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Measurement of inclusive charged-particle b-jet production in pp and p-Pb collisions at $$\sqrt{s_{\mathrm{NN}}}$$ = 5.02 TeV

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Abstract

A bstract A measurement of the inclusive b-jet production cross section is presented in pp and p-Pb collisions at $$\sqrt{s_{\mathrm{NN}}}$$ s NN = 5 . 02 TeV, using data collected with the ALICE detector at the LHC. The jets were reconstructed in the central rapidity region |η| < 0 . 5 from charged particles using the anti- k T algorithm with resolution parameter R = 0 . 4. Identification of b jets exploits the long lifetime of b hadrons, using the properties of secondary vertices and impact parameter distributions. The p T -differential inclusive production cross section of b jets, as well as the corresponding inclusive b-jet fraction, are reported for pp and p-Pb collisions in the jet transverse momentum range 10 ≤ p T , ch jet ≤ 100 GeV/ c , together with the nuclear modification factor, $${R}_{\mathrm{pPb}}^{\mathrm{b}-\mathrm{jet}}$$ R pPb b − jet . The analysis thus extends the lower p T limit of b-jet measurements at the LHC. The nuclear modification factor is found to be consistent with unity, indicating that the production of b jets in p-Pb at $$\sqrt{s_{\mathrm{NN}}}$$ s NN = 5 . 02 TeV is not affected by cold nuclear matter effects within the current precision. The measurements are well reproduced by POWHEG NLO pQCD calculations with PYTHIA fragmentation.

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... The production of charm and beauty jets in pp collisions at √ s = 5.02 TeV and 13 TeV was measured with ALICE [1,2]. Charm jets were identified by the presence of a prompt D 0 meson (reconstructed via its hadronic decay D 0 → Kπ − ) among their constituents, while beauty jets were tagged exploiting the wider impact parameter distribution of beauty-hadron-decay particles. ...
... Within the experimental and theoretical uncertainties, the measurements are also in agreement with the POWHEG + PYTHIA 8 calculations. The p ch jet T -differential inclusive production cross section of b jets, as well as the corresponding inclusive b-jet fraction, are reported in [2] and the measurements are well reproduced by POWHEG calculations with PYTHIA8 fragmentation. ...
... The beauty-jet production cross section has been measured down to p ch jet T =10 GeV/c in p-Pb collisions [2]. The overall impact of cold nuclear matter effects on the resulting p ch jet T -differential cross section can be quantified by means of the nuclear modification factor R b−jet pPb defined as the ratio of the yield measured in p-Pb collisions and the expected yield that would be obtained from a superposition of independent pp collisions. ...
Preprint
The early production of heavy-flavor (HF, charm and beauty) quarks makes them an excellent probe of the dynamical evolution of quantum chromodynamics (QCD) systems. Jets tagged by the presence of a HF hadron give access to the kinematics of the heavy quarks, and along with correlation measurements involving HF hadrons allow for comparisons of their production, propagation and fragmentation across different systems. In this contribution the latest results on HF jets and correlations measured with the ALICE detector in pp, p--Pb and Pb--Pb collisions from the LHC Run 2 data are reported.
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... Corresponding measurements were performed by the ATLAS [35], CMS [36], and (very recently) ALICE [37] collaborations. The ALICE experiment can reconstruct the b-flavored jets down to extremely low transverse momenta, such as p T ≈ 10 GeV [37]. The CMS collaboration recently measured jet shapes for b jets in pp collisions for the first time [38]. ...
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... (Note that R influences the amount of background picked up by the jet clustering algorithm as well as the fraction of the full parton shower is typically contained in the jet. This analysis uses R = 0.4, a choice employed by several inclusive and heavy-flavour jet analyses [27,28]). The jets were required to be fully contained within the pseudorapidity region of |η| < 0.8. ...
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... The reconstructed jets were categorized in 20 different p jet T ranges, from 15 GeV up to 400 GeV. In the case of the charm and beauty jet samples, the corresponding heavy quark was required to fall within the cone of the selected jet, similarly to jet-tagging methods that are utilized in the experiment [29,30]. ...
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FastJet is a C++ package that provides a broad range of jet finding and analysis tools. It includes efficient native implementations of all widely used 2-to-1 sequential recombination jet algorithms for pp and e+e- collisions, as well as access to 3rd party jet algorithms through a plugin mechanism, including all currently used cone algorithms. FastJet also provides means to facilitate the manipulation of jet substructure, including some common boosted heavy-object taggers, as well as tools for estimation of pileup and underlying-event noise levels, determination of jet areas and subtraction or suppression of noise in jets.
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We calculate the transverse momentum dependence of the medium-induced gluon energy distribution radiated off massive quarks in spatially extended QCD matter. In the absence of a medium, the distribution shows a characteristic mass-dependent depletion of the gluon radiation for angles smaller than m/E, the so-called dead cone effect. Medium-modifications of this spectrum are calculated as a function of quark mass, initial quark energy, in-medium pathlength and density. Generically, medium-induced gluon radiation is found to fill the dead cone, but it is reduced at large gluon energies compared to the radiation off light quarks. We quantify the resulting mass-dependence for momentum-averaged quantities (gluon energy distribution and average parton energy loss), compare it to simple approximation schemes and discuss its observable consequences for nucleus-nucleus collisions at RHIC and LHC. In particular, our analysis does not favor the complete disappearance of energy loss effects from leading open charm spectra at RHIC.
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We present a next-to-leading order (NLO) global DGLAP analysis of nuclear parton distribution functions (nPDFs) and their uncertainties. Carrying out an NLO nPDF analysis for the first time with three different types of experimental input -- deep inelastic $\ell$+A scattering, Drell-Yan dilepton production in p+$A$ collisions, and inclusive pion production in d+Au and p+p collisions at RHIC -- we find that these data can well be described in a conventional collinear factorization framework. Although the pion production has not been traditionally included in the global analyses, we find that the shape of the nuclear modification factor $R_{\rm dAu}$ of the pion $p_T$-spectrum at midrapidity retains sensitivity to the gluon distributions, providing evidence for shadowing and EMC-effect in the nuclear gluons. We use the Hessian method to quantify the nPDF uncertainties which originate from the uncertainties in the data. In this method the sensitivity of $\chi^2$ to the variations of the fitting parameters is mapped out to orthogonal error sets which provide a user-friendly way to calculate how the nPDF uncertainties propagate to any factorizable nuclear cross-section. The obtained NLO and LO nPDFs and the corresponding error sets are collected in our new release called {\ttfamily EPS09}. These results should find applications in precision analyses of the signatures and properties of QCD matter at the LHC and RHIC.
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Nuclear parton distribution functions (NPDFs) are determined by global analyses of experimental data on structure-function ratios F_2^A/F_2^{A'} and Drell-Yan cross-section ratios \sigma_{DY}^A/\sigma_{DY}^{A'}. The analyses are done in the leading order (LO) and next-to-leading order (NLO) of running coupling constant \alpha_s. Uncertainties of the NPDFs are estimated in both LO and NLO for finding possible NLO improvement. Valence-quark distributions are well determined, and antiquark distributions are also determined at x<0.1. However, the antiquark distributions have large uncertainties at x>0.2. Gluon modifications cannot be fixed at this stage. Although the advantage of the NLO analysis, in comparison with the LO one, is generally the sensitivity to the gluon distributions, gluon uncertainties are almost the same in the LO and NLO. It is because current scaling-violation data are not accurate enough to determine precise nuclear gluon distributions. Modifications of the PDFs in the deuteron are also discussed by including data on the proton-deuteron ratio F_2^D/F_2^p in the analysis. A code is provided for calculating the NPDFs and their uncertainties at given x and Q^2 in the LO and NLO.
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This report introduces general ideas and some basic methods of the Bayesian probability theory applied to physics measurements. Our aim is to make the reader familiar, through examples rather than rigorous formalism, with concepts such as: model comparison (including the automatic Ockham's Razor filter provided by the Bayesian approach); parametric inference; quantification of the uncertainty about the value of physical quantities, also taking into account systematic effects; role of marginalization; posterior characterization; predictive distributions; hierarchical modelling and hyperparameters; Gaussian approximation of the posterior and recovery of conventional methods, especially maximum likelihood and chi-square fits under well defined conditions; conjugate priors, transformation invariance and maximum entropy motivated priors; Monte Carlo estimates of expectation, including a short introduction to Markov Chain Monte Carlo methods. Comment: 40 pages, 2 figures, invited paper for Reports on Progress in Physics
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After a brief review of the various scenarios for quarkonium production in ultra-relativistic nucleus-nucleus collisions we focus on the ingredients and assumptions underlying the statistical hadronization model. We then confront model predictions for J/$\psi$ phase space distributions with the most recent data from the RHIC accelerator. Analysis of the rapidity dependence of the J/$\psi$ nuclear modification factor yields first evidence for the production of J/$\psi$ mesons at the phase boundary. We conclude with predictions for charmonium production at the LHC.
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This is a review of the theoretical background, experimental techniques, and phenomenology of what is called the "Glauber Model" in relativistic heavy ion physics. This model is used to calculate "geometric" quantities, which are typically expressed as impact parameter (b), number of participating nucleons (N_part) and number of binary nucleon-nucleon collisions (N_coll). A brief history of the original Glauber model is presented, with emphasis on its development into the purely classical, geometric picture that is used for present-day data analyses. Distinctions are made between the "optical limit" and Monte Carlo approaches, which are often used interchangably but have some essential differences in particular contexts. The methods used by the four RHIC experiments are compared and contrasted, although the end results are reassuringly similar for the various geometric observables. Finally, several important RHIC measurements are highlighted that rely on geometric quantities, estimated from Glauber calculations, to draw insight from experimental observables. The status and future of Glauber modeling in the next generation of heavy ion physics studies is briefly discussed.
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We discuss the factorization theorems that enable one to apply perturbative calculations to many important processes involving hadrons. In this introductory section we state briefly what the theorems are, and in Sects. 2 to 4, we indicate how they are applied in calculations. In subsequent sections, we present an outline of how the theorems are established, both in the simple but instructive case of scalar field theory and in the more complex and physically interesting case of quantum chromodynamics (QCD).
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We perform a next to leading order QCD global analysis of nuclear deep inelastic scattering and Drell-Yan data using the convolution approach to parameterize nuclear parton densities. We find both a significant improvement in the agreement with data compared to previous extractions, and substantial differences in the scale dependence of nuclear effects compared to leading order analyses.
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We study Drell-Yan (DY) dilepton production in proton(deuterium)-nucleus and in nucleus-nucleus collisions within the light-cone color dipole formalism. This approach is especially suitable for predicting nuclear effects in the DY cross section for heavy ion collisions, as it provides the impact parameter dependence of nuclear shadowing and transverse momentum broadening, quantities that are not available from the standard parton model. For p(D)+A collisions we calculate nuclear shadowing and investigate nuclear modification of the DY transverse momentum distribution at RHIC and LHC for kinematics corresponding to coherence length much longer than the nuclear size. Calculations are performed separately for transversely and longitudinally polarized DY photons, and predictions are presented for the dilepton angular distribution. Furthermore, we calculate nuclear broadening of the mean transverse momentum squared of DY dileptons as function of the nuclear mass number and energy. We also predict nuclear effects for the cross section of the DY process in heavy ion collisions. We found a substantial nuclear shadowing for valence quarks, stronger than for the sea.
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Broadening of the transverse momentum of a parton propagating through a medium is treated using the color dipole formalism, which has the advantage of being a well developed phenomenology in deep-inelastic scattering and soft processes. Within this approach, nuclear broadening should be treated as color filtering, i.e. absorption of large-size dipoles leading to diminishing (enlarged) transverse separation (momentum). We also present a more intuitive derivation based on the classic scattering theory of Moli\ere. This derivation helps to understand the origin of the dipole cross section, part of which comes from attenuation of the quark, while another part is due to multiple interactions of the quark. It also demonstrates that the lowest-order rescattering term provides an A-dependence very different from the generally accepted A^{1/3} behavior. The effect of broadening increases with energy, and we evaluate it using different phenomenological models for the unintegrated gluon density. Although the process is dominated by soft interactions, the phenomenology we use is tested using hadronic cross section data. Comment: 27 pages of Latex including 5 figures. A few references are added
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We present an improved leading-order global DGLAP analysis of nuclear parton distribution functions (nPDFs), supplementing the traditionally used data from deep inelastic lepton-nucleus scattering and Drell-Yan dilepton production in proton-nucleus collisions, with inclusive high-$p_T$ hadron production data measured at RHIC in d+Au collisions. With the help of an extended definition of the $\chi^2$ function, we now can more efficiently exploit the constraints the different data sets offer, for gluon shadowing in particular, and account for the overall data normalization uncertainties during the automated $\chi^2$ minimization. The very good simultaneous fit to the nuclear hard process data used demonstrates the feasibility of a universal set of nPDFs, but also limitations become visible. The high-$p_T$ forward-rapidity hadron data of BRAHMS add a new crucial constraint into the analysis by offering a direct probe for the nuclear gluon distributions -- a sector in the nPDFs which has traditionally been very badly constrained. We obtain a strikingly stronger gluon shadowing than what has been estimated in previous global analyses. The obtained nPDFs are released as a parametrization called EPS08. Comment: 26 pages, 14 figures; for v2, we have revised the Table 1 and Fig. 13, and added the Fig. 14 and the Table 3 along with some more discussion
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The aim of this work is to describe in detail the POWHEG method, first suggested by one of the authors, for interfacing parton-shower generators with NLO QCD computations. We describe the method in its full generality, and then specify its features in two subtraction frameworks for NLO calculations: the Catani-Seymour and the Frixione-Kunszt-Signer approach. Two examples are discussed in detail in both approaches: the production of hadrons in e+e- collisions, and the Drell-Yan vector-boson production in hadronic collisions. Comment: 91 pages, 2 figures