Pair Of Partons Research Articles

One-loop five-parton amplitudes in the NMRK limit

We analyse the real part of one-loop five-parton amplitudes in the next-to-multi-Regge kinematic (NMRK) limit, to leading power, and to finite order in the dimensional regularisation parameter. To leading logarithmic (LL) accuracy, it is known that five-parton amplitudes in this limit are given to all-orders by a single factorised expression, in which the pair of partons which are not well-separated in rapidity are described by a two-parton emission vertex. In this study, we investigate the one-loop amplitudes at next-to-leading logarithmic (NLL) accuracy, and find that is has a more complex structure. In particular, it is found that the purely gluonic amplitudes are compatible with an analogous factorisation of individual colour structures. From the one-loop amplitudes we extract one-loop two-parton emission vertices, which are functions of a subset of the momenta of the amplitude. In the multi-Regge kinematic (MRK) limit, the vertices themselves factorise into the known one-loop single-parton emission vertices and Lipatov vertex, with rapidity dependence governed by the one-loop gluon Regge trajectory, as required by compatibility with the known MRK limit of amplitudes. The one-loop two-parton emission vertices are necessary ingredients for the construction of the next-to-next-to leading order (NNLO) jet impact factors in the BFKL framework.

Coherent gluon radiation: beyond leading-log accuracy

Results are presented for the medium-induced, soft coherent radiation spectrum for all 2 → 2 partonic channels in QCD, at leading-order in αs but beyond leading logarithmic accuracy. The general formula is valid in the full kinematic range of the underlying process, and reduces to previous results in special cases. The soft gluon radiation spectrum is expressed in terms of the color density matrix specific to each channel, quantifying the entanglement between the color components of the 2 → 2 production amplitude. Beyond the leading logarithm, the spectrum depends explicitly on the off-diagonal elements of this matrix, owing to the soft gluon’s ability to probe the internal color structure of the parton pair.

Evolution of colour correlated double parton distributions: a quantitative study

Double parton distributions satisfy the same evolution equations as ordinary single-parton densities, provided that the colours of the two partons are uncorrelated. The situation is different for colour correlated parton pairs, where evolution to higher scales results in a suppression by Sudakov double logarithms. We perform a detailed study of evolution for colour correlated double parton distributions, both analytically and numerically, at lowest order and beyond. When the two observed partons originate from the perturbative splitting of a single one, the Sudakov suppression of colour correlations at the cross section level is not as strong as one might expect.

Valence and sea parton correlations in double parton scattering from data

The effective cross section of double parton scattering in proton collisions has been measured by many experiments with rather different results. Motivated by this fact, we assumed that the parton correlations in the transverse plane are different whether we have valence or sea partons. With this simple approach, we were able to fit the available data and found that sea parton pairs are more correlated in the transverse plane than valence-sea parton pairs.

Optimizing the Parton Showers in PYTHIA8 for Xe–Xe collision at 5.44 TeV

We present the doubly differential invariant yield of primary charged particles [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text] in Xe–Xe collisions at [Formula: see text] = 5.44 TeV, as a function of [Formula: see text] in kinematic region of [Formula: see text] and for the pseudorapidity interval of [Formula: see text] by using PYTHIA8 tuned to different Parton Showers like Simple Showers (Time-Like, Space-Like), DIRE and VINCIA Showers. The predictions of hard and soft QCD processes of Parton Showers are then compared with the ALICE data at different centrality classes of 0–5%, 50–60% and 70–80%. The observed deviations are connected with the kinematics involved and are mostly due to the color coherence and soft gluon emission from parton pairs. As PYTHIA8 does not take into account the Cronin effect, thus the deviation at low [Formula: see text] is due to Cronin effect. The nuclear modification factor is constructed and it is observed that the signal is less than unity or suppression is seen which is coming from Parton Showers.

Search for high-mass resonances decaying to a jet and a Lorentz-boosted resonance in proton-proton collisions at [formula omitted

A search is reported for high-mass hadronic resonances that decay to a parton and a Lorentz-boosted resonance, which in turn decays into a pair of partons. The search is based on data collected with the CMS detector at the LHC in proton-proton collisions at s=13TeV, corresponding to an integrated luminosity of 138fb−1. The boosted resonance is reconstructed as a single wide jet with substructure consistent with a two-body decay. The high-mass resonance is thus considered as a dijet system. The jet substructure information and the kinematic properties of cascade resonance decays are exploited to disentangle the signal from the large quantum chromodynamics multijet background. The dijet mass spectrum is analyzed for the presence of new high-mass resonances, and is found to be consistent with the standard model background predictions. Results are interpreted in a warped extra dimension model where the high-mass resonance is a Kaluza–Klein gluon, the boosted resonance is a radion, and the final state partons are all gluons. Limits on the production cross section are set as a function of the Kaluza–Klein gluon and radion masses. These limits exclude at 95% confidence level models with Kaluza–Klein gluon masses in the range 2.0 to 4.3 TeV and radion masses in the range 0.20 to 0.74 TeV. By exploring a novel experimental signature, the observed limits on the Kaluza–Klein gluon mass are extended by up to about 1 TeV compared to previous searches.

Two-loop splitting in double parton distributions: the colour non-singlet case

At small inter-parton distances, double parton distributions receive their dominant contribution from the splitting of a single parton. We compute this mechanism at next-to-leading order in perturbation theory for all colour configurations of the observed parton pair. Rapidity divergences are handled either by using spacelike Wilson lines or by applying the δ regulator. We investigate the behaviour of the two-loop contributions in different kinematic limits, and we illustrate their impact in different channels.

PYTHIA and the preoccupied proton

The PYTHIA Monte Carlo (PMC) has been applied broadly to simulations of high-energy p-p and p- $ \bar p $ collisions. The PMC is based on several assumptions, such as that most hadrons result from jet production (multiple par-ton interactions or MPIs), that p-p centrality is relevant and that color reconnection (CR) strongly influences fragmentation to jets. An alternative description is provided by the two-component (soft + hard) model (TCM) of hadron production. TCM analysis of p-Pb ensemble-mean-pt data reveals centrality trends quite different from those estimated via a geometric Glauber model based on the eikonal approximation. Glauber estimates of binary-collision number are three times TCM estimates. Detailed study of p-Pb data conflicts with a basic Glauber assumption – that a projectile proton may interact simultaneously with multiple target nucleons. Instead, in both p-p and p-A collisions, a p-N collision once initiated is exclusive of other possible interactions (during that collision), and within the collision any pair of participant partons may interact –a p-N collision is thus “all or nothing.” In this presentation the PMC is challenged by an assortment of contradictory data, and evidence for p-N exclusivity is reviewed to make a case for the “preoccupied proton” of the title.

Nuclear p\u22a5-broadening of an energetic parton pair

We revisit the transverse momentum broadening of a fast parton pair crossing a nuclear medium, putting emphasis on the pair global color state, for any number of colors N and within the eikonal limit for parton propagation and the Gaussian approximation for the gluon field of the target. The pair transverse momentum probability distribution is derived in a kinetic equation approach, and is determined by an operator ℬ describing the possible transitions between the pair color states when crossing the medium. The exponential of ℬ encompasses the 4-point correlators of Wilson lines in the saturation formalism. We emphasize the relation of ℬ with the anomalous dimension matrices appearing in the study of soft gluon radiation associated to hard 2 → 2 partonic processes. In a well-chosen, orthonormal basis of the pair color states, we rederive ℬ for any type of parton pair, making maximal use of SU(N) invariants and using ‘birdtrack’ color pictorial notations, providing a quite economical derivation of all previously known 4-point correlators (or equivalently, anomalous dimension matrices for 2 → 2 parton scattering). We discuss some general features of the pair transverse momentum distribution. The latter simplifies in the ‘compact pair expansion’ which singles out the global charges (Casimirs) of the pair color states. This study should provide the necessary tools to address nuclear broadening of n-parton systems in phenomenology while highlighting the color structure of the process.

Hadronic structure from double parton scattering

In the present paper we consider the so-called effective cross section, a quantity which encodes the experimental knowledge on double parton scattering in hadronic collisions that has been accomulated so far. We show that the effective cross section, under some assumptions close to those adopted in its experimental extractions, can be used to obtain a range of mean transverse distance between an interacting parton pair in double parton scattering. Therefore we have proved that the effective cross section offers a way to access information on the hadronic structure.

The effect of momentum deposition during fireball evolution on flow anisotropy

The highest LHC energies give rise to the production of many pairs of hard partons which deposit four-momentum into the expanding fireball matter. We argue that it is necessary to include momentum deposition during fireball evolution into 3 + 1 dimensional hydrodynamic simulations of the collision. This influence cannot be accounted for simply by modifying the initial conditions of the simulation. The resulting contribution to flow anisotropy is correlated with the fireball geometry and causes an increase of the elliptic flow in non-central collisions. The results are presented for various scenarios with energy and momentum deposition to clearly demonstrate this effect.

Correlations in double parton distributions: perturbative and non-perturbative effects

The correct description of Double Parton Scattering (DPS), which represents a background in several channels for the search of new Physics at the LHC, requires the knowledge of double parton distribution functions (dPDFs). These quantities represent also a novel tool for the study of the three-dimensional nucleon structure, complementary to the possibilities offered by electromagnetic probes. In this paper we analyze dPDFs using Poincar\'e covariant predictions obtained by using a Light-Front constituent quark model proposed in a recent paper, and QCD evolution. We study to what extent factorized expressions for dPDFs, which neglect, at least in part, two-parton correlations, can be used. We show that they fail in reproducing the calculated dPDFs, in particular in the valence region. Actually measurable processes at existing facilities occur at low longitudinal momenta of the interacting partons; to have contact with these processes we have analyzed correlations between pairs of partons of different kind, finding that, in some cases, they are strongly suppressed at low longitudinal momenta, while for other distributions they can be sizeable. For example, the effect of gluon-gluon correlations can be as large as 20 $\%$. We have shown that these behaviors can be understood in terms of a delicate interference of non-perturbative correlations, generated by the dynamics of the model, and perturbative ones, generated by the model independent evolution procedure. Our analysis shows that at LHC kinematics two-parton correlations can be relevant in DPS, and therefore we address the possibility to study them experimentally.

Double Parton Scattering at the LHC

In this short presentation, we will briefly discuss the status of Double Parton Scattering (DPS) at the Large Hadron Collider (LHC). Often DPS, which can be described as perturbative process, can occur when two pairs of partons interact in a same and single p-p (proton-proton) collision. Until the advent of LHC, this kind of process was not very well understood, because collisions at very high energies are necessary for a more visible double scattering contribution. We start our presentation speaking about what is the DPS phenomena. Next, we discuss the cross section calculation for the DPS process with its details. Naively, the DPS process can be calculated by the multiplication of two independent parton cross sections and dividing the result by an effective cross section. We review this possibility. And finally, we explain the concept of effective cross section and show some experimental results measured by different experiments.

Improving identification of dijet resonances at hadron colliders.

The experimental detection of resonances has played a vital role in the development of subatomic physics. The overwhelming multijet backgrounds at the Large Hadron Collider (LHC) necessitate the invention of new techniques to identify resonances decaying into a pair of partons. In this Letter we introduce an observable that achieves a significant improvement in several key measurements at the LHC: the Higgs boson decay to a pair of b quarks; W±/Z0 vector-boson hadronic decay; and extensions of the standard model (SM) with a new hadronic resonance. Measuring the Higgs decay to b quarks is a central test of the fermion mass generation mechanism in the SM, whereas the W±/Z0 production rates are important observables of the electroweak sector. Our technique is effective in large parts of phase space where the resonance is mildly boosted and is particularly well suited for experimental searches dominated by systematic uncertainties, which is true of many analyses in the high-luminosity running of the LHC.

Identification of Parton Pairs in a Dijet Event and Investigation of Its Effects on Dijet Resonance Search

Being able to distinguish parton pair type in a dijet event could significantly improve the search for new particles that are predicted by the theories beyond the Standard Model at the Large Hadron Collider. To explore whether parton pair types manifesting themselves as a dijet event could be distinguished on an event-by-event basis, I performed a simulation based study considering observable jet variables. I found that using a multivariate approach can filter out about 80% of the other parton pairs while keeping more than half of the quark-quark or gluon-gluon parton pairs in an inclusive QCD dijet distribution. The effects of event-by-event parton pair tagging for dijet resonance searches were also investigated and I found that improvement on signal significance after applying parton pair tagging can reach up to 4 times for gluon-gluon resonances.

Stimulation of static deconfined medium by multiple hard partons

We investigate the response of non-expanding deconfined hot matter to energy and momentum deposition from a pair of partons moving with high energies. Several situations are examined with partons moving so that the generated wakes in the medium interact. It is shown that the streams generated in the wakes, which carry deposited momentum, interact and merge if the hard partons are close enough to each other. Such cases are relevant for nuclear collisions at the Large Hadron Collider, where several hard partons are produced in a single collision. The observed effect itself could lead to elliptic flow at the level of a few per cent.

Search for narrow resonances using the dijet mass spectrum inppcollisions ats=8 TeV

Results are presented of a search for the production of new particles decaying to pairs of partons (quarks, antiquarks, or gluons), in the dijet mass spectrum in proton-proton collisions at sqrt(s) = 8 TeV. The data sample corresponds to an integrated luminosity of 4.0 inverse femtobarns, collected with the CMS detector at the LHC in 2012. No significant evidence for narrow resonance production is observed. Upper limits are set at the 95% confidence level on the production cross section of hypothetical new particles decaying to quark-quark, quark-gluon, or gluon-gluon final states. These limits are then translated into lower limits on the masses of new resonances in specific scenarios of physics beyond the standard model. The limits reach up to 4.8 TeV, depending on the model, and extend previous exclusions from similar searches performed at lower collision energies. For the first time mass limits are set for the Randall-Sundrum graviton model in the dijet channel.

Anomalous dimensions of the double parton fragmentation functions

Double parton fragmentation is a process in which a pair of partons produced in the short-distance process hadronize into the final state hadron. This process is important for quarkonium production when the transverse momentum is much greater than the quark mass. Resummation of logarithms of the ratio of these two scales requires the evolution equations for double parton fragmentation functions (DPFF). In this paper we complete the one-loop evaluation of the anomalous dimensions for the DPFF. We also consider possible mixing between the DPFF and single parton power suppressed gluon fragmentation and show that such effects are subleading.

Systematics of quarkonium production at the LHC and double parton fragmentation

In this paper we discuss the systematics of quarkonium production at the LHC. In particular, we focus on the necessity to sum logs of the form log(Q/p_perp) and log(p_perp/m_Q). We show that the former contributions are power suppressed, while the latter, whose contribution in fragmentation is well known, also arise in the short distance (i.e., non-fragmentation) production mechanisms. Though these contributions are suppressed by powers of m_Q/p_perp, they can be enhanced by inverse powers of v, the relative velocity between heavy quarks in the quarkonium. In the limit p_perp >> m_Q short distance production can be thought of as the fragmentation of a pair of partons (i.e., the heavy quark and anti-quark) into the final state quarkonium. We derive an all order factorization theorem for this process in terms of double parton fragmentation functions (DPFF) and calculate the one-loop anomalous dimension matrix for the DPFF.

Soft function for exclusiveN-jet production at hadron colliders

The N-jettiness event shape divides phase space into N+2 regions, each containing one jet or beam. Using a geometric measure these regions correspond to jets with circular boundaries. We give a factorization theorem for the cross section fully differential in the (transverse) mass of each jet, and compute the corresponding soft function at next-to-leading order (NLO). The ultraviolet divergences are analytically extracted by exploiting hemispheres for interactions between each pair of hard partons, leaving only convergent integrals that are sensitive to the precise boundaries. This method can be applied in general to N-jet soft functions, including other observables. For N-jettiness, the final result for the soft function involves stable one-dimensional numerical integrals, and all ingredients are now available to extend NLO cross sections to resummed predictions at next-to-next-to-leading logarithmic order.