Heavy Flavour Results from ATLAS

This book describes the underlying ideas and modern developments of Regge theory, confronting the theory with quantum chromodynamics and a huge variety of experimental data. It covers forty years of research and provides a unique insight into the theory and its phenomenological development. The authors review experiments that suggest the existence of a soft pomeron, and give a detailed discussion of attempts at describing this through nonperturbative quantum chromodynamics. They suggest that a second, hard pomeron is responsible for the dramatic rise in energy observed in deep inelastic lepton scattering. The two-pomeron hypothesis is applied to a variety of interactions and is compared and contrasted with perturbative quantum chromodynamics, as well as with the dipole approach. This book will provide a valuable reference for experimental particle physicists all over the world. It is also suitable for graduate courses in particle physics, high-energy scattering, QCD and the standard model.

We review the present understanding of the spin structure of protons and neutrons, the fundamental building blocks of nuclei collectively known as nucleons. The field of nucleon spin provides a critical window for testing Quantum Chromodynamics (QCD), the gauge theory of the strong interactions, since it involves fundamental aspects of hadron structure which can be probed in detail in experiments, particularly deep inelastic lepton scattering on polarized targets.QCD was initially probed in high energy deep inelastic lepton scattering with unpolarized beams and targets. With time, interest shifted from testing perturbative QCD to illuminating the nucleon structure itself. In fact, the spin degrees of freedom of hadrons provide an essential and detailed verification of both perturbative and nonperturbative QCD dynamics.Nucleon spin was initially thought of coming mostly from the spin of its quark constituents, based on intuition from the parton model. However, the first experiments showed that this expectation was incorrect. It is now clear that nucleon physics is much more complex, involving quark orbital angular momenta as well as gluonic and sea quark contributions. Thus, the nucleon spin structure remains a most active aspect of QCD research, involving important advances such as the developments of generalized parton distributions (GPD) and transverse momentum distributions (TMD).Elastic and inelastic lepton-proton scattering, as well as photoabsorption experiments provide various ways to investigate non-perturbative QCD. Fundamental sum rules—such as the Bjorken sum rule for polarized photoabsorption on polarized nucleons—are also in the non-perturbative domain. This realization triggered a vigorous program to link the low energy effective hadronic description of the strong interactions to fundamental quarks and gluon degrees of freedom of QCD. This has also led to advances in lattice gauge theory simulations of QCD and to the development of holographic QCD ideas based on the AdS/CFT or gauge/gravity correspondence, a novel approach providing a well-founded semiclassical approximation to QCD. Any QCD-based model of the nucleon’s spin and dynamics must also successfully account for the observed spectroscopy of hadrons. Analytic calculations of the hadron spectrum, a long sought goal of QCD research, have now being realized using light-front holography and superconformal quantum mechanics, a formalism consistent with the results from nucleon spin studies.We begin this review with a phenomenological description of nucleon structure in general and of its spin structure in particular, aimed to engage non-specialist readers. Next, we discuss the nucleon spin structure at high energy, including topics such as Dirac’s front form and light-front quantization which provide a frame-independent, relativistic description of hadron structure and dynamics, the derivation of spin sum rules, and a direct connection to the QCD Lagrangian. We then discuss experimental and theoretical advances in the nonperturbative domain—in particular the development of light-front holographic QCD and superconformal quantum mechanics, their predictions for the spin content of nucleons, the computation of PDFs and of hadron masses.

Most of our visible universe is made up of hadronic matter. Quantum Chromodynamics (QCD) is the theory of strong interaction that describes the hadronic matter. However, QCD predicts that at high enough temperatures and/or densities ordinary hadronic matter ceases to exist and a new form of matter is created, the so-called Quark Gluon Plasma (QGP). Non-perturbative lattice QCD simulations shows that for high temperature and small densities the transition from the hadronic to the QCD matter is not an actual phase transition, rather it takes place via a rapid crossover. On the other hand, it is generally believed that at zero temperature and high densities such a transition is an actual first order phase transition. Thus, in the temperature-density phase diagram of QCD, the first order phase transition line emanating from the zero temperature high density region ends at some higher temperature where the transition becomes a crossover. The point at which the first order transition line turns into a crossover is a second order phase transition point belonging to three dimensional Ising universality class. This point is known as the QCD Critical End Point (CEP). For the last couple of years the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has been performing experiments at lower energies in search of the elusive QCD CEP. In general critical behaviors are manifested through appearance of long range correlations and increasing fluctuations associated with the presence of mass-less modes in the vicinity of a second order phase transition. Experimental signatures of the CEP are likely to be found in observables related to fluctuations and correlations. Thus, one of the major focuses of the RHIC low energy scan program is to measure various experimental observables connected to fluctuations and correlations. On the other hand, with the start of the RHIC low energy scan program, a flurry of activities are taking place to provide solid theoretical background for the search of the CEP using observables related to fluctuations and correlations. While new data are pouring in from the RHIC low energy scan program, many recent advances have also been made in the phenomenological and lattice gauge theory sides in order to have a better theoretical understanding of the wealth of new data. This workshop tried to create a synergy between the experimental, phenomenological and lattice QCD aspects of the fluctuation and correlation related studies of the RHIC low energy scan program. The workshop brought together all the leading experts from related fields under the same forum to share new ideas among themselves in order to streamline the continuing search of CEP in the RHIC low energy scan program.

The gold-plated theoretical predictions for polarized deep inelastic lepton-nucleon scattering will be discussed within the framework of Quantum Chromodynamics (QCD). Experimental data from the Stanford accelerator will be compared with theoretical predictions. The spin distribution inside the nucleon will be described in parton language. A very brief discussion of recent developments in non-perturbative QCD is included as a prelude to serious theoretical spin physics.

Quantum chromodynamics (QCD) predicts the existence of both nonperturbative intrinsic and perturbative extrinsic heavy quark contributions to the fundamental structure of hadrons. The existence of intrinsic charm at the 3-standard-deviation level in the proton has recently been established from structure function measurements by the NNPDF Collaboration. Here we revisit the physics of intrinsic heavy quarks using light-front holographic QCD (LFHQCD) - a novel comprehensive approach to hadron structure which provides detailed predictions for dynamical properties of the hadrons, such as form factors, distribution amplitudes, structure functions, etc. We will extend this nonperturbative light-front QCD approach to study the heavy quark-antiquark contribution to the electromagnetic properties of nucleon. Our framework is based on a study of the eigenfunctions of the QCD light-front Hamiltonian, the frame-independent light-front wave functions (LFWFs) underlying hadron dynamics. We analyze the heavy quark content in the proton, induced either directly by the nonperturbative $|uud+Q\bar Q\rangle$ Fock state or by the $|uud+g\rangle$ Fock state, where the gluon splits into a heavy quark-antiquark pair. The specific form of these LFWFs are derived from LFHQCD. Using these LFWFs, we construct light-front representations for the heavy quark-antiquark asymmetry, the electromagnetic form factors of nucleons induced by heavy quarks, including their magnetic moments and radii.

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.

Selected recent quantum chromodynamics (QCD) measurements are reviewed for Fermilab Run II Tevatron proton-antiproton collisions studied by the Collider Detector at Fermilab (CDF) and D0 Collaborations at a centre-of-mass energy of {radical}s = 1.96 TeV. Tantamount to Rutherford scattering studies at the TeV scale, inclusive jet and dijet production cross-section measurements are used to seek and constrain new particle physics phenomena, test perturbative QCD calculations, inform parton distribution function (PDF) determinations, and extract a precise value of the strong coupling constant, a{sub s}(m{sub Z}) = 0.1161{sub -0.0048}{sup +0.0041}. Inclusive photon production cross-section measurements reveal an inability of next-to-leading-order (NLO) perturbative QCD (pQCD) calculations to describe low-energy photons arising directly in the hard scatter. Events with {gamma} + 3-jet configurations are used to measure the increasingly important double parton scattering (DPS) phenomenon, with an obtained effective interaction cross section of {sigma}{sub eff} = 16.4 {+-} 2.3 mb. Observations of central exclusive particle production demonstrate the viability of observing the Standard Model Higgs boson using similar techniques at the Large Hadron Collider (LHC). Three areas of inquiry into lower energy QCD, crucial to understanding high-energy collider phenomena, are discussed: the examination of intra-jet track kinematics to infer that jet formation is dominatedmore » by pQCD, and not hadronization, effects; detailed studies of the underlying event and its universality; and inclusive minimum-bias charged-particle momentum and multiplicity measurements, which are shown to challenge the Monte Carlo generators.« less

Quantum Chromo Dynamics (QCD) provides the theoretical framework for any study of TeV scale physics at LHC. Being familiar with the basic concepts and techniques of QCD is therefore a must for any high-energy physicist. In these notes we consider Higgs production via gluon fusion as an example on how accurate and flexible predictions can be obtained in perturbative QCD. We start by illustrating how to calculate the total cross section at the leading order (yet one loop) in the strong coupling α S and go through the details of the nextto-leading order calculation eventually highlighting the limitations of fixedorder predictions at the parton level. Finally, we briefly discuss how more exclusive (and practical) predictions can be obtained through matching/merging fixed-order results with parton showers.

iHixs 2 — Inclusive Higgs cross sections

Using the quantum chromodynamics (QCD) equation of state (EoS) from lattice\ncalculations we investigate effects from QCD on primordial gravitational waves\n(PGWs) produced during the inflationary era. We also consider different cases\nfor vanishing and nonvanishing lepton asymmetry where the latter one is\nconstrained by cosmic microwave background experiments. Our results show that\nthere is up to a few percent deviation in the predicted gravitational wave\nbackground in the frequency range around the QCD transition ($10^{-10}-\n10^{-7}$~Hz) for different lattice QCD EoSs, or at larger frequencies for\nnonvanishing lepton asymmetry using perturbative QCD. Future gravitational wave\nexperiments with high enough sensitivity in the measurement of the amplitude of\nPGWs like SKA, EPTA, DECIGO and LISA can probe these differences and can shed\nlight on the real nature of the cosmic QCD transition and the existence of a\nnonvanishing lepton asymmetry in the early universe.\n

In light of the large quantity of data collected during the second operational run of the Large Hadron Collider (LHC) at CERN, which has made it possible to perform measurements at unprecedented energies with a high degree of statistical precision, the necessity of understanding and constraining the systematic effects on such measurements has become increasingly important. Precision measurements of jet observables in proton-induced collisions have proven instrumental in constraining the parton distribution functions (PDFs) which describe the internal structure of protons, and which remain one of the largest sources of uncertainty in many analyses performed at hadron colliders. This thesis presents the first triple-differential measurement of the dijet production cross section performed in proton-proton collisions at a center-of-mass energy of 13 TeV, based on a data sample of 35.9 fb$^{-1}$ recorded by the CMS experiment at the Large Hadron Collider at CERN. The cross section is measured using anti-$k_\text{T}$ jets with radius parameters of R = 0.4 and R = 0.8 as a function of the dijet rapidity separation $y^{*}$ , the total boost of the dijet system $y_\text{b}$ , and either the average transverse momentum $\langle p_\text{T}\rangle_{1,2}$ or the invariant mass $m_\text{jj}$ of the dijet system as the third variable. This choice of rapidity variables exploits the topology of the dijet system to achieve an increased sensitivity to the proton PDFs. After accounting for detector-induced systematic effects in a three-dimensional unfolding procedure, the measured spectra are compared to fixed-order theory predictions at next-to-next-to-leading order accuracy in perturbative quantum chromodynamics, obtained using several recent PDF sets. While the data are observed to be described by the theory within the experimental and theoretical uncertainties across a large portion of the phase space, potentially significant deviations are observed in areas where a heightened sensitivity to the PDFs is expected.

We will discuss applications of QCD using perturbation theory to baryons1,2 and to the baryon resonances. The subjects that nuclear physicists study are changing and Quantum Chromodynamics (QCD) is presumably the correct theory for the strong interactions, so that this is a suitable subject for this institute. At the same time, we should remember that many of the consequences of QCD, and all the ones to be mentioned in these lectures, are derived using perturbation theory and so are valid only at higher momentum transfers, perhaps at the upper reaches of what will be got at CEBAF. There are many motivations for pursuing these studies. The predictions are testable and this is always an encouragement to the experimenter. We can see what limits traditional nuclear physics must approach as the kinematics becomes more extreme. We may get some idea where the boundary lies between the “strong interaction region of QCD” (i.e., traditional nuclear modeling) and the “asymptotically free” region of QCD.

RHIC experiments carry out a comprehensive physics program which studies open heavy flavor and quarkonium production in relativistic heavy-ion collisions. The discovery at RHIC of large high-pT suppression and flow of electrons from heavy quarks flavors have altered our view of the hot and dense matter formed in central Au + Au collisions at SNN=200 GeV. These results suggest a large energy loss and flow of heavy quarks in the hot, dense matter. In recent years, the RHIC experiments upgraded the detectors; (1) PHENIX Collaboration installed silicon vertex tracker (VTX) at mid-rapidity region and forward silicon vertex tracker (FVTX) at the forward rapidity region, and (2) STAR Collaboration installed the heavy flavor tracker (HFT) and the muon telescope detector (MTD) both at the mid-rapidity region. With these new upgrades, both experiments have collected large data samples. These new detectors enhance the capability of heavy flavor measurements via precision tracking. The PHENIX experiments established measurements of ψ (1S ) and ψ (2S ) production as a function of system size, p + p , p + Al, p + Au, and 3 He + Au collisions at SNN=200 GeV. In p/3 He + A collisions at forward rapidity, we observe no difference in the ψ (2S )/ψ (1S ) ratio relative to p + p collisions. At backward rapidity, where the comoving particle density is higher, we find that the ψ (2S ) is preferentially suppressed by a factor of two. STAR Collaboration presents the first J/ψ and ϒ measurements in the di-muon decay channel in Au + Au collisions at SNN=200 GeV at mid-rapidity at RHIC. We observe clear J/ψ RAA suppression and qualitatively well described by transport models simultaneously accounting for dissociation and regeneration processes.

Study of correlations and fluctuations in relativistic nuclear collisions addresses fundamental aspects of quantum chromodynamics (QCD) and the properties of strongly-interacting matter at extreme density and temperature. Correlations and fluctuations reveal the nature of QCD, first through the structure of elementary collisions and then through the properties and dynamics of the colored medium produced in heavy ion (HI) collisions. Five years after first RHIC data we are experiencing a paradigm shift: from two-state indicators that the QCD phase boundary has been traversed to quantitative study of the structure of the QCD medium in the temperature interval Tc– 3Tc above the boundary. The expected role of correlations and fluctuations has changed, and has increased in importance relative to single-particle measures.In this workshop we have reviewed correlation and fluctuation measurements in the context of our current theoretical understanding of nuclear collisions and have explored the connections among different measurement approaches. Three themes have emerged, in descending order of correlated-particle transverse momentum pt: high-pt 'triggered' jet correlations and recombination (most interesting at intermediate pt but based on perturbative QCD concepts), fluctuations and correlations which focus on structure at lower pt but are nevertheless dominated by (low-Q2) parton fragment correlations, and critical fluctuations and equilibration which emphasize the long-time and large-scale behavior of the bulk QCD medium. Correlation measurements reveal that RHIC collisions are complex; local structure appears to be dominated by low-Q2 parton fragmentation. High-pt correlations probe the QCD medium at larger scales and shorter times. Provocative phenomena appear at SPS energies where quieter circumstances offer the possibility to observe significant critical fluctuations. New techniques provide unification of high-pt jet correlations with lower-pt fluctuation measurements, revealing a deeper picture of parton fragmentation over a broad range of Q2 down to the 'saturation' scale. Recombination theory, emphasizing intermediate-pt hadronization, is providing the first theoretical attempts to describe two-particle correlations from parton fragmentation.In this workshop we have begun to establish a common conceptual framework permitting the closest possible connection between experimental correlation observables and the physics of the nuclear collision process. We wish to span the entire conceptual interval between perturbative QCD and the statistical model through study of correlations and fluctuations, to understand nonperturbative hadronization and its effect on the transfer of information from the structured prehadronic medium to the hadronic final state. Guidance comes from recent astrophysical measurements in which correlation studies of the contemporary cosmic microwave background reveal the dynamics of the early universe.We are thankful for generous financial support from Brookhaven National Laboratory and the Massachusetts Institute of Technology. We especially thank Timothy Hallman of BNL for suggesting and encouraging this workshop. The organizers warmly thank the local MIT students and Anna Maria Convertino for making this workshop a very positive experience for all in attendance.

We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distribution functions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC. We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of FL), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly present proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques. The physics of parton distributions, especially within the context of deep-inelastic scattering (DIS), has been an active subject of detailed theoretical and experimental investigations since the origins of perturbative quantum chromodynamics (QCD), which, thanks to asymptotic freedom, allows one to determine perturbatively their scale dependence [1–5]. Since the advent of HERA, much progress has been made in determining the Parton Distribution Functions (PDFs) of the proton. A good knowledge of the PDFs is vital in order to make predictions for both Standard Model and beyond the Standard Model processes at hadronic colliders, specifically the LHC. Furthermore, PDFs must be known as precisely as possible in order to maximize the discovery potential for new physics at the LHC. Conversely, LHC data will lead to an improvement in the knowledge of PDFs. The main aim of this document is to provide a state-of-the art assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions can be extracted from data, in particular current and forthcoming HERA data. In Ref. [6] we set the stage by providing an overview of relevant LHC processes and a discussion of their experimental and theoretical accuracy. In Ref. [7] we turn to the experimental determination of PDFs, and in particular examine the improvements to be expected from forthcoming measurements at HERA, as well as from analysis methods which allow one to combine HERA data with each other, and also with data from existing (Tevatron) and forthcoming (LHC) hadron colliders. In Ref. [8] we discuss the state of the art in the extraction of parton distributions of the data by first reviewing recent progress in higher-order QCD corrections and their impact on the extraction of PDFs, and then discussing and comparing the determination of PDFs from global fits. Finally, in Ref. [9] we summarize the current status of resummed QCD computations which are not yet used in parton fits, but could lead to an improvement in the theoretical precision of PDF determinations. In addition to summarizing the state of the art, we also provide several new results, benchmarks and predictions obtained within the framework of the HERA–LHC workshop. 2 M. DITTMAR, S. FORTE, A. GLAZOV, S. MOCH, S. ALEKHIN, G. ALTARELLI, . . .