Large Invariant Mass Research Articles

Search for New Phenomena in Final States with Two Leptons and One or No b-Tagged Jets at sqrt[s]=13 TeV Using the ATLAS Detector.

A search for new phenomena is presented in final states with two leptons and one or no b-tagged jets. The event selection requires the two leptons to have opposite charge, the same flavor (electrons or muons), and a large invariant mass. The analysis is based on the full run-2 proton-proton collision dataset recorded at a center-of-mass energy of sqrt[s]=13 TeV by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 139 fb^{-1}. No significant deviation from the expected background is observed in the data. Inspired by the B-meson decay anomalies, a four-fermion contact interaction between two quarks (b, s) and two leptons (ee or μμ) is used as a benchmark signal model, which is characterized by the energy scale and coupling, Λ and g_{*}, respectively. Contact interactions with Λ/g_{*} lower than 2.0 (2.4)TeV are excluded for electrons (muons) at the 95%confidence level, still far below the value that is favored by the B-meson decay anomalies. Model-independent limits are set as a function of the minimum dilepton invariant mass, which allow the results to be reinterpreted in various signal scenarios.

Nonexclusive particle production by γγ interactions in pp collisions at the LHC

Particle production in two-photon interactions at hadronic collisions is becoming increasingly relevant in the LHC physics programme as a way to improve our understanding of the Standard Model and search for signals of New Physics. A key ingredient for the study of these interactions in $pp$ collisions is the description of the photon content of the proton, which allow us to derive predictions for the cross sections associated to events where occur the proton dissociation (non - exclusive processes) and for those where both incident protons remain intact (exclusive processes). In this paper, a detailed comparison of the different models for the elastic and inelastic photon distributions found in the literature is presented and the current theoretical uncertainty is estimated. The impact on the invariant mass distribution for the dimuon production is analyzed. Moreover, the relative contribution of non - exclusive events is estimated and its dependence on the invariant mass of the pair is presented. We demonstrate that the predictions for production of pairs with large invariant mass is strongly dependent on the model assumed to describe the elastic and inelastic photon distributions and that the ratio between non - exclusive and exclusive cross sections present a mild energy dependence. Finally, our results indicate that a future experimental analysis of the non - exclusive events will be useful to constrain the photon content of proton.

The {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil in HQET

In this paper we discuss the Standard Model and new physics sensitivity of the {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil, where ℓ± are massive leptons and Noverline{K}=left{{pK}^{-},n{overline{K}}^0right} . We provide a full angular distribution with a set of operators that includes the Standard Model operators and their chirality flipped counterparts, and new scalar and pseudo-scalar operators. The resulting angular distribution allows us to construct observables that we study in the Standard Model and in model-independent new physics scenarios. To reduce the hadronic effects emanating from the Λb→ Λ* (1520) transition form factors, we exploit the Heavy Quark Effective theory framework valid at low Λ∗(1520) recoil, i.e., large dilepton invariant mass squared {q}^2sim mathcal{O}left({m}_b^2right) . Working to the leading order in 1/mb and including mathcal{O}left({alpha}_sright) corrections, we compute the ‘improved Isgur-Wise relations’ between the form factors. The relations correlate the form factors and thereby allow the description of this decay at the low-recoil region with a smaller number of independent form factors.

Electroproduction of a large invariant mass photon pair

We study the exclusive electroproduction of a photon pair in the kinematical regime where the diphoton invariant mass is large and where the nucleon flies almost in its original direction. We discuss the relative importance of the QCD process where the two photons originate from a quark line compared to the single (double) Bethe-Heitler processes where one (two) photons originate from the lepton line. This process turns out to be a promising tool to study generalized parton distributions in the nucleon, both at the medium energy of JLab and at a high energy electron ion collider.

Search for GKK decays to pair of muons at 14 TeV

We study the Randall-Sundrum (RS) model that predict the existence of narrow resonances that appear in the di-lipton invariant mass distribution in proton-proton collisions. This gives final state with di-muon having large invariant mass from the decay of the RS1 graviton. A Large Hadron Collider (LHC) feasibility study for five-sigma discovery at 14 TeV was performed. The Kaluza-Klein graviton (GKK) production cross sections as a function of the graviton mass are presented and the limits on cross sections times branching ratio into muons to observe a 5 σ signal are presented. Our estimates based on integrated luminosity of 100 fb-1 and 1000 fb-1. The conclusion of our study is that the LHC can test the RS1 graviton existence if it is already existing with masses up to 2.6 TeV/c2 (3 TeV/c2) at luminosity of 100 fb-1 (1000 fb-1), when the parameter c (model parameter) is equal to 0.01. With the optimistic scenario, at c equal to 0.1, the GKK could be discovered if it is existing with mass up to 5 TeV/c2 with 100 fb-1 and mass up to 6.5 TeV/c2 with 1000 fb-1.

Observation of Electroweak Production of a Same-Sign W Boson Pair in Association with Two Jets in pp Collisions at sqrt[s]=13 TeV with the ATLAS Detector.

This Letter presents the observation and measurement of electroweak production of a same-sign W boson pair in association with two jets using 36.1 fb^{-1} of proton-proton collision data recorded at a center-of-mass energy of sqrt[s]=13 TeV by the ATLAS detector at the Large Hadron Collider. The analysis is performed in the detector fiducial phase-space region, defined by the presence of two same-sign leptons, electron or muon, and at least two jets with a large invariant mass and rapidity difference. A total of 122 candidate events are observed for a background expectation of 69±7 events, corresponding to an observed signal significance of 6.5 standard deviations. The measured fiducial signal cross section is σ^{fid}=2.89_{-0.48}^{+0.51}(stat)_{-0.28}^{+0.29}(syst) fb.

Search for anomalous electroweak production of vector boson pairs in association with two jets in proton-proton collisions at 13 TeV

A search for anomalous electroweak production of WW, WZ, and ZZ boson pairs in association with two jets in proton-proton collisions at s=13TeV at the LHC is reported. The data sample corresponds to an integrated luminosity of 35.9fb−1 collected with the CMS detector. Events are selected by requiring two jets with large rapidity separation and invariant mass, one or two leptons (electrons or muons), and a W or Z boson decaying hadronically. No excess of events with respect to the standard model background predictions is observed and constraints on the structure of quartic vector boson interactions in the framework of dimension-8 effective field theory operators are reported. Stringent limits on parameters of the effective field theory operators are obtained. The observed 95% confidence level limits for the S0, M0, and T0 operators are −2.7<fS0/Λ4<2.7, −1.0<fM0/Λ4<1.0, and −0.17<fT0/Λ4<0.16, in units of TeV−4. Constraints are also reported on the product of the cross section and branching fraction for vector boson fusion production of charged Higgs bosons as a function of mass from 600 to 2000 GeV. The results are interpreted in the context of the Georgi–Machacek model.

Erratum: Hard photoproduction of a diphoton with a large invariant mass [Phys. Rev. D 96 , 074008 (2017)

We correct the expression for the $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}{N}^{\ensuremath{'}}$ amplitude in the generalized Bjorken regime, where the diphoton is nearly forward and its invariant mass is the hard scale enabling to factorize the scattering amplitude in terms of generalized parton distributions. This amplitude has a very peculiar and interesting analytic structure, while the cross section estimates are only slightly changed.

HEJ 2: High energy resummation for hadron colliders

We present HEJ 2, a new implementation of the High Energy Jets formalism for high-energy resummation in hadron-collider processes as a flexible Monte Carlo event generator. In combination with a conventional fixed-order event generator, HEJ 2 can be used to obtain greatly improved predictions for a number of phenomenologically important processes by adding all-order logarithmic corrections in sˆ∕p⊥2. A prime example for such a process is the gluon-fusion production of a Higgs boson in association with widely separated jets, which constitutes the dominant background to Higgs boson production in weak-boson fusion. Program summaryProgram Title: HEJ 2Program Files doi:http://dx.doi.org/10.17632/24c9mb6pf9.1Licensing provisions: GPLv2 or laterProgramming language: C++Nature of problem: Conventional event generators for high-energy colliders are based on fixed-order perturbation theory, which converges poorly in regions of phase space with large invariant masses between jets. However, reliable predictions in these regions are of particular importance whenever cuts relevant to weak-boson fusion or weak-boson scattering are applied. One highly relevant example is the separation of gluon fusion and weak-boson fusion contributions to the production of a Higgs boson in association with two or more jets.Solution method: The perturbative description can be amended by resumming the high-energy logarithms associated with large invariant masses to all orders in perturbation theory. HEJ 2 implements this resummation following the High Energy Jets framework. Using HEJ 2 in conjunction with a conventional event generator allows one to obtain reliable predictions for processes involving multiple jets in high-energy regions of phase space.Additional comments including restrictions and unusual features: The current version of HEJ 2 implements multi-jet processes with at most a single Higgs boson in the final state. Matching to fixed-order perturbation theory is currently restricted to leading order.

Decay in supersymmetry with R-parity violation * * Supported by the National Natural Science Foundation of China (11775092, 11521064, 11435003, 11805077), XY is also supported in part by the startup research funding from CCNU

In past years, several hints of lepton flavor universality (LFU) violation have emerged from the and data. More recently, the Belle Collaboration has reported the first measurement of the longitudinal polarization fraction in the decay. Motivated by this intriguing result, along with the recent measurements of and polarization, we present the study of decays in supersymmetry (SUSY) with -parity violation (RPV). We consider , , and modes and focus on the branching ratios, LFU ratios, forward-backward asymmetries, polarizations of daughter hadrons, and the lepton. The RPV SUSY was capable of explaining the anomalies at the level, after taking into account various flavor constraints. In the allowed parameter space, the differential branching fractions and LFU ratios are largely enhanced by the SUSY effects, especially in the large dilepton invariant mass region. Moreover, a lower bound 10−6 is obtained. These observables could provide testable signatures at the high-luminosity LHC and SuperKEKB, and correlate with direct searches for SUSY.

Central exclusive diffractive production of K+K−K+K− via the intermediate ϕϕ state in proton-proton collisions

We present a study of the exclusive $pp \to pp K^{+} K^{-} K^{+} K^{-}$ reaction at high energies. We consider diffractive mechanisms with the intermediate $\phi \phi$ state with its decay into the $K^{+} K^{-} K^{+} K^{-}$ system. We include the $\phi(1020)$ $\hat{t}/\hat{u}$-channel exchanges and the $f_{2}(2340)$ $s$-channel exchange mechanism. This $f_{2}$ state is a candidate for a tensor glueball. We discuss the possibility to use the $pp \to pp \phi \phi$ process in identifying the odderon exchange. An upper limit for the $\mathbb{P} \mathbb{O} \phi$ coupling is extracted from the WA102 experimental data. The amplitudes for the processes are formulated within the tensor-pomeron and vector-odderon approach. We adjust parameters of our model to the WA102 data and present several predictions for the ALICE, ATLAS, CMS and LHCb experiments. Integrated cross sections of order of a few nb are obtained including the experimental cuts relevant for the LHC experiments. The distributions in the four-kaon invariant mass, rapidity distance between the two $\phi$ mesons, special ``glueball filter variable'', proton-proton relative azimuthal angle are presented. The distribution in rapidity difference of both $\phi$-mesons could shed light on the $f_{2}(2340) \to \phi \phi$ coupling, not known at present. We discuss the possible role of the $f_{0}(2100)$, $\eta(2225)$, and $X(2500)$ resonances observed in the $\phi \phi$ channel in radiative decays of $J/\psi$. Using typical kinematic cuts for LHC experiments we find from our model that the odderon-exchange contribution should be distinguishable from other contributions for large rapidity distance between the $\phi$ mesons and in the region of large four-kaon invariant masses. At least, it should be possible to derive an upper limit on the odderon contribution in this reaction.

Chiral partner structure of light nucleons in an extended parity doublet model

We study chiral partner structure of four light nucleons, $N(939)$, $N(1440)$, $N(1535)$ and $N(1650)$ using an effective chiral model based on the parity doublet structure. In our model we introduce four chiral representations, $({\bf 1},{\bf 2})$, $({\bf 2},{\bf 1})$, $({\bf 2},{\bf 3})$ and $({\bf 3},{\bf 2})$ under ${\rm SU}(2)_{\rm L} \otimes {\rm SU}(2)_{\rm R}$ symmetry. We determine the model parameters by fitting them to available experimental values of masses, widths and the axial charge of $N(939)$ together with the axial charges of $N(1535)$ and $N(1650)$ by lattice analyses. We find five groups of solutions: In a group the chiral partner to $N(939)$ is $N(1440)$ having small chiral invariant mass. In another group, the chiral partner is a mixture of $N(1535)$ and $N(1650)$ having a large chiral invariant mass. We claim that off-diagonal elements of axial-charge matrix can be used for distinguishing these groups. We also discuss changes of masses associated with chiral symmetry restoration, which could emerge in high density matter.

Searching for dark matter in final states with two jets and missing transverse energy

We reemphasise the usefulness of angular correlations in LHC searches for missing transverse energy (ETmiss) signatures that involve jet (j) pairs with large invariant mass. For the case of mono-jet production via gluon-fusion, we develop a realistic analysis strategy that allows to split the dark matter (DM) signal into distinct one jet-like and two jet-like event samples. By performing state-of-the-art Monte Carlo simulations of both the mono-jet signature and the standard model background, it is shown that the dijet azimuthal angle difference Delta {phi}_{j_1{j}_2} in 2j + ETmiss production provides a powerful discriminant in realistic searches. Employing a shape fit to Delta {phi}_{j_1{j}_2} , we then determine the LHC reach of the mono-jet channel in the context of spin-0 s-channel DM simplified models. The constraints obtained by the proposed Delta {phi}_{j_1{j}_2} shape fit turn out to be significantly more stringent than those that derive from standard ETmiss shape analyses.

Muon Identification using Neural Networks With the Muon Telescope Detector at STAR

The installation of the Muon Telescope Detector (MTD) at STAR allows a measurement of the dimuon (μ+μ−) production in heavy-ion collisions over a large invariant mass range for the first time. Data has been collected with the MTD from Au+Au collisions at sNN=200GeV and from p+p collisions at s=200GeV. These two datasets allow for new opportunities to measure the dimuon invariant mass spectra at STAR. Before any dimuon measurements can be made, muons must be identified. This contribution presents muon identification employing deep neural networks (DNN) and compares it with other multi-variate techniques. Applications of the DNN technique for data-driven purity measurements are discussed.

Probing Generalized Parton Distributions Through the Photoproduction of a $\gamma \pi $ Pair

We study in the framework of collinear QCD factorization the photoproduction of a $\gamma\,\pi$ pair with a large invariant mass and a small transverse momentum, as a new way to access generalized parton distributions. In the kinematics of JLab 12-GeV, we demonstrate the feasibility of this measurement and show the extreme sensitivity of the unpolarized cross section to the axial quark GPDs.

Probing axial quark generalized parton distributions through exclusive photoproduction of a \u03b3\u03c0\xb1 pair with a large invariant mass

Exclusive photoproduction of a γπ± pair in the kinematics where the pair has a large invariant mass and the final nucleon has a small transverse momentum is described in the collinear factorization framework. The scattering amplitude is calculated at leading order in αs and the differential cross sections for the process are estimated in the kinematics of the JLab 12-GeV experiments. The order of magnitude of the predicted cross-sections seems sufficient for a dedicated experiment to be performed. The process turns out to be very sensitive to the axial generalized parton distribution combination {tilde{H}}_u-{tilde{H}}_d .

Erratum to: Exclusive photoproduction of a γ ρ pair with a large invariant mass

An unfortunate misprint in our numerical code has led to an overestimate of the cross section for the exclusive photoproduction of a γ ρL pair in the kinematics where the pair has a large invariant mass and the final nucleon has a small transverse momentum, described in the collinear factorization framework. The rates are thus expected to be smaller but the possibility to disentangle the transversity GPDs through the process where the ρ−meson is transversely polarized is enhanced.

Transverse momentum resummation for t -channel single top quark production at the LHC

We study the effect of multiple soft gluon radiation on the kinematical distributions of the $t$-channel single top quark production at the LHC. By applying the transverse momentum dependent factorization formalism, large logarithms (of the ratio of large invariant mass $Q$ and small total transverse momentum $q_\perp$ of the single-top plus one-jet final state system) are resummed to all orders in the expansion of the strong interaction coupling at the accuracy of next-to-leading logarithm, including the complete next-to-leading order corrections. We show that the main difference from PYTHIA prediction lies on the inclusion of the exact color coherence effect between the initial and final states in our resummation calculation, which becomes more important when the final state jet is required to be in the forward region. We further propose a new experimental observable $\phi^*$ to test the effect of multiple gluon radiation in the single-top events. The effect of bottom quark mass is also discussed.

Search for Higgs boson decays into pairs of light (pseudo)scalar particles in the γγjj final state in pp collisions at [formula omitted] with the ATLAS detector

This Letter presents a search for exotic decays of the Higgs boson to a pair of new (pseudo)scalar particles, H→aa, where the a particle has a mass in the range 20–60 GeV, and where one of the a bosons decays into a pair of photons and the other to a pair of gluons. The search is performed in event samples enhanced in vector-boson fusion Higgs boson production by requiring two jets with large invariant mass in addition to the Higgs boson candidate decay products. The analysis is based on the full dataset of pp collisions at s=13TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36.7 fb−1. The data are in agreement with the Standard Model predictions and an upper limit at the 95% confidence level is placed on the production cross section times the branching ratio for the decay H→aa→γγgg. This limit ranges from 3.1 pb to 9.0 pb depending on the mass of the a boson.

Hard photoproduction of a diphoton with a large invariant mass

The electromagnetic probe has proven to be a very efficient way to access the $3-$dimensional structure of the nucleon, particularly thanks to the exclusive Compton processes. We explore the hard photoproduction of a large invariant mass diphoton in the kinematical regime where the diphoton is nearly forward and its invariant mass is the hard scale enabling to factorize the scattering amplitude in terms of generalized parton distributions. This amplitude has a very peculiar and interesting analytic structure. We calculate unpolarized cross sections and the angular asymmetry triggered by a linearly polarized photon beam.