High Jet Multiplicity Physics at the LHC

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EXPI Gotschuchen. Welios Science Center. Offene Stellen Konferenzen Veranstaltungen. DI Dr. Conversely, at short distances the coupling becomes small, and quarks exhibit asymptotic freedom , behaving as quasi-free particles. The large momentum transfers present in so-called "hard scattering" at high-energy colliders make such short distances accessible, and in regions where the coupling is much less than one, perturbative techniques may be used to make calculations that may be compared to the measurements.

When scattered at such high energies, quarks and gluons give rise to collimated sprays of particles known as jets. It is these properties that have made rigorous investigation of QCD challenging, but possible, at such colliders. This article describes key collider results relevant to QCD, starting with those processes with the largest cross sections and moving to smaller cross sections.

The internal structure of hadrons, the production and features of hadronic jets, the production of heavy quarks and of electroweak bosons will be addressed along the way. The rise in hadronic cross sections is understood within scattering theory as being due to a virtual exchange of vacuum quantum numbers, known within Regge theory as a Pomeron.

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Pomeron exchange is eikonalised that is, copied — multiple Pomeron exchanges are included at very high energies, avoiding the violation of unitarity. The component of the cross section mediated by Pomeron exchange is commonly referred to as the diffractive component. This may be elastic or quasi-elastic - that is, one or both hadrons may disassociate and fragment. However, the fact that vacuum quantum numbers that is, no charge or color are exchanged leads to the presence of regions in rapidity that are unpopulated by particles, even in the cases where the hadrons have fragmented.

Particle production in processes in which color is exchanged populate rapidity uniformly, leading to the exponential suppression of rapidity gaps. This will be discussed in subsequent sections. Hadrons are extended objects consisting of quarks, bound together by gluons. Quarks and gluons are collectively referred to as partons, and the distribution of partons, and their momenta, inside the hadron is a critical factor in physics at hadron colliders.

Thus, knowledge of the PDFs is needed in order to predict cross sections, and conversely, precise measurements of cross sections can be used to constrain PDFs. However, the collinear radiation of gluons from quarks and the splitting of gluons into quark-antiquark pairs at high scales is predicted in QCD and described by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations. The interdependence thus introduced allows measurements of quark densities to be translated into information on the gluon content of the proton, principally via scaling violations.

Further constraints on PDFs come from:.

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Events tagged this way can be used to extract so-called diffractive PDFs of the proton. This gives information on the QCD structure of the colour singlet Pomeron exchange discussed in the previous section.

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Monaco , M. Lazzizzera , M. Lehti , T. Zito , G. All rights reserved.

However, although diffractive processes are also measured at hadron-hadron colliders, the applicability of such diffractive PDFs is complicated by the presence of secondary interactions between the proton remnants - i. Soft QCD forms the final stage of any high-energy collision involving strongly interacting particles in the initial or final state. At lepton colliders, precise measurements of particle multiplicities in hadronic final states provide stringent constraints on models for hadronization.

These models then form the starting point for understanding particle production in the more complex environment of hadronic collisions. The vast majority of hadron collisions can be characterised as peripheral scatters between hadrons in which no particular hard energy scale exists.

Such events give no insight into short-distance physics, but they can shed light on the process of hadronization.

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They also need to be measured and understood, at least phenomenologically, because they form a background to all other studies at hadron colliders, often appearing in conjunction with events containing a harder scatter in the same, or adjacent, time window of the experiment an effect known as 'pile up'. These events are often called 'minimum bias', since they generally pass even the loosest selection criteria of the experiment.

Studying short and long-range correlations in such events constrains models of particle-production implemented in Monte Carlo simulations. As well as being useful input for other physics studies at colliders, such modelling is useful for comparisons with studies of quark-gluon matter in heavy-ion collisions, and for understanding high-energy cosmic ray air showers. Measurements of identical-particle production have shown evidence for Bose-Einstein correlations, which are generally insensitive to the short-distance hard scattering and can give information on the radial distance from the collision at which hadronization occurs.

At the mid-to-high transverse momentum end of the 'minimum-bias' spectrum, scatters involve momentum transfers of a few GeV, sufficient to resolve partons inside the hadrons. A naive convolution of the parton densities with the cross sections for parton-parton scattering shows that at high hadron-hadron centre-of-mass energies and low parton-parton centre-of-mass energy the parton-parton cross section becomes bigger than the hadron-hadron cross section.

This implies that each hadron-hadron collision may contain more than one parton-parton scatter, a phenomenon known as multiple parton interactions. These additional partonic scatters may also accompany a much harder parton scattering processes, in which case they form part of the so-called 'underlying event'. Both hadron-hadron collisions and lepton-hadron collisions at high energy must be understood in terms of partons, the constituents of those hadrons. In the parton model, a typical hadron collision will consist of an initial "hard", high momentum-transfer scattering involving one constituent from each initial hadron there can be more then one of these scatters in the case of multiple parton interactions discussed above.

In the case of a pure QCD interaction, this hard scatter may produce two or more outgoing high-energy partons, which may then radiate further gluons, and any gluons may split into further quark anti-quark pairs. As the strong coupling constant increases at lower scales, this process repeats more frequently at lower and lower energies, producing additional particles at smaller and smaller angles to the initial parton directon.

Eventually, the partons reach low enough energies that they are confined into hadrons hadronization. As a result of this process, the influence of the initial high energy partons is strongly seen in the eventual distribution of much lower energy final-state particles.

This means that careful measurement of the distribution of final-state particles can rather directly yield information about the short distance physics occurring in a collision. The most common method for mapping final-state particle measurements to partonic physics is to use algorithmically defined event-shape variables, designed to be insensitive to soft, long distance physics but to preserve those features of an event which reflect the momenta of high energy quarks and gluons.

Global variables such as thrust or sphericity have been used, especially in leptonic collisions. However, the dominance of soft QCD radiation at angles close to the direction of an initial parton direction motivates the most commonly-used event-shape concept: a "jet", or a more-or-less collimated spray of particles and energy. Each of these sprays of particles can be combined into a composite object the "jet" , whose kinematic properties reflect those of an initiating unmeasurable parton.

The reconstruction of jets from the hadrons produced in a typical collision is carried out using a recombination or geometric jet algorithm , and any such algorithm must meet a number of requirements. Formally, to enable them to be implemented to arbitrary order in QCD calculation, jet algorithms must be insensitive to very soft QCD radiation and to collinear splitting of partons infrared and collinear safety producing additional soft particles.

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This book describes research in two different areas of state-of-the-art hadron collider physics, both of which are of central importance in the field of particle. The Large Hadron Collider at CERN completed its first data-taking phase in , after three years of remarkable performance.

Important experimental properties include speed and geometrical regularity, as well as a well-defined relationship between the reconstructed jet and input particles, allowing the resulting jets to be calibrated. Accurate measurement of jets presents particular l challenges for detector design, principally because in general the response of a detector to hadrons will differ from that to photons, and jets typically contain a statistically-fluctuating sample of both, since photons are produced in the decay of neutral pions.

Many jet algorithms have been proposed and used in experiments, and take as input typically a list of energy or momentum measurements from a particle detector, or a list of simulated or reconstructed final-state particles. Successive elements in this list are the compared, and if they meet a given criteria, are merged; this process is repeated until a stable list composite objects, the jets, is obtained.

The angular cut-off, or distance parameter, can be adjusted to yield larger or smaller jets, with 0. As colliding beam energies have increased, the increasing phase space for jet production increases sensitivity to QCD effects and mandates calculations of increasing sophistication.

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Higher order calculations in the strong coupling are needed both to describe high jet multiplicities and to obtain precise predictions of cross sections. In some regions of phase space, large kinematic factors can enter into the calculations, usually in the form of logarithms of ratios of momentum scales. These terms can disrupt the convergence of a perturbative expansion in the strong coupling; practically this is often manifested as high multiplicity gluon radiation.

Such contributions can often be recast as an exponential series and resummed, incorporating a portion of the calculation to infinite order in the strong coupling. Increasingly, such effects are implemented in Monte Carlo event generators , in which a fixed and often next-to-leading order matrix element is matched to a logarithmic parton shower which models high-multiplicity QCD radiation and in which models for soft physics such as hadronization and underlying event are also incorporated, giving a complete prediction for the final state.

Measurements of jets at colliders have demonstrated the accuracy of QCD as a theory of the strong interaction, including the above effects, and demonstrating key features of QCD, including coherent radiation, the spins of the quark and gluon, and the color factors involved in their couplings.