Precision Predictions of Exclusive Jet Cross Sections at the LHC

Gangal, Shireen

Report/Dissertation / PhD Thesis

PUBDB-2015-04278

Precision Predictions of Exclusive Jet Cross Sections at the LHC

Gangal, S. (Corresponding author)DESY*

2015
Verlag Deutsches Elektronen-Synchrotron Hamburg

Hamburg : Verlag Deutsches Elektronen-Synchrotron, DESY-THESIS 159 pp. (2015) [10.3204/DESY-THESIS-2015-042] = Universität Hamburg, Diss., 2015

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Please use a persistent id in citations: doi:10.3204/DESY-THESIS-2015-042

Report No.: DESY-THESIS-2015-042

Abstract: With the discovery of the Higgs boson, a central objective of the LHC Higgs program is to study its properties in detail by exploring different production and decay channels. This requires precise theoretical predictions of inclusive cross sections as well as differential and exclusive cross sections. In this thesis, we study perturbative uncertainties in the fixed-order (FO) predictions of exclusive jet cross sections and obtain resummed predictions for a new class of rapidity-dependent jet veto observables, focusing on Higgs production via gluon gluon fusion (ggF) at the LHC. Experimental analyses at the LHC often use jet binning and jet selection cuts to distinguish between different Higgs production mechanisms and to separate signal from backgrounds. Such jet vetoes and jet selection cuts induce Sudakov logarithms of the ratio of the veto scale and the hard scale in the process. In the limit of very tight jet vetoes, these logarithms can become large and introduce large uncertainties in the FO predictions of cross sections. By resumming these large logarithms to all orders, the perturbative uncertainties can be considerably reduced. Whether in FO or resummed predictions, a consistent treatment of uncertainties in different jet bins is required. In the first part of the thesis, we studied in detail the perturbative uncertainties in the NLO predictions for pp -> H+2-jets via ggF for the vector boson fusion (VBF) selection cuts used by ATLAS and CMS in their H -> $\gamma\gamma$ analyses. Our study shows that, while applying strong restrictions on additional emissions is expected to increase the sensitivity to the VBF signal and reduce the ggF contribution, it is not necessarily beneficial for distinguishing the VBF and ggF production modes because of the quickly increasing ggF uncertainties. In the second part of the thesis, we introduce rapidity-dependent jet veto observables for which the transverse momentum of a jet is weighted by a smooth function of the jet rapidity. These jet-based observables provide natural and clean ways to veto central jets and can yield valuable complementary information in the exclusive jet bins. Using Soft Collinear Effective Theory (SCET), we study the factorization and resummation properties of these rapidity-dependent observables and obtain predictions for the resummed H+0-jet cross section at NLL$^\prime$ with a veto on these observables. Because the experimentally relevant region is an intermediate one, where both the resummed and the FO contributions are important, we calculate the FO corrections at NLO and combine them with our resummed predictions to obtain the full NLL$^\prime$ + NLO result for the H+0-jet cross section. We compare our numerical predictions with the differential cross section measurement by ATLAS in the H-> $\gamma \gamma$ channel and find good agreement. At $O(\alpha_s^2)$, these jet-based observables have a non trivial dependence on the jet-algorithm due to clustering effects. In the final part, we consider the corrections due to clustering of two collinear or soft particles into a single jet which are an important input for predicting the cross section at NNLL$^\prime$ + NNLO. These corrections are numerically significant for the jet radii currently used in experiments.

Note: Universität Hamburg, Diss., 2015

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