
On the phenomenology of sphaleroninduced processes at the LHC and beyond. (arXiv:1910.04761v1 [hepph])
Authors: Andreas Papaefstathiou, Simon Plätzer, Kazuki Sakurai
We investigate the phenomenological aspects of nonperturbative baryon and
leptonnumberviolating 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 zerotemperature highenergy 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
generalpurpose 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
protonproton centreofmass energy, a potential highenergy upgrade at 27 TeV
and the proposed Future Circular Collider (FCChh) 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/sphaleroninduced processes at various energy
scales.

Complete Form Factors in YangMills from Unitarity and Spinor Helicity in Six Dimensions. (arXiv:1910.04772v1 [hepth])
Authors: Manuel Accettulli Huber, Andreas Brandhuber, Stefano De Angelis, Gabriele Travaglini
We present a systematic procedure to compute complete, analytic form factors
of gaugeinvariant operators at loop level in pure YangMills. 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
sixdimensional spinorhelicity 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 oneloop minimal and
nonminimal 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 sixdimensional
quantities written in the spinorhelicity formalism.

Cosmological implications of electromagnetically interacting dark matter: millicharged particles and atoms with singly and doubly charged dark matter. (arXiv:1910.04779v1 [astroph.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) millicharged 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 publiclyavailable 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 [hepph])
Authors: ChiaFeng Chang, Yanou Cui
Global cosmic strings are generically predicted in particle physics beyond
the Standard Model, e.g., a postinflationary global $U(1)$ symmetry breaking
which may associate with axionlike 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 Righthanded Neutrinos. (arXiv:1910.04782v1 [hepph])
Authors: Farinaldo S. Queiroz, Clarissa Siqueira
We have witnessed in the past decade the observation of a puzzling cosmicray
excess at energies larger than $10$~GeV. The AMS02 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
AMS02 collaboration in their analysis, one can conclude that previous
explanations to the new AMS02 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 righthanded neutrino portal might reside the answer. We
discuss a decaying twocomponent dark matter scenario where the decay products
are righthanded 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 spinflip with Coulombnuclear interference. (arXiv:1910.04799v1 [hepph])
Authors: B. Z. Kopeliovich, M. Krelina
Brandnew highprecision data for singlespin 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 spinflip to
nonflip ratio $r_5(t)$ in a wide energy range. We introduced essential
modification in the Coulombnuclear interference (CNI) mechanism, missed in
previous analyses. In particular, absorptive corrections make the proton
electromagnetic vertex different from the formfactor, measured in
electronproton 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 spinflip amplitude, which steeply rises with
energy. We found the spinflip to nonflip 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 [grqc])
Authors: Pratik Wagle, Alexander Saffer, Nicolas Yunes
The observation of the inspiral and merger of compact binaries by the
LIGOVirgo collaboration has allowed for new tests of Einstein's theory in the
extreme gravity regime, where gravitational interactions are simultaneously
strong, nonlinear, 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 ChernSimons and EinsteindilatonGaussBonnet
gravity cannot be differentiated from general relativity based on the detection
of polarization modes alone. To prove this result, we use the NewmanPenrose
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 [hepth])
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 KaluzaKlein 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 shortdistance cutoff on local EFT. The
bound is qualitatively saturated by the dilatonic black holes and KaluzaKlein
monopoles.

Extraction of DVCS form factors with uncertainties. (arXiv:1910.04806v1 [hepph])
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 [heplat])
Authors: Pedro Bicudo, Marco Cardoso, Nuno Cardoso, Marc Wagner
We discuss, how to study $I = 0$ quarkonium resonances decaying into pairs of
heavylight 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 heavylight mesonmeson decay channel.
We discuss our results in the context of corresponding experimental results, in
particular for $\Upsilon (10860)$ and $\Upsilon (11020)$.

The DodelsonWidrow Mechanism In the Presence of SelfInteracting Neutrinos. (arXiv:1910.04901v1 [hepph])
Authors: André de Gouvêa, Manibrata Sen, Walter Tangarife, Yue Zhang
keVscale gaugesinglet fermions, allowed to mix with the active neutrinos,
are elegant dark matter (DM) candidates. They are produced in the early
universe via the DodelsonWidrow mechanism and can be detected as they decay
very slowly, emitting Xrays. 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
nextgeneration experiments, including DUNE.

The viability of the 3+1 neutrino model in the supernova neutrino process. (arXiv:1910.04984v1 [hepph])
Authors: Heamin Ko, Dukjae Jang, Motohiko Kusakabe, MyungKi Cheoun
Adopting the 3+1 neutrino mixing parameters by the IceCube and shortbase line
experiments, we investigate the sterileactive 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 neutrinoinduced 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 [hepph])
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 $T_C$: $S_3/T \propto (1T/T_C)^{\gamma}$, 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 signaltonoise
ratio (SNR) for DeciHertz 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 [hepph])
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 CPodd, parityviolating coupling which leads to a
momentumsuppression of the DMnucleon crosssection. All of our results,
samples and input files are publicly available via
$\href{https://www.zenodo.org/communities/gambitofficial/}{\textsf{Zenodo}}$.

Modified interactions in the topquark electroweak sector: exploiting unitarity violating effects at the amplitude level to probe New Physics. (arXiv:1910.05053v1 [hepph])
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 [hepph])
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 SUSYbreaking soft terms. We consider $\mu<0$ such that the
SUSY loopthreshold 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 SMlike 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 topbottom Yukawa unification. Nevertheless, with moderate
deviations of a few percent from exact topbottom Yukawa unification (stemming
e.g.\ from GUTthreshold corrections or higherdimensional 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 [hepph])
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 longlived 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 GeVscale 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 Bdecays 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 [hepph])
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 fourquark (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 [astroph.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 redtilted (spectral index $n_s<1$) at the large scale (small
$k$) and is consistent with the $n_sr$ values allowed from the CMB
observations. Along with that, it has a bluetilt ($n_s>1$) 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 [hepph])
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: Powercounting for small$c_s$ Cosmology. (arXiv:1910.05277v1 [grqc])
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 higherderivative interactions like $\mathfrak{L}_{\rm eff}
\sim (\partial \Phi)^4$ compete with lowerderivative 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 $P(X) \propto \sqrt{1kX}$  since these keep all orders in
$\partial \Phi$ while dropping $\partial^n \Phi$ for $n > 1$. We here find a
sensible powercounting scheme for DBI models that gives a controlled expansion
in powers of three types of small parameters: $H/M_p$, slowroll 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 powercounting 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 powercounting 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 $1c_s$ to be larger than derivativemeasuring
quantities like $(H/M_p)^2$.

MediumInduced Transverse Momentum Broadening via Forward Dijet Correlations. (arXiv:1910.05290v1 [hepph])
Authors: Jiangyong Jia, ShuYi Wei, BoWen Xiao, Feng Yuan
Dijet azimuthal angle correlation is arguably one of the most direct probes
of the mediuminduced broadening effects. The evidence for such broadening,
however, is not yet clearly observed within the precision of current
midrapidity 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 highluminosity 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 [hepex])
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 QuarkDiquark Symmetry Prediction for Hyperfine Splitting. (arXiv:1910.05337v1 [hepph])
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 quarkdiquark 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 [hepph] UPDATED)
Authors: Eric Braaten, LiPing He, Kevin Ingles
The claim that the $X(3872)$ meson cannot be a charmmeson 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 charmmeson pairs.
Assuming $X$ is sufficiently weakly bound, we derive an equality between the
$X$ cross section and a charmmeson 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 orderofmagnitude estimate of
the $X$ cross section in terms of a naive charmmeson 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 TeVscale scalar Leptoquarks in the EFT. (arXiv:1812.03178v4 [hepph] UPDATED)
Authors: Shaouly BarShalom, 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 LQleptonquark interaction Lagrangian can
address the persistent hints for physics beyond the Standard Model in the
Bdecays $\bar B \to D^{(*)} \tau \bar\nu$, $\bar B \to \bar K \ell^+ \ell^$
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 oneloop subeV
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 samecharge $\phi_3 \phi_3$ pairproduction signal, which
leads to low background samesign 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 $pp \to
\phi_3 \phi_3 \to \tau^+ \tau^+ + 2 \cdot j_b$ ($j_b$=bjet) 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 samesign lepton signals have a rate
which is several times larger than the QCD LQmediated oppositesign leptons
signals, $gg,q \bar q \to \phi_3 \phi_3^* \to \ell^+ \ell^ +X$. We also
consider the samesign charged lepton signals in the LQ EFT framework at higher
energy hadron colliders such as a 27 TeV HELHC and a 100 TeV FCChh.

Lattice study of QCD at finite chiral density: topology and confinement. (arXiv:1902.09325v2 [heplat] 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 [hepth] 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 socalled
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 FeynmanVernon influence functional. In particular, we employ a novel
approach akin to the LSZ reduction more familiar to scatteringmatrix theory.
The resulting equations allow the analysis, for instance, of decoherence
induced in atominterferometry experiments by these classes of modified
theories of gravity.

Quark fragmentation as a probe of dynamical mass generation. (arXiv:1903.04458v2 [hepph] 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 coloraveraged 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 gaugeinvariant
definition for the mass of the coloraveraged dressed quark and decompose this
into the sum of a current and an interactiondependent component. The latter,
which we argue is an order parameter for dynamical chiral symmetry breaking,
also appears in the sum rule for the twist3 $\tilde{E}$ 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 [hepph] 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 lightbylight (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
onshell. However, the spacelike electromagnetic and transition form factors
are not determined from analytic continuation of timelike data, but directly
calculated within the functional approach to QCD using DysonSchwinger and
BetheSalpeter 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 $a_\mu^{\text{PSpole}}
= 91.6 \,(1.9) \times 10^{11}$ and for the pionbox 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 opensource $\texttt{C++}$ 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 integrationbyparts 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 [hepph] 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
redefined 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 PointLike Sources. (arXiv:1905.03796v2 [astroph.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 pointlike 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 MilkyWay 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 highcadence monitoring of $\sim 10^5$
stars on the far side of Andromeda over a yearlong 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
timedomain 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 dataset 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 [hepph] UPDATED)
Authors: Kan Chen, Rui Chen, ZhiFeng 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 antistrange
meson $(\bar{K}^{(*)})$ with a strange or doubly strange ground octet baryon
$\mathcal{B}$ ($\Lambda$, $\Sigma$, and $\Xi$), where the spinorbit force and
the recoil correction are considered in the adopted onebosonexchange 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 [hepph] 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
fluctuationdissipation 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 wellknown 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 welldefined 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
highenergy physics, like for cosmological phase transitions.

Critical behavior of the bulk viscosity in QCD. (arXiv:1906.11306v2 [hepph] 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 noncritical 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 noncritical 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 quarkgluon plasma using the Complex Langevin equation. (arXiv:1907.08712v2 [heplat] UPDATED)
Authors: Dénes Sexty
The pressure and energy density of the quarkgluon 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 $m_\pi=500700$ MeV are used with a
treelevel 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.

PseudoNambuGoldstone dark matter and twoHiggsdoublet models. (arXiv:1907.09684v3 [hepph] UPDATED)
Authors: XueMin Jiang, Chengfeng Cai, ZhaoHuan Yu, YuPan Zeng, HongHao 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 pseudoNambuGoldstone boson
acting as a dark matter candidate. The pseudoNambuGoldstone nature of the
boson leads to the vanishing of its treelevel 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 [hepph] UPDATED)
Authors: Gang Lü, QinQin Zhi, Qiushi 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 ud 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 [hepph] 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
newphysics 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 lightquark resonances, nonfactorisable
power corrections, and cascade decays. We then apply these results to a
stateoftheart phenomenological study of $\bar{B}\to X_d \ell^+\ell^$,
including also logarithmically enhanced QED corrections and the recently
calculated fivebody contributions. To fully exploit the newphysics potential
of inclusive flavourchanging 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 [hepph] 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 [hepph] 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
nonsupersymmetric 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 nonperturbative 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 twoloop 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 [astroph.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 lineofsight (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
highquality 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,
nondetections 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.

Pomeronpomeron scattering. (arXiv:1910.02503v2 [hepph] 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 nonlinear 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 [hepph] 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 higherorder corrections. The
coherences reveal that, the weakscale 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 heavyion collisions and spacetimemomentum conditions for pion emission. (arXiv:1910.04544v1 [nuclth] 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
spacetimemomentum conditions for pion emission. The calculation is performed
in the framework of the firestreak 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 firestreak 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 firestreak 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 $x_F$.
