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From Ds± Production Asymmetry at the LHC to Prompt Ντ at Icecube

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From Ds± Production Asymmetry at the LHC to Prompt Ντ at Icecube Physics Letters B 794 (2019) 29–35 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb ± From Ds production asymmetry at the LHC to prompt ντ at IceCube ∗ ∗ Victor P. Goncalves a, , Rafał Maciuła b, Antoni Szczurek b, ,1 a Instituto de Física e Matemática, Universidade Federal de Pelotas (UFPel), Caixa Postal 354, CEP 96010-900, Pelotas, RS, Brazil b Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, PL-31-342 Kraków, Poland a r t i c l e i n f o a b s t r a c t Article history: The description of the heavy meson production at large energies and forward rapidities at the LHC Received 18 February 2019 is fundamental to derive realistic predictions of the prompt atmospheric neutrino flux at the IceCube Received in revised form 29 April 2019 Observatory. In particular, the prompt tau neutrino flux is determined by the decay of Ds mesons Accepted 17 May 2019 produced in cosmic ray–air interactions at high energies and large values of the Feynman-xF variable. Available online 21 May 2019 + − Recent data of the LHCb Collaboration indicate a production asymmetry for D and D mesons, which Editor: A. Ringwald s s cannot be explained in terms of the standard modeling of the hadronization process. In this paper we demonstrate that this asymmetry can be described assuming an asymmetric strange sea (s(x) = s¯(x)) in the proton wave function and taking into account the dominant charm and subdominant strange fragmentation into Ds mesons. Moreover, we show that the strange quark fragmentation contribution is dominant at large-xF (≥ 0.3). The prompt ντ flux is calculated and the enhancement associated with the strange quark fragmentation contribution, disregarded in previous calculations, is estimated. The considered scenario leads to quite sizable enhancement of the high-energy τ -neutrino flux. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license 3 (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP . 1. Introduction tion between the LHC and IceCube results and provide more pre- cise predictions for the Ds production at the LHC and the prompt The experimental results obtained in recent years by the LHC, tau neutrino flux at the IceCube. the Pierre Auger and IceCube Neutrino Observatories have chal- The atmospheric neutrinos are produced in cosmic-ray inter- lenged and improved our understanding of Particle Physics. The actions with nuclei in Earth’s atmosphere [8]. At low neutrino 5 discovery of the Higgs boson [1] completed the Standard Model energies (Eν 10 GeV), these neutrinos arise from the decay of (SM), which is now a self-consistent theory, although, there is still light mesons (pions and kaons), and the associated flux is denoted a small room for Beyond Standard Model (BSM) behavior. On the as the conventional atmospheric neutrino flux [9]. On the other 5 7 other hand, the detection of astrophysical neutrinos by the IceCube hand, in the energy range 10 GeV < Eν < 10 GeV, it is expected Neutrino Observatory sets the beginning of the neutrino astronomy that the prompt atmospheric neutrino flux associated with the de- [2]. In addition, the data from the Pierre Auger Observatory pro- cay of hadrons containing heavy flavors become important [10]. In vide an unique opportunity to test Particle Physics at energies well the particular case of the tau neutrino ντ flux, it is dominated at beyond current accelerators [3]. Such results motivated, in par- low energies by the conventional atmospheric flux, via νμ → ντ ticular, the development of new and/or more precise approaches 4 oscillations. On the other hand, for Eν > 10 GeV, this contri- to describe the perturbative and nonperturbative regimes of the bution becomes negligible and the prompt ντ flux is determined Quantum Chromodynamics (QCD). One example is the recent im- by the decay of Ds mesons, which have a leptonic decay channel provement in the description of the heavy meson production in Ds → τντ with a branching ratio of a few percent, with the sub- hadronic collisions at the LHC, directly influenced by the need to sequent τ decay that also contributes to the flux [11]. A precise constrain the magnitude of the prompt neutrino flux, which is cru- determination of the prompt ντ flux is crucial to identify the tau cial for a precise determination of the cosmic neutrino flux at the neutrinos of cosmic origin, which is considered another important IceCube [4–7]. In what follows we will explore this direct connec- signature of the cosmic ray origin of the highest neutrino flux. As demonstrated in Ref. [6], the prompt neutrino flux is determined by the heavy meson production at high energies and very forward Corresponding authors. * rapidities. Therefore, the description of the D production in the E-mail addresses: [email protected] (V.P. Goncalves), [email protected] s (R. Maciuła), [email protected] (A. Szczurek). kinematical region probed by the LHCb Collaboration is a requisite 1 Also at University of Rzeszów, PL-35-959 Rzeszów, Poland. to obtain a precise prediction of the prompt ντ flux. https://doi.org/10.1016/j.physletb.2019.05.026 0370-2693/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 30 V.P. Goncalves et al. / Physics Letters B 794 (2019) 29–35 + − During the last years, the LHCb Collaboration released a large identical amount of Ds and Ds mesons, which implies that the set of data associated with the D and B meson production. The charge asymmetry defined by data for the transverse momentum and rapidity distributions are, + − in general, quite well described by theoretical approaches. On the + σ (D ) − σ (D ) A (D ) = s s (1) other hand, the description of the experimental data for the charge P s + + − σ (Ds ) σ (Ds ) production asymmetries [12,13]is still a challenge for the great majority of the theoretical approaches. In general, these production will be zero at this approximation. Consequently, the asymme- asymmetry are interpreted as arising during the nonperturbative tries are expected to be generated by subleading partonic subpro- process of hadronization as implemented e.g. in the PYTHIA Monte cesses, initial state asymmetries and/or a distinct description of the hadronization process. Here we extend the approach proposed in Carlo, which is based on the Lund string fragmentation model. + − However, this approach fails to describe the recent LHCb data for Ref. [14], which explains the D /D asymmetry in terms of the ± unfavored fragmentation functions, which are responsible for light the Ds production asymmetries, which have found evidence for a nonzero asymmetry. In particular, in contrast with PYTHIA that quark/antiquark fragmentation to D mesons, for Ds production. predicts a positive value, the experimental data indicate that the The fact that u = d in the incident protons, naturally leads to the + − asymmetry is negative. Very recently, two of us have proposed D /D asymmetry when the subleading contribution for the frag- in Ref. [14]an alternative approach to describe the asymmetry mentation is taken into account. In contrast, in the case of Ds pro- + − present in the D and D production [12]. The basic idea is that duction, the inclusion of the subleading fragmentation, associated → − ¯ → + + = subleading contributions for the parton fragmentation are nonneg- to the s Ds and s Ds transformations, implies A P (Ds ) 0if = ¯ ± ligible at the LHC energies and that the asymmetry comes from the s s. Therefore, our interpretation of the LHCb data is that the Ds asymmetry of the u and d valence distributions inherent in the in- asymmetry arises due to an asymmetry in the strange quark sea. ± As explained before, such a behavior is predicted by some theoreti- cident protons. In this paper we extend the approach for the Ds production and demonstrate that the LHCb data can be described cal models and is not excluded by the recent QCD global analysis of if we assume that the strange quark sea in the proton is asym- the experimental data. In fact, the global fits do not include the re- metric, with s = s¯. Such asymmetry is predicted e.g. by the Meson cent LHCb experimental data. In particular, the CTEQ Collaboration Cloud Model (see. e.g. Refs. [15–19]) and is not excluded by the has performed a dedicated study of the strange parton distribu- recent data and by QCD global analyses. In reality, the strange sea tion of the proton in Ref. [24]and obtained that the experimental ¯ in the proton is only poorly known, with its behavior being deter- data is quite well described also assuming s = s. In our analysis mined in a great extent by the neutrino and antineutrino-induced we use rather old CTEQ6 PDF parametrization, however, the most DIS data on charm production obtained by the CCFR/NuTeV and recent analyzes published by the various PDF collaborations use, NOMAD experiments [20,21], which are characterized by the pres- as input for their fits, symmetric strange/antistrange quark dis- + − ence of μ μ in the final state. In particular, these experiments tributions. In several papers related to PDFs, it was shown that have studied the difference between the neutrino and antineutrino different data seem to point towards different values of the asym- induced dimuon differential cross sections, which is sensitive to metry. In particular, data from some experiments seemed to prefer the strange quark/antiquark asymmetry s(x) − s¯(x), and found a negative asymmetries, whereas those from other ones seemed to non-zero value for the asymmetry, which could partially explain prefer positive asymmetries.
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