1,563 research outputs found

    Exclusive processes in proton-proton collisions with the CMS experiment at the LHC

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    We present the recent measurements of exclusive processes performed in the CMS experiment at the LHC using data collected at a centre of mass energy of 7 TeV. These measurements include the double-pomeron production of photon pairs, the two-photon production of leptons pairs, and the previously undetected two-photon production of W boson pairs. While in case of the two first processes that enables to set limits on production cross-section, in the later case it provides also stringent limits on the anomalous quartic gauge couplings.Comment: Presented at the Low x workshop, May 30 - June 4 2013, Rehovot and Eilat, Israe

    CepGen - A generic central exclusive processes event generator for hadron-hadron collisions

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    We present an event generator for the simulation of central exclusive processes in hadron-hadron reactions. Among others, it implements the two-photon production of lepton pairs previously introduced in LPAIR. As a proof of principle, we show that the two approaches are numerically consistent. The k(T)-factorized description of this process is also handled, along with the two-photon production of a quark, or a W-+/- gauge boson pair. This toolbox may be used as a common framework for the definition of many other processes following this approach. Additionally, photoproduction and other photon induced processes are also considered, or being implemented. Program summary Program title: CepGen CPC Library link to program files: https://doi.org/10.17632/24jg665g65.1 Developer's repository link: https://github.com/cepgen/cepgen Licensing provisions: GNU General Public License 3 Programming language: C++/Python External routines/libraries: GSL [1] for MC integration and histogramming, optional wrappers for LHAPDF [2] for the partonic proton structure functions evaluation, or ROOT [3], Delphes [4] for the output treatment. Nature of problem: The simulation of central exclusive, and in particular two-photon induced processes is becoming increasingly topical given its potential source of contamination for electroweak studies and resonance searches at LHC and future colliders. However, most of simulation tools available are only accounting for the production of photons collinear to the incoming proton beams. Legacy codes such as LPAIR, have however shown their effectiveness in predicting such processes at LHC energies. Unfortunately, they are barely maintained nor maintainable with modern computing infrastructures. Solution method: CepGen provides a modern implementation of legacy photon-induced matrix elements (gamma gamma -> l(+)l(-), and W+ W-, with more to be added), including standard e(+) e(-), or ppbeams (both elastic and dissociative final beam states for the latter). For the modern implementation of LPAIR, it inherits from the former fine treatment of the low-|t| region accounting for a large fraction of the cross section. It also introduces a general wrapping framework to define new photon-induced and diffractive processes, either in C++ or in Fortran. This wrapper provides the k(T) factorization procedure for 2 -> 4 process computation, and a highly flexible 2 -> N process placeholder. A user-defined taming of the matrix element is also included to study the effect of kinematic variables-dependent survival factors observed experimentally. Additional comments including restrictions and unusual features: Depending on the complexity of the central process, memory and CPU time. Currently event generation runs only in single-threaded mode, development ongoing to support multi-threading. (C) 2021 The Author. Published by Elsevier B.V.Peer reviewe

    Central ÎŒ+Ό−\mu^{+}\mu^{-} production via photon-photon fusion in proton-proton collisions with proton dissociation

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    We present a formalism which uses fluxes of equivalent photons including transverse momenta of the intermediate photons. The formalism reminds the familiar ktk_t-factorization approach used, e.g., to study the two-photon production of ccˉc\bar{c} or bbˉb\bar{b} pairs. The results of the new method are compared with those obtained using the code LPAIR, and a good agreement is obtained. The inclusion of the photon transverse momenta is necessary in studies of correlation observables. We present distributions for the dimuon invariant mass, transverse momentum of the muon pair and relative azimuthal angle between muons separately for elastic-elastic, elastic-inelastic, inelastic-elastic and inelastic-inelastic mechanisms. For typical experimental cuts all mechanisms give similar contributions. The results are shown for different sets of cuts relevant for the LHC experiments. The cross sections in different regions of phase space depend on F2F_2 structure function in different regions of xx and Q2Q^2. A comment on F2F_2 is made.Comment: 24 pages, 36 figures, 2 table

    Production of tt/mml:mover> pairs via fusion with photon transverse momenta and proton dissociation

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    12 pages, 11 figuresWe discuss the production of t t quark- antiquark pairs in proton- proton collisions via the fusion mechanism. We include topologies in which both protons stay intact or one or even both of them undergo dissociation. The calculations are performed within the k T - factorisation approach, including transverse momenta of intermediate photons. Photon uxes associated with inelastic ( dissociative) processes are calculated based on modern parameterisations of proton structure functions. We fi nd an integrated cross section of about 2.36 fb at p s = 13 TeV for all contributions ( without requirement of rapidity gap). The cross section for the fully elastic process is the smallest. Inelastic contributions are signi fi cantly reduced when a veto on outgoing jets is imposed. We present several di ff erential distributions in rapidity and transverse momenta of single t or t quarks/ antiquarks as well as distributions in invariant mass of both the t t and masses of dissociated systems. A few two- dimensional distributions are presented in addition.Peer reviewe

    Characterisation of the dip-bump structure observed in proton-proton elastic scattering at root s=8 TeV

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    We describe an analysis comparing the p (p) over bar elastic cross section as measured by the D0 Collaboration at a center-of-mass energy of 1.96 TeV to that in pp collisions as measured by the TOTEM Collaboration at 2.76, 7, 8, and 13 TeVusing a model-independent approach. The TOTEM cross sections, extrapolated to a center-of-mass energy of root s = 1.96 TeV, are compared with the D0 measurement in the region of the diffractive minimum and the second maximum of the pp cross section. The two data sets disagree at the 3.4s level and thus provide evidence for the t-channel exchange of a colorless, C-odd gluonic compound, also known as the odderon. We combine these results with a TOTEM analysis of the same C-odd exchange based on the total cross section and the ratio of the real to imaginary parts of the forward elastic strong interaction scattering amplitude in pp scattering for which the significance is between 3.4s and 4.6s. The combined significance is larger than 5 sigma and is interpreted as the first observation of the exchange of a colorless, C-odd gluonic compound.Peer reviewe

    Odderon Exchange from Elastic Scattering Differences between pp and p(p)over-bar Data at 1.96 TeV and from pp Forward Scattering Measurements

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    We describe an analysis comparing the p (p) over bar elastic cross section as measured by the D0 Collaboration at a center-of-mass energy of 1.96 TeV to that in pp collisions as measured by the TOTEM Collaboration at 2.76, 7, 8, and 13 TeVusing a model-independent approach. The TOTEM cross sections, extrapolated to a center-of-mass energy of root s = 1.96 TeV, are compared with the D0 measurement in the region of the diffractive minimum and the second maximum of the pp cross section. The two data sets disagree at the 3.4s level and thus provide evidence for the t-channel exchange of a colorless, C-odd gluonic compound, also known as the odderon. We combine these results with a TOTEM analysis of the same C-odd exchange based on the total cross section and the ratio of the real to imaginary parts of the forward elastic strong interaction scattering amplitude in pp scattering for which the significance is between 3.4s and 4.6s. The combined significance is larger than 5 sigma and is interpreted as the first observation of the exchange of a colorless, C-odd gluonic compound.Peer reviewe

    Search for supersymmetry in events with opposite-sign dileptons and missing transverse energy using an artificial neural network

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    In this paper, a search for supersymmetry (SUSY) is presented in events with two opposite-sign isolated leptons in the final state, accompanied by hadronic jets and missing transverse energy. An artificial neural network is employed to discriminate possible SUSY signals from a standard model background. The analysis uses a data sample collected with the CMS detector during the 2011 LHC run, corresponding to an integrated luminosity of 4.98  fb-1 of proton-proton collisions at the center-of-mass energy of 7 TeV. Compared to other CMS analyses, this one uses relaxed criteria on missing transverse energy (EÌžT>40  GeV) and total hadronic transverse energy (HT>120  GeV), thus probing different regions of parameter space. Agreement is found between standard model expectation and observations, yielding limits in the context of the constrained minimal supersymmetric standard model and on a set of simplified model

    Search for supersymmetry in proton-proton collisions at <mml:msqrt>s</mml:msqrt>=13 TeV in events with high-momentum Z bosons and missing transverse momentum

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    A search for new physics in events with two highly Lorentz-boosted Z bosons and large missing transverse momentum is presented. The analyzed proton-proton collision data, corresponding to an integrated luminosity of 137 fb(-1), were recorded at s = 13 TeV by the CMS experiment at the CERN LHC. The search utilizes the substructure of jets with large radius to identify quark pairs from Z boson decays. Backgrounds from standard model processes are suppressed by requirements on the jet mass and the missing transverse momentum. No significant excess in the event yield is observed beyond the number of background events expected from the standard model. For a simplified supersymmetric model in which the Z bosons arise from the decay of gluinos, an exclusion limit of 1920 GeV on the gluino mass is set at 95% confidence level. This is the first search for beyond-standard-model production of pairs of boosted Z bosons plus large missing transverse momentum.Peer reviewe
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