Next-to-Next-to Leading Order QCD Predictions for Single Jet Inclusive Production at the LHC

Next-to-Next-to-Leading Order Study of Three-Jet Production at the LHC

Multijet rates at hadron colliders provide a unique possibility for probing quantum chromodynamics (QCD), the theory of strong interactions. By comparing theory predictions with collider data, one can directly test perturbative QCD, extract fundamental parameters like the strong coupling α_{s}, and search for physics beyond the standard model. In this work we calculate, for the first time, the next-to-next-to-leading order (NNLO) QCD corrections to typical three-jet observables and to differential three-to-two jet ratios. The calculation is complete apart from the three-jet double virtual contributions which are included in the leading-color approximation. We demonstrate that the inclusion of the NNLO corrections significantly reduces the dependence of those observables on the factorization and renormalization scales. Besides its phenomenological value, this proof-of-principle computation represents a milestone in perturbative QCD.

Discovering Partonic Rescattering in Light Nucleus Collisions

We demonstrate that oxygen-oxygen collisions at the LHC provide unprecedented sensitivity to parton energy loss in a system whose size is comparable to those created in very peripheral heavy-ion collisions. With leading and next-to-leading order calculations of nuclear modification factors, we show that the baseline in the absence of partonic rescattering is known with up to 2% theoretical accuracy in inclusive oxygen-oxygen collisions. Surprisingly, a Z-boson normalized nuclear modification factor does not lead to higher theoretical accuracy within current uncertainties of nuclear parton distribution functions. We study a broad range of parton energy loss models and we find that the expected signal of partonic rescattering can be disentangled from the baseline by measuring charged hadron spectra in the range 20  GeV<p_{T}<100  GeV.

Jet cross sections at the LHC and the quest for higher precision

We perform a phenomenological study of Z plus jet, Higgs plus jet and di-jet production at the Large Hadron Collider. We investigate in particular the dependence of the leading jet cross section on the jet radius as a function of the jet transverse momentum. Theoretical predictions are obtained using perturbative QCD calculations at the next-to and next-to-next-to-leading order, using a range of renormalization and factorization scales. The fixed order predictions are compared to results obtained from matching next-to-leading order calculations to parton showers. A study of the scale dependence as a function of the jet radius is used to provide a better estimate of the scale uncertainty for small jet sizes. The non-perturbative corrections as a function of jet radius are estimated from different generators.

Triple Differential Dijet Cross Section at the LHC

The measurement of the triple-differential dijet production cross section as a function of the average transverse momentum p_{T,avg}, half the rapidity separation y^{*}, and the boost y_{b} of the two leading jets in the event enables a kinematical scan of the underlying parton momentum distributions. We compute for the first time the second-order perturbative QCD corrections to this triple-differential dijet cross section, at leading color in all partonic channels, thereby enabling precision studies with LHC dijet data. A detailed comparison with experimental CMS 8 TeV data is performed, demonstrating how the shape of this differential cross section probes the parton densities in different kinematical ranges.

Determination of αs in NNLO QCD using H1 jet cross section measurements

Measurements of jet cross sections in neutral current deep-inelastic scattering (NC DIS) using data taken with the H1 detector at HERA are accomplished by the precision measurement of double-differential inclusive jet, dijet and trijet cross sections at low photon virtualities 5.5 < Q2 < 80 GeV2, and by extending previous inclusive jet measurements in the range 150 < Q2 < 15000 GeV2 to low transverse jet momenta 5 < PT < 7 GeV. The strong coupling constant at the Z-boson mass, αs(mZ), is determined in next-to-next-to-leading order (NNLO) QCD using H1 inclusive jet and dijet cross section measurements. Complementary, αs(mZ) is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The running of the strong coupling is tested at different values of the renormalisation scale and the results are found to be in agreement with the QCD expectations.

Towards ultimate parton distributions at the high-luminosity LHC

Since its start of data taking, the LHC has provided an impressive wealth of information on the quark and gluon structure of the proton. Indeed, modern global analyses of parton distribution functions (PDFs) include a wide range of LHC measurements of processes such as the production of jets, electroweak gauge bosons, and top quark pairs. In this work, we assess the ultimate constraining power of LHC data on the PDFs that can be expected from the complete dataset, in particular after the High-Luminosity (HL) phase, starting in around 2025. The huge statistics of the HL-LHC, delivering L = 3 ab - 1 to ATLAS and CMS and L = 0.3 ab - 1 to LHCb, will lead to an extension of the kinematic coverage of PDF-sensitive measurements as well as to an improvement in their statistical and systematic uncertainties. Here we generate HL-LHC pseudo-data for different projections of the experimental uncertainties, and then quantify the resulting constraints on the PDF4LHC15 set by means of the Hessian profiling method. We find that HL-LHC measurements can reduce PDF uncertainties by up to a factor of 2 to 4 in comparison to state-of-the-art fits, leading to few-percent uncertainties for important observables such as the Higgs boson transverse momentum distribution via gluon-fusion. Our results illustrate the significant improvement in the precision of PDF fits achievable from hadron collider data alone, and motivate the continuation of the ongoing successful program of PDF-sensitive measurements by the LHC collaborations.

Charged hadron fragmentation functions from collider data: NNPDF Collaboration

We present NNFF1.1h, a new determination of unidentified charged-hadron fragmentation functions (FFs) and their uncertainties. Experimental measurements of transverse-momentum distributions for charged-hadron production in proton-(anti)proton collisions at the Tevatron and at the LHC are used to constrain a set of FFs originally determined from electron-positron annihilation data. Our analysis is performed at next-to-leading order in perturbative quantum chromodynamics. We find that the hadron-collider data is consistent with the electron-positron data and that it significantly constrains the gluon FF. We verify the reliability of our results upon our choice of the kinematic cut in the hadron transverse momentum applied to the hadron-collider data and their consistency with NNFF1.0, our previous determination of the FFs of charged pions, kaons, and protons/antiprotons.

Precision determination of the strong coupling constant within a global PDF analysis: NNPDF Collaboration

We present a determination of the strong coupling constant α s ( m Z ) based on the NNPDF3.1 determination of parton distributions, which for the first time includes constraints from jet production, top-quark pair differential distributions, and the Z p T distributions using exact NNLO theory. Our result is based on a novel extension of the NNPDF methodology - the correlated replica method - which allows for a simultaneous determination of α s and the PDFs with all correlations between them fully taken into account. We study in detail all relevant sources of experimental, methodological and theoretical uncertainty. At NNLO we find α s ( m Z ) = 0.1185 ± 0 . 0005 (exp) ± 0 . 0001 (meth) , showing that methodological uncertainties are negligible. We conservatively estimate the theoretical uncertainty due to missing higher order QCD corrections (N 3 LO and beyond) from half the shift between the NLO and NNLO α s values, finding Δ α s th = 0.0011 .

The impact of LHC jet data on the MMHT PDF fit at NNLO

We investigate the impact of the high precision ATLAS and CMS 7 TeV measurements of inclusive jet production on the MMHT global PDF analysis at next-to-next-to-leading order (NNLO). This is made possible by the recent completion of the long-term project to calculate the NNLO corrections to the hard cross section. We find that a good description of the ATLAS data is not possible with the default treatment of experimental systematic errors, and propose a simplified solution that retains the dominant physical information of the data. We then investigate the fit quality and the impact on the gluon PDF central value and uncertainty when the ATLAS and CMS data are included in a MMHT fit. We consider both common choices for the factorization and renormalization scale, namely the inclusive jet transverse momentum, p ⊥ , and the leading jet p ⊥ , as well as the different jet radii for which the ATLAS and CMS data are made available. We find that the impact of these data on the gluon is relatively insensitive to these inputs, in particular the scale choice, while the inclusion of NNLO corrections tends to improve the data description somewhat and has a qualitatively similar though not identical impact on the gluon in comparison to NLO.

Cross-section measurements of final states with photons and jets with the ATLAS experiment

The ATLAS Collaboration has performed precise measurements of the cross-section of final states with photons and/or jets at centre-of-mass energies of 8 and 13 TeV. The results are compared with state-of-the-art theory predictions and with predictions of several Monte Carlo generators. We also present new measurements of transverse energy-energy correlations and their associated asymmetries in multi-jet events at 8 TeV. Both measurements are used to extract the strong coupling constant and test the renormalization group equations.

Threshold and Jet Radius Joint Resummation for Single-Inclusive Jet Production

We present the first threshold and jet radius jointly resummed cross section for single-inclusive hadronic jet production. We work at next-to-leading logarithmic accuracy and our framework allows for a systematic extension beyond the currently achieved precision. Long-standing numerical issues are overcome by performing the resummation directly in momentum space within soft collinear effective theory. We present the first numerical results for the LHC and observe an improved description of the available data. Our results are of immediate relevance for LHC precision phenomenology including the extraction of parton distribution functions and the QCD strong coupling constant.

Determination of the strong coupling constant α s ( m Z ) in next-to-next-to-leading order QCD using H1 jet cross section measurements: H1 Collaboration

The strong coupling constant α s is determined from inclusive jet and dijet cross sections in neutral-current deep-inelastic ep scattering (DIS) measured at HERA by the H1 collaboration using next-to-next-to-leading order (NNLO) QCD predictions. The dependence of the NNLO predictions and of the resulting value ofα s ( m Z ) at the Z-boson mass m Z are studied as a function of the choice of the renormalisation and factorisation scales. Using inclusive jet and dijet data together, the strong coupling constant is determined to beα s ( m Z ) = 0.1157 ( 20 ) exp ( 29 ) th . Complementary,α s ( m Z ) is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The valueα s ( m Z ) = 0.1142 ( 28 ) tot obtained is consistent with the determination from jet data alone. The impact of the jet data on the PDFs is studied. The running of the strong coupling is tested at different values of the renormalisation scale and the results are found to be in agreement with expectations.

Measurement of the triple-differential dijet cross section in proton-proton collisions at s = 8 TeV and constraints on parton distribution functions

A measurement is presented of the triple-differential dijet cross section at a centre-of-mass energy of 8TeV using 19.7fb -1 of data collected with the CMS detector in proton-proton collisions at the LHC. The cross section is measured as a function of the average transverse momentum, half the rapidity separation, and the boost of the two leading jets in the event. The cross section is corrected for detector effects and compared to calculations in perturbative quantum chromodynamics at next-to-leading order accuracy, complemented with electroweak and nonperturbative corrections. New constraints on parton distribution functions are obtained and the inferred value of the strong coupling constant isα S ( M Z ) = 0.1199 ± 0.0015 ( exp ) - 0.0020 + 0.0031 ( theo ) , where M Z is the mass of the Z boson.

Precise Predictions for Dijet Production at the LHC

We present the calculation of dijet production, doubly differential in dijet mass m_{jj} and rapidity difference |y^{*}|, at leading color in all partonic channels at next-to-next-to-leading order (NNLO) in perturbative QCD. We consider the long-standing problems associated with scale choice for dijet production at next-to-leading order (NLO) and investigate the impact of including the NNLO contribution. We find that the NNLO theory provides reliable predictions, even when using scale choices that display pathological behavior at NLO. We choose the dijet invariant mass as the theoretical scale on the grounds of perturbative convergence and residual scale variation and compare the predictions to the ATLAS 7 TeV 4.5  fb^{-1} data.

Parton distributions from high-precision collider data: NNPDF Collaboration

We present a new set of parton distributions, NNPDF3.1, which updates NNPDF3.0, the first global set of PDFs determined using a methodology validated by a closure test. The update is motivated by recent progress in methodology and available data, and involves both. On the methodological side, we now parametrize and determine the charm PDF alongside the light-quark and gluon ones, thereby increasing from seven to eight the number of independent PDFs. On the data side, we now include the D0 electron and muon W asymmetries from the final Tevatron dataset, the complete LHCb measurements of W and Z production in the forward region at 7 and 8 TeV, and new ATLAS and CMS measurements of inclusive jet and electroweak boson production. We also include for the first time top-quark pair differential distributions and the transverse momentum of the Z bosons from ATLAS and CMS. We investigate the impact of parametrizing charm and provide evidence that the accuracy and stability of the PDFs are thereby improved. We study the impact of the new data by producing a variety of determinations based on reduced datasets. We find that both improvements have a significant impact on the PDFs, with some substantial reductions in uncertainties, but with the new PDFs generally in agreement with the previous set at the one-sigma level. The most significant changes are seen in the light-quark flavor separation, and in increased precision in the determination of the gluon. We explore the implications of NNPDF3.1 for LHC phenomenology at Run II, compare with recent LHC measurements at 13 TeV, provide updated predictions for Higgs production cross-sections and discuss the strangeness and charm content of the proton in light of our improved dataset and methodology. The NNPDF3.1 PDFs are delivered for the first time both as Hessian sets, and as optimized Monte Carlo sets with a compressed number of replicas.

Nested soft-collinear subtractions in NNLO QCD computations

We discuss a modification of the next-to-next-to-leading order (NNLO) subtraction scheme based on the residue-improved sector decomposition that reduces the number of double-real emission sectors from five to four. In particular, a sector where energies and angles of unresolved particles vanish in a correlated fashion is redundant and can be discarded. This simple observation allows us to formulate a transparent iterative subtraction procedure for double-real emission contributions, to demonstrate the cancellation of soft and collinear singularities in an explicit and (almost) process-independent way and to write the result of a NNLO calculation in terms of quantities that can be computed in four space-time dimensions. We illustrate this procedure explicitly in the simple case of [Formula: see text] gluonic corrections to the Drell-Yan process of [Formula: see text] annihilation into a lepton pair. We show that this framework leads to fast and numerically stable computation of QCD corrections.