A. D. Martin, W. J. Stirling, R. S. Thorne, G. Watt
备注：
157 pages, 70 figures. Code can be found at http://projects.hepforge.org/mstwpdf/ and in LHAPDF V5.7.0. v3: final version published in EPJC with extended Section 12
We present updated leading-order, next-to-leading order and next-to-next-to-leading order parton distribution functions ("MSTW 2008") determined from global analysis of hard-scattering data within the standard framework of leading-twist fixed-order collinear factorisation in the MSbar scheme. These parton distributions supersede the previously available "MRST" sets and should be used for the first LHC data-taking and for the associated theoretical calculations. New data sets fitted include CCFR/NuTeV dimuon cross sections, which constrain the strange quark and antiquark distributions, and Tevatron Run II data on inclusive jet production, the lepton charge asymmetry from W decays and the Z rapidity distribution. Uncertainties are propagated from the experimental errors on the fitted data points using a new dynamic procedure for each eigenvector of the covariance matrix. We discuss the major changes compared to previous MRST fits, briefly compare to parton distributions obtained by other fitting groups, and give predictions for the W and Z total cross sections at the Tevatron and LHC.
The theory of the perturbative pomeron, due to Lipatov and collaborators, is used to compute the probability of observing parton-parton elastic scattering and rapidity gaps between jets in hadron collisions at SSC and LHC energies.
Double parton scattering, i.e. two parton hard scattering processes in the same hadron-hadron collision, may constitute an important background for Higgs and other new particle searches at the LHC. We point out that like-sign W pair production provides a relatively clean way of searching for, and calibrating, double parton scattering at the LHC.
In this contribution we briefly report on the progress and open problems in parton distribution functions (PDFs), with emphasis on their implications for LHC phenomenology. Then we study the impact of the recent ATLAS and CMS W lepton asymmetry data on the NNPDF2.1 parton distributions. We show that these data provide the first constrains on PDFs from LHC measurements.
The phenomenology of gluon saturation at small parton momentum fraction, Bjorken-x, in the proton and in the nucleus is introduced. The experimentally-accessible kinematic domains at the nucleus-nucleus colliders RHIC and LHC are discussed. Finally, the saturation hints emerging from measurements at RHIC and the perspectives for LHC are described.
A sizable rate of events where two pairs of b-quarks are produced contemporarily is foreseen at the CERN LHC, as a consequence of the large parton luminosity. At very high energies both single and the double parton scatterings contribute to the process, the latter mechanisms, although power suppressed, giving the dominant contribution to the integrated cross section.
We analyse the multiplicity distributions of charged particles at Tevatron (p\bar p) and LHC (pp) energies in the framework of the independent pair parton interactions (IPPI) model. It is shown that the number of soft pair parton interactions (and therefore the density of the partonic medium) is large and increases with energy. The mean multiplicity at each parton interaction grows also with energy. This growth depends on the width of the rapidity window. The similar conclusions are obtained in the multiladder exchange model (QGSM).
I review some issues related to short distance QCD and its relation to the experimental program of the Large Hadron Collider (LHC) now under construction in Geneva.
We discuss charm production at LHC. The production of single $c \bar c$ pairs is calculated in the $k_t$-factorization approach. We use several unintegrated gluon distributions from the literature. Differential distributions for several charmed mesons are presented and compared to recent results of the ALICE and LHCb collaborations. Some missing strength can be observed. Furthermore we discuss production of two $c \bar c$ pairs within a simple formalism of double-parton scattering (DPS). Surprisingly large cross sections, comparable to single-parton scattering (SPS) contribution to $c \bar c$ …
The production of $c\bar{c}c\bar{c}$, $b\bar{b}b\bar{b}$ and $c\bar{c}b\bar{b}$ pairs considering double parton scattering at LHC energies is investigated. We estimate the contribution of saturation effects to the different final states and predict the energy dependence of the cross sections. Moreover, we estimate the ratio between the double and single parton scattering cross sections for the full rapidity range of the LHC and for the rapidity range of the LHCb experiment. For the full rapidity range we confirm a previous prediction, namely that for charm production the double parton scatteri…
We examine the contribution of Multiple Parton Interactions to Z+n-jets production at the LHC, n=2,3,4, where the Z boson is assumed to decay leptonically. We compare the results obtained with the correlated GS09 double parton distribution function with those obtained with two instances of fully factorized single parton distribution functions: MSTW2008LO and CTEQ6LO. It appears quite feasible to measure the MPI contribution to Z+2/3/4 jets already in the first phase of the LHC with a total luminosity of one inverse femtobarn at 7 TeV. If as expected the trigger threshold for single photons is …
Associated production of charged gauge bosons and charged charmed mesons at the LHC is considered in the framework of $k_T$-factorization approach. Theoretical predictions are compared with ATLAS data, and reasonably good agreement is found. Predictions on the same-sign $W^{\pm}D^{\pm}$ configurations are presented including single parton scattering and double parton scattering contributions. The latter are shown to dominate over the former, thus giving evidence that the proposed process can serve as another indicator of double parton interactions.
The experimental capability of recognizing the presence of b quarks in complex hadronic final states has addressed the attention towards final states with b\bar{b} pairs for observing the production of the Higgs boson at the LHC, in the intermediate Higgs mass range.We point out that double parton scattering processes are going to represent a sizeable background to the process.
I report on precision comparisons of theoretical predictions for Drell-Yan (DY) transverse momentum spectra from two approaches: the analytical resummation approach based on Collins-Soper-Sterman (CSS) formalism and the parton branching (PB) approach to the evolution of transverse momentum dependent (TMD) parton distributions.
We consider the impact that can be made on our understanding of parton distributions (PDFs) and QCD from early measurements at the LHCb experiment. The high rapidity values make the experiment uniquely suited to a detailed study of small-x parton distributions and hence will make a significant contribution towards the clarification of both experimental and theoretical uncertainties on PDFs and their applications.
I review the current status of the nucleon PDFs determined from global fits with emphasis on the impact of recent experimental data and the remaining theoretical challenges.
We review the recent progress in the theoretical description and experimental observation of multiple parton interactions. Subjects covered include experimental measurements of minimum bias interactions and of the underlying event, models of soft physics implemented in Monte Carlo generators, developments in the theoretical description of multiple parton interactions and phenomenological studies of double parton scattering. This article stems from contributions presented at the Helmholtz Alliance workshop on "Multi-Parton Interactions at the LHC", DESY Hamburg, 13-15 September 2010.
We study inclusive dijet azimuthal decorrelations in proton-proton collisions at the CERN LHC invoking the hypothesis of parton Reggeization in t-channel exchanges at high energies. In the parton Reggeization approach, the main contribution to the azimuthal angle difference between the two most energetic jets is due to the Reggeon-Reggeon-Particle-Particle scattering, when the fusion of two Reggeized gluons into a pair of Yang-Mills gluons dominates. Using a high-energy factorization scheme with the Kimber-Martin-Ryskin unintegrated parton distribution functions and the Fadin-Lipatov effective…
The predictions of Drell-Yan production of low-mass, lepton-pairs, at high rapidity at the LHC, are known to depend sensitively on the choice of factorization and renormalization scales. We show how this sensitivity can be greatly reduced by fixing the factorization scale of the LO contribution based on the known NLO matrix element, so that observations of this process at the LHC can make direct measurements of parton distribution functions in the low x domain; x less than about 10^{-4}.
W and Z bosons will be produced copiously at the LHC proton-proton collider. We study the parton distribution dependence of the total production cross sections and rapidity distributions, paying particular attention to the uncertainties arising from uncertainties in the parton distributions themselves. Variations in the gluon, the strong coupling, the sea quarks and the overall normalisation are shown to lead to small but non-negligible variations in the cross section predictions. Ultimately, therefore, the measurement of these cross sections will provide a powerful cross check on our knowledg…
We discuss the parton branching (PB) evolution equation for transverse momentum dependent (TMD) parton distribution functions (PDFs), especially we concentrate on the angular ordering constrain and its effect on the TMD distributions. We discuss application of the PB TMDs to precise prediction of Drell-Yan transverse momentum spectrum at LHC, stressing the role of angular ordering in the low transverse momentum region. We compare the PB implementation of the angular ordering condition with the Kimber-Martin-Ryskin-Watt (KMRW), both at the analytical and numerical level.
The average number of minijets and the corresponding transverse energy produced in heavy ion collisions are evaluated by including explicitly semi-hard parton rescatterings in the dynamics of the interaction. At the LHC semi-hard rescatterings have a sizable effect on global characteristics of the typical inelastic event. An interesting feature is that the dependence on the cutoff which separates soft and hard parton interactions becomes less critical after taking rescatterings into account.
We present the general introduction to the Parton Reggeization Approach and discuss some new results on large-p_T production for heavy quarks at the LHC. We concentrate here on study of different correlation spectra for b\barb-jet pairs, D\barD-meson pairs and DD-meson pairs.
We study the production of same-sign W boson pairs at the LHC in double parton interactions. Compared with simple factorised double parton distributions (dPDFs), we show that the recently developed dPDFs, GS09, lead to non-trivial kinematic correlations between the W bosons. A numerical study of the prospects for observing this process using same-sign dilepton signatures, including same-sign WWjj, di-boson and heavy flavour backgrounds, at 14 TeV centre-of-mass energy is then performed. It is shown that a small excess of same-sign dilepton events from double parton scattering over a background…
We study angular correlations in associated hadroproduction of $Υ(1S)$ with the $D^{\pm}$ and $D^0$-mesons at the LHC in the Leading Order of the parton Reggeization approach. Hadronization of $b\bar{b}$-pair to $Υ(1S)$ is described within the NRQCD-factorization framework. Production of $D-$mesons is described in the fragmentation model with scale-dependent fragmentaion functions. We have found good agreement with LHCb data for various differential distributions, except for the case of spectra on azimuthal angle differences at the small $\triangleφ$ values. The total cross-section in our Sing…
At the LHC we are colliding protons, but it is not the protons that are doing the interacting. It is their constituents the quarks, antiquarks and gluons-collectively known as partons. We need to know how what fractional momentum of the proton each of these partons takes at the energy scale of LHC collisions, in order to understand LHC physics. Such parton momentum distributions are known as PDFs (Parton Distribution Functions) and are a field of study in their own right. However, it is now the case that the uncertainties on PDFs are a major contributor to the background to the discovery of ph…
We present the first NNPDF full set of Parton Distribution Functions from a comprehensive DIS analysis. This approach, combining a Monte Carlo sampling of the probability measure in the space of PDFs with the use of neural networks as interpolating functions, provides a faithful and statistically sound determination of the uncertainty in parton distributions. The features of the fit and the results are discussed in details as well as some preliminary phenomenological analysis
Inclusive jet production will dominate the high $Q^{2}$ final states at the LHC.~In this work we try to estimate the up-to-date expectations, for high $E_{T}$ jets and their expected origin from the various parton-parton scattering processes. For these studies we have used a standard Parton Distribution Function (PDF) and simulated millions of events with the PYTHIA8 event genertor. The results are compared with simulations for center-of-mass energies of 0.9 TeV, 2.36 TeV, 7 TeV and 14 TeV corresponding to existing and future LHC runs. We present some expectations for the relative cross sectio…
Heavy quarkonium production in the framework of the nonrelativistic quantum chromodynamics and leading order of the parton Reggeization approach at the Tevatron and LHC Colliders is discussed. The new results, which are reviewed in this report, include the comparison with recent data on chi_{c1,c2} production from ATLAS Collaboration and special discussion of heavy quarkonium polarization issues in the considered framework.
We study the contribution from double parton scattering (DPS) to four-jet production at the LHC, both at the leading order accuracy and after incorporating the effects of QCD radiation. Apart from DPS, we also include and discuss the contribution from single parton scattering (SPS). We find that the QCD radiation impacts theoretical predictions significantly, with DPS contributions more affected than the SPS. We also examine a number of observables in regard to their effectiveness for discrimination between DPS and SPS events and propose sets of kinematical cuts to improve the prospects of mea…
The Drell-Yan massive dilepton production in hadron-hadron collisions provides a unique tool, complementary to Deep Inelastic Scattering, for improving our understanding of hadronic substructure and in particular for testing parton distributions. We will consider measurements of the differential and double-differential Drell-Yan cross sections from FNAL Tevatron up to CERN LHC energies and they will be compared to the predictions of perturbative QCD calculations using most recent sets (CT14 and MMHT14) of parton distribution functions, as well as those provided by the statistical approach.
Double parton scattering (DPS) is studied at the example of $J/ψ$ pair-production in the LHCb and ATLAS experiments of the Large Hadron Collider (LHC) at centre-of-mass energies of $\sqrt{S}=$ 7, 8, and 13 TeV. We report theoretical predictions delivered to the LHCb and ATLAS collaborations adjusted for the fiducial volumes of the corresponding measurements during Run I and provide new predictions at 13 TeV collision energy. It is shown that DPS can lead to noticeable contributions in the distributions of longitudinal variables of the di-$J/ψ$ system, especially at 13 TeV. The increased DPS ra…
We perform a calculation of inclusive $Z$ boson production in proton-lead collisions at the LHC taking into account the transverse momenta of the initial partons. We use the framework of $k_T$-factorization combining transverse momentum dependent parton distributions (TMDs) with off-shell matrix elements. In order to do it we need to construct appropriate TMDs for lead nuclei which is done using the parton branching method. Our computations are compared with data from CMS taken at $\sqrt{s}=5.02$ TeV. The results are in good agreement with the measurements especially the transverse momentum di…
The new methodology of adding QCD NLO corrections in the initial state Monte Carlo parton shower (hard process part) is presented using process of the heavy boson production at the LHC as an example. Despite the simplified model of the process, presented numerical results prove that the basic concept of the new methodology works correctly in the numerical environment of the Monte Carlo parton shower event generator. The presented method is an alternative to the well established methods, MC@NLO and POWHEG. Refinements of the new method with better computer CPU time efficiency are also discussed…
Despite a wealth of experimental data for high p_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. One important reason is that the pQCD parton spectrum at RHIC kinematics is so steeply falling that distinguishing even a moderate shift in parton energy from complete parton absorption is almost impossible in observable quantities. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and as a result, only the pathlength dependence of energy loss offers some discriminating p…
Despite a wealth of experimental data for high-P_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. A key reason is that the pQCD parton spectrum at RHIC is falling so steeply that distinguishing even a moderate shift in parton energy from complete parton absorption is essentially impossible. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and, as a result, at RHIC energies only the pathlength dependence of energy loss offers some discriminating power. At LHC however,…
Predictions for Drell--Yan lepton pair production at low dilepton mass and small x at the LHC usually have a large scale dependence. This can be decreased by determining an "optimal factorization scale". In this paper, we reduce this scale by imposing a cutoff in azimuthal angle between the transverse momentum of the leptons, properly taking into account Sudakov effects. This allows one to probe the parton distributions at smaller scales eliminating most of the current theoretical uncertainty.
We analyze the role of nuclear modifications of parton distributions, notably, the nuclear shadowing and antishadowing corrections, in production of lepton pairs from decays of neutral electroweak gauge bosons in proton-lead and lead-collisions at the LHC. Using the Collins-Soper-Sterman resummation formalism that we extended to the case of nuclear parton distributions, we observed a direct correlation between the predicted behavior of the transverse momentum and rapidity distributions of the produced vector bosons and the pattern of quark and gluon nuclear modifications. This makes Drell-Yan …
The expected rate for Multiple Parton Interactions (MPI) at the LHC is large. This requires an estimate of their impact on all measurement foreseen at the LHC while opening unprecendented opportunities for a detailed study of these phenomena. In this paper we examine the MPI background to top-antitop production, in the semileptonic channel, in the early phase of data taking when the full power of $b$--tagging will not be available. The MPI background turns out to be small but non negligible, of the order of 20% of the background provided by W+4j production through a Single Parton Interaction. …
We study triple-parton scattering effects in open charm production in proton-proton collisions at the LHC. Predictions for one, two and three $c\bar c$ pairs production are given for $\sqrt{s}= 7$ TeV and $\sqrt{s}= 13$ TeV. Quite large cross sections, of the order of milibarns, for the triple-parton scattering mechanism are obtained. We suggest a measurement of three $D^{0}$ or three $\bar{D^{0}}$ mesons by the LHCb collaboration. The predicted visible cross sections are of the order of a few nanobarns. The counting rates including $D^{0} \to K^{-}π^{+}$ branching fractions are also given. We…
The experimental errors in current and future hadron colliders are expected to decrease to a level that will challenge the uncertainties in the theoretical calculations. One important component in the prediction uncertainties comes from the Parton Density Functions of the (anti)proton. In this work we develop an interface from the Les Houches Accord Parton Density Functions (LHAPDF) package to the very popular Monte Carlo generator {\tt PYTHIA} version 6.2. Then we proceed to estimate the PDF uncertainties for the production of Drell-Yan pairs from the Z pole to masses above 1 $\TeV$ and for H…
We study the inclusive DbarD and DD pair production in proton-proton collisions at the LHC at lead- ing order of the parton Reggeization approach endowed with universal scale-depended fragmen- tation functions for c-quark to D-meson and for gluon to D-meson transitions. We have described DbarD and DD distributions in azimuthal angle, as well as transverse momentum, rapidity distance, and invariant mass measured in the region of large rapidity 2 < y < 4 by the LHCb Collabora- tion at the LHC without free parameters. We have used Reggeized amplitudes for the processes RR - gg and RR - c barc whi…
In this talk, we present the first, numerically stable, results for the one-loop amplitudes needed for computing W,Z + 3 jet cross sections at the LHC to next-to-leading order in the QCD coupling. We implemented these processes in BlackHat, an automated program based on on-shell methods. These methods scale very well with increasing numbers of external partons, and are applicable to a wide variety of problems of phenomenological interest at the LHC.
Associated production of prompt $Υ(1S)$ and $D^{+,0}$ mesons has been proposed as a golden channel for the search of Double Parton Scattering, because Single Parton Scattering contribution to the cross-section is believed to be negligible on a basis of leading order calculations in the Collinear Parton Model. We study this process in the leading order approximation of the Parton Reggeization Approach. Hadronization of $b\bar{b}$-pair into bottomonium states is described within framework of the NRQCD-factorization approach while production of $D$ mesons is described in the fragmentation model w…
We examine present data for double parton scattering at LHC and discuss their energy dependence from its earliest measurements at the ISR. Different models for the effective cross-section are considered and their behavior studied for a variety of selected final states. We point out that data for pp ->4 jets or pp -> quarkonium pair indicate the effective cross-section to increase with energy. We compare this set of data with different models, including one inspired by our soft gluon resummation model for the impact parameter distribution of partons.
Fully dynamic simulations of heavy ion collisions at RHIC and at LHC energies within the perturbative QCD-based partonic transport model BAMPS (Boltzmann Approach to Multi-Parton Scatterings) are presented, focusing on the simultaneous investigation of jet quenching and elliptic flow. The model features inelastic 2 <-> 3 processes based on the Gunion-Bertsch matrix element and has recently been extended to include light quark degrees of freedom, allowing for direct comparison to hadronic data on the nuclear modification factor via a fragmentation scheme for high-pT partons. The nuclear modific…
Parton luminosities are convenient for estimating how the physics potential of Large Hadron Collider experiments depends on the energy of the proton beams. I present parton luminosities, ratios of parton luminosities, and contours of fixed parton luminosity for $gg$, $u\bar{d}$, and $qq$ interactions over the energy range relevant to the Large Hadron Collider, along with example analyses for specific processes.
We present hadron-level predictions from the Monte Carlo generator Cascade and parton level calculations of open b quark, b-flavored hadron and inclusive b-jet production in the framework of the kt-factorization QCD approach for the LHC energies. The unintegrated gluon densities in a proton are determined using the CCFM evolution equation and the Kimber-Martin-Ryskin (KMR) prescription. Our predictions are compared with the first data taken by the CMS and LHCb collaborations at 7 TeV. We study the theoretical uncertainties of our calculations and investigate the effects coming from parton show…
We construct a model for double parton distribution functions (dPDFs) based on the notion of self-similarity, pursued earlier for small \textit{x} physics at HERA. The most general form of dPDFs contains total thirteen parameters to be fitted from data of proton-proton collision at LHC. It is shown that the constructed dPDF does not factorize into two single PDFs in conformity with QCD expectation, and it satisfies the condition that at the kinematic boundary $x_{1}+x_{2}=1 $ (where $x_{1}$ and $x_{2}$ are the longitudinal fractional momenta of two partons), the dPDF vanishes.
In this paper we explore the use of results on forward particle production at the LHC to constrain nuclear Parton Density Functions (nPDFs). The case study here is based on a possible future measurement of forward photon production with the Forward Calorimeter that is currently under discussion as an upgrade of the ALICE experiment. As a starting point, we use the recent nNNPDF 1.0 nuclear PDFs, which have been determined using fixed target neutral-current DIS data, which constrain the gluon density at x > $10^{-2}$. The Bayesian reweighting technique is used to include the constraints from th…
We discuss the prospects of observing double parton scattering (DPS) processes with purely leptonic final states at the LHC. We first study same-sign W pair production, which is particularly suited for studying momentum and valence number conservation effects, followed by discussions on double Drell-Yan and production of J/psi pairs. The effects of initial state and intrinsic transverse momentum smearing on pair-wise transverse momentum balance characteristic to DPS are studied quantitatively. We also present a new technique, based on rapidity differences, to extract the DPS component from a d…
We investigate the potential to observe double parton scattering at the Large Hadron Collider in pp -> W bb X -> mu nu bb X at 7 TeV. Our analysis tests the efficacy of several kinematic variables in isolating the double parton process of interest from the single parton process and relevant backgrounds for the first 10/fb of integrated luminosity. These variables are constructed to expose the independent nature of the two subprocesses in double parton scattering, pp -> mu nu and pp -> bb X. We use next-to-leading order perturbative predictions for the double parton and single parton scattering…
Parton distribution functions (pdfs) are an important ingredient for LHC phenomenology. Recent progress in determining pdfs from global analyses is reviewed, and some of the most important outstanding issues are highlighted. Particular attention is paid to the precision with which predictions for LHC `standard-candle' cross sections can be made, and also to new information that LHC can provide on pdfs.
An iso-singlet W' carrying hypercharge one is considered within an effective Lagrangian approach. The predictions for the exclusion in the (M_{W'},g_q/g) parameter space presented in Ref. [1] for the search in the di-jet invariant mass spectrum are shown to be in reasonable agreement with the LHC results based on 1 fb^{-1} of integrated luminosity.
We present a preliminary set of updated NLO parton distributions. For the first time we have a quantitative extraction of the strange quark and antiquark distributions and their uncertainties determined from CCFR and NuTeV dimuon cross sections. Additional jet data from HERA and the Tevatron improve our gluon extraction. Lepton asymmetry data and neutrino structure functions improve the flavour separation, particularly constraining the down quark valence distribution.
We study the inclusive hadroproduction of D^0, D^+, D^star+, and D_s^+ mesons at leading order in the parton Reggeization approach endowed with universal fragmentation functions fitted to $e^+e^-$ annihilation data from CERN LEP1. We have described $D$-meson transverse momentum distributions measured in the central region of rapidity by the CDF Collaboration at Tevatron (|y|<1) and ALICE Collaboration at LHC (|y|<0.5) within uncertainties and without free parameters, using Kimber-Martin-Ryskin unintegrated gluon distribution function in a proton. The forward D-meson production (|y|>2.0) measur…
We calculate $c\bar{c}c\bar{c}$ and $b\bar{b}b\bar{b}$ production cross section in Double Parton Scattering (DPS) processes at the energies of LHC. In our calculation we include saturation effects in the gluon distribution. For charm production, our results confirm the prediction of a previous work in which saturation effects were neglected, namely that at the energies of LHC charm production in DPS processes becomes comparable with charm production in SPS. We also estimate the production of $c\bar{c}b\bar{b}$ in DPS. Our results indicate that at $\sqrt{s}=14$ TeV half of the total amount of b…
