QCD with Jets and Heavy Flavour in pp and PbPb Collisions in ATLAS

ATLAS has studied different aspects of QCD in pp and PbPb collisions. A summary of interesting recent results at centre-of-mass energies of 7 TeV, 8 TeV and 13 TeV (pp), and 2.76 TeV (PbPb) per nucleon pair is presented. Measurements using pp collisions of Z, W, photon, quarkonia and open charm production differential cross sections in a variety of variables are presented. Jet and heavy flavour muon measurements in PbPb collisions, aimed to test the properties of the Quark Gluon Plasma with the view of better understanding of jet quenching, are also presented.


Introduction
The Large Hadron Collider (LHC) is the energy frontier of high energy particle physics, providing the highest energy and highest luminosity environment for proton-proton (pp) and heavy ion (HI) collisions to date. ATLAS is a multipurpose particle detector with a broad physics program that makes use of precision tracking, calorimetry and muon detectors [1]. Signals arising from collisions of coloured constituents of the matter are dominated by Quantum Chromodynamic (QCD) processes. Measuring broad aspects of QCD production provides constraints on QCD theoretical predictions as well as requisites for the empirical description of the proton and its constituents. Finally, having an accurate description of the QCD background of non-QCD measurements, such as Higgs measurements or searches beyond the Standard Model, is critical to their success. measurements display a good agreement with predictions from NNLO QCD and NLO+EW corrections. The electron and muon channels measured separately in each case are found to be consistent. Figure 1 (left), displaying the ratio of predicted to measured cross sections shows tension between different PDFs. In Figure 1 (right), the W + to W − cross section ratio enhances this tension due to cancellations of correlated measurement uncertainties.

Inclusive Z p T at 8 TeV
The measurement of the inclusive Z boson p T spectrum at 8 TeV provides a comprehensive test of QCD predictions spanning the non-perturbative regime at low p T to the perturbative regime at high p T [3]. W and Z bosons constitute a significant background in many analyses and previous simulations of these backgrounds have required data-driven corrections [4,5]. The measurement is conducted by selecting dilepton final states, a clean signature that benefits from a precise p T determination and low background. Measurements were compared to the predictions from Pythia and Sherpa, as shown in Figure 2 (left), both displaying a poor agreement in the high p T tail. Comparisons to a higher order pQCD computation DYNNLO yielded a better description of the shape but with a constant offset, as seen in Figure 2 (right).

Inclusive photon E T at 8 TeV
Prompt photons originating from hard scattering are a clean probe of the parton level dynamics and of the gluon PDF. Arising predominantly through Compton process like qg → qγ scattering, prompt photon signatures are targeted experimentally by requiring that the photon is isolated, i.e. well separated from other calorimetric signatures. Events simulated using Pythia and CTEQ6L1 PDF are used to correct for detector effects and signal leakage in order to extract the central value of the measurements [6]. A significant improvement in experimental uncertainties, compared to previous results, is obtained thanks to improved photon calibrations [7]. The result is compared to theory predictions at NLO from JetPhox using various PDF sets, and at NLO+NNNLL (which is comparable to NNLO) from PeTeR, as seen in Figure 3. Comparisons yield a good shape agreement and a better normalization with PeTeR. This measurement provides an important constraint on the global fit of the gluon PDF. 3 Heavy flavour production measurements

J/ψ and ψ(2S ) at 7 and 8 TeV
Heavy flavour QCD bound states (quarkonia) probe the boundary of perturbative and non-perturbative QCD. Charmonium production can arise through "prompt" short lived sources or through "nonprompt" long lived sources such as the decay of a beauty hadron. The ψ(2S ) state is of interest because it is the only vector charmonium state that has mostly direct production. The measurements are performed by selecting di-muon decay events, a final state that exhibits efficient triggering and clean reconstruction [8]. The pseudo-proper time τ observable measures the displacement of the hadron before its di-muon decay, as seen in Figure 4 (left). A two dimensional maximum likelihood fit is performed over the invariant mass and τ, the former discriminating signal from background by isolating the bound state resonance, the latter separating the prompt from the non-prompt components.
The results of these cross section measurements agree well with results from other LHC experiment measurements. The comparison of the data to NLO NRQCD predictions for prompt production agree well as seen in Figure 4 (middle), the comparison to FONLL predictions for non-prompt production shows that the predictions slightly overestimate the data at high p T , as seen in Figure 4 (right).

Prompt and non-prompt J/ψ at 13 TeV
During the long shut down between Run 1 and Run 2 ATLAS installed additional layers of silicon pixel tracking by virtue of shrinking the beam pipe. This new "Inner B Layer" (IBL) provides enhanced vertexing and heavy flavour tagging capabilities in anticipation of more challenging pile-up conditions for Run 2. The ratio of prompt to non-prompt J/ψ production was measured at 13 TeV using a small preliminary dataset [9] and found to be consistent with the Run 1 results in all pseudorapidity regions, providing a promising first look of ATLAS heavy flavour performance in Run 2.

Open charm production at 7 TeV
D mesons arising in the fragmentation of c and b quarks provide a test of pQCD heavy flavour production, of heavy flavour content of the proton, and serve as an important constraint for heavy flavour backgrounds for Higgs measurements. At LHC the charm cross section is large cross and a clean signatures is provided by the precise tracking and vertexing capability of ATLAS [10]. Comparing the results to various predictions, we see that they typically underestimate the data, with FONLL and POWHEG showing the best shape agreement, as shown in Figure 5. The total measured cross section is consistent with those reported by ALICE [11]. The strangeness suppression factor and the charged non-strange vector D-meson fraction are consistent with previous results from ALICE [12].

Heavy ion measurements
Heavy Ion (HI) collisions create a hot dense medium of strong nuclear matter with deconfined color charges referred to as the Quark Gluon Plamsa (QGP). This new testing ground for QCD bears an important correspondence to the state and evolution of the early universe. Jet quenching, a recently discovered QGP phenomena, refers to jets interacting with the QGP and becoming more diffuse.
Measurements are compared to analogous measurements derived from pp collisions in place of HI simulations. HI collisions are parametrized in terms of the Centrality observable. Centrality is measured in ATLAS using the total energy collected in the forward calorimeters: the more central or head-on the collision, the more interacting nucleons there are, and in turn, the more outgoing particles and energy deposited [13].

Dijet p T correlations in PbPb at 2.76 TeV
Dijets provide a convenient probe of the QGP since in pp collisions they are typically well balanced. The dijet asymmetry, , was measured to quantify the effect of jet quenching as a function of jet p T and centrality [14]. The x J distribution was found to strongly deviate from the nominal distribution observed in pp collisions with decreasing jet p T and decreasing collision centrality, as shown in Figure  6 (left). The low p T and high centrality region show the strongest asymmetry with a peak at x J = 0.5.

Internal jet structure in PbPb at 2.76 TeV
The internal structure of jets in PbPb collisions is measured to provide constraints on jet quenching models and in-medium modifications for parton shower models [15]. Charged particles with p T down to 1 GeV falling inside a jet are measured, correcting for tracking inefficiencies. The resulting jet fragmentation functions are reported as a function of longitudinal and transverse charged particle p T , with respect to the jet axis, in different regions of centrality, jet η and jet p T . In comparison to pp collision measurements, as shown in Figure 6 (right), an enhancement is observed at low and high charged particle p T and a suppression in the 4 < p T < 25 GeV range. The enhancement at high p T diminishes with increasing jet p T and with increasing jet η, and, as would be expected, all the effects diminish with decreasing centrality.

Heavy flavour muons in PbPb
Heavy flavour signals are produced perturbatively in the hard scatter (as their masses are much greater than the temperature of the medium) and subsequently interact with the medium [16]. Heavy flavour muons are identified using an energy loss variable that takes the difference between the p T measured by the inner detector and by the muon system. The yields show a centrality dependent suppression that is slightly more significant than that measured by the inclusive jet HI result [17]. Following its creation the QGP expands and cools. By measuring the distribution of the heavy flavour muons in azimuthal angle φ, the azimuthal anisotropy, relevant to elliptical flow, is extracted by fitting a 2 nd order Fourier coefficient, v 2 . v 2 is measured to be up to 8% at low p T and the overall variation is consistent with previous measurements [18]. Overall the findings are consistent with, and much more precise than, measurements performed by ALICE in the forward region (2.5 < y < 4.0) [19].