hep-ph updates on
High Energy Physics - Phenomenology (hep-ph) updates on the e-print archive
  • On the phenomenology of sphaleron-induced processes at the LHC and beyond. (arXiv:1910.04761v1 [hep-ph])

    Authors: Andreas Papaefstathiou, Simon Plätzer, Kazuki Sakurai

    We investigate the phenomenological aspects of non-perturbative baryon- and lepton-number-violating processes at hadron colliders. Such processes, induced by instanton/sphaleron configurations of the electroweak gauge fields, are believed to play a crucial role in the generation of baryon asymmetry in the early Universe at finite temperature. On the other hand, at colliders (that represent the zero-temperature high-energy regime) the rate and observability of such processes are still under debate. Motivated by current theoretical considerations, we construct a modern event generator within the general-purpose Herwig Monte Carlo framework, that aims to capture the most relevant features of the dominant processes. We perform a detailed phenomenological analysis focussing on the Large Hadron Collider, at 13 TeV proton-proton centre-of-mass energy, a potential high-energy upgrade at 27 TeV and the proposed Future Circular Collider (FCC-hh) at 100 TeV. We derive constraints on the expected rates for various parametrisations of our model. We find that all three colliders are capable of providing meaningful information on the nature of instanton/sphaleron-induced processes at various energy scales.

  • Complete Form Factors in Yang-Mills from Unitarity and Spinor Helicity in Six Dimensions. (arXiv:1910.04772v1 [hep-th])

    Authors: Manuel Accettulli Huber, Andreas Brandhuber, Stefano De Angelis, Gabriele Travaglini

    We present a systematic procedure to compute complete, analytic form factors of gauge-invariant operators at loop level in pure Yang-Mills. We consider applications to operators of the form $\mathrm{Tr}\, F^n$ where $F$ is the gluon field strength. Our approach is based on an extension to form factors of the dimensional reconstruction technique, in conjunction with the six-dimensional spinor-helicity formalism and generalised unitarity. For form factors this technique requires the introduction of additional scalar operators, for which we provide a systematic prescription. We also discuss a generalisation of dimensional reconstruction to any number of loops, both for amplitudes and form factors. Several novel results for one-loop minimal and non-minimal form factors of $\mathrm{Tr}\, F^n$ with $n>2$ are presented. Finally, we describe the \texttt{Mathematica} package \texttt{SpinorHelicity6D}, which is tailored to handle six-dimensional quantities written in the spinor-helicity formalism.

  • Cosmological implications of electromagnetically interacting dark matter: milli-charged particles and atoms with singly and doubly charged dark matter. (arXiv:1910.04779v1 [astro-ph.CO])

    Authors: Gautham A P, Shiv Sethi

    While the behavior of the dominant component of the dark matter is reasonably well established by cosmological observables, its particle nature and interactions with the rest of the matter are not known. We consider three dark matter models that admit electromagnetic interaction between baryons and dark matter: (a) milli-charged particle (CCDM) of charge $q_{\rm ccdm}$ and mass $m_{\rm ccdm}$, (b) a neutral atom of two charged particles of mass $m_{\rm dd}$ (DD), and (c) a neutral atom of doubly charged particle and helium nucleus (HeD). We derive and discuss in detail the formation, stability, and interaction of these atoms with baryons. These new interactions are incorporated into the publicly-available code CLASS to obtain the matter power spectra and CMB anisotropies. We carry out MCMC analysis to constrain the fraction of interacting dark matter allowed by Planck data. For the range of allowed parameters, all the cold dark matter could be the form of HeD atoms or DD atoms if $m_{\rm dd} \gtrsim 25 \, \rm GeV$. The MCMC analysis suggests that the current data prefers, at 1$\sigma$ level, a fraction of nearly 5% of either CCDM or DD dark matter for the following parameters: $q_{\rm ccdm} = 10^{-6}e$, $m_{\rm ccdm} = 50 \, \rm MeV$ or $m_{\rm dd} = 10 \, \rm GeV$.

  • Stochastic Gravitational Wave Background from Global Cosmic Strings. (arXiv:1910.04781v1 [hep-ph])

    Authors: Chia-Feng Chang, Yanou Cui

    Global cosmic strings are generically predicted in particle physics beyond the Standard Model, e.g., a post-inflationary global $U(1)$ symmetry breaking which may associate with axion-like dark matter. We demonstrate that although subdominant to Goldstone emission, gravitational waves (GWs) radiated from global strings can be observable with current/future GW detectors. The frequency spectrum of such GWs is also shown to be a powerful tool to probe the Hubble expansion rate of the Universe at times prior to the Big Bang nucleosynthesis where the standard cosmology has yet to be tested.

  • Explaining the AMS positron excess via Right-handed Neutrinos. (arXiv:1910.04782v1 [hep-ph])

    Authors: Farinaldo S. Queiroz, Clarissa Siqueira

    We have witnessed in the past decade the observation of a puzzling cosmic-ray excess at energies larger than $10$~GeV. The AMS-02 data published this year has new ingredients such as the bump around $300$~GeV followed by a drop at $800$~GeV, as well as smaller error bars. Adopting the background used by the AMS-02 collaboration in their analysis, one can conclude that previous explanations to the new AMS-02 such as one component annihilating and decaying dark matter as well as pulsars seem to fail at reproducing the data. Here, we show that in the right-handed neutrino portal might reside the answer. We discuss a decaying two-component dark matter scenario where the decay products are right-handed neutrinos that have their decay pattern governed by the type I seesaw mechanism. This setup provides a very good fit to data, for example, for a conservative approach including just statistical uncertainties leads to $\chi^2/d.o.f \sim 2.3$ for $m_{DM_1}=2150$~GeV with $\tau_{1}=3.78 \times 10^{26}$ s and $m_{DM_2}=300$ with $\tau_{2}=5.0 \times 10^{27}$ s for $M_N=10$ GeV, and, in an optimistic case, including systematic uncertainties, we find $\chi^2/d.o.f \sim 1.12$, for $M_N = 10$ GeV, with $m_{DM_1}=2200$ GeV with $\tau_{1}=3.8 \times 10^{26}$ s and $m_{DM_2}=323$ GeV with $\tau_{2}=1.68 \times 10^{27}$ s.

  • Probing the Pomeron spin-flip with Coulomb-nuclear interference. (arXiv:1910.04799v1 [hep-ph])

    Authors: B. Z. Kopeliovich, M. Krelina

    Brand-new high-precision data for single-spin asymmetry $A_N(t)$ in small angle elastic $pp$ scattering from the fixed target experiment HJET at BNL at $E_{lab}=100$ and $255$ GeV, as well as high energy STAR measurements at $\sqrt{s}=200$ GeV, for the first time allowed to determine the spin-flip to non-flip ratio $r_5(t)$ in a wide energy range. We introduced essential modification in the Coulomb-nuclear interference (CNI) mechanism, missed in previous analyses. In particular, absorptive corrections make the proton electromagnetic vertex different from the formfactor, measured in electron-proton scattering. Introduction of absorptive corrections strongly affect the results for $r_5(t)$. The Regge analysis allowed to single out the Pomeron contribution to the spin-flip amplitude, which steeply rises with energy. We found the spin-flip to non-flip ratio of the Pomeron amplitudes to be nearly $-10\%$, rising with energy as $s^{0.34}$, both in good accord with theoretical expectations.

  • Polarization modes of gravitational waves in Quadratic Gravity. (arXiv:1910.04800v1 [gr-qc])

    Authors: Pratik Wagle, Alexander Saffer, Nicolas Yunes

    The observation of the inspiral and merger of compact binaries by the LIGO-Virgo collaboration has allowed for new tests of Einstein's theory in the extreme gravity regime, where gravitational interactions are simultaneously strong, non-linear, and dynamical. Theories beyond Einstein's can also be constrained by detecting the polarization modes of gravitational waves. In this paper, we show that dynamical Chern-Simons and Einstein-dilaton-Gauss-Bonnet gravity cannot be differentiated from general relativity based on the detection of polarization modes alone. To prove this result, we use the Newman-Penrose method and an irreducible decomposition method to find that only the tensorial modes can be detected in both these theories.

  • Transplanckian Censorship and the Local Swampland Distance Conjecture. (arXiv:1910.04804v1 [hep-th])

    Authors: Patrick Draper, Szilard Farkas

    The swampland distance conjecture (SDC) addresses the ability of effective field theory to describe distant points in moduli space. It is natural to ask whether there is a local version of the SDC: is it possible to construct local excitations in an EFT that sample extreme regions of moduli space? In many cases such excitations exhibit horizons or instabilities, suggesting that there are bounds on the size and structure of field excitations that can be achieved in EFT. Static bubbles in ordinary Kaluza-Klein theory provide a simple class of examples: the KK radius goes to zero on a smooth surface, locally probing an infinite distance point, and the bubbles are classically unstable against radial perturbations. However, it is also possible to stabilize KK bubbles at the classical level by adding flux. We study the impact of imposing the Weak Gravity Conjecture (WGC) on these solutions, finding that a rapid pair production instability arises in the presence of charged matter with $q/m\gtrsim 1$. We also analyze 4d electrically charged dilatonic black holes. Small curvature at the horizon imposes a bound $\log(M_{BH})\gtrsim |\Delta\phi|$, independent of the WGC, and the bound can be strengthened if the particle satisfying the WGC is sufficiently light. We conjecture that quantum gravity in asymptotically flat space requires a general bound on large localized moduli space excursions of the form $ |\Delta\phi|\lesssim |\log(R\Lambda)|$, where $R$ is the size of the minimal region enclosing the excitation and $\Lambda^{-1}$ is the short-distance cutoff on local EFT. The bound is qualitatively saturated by the dilatonic black holes and Kaluza-Klein monopoles.

  • Extraction of DVCS form factors with uncertainties. (arXiv:1910.04806v1 [hep-ph])

    Authors: Kresimir Kumericki

    We discuss recent attempts to extract deeply virtual Compton scattering form factors with emphasis on their uncertainties, which turn out to be most reliably provided by method of neural networks.

  • Bottomonium resonances with $I = 0$ from lattice QCD correlation functions with static and light quarks. (arXiv:1910.04827v1 [hep-lat])

    Authors: Pedro Bicudo, Marco Cardoso, Nuno Cardoso, Marc Wagner

    We discuss, how to study $I = 0$ quarkonium resonances decaying into pairs of heavy-light mesons using static potentials from lattice QCD. These static potentials can be obtained from a set of correlation functions containing both static and light quarks. As a proof of concept we focus on bottomonium with relative orbital angular momentum $L = 0$ of the $\bar{b} b$ pair corresponding to $J^{P C} = 0^{- +}$ and $J^{P C} = 1^{- -}$. We use static potentials from an existing lattice QCD string breaking study and compute phase shifts and $\mbox{T}$ matrix poles for the lightest heavy-light meson-meson decay channel. We discuss our results in the context of corresponding experimental results, in particular for $\Upsilon (10860)$ and $\Upsilon (11020)$.

  • The Dodelson-Widrow Mechanism In the Presence of Self-Interacting Neutrinos. (arXiv:1910.04901v1 [hep-ph])

    Authors: André de Gouvêa, Manibrata Sen, Walter Tangarife, Yue Zhang

    keV-scale gauge-singlet fermions, allowed to mix with the active neutrinos, are elegant dark matter (DM) candidates. They are produced in the early universe via the Dodelson-Widrow mechanism and can be detected as they decay very slowly, emitting X-rays. In the absence of new physics, this hypothesis is virtually ruled out by astrophysical observations. Here, we show that new interactions among the active neutrinos allow these sterile neutrinos to make up all the DM while safely evading all current experimental bounds. The existence of these new neutrino interactions may manifest itself in next-generation experiments, including DUNE.

  • The viability of the 3+1 neutrino model in the supernova neutrino process. (arXiv:1910.04984v1 [hep-ph])

    Authors: Heamin Ko, Dukjae Jang, Motohiko Kusakabe, Myung-Ki Cheoun

    Adopting the 3+1 neutrino mixing parameters by the IceCube and shortbase line experiments, we investigate the sterile-active neutrino oscillation effects on the supernova neutrino process. For the sterile neutrino ($\nu_s$), we study two different luminosity models. First, we presume that the $\nu_s$ does not interact with other particles through the standard interactions apart from the oscillation with the active neutrinos. Second, we consider that $\nu_s$ can be directly produced by $\nu_e$ scattering with matter. In both cases, we find that the pattern of neutrino oscillations can be changed drastically by the $\nu_s$ in supernova environments. Especially multiple resonances occur, and consequently affect thermal neutrino-induced reaction rates. As a result, $^7$Li, $^7$Be, $^{11}$B, $^{11}$C, $^{92}$Nb, $^{98}$Tc and $^{138}$La yields in the $\nu$-process are changed. Among those nuclei, $^7$Li and $^{11}$B yields can be constrained by the analysis of observed SiC X grains. Based on the meteoritic data, we conclude that the second model can be allowed while first model is excluded. The viability of the second model depends on the sterile neutrino temperature and the neutrino mass hierarchy.

  • Gravitational waves from chiral phase transition in a conformally extended standard model. (arXiv:1910.05025v1 [hep-ph])

    Authors: Mayumi Aoki (Kanazawa U.), Jisuke Kubo (Heidelberg, Max Planck Inst. & Toyama U.)

    The gravitational wave (GW) background produced at the cosmological chiral phase transition in a conformal extension of the standard model is studied. To obtain the bounce solution of coupled field equations we implement an iterative method. We find that the corresponding $O(3)$ symmetric Euclidean action $S_3$ divided by the temperature $T$ has a simple behavior near the critical temperature $T_C$: $S_3/T \propto (1-T/T_C)^{-\gamma}$, which is subsequently used to determine the transition's inverse duration $\beta$ normalized to the Hubble parameter $H$. It turns out that $\beta/H \gtrsim 10^3$, implying that the sound wave period $\tau_\text{sw}$ as an active GW source, too, can be much shorter than the Hubble time. We therefore compute $\tau_\text{sw} H$ and use it as the reduction factor for the sound wave contribution. The signal-to-noise ratio (SNR) for Deci-Hertz Interferometer Gravitational Wave Observatory (DECIGO) and Big Bang Observer (BBO) is evaluated, with the result: SNR$^\text{DECIGO} \lesssim 1.2$ and SNR$^\text{BBO} \lesssim 12.0$ for five years observation, from which we conclude that the GW signal predicted by the model in the optimistic case could be detected at BBO.

  • Global study of effective Higgs portal dark matter models using GAMBIT. (arXiv:1910.05051v1 [hep-ph])

    Authors: Ankit Beniwal (for the GAMBIT Collaboration)

    In this proceeding, we present frequentist and Bayesian results from a global fit of effective vector and Majorana fermion Higgs portal dark matter (DM) models using the $\mathsf{GAMBIT}$ software. We systematically explore the parameter space of these models using advanced sampling techniques to simultaneously satisfy the observed DM abundance, Higgs invisible decay, and indirect and direct detection limits. In addition, we take account of a set of nuisance parameters arising from Standard Model, nuclear physics, DM halo and velocity distribution. For the vector DM model viable solutions are found at low and high vector masses. The Majorana fermion model requires a strong preference for a CP-odd, parity-violating coupling which leads to a momentum-suppression of the DM-nucleon cross-section. All of our results, samples and input files are publicly available via $\href{}{\textsf{Zenodo}}$.

  • Modified interactions in the top-quark electroweak sector: exploiting unitarity violating effects at the amplitude level to probe New Physics. (arXiv:1910.05053v1 [hep-ph])

    Authors: Fabio Maltoni, Luca Mantani, Ken Mimasu

    We present a broad study of collider processes that embed $2 \to 2$ scattering amplitudes involving top quarks in the Electroweak sector. We parametrise the modified interactions using the Standard Model Effective Field Theory framework and discuss how the New Physics effects lead to unitarity violating behaviour at the amplitude level. For each scattering amplitude we compute the helicity amplitudes in the high energy limit paying special attention to the effects of the higher dimensional operators. We also discuss whether and to what extent the unitarity violating effects are retained in physical processes at colliders.

  • Comparatively Light Extra Higgs States as Signature of SUSY $\mathrm{SO}(10)$ GUTs with 3rd Family Yukawa Unification. (arXiv:1910.05191v1 [hep-ph])

    Authors: Stefan Antusch, Christian Hohl, Vasja Susic

    We study $3$rd family Yukawa unification in the context of supersymmetric (SUSY) $\mathrm{SO}(10)$ GUTs and $\mathrm{SO}(10)$-motivated boundary conditions for the SUSY-breaking soft terms. We consider $\mu<0$ such that the SUSY loop-threshold effects enable a good fit to all third family masses of the charged Standard Model (SM) fermions. We find that fitting the third family masses together with the mass of the SM-like Higgs particle, the scenario predicts the masses of the superpartner particles and of the extra Higgs states of the MSSM: while the sparticles are predicted to be comparatively heavy (above the present LHC bound but within reach of future colliders), the spectrum has the characteristic feature that the lightest new particles are the extra MSSM Higgses. We show that this effect is rather robust with respect to many deformations of the GUT boundary conditions, but turns out to be sensitive to the exactness of top-bottom Yukawa unification. Nevertheless, with moderate deviations of a few percent from exact top-bottom Yukawa unification (stemming e.g.\ from GUT-threshold corrections or higher-dimensional operators), the scenario still predicts extra MSSM Higgs particles with masses not much above $1.5\,\mathrm{TeV}$, which could be tested e.g.\ by future LHC searches for ditau decays $H^0/A^{0}\to\tau\tau$. Finding the extra MSSM Higges before the other new MSSM particles could thus be a smoking gun for a Yukawa unified $\mathrm{SO}(10)$ GUT.

  • Identifying Exclusive Displaced Hadronic Signatures in the Forward Region of the LHC. (arXiv:1910.05225v1 [hep-ph])

    Authors: Xabier Cid Vidal, Yuhsin Tsai, Jose Zurita

    The LHCb detector provides accurate vertex reconstruction and hadronic particle identification, which make the experiment an ideal place to look for light long-lived particles (LLP) decaying into Standard Model (SM) hadrons. In contrast with the typical search strategy relying on energetic jets and a high multiplicity of tracks from the LLP decay, LHCb can identify LLPs in exclusive, specific hadronic final states. To illustrate the idea, we study the sensitivity of LHCb to an exotic Higgs decay $h\to SS$, followed by the displaced decay of GeV-scale scalars into charged kaons $S\to K^+K^-$. We show that the reconstruction of kaon vertices in narrow invariant mass windows can efficiently eliminate the combinatorial backgrounds from $B$-meson decays. While the same signal is extremely difficult to probe in the existing displaced jet searches at ATLAS/CMS, the LHCb search we propose can probe the branching ratio BR$(h\to SS)$ down to $0.1\%$ ($0.02\%$) level with $15$ ($300$) fb$^{-1}$ of data. We also apply this projected bound to two scenarios with Higgs portal couplings, where the scalar mediator $S$ either couples to a) the SM quarks only, or b) to both quarks and leptons in the minimal flavor violation paradigm. In both scenarios we compare the reach of our proposed search with the expected constraints from ATLAS and CMS on the invisible Higgs width and with the constraints from rare B-decays studies at LHCb. We find that for 1 GeV $< m_S < $ 2 GeV and $0.5~{\rm mm} \lesssim c \tau \lesssim 10$ mm our proposed search will be competitive with the ATLAS and CMS projections, while at the same time providing crucial information of the hadronic interactions of $S$, which can not be obtained from the {\it indirect} measurement of the Higgs invisible width.

  • The Heavy Quark Expansion for Inclusive Semileptonic Charm Decays Revisited. (arXiv:1910.05234v1 [hep-ph])

    Authors: Matteo Fael, Thomas Mannel, K. Keri Vos

    The Heavy Quark Expansion (HQE) has become an extremely powerful tool in flavor physics. For charm decays, where the expansion parameters $\alpha_s(m_c)$ and $\Lambda_{\rm QCD}/m_c$ are bigger than for bottom decays, it remains to be seen if the HQE can be applied with similar success. Nevertheless, to make optimal use of the plethora of data already available and coming in the near future, a better understanding of HQE for charm decays is crucial. This paper discusses in detail how the HQE for charm decays is set up, what is the role of four-quark (weak annihilation) operators and how this compares to the well understood bottom decays. Subtleties concerning radiative corrections and the charm mass scheme are briefly discussed. An experimental study of the relevant HQE hadronic matrix elements will then show if the HQE expansion for charm converges well enough. Besides serving as an important cross check for inclusive $B$ decays, in the end, this study might open the road for inclusive $|V_{cs}|$ and $|V_{cd}|$ extractions.

  • Formation of Primordial Black Holes from Warm Inflation. (arXiv:1910.05238v1 [astro-ph.CO])

    Authors: Richa Arya

    Primordial Black Holes (PBHs) serve as a unique probe to the physics of the early Universe, particularly inflation. In light of this, we study the formation of PBHs by the collapse of overdense perturbations generated during a model of warm inflation. For our model, we find that the primordial curvature power spectrum is red-tilted (spectral index $n_s<1$) at the large scale (small $k$) and is consistent with the $n_s-r$ values allowed from the CMB observations. Along with that, it has a blue-tilt ($n_s>1$) for the small PBH scales (large $k$), with a sufficiently large amplitude of the primordial curvature power spectrum required to form PBHs. These features originate because of the inflaton's coupling with the other fields during warm inflation. We discuss the role of the inflaton dissipation to the enhancement in the primordial power spectrum at the PBH scales. We find that for some parameter range of our warm inflation model, PBHs with mass $\sim 10^3$ g can be formed with significant abundance. Such tiny mass PBHs have a short lifetime $\sim 10^{-19}$ s and would have evaporated into Hawking radiation in the early Universe. Further in this study, we discuss the evaporation constraints on the initial mass fraction of the generated PBHs and the possibility of Planck mass PBH relics to constitute the dark matter.

  • On the Measurement of Sachs Form Factors in Processes without and with Proton Spin Flip. (arXiv:1910.05267v1 [hep-ph])

    Authors: M.V. Galynskii

    The physical meaning of the decomposition of the Rosenbluth formula into two terms containing only squares of Sachs form factors has been established. A new method has been proposed for their independent measurement in the $e \vec{p} \to e \vec{p}$ elastic process when the initial proton at rest is fully polarized along the direction of motion of the final proton.

  • Keeping an Eye on DBI: Power-counting for small-$c_s$ Cosmology. (arXiv:1910.05277v1 [gr-qc])

    Authors: Ivana Babic, C.P. Burgess, Ghazal Geshnizjani

    Inflationary mechanisms for generating primordial fluctuations ultimately compute them as the leading contributions in a derivative expansion, with corrections controlled by powers of derivatives like the Hubble scale over Planck mass: $H/M_p$. At face value this derivative expansion breaks down for models with a small sound speed, $c_s$, to the extent that $c_s \ll 1$ is obtained by having higher-derivative interactions like $\mathfrak{L}_{\rm eff} \sim (\partial \Phi)^4$ compete with lower-derivative propagation. This concern arises more generally for models whose lagrangian is given as a function $P(X)$ for $X = -\partial_\mu \Phi \partial^\mu \Phi$ --- including in particular DBI models for which $P(X) \propto \sqrt{1-kX}$ --- since these keep all orders in $\partial \Phi$ while dropping $\partial^n \Phi$ for $n > 1$. We here find a sensible power-counting scheme for DBI models that gives a controlled expansion in powers of three types of small parameters: $H/M_p$, slow-roll parameters (possibly) and $c_s \ll 1$. We do not find a similar expansion framework for generic small-$c_s$ or $P(X)$ models. Our power-counting result quantifies the theoretical error for any prediction (such as for inflationary correlation functions) by fixing the leading power of these small parameters that is dropped when not computing all graphs (such as by restricting to the classical approximation); a prerequisite for meaningful comparisons with observations. The new power-counting regime arises because small $c_s$ alters the kinematics of free fluctuations in a way that changes how interactions scale at low energies, in particular allowing $1-c_s$ to be larger than derivative-measuring quantities like $(H/M_p)^2$.

  • Medium-Induced Transverse Momentum Broadening via Forward Dijet Correlations. (arXiv:1910.05290v1 [hep-ph])

    Authors: Jiangyong Jia, Shu-Yi Wei, Bo-Wen Xiao, Feng Yuan

    Dijet azimuthal angle correlation is arguably one of the most direct probes of the medium-induced broadening effects. The evidence for such broadening, however, is not yet clearly observed within the precision of current mid-rapidity measurements at RHIC and the LHC. We show that the dijet correlation in forward rapidity from the future LHC RUN3, aided by forward detector upgrades, can reveal this broadening thanks to the steeper jet spectra, suppressed vacuum radiations and lower underlying event background, with a sensitivity comparable to that of the future high-luminosity Au+Au run at RHIC. Dijet correlation measurements at the two facilities together can provide powerful constraints on the temperature dependence of medium transport properties.

  • Identifying hadronic charmonium decays in hadron colliders. (arXiv:1910.05334v1 [hep-ex])

    Authors: Nicolo de Groot, Sergi Castells

    Identification of charmonium states at hadron colliders has mostly been limited to leptonic decays of the J/{\Psi}. In this paper we present and algorithm to identify hadronic decays of charmonium states (J/{\Psi}, {\Psi}(2S), \chi_{c0,1,2}) which make up the large majority of all decays.

  • Double Heavy baryons and Corrections to Heavy Quark-Diquark Symmetry Prediction for Hyperfine Splitting. (arXiv:1910.05337v1 [hep-ph])

    Authors: Thomas Mehen, Abhishek Mohapatra

    In the $m_Q\rightarrow\infty$ limit, the hyperfine splittings in the ground state doubly heavy baryons $\left(QQq\right)$ and single heavy antimesons $(\bar{Q}q)$ are related by heavy quark-diquark symmetry (HQDQ) as the light degrees of freedom in both the hadrons are expected to have identical configurations. In this article, working within the framework of nonrelativistic QCD (NRQCD), we study the perturbative and nonperturbative corrections to the HQDQ symmetry hyperfine splitting relation that scale as ${\cal O}\left(\alpha_s^2\right)$ and $\Lambda_{\rm QCD}^2/m_Q^2$ respectively. In the extreme heavy quark limit, the perturbative corrections to hyperfine splitting of doubly charm or bottom baryons are a few percent or smaller. The nonperturbative corrections to hyperfine splitting are of order $10\%$ in the case of doubly charm baryons and $1\%$ or smaller in doubly bottom baryons.

  • Estimates of the $X(3872)$ Cross Section at a Hadron Collider. (arXiv:1811.08876v2 [hep-ph] UPDATED)

    Authors: Eric Braaten, Li-Ping He, Kevin Ingles

    The claim that the $X(3872)$ meson cannot be a charm-meson molecule because its prompt production cross section at hadron colliders is too large is based on an upper bound in terms of a cross section for producing charm-meson pairs. Assuming $X$ is sufficiently weakly bound, we derive an equality between the $X$ cross section and a charm-meson pair cross section that takes into account the threshold enhancement from the $X$ resonance. The cross section for producing $X$ is equal to that for producing $D^{*0} \bar{D}^0$ integrated up to a relative momentum $k_\mathrm{max} = 7.7\,\gamma_X$, where $\gamma_X$ is the binding momentum of $X$. We also derive an order-of-magnitude estimate of the $X$ cross section in terms of a naive charm-meson pair cross section that does not take into account the threshold enhancement, such as that produced by a Monte Carlo event generator. The cross section for producing $X$ can be approximated by the naive cross section for producing $D^{*0} \bar{D}^0$ integrated up to a relative momentum $k_\mathrm{max}$ of order $(m_\pi^2 \gamma_X)^{1/3}$. The estimates of the prompt $X$ cross section at hadron colliders are consistent with the cross sections observed at the Tevatron and the LHC.

  • Phenomenology of TeV-scale scalar Leptoquarks in the EFT. (arXiv:1812.03178v4 [hep-ph] UPDATED)

    Authors: Shaouly Bar-Shalom, Jonathan Cohen, Amarjit Soni, Jose Wudka

    We examine new aspects of leptoquark (LQ) phenomenology using effective field theory (EFT). We construct a complete set of leading effective operators involving SU(2) singlets scalar LQ and the SM fields up to dimension six. We show that, while the renormalizable LQ-lepton-quark interaction Lagrangian can address the persistent hints for physics beyond the Standard Model in the B-decays $\bar B \to D^{(*)} \tau \bar\nu$, $\bar B \to \bar K \ell^+ \ell^-$ and in the measured anomalous magnetic moment of the muon, the LQ higher dimensional effective operators may lead to new interesting effects associated with lepton number violation. These include the generation of one-loop sub-eV Majorana neutrino masses, mediation of neutrinoless double-$\beta$ decay and novel LQ collider signals. For the latter, we focus on 3rd generation LQ ($\phi_3$) in a framework with an approximate $Z_3$ generation symmetry, and show that one class of the dimension five LQ operators may give rise to a striking asymmetric same-charge $\phi_3 \phi_3$ pair-production signal, which leads to low background same-sign leptons signals at the LHC. For example, with $M_{\phi_3} \sim 1$ TeV and a new physics scale of $\Lambda \sim 5$ TeV, we expect about $5000$ positively charged $\tau^+ \tau^+$ events via $pp \to \phi_3 \phi_3 \to \tau^+ \tau^+ + 2 \cdot j_b$ ($j_b$=b-jet) at the 13 TeV LHC with an integrated luminosity of 300 fb$^{-1}$. It is interesting to note that, in the LQ EFT framework, the expected same-sign lepton signals have a rate which is several times larger than the QCD LQ-mediated opposite-sign leptons signals, $gg,q \bar q \to \phi_3 \phi_3^* \to \ell^+ \ell^- +X$. We also consider the same-sign charged lepton signals in the LQ EFT framework at higher energy hadron colliders such as a 27 TeV HE-LHC and a 100 TeV FCC-hh.

  • Lattice study of QCD at finite chiral density: topology and confinement. (arXiv:1902.09325v2 [hep-lat] UPDATED)

    Authors: N.Yu. Astrakhantsev, V.V. Braguta, A.Yu. Kotov, D.D. Kuznedelev, A.A. Nikolaev

    In this paper we study the properties of QCD at nonzero chiral density $\rho_5$, which is introduced through chiral chemical potential $\mu_5$. The study is performed within lattice simulation of QCD with dynamical rooted staggered fermions. We first check that $\rho_5$ is generated at nonzero $\mu_5$ and in the chiral limit observe $\rho_5 \sim \Lambda_{QCD}^2 \mu_5$. We also test the possible connection between confinement and topological fluctuations. To this end, we measured the topological susceptibility $\chi_{\mbox{\footnotesize top}}$ and string tension $\sigma$ for various values of $\mu_5$. We observed that both string tension and chiral susceptibility grow with $\mu_5$ and there is a strong correlation between these quantities. We thus conclude that the chiral chemical potential enhances topological fluctuations and that these fluctuations can indeed be closely related to the strength of confinement.

  • Influence functionals, decoherence and conformally coupled scalars. (arXiv:1902.09607v2 [hep-th] UPDATED)

    Authors: Clare Burrage, Christian Käding, Peter Millington, Jiří Minář

    Some of the simplest modifications to general relativity involve the coupling of additional scalar fields to the scalar curvature. By making a Weyl rescaling of the metric, these theories can be mapped to Einstein gravity with the additional scalar fields instead being coupled universally to matter. The resulting couplings to matter give rise to scalar fifth forces, which can evade the stringent constraints from local tests of gravity by means of so-called screening mechanisms. In this talk, we derive evolution equations for the matrix elements of the reduced density operator of a toy matter sector by means of the Feynman-Vernon influence functional. In particular, we employ a novel approach akin to the LSZ reduction more familiar to scattering-matrix theory. The resulting equations allow the analysis, for instance, of decoherence induced in atom-interferometry experiments by these classes of modified theories of gravity.

  • Quark fragmentation as a probe of dynamical mass generation. (arXiv:1903.04458v2 [hep-ph] UPDATED)

    Authors: Alberto Accardi (Hampton U. and Jefferson Lab), Andrea Signori (Argonne National Laboratory)

    We address the propagation and hadronization of a struck quark by studying the gauge invariance of the color-averaged cut quark propagator, and by relating this to the single inclusive quark fragmentation correlator by means of new sum rules. Using suitable Wilson lines, we provide a gauge-invariant definition for the mass of the color-averaged dressed quark and decompose this into the sum of a current and an interaction-dependent component. The latter, which we argue is an order parameter for dynamical chiral symmetry breaking, also appears in the sum rule for the twist-3 $\tilde{E}$ fragmentation function, providing a specific experimental way to probe the dynamical generation of mass in Quantum Chromo Dynamics.

  • Single pseudoscalar meson pole and pion box contributions to the anomalous magnetic moment of the muon. (arXiv:1903.10844v2 [hep-ph] UPDATED)

    Authors: Gernot Eichmann, Christian S. Fischer, Esther Weil, Richard Williams

    We present results for single pseudoscalar meson pole contributions and pion box contributions to the hadronic light-by-light (LBL) correction of the muon's anomalous magnetic moment. We follow the recently developed dispersive approach to LBL, where these contributions are evaluated with intermediate mesons on-shell. However, the space-like electromagnetic and transition form factors are not determined from analytic continuation of time-like data, but directly calculated within the functional approach to QCD using Dyson-Schwinger and Bethe-Salpeter equations. This strategy allows for a systematic comparison with a strictly dispersive treatment and also with recent results from lattice QCD. Within error bars, we obtain excellent agreement for the pion electromagnetic and transition form factor and the resulting contributions to LBL. In addition, we present results for the $\eta$ and $\eta'$ pole contributions and discuss the dynamical effects in the $\eta-\eta'$ mixing due to the strange quarks. Our result for the total pseudoscalar pole contributions is $a_\mu^{\text{PS-pole}} = 91.6 \,(1.9) \times 10^{-11}$ and for the pion-box contribution we obtain $a_\mu^{\pi-\text{box}} = -16.3 \,(2)(4) \times 10^{-11}$.

  • Reconstructing Rational Functions with $\texttt{FireFly}$. (arXiv:1904.00009v2 [cs.SC] UPDATED)

    Authors: Jonas Klappert, Fabian Lange

    We present the open-source $\texttt{C++}$ library $\texttt{FireFly}$ for the reconstruction of multivariate rational functions over finite fields. We discuss the involved algorithms and their implementation. As an application, we use $\texttt{FireFly}$ in the context of integration-by-parts reductions and compare runtime and memory consumption to a fully algebraic approach with the program $\texttt{Kira}$.

  • Sivers distribution functions of sea quark in proton with chiral Lagrangian. (arXiv:1904.06815v2 [hep-ph] UPDATED)

    Authors: Fangcheng He, P. Wang

    We propose a mechanism for the Sivers distribution function in proton with chiral Lagrangian. By introducing the gauge link of the vector meson, the transverse momentum dependent distribution of a pion in the nucleon is re-defined which is locally $SU(2)_V$ invariant as the Lagrangian. The eikonal propagator is generated from the gauge link and this scenario is proved to be equivalent to the final state interaction. By combining the calculated splitting function and the valence $\bar{q}$ distribution in $\pi$ from the recent fit, the sea quark Sivers function in proton is obtained. We find reasonable numerical results for the first momentum $x\Delta^Nf_{\bar{q}}^{(1)}(x)$ without any fine tuning of the free parameters.

  • Prospects of Cosmic Superstring Detection through Microlensing of Extragalactic Point-Like Sources. (arXiv:1905.03796v2 [astro-ph.CO] UPDATED)

    Authors: David F. Chernoff, Ariel Goobar, Janina J. Renk

    The existence of cosmic superstrings may be probed by astronomical time domain surveys. When crossing the line of sight to point-like sources, strings produce a distinctive microlensing signature. We consider two avenues to hunt for a relic population of superstring loops: frequent monitoring of (1) stars in Andromeda, lensed by loops in the haloes of the Milky-Way and Andromeda and (2) supernovae at cosmological distances, lensed by loops in the intergalactic medium. We assess the potential of such experiments to detect and/or constrain strings with a range of tensions, $10^{-15} \lesssim G \mu/c^2 \lesssim 10^{-6}$. The practical sensitivity is tied to cadence of observations which we explore in detail. We forecast that high-cadence monitoring of $\sim 10^5$ stars on the far side of Andromeda over a year-long period will detect microlensing events if $G\mu/c^2 \sim 10^{-13}$, while $\sim 10^6$ stars will detect events if $10^{-13.5} <G\mu/c^2 < 10^{-11.5}$; the upper and lower bounds of the accessible tension range continue to expand as the number of stars rises. We also analyse the ability to reject models in the absence of fluctuations. While challenging, these studies are within reach of forthcoming time-domain surveys. Supernova observations can hypothetically constrain models with $10^{-12} < G\mu/c^2 < 10^{-6}$ without any optimisation of the survey cadence. However, the event rate forecast suggests it will be difficult to reject models of interest. As a demonstration, we use observations from the Pantheon Type Ia supernova cosmology data-set to place modest constraints on the number density of cosmic superstrings in a poorly tested region of the parameter space.

  • $\bar{K}\Lambda$ molecular explanation to the newly observed $\Xi(1620)^0$. (arXiv:1906.05553v2 [hep-ph] UPDATED)

    Authors: Kan Chen, Rui Chen, Zhi-Feng Sun, Xiang Liu

    The newly observed $\Xi(1620)^0$ by the Belle Collaboration inspires our interest in performing a systematic study on the interaction of an anti-strange meson $(\bar{K}^{(*)})$ with a strange or doubly strange ground octet baryon $\mathcal{B}$ ($\Lambda$, $\Sigma$, and $\Xi$), where the spin-orbit force and the recoil correction are considered in the adopted one-boson-exchange model. Our results indicate that $\Xi(1620)^0$ can be explained as a $\bar{K}\Lambda$ molecular state with $I(J^P)=1/2(1/2^-)$ and the intermediate force from $\sigma$ exchange plays an important role. Additionally, we also predict several other possible molecular candidates, i.e., the $\bar{K}\Sigma$ molecular state with $I(J^P)=1/2(1/2^-)$ and the triply strange $\bar{K}\Xi$ molecular state with $I(J^P)=0(1/2^-)$.

  • Formulating the Kramers problem in field theory. (arXiv:1906.08684v2 [hep-ph] UPDATED)

    Authors: Arjun Berera, Joel Mabillard, Bruno W. Mintz, Rudnei O. Ramos

    The escape problem is defined in the context of quantum field theory. The escape rate is explicitly derived for a scalar field governed by fluctuation-dissipation dynamics, through generalizing the standard Kramers problem. In the presence of thermal fluctuations, there is a nonvanishing probability for a classical background field, initially located at a minimum of its potential in a homogeneous configuration, to escape from the well. The simple and well-known related problem of the escape of a classical point particle due to random forces is first reviewed. We then discuss the difficulties associated with a well-defined formulation of an escape rate for a scalar field and how these can be overcome. A definition of the Kramers problem for a scalar field and a method to obtain the rate are provided. Finally, we discuss some of the potential applications of our results, which can range from condensed matter systems, i.e., nonrelativistic fields, to applications in high-energy physics, like for cosmological phase transitions.

  • Critical behavior of the bulk viscosity in QCD. (arXiv:1906.11306v2 [hep-ph] UPDATED)

    Authors: Mauricio Martinez, Thomas Schaefer, Vladimir Skokov

    We study the behavior of the bulk viscosity $\zeta$ in QCD near a possible critical endpoint in the phase diagram. We verify the expectation that $(\zeta/s)\sim a(\xi/\xi_0)^{x_\zeta}$, where $s$ is the entropy density, $\xi$ is the correlation length, $\xi_0$ is the non-critical correlation length, $a$ is a constant and $x_\zeta\simeq 3$. Using a recently developed equation of state that includes a critical point in the universality class of the Ising model we estimate the constant of proportionality $a$. We find that $a$ is typically quite small, $a\sim O(10^{-4})$. We observe, however, that the result is sensitive to the commonly made assumption that the Ising temperature axis is approximately aligned with the QCD baryon chemical potential axis. If this is not the case, then the critical $\zeta/s$ can approach the non-critical value of $\eta/s$, where $\eta$ is the shear viscosity, even if the enhancement of the correlation length is modest, $\xi/\xi_0\sim 2$.

  • Calculating the EoS of the dense quark-gluon plasma using the Complex Langevin equation. (arXiv:1907.08712v2 [hep-lat] UPDATED)

    Authors: Dénes Sexty

    The pressure and energy density of the quark-gluon plasma at finite baryon chemical potential are calculated using the Complex Langevin equation. The stout smearing procedure is generalized for the SL(3,$\mathcal{C}$) manifold allowing the usage of an improved action in the Complex Langevin setup. Four degenerate flavors of staggered quarks with $m_\pi=500-700$ MeV are used with a tree-level Symanzik improved gauge action on $16^3 \times 8 $ lattices. Results are compared to the Taylor expansion and good agreement is found for small chemical potentials.

  • Pseudo-Nambu-Goldstone dark matter and two-Higgs-doublet models. (arXiv:1907.09684v3 [hep-ph] UPDATED)

    Authors: Xue-Min Jiang, Chengfeng Cai, Zhao-Huan Yu, Yu-Pan Zeng, Hong-Hao Zhang

    We study a dark matter model with one singlet complex scalar and two Higgs doublets. The scalar potential respects a softly broken global symmetry, which makes the imaginary part of the singlet become a pseudo-Nambu-Goldstone boson acting as a dark matter candidate. The pseudo-Nambu-Goldstone nature of the boson leads to the vanishing of its tree-level scattering amplitude off nucleons at zero momentum transfer. Therefore, although the interaction strength could be sufficiently large to yield a viable relic abundance via thermal mechanism, direct detection is incapable of probing this candidate. We further investigate the constraints from Higgs measurements, relic abundance observation, and indirect detection.

  • Isospin symmetry breaking effects for the mode ${B}\rightarrow \pi\pi(K)$ in Perturbative QCD. (arXiv:1907.11550v2 [hep-ph] UPDATED)

    Authors: Gang Lü, Qin-Qin Zhi, Qiu-shi Li, Ying Wang

    We calculate the direct $CP$ violation for the decay process of ${B}^{0}\rightarrow \pi^{0}\pi^{0}$, ${B}^{+}\rightarrow \pi^{0}\pi^{+}$, ${B}^{0}\rightarrow K^{0}\pi^{0}$ and ${B}^{+}\rightarrow K^{+}\pi^{0}$ via isospin symmetry breaking effects from the $\pi^{0}-\eta-\eta'$ mixing mechanism in perturbative QCD approach. Isospin symmetry breaking originates from the electroweak interaction and the u-d quark mass difference through the strong interaction which are known to be tiny. However, we find that isospin symmetry breaking at the leading order changes the $CP$ violation from the new strong phases. Our calculation results for the $CP$ violation are within or including the range of experimental results. We also compare our results with those from the QCD factorization and the perturbative QCD schemes without regard to isospin symmetry breaking effects.

  • Long distance effects in inclusive rare $B$ decays and phenomenology of $\bar{B}\to X_d \ell^+\ell^-$. (arXiv:1908.07507v2 [hep-ph] UPDATED)

    Authors: Tobias Huber, Tobias Hurth, Jack Jenkins, Enrico Lunghi, Qin Qin, K. Keri Vos

    Rare inclusive $B$ decays such as $\bar{B}\to X_{s(d)} \ell^+\ell^-$ are interesting probes for physics beyond the Standard Model. Due to the complementarity to their exclusive counterparts, they might shed light on the anomalies currently seen in exclusive $b \to s$ transitions. Distinguishing new-physics effects from the Standard Model requires precise predictions and necessitates the control of long distance effects. In the present work we revisit and improve the description of various long distance effects in inclusive decays such as charmonium and light-quark resonances, nonfactorisable power corrections, and cascade decays. We then apply these results to a state-of-the-art phenomenological study of $\bar{B}\to X_d \ell^+\ell^-$, including also logarithmically enhanced QED corrections and the recently calculated five-body contributions. To fully exploit the new-physics potential of inclusive flavour-changing neutral current decays, the $\bar{B}\to X_d \ell^+\ell^-$ observables should be measured in a dedicated Belle II analysis.

  • One loop Volkov propagator in the Lorentz class of gauges. (arXiv:1908.10228v2 [hep-ph] UPDATED)

    Authors: Martin Lavelle, David McMullan

    We calculate the ultraviolet divergences in a weak field expansion of the Volkov propagator. This is done for the full Lorentz class of gauges. The expected gauge invariance of the vacuum mass shift in each sideband is recovered. However, the renormalisation of the background induced mass shift is shown to be gauge dependent. In particular, we show that it vanishes in Landau gauge. We find that only in that gauge does the vacuum renormalisation remove all ultraviolet divergences.

  • Predicting the SUSY breaking scale in SUGRA models with degenerate vacua. (arXiv:1909.02124v2 [hep-ph] UPDATED)

    Authors: C. D. Froggatt, R. Nevzorov, H. B. Nielsen, A. W. Thomas

    In N=1 supergravity the scalar potential may have supersymmetric (SUSY) and non-supersymmetric Minkowski vacua (associated with supersymmetric and physical phases) with vanishing energy density. In the supersymmetric Minkowski (second) phase some breakdown of SUSY may be induced by non-perturbative effects in the observable sector that give rise to a tiny positive vacuum energy density. Postulating the exact degeneracy of the physical and second vacua as well as assuming that at high energies the couplings in both phases are almost identical, one can estimate the dark energy density in these vacua. It is mostly determined by the SUSY breaking scale M_S in the physical phase. Exploring the two-loop renormalization group (RG) flow of couplings in these vacua we find that the measured value of the cosmological constant can be reproduced if M_S varies from 20 TeV to 400 TeV. We also argue that this prediction for the SUSY breaking scale is consistent with the upper bound on M_S in the higgsino dark matter scenario.

  • Circumventing Lens Modeling to Detect Dark Matter Substructure in Strong Lens Images with Convolutional Neural Networks. (arXiv:1910.00015v2 [astro-ph.CO] UPDATED)

    Authors: Ana Diaz Rivero, Cora Dvorkin

    Strong gravitational lensing is a promising way of uncovering the nature of dark matter, by finding perturbations to images that cannot be well accounted for by modeling the lens galaxy without additional structure, be it subhalos (smaller halos within the smooth lens) or line-of-sight (LOS) halos. We present results attempting to infer the presence of substructure from images, without doing any intermediate lens modeling, using a simple convolutional neural network (CNN). We find that the network is only able to infer the presence of subhalos with $>75\%$ accuracy when they have masses of $\geq 5 \times 10^9$M$_{\odot}$ if they lie within the main lens galaxy. Since less massive foreground LOS halos can have the same effect as higher mass subhalos, the CNN can probe lower masses in the halo mass function. The accuracy does not improve significantly if we add a population of less massive subhalos. With the expectation of experiments such as HST and Euclid yielding thousands of high-quality strong lensing images in the next years, having a way of analyzing images quickly to identify candidates that merit further analysis to determine individual subhalo properties while preventing extensive resources being used for images that would yield null detections could be very useful. By understanding the sensitivity as a function of substructure mass, non-detections could be combined with the information from images with substructure to constrain the cold dark matter scenario, in particular if the sensitivity can be pushed to lower masses.

  • Pomeron-pomeron scattering. (arXiv:1910.02503v2 [hep-ph] UPDATED)

    Authors: István Szanyi, Volodymyr Svintozelskyi

    Central exclusive diffractive (CED) production of meson resonances potentially is a factory producing new particles, in particular a glueball. The produced resonances lie in trajectories with vacuum quantum numbers, essentially on the pomeron trajectory. A tower of resonance recurrences, the production cross section and the resonances widths are predicted. A new feature is the form of the non-linear pomeron trajectory, producing resonances (glueballs) with increasing widths. At LHC energies, in the nearly forward direction the $t$-channel both in elastic, single or double diffraction dissociation as well as in CED is dominated by pomeron exchange (the role of secondary trajectories is negligible, however a small contribution from the odderon may be present).

  • Probing Unification With Precision Higgs Physics. (arXiv:1910.02853v2 [hep-ph] UPDATED)

    Authors: Sibo Zheng

    We propose a novel approach of probing grand unification through precise measurements on the Higgs Yukawa couplings at the LHC, which is well motivated by the appearance of effective operators not suppressed by the mass scale of unification $M_{\rm{U}}$ in realistic models of unification with minimal Yukawa sector. These operators modify the Higgs Yukawa couplings in correlated patterns at scale $M_{\rm{U}}$ that hold up to higher-order corrections. The coherences reveal that, the weak-scale effect on tau Yukawa coupling is the largest among the third generation, which if verified by the future LHC, can serve as a hint of unification.

  • Spectator induced electromagnetic effects in heavy-ion collisions and space-time-momentum conditions for pion emission. (arXiv:1910.04544v1 [nucl-th] CROSS LISTED)

    Authors: V.Ozvenchuk, A.Rybicki, A.Szczurek, A.Marcinek, M.Kiełbowicz

    We present our calculation of electromagnetic effects, induced by the spectator charge on Feynman-$x_F$ distributions of charged pions in peripheral $Pb+Pb$ collisions at CERN SPS energies, including realistic initial space-time-momentum conditions for pion emission. The calculation is performed in the framework of the fire-streak model, adopted to the production of both $\pi^-$ and $\pi^+$ mesons. Isospin effects are included to take into account the asymmetry in production of $\pi^+$ and $\pi^-$ at high rapidity. A comparison to a simplified model from the literature is made. We obtain a good description of the NA49 data on the $x_F$- and $p_T$-dependence of the ratio of cross sections $\pi^+/\pi^-$. The experimental data favors short times ($0.5<\tau<2$ fm/$c$) for fast pion creation in the local fire-streak rest frame. The possibility of the expansion of the spectators is considered in our calculation, and its influence on the electromagnetic effect observed for the $\pi^+/\pi^-$ ratio is discussed. We conclude that the fire-streak model, which properly describes the centrality dependence of $\pi^-$ rapidity spectra at CERN SPS energies, also provides realistic initial conditions for pion production. Consequently, it provides a quantitative description of the electromagnetic effect on the $\pi^+/\pi^-$ ratio as a function of $x_F$.

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