arXiv subjects

E. G. de Oliveira

Publications and source records attributed to E. G. de Oliveira.

At least 19 recordsLinked to original sources

arXiv2019

Studying low-$x$ structure function models with astrophysical tau neutrinos: double bang, lollipop and sugardaddy topologies

Despite not have been yet identified by the IceCube detector, events generated from $ν_τ$ deep inelastic neutrino scattering in ice with varied topologies, such as double cascades (often called \textit{double bangs}), \textit{lollipops} and \textit{sugardaddies}, constitute a potential laboratory for low-x parton studies. Here we investigate these events, analyzing the effect of next-to-next-to-leading order (NNLO) Parton Distribution Function (PDFs) in the total neutrino--nucleon cross section, as compared with the color dipole formalism, where saturation effects play a major role. Energy deposit profiles in the `bangs' are also analysed in terms of virtual $W$-boson and tauon energy distributions and are found to be crucial in establishing a clear signal for gluon distribution determination at very small $x$. By taking the average (all flavor) neutrino flux ($Φ_ν\sim E_ν^{-2.3}$) into differential cross sections as a function of $τ$ and $W$ energies, we find significant deviations from pure DGLAP parton interactions for neutrino energies already at a few PeV. With these findings one aims at providing not only possible observables to be measured in large volume neutrino detectors in the near future, but also theoretical ways of unraveling QCD dynamics using unintegrated neutrino-nucleon cross sections in the ultrahigh-energy frontier.

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arXiv2019

Absorptive effects and power corrections in low $x$ DGLAP evolution

We address a long standing problem concerning the scale behaviour of parton densities in the low $x$, low $Q^2$ domain. We emphasize the important role of absorptive corrections at low $x$ and use knowledge of diffractive deep inelastic scattering to exclude the absorptive effect from conventional deep inelastic data. In this way we obtain a significantly different low $x$ behaviour of the gluon density, which is now much better described by linear DGLAP evolution. Accounting also for a second power correction, which arises from the freezing of $α_s$ at low $Q^2$, leads to an essentially flat behaviour of the low $x$ gluon density.

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arXiv2018

Open charm production and low x gluons

We compare the rapidity, $y$, and the beam energy, $\sqrt{s}$, behaviours of the cross section of the data for $D$ meson production in the forward direction that were measured by the LHCb collaboration. We describe the observed cross sections using NLO perturbative QCD, and choose the optimal factorization scale for the LO contribution which provides the resummation of the large double logarithms. We emphasize the inconsistency observed in the $y$ and $\sqrt{s}$ behaviours of the $D$ meson cross sections. The $y$ behaviour indicates a very {\it flat} $x$ dependence of the gluon PDF in the unexplored low $x$ region around $x\sim 10^{-5}$. However, to describe the $\sqrt{s}$ dependence of the data we need a steeper gluon PDF with decreasing $x$. Moreover, an even steeper behaviour is needed to provide an extrapolation which matches on to the well known gluons found in the global PDF analyses for $x\sim 10^{-3}$. The possible role of non-perturbative effects is briefly discussed.

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arXiv2017

Low x gluons determined by open charm production

We use the LHCb data on the forward production of $D$ mesons at 7, 13 and 5 TeV to make a direct determination of the gluon distribution, $xg$, at NLO in the $10^{-5} \lesssim x \lesssim 10^{-4}$ region. We first use a simple parametrization of the gluon density in each of the four transverse momentum intervals of the detected $D$ mesons. Second, we use a double log parametrization to make a combined fit to all the LHCb $D$ meson data. The values found for $xg$ in the above $x$ domain are of the similar magnitude (or a bit larger) than the central values of extrapolations of the gluons obtained by the global PDF analyses into this small $x$ region. However, in contrast, we find $xg$ has a weak dependence on $x$.

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arXiv2017

Scale dependence of open $c\bar{c}$ and $b\bar{b}$ production in the low $x$ region

The `optimal' factorization scale $μ_0$ is calculated for open heavy quark production. We find that the optimal value is $μ_F=μ_0\simeq 0.85\sqrt{p^2_T+m_Q^2} $; a choice which allows us to resum the double-logarithmic, $(α_s\lnμ^2_F\ln(1/x))^n$ corrections (enhanced at LHC energies by large values of $\ln(1/x)$) and to move them into the incoming parton distributions, PDF$(x,μ_0^2)$. Besides this result for the single inclusive cross section (corresponding to an observed heavy quark of transverse momentum $p_T$), we also determined the scale for processes where the acoplanarity can be measured; that is, events where the azimuthal angle between the quark and the antiquark may be determined experimentally. Moreover, we discuss the important role played by the $2\to 2$ subprocesses, $gg\to Q\bar{Q}$ at NLO and higher orders. In summary, we achieve a better stability of the QCD calculations, so that the data on $c\bar{c}$ and $b\bar{b}$ production can be used to further constrain the gluons in the small $x$, relatively low scale, domain, where the uncertainties of the global analyses are large at present.

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arXiv2014

Evolution in opening angle combining DGLAP and BFKL logarithms

We present an evolution equation which simultaneously sums the leading BFKL and DGLAP logarithms for the integrated gluon distribution in terms of a single variable, namely the emission angle of the gluon. This form of evolution is appropriate for Monte Carlo simulations of events of high energy pp (and p\bar{p}) interactions, particularly where small x events are sampled.

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arXiv2014

BFKL equation for an integrated gluon density

We show how it is possible to rewrite the BFKL equation for the unintegrated gluon distribution, in terms of integrated gluons, similar to that used in DGLAP. We add to our equation the next-to-leading log terms which provide exact energy-momentum conservation and account for the kinematic constraint in real gluon emission. In this way the equation includes the major part of the higher-order corrections to BFKL evolution. We discuss the possibility to obtain a unified BFKL-DGLAP evolution equation relevant to processes at the LHC where both log(1/x) and logQ^2 are large simultaneously.

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arXiv2013

Physical factorisation scheme for PDFs for non-inclusive applications

We introduce the physical factorisation scheme, which is necessary to describe observables which are `not completely inclusive'. We derive the formulae for NLO DGLAP evolution in this scheme, and also for the `rotation' of the conventional MSbar PDFs into the physical representation. Unlike, the MSbar prescription, where, for example, the gluon PDF at NLO obtains an admixture of the quark-singlet PDF, and vice-versa, the physical approach does not mix parton PDFs of different types. That is, the physical approach retains the precise quantum numbers of each PDF. The NLO corrections to DGLAP evolution in the physical scheme are less than those in the MSbar case, indicating a better convergence of the perturbative series.

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arXiv2013

Treatment of the infrared contribution: NLO QED evolution as a pedagogic example

We show that the conventional prescription used for DGLAP parton evolution at NLO has an inconsistent treatment of the contribution from the infrared (IR) region. We illustrate the problem by studying the simple example of QED evolution, treating the electron and photon as partons. The deficiency is not present in a physical approach which removes the IR divergency and allows calculation in the normal 4-dimensional space.

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arXiv2013

Treatment of heavy quarks in QCD

We show that to correctly describe the effects of the heavy-quark mass, m_h, in DGLAP evolution, it is necessary to work in the so-called `physical' scheme. In this way, we automatically obtain a smooth transition through the heavy-quark thresholds. Moreover, we show that to obtain NLO accuracy, it is sufficient to account for the heavy-quark mass, m_h, just in the LO (one-loop) splitting function. The use of the MS-bar factorisation scheme is not appropriate, since at NLO we deal with a mixture of quarks and gluon (that is, the mass of the heavy parton is not well-defined). The formulae for the explicit m_h dependence of the splitting functions, and for alpha_s, are presented.

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arXiv2013

Unintegrated parton distributions in nuclei

We study how unintegrated parton distributions in nuclei can be calculated from the corresponding integrated partons using the EPS09 parametrization. The role of nuclear effects is presented in terms of the ratio $R^A=\text{uPDF}^A/A\cdot \text{PDF}^N$ for both large and small $x$ domains.

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arXiv2013

Heavy quarkonium production in a strong magnetic field

It is well known that in noncentral heavy-ion collisions a transient strong magnetic field is generated in the direction perpendicular to the reaction plane. The maximal strength of this field is estimated to be $eB \sim m^2_π \sim 0.02 \text{GeV}^2$ at the RHIC and $eB \sim 15 m^2_π \sim 0.3 \text{GeV}^2$ at the LHC. We investigate the effects of a strong magnetic field on $B$ and $D$ mesons, focusing on the changes of the energy levels and the masses of the bound states. Using the Color Evaporation Model we discuss the possible changes in the production of $J/ψ$ and $Υ$.

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arXiv2013

Inclusive hadron and photon production at LHC in dipole momentum space

Using a momentum space model for the dipole scattering amplitude we present an analysis of the saturation effects at LHC energies, describing the data on proton-proton and proton-lead collisions. The model is based on the asymptotic solutions of the Balitsky-Kovchegov equation, being ideal in the saturation domain where the target wave function has a high occupation number. We also make predictions for the nuclear modification ratios on charged hadron and prompt photon production in the forward region, where the high parton density effects are important.

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arXiv2013

Improving the Drell-Yan probe of small x partons at the LHC via a k_t cut

We show that the observation of the Drell-Yan production of low-mass lepton-pairs (M < 20 GeV) at high rapidities (Y > 3) at the LHC can make a direct measurement of parton distribution functions (PDFs) in the low x region, x < 10^{-4}. We describe a procedure that greatly reduces the sensitivity of the predictions to the choice of the factorization scale and, in particular, show how, by imposing a cutoff on the transverse momentum of the lepton-pair, the data are able to probe PDFs in the important low scale, low x domain. We include the effects of the Sudakov suppression factor.

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arXiv2012

The LHC can probe small x PDFs; the treatment of the infrared region

First, we show how to reduce the sensitivity of the NLO predictions of the Drell-Yan production of low-mass, lepton-pairs, at high rapidity, to the choice of factorization scale. In this way, observations of this process at the LHC can make direct measurements of parton distribution functions in the low x domain; x < 10^{-4}. Second, we find an inconsistency in the conventional NLO treatment of the infrared region. We illustrate the problem using the NLO coefficient function of Drell-Yan production.

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arXiv2012

Running coupling and pomeron loop effects on inclusive and diffractive DIS cross sections

Within the framework of a (1+1)--dimensional model which mimics high energy QCD, we study the behavior of the cross sections for inclusive and diffractive deep inelastic $γ^*h$ scattering cross sections. We analyze the cases of both fixed and running coupling within the mean field approximation, in which the evolution of the scattering amplitude is described by the Balitsky-Kovchegov equation, and also through the pomeron loop equations, which include in the evolution the gluon number fluctuations. In the diffractive case, similarly to the inclusive one, the suppression of the diffusive scaling, as a consequence of the inclusion of the running of the coupling, is observed.

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arXiv2012

The treatment of the infrared region in perturbative QCD

We discuss the contribution coming from the infrared region to NLO matrix elements and/or coefficient functions of hard QCD processes. Strictly speaking, this contribution is not known theoretically, since it is beyond perturbative QCD. For DGLAP evolution all the infrared contributions are collected in the phenomenological input parton distribution functions (PDFs), at some relatively low scale Q_0; functions which are obtained from a fit to the `global' data. However dimensional regularization sometimes produces a non-zero result coming from the infrared region. Instead of this conventional regularization treatment, we argue that the proper procedure is to first subtract from the NLO matrix element the contribution already generated at the same order in α_s by the LO DGLAP splitting function convoluted with the LO matrix element. This prescription eliminates the logarithmic infrared divergence, giving a well-defined result which is consistent with the original idea that everything below Q_0 is collected in the PDF input. We quantify the difference between the proposed treatment and the conventional approach using low-mass Drell-Yan production and deep inelastic electron-proton scattering as examples; and discuss the potential impact on the `global' PDF analyses. We present arguments to show that the difference cannot be regarded as simply the use of an alternative factorization scheme.

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arXiv2012

Nuclear shadowing in deep inelastic scattering on nuclei: a closer look

The measurement of the nuclear structure function $F_2^A (x,Q^2)$ at the future electron-ion collider (EIC) will be of great relevance to understand the origin of the nuclear shadowing and to probe gluon saturation effects. Currently there are several phenomenological models, based on very distinct approaches, which describe the scarce experimental data quite successfully. One of main uncertainties comes from the schemes used to include the effects associated to the multiple scatterings and to unitarize the cross section. In this paper we compare the predictions of three different unitarization schemes of the nuclear structure function which use the same theoretical input to describe the projectile-nucleon interaction. In particular, we consider as input the predictions of the Color Glass Condensate formalism, which reproduce the inclusive and diffractive $ep$ HERA data. Our results suggest that the experimental analysis of $F_2^A$ will be able to discriminate between the unitarization schemes.

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