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Physics at the Terascale
Research Topics
RT1: Physics Analysis
Analysis Centre
Theorist of the Month
Slides
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Slides
Collisions at the LHC are dominated by jets, collimated sprays of hadrons that are proxies for short-distance quarks and gluons. With the remarkable performance of the ATLAS and CMS detectors, jets can now be characterized not just by their overall direction and energy but also by their substructure. In this talk, I highlight the ways that jet substructure can enhance the search for new physics at the LHC, especially at higher energies and luminosities. I also explain how theoretical studies of jet substructure have taught us surprising lessons about QCD.
The past years have witnessed a tremendous improvement in our capabilities to calculate multi-parton processes at next-to-leading order. NLO calculations are essential for precision physics at the LHC. I will explain the major developments.
The recent discovery of a 125 GeV Higgs boson --coupled with lack of SUSY signal at LHC-- has led many to question SUSY naturalness. In radiatively-driven natural SUSY, SUSY is perfectly natural with a hallmark signature of light higgsinos with mass 100-200 GeV. The resultant spectrum may well elude LHC searches. However, the International Linear Collider operating with E>2m(higgsino) would be a higgsino factory and serve as a SUSY discovery machine! Thermally produced neutralinos comprise only a small portion of dark matter, with the remainder is composed of axions. The SUSY DFSZ axion model elegantly accommodates a hierarchy between Peccei-Quinn (PQ) and SUSY breaking scales, and correlates the Higgs mass with the PQ scale. Both axion and higgsino-like wimp detection are expected, but it may take a ton-scale noble liquid detector to completely explore the wimp possibility.
A large class of production processes at the LHC can be thought and initiated by b-quarks: single-top, (pseudo)-scalars at large tan-beta, gamma, W and Z in association with b-jets. Over the years, predictions in QCD have become available in 4- and 5-flavour schemes leading often to quite different and therefore puzzling results. I discuss how one can think about these schemes in an easy way so that to understand and solve the apparent differences and to make accurate and reliable predictions for the LHC.
We first discuss the motivation of supersymmetric models with extended gauge symmetries: origin of R-parity and neutrino physics. Several of these models contain additional tree-level and/or loop contributions to the mass of the lightest Higgs boson. This allows an explanation of the observed Higgs mass close to 125 GeV without the need to go to very specific parameter combinations as in the CMSSM. We will discuss different model variants in this context. Moreover we will show how supersymmetry changes the properties of an additional Z-boson and that the existing bounds can be substantially changed. Last but not least we will discuss how the LHC phenomenology of supersymmetric particles get altered in such models.
Investigations of scattering in the limit of large centre-of-mass energies have spurred great advances over the last decades in our understanding of theories of fundamental interactions. This talk focuses on LHC applications of ideas developed in the context of QCD high-energy scattering, and on their role to uncover new aspects of the Standard Model.
The POWHEG BOX is a framework for building generators interfaced to shower Monte Carlo that include next-to-leading corrections to the basic hard process. I will discuss recent progress in this framework, with particular attention to a newly proposed procedure (dubbed MiNLO, for Multiscale improved Next-to-Leading Order) for the choice of the factorization and renormalization scales in NLO calculations. This procedure is particularly suited for processes involving several, widely different scales due to associated jet production, and it is most useful in the framework of the POWHEG method. I will discuss the MiNLO improved POWHEG generators for processes like Higgs, W or Z production with up to two associated jets.
An important ingredient in a successful physics programme at the LHC is the systematic use of precise computations, which help reduce the theoretical bias on experimental analyses, thanks to the possibility of giving reliable predictions with a minimal amount of tuning on real data. Significant progress towards this situation has been made recently, owing to the possibility of computing an arbitrary process to the next-to-leading order accuracy in QCD, including the matching to parton shower Monte Carlos, in a fully automated manner. I shall describe the theoretical ideas which have been used, illustrate some selected applications to LHC physics, and briefly discuss future plans, including extensions to theories other than QCD.
New results have been pouring in from the LHC during the last two years. Searches for new physics have produced many null results, leading to new interesting limits, while a new particle at 125GeV has been discovered, compatible with the Standard Model Higgs boson. These conclusions are reshaping our knowledge of Beyond the Standard Model Physics at the TeV scale. In this talk I will present past and current work on the implications of ATLAS and CMS results on new physics models, such as supersymmetry and weakly interacting (WIMP) Dark Matter models. I will also discuss how the task of comparing of theoretical models with experimental searches can be simplified in our current data rich era.
Jets are almost everywhere in LHC physics. We shall review the status of our present theoretical understanding, describe the main measurements performed so far, and discuss the role that jets (and their substructure) can play in future analyses and searches.
Understanding the physics of jets is important for many analyses at the LHC. Two examples are i) channels with a fixed number of jets used in Higgs analyses and searches for new physics, and ii) the exploration of jet substructure to distinguish boosted heavy objects or quark vs. gluon initiated jets. Jets also play an important role in ep collisions, where final state jets are used to measure alphas(mZ) and constrain parton distributions. Using new technology, next-to-next-to-leading log (NNLL) predictions have recently become available for many jet observables, pushing precision phenomenology into a new regime. I will use NNLL jet mass distributions for the LHC to explore the dependence on jet kinematics, jet size, and partonic channels, and will use NNLL predictions for 2-jets in DIS to explore the sensitivity to measuring the transverse momentum distribution of initial state radiation from the proton. At this precision power corrections beyond those in the parton distributions also become relevant, and the question of whether they can be described by universal parameters will be discussed. Interestingly the answer to this question depends on how hadron masses are treated in the experimental measurements.
The Large Hadron Collider LHC has probed the structure of matter at the Terascale with unprecedented breadth and precision. The wealth of exciting results include the discovery of a Higgs-like particle at a mass of around 125 GeV, but no sign for physics beyond the standard model of particle physics could be established. I will review the theoretical motivation for new physics at the Terascale, summarise the current searches at the LHC and their implications for popular new physics models like supersymmetry, and discuss the prospects for physics at the LHC in the next decade.
I motivate the need for precision calculations in perturbative quantum field theory and review some recent progress towards computing the dijet cross section at the LHC at next-to-next-to-leading order. I review the sorts of observables that might benefit from such precision and the measurements that might be made at the LHC.
We review the current state of the art for QCD calculations (both perturbative and non-perturbative) and tools for jet physics with a particular emphasis on recent developments relevant for new physics studies at the LHC. We also highlight future issues and challenges in this context.
Multiple parton interactions (MPI) when in a single collision of hadrons more than one parton pair enter hard scattering, are rare. Nevertheless, such collisions have specific final state topology, so that in a well defined part of phase space MPI turn out to be a significant - and even dominant - source of multi-jet production.
There are underwater stones that sunk many theoretical expeditions that aimed at predicting MPIs. To understand this physics, one has to critically reexamine the familiar momentum-space Feynman diagram technique for calculating particle scattering amplitudes, and to pay attention to the geometry of the process. Then the MPI physics becomes rather transparent and predictable.
To study MPIs is important not only for evaluating QCD backgrounds at LHC new physics searches, but also for better understanding the structure of the proton.
