Quality of Jet Measurements and Impact on a Search for New Physics at CMS

Matthias Schröder
Nov 2012

Thesis Type:

The search for new physics beyond the Standard Model is one of the primary enterprises in particle physics today. Many models predict the production of new, coloured particles at the LHC. These particles would eventually decay into quarks and gluons, which manifest as particle jets in the detector. Hence, a precise understanding of the properties of jets is essential to study the underlying hard interaction process of hadronic final-states in order to probe the Standard Model and enter the territory beyond.

In this thesis, a measurement of the jet transverse-momentum (pT) response function at the CMS experiment at the CERN LHC is presented, and its impact on a QCD-background estimation method in a search for new physics is described.

The jet-pT response function is determined in two steps from dijet data collected in proton-proton collisions at a centre-of-mass energy of 7 TeV and corresponding to an integrated luminosity of up to 4.90 fb-1. The jets are clustered with the anti-kT algorithm from individually measured particles that are reconstructed
with the Particle-Flow algorithm combining the information from all relevant subdetectors. Firstly, the relative jet-pT
resolution σ/pT is measured using an unbinned maximum-likelihood approach that is based on the pT balance in QCD-dijet events and incorporates the jet-pT spectrum. In the central detector region, the relative resolution in data is approximately 10% at pT = 100 GeV and improves to 5.5% at 600
GeV. It is approximately 5% larger on average than in the simulation. Secondly, the tails of the jet-pT response function are measured studying the dijet pT-asymmetry. The tails are caused by semileptonically decaying heavy-flavour quarks as well as instrumental effects and contribute at the percent level to the response function. Their relative size in data is larger than in the simulation by 20 - 60% in the central detector region at medium to high pT.

Furthermore, a search for new physics in a multijet final-state with no leptons and a large pT imbalance is reviewed, which has been conducted by CMS with 4.98 fb-1 of data. The QCD-multijet background is estimated from data in a control region by modelling the jet-pT mismeasurements using the response function determined from dijet events. Taking into account the uncertainties on the response-function measurement, a prediction of the QCD background in the
search region with a typical uncertainty of 60 - 70% is achieved. No excess in the number of events has been observed. Derived exclusion limits on the parameters of the Constrained Minimal Supersymmetric Standard Model correspond to a lower limit on equal squark and gluino masses of approximately 1300 GeV.

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