Giovanna Davatz

Combining Monte Carlo generators with next-to-next-to-leading order calculations: Event reweighting for Higgs boson production at the LHC

We study a phenomenological ansatz for merging next-to-next-to-leading order (NNLO) calculations with Monte Carlo event generators. We reweight them to match bin-integrated NNLO differential distributions. To test this procedure, we study the Higgs boson production cross-section at the LHC, for which a fully differential partonic NNLO calculation is available. We normalize PYTHIA and MC@NLO Monte Carlo events for Higgs production in the gluon fusion channel to reproduce the bin integrated NNLO double differential distribution in the transverse momentum and rapidity of the Higgs boson. These events are used to compute differential distributions for the photons in the pp→H→γγ decay channel, and are compared to predictions from fixed-order perturbation theory at NNLO. We find agreement between the reweighted generators and the NNLO result in kinematic regions where we expect a good description using fixed-order perturbation theory. Kinematic boundaries where resummation is required are also modeled correctly using this procedure. We then use these events to compute distributions in the pp→H→W+W−→l+l−ν channel, for which an accurate description is needed for measurements at the LHC. We find that the final state lepton distributions obtained from PYTHIA are not significantly changed by the reweighting procedure.

Fast neutron detection with pressurized 4He scintillation detectors

Measurement result and performance parameters are presented for fast neutron detectors exploiting the scintillation of natural helium at high pressure. This detection medium has a very low electron density, minimizing the sensitivity to gamma radiation and thus enabling neutron detection also in high gamma radiation environment. Contrary to proportional counters, scintillation detection enables fast (nanosecond) timing and pulse shape discrimination, a technique that enables a lower neutron detection threshold. In this work, the basic principles of the detector are described, followed by a study of gamma rejection capabilities. Methods to calibrate the detector are discussed. Finally, a brief description of a 4He scintillation based detector system including data acquisition electronics is given.

Effective K-factors for gg→H→WW→ℓνℓν at the LHC

A simulation of the search for the Standard Model Higgs boson at the LHC, in the channel gg→H→WW→lνlν, is described. Higher-order QCD corrections are taken into account by using a reweighting procedure, which allows us to combine event rates obtained with the PYTHIA Monte Carlo program with the most up-to-date theoretical predictions for the transverse-momentum spectra of the Higgs signal and its corresponding WW background. With this method the discovery potential for Higgs masses between 140 and 180 GeV is recalculated and the potential statistical significance of this channel is found to increase considerably. For a Higgs mass of 165 GeV a signal-to-background ratio of almost 2:1 can be obtained. A statistical significance of five standard deviations might already be achieved with an integrated luminosity close to 0.4 fb−1. Using this approach, an experimental effective K-factor of about 2.04 is obtained for the considered Higgs signature, which is only about 15% smaller than the theoretical inclusive K-factor.A simulation of the search for the Standard Model Higgs boson at the LHC, in the channel gg -> H -> WW -> lnu lnu, is described. Higher-order QCD corrections are taken into account by using a reweighting procedure, which allows us to combine event rates obtained with the PYTHIA Monte Carlo program with the most up-to-date theoretical predictions for the transverse-momentum spectra of the Higgs signal and its corresponding WW background. With this method the discovery potential for Higgs masses between 140 and 180 GeV is recalculated and the potential statistical significance of this channel is found to increase considerably. For a Higgs mass of 165 GeV a signal-to-background ratio of almost 2:1 can be obtained. A statistical significance of five standard deviations might already be achieved with an integrated luminosity close to 0.4 fb^{-1}. Using this approach, an experimental effective K-factor of about 2.04 is obtained for the considered Higgs signature, which is only about 15 % smaller than the theoretical inclusive K-factor.

Effective K factors for gg ---> H ---> WW ---> l nu l nu at the LHC

A simulation of the search for the Standard Model Higgs boson at the LHC, in the channel gg→H→WW→lνlν, is described. Higher-order QCD corrections are taken into account by using a reweighting procedure, which allows us to combine event rates obtained with the PYTHIA Monte Carlo program with the most up-to-date theoretical predictions for the transverse-momentum spectra of the Higgs signal and its corresponding WW background. With this method the discovery potential for Higgs masses between 140 and 180 GeV is recalculated and the potential statistical significance of this channel is found to increase considerably. For a Higgs mass of 165 GeV a signal-to-background ratio of almost 2:1 can be obtained. A statistical significance of five standard deviations might already be achieved with an integrated luminosity close to 0.4 fb−1. Using this approach, an experimental effective K-factor of about 2.04 is obtained for the considered Higgs signature, which is only about 15% smaller than the theoretical inclusive K-factor.A simulation of the search for the Standard Model Higgs boson at the LHC, in the channel gg -> H -> WW -> lnu lnu, is described. Higher-order QCD corrections are taken into account by using a reweighting procedure, which allows us to combine event rates obtained with the PYTHIA Monte Carlo program with the most up-to-date theoretical predictions for the transverse-momentum spectra of the Higgs signal and its corresponding WW background. With this method the discovery potential for Higgs masses between 140 and 180 GeV is recalculated and the potential statistical significance of this channel is found to increase considerably. For a Higgs mass of 165 GeV a signal-to-background ratio of almost 2:1 can be obtained. A statistical significance of five standard deviations might already be achieved with an integrated luminosity close to 0.4 fb^{-1}. Using this approach, an experimental effective K-factor of about 2.04 is obtained for the considered Higgs signature, which is only about 15 % smaller than the theoretical inclusive K-factor.

A scalable DAQ system based on the DRS4 waveform digitizing chip

Many current and future experiments require the highest temporal resolution together with large numbers of channels in the data acquisition system (DAQ) for a minimum cost. The DRS4 waveform digitizing chip allows data sampling at up to 5 Giga-samples per second (GSPS) with high amplitude resolution. The domino wave sampling method offers a significant cost and power reduction compared to a traditional flash analog-to-digital converter (ADC). This work presents a new DAQ system based on the DRS4 chip that allows continuous digitization of analog signals at 120 Mega-samples per second (MSPS) with the possibility to sample a region of interest up to a rate of 5 GSPS, thereby allowing a long event record with small dead-time in the read-out. The signal-to-noise ratio (SNR) of the system is measured to be 9.3 bit for the 120 MSPS signal and 9.6 bit for the DRS4 readout signal. Arbitrarily complex trigger logic can be built entirely in the digital domain in the read-out field-programmable gate array (FPGA). A Gigabit Ethernet link provides high-speed connectivity from the DAQ board to the backend system. Built-in board-to-board communication and the modular design of the system offer great scalability and flexibility with respect to the number of supported data channels.

Standard Model Higgs discovery potential of CMS in the H → WW → ℓνℓν channel

The discovery potential of the CMS detector for the Standard Model Higgs boson in the H → WW → lνlν channel is assessed using a full detector simulation. Sources of systematic uncertainties as well as methods to determine backgrounds from data are discussed. If the Standard Model Higgs boson has a mass between 150 GeV and 180 GeV, it should be observed with a significance of more than 5σ with a luminosity of about 10 fb−1.

Simulation of a cross section and mass measurement of a standard model Higgs boson in the gg → H → WW → ℓ ν ℓ ν channel at the CERN LHC

The potential to discover a Standard-Model-like Higgs boson at the LHC in the mass range from 150-180 GeV, decaying into a pair of W bosons with subsequent leptonic decays, has been established during the last 10 years. Assuming that such a signal will eventually be observed, the analysis described in this paper investigates how accurate the signal cross section can be measured and how the observable lepton pt spectra can be used to constrain the mass of the Higgs boson. Combining the signal cross section with the analysis of the lepton pt spectra and assuming the SM Higgs cross section is known with an accuracy of +-5%, our study indicates that an integrated luminosity of about 10 fb-1 allows to measure the mass of a SM Higgs boson with an accuracy between 2 and 2.5 GeV.

Fast neutron detection in homeland security applications

Calculations are presented to support the claim that fast neutron detection systems can achieve higher performance in detecting weak neutron sources than conventional thermal neutron detection systems involving moderators. Minimum Detectable Limits (MDL) are used as a comparative metric, more representative than the metric of absolute sensitivity, which does not take into account the influence of natural backgrounds. Monte Carlo simulations are used to show that heavily shielded neutron sources emit a substantial fraction of fast neutrons. Arguments are presented to support the claim that fast neutron detection systems are superior at defeating heavy neutron shielding than thermal systems.

Gamma-insensitive fast neutron detector with spectral source identification potential

A neutron scintillation detector based on high-pressure 4He has been developed and tested. The detector responds to elastic scattering of unmoderated fast neutrons, preserving neutron energy, position, and timing information. Neutrons from three different sources, AmBe, 252Cf, and from the ambient background, have been measured with clearly distinct energy spectra. Exposing the detector to different levels of gamma radiation up to 1 mSv/h from a 60Co source has demonstrated that gamma exposure does not affect neutron detection performance within measurement statistics. Applications in the field of radiation portal monitoring are discussed.

Noble Gas Scintillation-Based Radiation Portal Monitors and Active Interrogation Systems

Noble gas scintillation detectors (NGSD) combine good scintillation properties such as high light yields and good energy resolution with cost-effective scalability to large detector masses. A unique capability of noble gas scintillators is outstanding discrimination between electron recoils (gamma-like events) and nuclear recoils (n-like events) down to low energy deposits of the order of tens of keVs. This capability allows the construction of fast neutron detectors with good timing capability. This work points out the potential advantages of using NGSDs for Homeland Security applications such as portal monitoring or active interrogation.