Pair Production Cross Section Research Articles

Measurement of diffractive and exclusive processes with the ATLAS detector

The ATLAS Collaboration has carried out a study of di ractive dijet produc- tion in proton-proton collisions at a centre-of-mass energy of p s = 7 TeV at the LHC. The data distributions are compared with Monte Carlo models and the rapidity gap sur- vival probability has been estimated in the kinematic region with high di ractive contri- bution. Prospects for exclusive jet production studies with the forward proton tagging capability of the AFP sub-detector of ATLAS are also discussed. First results based on data taken jointly with the ATLAS and the LHCf detectors in a p+Pb run will also be shown. In addition, the measurement of the cross-section for the exclusive production of di-lepton pairs in pp collisions at p s = 7 TeV is discussed.

Measurement of hadron pair production cross sections at BESIII

In this paper, two results using data samples collected with the BESIII detector at BEPCII collider are presented. (1) the Born cross section of e+e−→pp‾ at 12 center-of-mass energies from 2232.4 to 3671.0 MeV are measured; the effective electromagnetic form factor is deduced under the assumption |GE|=|GM|; the ratio |GE/GM| and |GM| are extracted by fitting the polar angle distribution; the results show that the |GE/GM| ratios are close to unity, which indicate the data are consistent with the assumption |GE|=|GM| within uncertainties. (2) The cross section of e+e−→π+π− in the effective center-of-mass energy region between 600 and 900 MeV is measured by the method of the initial state radiation using the data set with an integrated luminosity of 2.93fb-1 taken at center-of-mass energy of 3773 MeV; the pion form factor |Fπ|2 is extracted; and the contribution to (g−2)μ order hadronic vacuum polarization is calculated, which shows aμππ,LO(600−900 MeV)=(370.0±2.5stat±3.3sys)⋅10−10.

Search for supersymmetry in events with soft leptons, low jet multiplicity, and missing transverse energy in proton–proton collisions at s=8 TeV

Results are presented from a search for supersymmetric particles in scenarios with a compressed mass spectrum. The data sample corresponds to 19.7 inverse femtobarns of proton-proton collisions recorded by the CMS experiment at sqrt(s) = 8 TeV. The search targets top squark pair production in scenarios with mass differences Delta m = m(top squark) - m(neutralino) below the W-boson mass and with top-squark decays in the four-body mode (top squark to b l nu neutralino), where the neutralino is assumed to be the lightest supersymmetric particle (LSP). The signature includes a high transverse momentum (pt) jet associated with initial-state radiation, one or two low-pt leptons, and significant missing transverse energy. The event yields observed in data are consistent with the expected background contributions from standard model processes. Limits are set on the cross section for top squark pair production as a function of the top squark and LSP masses. Assuming a 100% branching fraction for the four-body decay mode, top-squark masses below 316 GeV are excluded for Delta m = 25 GeV at 95% CL. The dilepton data are also interpreted under the assumption of chargino-neutralino production, with subsequent decays to sleptons or sneutrinos. Assuming a difference between the common lightest chargino/next lightest neutralino mass and the LSP mass of 20 GeV and a tau-enriched decay scenario, masses in the range m(chargino) < 307 GeV are excluded at 95% CL.

Production Cross-Section Estimates for Strongly-Interacting Electroweak-Symmetry Breaking Sector Resonances at Particle Colliders**Supported by the Spanish Excellence Network on Hadronic Physics FIS2014-57026-REDT, by Spanish Grants Universidad Complutense UCM:910309 and Ministerio de Economia y Competitividad MINECO:FPA2011-27853-C02-01, MINECO:FPA2014-53375-C2-1-P, by the Deutsche Forschungsgemeinschaft and National Natural Science Foundation of China through Funds Provided to the Sino-German CRC 110 “Symmetries and the Emergence of Structure

We are exploring a generic strongly-interacting Electroweak Symmetry Breaking Sector (EWSBS) with the low-energy effective field theory for the four experimentally known particles (W±L, ZL, h) and its dispersion-relation based unitary extension. In this contribution we provide simple estimates for the production cross-section of pairs of the EWSBS bosons and their resonances at proton-proton colliders as well as in a future e−e+ (or potentially a μ−μ+) collider with a typical few-TeV energy. We examine the simplest production mechanisms, tree-level production through a W (dominant when quantum numbers allow) and the simple effective boson approximation (in which the electroweak bosons are considered as collinear partons of the colliding fermions). We exemplify with custodial isovector and isotensor resonances at 2 TeV, the energy currently being discussed because of a slight excess in the ATLAS 2-jet data. We find it hard, though not unthinkable, to ascribe this excess to one of these WLWL rescattering resonances. An isovector resonance could be produced at a rate smaller than, but close to earlier CMS exclusion bounds, depending on the parameters of the effective theory. The ZZ excess is then problematic and requires additional physics (such as an additional scalar resonance). The isotensor one (that would describe all charge combinations) has smaller cross-section.

Measurement of the differential cross section for top quark pair production in pp collisions at [Formula: see text

The normalized differential cross section for top quark pair ({mathrm{t}}overline{{mathrm{t}}}) production is measured in pp collisions at a centre-of-mass energy of 8,text {TeV} at the CERN LHC using the CMS detector in data corresponding to an integrated luminosity of 19.7,text {fb}^{-1}. The measurements are performed in the lepton+jets (mathrm {e}/mu +jets) and in the dilepton (mathrm {e}^+mathrm {e}^-, mu ^+ mu ^- , and mathrm {e}^pm mu ^{mp }) decay channels. The {mathrm{t}}overline{{mathrm{t}}} cross section is measured as a function of the kinematic properties of the charged leptons, the jets associated to b quarks, the top quarks, and the {mathrm{t}}overline{{mathrm{t}}} system. The data are compared with several predictions from perturbative quantum chromodynamic up to approximate next-to-next-to-leading-order precision. No significant deviations are observed relative to the standard model predictions.Electronic supplementary materialThe online version of this article (doi:10.1140/epjc/s10052-015-3709-x) contains supplementary material, which is available to authorized users.

Measurement of the t t ¯ W $$ t\overline{t}W $$ and t t ¯ Z $$ t\overline{t}Z $$ production cross sections in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$. The dataset corresponds to an integrated luminosity of $20.3\ \mathrm{fb}^{-1}$ collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the $t\bar{t}W$ and $t\bar{t}Z$ processes simultaneously yields a significance of $5.0\sigma$ ($4.2\sigma$) over the background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The measured cross sections are $\sigma_{t\bar{t}W} = 369^{+100}_{-91}$ fb and $\sigma_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1\sigma$. All measurements are consistent with next-to-leading-order calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.

High-energy behavior of photon, neutrino, and proton cross sections

By combining the color dipole model of the nucleon with the assumption that cross sections behave asymptotically as $\ln^2(s)$, we are able to describe the data for photon, neutrino and hadron interactions with protons at all energies, $s$ is the center-of-mass energy of the interacting particles. Specifically, we extrapolate the perturbative QCD calculations into the regime of small fractional parton momenta $x$ using a color dipole description of the proton target that guarantees an asymptotic $\ln^2(s)$ behavior of all cross sections. The ambiguity of introducing a parametrization associated with the dipole approximation is mitigated by the requirement that the saturation of the small-$x$ structure functions produces $\ln^2(s)$-behaved asymptotic cross sections, in agreement with the data. The same formalism allows us to calculate the cross section for the hadronic pair production of charm particles. The results, in particular those for the high-energy neutrino and charm cross sections, are relevant for evaluating the sensitivity as well as the background in neutrino telescopes.

Determination of the top-quark pole mass using t t ¯ $$ t\overline{t} $$ + 1-jet events collected with the ATLAS experiment in 7 TeV pp collisions

The normalized differential cross section for top-quark pair production in association with at least one jet is studied as a function of the inverse of the invariant mass of the $t \bar{t}+1$-jet system. This distribution can be used for a precise determination of the top-quark mass since gluon radiation depends on the mass of the quarks. The experimental analysis is based on proton-proton collision data collected by the ATLAS detector at the LHC with a centre-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.6 fb$^{-1}$. The selected events were identified using the lepton+jets top-quark-pair decay channel, where lepton refers to either an electron or a muon. The observed distribution is compared to a theoretical prediction at next-to-leading-order accuracy in quantum chromodynamics using the pole-mass scheme. With this method, the measured value of the top-quark pole mass, $m_t^{\rm pole}$, is: $m_t^{\rm pole}$ =173.7 $\pm$ 1.5 (stat.) $\pm$ 1.4 (syst.) $^{+ 1.0}_{-0.5}$ (theory) GeV. This result represents the most precise measurement of the top-quark pole mass to date.

Electroweak SUSY Searches at CMS using LHC-data collected at [formula omitted

The searches presented in this summary correspond to the electroweak (EWK) pair production of neutralinos (χ˜i0) and charginos (χ˜1±) decaying to Higgs (h), Z, and W bosons, and the lightest supersymmetric particle (LSP). The searches were performed using 19.5 fb-1 of data from proton-proton collisions at the LHC, at a center of mass energy of 8 TeV collected in 2012 by the CMS detector. The main production modes correspond to pair production of χ˜i0, where each χ˜i0 decays to a h or a Z boson and a LSP, resulting in final states with hh, hZ and ZZ, with associated missing transverse energy (ETmiss). A second mechanism is the pair production χ˜1±χ˜i0, which leads to hW states in addition to ETmiss. The observed data is consistent with the estimation of the SM backgrounds. Therefore, no evidence of supersymmetric particles is found, and 95% confidence level upper limits are set on the pair production cross section of the respective process as a function of the χ˜i0 and χ˜1± masses.

Higgs boson pair production: Top quark mass effects at NLO and NNLO

We compute next-to-next-to-leading order QCD corrections to the gluon-induced production cross section of Higgs boson pairs in the large top quark mass limit using the soft-virtual approximation. In the limit of infinitely-heavy top quark we confirm the results in the literature. We add two more expansion terms in the inverse top quark mass to the Mt→∞ result. Since the 1/Mt expansion converges poorly, we try to improve on it by factorizing the exact leading order cross section. We discuss two ways of doing that and conclude that the finite top quark mass effects shift the cross section at most by about 10% at next-to-leading order and by about 5% at next-to-next-to-leading order.

Study of asymmetry parameters of decays in a τ-charm factory

An enhancement of cross section for pair production near their threshold provides an opportunity to study the charm baryon at a τ-charm factory with collisions. We estimate the prospects of searching for the CP violation in the decays and . Helicity amplitude analysis shows that the asymmetry parameters, and , can be directly accessed by looking at the angular proton and antiproton distributions in their helicity systems. A Monte Carlo estimation shows that data with the integrated luminosity of yields a statistical precision of about 25% for the asymmetry parameter . To improve the precision to better than 0.1%, an integrated luminosity of at least 1800 fb−1 of data is required.

Top quark anomalous electromagnetic couplings in photon-photon scattering at the LHC

The capability of the LHC to study the electromagnetic dipole moments of the top quark is discussed. In particular, the process $pp\to p\gamma\gamma p\to pt\bar{t}p$, which is supposed to be tagged by the forward/backward detectors at the LHC experiments, is used to explore the top quark electric and magnetic moments. We perform analytical calculations and then a numerical analysis on the sensitivity of the total cross section of the top quark pair production in $\gamma\gamma$ scattering at the LHC to the anomalous top quark couplings with photon. It is shown that improvements in the bounds on the electromagnetic dipole moments of the top quark can be achieved in this channel in comparison with the constraints from the former studies.

One-loop QCD correction to top pair production in the littlest Higgs model with T-parity at the LHC

In this work, we investigate the one-loop QCD correction to top pair production in the littlest Higgs model with T-parity at the LHC. We calculate the relative correction of the top pair production cross-section and top-antitop spin correlation at the LHC for . We find that the relative corrections of top pair production cross-section can reach about , and the top-antitop spin correlation can reach at the 8(14) TeV LHC in the favorable parameter space.

Measurement of the top pair production cross section in 8 TeV proton-proton collisions using kinematic information in thelepton+jetsfinal state with ATLAS

A measurement is presented of the $t\bar{t}$ inclusive production cross-section in $pp$ collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV using data collected by the ATLAS detector at the CERN Large Hadron Collider. The measurement was performed in the lepton+jets final state using a data set corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The cross-section was obtained using a likelihood discriminant fit and $b$-jet identification was used to improve the signal-to-background ratio. The inclusive $t\bar{t}$ production cross-section was measured to be $260\pm 1{\textrm{(stat.)}} ^{+22}_{-23} {\textrm{(syst.)}}\pm 8{\textrm{(lumi.)}}\pm 4{\mathrm{(beam)}}$ pb assuming a top-quark mass of 172.5 GeV, in good agreement with the theoretical prediction of $253^{+13}_{-15}$ pb. The $t\bar{t}\to (e,\mu)+{\mathrm{jets}}$ production cross-section in the fiducial region determined by the detector acceptance is also reported.

Pinning down top dipole moments with ultraboosted tops

We investigate existing and future hadron-collider constraints on the top dipole chromomagnetic and chromoelectric moments, two quantities that are expected to be modified in the presence of new physics. We focus first on recent measurements of the inclusive top pair production cross section at the Tevatron and at the Large Hadron Collider. We then analyse the role of top-antitop events produced at very large invariant masses, in the context of the forthcoming 13-14 TeV runs of the LHC, and at a future 100 TeV proton-proton collider. In this latter case, the selection of semileptonic decays to hard muons allows to tag top quarks boosted to the multi-TeV regime, strongly reducing the QCD backgrounds and leading to a significant improvement in the sensitivity to anomalous top couplings.

Top-quark production in proton–nucleus and nucleus–nucleus collisions at LHC energies and beyond

Single and pair top-quark production in proton–lead (p–Pb) and lead–lead (Pb–Pb) collisions at the CERN Large Hadron Collider (LHC) and Future Circular Collider (FCC) energies, are studied with next-to-leading-order perturbative QCD calculations including nuclear parton distribution functions. At the LHC, the pair-production cross sections amount to σtt¯=3.4 μb in Pb–Pb at sNN=5.5 TeV, and σtt¯=60 nb in p–Pb at sNN=8.8 TeV. At the FCC energies of sNN=39 and 63 TeV, the same cross sections are factors of 90 and 55 times larger respectively. In the leptonic final-state tt¯→W+bW−b¯→bb¯ℓℓνν with ℓ=e±,μ±, after typical acceptance and efficiency cuts, one expects about 90 and 300 top-quarks per nominal LHC-year and 4.7⋅104 and 105 per FCC-year in Pb–Pb and p–Pb collisions respectively. The total tt¯ cross sections, dominated by gluon fusion processes, are enhanced by 3–8% in nuclear compared to p–p collisions due to an overall net gluon antishadowing, although different regions of their differential distributions are depleted due to shadowing or EMC-effect corrections. The rapidity distributions of the decay leptons in tt¯ processes can be used to reduce the uncertainty on the Pb gluon density at high virtualities by up to 30% at the LHC (full heavy-ion programme), and by 70% per FCC-year. The cross sections for single-top production in electroweak processes are also computed, yielding about a factor of 30 smaller number of measurable top-quarks after cuts, per system and per year.

Precision top-quark physics with applications

We review the recent progress in the determination of top-quark pair production cross sections at hadron colliders, in particular the Tevatron and the LHC. We discuss the theoretical developments, which lead to the determination of next-to-next-to-leading (NNLO) corrections to this process. Furthermore, we describe some applications which follow from a comparison of theory predictions with data, e.g. the determination of the top-quark pole mass, constraints on the gluon distribution function and bounds on light top squarks. Finally, we present the most recent analysis of the Tevatron forward-backward asymmetry, where we demonstrate agreement between the Standard Model and the measurement.

Gamma Ray Buildup Factor for Finite Media in Energy Range (4-10) MeV for Al and Pb

A computer program based on Monte Carlo method had been designed and written in visual basic computer language and utilized for simulating the classic problem of gamma ray beam incident on finite plane slabs of absorbing materials. The source geometry adopted in this program is plane normal source. Dose buildup factor of gamma photons in the absence and presence pair production effect have been calculated in the energy range (4-10) MeV for Aluminum and Lead up to 5 mean free path thickness. Dose buildup factor in the presence of pair production is higher than dose buildup factor in the absence of pair production effect. The deviation between the values of dose buildup factor in the presence and absence pair production is increased when the energy is increased within the studied energy range because the cross section for pair production is increased within the studied energy range.

Precision Measurement of the Mass of the τ Lepton at BESIII

An energy scan near the τ pair production threshold has been performed using the BESIII detector. About 24 pb−1 of data, distributed over four scan points, was collected. This analysis is based on τ pair decays to ee, eμ, eh, μμ, μh, hh, eρ, μρ and πρ final states, where h denotes a charged π or K. The mass of the τ lepton is measured from a maximum likelihood fit to the τ pair production cross section data to be mτ=(1776.91±0.12−0.13+0.10) MeV/c2, which is currently the most precise value in a single measurement.

Modification of Gaussian mixture models for data classification in high energy physics

In high energy physics, we deal with demanding task of signal separation from background. The Model Based Clustering method involves the estimation of distribution mixture parameters via the Expectation-Maximization algorithm in the training phase and application of Bayes' rule in the testing phase. Modifications of the algorithm such as weighting, missing data processing, and overtraining avoidance will be discussed. Due to the strong dependence of the algorithm on initialization, genetic optimization techniques such as mutation, elitism, parasitism, and the rank selection of individuals will be mentioned. Data pre-processing plays a significant role for the subsequent combination of final discriminants in order to improve signal separation efficiency. Moreover, the results of the top quark separation from the Tevatron collider will be compared with those of standard multivariate techniques in high energy physics. Results from this study has been used in the measurement of the inclusive top pair production cross section employing DØ Tevatron full Runll data (9.7 fb−1).