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Register a new userEffects of QCD Resummation on $W^+h$ and $tbar b$ Production at the Tevatron
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S. Mrenna
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The resummation of multiple soft gluon emission affects the production rate and kinematic distributions of $W^+h$ (where h is a Higgs boson) and $t bar b$ pairs at the Tevatron with $sqrt{s}=2$ TeV. Using the Collins-Soper-Sterman resummation formalism, the production rate is enhanced over the next-to-leading-order (NLO) prediction by 2-3% for the $W^+h$ process, for Higgs boson masses between 80-120 GeV, and over 3% for the $tbar b$ process for $m_t=175$ GeV. After resummation, the $tbar b$ rate changes by 12-13% when $m_t$ is varied by $pm 5$ GeV. Various kinematic distributions are presented for the individual final state particles and for the pair. The explicit radiation of hard gluons in NLO QCD is included also for the $tbar b$ final state.
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Recent measurements of the $W$-boson charge asymmetry and of the $Z$-boson production cross sections, performed at the Tevatron collider in Run II by the D0 and CDF collaborations, are studied using the HERAFitter framework to assess their impact on the proton parton distribution functions (PDFs). The Tevatron measurements, together with deep-inelastic scattering data from HERA, are included in a QCD analysis performed at next-to-leading order, and compared to the predictions obtained using other PDF sets from different groups. Good agreement between measurements and theoretical predictions is observed. The Tevatron data provide significant constraints on the $d$-valence quark distribution.
We study the kinematic distributions of top--antitop quark pairs produced at the Tevatron, including the effects of initial state and final state multiple soft gluon emission, using the Collins--Soper--Sterman resummation formalism. The resummed results are compared with those predicted by the showering event generator PYTHIA for various distributions involving the top--antitop quark pair and the individual top quark or antiquark. The comparison between the experimental and predicted distributions will be a strong test of our understanding and application of perturbative QCD. Our results indicate that the showering event generators do not produce enough radiation. We reweight the PYTHIA distributions to agree with our resummed calculation, then use the reweighted events to better estimate the true hadronic activity in top--antitop quark pair production at hadron colliders.
The merging procedure of tree-level matrix elements and the subsequent parton shower as implemented in the new event generator SHERPA will be validated for the example of W/Z+jets production at the Tevatron. Comparisons with results obtained from other approaches and programs and with experimental results clearly show that the merging procedure yields relevant and correct results at both the hadron and parton levels.
A feasibility study for an experimental analysis searching for $tbar{t}H(Hrightarrow bbar{b})$ production at the LHC and its high luminosity phase is presented in this note. Unlike search strategies currently being used in experimental collaborations, the present analysis exploits jet substructure techniques and focuses on the reconstruction of boosted Higgs bosons, to obtain sensitivity to the signal in a simple cut-based analysis. The $tbar{t} +$ jets background may be constrained in the proposed analysis through a control region with very small signal contamination. Using this analysis strategy, the $tbar{t}H(Hrightarrow bbar{b})$ process could be observed at the LHC, in the semi-leptonic channel alone, with a significance of $5.41pm 0.12$ for $mathcal{L}=300,mbox{fb}^{-1}$. For the same integrated luminosity, in the High Luminosity LHC scenario with an upgraded detector, a significance of $6.13pm 0.11$ may be obtained. The top Yukawa coupling could be measured with a 35% uncertainty using $mathcal{L}=300,mbox{fb}^{-1}$ of LHC data and of 17% at the HL-LHC scenario with $mathcal{L}=3000,mbox{fb}^{-1}$. In the same luminosity scenarios, the signal strength is equally expected to have a 18$%$ and 5$%$ uncertainty, respectively. Finally, it was found that re-clustered jets may be used without loss of efficiency.
In the framework of the large extra dimensions (LED) model, the effects of LED on the processes rrtth and eetth at future linear colliders are investigated in both polarized and unpolarized collision modes. The results show that the virtual Kaluza-Klein (KK) graviton exchange can significantly modify the standard model expectations for these processes with certain polarizations of initial states. The process rrtth with $sqrt{s}=3.5 TeV$ allows the effective scale $Lambda_T$ to be probed up to 7.8 and 8.6 TeV in the unpolarized and $P_{gamma} = 0.9$, J=2 polarized $gamma gamma$ collision modes, respectively. For the eetth process with $sqrt{s}=3.5 TeV$, the upper limits of $Lambda_T$ to be observed can be 6.7 and 7.0 TeV in the unpolarized and $P_{e^+} = 0.6$, $P_{e^-} = 0.8$, $-+$ polarized $e^+e^-$ collision modes, respectively. We find the rrtth channel in J=2 polarized photon collision mode provides a possibility to improve the sensitivity to the graviton tower exchange.
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