Hadron Production In Collisions Research Articles

Threshold resummation at NNLL for single-particle production in hadronic collisions

We develop threshold resummation for single-particle inclusive cross sections in hadron-hadron collisions to the level of next-to-next-to-leading logarithm, up to full matching with two-loop hard functions. We define and calculate all one-loop soft functions for all partonic channels. This enables us to separate the hard and soft functions at one loop. Along with these results, the one-loop finite parts of jet functions are used to check that the full soft, collinear and virtual corrections are reproduced to one loop for all partonic reactions. We exhibit these NLO results explicitly. NLO expansions of the resummed cross section match the exact NLO results extremely well numerically, and two loop expansions result in substantial corrections over many kinematic configurations. Explicit results are given in Mellin moment space, and a number of options for generating resummed cross sections are discussed.

Measurements of very-forward energy with the CASTOR calorimeter of CMS

The very-forward energy production in hadron collisions is of paramount importance for the understanding of ultra-high energy cosmic ray air showers. The CASTOR calorimeter of CMS is located at -6:6 < η < -5:2 in the phase-space where the peak of energy is deposited at LHC. The composition and characteristics of the particles in this phase-space have a determining impact on the formation of air shower cascades. An overview of various energy measurements performed with CASTOR is reported and possible implications for cosmic ray physics are outlined.

Diffractive dijet production: Breakdown of factorization

We analyse the origin of dramatic breakdown of diffractive factorisation, observed in single-diffractive (SD) dijet production in hadronic collisions. One of the sources is the application of the results of measurements of the diagonal diffractive DIS to the off-diagonal hadronic diffractive process. The suppression caused by a possibility of inelastic interaction with the spectator partons is calculated at the amplitude level, differently from the usual probabilistic description. It turns out, however, that interaction with the spectator partons not only suppresses the SD cross section, but also gives rise to the main mechanism of SD dijet production, which is another important source of factorization failure. Our parameter-free calculations of SD-to-inclusive cross section ratio, performed in the dipole representation, agrees with the corresponding CDF Tevatron (Run II) data at $\sqrt{s}=1.96$ TeV in the relevant kinematic regions. The energy and hard scale dependences demonstrate a trend, opposite to the factorisation-based expectations, similarly to the effect observed earlier in diffractive Abelian radiation.

Transverse-momentum resummation for top-quark pair production at the LHC

We consider transverse-momentum resummation for top-quark pair production in hadron collisions. At small transverse momenta of the top-quark pair, the logarithmically-enhanced QCD contributions are resummed to all orders up to next-to-leading logarithmic accuracy. At intermediate and large values of transverse momenta, the resummation is consistently combined with the complete result at fixed perturbative order. We present numerical results for the transverse-momentum distribution of top-quark pairs at LHC energies. We perform a detailed study of the scale dependence of the results to estimate their perturbative uncertainty. We comment on the comparison with ATLAS and CMS data.

Soft-gluon corrections in top-quark production

I review calculations of soft-gluon corrections for top-quark production in hadron collisions. I describe theoretical formalisms for their resummation and for finite-order expansions. I show that soft-gluon corrections are dominant for a large number of top-quark processes. I discuss top–antitop pair production as well as single-top production, including total cross sections and differential distributions, and compare with data from the LHC and the Tevatron. I also discuss top-quark production in association with charged Higgs bosons, Z bosons and other particles in models of new physics.

Direct photon production and PDF fits reloaded

Direct photon production in hadronic collisions provides a handle on the gluon PDF by means of the QCD Compton scattering process. In this work we revisit the impact of direct photon production on a global PDF analysis, motivated by the recent availability of the next-to-next-to-leading (NNLO) calculation for this process. We demonstrate that the inclusion of NNLO QCD and leading-logarithmic electroweak corrections leads to a good quantitative agreement with the ATLAS measurements at 8 and 13 TeV, except for the most forward rapidity region in the former case. By including the ATLAS 8 TeV direct photon production data in the NNPDF3.1 NNLO global analysis, we assess its impact on the medium-x gluon. We also study the constraining power of the direct photon production measurements on PDF fits based on different datasets, in particular on the NNPDF3.1 no-LHC and collider-only fits. We also present updated NNLO theoretical predictions for direct photon production at 13 TeV that include the constraints from the 8 TeV measurements.

GR@PPA 2.9: Radiation matching for simulating photon production processes in hadron collisions

We release an event generator package GR@PPA 2.9, for simulating the direct (single) photon and diphoton (double photon) production in hadron collisions. The included programs were used in our previous studies, in which we have explicitly shown large contributions from parton-associated processes. The programs consistently combine simulations based on matrix elements with parton-shower simulations that reproduce the multiple parton radiation and quark fragmentation to photons. The matrix elements include associated parton production processes up to two partons. We provide instructions for the installation and execution of the programs in this article. The practical performance is also presented. Program summaryProgram Title: GR@PPA 2.9Program Files doi:http://dx.doi.org/10.17632/7838c8djsr.1Licensing provisions: GPLv3Programming language: Fortran; with some included libraries coded in C and C++External routines/libraries: bash and Perl for the setup, and CERNLIB, ROOT, LHAPDF 5, PYTHIA 6.4 according to the choice of usersJournal reference of previous version: Comput. Phys. Comm. 183 (2012) 1014–1028Does the new version supersede the previous version?: No, this version supports processes not included in previous versions.Nature of problem: It is necessary to include jet-associated processes up to two jets for realistic simulations of photon production processes in high-energy hadron collisions. Photons also need to be considered as radiation. An appropriate matching method has to be introduced for combining those processes having different jet and photon multiplicities and parton shower simulations.Solution method: The limited leading-log (LLL) subtraction method has been extended to multi-jet processes and those including photon radiation. The final-state parton shower simulation has been improved to support QED photon radiation.Reasons for the new version: New processes have been supported.Summary of revisions: This program package provides event generators for realistic simulations of single and double photon production processes in high-energy hadron collisions. Users can consistently combine different jet- and photon-multiplicity processes up to two jets and two photons.

Matching factorization theorems with an inverse-error weighting

We propose a new fast method to match factorization theorems applicable in different kinematical regions, such as the transverse-momentum-dependent and the collinear factorization theorems in Quantum Chromodynamics. At variance with well-known approaches relying on their simple addition and subsequent subtraction of double-counted contributions, ours simply builds on their weighting using the theory uncertainties deduced from the factorization theorems themselves. This allows us to estimate the unknown complete matched cross section from an inverse-error-weighted average. The method is simple and provides an evaluation of the theoretical uncertainty of the matched cross section associated with the uncertainties from the power corrections to the factorization theorems (additional uncertainties, such as the nonperturbative ones, should be added for a proper comparison with experimental data). Its usage is illustrated with several basic examples, such as Z boson, W boson, H0 boson and Drell–Yan lepton-pair production in hadronic collisions, and compared to the state-of-the-art Collins–Soper–Sterman subtraction scheme. It is also not limited to the transverse-momentum spectrum, and can straightforwardly be extended to match any (un)polarized cross section differential in other variables, including multi-differential measurements.

Heavy quark radiation in NLO+PS POWHEG generators

In this paper we deal with radiation from heavy quarks in the context of next-to-leading order calculations matched to parton shower generators. A new algorithm for radiation from massive quarks is presented that has considerable advantages over the one previously employed. We implement the algorithm in the framework of the POWHEG-BOX, and compare it with the previous one in the case of the hvq generator for bottom production in hadronic collisions, and in the case of the bb4l generator for top production and decay.

Transverse vetoes with rapidity cutoff in SCET

We consider di-jet production in hadron collisions where a transverse veto is imposed on radiation for (pseudo-)rapidities in the central region only, where this central region is defined with rapidity cutoff. For the case where the transverse measurement (e.g., transverse energy or min pT for jet veto) is parametrically larger relative to the typical transverse momentum beyond the cutoff, the cross section is insensitive to the cutoff parameter and is factorized in terms of collinear and soft degrees of freedom. The virtuality for these degrees of freedom is set by the transverse measurement, as in typical transverse-momentum dependent observables such as Drell-Yan, Higgs production, and the event shape broadening. This paper focuses on the other region, where the typical transverse momentum below and beyond the cutoff is of similar size. In this region the rapidity cutoff further resolves soft radiation into (u)soft and soft-collinear radiation with different rapidities but identical virtuality. This gives rise to rapidity logarithms of the rapidity cutoff parameter which we resum using renormalization group methods. We factorize the cross section in this region in terms of soft and collinear functions in the framework of soft-collinear effective theory, then further refactorize the soft function as a convolution of the (u)soft and soft-collinear functions. All these functions are calculated at one-loop order. As an example, we calculate a differential cross section for a specific partonic channel, qq′ → qq′, for the jet shape angularities and show that the refactorization allows us to resum the rapidity logarithms and significantly reduce theoretical uncertainties in the jet shape spectrum.

Measurement of open heavy-flavour production as a function of charged-particle multiplicity with ALICE at the LHC

Heavy quarks are produced in the early stages of ultra-relativistic hadron collisions via hard scatterings and are an important tool for studying different aspects of Quantum Chromodynamics (QCD) in hadronic collisions. Charged-particle multiplicity gives information on the global characteristics of the event and could be used to characterize particle production mechanisms. In hadronic collisions at Large Hadron Collider (LHC) energies, there is a significant contribution of multi-parton interactions. The measurement of heavy-flavour yields as a function of charged-particle multiplicity gives insight into the mechanisms influencing their production in hadronic collisions at these energies and it is a tool to test the possible influence of multi-parton interactions. Furthermore, the charged-particle multiplicity dependence of open heavy flavours is used to test the ability of QCD theoretical models to describe the data. In ALICE, heavy-flavour production is measured via the hadronic and semi-leptonic decay channels (electrons at central rapidity and muons at forward rapidity). Charged-particle multiplicity is measured at central and forward rapidity. We will present the results on open heavy-flavour production as a function of the charged-particle multiplicity in pp and p–Pb collisions. Results will be compared to quarkonia measurements as well as theoretical model calculations.

W\xb1Z production at the LHC: fiducial cross sections and distributions in NNLO QCD

We report on the first fully differential calculation for W±Z production in hadron collisions up to next-to-next-to-leading order (NNLO) in QCD perturbation theory. Leptonic decays of the W and Z bosons are consistently taken into account, i.e. we include all resonant and non-resonant diagrams that contribute to the process pp → ℓ′±νℓ′ ℓ+ℓ− +X both in the same-flavour (ℓ′ = ℓ) and the different-flavour (ℓ′ ≠ ℓ) channel. Fiducial cross sections and distributions are presented in the presence of standard selection cuts applied in the experimental W±Z analyses by ATLAS and CMS at centre-of-mass energies of 8 and 13 TeV. As previously shown for the inclusive cross section, NNLO corrections increase the NNLO result by about 10%, thereby leading to an improved agreement with experimental data. The importance of NNLO accurate predictions is also shown in the case of new-physics scenarios, where, especially in high-pT categories, their impact can reach mathcal{O}left(20%right) . The availability of differential NNLO predictions will play a crucial role in the rich physics programme that is based on precision studies of W±Z signatures at the LHC.

Predictions for the Isolated Prompt Photon Production at the LHC ats= 13 TeV

The prompt photon production in hadronic collisions has a long history of providing information on the substructure of hadrons and testing the perturbative techniques of QCD. Some valuable information about the parton densities in the nucleon and nuclei, especially of the gluon, can also be achieved by analysing the measurements of the prompt photon production cross section whether inclusively or in association with heavy quarks or jets. In this work, we present predictions for the inclusive isolated prompt photon production in pp collisions at center-of-mass energy of 13 TeV using various modern PDF sets. The calculations are presented as a function of both photon transverse energyETγand pseudorapidityηγfor the ATLAS kinematic coverage. We also study in detail the theoretical uncertainty in the cross sections due to the variation of the renormalization, factorization, and fragmentation scales. Moreover, we introduce and calculate the ratios of photon momenta for different rapidity regions and study the impact of various input PDFs on such quantity.

Consistent simulation of nonresonant diphoton production in hadron collisions including associated jet production up to two jets

An event generator for diphoton ($\gamma\gamma$) production in hadron collisions that includes associated jet production up to two jets has been developed using a subtraction method based on the LLL subtraction. The parton shower (PS) simulation to restore the subtracted divergent components involves both QED and QCD radiation, and QED radiation at very small $Q^{2}$ are simulated by referring to a fragmentation function (FF). The PS/FF simulation has the ability to enforce the radiation of a given number of energetic photons. The generated events can be fed to PYTHIA to obtain particle (hadron)-level event information, which enables us to perform realistic simulations of photon isolation and hadron-jet reconstruction. The simulated events, in which the loop-mediated $gg \rightarrow \gamma\gamma$ process is involved, reasonably reproduce the diphoton kinematics measured at the LHC. Using the developed simulation, we found that the 2-jet processes significantly contribute to diphoton production. A large 2-jet contribution can be considered as a common feature in electroweak-boson production in hadron collisions although the reason is yet to be understood. Discussion concerning the treatment of the underlying events in photon isolation is necessary for future higher precision measurements.

Influence of quantum conservation laws on particle production in hadron collisions

Conservation laws strongly influence production of particles in high-energy particle collisions. Effects connected to these mechanisms were studied in details using correlation techniques in e+e− collisions. At the time, models were tuned to correctly reproduce the measurements. Similar studies for hadron-hadron collisions have never been performed, until recent ALICE measurements. ALICE has reported on studies of untriggered two-particle angular correlations of identified particles (π, K, and p) measured in pp collisions at center-of-mass energy of s=7 TeV. Those preliminary results confirm that also in hadron-hadron collisions, at much higher energies, conservation laws strongly influence the shape of the correlation functions for different particle types and must be taken into account while analysing the data. Moreover, they show that the contemporary models (PYTHIA, PHOJET) no longer reproduce the experimental data well.

Transverse-momentum-dependent gluon distributions from JIMWLK evolution

Transverse-momentum-dependent (TMD) gluon distributions have different operator definitions, depending on the process under consideration. We study that aspect of TMD factorization in the small-x limit, for the various unpolarized TMD gluon distributions encountered in the literature. To do this, we consider di-jet production in hadronic collisions, since this process allows to be exhaustive with respect to the possible operator definitions, and is suitable to be investigated at small x. Indeed, for forward and nearly back-to-back jets, one can apply both the TMD factorization and Color Glass Condensate (CGC) approaches to compute the di-jet cross-section, and compare the results. Doing so, we show that both descriptions coincide, and we show how to express the various TMD gluon distributions in terms of CGC correlators of Wilson lines, while keeping Nc finite. We then proceed to evaluate them by solving the JIMWLK equation numerically. We obtain that at large transverse momentum, the process dependence essentially disappears, while at small transverse momentum, non-linear saturation effects impact the various TMD gluon distributions in very different ways. We notice the presence of a geometric scaling regime for all the TMD gluon distributions studied: the "dipole" one, the Weizs\"acker-Williams one, and the six others involved in forward di-jet production.

From soft photon study at Nuclotron and U-70 to NICA

Experimental and theoretical studies of direct photon production in hadronic collisions essentially expand our insights in multiparticle production mechanisms. These photons are useful probes to investigate nuclear matter at all stages of the interaction. Soft photons play a particular role in these studies. Until now we have no explanation for the experimentally observed excess of soft photons. These photons have low transverse momenta \( p_{T} < 0.1\) GeV/c, \( \vert x\vert < 0.01\) . In this domain their yield exceeds the theoretical estimates by 5-8 times. The registration of soft photons at Nuclotron (LHEP, JINR) has been carried out by the electromagnetic calorimeter built by the SVD-2 Collaboration. Soft photon electromagnetic calorimeter was tested at U-70, IHEP (Protvino). For the first time the soft photon yield at interactions of 3.5A GeV/c per nucleon deuterium and lithium beams has been measured. The obtained energy spectra confirm the increased yield of soft photons with their energy less than 50MeV (in the laboratory system) in comparison with theoretical predictions and agree with previous experiments at high-energy interactions. It is planned to continue soft photon study at the future accelerator complex NICA with heavy-ion beams.

Consistent simulation of direct-photon production in hadron collisions including associated two-jet production

We have developed an event generator for direct-photon production in hadron collisions, including associated 2-jet production in the framework of the GR@PPA event generator. The event generator consistently combines [Formula: see text] + 2-jet production processes with the lowest-order [Formula: see text] + jet and photon-radiation (fragmentation) processes from quantum chromodynamics (QCD) 2-jet production using a subtraction method. The generated events can be fed to general-purpose event generators to facilitate the addition of hadronization and decay simulations. Using the obtained event information, we can simulate photon isolation and hadron-jet reconstruction at the particle (hadron) level. The simulation reasonably reproduces measurement data obtained at the large hadron collider (LHC) concerning not only the inclusive photon spectrum, but also the correlation between the photon and jet. The simulation implies that the contribution of the [Formula: see text] + 2-jet is very large, especially in low photon-[Formula: see text] ([Formula: see text] 50 GeV) regions. Discrepancies observed at low [Formula: see text], although marginal, may indicate the necessity for the consideration of further higher-order processes. Unambiguous particle-level definition of the photon-isolation condition for the signal events is desired to be given explicitly in future measurements.

Tetraquark production in double parton scattering

We develop a model to study tetraquark production in hadronic collisions. We focus on double parton scattering and formulate a version of the color evaporation model for the production of the $X(3872)$ and of the $T_{4c}$ tetraquark, a state composed by the $c \bar{c} c \bar{c}$ quarks. We find that the production cross section grows rapidly with the collision energy $\sqrt{s}$ and make predictions for the forthcoming higher energy data of the LHC.

QCD factorization for quarkonium production in hadron collisions at low transverse momentum

Inclusive production of a quarkonium $\eta_{c,b}$ in hadron collisions at low transverse momentum can be used to extract various Transverse-Momentum-Dependent(TMD) gluon distributions of hadrons, provided the TMD factorization for the process holds. The factorization involving unpolarized TMD gluon distributions of unpolarized hadrons has been examined with on-shell gluons at one-loop level. In this work we study the factorization at one-loop level with diagram approach in the most general case, where all TMD gluon distributions at leading twist are involved. We find that the factorization holds and the perturbative effects are represented by one perturbative coefficient. Since the initial gluons from hadrons are off-shell in general, there exists the so-called super-leading region found recently. We find that the contributions from this region can come from individual diagrams at one-loop level, but they are cancelled in the sum. Our factorized result for the differential cross-section is explicitly gauge-invariant.