Perturbative QCD Calculations Research Articles

Associated production of heavy flavors and W, Z bosons at CMS

The mechanism of production of heavy-flavoured jets, originated by the hadronization of b or c quarks, in association with vector bosons, W or Z, in the Standard Model is of primary importance. The study of events with one or two well-identified and isolated leptons accompanied by b-jets or displaced secondary vertices is therefore crucial to refine the theoretical calculations in perturbative QCD, as well as validate associated predictions from simulation. The understanding of these processes is furthermore required by Higgs and Beyond the Standard Model searches with similar final states. Using the LHC proton-proton collision data collected at a center of mass energy of 7 TeV by the CMS detector, measurements of the W+b, W+c, Z+b and Z+B hadrons cross sections are presented, comparing experimental data with several theoretical predictions in quantum chromodynamics.

Transport coefficients of heavy quarks aroundTcat finite quark chemical potential

The interactions of heavy quarks with the partonic environment at finite temperature $T$ and finite quark chemical potential $\mu_q$ are investigated in terms of transport coefficients within the Dynamical Quasi-Particle model (DQPM) designed to reproduce the lattice-QCD results (including the partonic equation of state) in thermodynamic equilibrium. These results are confronted with those of nuclear many-body calculations close to the critical temperature $T_c$. The hadronic and partonic spatial diffusion coefficients join smoothly and show a pronounced minimum around $T_c$, at $\mu_q=0$ as well as at finite $\mu_q$. Close and above $T_c$ its absolute value matches the lQCD calculations for $\mu_q=0$. The smooth transition of the heavy quark transport coefficients from the hadronic to the partonic medium corresponds to a cross over in line with lattice calculations, and differs substantially from perturbative QCD (pQCD) calculations which show a large discontinuity at $T_c$. This indicates that in the vicinity of $T_c$ dynamically dressed massive partons and not massless pQCD partons are the effective degrees-of-freedom in the quark-gluon plasma.

Centrality, rapidity and pT dependence of isolated prompt photon production in lead–lead collisions at sNN=2.76 TeV with the ATLAS detector at the LHC

Abstract The ATLAS experiment at the LHC has measured prompt photon production in s NN = 2.76 TeV Pb + Pb collisions using data collected in 2011 with an integrated luminosity of 0.14 nb − 1 . The measurement is performed with a hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolution, and detailed information about the shower shape of each measured photon. Using a selection based on shower shapes and an isolation criterion gives measured purities between approximately 50–90%, depending on p T , η and centrality. Photon yields, scaled by the mean nuclear thickness function, are determined as a function of collision centrality, pseudorapidity (in two intervals | η | 1.37 and 1.52 | η | 2.37 ) and transverse momentum ( 22 p T 280 GeV ). The scaled yields are shown compared to expectations from JETPHOX (perturbative QCD calculations at next to leading order), as are the ratios of the forward yields to those near mid-rapidity (for 22 p T 140 GeV ). The observed photon yields agree well with the predictions for proton–proton within statistical and systematic uncertainties, but are unable to distinguish between these calculations and others including isospin and nuclear PDF effects.

Measurements of open heavy flavor in STAR experiment

We present recent open heavy-flavor measurements performed with the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). The production cross sections of charm quarks in p+p collisions at s = 200 and 500 GeV are determined through hadronic decay channels of D 0 and D ⁎ + mesons. These results are consistent with perturbative QCD calculations. Using p+p collisions as a reference, the nuclear modification factors R AA of D 0 meson production are extracted from Au+Au collisions at s NN = 200 GeV and U+U collisions at s NN = 193 GeV . Model calculations including charm–medium interactions and hadronization via both fragmentation and coalescence describe the measured D 0 R AA . Results are also reported on R AA and elliptic flow v 2 of non-photonic electrons (NPE) from semileptonic decays of open heavy-flavor hadrons. None of the existing model calculations simultaneously describe the NPE R AA and v 2 . Finally we give an outlook for heavy-flavor measurements with the newly upgraded STAR detector.

Measurement of differential production cross-sections for a Z boson in association with b-jets in 7 TeV proton-proton collisions with the ATLAS detector

Measurements of differential production cross-sections of a $Z$ boson in association with $b$-jets in $pp$ collisions at $\sqrt{s}=7$ TeV are reported. The data analysed correspond to an integrated luminosity of 4.6 fb$^{-1}$ recorded with the ATLAS detector at the Large Hadron Collider. Particle-level cross-sections are determined for events with a $Z$ boson decaying into an electron or muon pair, and containing $b$-jets. For events with at least one $b$-jet, the cross-section is presented as a function of the $Z$ boson transverse momentum and rapidity, together with the inclusive $b$-jet cross-section as a function of $b$-jet transverse momentum, rapidity and angular separations between the $b$-jet and the $Z$ boson. For events with at least two $b$-jets, the cross-section is determined as a function of the invariant mass and angular separation of the two highest transverse momentum $b$-jets, and as a function of the $Z$ boson transverse momentum and rapidity. Results are compared to leading-order and next-to-leading-order perturbative QCD calculations.

Measurement of the differential [formula omitted]-jet cross section and the ratio [formula omitted] in [formula omitted] collisions at [formula omitted

We present the first measurements of the differential cross section dσ/dpTγ for the production of an isolated photon in association with at least two b-quark jets. The measurements consider photons with rapidities |yγ|<1.0 and transverse momenta 30<pTγ<200 GeV. The b-quark jets are required to have pTjet>15 GeV and |yjet|<1.5. The ratio of differential production cross sections for γ+2b-jets to γ+b-jet as a function of pTγ is also presented. The results are based on the proton–antiproton collision data at s=1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The measured cross sections and their ratios are compared to the next-to-leading order perturbative QCD calculations as well as predictions based on the kT-factorization approach and those from the sherpa and pythia Monte Carlo event generators.

What have we learned about the quark–gluon plasma with the ATLAS detector at the LHC?

Recent results from lead–lead and proton–lead collisions, measured by the ATLAS experiment at the LHC, are presented. In lead–lead collisions, electroweak bosons are found to be produced proportionally to the number of binary nucleon–nucleon collisions, and to have rapidity distributions compatible with perturbative QCD calculations, suggesting no need for large nuclear PDF effects. Conversely, the large suppression of inclusive jets, the elliptic flow of hadrons at high p T and the direct measurements of jet v 2 support the need for a path-length-dependent energy loss in the hot, dense medium. Proton–lead measurements provide new insights into particle production in small, longitudinally asymmetric systems, but require further insights into the fluctuating nature of proton–proton collisions. The modification factors for charged hadrons show a non-trivial dependence on centrality and rapidity, with a “Cronin” peak appearing only in the most central events, and in the lead-going direction. Finally the measurements of inclusive jets in proton–lead show a striking scaling in the R CP suppression variable that is only a function of the jet momentum, while the suppression factor relative to PYTHIA jet cross sections shows an enhanced yield in peripheral events, and a suppressed yield in central events.

Measurement of the triple-differential cross section for photon + jets production in proton-proton collisions at $ \sqrt{s} $ = 7 TeV

A measurement of the triple-differential cross section (sigma as a function of the photon pt and eta and the jet eta) in photon + jets final states using a data sample from proton-proton collisions at sqrt(s) = 7 TeV is presented. This sample corresponds to an integrated luminosity of 2.14 inverse femtobarns collected by the CMS detector at the LHC. Photons and jets are reconstructed within a pseudorapidity range of abs(eta) < 2.5, and are required to have transverse momenta in the range 40 < pt(gamma) < 300 GeV and pt(jet) > 30 GeV, respectively. The measurements are compared to theoretical predictions from the SHERPA leading-order QCD Monte Carlo event generator and the next-to-leading-order perturbative QCD calculation from JETPHOX. The predictions are found to be consistent with the data over most of the examined kinematic region.

Open-charm production as a function of charged particle multiplicity in pp collisions at = 7 TeV with ALICE

Heavy quarks (charm and beauty) are an effective tool to investigate the properties of the Quark-Gluon Plasma created in heavy-ion collisions as they are produced in initial hard scattering processes and as they experience all the stages of the medium evolution. The measurement of heavy-flavour production cross sections in pp collisions at the LHC, besides providing a reference for heavy-ion studies, allows one to test perturbative QCD calculations. A brief review of ALICE results on the production of heavy-flavoured hadrons measured from fully reconstructed hadronic decay topologies in pp collisions at = 7 TeV is presented. Furthermore, heavy-flavour production was also studied as a function of the particle multiplicity in pp collisions. This could provide insight into multi-parton scatterings. A measurement of the inclusive J/ψ yield as a function of the charged-particle pseudorapidity density was performed by the ALICE Collaboration at the LHC in pp collisions at = 7 TeV. An increase of the J/ψ yield with increasing multiplicity was observed. In this context, the study of the yield of D mesons as a function of the charged-particle multiplicity could provide a deeper insight into charm-quark production in pp collisions. We will present the first results obtained for prompt D0, D+, and D*+ mesons using hadronic decay channels at midrapidity in pp collisions at = 7 TeV as a function of the charged-particle multiplicity. The prompt D-meson yields as a function of multiplicity are measured in different pT intervals. These yields will be compared to the results obtained for inclusive and non-prompt J/ψ.

Measurement of dijet cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector

Double-differential dijet cross sections measured in pp collisions at the LHC with a 7 TeV centre-of-mass energy are presented as functions of dijet mass and rapidity separation of the two highest-pT jets. These measurements are obtained using data corresponding to an integrated luminosity of 4.5/fb, recorded by the ATLAS detector in 2011. The data are corrected for detector effects so that cross sections are presented at the particle level. Cross sections are measured up to 5 TeV dijet mass using jets reconstructed with the anti-kt algorithm for values of the jet radius parameter of 0.4 and 0.6. The cross sections are compared with next-to-leading-order perturbative QCD calculations by NLOJET++ corrected to account for non-perturbative effects. Comparisons with POWHEG predictions, using a next-to-leading-order matrix element calculation interfaced to a parton-shower Monte Carlo simulation, are also shown. Electroweak effects are accounted for in both cases. The quantitative comparison of data and theoretical predictions obtained using various parameterizations of the parton distribution functions is performed using a frequentist method. An example setting a lower limit on the compositeness scale for a model of contact interactions is presented, showing that the unfolded results can be used to constrain contributions to dijet production beyond that predicted by the Standard Model.

Confronting current NLO parton fragmentation functions with inclusive charged-particle spectra at hadron colliders

The inclusive spectra of charged particles measured at high transverse momenta (pT≳2 GeV/c) in proton–proton and proton–antiproton collisions in the range of center-of-mass energies s=200–7000 GeV are compared with next-to-leading order perturbative QCD calculations using seven recent sets of parton-to-hadron fragmentation functions (FFs). Accounting for the uncertainties in the scale choices and in the parton distribution functions, we find that most of the theoretical predictions tend to overpredict the measured LHC and Tevatron cross sections by up to a factor of two. We identify the currently too-hard gluon-to-hadron FFs as the probable source of the problem, and justify the need to refit the FFs using the available LHC and Tevatron data in a region of transverse momenta, pT≳10 GeV/c, which is supposedly free from additional non-perturbative contributions and where the scale uncertainty is only modest.

Getting more flavor out of one-flavor QCD

We argue that no notion of flavor is necessary when performing amplitude calculations in perturbative QCD with massless quarks. We show this explicitly at tree level, using a flavor recursion relation to obtain multiflavored QCD from one-flavor QCD. The method relies on performing a color decomposition, under which the one-flavor primitive amplitudes have a structure which is restricted by planarity and cyclic ordering. An understanding of $SU(3{)}_{c}$ group theory relations between QCD primitive amplitudes and their organization around the concept of a Dyck tree is also necessary. The one-flavor primitive amplitudes are effectively $\mathcal{N}=1$ supersymmetric, and a simple consequence is that all of tree-level massless QCD can be obtained from Drummond and Henn's closed form solution to tree-level $\mathcal{N}=4$ super Yang-Mills theory.

Measurement of the inclusive isolated prompt photons cross section inppcollisions ats=7 TeVwith the ATLAS detector using4.6 fb−1

A measurement of the cross section for the production of isolated prompt photons in pp collisions at a center-of-mass energy s=7 TeV is presented. The results are based on an integrated luminosity of 4.6 fb−1 collected with the ATLAS detector at the LHC. The cross section is measured as a function of photon pseudorapidity ηγ and transverse energy ETγ in the kinematic range 100≤ETγ<1000 GeV and in the regions |ηγ|<1.37 and 1.52≤|ηγ|<2.37. The results are compared to leading-order parton-shower Monte Carlo models and next-to-leading-order perturbative QCD calculations. Next-to-leading-order perturbative QCD calculations agree well with the measured cross sections as a function of ETγ and ηγ.Received 7 November 2013DOI:https://doi.org/10.1103/PhysRevD.89.052004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2014 CERN, for the ATLAS Collaboration

Statistical issues in the parton distribution analysis of the Tevatron jet data

We analyse a tension between the D0 and CDF inclusive jet data and the perturbative QCD calculations, which are based on the ABKM09 and ABM11 parton distribution functions (PDFs) within the nuisance parameter framework. Particular attention is paid on the uncertainties in the nuisance parameters due to the data fluctuations and the PDF errors. We show that with account of these uncertainties the nuisance parameters do not demonstrate a statistically significant excess. A statistical bias of the estimator based on the nuisance parameters is also discussed.

Measurement of Associated Production ofZBosons with Charm Quark Jets inpp¯Collisions ats=1.96 TeV

We present the first measurements of the ratios of cross sections σ(pp → Z+c jet)/σ(pp → Z+jet) and σ(pp → Z+c jet)/σ(pp → Z+b jet) for the associated production of a Z boson with at least one charm or bottom quark jet. Jets have transverse momentum ​pT(jet)​>20 GeV and pseudorapidity |​η(jet)​|<2.5. These cross section ratios are measured differentially as a function of jet and Z boson transverse momenta, based on 9.7 fb(-1) of pp collisions collected with the D0 detector at the Fermilab Tevatron Collider at √s=1.96 TeV. The measurements show significant deviations from perturbative QCD calculations and predictions from various event generators.

Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions ats=200 GeV

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8<η<2.0 in polarized proton-proton collisions at s=200 GeV are presented. Neutral pions were detected using the end cap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5<pT<16 GeV/c and is found to agree with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry ALL is measured in the same pseudorapidity range and spans a range of Bjorken-x down to x≈0.01. The measured ALL is consistent with model predictions for varying degrees of gluon polarization. The parity-violating asymmetry AL is also measured and found to be consistent with zero. The transverse single-spin asymmetry AN is measured over a previously unexplored kinematic range in Feynman-x and pT. Such measurements may aid our understanding of the onset and kinematic dependence of the large asymmetries observed at more forward pseudorapidity (η≈3) and their underlying mechanisms. The AN results presented are consistent with a twist-3 model prediction of a small asymmetry over the present kinematic range.2 MoreReceived 6 September 2013DOI:https://doi.org/10.1103/PhysRevD.89.012001© 2014 American Physical Society

Open heavy-flavour measurements in pp and Pb-Pb collisions with ALICE at the LHC

We present an overview of measurements related to open heavy-flavour pro- duction with the ALICE experiment at the LHC. Studies are performed using single lep- tons (electrons at mid-rapidity and muons at forward-rapidity) and D mesons, which are reconstructed via their hadronic decay channels. The measured differential production cross sections in proton-proton collisions at √ s = 2.76 and 7 TeV are in agreement with perturbative QCD calculations. Results from Pb-Pb collisions at √ sNN = 2.76 TeV on the nuclear modification factor RAA are shown, along with the elliptic flow ν2.

Measurement of the differential and double-differential Drell-Yan cross sections in proton-proton collisions at $ \sqrt{s} $ = 7 TeV

Abstract Measurements of the differential and double-differential Drell-Yan cross sections are presented using an integrated luminosity of 4.5 (4.8) fb−1 in the dimuon (dielectron) channel of proton-proton collision data recorded with the CMS detector at the LHC at $ \sqrt{s} $ = 7 TeV. The measured inclusive cross section in the Z-peak region (60–120 GeV) is σ(ℓℓ) = 986.4 ± 0.6 (stat.) ± 5.9 (exp. syst.) ± 21.7 (th. syst.) ± 21.7 (lum.) pb for the combination of the dimuon and dielectron channels. Differential cross sections dσ/dm for the dimuon, dielectron, and combined channels are measured in the mass range 15 to 1500 GeV and corrected to the full phase space. Results are also presented for the measurement of the double-differential cross section d2σ/dm d|y| in the dimuon channel over the mass range 20 to 1500 GeV and absolute dimuon rapidity from 0 to 2.4. These measurements are compared to the predictions of perturbative QCD calculations at next-to-leading and next-to-next-to-leading orders using various sets of parton distribution functions.

Challenges for QCD theory: some personal reflections

At the LHC all processes are QCD ones, whether ‘signal’ or ‘background’. In this review the frontiers of current QCD research are addressed, towards increased understanding, improved calculational precision, and role in potential future discoveries. Issues raised include: the limits of perturbative QCD calculations and parton distribution usage, the nature of multiparton interactions, the impact of colour reconnection on physical observables, the need for progress on hadronization modelling, the improvements of parton showers and their combination with the matrix-element description, the use of QCD concepts in Beyond-the-Standard-Model scenarios and the key position of event generators and other software in the successful exploration of LHC physics. On the way, several questions are posed, where further studies are needed.

Isolated Photons + Jets in DIS and Photoproduction at ZEUS

In this report, recent measurements of the production of isolated photons accompanied by jets in DIS and photo-production at HERA are presented. The measurements are compared to the perturbative QCD calculations and to the predictions made within the kT-factorisation QCD approach. A reasonable level of agreement between data and the predictions of both types is observed, however there is still room for the theories to be improved.