Browsing by Subject "hep-ph"

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  • Forthomme, Laurent (2022)
    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: Developer's repository link: 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.
  • Herranen, Matti; Kainulainen, Kimmo; Matti Rahkila, Pyry (2010)
    We reformulate and extend our recently introduced quantum kinetic theory for interacting fermion and scalar fields. Our formalism is based on the coherent quasiparticle approximation (cQPA) where nonlocal coherence information is encoded in new spectral solutions at off-shell momenta. We derive explicit forms for the cQPA propagators in the homogeneous background and show that the collision integrals involving the new coherence propagators need to be resummed to all orders in gradient expansion. We perform this resummation and derive generalized momentum space Feynman rules including coherent propagators and modified vertex rules for a Yukawa interaction. As a result we are able to set up self-consistent quantum Boltzmann equations for both fermion and scalar fields. We present several examples of diagrammatic calculations and numerical applications including a simple toy model for coherent baryogenesis.
  • Herranen, Matti; Kainulainen, Kimmo; Rahkila, Pyry M. (2010)
    We show that the dynamical Wigner functions for noninteracting fermions and bosons can have complex singularity structures with a number of new solutions accompanying the usual mass-shell dispersion relations. These new shell solutions are shown to encode the information of the quantum coherence between particles and antiparticles, left and right moving chiral states and/or between different flavour states. Analogously to the usual derivation of the Boltzmann equation, we impose this extended phase space structure on the full interacting theory. This extension of the quasiparticle approximation gives rise to a self-consistent equation of motion for a density matrix that combines the quantum mechanical coherence evolution with a well defined collision integral giving rise to decoherence. Several applications of the method are given, for example to the coherent particle production, electroweak baryogenesis and study of decoherence and thermalization.
  • Keus, Venus; King, Stephen F.; Moretti, Stefano; Sokolowska, Dorota (2014)
    Following the discovery of a Higgs boson, there has been renewed interest in the general 2-Higgs-Doublet Model (2HDM). A model with One Inert Doublet plus One Higgs Doublet (I(1+1)HDM), where one of the scalar doublets is "inert" (since it has no vacuum expectation value and does not couple to fermions) has an advantage over the 2HDM since it provides a good Dark Matter (DM) candidate, namely the lightest inert scalar. Motivated by the existence of three fermion families, here we consider a model with two scalar doublets plus one Higgs doublet (I(2+1)HDM), where the two scalar doublets are inert. The I(2+1)HDM has a richer phenomenology than either the I(1+1)HDM or the 2HDM. We discuss the new regions of DM relic density in the I(2+1)HDM with simplified couplings and address the possibility of constraining the model using recent results from the Large Hadron Collider (LHC) and DM direct detection experiments.
  • Biswas, Sanjoy; Gabrielli, Emidio; Heikinheimo, Matti; Mele, Barbara (2017)
    We study the $ZH$ associated production followed by the Higgs $H\to \gamma \bar{\gamma}$ decay into a photon plus an invisible and massless dark photon, at future high-energy $e^+e^-$ facilities. Large $H\to \gamma \bar{\gamma}$ decay rates (with branching ratios up to a few percent) are allowed, thanks to possible non-decoupling properties of the Higgs boson under specific conditions, and unsuppressed dark-photon couplings in the dark sector. Such large decay rates can be obtained in the framework of recent flavor models that aim to naturally explain the observed spread in the fermion mass spectrum. We analyze the experimental prospects for observing the $e^+e^-\rightarrow ZH$ process followed by the semi invisible Higgs decay into a photon plus a massless invisible system. Search strategies for both the leptonic and the hadronic final states (arising from $Z\rightarrow \mu^+\mu^-$ and $Z\rightarrow q\bar{q}$, respectively) are outlined. We find that a $5\sigma$ sensitivity to a branching fraction $BR_{\gamma\bar{\gamma}}\sim 3\times 10^{-4}$ can be achieved by combining the two channels with an integrated luminosity of 10 ab$^{-1}$ at a c.m. energy of 240 GeV. This is considerably better than the corresponding sensitivity in alternative channels previously studied at lepton colliders. The analysis is model independent, and its results can be straightforwardly applied to the search of any Higgs two-body decay into a photon plus an undetected light particle.
  • Kahara, Topi; Tuominen, Kimmo (2010)
    We study effective models of chiral fields and Polyakov loop expected to describe the dynamics responsible for the phase structure of two-flavor QCD at finite temperature and density. We consider chiral sector described either using linear sigma model or Nambu-Jona-Lasinio model and study the phase diagram and determine the location of the critical point as a function of the explicit chiral symmetry breaking (i.e. the bare quark mass $m_q$). We also discuss the possible emergence of the quarkyonic phase in this model.
  • Holopainen, Hannu; Räsänen, Sami; J. Eskola, Kari (2011)
    We calculate the thermal photon transverse momentum spectra and elliptic flow in $\sqrt{s_{NN}} = 200$ GeV Au+Au collisions at RHIC and in $\sqrt{s_{NN}} = 2.76$ TeV Pb+Pb collisions at the LHC, using an ideal-hydrodynamical framework which is constrained by the measured hadron spectra at RHIC and LHC. The sensitivity of the results to the QCD-matter equation of state and to the photon emission rates is studied, and the photon $v_2$ is discussed in the light of the photonic $p_T$ spectrum measured by the PHENIX Collaboration. In particular, we make a prediction for the thermal photon $p_T$ spectra and elliptic flow for the current LHC Pb+Pb collisions.
  • Kuusela, Mikael; Lämsä, Jerry W.; Malmi, Eric; Mehtälä, Petteri; Orava, Risto (2010)
    Close to one half of the LHC events are expected to be due to elastic or inelastic diffractive scattering. Still, predictions based on extrapolations of experimental data at lower energies differ by large factors in estimating the relative rate of diffractive event categories at the LHC energies. By identifying diffractive events, detailed studies on proton structure can be carried out. The combined forward physics objects: rapidity gaps, forward multiplicity and transverse energy flows can be used to efficiently classify proton-proton collisions. Data samples recorded by the forward detectors, with a simple extension, will allow first estimates of the single diffractive (SD), double diffractive (DD), central diffractive (CD), and non-diffractive (ND) cross sections. The approach, which uses the measurement of inelastic activity in forward and central detector systems, is complementary to the detection and measurement of leading beam-like protons. In this investigation, three different multivariate analysis approaches are assessed in classifying forward physics processes at the LHC. It is shown that with gene expression programming, neural networks and support vector machines, diffraction can be efficiently identified within a large sample of simulated proton-proton scattering events. The event characteristics are visualized by using the self-organizing map algorithm.
  • Karavirta, Tuomas; Mykkänen, Anne; Rantaharju, Jarno; Rummukainen, Kari; Tuominen, Kimmo (2011)
  • Mieskolainen, Mikael; Orava, Risto (American Institute of Physics, 2017)
    AIP Conference Proceedings
    A new combinatorial vector space measurement model is introduced for soft QCD diffraction. The model independent mathematical construction resolves experimental complications; the theoretical framework of the approach includes the Good-Walker view of diffraction, Regge phenomenology together with AGK cutting rules and random fluctuations.
  • Aaltonen, T.; Mehtälä, Petteri; Orava, Risto; Mäki, Tuula; Saarikko, Heimo; Österberg, Kenneth; CDF Collaboration (2009)
    We search for new charmless decays of neutral b hadrons to pairs of charged hadrons, using 1  fb-1 of data collected by the CDF II detector at the Fermilab Tevatron. We report the first observation of the Bs0→K-π+ decay and measure B(Bs0→K-π+)=(5.0±0.7(stat)±0.8(syst))×10-6. We also report the first observation of charmless b-baryon decays, and measure B(Λb0→pπ-)=(3.5±0.6(stat)±0.9(syst))×10-6 and B(Λb0→pK-)=(5.6±0.8(stat)±1.5(syst))×10-6. No evidence is found for other modes, and we set the limit B(Bs0→π+π-)<1.2×10-6 at 90% C.L.
  • Rubio Pena, Javier; Tomberg, Eemeli S. (2019)
    We study the details of preheating in Palatini Higgs inflation. We show, that contrary to what happens in the metric formulation of the model, the Universe does not reheat through the creation of gauge bosons only, but also through the tachyonic production of Higgs excitations. The latest entropy production channel turns out to be very efficient and leads to an almost instantaneous onset of radiation domination after the end of inflation. As compared to the metric case, this reduces the number of e-folds needed to solve the usual hot big bang problems while leading to a smaller spectral index for the primordial spectrum of density perturbations.
  • Łuszczak, Marta; Forthomme, Laurent; Schäfer, Wolfgang; Szczurek, Antoni (2019)
    We 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.
  • Panero, Marco (2012)
    Generalizations of QCD in which the number of colors N is taken to infinity are characterized by profound mathematical properties, with far-reaching implications for fundamental problems and for phenomenological issues alike. In this contribution, after a brief introduction to the theoretical motivation for studying the large-N limit, the role of lattice computations in large-N gauge theories is discussed, and a selection of interesting results obtained in recent years is highlighted. Finally, some promising research directions for future studies are pointed out.
  • Spethmann, Christian; Veermäe, Hardi; Sepp, Tiit; Heikinheimo, Matti; Deshev, Boris; Hektor, Andi; Raidal, Martti (2017)
    Dark plasma is an intriguing form of self-interacting dark matter with an effective fluid-like behavior, which is well motivated by various theoretical particle physics models. We aim to find an explanation for an isolated mass clump in the Abell 520 system, which cannot be explained by traditional models of dark matter, but has been detected in weak lensing observations. We performed N-body smoothed particle hydrodynamics simulations of galaxy cluster collisions with a two component model of dark matter, which is assumed to consist of a predominant non-interacting dark matter component and a 10-40 percent mass fraction of dark plasma. The mass of a possible dark clump was calculated for each simulation in a parameter scan over the underlying model parameters. In two higher resolution simulations shock-waves and Mach cones were observed to form in the dark plasma halos. By choosing suitable simulation parameters, the observed distributions of dark matter in both the Bullet Cluster (1E 0657-558) and Abell 520 (MS 0451.5+0250) can be qualitatively reproduced.
  • Caselle, Michele; Castagnini, Luca; Feo, Alessandra; Gliozzi, Ferdinando; Panero, Marco (2010)
  • M Godbole, Rohini; D Rindani, Saurabh; Rao, Kumar; K Singh, Ritesh (2010)
    We investigate the effects of new physics scenarios containing a high mass vector resonance on top pair production at the LHC, using the polarization of the produced top. In particular we use kinematic distributions of the secondary lepton coming from top decay, which depends on top polarization, as it has been shown that the angular distribution of the decay lepton is insensitive to the anomalous tbW vertex and hence is a pure probe of new physics in top quark production. Spin sensitive variables involving the decay lepton are used to probe top polarization. Some sensitivity is found for the new couplings of the top.