on boosting sbottoms through an energetic initial state radiation jet. In the final state, we require a large missing transverse energy and one or two b-jets besides the initial state radiation jet. We also define a few kinematic variables to further increase the discovery reach. For the case that the sbottom mainly decays into the bottom quark and the stable neutralino, we have found that even for a mass splitting as small as 10 GeV sbottoms with masses up to around 400 GeV can be excluded at the 95% confidence level with 20 inverse femtobarn data at the 8 TeV LHC.

hep-ph↗