Precision QCD Research Articles

Hard and precision QCD measurements at HERA

Recent results on the combination of open charm and beauty production data in deep-inelastic scattering and their mparison to perturbative QCD calculations at next-to-leading order and approximate next-to-next-to-leading order are presented. The combined data are used to extract the masses of the charm and beauty quarks at next-to-leading order. Inclusive jet and dijet cross sections in deep-inelastic scattering are used to extract the strong coupling αs(mZ) at next-to-next-to-leading order for the first time. Finally, further new observables measured for the description of prompt photons plus jet production in deep-inelastic scattering and their comparison to perturbative QCD calculations are shown.

SCET for jet physics in the vacuum and the medium

In this plenary talk I discuss soft-collinear effective theory (SCET) as a framework for precision QCD phenomenology. Emphasis is placed on jet and heavy flavor observables accessible at current and future collider facilities. One of the principal challenges that calculations of hard probes in heavy ion reactions face is the ambiguity associated with the implementation of medium-induced radiative effects. I demonstrate how extension of SCET to describe parton propagation in QCD matter has helped quantify and reduce the theoretical uncertainty in jet quenching calculations.

NNLO QCD corrections to jet production in deep inelastic scattering

Hadronic jets in deeply inelastic electron-proton collisions are produced by the scattering of a parton from the proton with the virtual gauge boson mediating the interaction. The HERA experiments have performed precision measurements of inclusive single jet production and di-jet production in the Breit frame, which provide important constraints on the strong coupling constant and on parton distributions in the proton. We describe the calculation of the next-to-next-to-leading order (NNLO) QCD corrections to these processes, and assess their size and impact. A detailed comparison with data from the H1 and ZEUS experiments highlights that inclusive single jet production displays a better perturbative convergence than di-jet production. We also observe that the event selection cuts in some of the di-jet measurements of both H1 and ZEUS induce an infrared sensitivity that destabilises the perturbative stability of the predictions. Our results open up new opportunities for QCD precision studies with jet production observables in deep inelastic scattering.

Precision QCD measurements in DIS at HERA

New and recent results on QCD measurements from the H1 and ZEUS experiments at the HERA ep collider are reviewed. The final results on the combined deep-inelastic neutral and charged current cross-sections are presented and their role in the extractions of parton distribution functions (PDFs) is studied. The PDF fits give insight into the compatibility of QCD evolution and heavy flavor schemes with the data as a function of kinematic variables such as the scale Q2. Measurements of jet production cross-sections in ep collisions provide direct proves of QCD and extractions of the strong coupling constants are performed. Charm and beauty cross-section measurements are used for the determination of the heavy quark masses. Their role in PDF fits is investigated. In the regime of diffractive DIS and photoproduction, dijet and prompt photon production cross-sections provide insights into the process of factorization and the nature of the diffractive exchange.

Precise QCD Predictions for the Production of Dijet Final States in Deep Inelastic Scattering.

The production of two-jet final states in deep inelastic scattering is an important QCD precision observable. We compute it for the first time to next-to-next-to-leading order (NNLO) in perturbative QCD. Our calculation is fully differential in the lepton and jet variables and allows one to impose cuts on the jets in both the laboratory and the Breit frame. We observe that the NNLO corrections are moderate in size, except at kinematical edges, and that their inclusion leads to a substantial reduction of the scale variation uncertainty on the predictions. Our results will enable the inclusion of deep inelastic dijet data in precision phenomenology studies.

New approach to hard corrections in precision QCD for LHC and FCC physics

We present a new approach to the realization of hard fixed-order corrections in predictions for the processes probed in high energy colliding hadron beam devices, with some emphasis on the large hadron collider (LHC) and the future circular collider (FCC) devices. We show that the usual unphysical divergence of such corrections as one approaches the soft limit is removed in our approach, so that we would render the standard results to be closer to the observed exclusive distributions. We use the single [Formula: see text] production and decay to lepton pairs as our prototypical example, but we stress that the approach has general applicability. In this way, we open another part of the way to rigorous baselines for the determination of the theoretical precision tags for LHC physics, with an obvious generalization to the FCC as well.

UsingΛb→Λμ+μ−data within a Bayesian analysis of|ΔB|=|ΔS|=1decays

We study the impact of including the baryonic decay $\Lambda_b\to \Lambda(\to p \pi^-)\mu^+\mu^-$ in a Bayesian analysis of $|\Delta B | = |\Delta S| = 1$ transitions. We perform fits of the Wilson coefficients $C_{9}$, $C_{9'}$, $C_{10}$ and $C_{10'}$, in addition to the relevant nuisance parameters. Our analysis combines data for the differential branching fraction and three angular observables of $\Lambda_b\to \Lambda(\to p \pi^-)\mu^+\mu^-$ with data for the branching ratios of $B_s \to \mu^+\mu^-$ and inclusive $b \to s\ell^+\ell^-$ decays. Newly available precise lattice QCD results for the full set of $\Lambda_b \to \Lambda$ form factors are used to evaluate the observables of the baryonic decay. Our fits prefer shifts to $C_{9}$ that are opposite in sign compared to those found in global fits of only mesonic decays, and the posterior odds show no evidence of physics beyond the Standard Model. We investigate a possible hadronic origin of the observed tensions between theory and experiment.

The NNLO QCD corrections to Z boson production at large transverse momentum

The transverse momentum distribution of massive neutral vector bosons can be measured to high accuracy at hadron colliders. The transverse momentum is caused by a partonic recoil, and is determined by QCD dynamics. We compute the single and double-differential transverse momentum distributions for fully inclusive $Z/\gamma^*$ production including leptonic decay to next-to-next-to-leading order (NNLO) in perturbative QCD. We also compute the same distributions normalised to the cross sections for inclusive $Z/\gamma^*$ production, i.e. integrated over the transverse momentum of the lepton pair. We compare our predictions for the fiducial cross sections to the 8 TeV data set from the ATLAS and CMS collaborations, which both observed a tension between data and NLO theory predictions, using the experimental cuts and binning. We find that the inclusion of the NNLO QCD effects does not fully resolve the tension with the data for the unnormalised $p^Z_T$ distribution. However, we observe that normalising the NNLO $Z$-boson transverse momentum distribution by the NNLO Drell-Yan cross section substantially improves the agreement between experimental data and theory, and opens the way for precision QCD studies of this observable.

Comparisons of Exact Amplitude-Based Resummation Predictions and LHC Data

Using the MC Herwiri1.031, we present the current status of the comparisons with LHC data of the predictions of our approach of exact amplitude-based resummation for precision QCD calculations.

Nonglobal correlations in collider physics

Despite their importance for precision QCD calculations, correlations between in- and out-of-jet regions of phase space have never directly been observed. These so-called non-global effects are present generically whenever a collider physics measurement is not explicitly dependent on radiation throughout the entire phase space. In this paper, we introduce a novel procedure based on mutual information, which allows us to isolate these non-global correlations between measurements made in different regions of phase space. We study this procedure both analytically and in Monte Carlo simulations in the context of observables measured on hadronic final states produced in $e^+e^-$ collisions, though it is more widely applicable. The procedure exploits the sensitivity of soft radiation at large angles to non-global correlations, and we calculate these correlations through next-to-leading logarithmic accuracy. The bulk of these non-global correlations are found to be described in Monte Carlo simulation. They increase by the inclusion of non-perturbative effects, which we show can be incorporated in our calculation through the use of a model shape function. This procedure illuminates the source of non-global correlations and has connections more broadly to fundamental quantities in quantum field theory.

Setting the renormalization scale in perturbative QCD: Comparisons of the principle of maximum conformality with the sequential extended Brodsky-Lepage-Mackenzie approach

A key problem in making precise perturbative QCD (pQCD) predictions is how to set the renormalization scale of the running coupling unambiguously at each finite order. The elimination of the uncertainty in setting the renormalization scale in pQCD will greatly increase the precision of collider tests of the Standard Model and the sensitivity to new phenomena. Renormalization group invariance requires that predictions for observables must also be independent on the choice of the renormalization scheme. The well-known Brodsky-Lepage-Mackenzie (BLM) approach cannot be easily extended beyond next-to-next-to-leading order of pQCD. Several suggestions have been proposed to extend the BLM approach to all-orders. In this paper we discuss two distinct methods. One is based on the "Principle of Maximum Conformality" (PMC), which provides a systematic all-orders method to eliminate the scale- and scheme- ambiguities of pQCD. The PMC extends the BLM procedure to all orders using renormalization group methods; as an outcome, it significantly improves the pQCD convergence by eliminating renormalon divergences. An alternative method is the "sequential extended BLM" (seBLM) approach, which has been primarily designed to improve the convergence of pQCD series. The seBLM, as originally proposed, introduces auxiliary fields and follows the pattern of the $\beta_0$-expansion to fix the renormalization scale. However, the seBLM requires a recomputation of pQCD amplitudes including the auxiliary fields; due to the limited availability of calculations using these auxiliary fields, the seBLM has only been applied to a few processes at low-orders......

Precise QCD predictions for jet production at the LHC

Theoretical predictions used in experimental analysis of LHC data have an inherent theoretical uncertainty associated to the relevant order in the perturbative expansion that the observable is computed. In this talk we briefly introduce and motivate the topic of QCD radiative corrections stressing the impact of including higher-order QCD effects in the theory predictions and in particular we consider two-loop QCD corrections. We present numerical results at NNLO for gluonic jet production where jets are reconstructed using the anti-$k_T$ jet algorithm from gluon-gluon and quark-antiquark scattering and discuss the inclusion of leading-$N_F$ quark contributions in the final-state.

The study of QCD in high pile-up environments: Higgs boson and Drell-Yan production

In the forthcoming high-luminosity phase of the LHC many of the most interesting measurements for precision QCD studies are hampered by large pile-up conditions, especially at not very high transverse momenta. However, with the recently discovered Higgs boson, which couples in the heavy top limit directly to gluons, we have access to a novel production process to probe QCD by a colour-singlet current. In this study we compare observables in Higgs boson and Drell-Yan production and investigate whether measuring ratios or subtractions can yield results that are stable in high pile-up environments, and yet sensitive to (small-p T ) QCD physics in gluon fusion processes. We present results of Monte Carlo event generator calculations for a few specific examples.

Sea quark contributions to the electric polarizability of hadrons

We present a lattice QCD calculation of the polarizability of the neutron and other neutral hadrons that includes the effects of the background field on the sea quarks. This is done by perturbatively reweighting the charges of the sea quarks to couple them to the background field. The main challenge in such a calculation is stochastic estimation of the weight factors, and we discuss the difficulties in this estimation. Here we use an extremely aggressive dilution scheme to reduce the stochastic noise to a manageable level. The pion mass in our calculation is 306 MeV and the lattice size is 3 fm. For neutron, we find that $\alpha_E = 2.70(55) * 10^{-4}fm^3$, which is the most precise lattice QCD determination of the polarizability to date that includes sea effects.

Jet cleansing: Separating data from secondary collision induced radiation at high luminosity

One of the greatest impediments to extracting useful information from high luminosity hadron-collider data is radiation from secondary collisions (i.e. pileup) which can overlap with that of the primary interaction. In this paper we introduce a simple jet-substructure technique termed cleansing which can consistently correct for large amounts of pileup in an observable independent way. Cleansing works at the subjet level, combining tracker and calorimeter-based data to reconstruct the pileup-free primary interaction. The technique can be used on its own, with various degrees of sophistication, or in concert with jet grooming. We apply cleansing to both kinematic and jet shape reconstruction, finding in all cases a marked improvement over previous methods both in the correlation of the cleansed data with uncontaminated results and in measures like S/rt(B). Cleansing should improve the sensitivity of new-physics searches at high luminosity and could also aid in the comparison of precision QCD calculations to collider data.

EERAD3: Event shapes and jet rates in electron–positron annihilation at order [formula omitted

The program EERAD3 computes the parton-level QCD contributions to event shapes and jet rates in electron–positron annihilation to order αs3. For three-jet production and related observables, this corresponds to next-to-next-to-leading order corrections, and allows for precision QCD studies. We describe the program and its usage in detail. Program summaryProgram title: EERAD3Catalogue identifier: AEUC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEUC_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 119253No. of bytes in distributed program, including test data, etc.: 1368922Distribution format: tar.gzProgramming language: FORTRAN77 (with some FORTRAN90 features).Computer: All.Operating system: All.Has the code been vectorised or parallelised?: Program code allows the storage and adaptation of Monte Carlo information for producing statistically independent results on many cores. A tool for the combination of these is provided with the code.RAM: 6 MBytesClassification: 11.2, 11.5.Nature of problem:Computation of second-order QCD corrections to event shapes and jet cross sections in electron–positron annihilation.Solution method:Generation of events at parton-level, supplemented by a subtraction method to combine infrared singular contributions among different partonic channels.Running time:The high-precision/high-resolution distributions in [1] required about 700 CPU-days on an Intel Q6800 2.9 GHz. Reasonable precision can be obtained with run times of about 100 hours per coefficient for distributions, or within 5 hours if only single moments are computed.

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The experimental value for the isospin amplitude mathrm{Re}A_2 in Krightarrow pi pi decays has been successfully explained within the standard model (SM), both within the large N approach to QCD and by QCD lattice calculations. On the other hand within the large N approach the value of mathrm{Re}A_0 is by at least 30,% below the data. While this deficit could be the result of theoretical uncertainties in this approach and could be removed by future precise QCD lattice calculations, it cannot be excluded that the missing piece in mathrm{Re}A_0 comes from new physics (NP). We demonstrate that this deficit can be significantly softened by tree-level FCNC transitions mediated by a heavy colourless Z^prime gauge boson with a flavour-violating left-handed coupling Delta ^{sd}_L(Z^prime ) and an approximately universal flavour diagonal right-handed coupling Delta ^{qq}_R(Z^prime ) to the quarks. The approximate flavour universality of the latter coupling assures negligible NP contributions to mathrm{Re}A_2. This property, together with the breakdown of the GIM mechanisms at tree level, allows one to enhance significantly the contribution of the leading QCD-penguin operator Q_6 to mathrm{Re}A_0. A large fraction of the missing piece in the Delta I=1/2 rule can be explained in this manner for M_{Z^prime } in the reach of the LHC, while satisfying the constraints from varepsilon _K, varepsilon '/varepsilon , Delta M_K, LEP-II and the LHC. The presence of a small right-handed flavour-violating coupling Delta ^{sd}_R(Z^prime )ll Delta ^{sd}_L(Z^prime ) and of enhanced matrix elements of Delta S=2 left–right operators allows one to satisfy simultaneously the constraints from mathrm{Re}A_0 and Delta M_K, although this requires some fine-tuning. We identify the quartic correlation between Z^prime contributions to mathrm{Re}A_0, varepsilon '/varepsilon , varepsilon _K and Delta M_K. The tests of this proposal will require much improved evaluations of mathrm{Re}A_0 and Delta M_K within the SM, of langle Q_6 rangle _0 as well as precise tree-level determinations of |V_{ub}| and |V_{cb}|. We present correlations between varepsilon '/varepsilon , K^+rightarrow pi ^+nu bar{nu } and K_{L}rightarrow pi ^0nu bar{nu } with and without the Delta I=1/2 rule constraint and generalise the whole analysis to Z^prime with colour (G^prime ) and Z with FCNC couplings. In the latter case no improvement on mathrm{Re}A_0 can be achieved without destroying the agreement of the SM with the data on mathrm{Re}A_2. Moreover, this scenario is very tightly constrained by varepsilon '/varepsilon . On the other hand, in the context of the Delta I=1/2 rule G^prime is even more effective than Z^prime : it provides the missing piece in mathrm{Re}A_0 for M_{G^prime }=(3.5–4.0), mathrm{TeV}.

Precision QCD measurements at HERA

A review of recent experimental results on perturbative QCD from the HERA experiments H1 and ZEUS is presented. All inclusive deep inelastic cross sections mea- sured by the H1 and ZEUS collaborations in neutral and charged current unpolarised ep scattering are combined. They span six orders of magnitude in negative four-momentum- transfer squared, Q 2 , and in Bjorken x. This data set is used as the sole input to NLO and NNLO QCD analyses to determine new sets of parton distributions, HERAPDF2.0, with small experimental uncertainties and an estimate of model and parametrisation un- certainties. Also shown are new results on inclusive jet, dijet and trijet differential cross sections measured in neutral current deep inelastic scattering. The precision jet data is used to extract the strong coupling αs at NLO with small experimental errors.

ATLAS QCD measurements for Higgs studies and New Physics searches

Our understanding of the modelling of Standard Model processes plays an important role for Higgs physics and searches beyond the Standard Model. Measurements such as vector boson plus jet production are essential for the understanding of backgrounds, modelling of double parton interactions, heavy flavour production from gluon splitting and modelling of the underlying event. Precision QCD measurements can also be used to constrain parton density functions, which are important for the understanding of the Higgs boson production cross section and the Standard Model backgrounds at the LHC kinematic limits. A review of recent ATLAS measurements is presented.

Higgs boson as a gluon trigger

In the forthcoming high-luminosity phase at the LHC many of the most interesting measurements for precision QCD studies are hampered by conditions of large pile-up, particularly at not very high transverse momenta. We study observables based on measuring ratios of color-singlet currents via Higgs boson and Drell-Yan production, which may be accessed also at large pile-up, and used for an experimental program on QCD physics of gluon fusion processes in the LHC high-luminosity runs. We present results of Monte Carlo calculations for a few specific examples.