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Search for New Phenomena in Events with Large Jet Multiplicities

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High Jet Multiplicity Physics at the LHC

Part of the book series: Springer Theses ((Springer Theses))

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Abstract

This chapter describes a search for new particles hidden in 8 TeV collisions producing many jets. After a first overview of the analysis, the next sections describe the supersymmetric signals, the data, and the triggers used, as well as the main reconstructed objects, kinematic variables and cleaning cuts used in the analysis. After that, the main backgrounds to the search are described, and the methods used to estimate them discussed, including the determination of their uncertainties. Finally, the last section includes an overview of the statistical procedure used to interpret the result, and the corresponding model-independent and model-dependent exclusion limits on new physics.

‘Do you know, I always thought unicorns were fabulous monsters, too? I never saw one alive before!’

‘Well, now that we have seen each other,’ said the unicorn, ‘if you’ll believe in me, I’ll believe in you.’

Lewis Carroll, Through the Looking-Glass

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Notes

  1. 1.

    More details on variable definitions will be given in Sect. 3.2.4.

  2. 2.

    The small inefficiency seen in Fig. 3.4 for events with the sixth jet \(p_{\mathrm {T}}\) between 50 and 55 GeV could only affect the signal regions indirectly via the multi-jet background prediction. A dedicated systematic uncertainty is derived, and found to be negligible. The uncertainty will be discussed in Sect. 3.5.4.

  3. 3.

    Closure here refers to the self-consistency of the calibration procedure, i.e. the recovery of the truth-level jet energy scale when the calibration procedure is applied on the simulated events used to derive it.

  4. 4.

    Other than its own.

  5. 5.

    The signal fraction in these regions could have been significant in the first iterations of the analysis. As SUSY phase space is getting excluded, the cross sections of the available phase space get smaller. This is why the signal fraction is now expected to be very small in these regions, and they are considered simply as confirmation regions.

  6. 6.

    The convention used by ATLAS and CMS is to take the central value defined by the positive and negative cross section uncertainties as the best estimate of the cross section. This approach also results in a symmetric uncertainty.

  7. 7.

    Although the ‘vanilla’ approach would be to only use the data in the control regions for this calculation, for the fitting algorithm control and signal regions are by construction equivalent. The main difference between regions is the relative contribution from signal and background, which is what determines whether they can be used to constrain the background; this way, all the data available is used in order to obtain the best possible background description. From this perspective the use of the terms ‘control’ and ‘signal’ partially loses its meaning (inherited from older statistical approaches), although the nomenclature is still useful at the time of developing the analysis.

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  1. Memorial Sloan Kettering Cancer Center, New York, NY, USA

    Mireia Crispín Ortuzar

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Ortuzar, M. . (2016). Search for New Phenomena in Events with Large Jet Multiplicities. In: High Jet Multiplicity Physics at the LHC. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-43461-2_3

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