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Steven Green, Mark Thomson

Linear Collider Physics

Separation of hadronic decays of WW/ZZ events in the di-jet invariant mass plane.

Separation of hadronic decays of WW/ZZ events in the di-jet invariant mass plane. The blue region represents WW events, whilst the red region represents ZZ events.

The center-of-mass dependencies of the cross sections for the main Higgs production processes
          at an e+e− collider

The center-of-mass dependencies of the cross sections for the main Higgs production processes at an e+e− collider

To maximally exploit the benefits of particle flow calorimetry it is important to be able to associate all energy deposits to the correct particle leaving energy in the calorimeter. This means that pattern recognition plays an essential role in the particle flow paradigm. The topology of the events being reconstructed will, to some extent, dictate the precision of the collider. The 'clean' topologies associated to electron-position colliders, as opposed to the 'busy' topologies associated to proton-proton colliders such as the LHC, mean that the pattern recognition is easier to implement at the linear collider and so the particle flow approach can offer more precision that at a proton-proton collider.

The precision of the linear collider allows for detailed physics studies. For example the Higgs boson mass can be determined to better than 100 MeV at CLIC operating at √s = 350 GeV from either the Z recoil mass distribution or from the direct reconstruction of the decay products. At higher center-of-mass energies the large samples of H → bb decays would allow the Higgs mass to be determined with a statistical precision of about ±30 MeV. A linear collider also provides the possibility of detecting CP violation in the Higgs sector, where a priori the observed Higgs state with mH = 125 GeV can be an admixture of CP even and CP odd states.

Top physics is an integral part of the linear collider physics program. The first stage will provide a precise measurement of the top quark mass on the 100 MeV level and measurements of other top properties such as the width, while higher energy stages give access to various measurements sensitive to new physics. The achievable precision for the mass measurements has already been investigated in detail in full simulations, including incomplete studies of systematic uncertainties. The potential to use top quarks as a probe for new physics will be the subject of studies in the near future, which will include:
• top quark production asymmetries;
• top couplings to γ , Z, W and H bosons;
• CP violation in the top sector;
• flavour changing top decays;

This physics is characterised by high multiplicity final states, often 6/8 jets, and small cross-sections, e.g. σ(e+e-ZHH) = 0.3fb.

There is a strong desire to separate W and Z hadronic decays, in the study of physics. This desire originates from the fact that many physics processes involve the decay of either a W or Z boson and to probe the underlying physics you need to be able to distinguish between these decays. For the case of leptonic decays, the leptons make it easy to identify whether a W or Z has decayed. However, for hadronic decays it is necessary to form jets and use the invariant mass of those jet combinations to determine whether the parent particle was a W or a Z. The invariant mass of jets depends very strongly on both the intrinsic energy resolution of the detector as well as the quality of the pattern recognition applied in the reconstruction. Therefore, excellent jet energy resolution is needed to be able to properly understand the bulk of physics processes.

Hence high luminosity, i.e. ILC/CLIC and detector optimisation for precision physics in multi-jet environment are both required. In particular, 3-4% jet energy resolution gives decent 2.6-2.3σ W/Z separation.

This sets a reasonable choice for LC jet energy minimal goal ~3.5%. for 50 – 500 GeV jets, which calls for new approach to calorimetry.

Separation of W/Z bosons for different jet resolution. From left to right, the jet resolution of LEP , 3% and perfect resolution