Dimuon Invariant Mass Research Articles

Curvature-bias corrections using a pseudomass method

Momentum measurements for very high momentum charged particles, such as muons from electroweak vector boson decays, are particularly susceptible to charge-dependent curvature biases that arise from misalignments of tracking detectors. Low momentum charged particles used in alignment procedures have limited sensitivity to coherent displacements of such detectors, and therefore are unable to fully constrain these misalignments to the precision necessary for studies of electroweak physics. Additional approaches are therefore required to understand and correct for these effects. In this paper the curvature biases present at the LHCb detector are studied using the pseudomass method in proton-proton collision data recorded at centre of mass energy √(s)=13 TeV during 2016, 2017 and 2018. The biases are determined using Z→μ + μ - decays in intervals defined by the data-taking period, magnet polarity and muon direction. Correcting for these biases, which are typically at the 10-4 GeV-1 level, improves the Z→μ + μ - mass resolution by roughly 18% and eliminates several pathological trends in the kinematic-dependence of the mean dimuon invariant mass.

Evaluation of gradient boosting and deep learning algorithms in dimuon production

Usage of artificial intelligence (AI) and gradient boosting based machine learning (ML) algorithms in various fields has been favored to advance the human performances due to their accomplishments in real-world issues requiring intensive computation. For this reason, these methods have been started to be applied in physics fields such as material science, particle, and nuclear physics. Dimuons are muon (µ−) and anti-muon (µ+) pairs produced at the different stages of the medium created in high-energy collisions which can be used to understand the formation of the universe. Since they do not interact strongly, they can be utilized to scrutinize properties of the formed medium such as the thermal radiation and production mechanism of various particles. The success of AI and gradient boosting based ML methods suggested their adaptations in particle analysis. In this study, LightGBM, a gradient boosting framework, and Deep Neural Networks (DNNs) performances were examined for the determination of dimuon mass spectrum in proton-proton collisions at the LHC which can be an example for other spectrum studies. From the comparison of the models it is revealed that DNNs performed 99.993% success to determine dimuon invariant mass spectrum with 99.989% sensitivity and 99.989% precision. The results demonsrate the power of nonlinearity property of DNNs to find spectrum in physics studies.

Taming the long distance effects in the Ds+→π+ℓ−ℓ+ decay

The semileptonic decay ${D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}{\ensuremath{\ell}}^{+}$, where $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, is used as a reference channel when looking for suppressed or forbidden processes of the standard model of the form ${D}_{s}\ensuremath{\rightarrow}P{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{\ensuremath{'}+}$. The process is dominated by hadronic resonances from the $\ensuremath{\eta}$ to the $\ensuremath{\phi}$ meson region, usually excluded due to the large uncertainties associated. In this work, we explore the role of the parameters involved in the resonance region. We focus in the relative strong phase between the $\ensuremath{\rho}$ and the $\ensuremath{\phi}$ meson (${\ensuremath{\delta}}_{\ensuremath{\rho}\ensuremath{\phi}}$). We argue that this parameter can be bound from LHCb data of the dimuon invariant mass in the ${D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{+}$ decay and we perform a fit to provide a first approach to it. We obtain ${\ensuremath{\delta}}_{\ensuremath{\rho}\ensuremath{\phi}}=(0.44\ifmmode\pm\else\textpm\fi{}0.24)\ensuremath{\pi}$. In contradistinction, it would be useful to consider observables insensitive to this phase. We first compute the invariant mass of the lepton pair at a given angle of one of the leptons emission with respect to the pion, and then we compute the forward-backward distribution at such angle. We show that for the latter observable the resonance region is smeared, exhibiting no dependence on the ${\ensuremath{\delta}}_{\ensuremath{\rho}\ensuremath{\phi}}$ phase, although global effects are observed by comparing with the pure phase space approach. In order to consider this observable in more general scenarios, we analyze the behavior for the resonant background process ${D}_{s}^{+}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\pi}$, the short distance contribution in the standard model, and the charged Higgs mediated process as given by the two Higgs doublet model, which exhibit distinguishable features among themselves.

Precision measurement of forward Z boson production in proton-proton collisions at sqrt{mathrm{s}} = 13 TeV

A precision measurement of the Z boson production cross-section at sqrt{mathrm{s}} = 13 TeV in the forward region is presented, using pp collision data collected by the LHCb detector, corresponding to an integrated luminosity of 5.1 fb−1. The production cross-section is measured using Z → μ+μ− events within the fiducial region defined as pseudorapidity 2.0 < η < 4.5 and transverse momentum pT> 20 GeV/c for both muons and dimuon invariant mass 60 < Mμμ< 120 GeV/c2. The integrated cross-section is determined to beσZ→μ+μ−=196.4±0.2±1.6±3.9pb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sigma \\left(Z\ o {\\mu}^{+}{\\mu}^{-}\\right)=196.4\\pm 0.2\\pm 1.6\\pm 3.9\\ \\mathrm{pb}, $$\\end{document}where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measured results are in agreement with theoretical predictions within uncertainties.

Angular Analysis of D0→π+π−μ+μ− and D0→K+K−μ+μ− Decays and Search for CP Violation

The first full angular analysis and an updated measurement of the decay-rate CP asymmetry of the D^{0}→π^{+}π^{-}μ^{+}μ^{-} and D^{0}→K^{+}K^{-}μ^{+}μ^{-} decays are reported. The analysis uses proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13TeV. The dataset corresponds to an integrated luminosity of 9 fb^{-1}. The full set of CP -averaged angular observables and their CP asymmetries are measured as a function of the dimuon invariant mass. The results are consistent with expectations from the standard model and with CP symmetry.

Search for Higgs boson decays into a pair of pseudoscalar particles in the bbμμ final state with the ATLAS detector in pp collisions at s=13 TeV

This paper presents a search for decays of the Higgs boson with a mass of 125 GeV into a pair of new pseudoscalar particles, $H\rightarrow aa$, where one $a$-boson decays into a $b$-quark pair and the other into a muon pair. The search uses 139 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}=13$ TeV recorded between 2015 and 2018 by the ATLAS experiment at the LHC. A narrow dimuon resonance is searched for in the invariant mass spectrum between 16 GeV and 62 GeV. The largest excess of events above the Standard Model backgrounds is observed at a dimuon invariant mass of 52 GeV and corresponds to a local (global) significance of $3.3 \sigma$ ($1.7 \sigma$). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the $bb\mu\mu$ final state, $\mathcal{B}(H\rightarrow aa\rightarrow bb\mu\mu)$, and are in the range $\text{(0.2-4.0)} \times 10^{-4}$, depending on the signal mass hypothesis.

Angular analysis of the rare decay {B}_s^0 \u2192 \u03d5\u03bc+\u03bc\u2212

An angular analysis of the rare decay {B}_s^0 → ϕμ+μ− is presented, using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 8.4 fb−1. The observables describing the angular distributions of the decay {B}_s^0 → ϕμ+μ− are determined in regions of q2, the square of the dimuon invariant mass. The results are consistent with Standard Model predictions.

Branching Fraction Measurements of the Rare B_{s}^{0}→ϕμ^{+}μ^{-} and B_{s}^{0}→f_{2}^{'}(1525)μ^{+}μ^{-} Decays.

The branching fraction of the rare B_{s}^{0}→ϕμ^{+}μ^{-} decay is measured using data collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13TeV, corresponding to integrated luminosities of 1, 2, and 6 fb^{-1}, respectively. The branching fraction is reported in intervals of q^{2}, the square of the dimuon invariant mass. In the q^{2} region between 1.1 and 6.0 GeV^{2}/c^{4}, the measurement is found to lie 3.6 standard deviations below a standard model prediction based on a combination of light cone sum rule and lattice QCD calculations. In addition, the first observation of the rare B_{s}^{0}→f_{2}^{'}(1525)μ^{+}μ^{-} decay is reported with a statistical significance of 9 standard deviations and its branching fraction is determined.

Angular Analysis of the B^{+}→K^{*+}μ^{+}μ^{-} Decay.

We present an angular analysis of the B^{+}→K^{*+}(→K_{S}^{0}π^{+})μ^{+}μ^{-} decay using 9 fb^{-1} of pp collision data collected with the LHCb experiment. For the first time, the full set of CP-averaged angular observables is measured in intervals of the dimuon invariant mass squared. Local deviations from standard model predictions are observed, similar to those in previous LHCb analyses of the isospin-partner B^{0}→K^{*0}μ^{+}μ^{-} decay. The global tension is dependent on which effective couplings are considered and on the choice of theory nuisance parameters.

Angular analysis of the decay B+\u2192 K\u2217(892)+\u03bc+\u03bc\u2212 in proton-proton collisions at sqrt{mathrm{s}} = 8 TeV

Angular distributions of the decay B+→ K∗(892)+μ+μ− are studied using events collected with the CMS detector in sqrt{mathrm{s}} = 8 TeV proton-proton collisions at the LHC, corresponding to an integrated luminosity of 20.0 fb−1. The forward-backward asymmetry of the muons and the longitudinal polarization of the K∗(892)+ meson are determined as a function of the square of the dimuon invariant mass. These are the first results from this exclusive decay mode and are in agreement with a standard model prediction.

Testing the HyperCP anomaly at LHCb: evidence for the Σ+ → pµ + µ − decay

A search for Σ+ → pµ + µ − decays with LHCb Run 1 data is presented, yielding an evidence for this decay and a measurement of a branching fraction compatible with the Standard Model. The HyperCP anomaly in the dimuon invariant mass distribution is not confirmed, and its branching fraction central value is excluded at 95% CL. Prospects for the future of this measurement in LHCb are also presented.

Polarization Observables in WZ Production at the 13 TeV LHC: Inclusive Case

We present a study of the polarization observables of the $W$ and $Z$ bosons in the process \(p p \to W^\pm Z\to e^\pm \nu_e \mu^+\mu^-\) at the 13 TeV Large Hadron Collider. The calculation is performed at next-to-leading order, including the full QCD corrections as well as the electroweak corrections, the latter being calculated in the double-pole approximation. The results are presented in the helicity coordinate system adopted by ATLAS and for different inclusive cuts on the di-muon invariant mass. We define left-right charge asymmetries related to the polarization fractions between the \(W^+ Z\) and \(W^- Z\) channels and we find that these asymmetries are large and sensitive to higher-order effects. Similar findings are also presented for charge asymmetries related to a P-even angular coefficient.

Study of electromagnetic Dalitz decays χcJ→μ+μ−J/ψ

Using $4.48 \times 10^{8}$ $\psi(3686)$ events collected with the BESIII detector, we search for the decays $\chi_{cJ} \rightarrow \mu^{+}\mu^{-}J/\psi$ through the radiative decays $\psi(3686) \rightarrow \gamma\chi_{cJ}$, where $J=0,1,2$. The decays $\chi_{c1,2} \rightarrow \mu^{+}\mu^{-}J/\psi$ are observed, and the corresponding branching fractions are measured to be $\mathcal{B}(\chi_{c1} \rightarrow \mu^{+}\mu^{-}J/\psi) = (2.51 \pm 0.18 \pm 0.20)\times10^{-4}$ and $\mathcal{B}(\chi_{c2} \rightarrow \mu^{+}\mu^{-}J/\psi) = (2.33 \pm 0.18 \pm 0.29)\times10^{-4}$, where the first uncertainty is statistical and the second one systematic. No significant $\chi_{c0} \rightarrow \mu^{+}\mu^{-}J/\psi$ decay is observed, and the upper limit on the branching fraction is determined to be $2.0\times10^{-5}$ at 90% confidence level. Also, we present a study of di-muon invariant mass dependent transition form factor for the decays $\chi_{c1,2} \rightarrow \mu^{+}\mu^{-}J/\psi$.

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.

Angular analyses of b → sll transitions at CMS

The flavour changing neutral current decays are interesting probes for searching for new physics. Angular distributions of b → sℓ+ℓ− transition processes of both B0 → K*0µ+µ− and B+ → K+µ+µ− are studied using a sample of proton-proton collisions at collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 20.5 fb−1. Angular analyses are performed to determine P1 and angular parameters for B0 → K*0µ+µ− and AFB and FH parameters for B+ → K+µ+µ−, all as functions of the dimuon invariant mass squared. All of the measurements are consistent with the standard model predictions.

Decay rate and asymmetries of \u03a3+ \u2192 p\u03bc+\u03bc\u2212

The LHCb Collaboration has recently reported evidence for the rare hyperon decay Σ+ → pμ+μ− that is consistent with the standard model expectation. Motivated by this new result we revisit the calculation of this mode including both long and short distance contributions. In the standard model this mode is completely dominated by long distance physics and thus subject to large uncertainties. We present a range of predictions for the rate, the dimuon invariant mass spectrum, and a muon polarization asymmetry that covers these uncertainties as well as known constraints. We study the interplay between short and long distance contributions which could result in additional asymmetries, but we find that they are negligible within the standard model. We propose a parameterization of these asymmetries in terms of a couple of constants that can arise from new physics.

Erratum to: Differential branching fraction and angular analysis of Λ b 0 → Λμ + μ − decays

The differential branching fraction of the rare decay $\Lambda^{0}_{b} \rightarrow \Lambda \mu^+\mu^-$ is measured as a function of $q^{2}$, the square of the dimuon invariant mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of $3.0 \mbox{ fb}^{-1}$, collected by the LHCb experiment. Evidence of signal is observed in the $q^2$ region below the square of the $J/\psi$ mass. Integrating over $15 < q^{2} < 20 \mbox{ GeV}^2/c^4$ the branching fraction is measured as $d\mathcal{B}(\Lambda^{0}_{b} \rightarrow \Lambda \mu^+\mu^-)/dq^2 = (1.18 ^{+ 0.09} _{-0.08} \pm 0.03 \pm 0.27) \times 10^{-7} ( \mbox{GeV}^{2}/c^{4})^{-1}$, where the uncertainties are statistical, systematic and due to the normalisation mode, $\Lambda^{0}_{b} \rightarrow J/\psi \Lambda$, respectively. In the $q^2$ intervals where the signal is observed, angular distributions are studied and the forward-backward asymmetries in the dimuon ($A^{l}_{\rm FB}$) and hadron ($A^{h}_{\rm FB}$) systems are measured for the first time. In the range $15 < q^2 < 20 \mbox{ GeV}^2/c^4$ they are found to be $A^{l}_{\rm FB} = -0.05 \pm 0.09 \mbox{ (stat)} \pm 0.03 \mbox{ (syst)}$ and $A^{h}_{\rm FB} = -0.29 \pm 0.07 \mbox{ (stat)} \pm 0.03 \mbox{ (syst)}$.

Angular moments of the decay \u039bb0\u2009\u2192\u2009\u039b\u03bc+\u03bc\u2212 at low hadronic recoil

An analysis of the angular distribution of the decay Λb0 → Λμ+μ− is presented, using data collected with the LHCb detector between 2011 and 2016 and corresponding to an integrated luminosity of approximately 5 fb−1. Angular observables are determined using a moment analysis of the angular distribution at low hadronic recoil, corresponding to the dimuon invariant mass squared range 15 < q2 < 20 GeV2/c4. The full basis of observables is measured for the first time. The lepton-side, hadron-side and combined forward-backward asymmetries of the decay are determined to beAFBℓ=−0.39±0.04stat±0.01syst,AFBh=−0.30±0.05stat±0.02syst,AFBℓh=+0.25±0.04stat±0.01syst.\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\begin{document}$$ \\begin{array}{l}{A}_{\\mathrm{FB}}^{\\ell }=-0.39\\pm 0.04\\left(\\mathrm{stat}\\right)\\pm 0.01\\left(\\mathrm{syst}\\right),\\hfill \\\\ {}{A}_{\\mathrm{FB}}^h=-0.30\\pm 0.05\\left(\\mathrm{stat}\\right)\\pm 0.02\\left(\\mathrm{syst}\\right),\\hfill \\\\ {}{A}_{\\mathrm{FB}}^{\\ell h}=+0.25\\pm 0.04\\left(\\mathrm{stat}\\right)\\pm 0.01\\left(\\mathrm{syst}\\right).\\hfill \\end{array} $$\\end{document}The measurements are consistent with Standard Model predictions.

Measurement of Angular and CP Asymmetries in D^{0}→π^{+}π^{-}μ^{+}μ^{-} and D^{0}→K^{+}K^{-}μ^{+}μ^{-} Decays.

The first measurements of the forward-backward asymmetry of the dimuon pair (A_{FB}), the triple-product asymmetry (A_{2ϕ}), and the charge-parity-conjugation asymmetry (A_{CP}), in D^{0}→π^{+}π^{-}μ^{+}μ^{-} and D^{0}→K^{+}K^{-}μ^{+}μ^{-} decays are reported. They are performed using data from proton-proton collisions collected with the LHCb experiment from 2011 to 2016, corresponding to a total integrated luminosity of 5 fb^{-1}. The asymmetries are measured to be A_{FB}(D^{0}→π^{+}π^{-}μ^{+}μ^{-})=(3.3±3.7±0.6)%, A_{2ϕ}(D^{0}→π^{+}π^{-}μ^{+}μ^{-})=(-0.6±3.7±0.6)%, A_{CP}(D^{0}→π^{+}π^{-}μ^{+}μ^{-})=(4.9±3.8±0.7)%, A_{FB}(D^{0}→K^{+}K^{-}μ^{+}μ^{-})=(0±11±2)%, A_{2ϕ}(D^{0}→K^{+}K^{-}μ^{+}μ^{-})=(9±11±1)%, A_{CP}(D^{0}→K^{+}K^{-}μ^{+}μ^{-})=(0±11±2)%, where the first uncertainty is statistical and the second systematic. The asymmetries are also measured as a function of the dimuon invariant mass. The results are consistent with the standard model predictions.

The decay in the model

We study the four-body decay in the Randall–Sundrum model with custodial protection (RSc). By considering the constraints coming from the direct searches of the lightest Kaluza–Klein (KK) excitation of the gluon, electroweak precision tests, the measurements of the Higgs signal strengths at the LHC and from ΔF = 2 flavor observables, we perform a scan of the parameter space of the RSc model and obtain the maximum allowed deviations of the Wilson coefficients for different values of the lightest KK gluon mass . Later, their implications on the observables such as differential branching fraction, longitudinal polarization of the daughter baryon Λ, forward–backward asymmetry with respect to leptonic, hadronic and combined lepton–hadron angles are discussed where we present the analysis of these observables in different bins of di-muon invariant mass squared s ( = q2). It is observed that with the current constraints the Wilson coefficients in the RSc model show slight deviations from their Standard Model values and hence cannot accommodate the discrepancies between the Standard Model calculations of various observables and the LHCb measurements in Λb decays.