Multiparton Interactions Research Articles

Latest Minimum Bias and Underlying Event measurements with the ATLAS Detector

The modelling of Minimum Bias and Underlying Event is a crucial component in the description of soft Quantum Chromodynamics processes. They are both described by multi-parton interaction models, the result of proton collisions containing more than one partonic interaction due to collective and beam remnant effects. Recent studies by the ATLAS Experiment at the LHC aiming at measuring Charged-Particle distributions and the properties of the Underlying Event are presented.

QCD Soft and Forward Physics at CMS

We present recent results on minimum bias collisions, underlying event activity and double parton scattering using data recorded by the CMS detector at the LHC. The results on the measurement of the underlying event (UE) using leading tracks, jets, and Drell-Yan processes are presented. Double parton scattering is investigated in several final states including vector bosons, and the results are compared to other experiments and to multi parton interaction (MPI) models tuned to recent underlying event measurements at CMS.

Azimuthal correlations for inclusive 2-jet, 3-jet, and 4-jet events in pp collisions at sqrt{s}= 13~hbox {TeV}

Azimuthal correlations between the two jets with the largest transverse momenta p_{mathrm {T}} in inclusive 2-, 3-, and 4-jet events are presented for several regions of the leading jet p_{mathrm {T}} up to 4,text {Te}text {V}. For 3- and 4-jet scenarios, measurements of the minimum azimuthal angles between any two of the three or four leading p_{mathrm {T}} jets are also presented. The analysis is based on data from proton–proton collisions collected by the CMS Collaboration at a centre-of-mass energy of 13,text {Te}text {V}, corresponding to an integrated luminosity of 35.9,text {fb}^{-1}. Calculations based on leading-order matrix elements supplemented with parton showering and hadronization do not fully describe the data, so next-to-leading-order calculations matched with parton shower and hadronization models are needed to better describe the measured distributions. Furthermore, we show that azimuthal jet correlations are sensitive to details of the parton showering, hadronization, and multiparton interactions. A next-to-leading-order calculation matched with parton showers in the MC@NLO method, as implemented in herwig 7, gives a better overall description of the measurements than the powheg method.

Investigation of the energy dependence of the p_{text {T0}} parameter in the Pythia\xa08 Monte Carlo event generator

By using predictions from the Pythia 8 Monte Carlo event generator, we determine the energy-dependent turn-over values (p_{text {T0}}) of the partonic cross section for the simulation of multiparton interactions. Since the observed energy dependence of the p_{text {T0}} values is not well described by a power-law function, we introduce an additional energy-dependent term, to better describe the experimental observations in the energy range sqrt{s} = 0.3-13 , hbox {TeV}. We obtain a similar level of agreement for predictions using various parton densities.

Role of multiparton interactions on J/ψ production in p+p collisions at LHC energies

The production mechanism of quarkonia states in hadronic collisions is still to be understood by the scientific community. In high-multiplicity $p+p$ collisions, Underlying Event (UE) observables are of major interest. The Multi-Parton Interactions (MPI) is a UE observable, where several interactions occur at the partonic level in a single $p+p$ event. This leads to dependence of particle production on event multiplicity. If the MPI occurs in a harder scale, there will be a correlation between the yield of quarkonia and total charged particle multiplicity. The ALICE experiment at the Large Hadron Collider (LHC) in $p+p$ collisions at $\sqrt{s}$ = 7 and 13 TeV has observed an approximate linear increase of relative $J/\psi$ yield ($\frac{dN_{J/\psi}/dy}{<dN_{J/\psi}/dy>}$) with relative charged particle multiplicity density ($\frac{dN_{ch}/dy}{<dN_{ch}/dy>}$). In our present work we have performed a comprehensive study of the production of charmonia as a function of charged particle multiplicity in $p+p$ collisions at LHC energies using pQCD-inspired multiparton interaction model, PYTHIA8 tune 4C, with and without Color Reconnection (CR) scheme. A detail multiplicity and energy dependent study is performed to understand the effects of MPI on $J/\psi$ production. The ratio of $\psi(2S)$ to $J/\psi$ is also studied as a function of charged particle multiplicity at LHC energies.

Resonance suppression from color reconnection

We present studies that show how multi-parton interaction and color reconnection affect the hadro-chemistry in proton-proton (pp) collisions with special focus on the production of resonances using the PYTHIA8 event generator. We find that color reconnection suppresses the relative production of meson resonances such as $\rho_{0}$ and K*, providing an alternative explanation for the K*/K decrease observed in proton-proton collisions as a function of multiplicity by the ALICE collaboration. Detailed studies of the underlying mechanism causing meson resonance suppression indicate that color reconnection leads to shorter, less energetic strings whose fragmentation is less likely to produce more massive hadrons for a given quark content, therefore reducing ratios such as K*/K and $\rho_0/\pi$ in high-multiplicity pp collisions. In addition, we have also studied the effects of allowing string junctions to form and found that these may also contribute to resonance suppression.

Collectivity from interference

In hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant αs, it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics, e.g., in the analysis of azimuthal anisostropy coefficients vn at the LHC. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of m particles in hadronic collisions with N parton-parton interactions (“sources”). In an expansion in powers of 1/(Nc2 − 1) and to leading order in the number of sources N , we calculate interference effects in the m-particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies vn{2} and vn{4}. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for vn{2} and vn{4}, n even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured vn coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

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.

Multiparton interactions: From pp to pA

In the process of understanding nuclear collisions, reliable extrapolations from pp collisions, based on Glauber models, are highly desirable, though seldomly accurate. We review the inclusion of diffractive excitations and argue that they provide an important contribution to centrality observables in pA collisions. We present a method for distinguishing between diffractively and non-diffractively wounded nucleons, and a proof-of-principle for an extrapolation of multiparton interaction models built on this.

Measurement of the inclusive energy spectrum in the very forward direction in proton-proton collisions at sqrt{s}=13 TeV

The differential cross section for inclusive particle production as a function of energy in proton-proton collisions at a center-of-mass energy of 13 TeV is measured in the very forward region of the CMS detector. The measurement is based on data collected with the CMS apparatus at the LHC, and corresponds to an integrated luminosity of 0.34 μb−1. The energy is measured in the CASTOR calorimeter, which covers the pseudorapidity region −6.6 < η < −5.2. The results are given as a function of the total energy deposited in CASTOR, as well as of its electromagnetic and hadronic components. The spectra are sensitive to the modeling of multiparton interactions in pp collisions, and provide new constraints for hadronic interaction models used in collider and in high energy cosmic ray physics.

Interplay of soft and perturbative correlations in multiparton interactions at central rapidities

We study the role of soft/nonperturbative correlations in the multi parton interactions in the central kinematics relevant for double parton scattering (DPS) and underlying event (UE) measurements at ATLAS and CMS. We show that the effect of soft correlations is negligible for DPS regime (typical transverse momenta larger than 10–20 GeV), but may be important for UE (several GeV scale). The characteristic scale where soft correlations become important increases with decrease of x (energy increase) leading to approximately constant σeff at small x.

Multi-parton interactions and rapidity gap survival probability in jet–gap–jet processes

We discuss an application of dynamical multi-parton interaction model, tuned to measurements of underlying event topology, for a description of destroying rapidity gaps in the jet–gap–jet processes at the LHC. We concentrate on the dynamical origin of the mechanism of destroying the rapidity gap. The cross section for jet–gap–jet is calculated within LL BFKL approximation. We discuss the topology of final states without and with the MPI effects. We discuss some examples of selected kinematical situations (fixed jet rapidities and transverse momenta) as distributions averaged over the dynamics of the jet–gap–jet scattering. The colour-singlet ladder exchange amplitude for the partonic subprocess is implemented into the Pythia 8 generator, which is then used for hadronisation and for the simulation of the MPI effects. Several differential distributions are shown and discussed. We present the ratio of cross section calculated with and without MPI effects as a function of rapidity gap in between the jets.

Single inclusive jet transverse momentum and energy spectra at very forward rapidity in proton–proton collisions with [formula omitted

We present predictions of single inclusive jet transverse momentum, energy, and Feynman-x spectra at forward rapidity (5.2<y<6.6) in proton–proton collisions with s=7 and 13 TeV. Calculations based on high-energy factorization and kT-dependent parton densities are compared to simulations using the Pythia event generator. Effects from parton density evolution, parton shower dynamics, multi-parton interactions, and saturation of parton densities are investigated.

Exploring minijets beyond the leading power

The crucial parameter of the current Monte Carlo models of high energy hadron-hadron interaction is the transverse momentum cutoff $p_{T0}$ for parton-parton interactions which slowly grows with energy and regularizes the cross section. This modification of the collinear factorization formula goes beyond the leading power and thus a natural question arises if such cutoff can be extracted from a formalism which takes into account power corrections. In this work, we consider the High Energy Factorization (HEF) valid at small $x$ and a new model, based on a similar principle to HEF, which in addition has a limit respecting the Dokshitzer-Dyakonov-Troyan formula for the dijet momentum disbalance spectrum. Minijet cross section and its suppression is then analyzed in two ways. First, we study minijets directly in the low-$p_T$ region, and demonstrate that higher twist corrections do generate suppression of the inclusive jet production cross section though these effects are not leading to the increase of the cutoff with incident energy. Second, we consider hard inclusive dijet production where Multi Parton Interactions (MPIs) with minijets produce power corrections. We introduce an observable constructed from differential cross section in the ratio $\tau$ of dijet disbalance to the average dijet $p_T$ and demonstrate that the $\tau>1$ region is sensitive to the cutoff $p_{T0}$ in the MPI minijet models. The energy dependence of the cutoff is reflected in the energy dependence of the bimodality coefficient $b$ of the $\tau$ distribution. We compare $b$ calculated from $\mathsf{pythia}$, where one can conveniently control MPIs by the program parameters, and HEF for a few unintegrated gluon distributions (UGDs). We find that the energy dependence of $b$ is very sensitive to the particular choice of UGD and in some models it resembles predictions of the Monte Carlo models.

Multiple parton interactions and production of charged particles up to the intermediate- pT range in high-multiplicity pp events at the LHC

Satisfactory description of data by hydrodynamics-motivated models, as has been reported recently by experimental collaborations at the LHC, confirm ``collectivity'' in high-multiplicity proton-proton $(pp)$ collisions. Notwithstanding this, a detailed study of high-multiplicity $pp$ data in other approaches or models is essential for better understanding of the specific phenomenon. In this study, the focus is on a pQCD-inspired multiparton interaction (MPI) model, including a color reconnection (CR) scheme as implemented in the Monte Carlo code, PYTHIA8 tune 4C. The MPI with the color reconnection reproduces the dependence of the mean transverse momentum $⟨{p}_{T}⟩$ on the charged particle multiplicity ${N}_{\mathrm{ch}}$ in $pp$ collisions at the LHC, providing an alternate explanation to the signature of ``hydrodynamic collectivity'' in $pp$ data. It is, therefore, worth exploring how this model responds to other related features of high-multiplicity $pp$ events. This comparative study with recent experimental results demonstrates the limitations of the model in explaining some of the prominent features of the final-state charged particles up to the intermediate-${p}_{T}$ (${p}_{T}&lt;2.0\text{ }\text{ }\mathrm{GeV}/c$) range in high-multiplicity $pp$ events.

Studies of inclusive four-jet production with two b -tagged jets in proton-proton collisions at 7 TeV

Measurements are presented of the cross section for the production of at least four jets, of which at least two originate from b quarks, in proton-proton collisions. Data collected with the CMS detector at the LHC at a center-of-mass energy of 7 TeV are used, corresponding to an integrated luminosity of 3 inverse picobarns. The cross section is measured as a function of the jet transverse momentum for pt > 20 GeV, and of the jet pseudorapidity for abs(eta) < 2.4 (b jets), 4.7 (untagged jets). The correlations in azimuthal angle and pt between the jets are also studied. The inclusive cross section is measured to be sigma(pp to 2 b + 2 j + X) = 69 +/- 3 (stat) +/- 24 (syst) nb. The eta and pt distributions of the four jets and the correlations between them are well reproduced by event generators that combine perturbative QCD calculations at next-to-leading-order accuracy with contributions from parton showers and multiparton interactions.

Effects produced by multi-parton interactions and color reconnection in small systems

Multi-parton interactions and color reconnection can produce QGP-like effects in small systems, specifically, radial flow-like patterns. For pp collisions simulated with Pythia 8.212, in this work we investigate their effects on different observables like event multiplicity, event shapes and transverse momentum distributions.

Diffractive and non-diffractive wounded nucleons and final states in pA collisions

We review the state-of-the-art of Glauber-inspired models for estimating the distribution of the number of participating nucleons in pA and AA collisions. We argue that there is room for improvement in these models when it comes to the treatment of diffractive excitation processes, and present a new simple Glauber-like model where these processes are better taken into account. We also suggest a new way of using the number of participating, or wounded, nucleons to extrapolate event characteristics from pp collisions, and hence get an estimate of basic hadronic final-state properties in pA collisions, which may be used to extract possible nuclear effects. The new method is inspired by the Fritiof model, but based on the full, semi-hard multiparton interaction model of Pythia 8.

Measurement of dijet azimuthal decorrelation in ppcollisions at [Formula: see text

A measurement of the decorrelation of azimuthal angles between the two jets with the largest transverse momenta is presented for seven regions of leading jet transverse momentum up to 2.2,mathrm{TeV}. The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8,mathrm{TeV} corresponding to an integrated luminosity of 19.7,text {fb}^{-1}. The dijet azimuthal decorrelation is caused by the radiation of additional jets and probes the dynamics of multijet production. The results are compared to fixed-order predictions of perturbative quantum chromodynamics (QCD), and to simulations using Monte Carlo event generators that include parton showers, hadronization, and multiparton interactions. Event generators with only two outgoing high transverse momentum partons fail to describe the measurement, even when supplemented with next-to-leading-order QCD corrections and parton showers. Much better agreement is achieved when at least three outgoing partons are complemented through either next-to-leading-order predictions or parton showers. This observation emphasizes the need to improve predictions for multijet production.

Measurement of the double-differential inclusive jet cross section in proton-proton collisions at [Formula: see text

A measurement of the double-differential inclusive jet cross section as a function of jet transverse momentum p_{mathrm {T}} and absolute jet rapidity |y | is presented. The analysis is based on proton–proton collisions collected by the CMS experiment at the LHC at a centre-of-mass energy of 13,text {TeV}. The data samples correspond to integrated luminosities of 71 and 44,text {pb}^text {-1} for |y |<3 and 3.2<|y |<4.7, respectively. Jets are reconstructed with the anti-k_{mathrm {t}} clustering algorithm for two jet sizes, R, of 0.7 and 0.4, in a phase space region covering jet p_{mathrm {T}} up to 2,text {TeV} and jet rapidity up to |y | = 4.7. Predictions of perturbative quantum chromodynamics at next-to-leading order precision, complemented with electroweak and nonperturbative corrections, are used to compute the absolute scale and the shape of the inclusive jet cross section. The cross section difference in R, when going to a smaller jet size of 0.4, is best described by Monte Carlo event generators with next-to-leading order predictions matched to parton showering, hadronisation, and multiparton interactions. In the phase space accessible with the new data, this measurement provides a first indication that jet physics is as well understood at sqrt{s}=13,text {TeV} as at smaller centre-of-mass energies.