Publications

2017
TSA Underwood, W Sung, CH McFadden, SJ McMahon, DC Hall, AL McNamara, H Paganetti, GO Sawakuchi, and J Schuemann. 2017. “Comparing stochastic proton interactions simulated using TOPAS-nBio to experimental data from fluorescent nuclear track detectors.” Physics in Medicine and Biology, 8, 62: 3237. Publisher's Version Abstract

Whilst Monte Carlo (MC) simulations of proton energy deposition have been well-validated at the macroscopic level, their microscopic validation remains lacking. Equally, no gold-standard yet exists for experimental metrology of individual proton tracks. In this work we compare the distributions of stochastic proton interactions simulated using the TOPAS-nBio MC platform against confocal microscope data for Al2O3:C,Mg fluorescent nuclear track detectors (FNTDs). We irradiated $8\times 4\times 0.5$  mm3 FNTD chips inside a water phantom, positioned at seven positions along a pristine proton Bragg peak with a range in water of 12 cm. MC simulations were implemented in two stages: (1) using TOPAS to model the beam properties within a water phantom and (2) using TOPAS-nBio with Geant4-DNA physics to score particle interactions through a water surrogate of Al2O3:C,Mg. The measured median track integrated brightness (IB) was observed to be strongly correlated to both (i) voxelized track-averaged linear energy transfer (LET) and (ii) frequency mean microdosimetric lineal energy, $\overline{{{y}_{F}}}$ , both simulated in pure water. Histograms of FNTD track IB were compared against TOPAS-nBio histograms of the number of terminal electrons per proton, scored in water with mass-density scaled to mimic Al2O3:C,Mg. Trends between exposure depths observed in TOPAS-nBio simulations were experimentally replicated in the study of FNTD track IB. Our results represent an important first step towards the experimental validation of MC simulations on the sub-cellular scale and suggest that FNTDs can enable experimental study of the microdosimetric properties of individual proton tracks.

DC Hall, AV Trofimov, BA Winey, NJ Liebsch, and H Paganetti. 2017. “Predicting patient-specific dosimetric benefits of proton therapy for skull-base tumors using a geometric knowledge-based method.” International Journal of Radiation Oncology • Biology • Physics, 5, 97: 1087-1094. Publisher's Version Abstract

Purpose: To predict the organ at risk (OAR) dose levels achievable with proton beam therapy (PBT), solely based on the geometric arrangement of the target volume in rela- tion to the OARs. A comparison with an alternative therapy yields a prediction of the patient-specific benefits offered by PBT. This could enable physicians at hospitals without proton capabilities to make a better-informed referral decision or aid patient selection in model-based clinical trials.

Methods and Materials: Skull-base tumors were chosen to test the method, owing to their geometric complexity and multitude of nearby OARs. By exploiting the correla- tions between the dose and distance-to-target in existing PBT plans, the models were independently trained for 6 types of OARs: brainstem, cochlea, optic chiasm, optic nerve, parotid gland, and spinal cord. Once trained, the models could estimate the feasible doseevolume histogram and generalized equivalent uniform dose (gEUD) for OAR structures of new patients. The models were trained using 20 patients and validated using an additional 21 patients. Validation was achieved by comparing the predicted gEUD to that of the actual PBT plan.

Results: The predicted and planned gEUD were in good agreement. Considering all OARs, the prediction error was þ1.4 ` 5.1 Gy (mean ` standard deviation), and Pearson’s correlation coefficient was 93%. By comparing with an intensity modulated photon treatment plan, the model could classify whether an OAR structure would experience a gain, with a sensitivity of 93% (95% confidence interval: 87%-97%) and specificity of 63% (95% confidence interval: 38%-84%).

Conclusions: We trained and validated models that could quickly and accurately pre- dict the patient-specific benefits of PBT for skull-base tumors. Similar models could be developed for other tumor sites. Such models will be useful when an estimation of the feasible benefits of PBT is desired but the experience and/or resources required for treatment planning are unavailable. 

 

2016

This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in the HWWeνμν channel, using 20.3 fb1 of proton-proton collision data. The data were produced at a centre-of-mass energy of √s=8 TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observed HWWeνμν signal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.

DC Hall, A Makarova, H Paganetti, and B Gottschalk. 2016. “Validation of nuclear models in Geant4 using the dose distribution of a 177 MeV proton pencil beam.” Physics in Medicine and Biology, 1, 61: N1-10. Publisher's Version Abstract

A proton pencil beam is associated with a surrounding low-dose envelope, originating from nuclear interactions. It is important for treatment planning systems to accurately model this envelope when performing dose calculations for pencil beam scanning treatments, and Monte Carlo (MC) codes are commonly used for this purpose. This work aims to validate the nuclear models employed by the Geant4 MC code, by comparing the simulated absolute dose distribution to a recent experiment of a 177 MeV proton pencil beam stopping in water.

Striking agreement is observed over five orders of magnitude, with both the shape and normalisation well modelled. The normalisations of two depth dose curves are lower than experiment, though this could be explained by an experimental positioning error. The Geant4 neutron production model is also verified in the distal region. The entrance dose is poorly modelled, suggesting an unaccounted upstream source of low-energy protons. Recommendations are given for a follow-up experiment which could resolve these issues.

2015
D Hall. 2015. Discovery and measurement of the Higgs boson in the WW decay channel. Switzerland: Springer International Publishing. Publisher's Version Abstract

In the Standard Model of particle physics, the non-zero masses of the W and Z bosons and the fermions are generated through interactions with the Higgs field, excitations of which correspond to Higgs bosons. Thus, the experimental discovery of the Higgs boson is of prime importance to physics, and would confirm our understanding of fundamental mass generation.

This thesis describes a search for the ggHWWlνlν process of Higgs boson production and decay. It uses the LHC Run I dataset of pp collisions recorded by the ATLAS detector, which corresponds to an integrated luminosity of 4.5 fb−1 at 7TeV and 20.3 fb-1 at 8 TeV. An excess of events is observed with a significance of 4.8 standard deviations, which is consistent with Higgs boson production. The significance is extended to 6.1 standard deviations when the vector boson fusion production process is included. The measured signal strength is 1.11+0.23-0.21 at mH = 125 GeV. A cross section measurement of WW production, a major background to this search, is also presented using the 7 TeV dataset only.

We report the observation of Higgs boson decays to WW based on an excess over background of 6.1 standard deviations in the dilepton final state, where the Standard Model expectation is 5.8 standard deviations. Evidence for the vector-boson fusion (VBF) production process is obtained with a significance of 3.2 standard deviations. The results are obtained from a data sample corresponding to an integrated luminosity of 25fb1 from √s=7 and 8 TeV pp collisions recorded by the ATLAS detector at the LHC. For a Higgs boson mass of 125.36 GeV, the ratio of the measured value to the expected value of the total production cross section times branching fraction is 1.09+0.160.15(stat)+0.170.14(syst). The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are 1.02±0.19(stat)+0.220.18(syst) and 1.27+0.440.40(stat)+0.300.21(syst), respectively. At √s=8TeV, the total production cross sections are measured to be σ(ggHWW)=4.6±0.9(stat)+0.80.7(syst)pb and σ(VBHWW)=0.51+0.170.15(stat)+0.130.08(syst)pb. The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.

2013
LHC Higgs Cross Section Working Group. 2013. “Handbook of LHC Higgs Cross Sections: 3. Higgs Properties.” CERN-2013-004. Publisher's Version

This paper presents a measurement of the W+W production cross section in pp collisions at √s=7TeV. The leptonic decay channels are analyzed using data corresponding to an integrated luminosity of 4.6fb1 collected with the ATLAS detector at the Large Hadron Collider. The W+W production cross section σ(ppW+W+X) is measured to be 51.9±2.0(stat)±3.9(syst)±2.0(lumi)pb, compatible with the Standard Model prediction of 44.7+2.11.9pb. A measurement of the normalized fiducial cross section as a function of the leading lepton transverse momentum is also presented. The reconstructed transverse momentum distribution of the leading lepton is used to extract limits on anomalous WWZ and WWγ couplings.

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H→γγH→ZZ→4ℓ and H→WW→ℓνℓν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of View the MathML source and View the MathML source, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson.

2012

This Letter reports a measurement of the WW production cross section in pp collisions using data corresponding to an integrated luminosity of 1.02 fb−1 collected with the ATLAS detector. Using leptonic decays of oppositely charged W bosons, the total measured cross section is σ(pp→WW) = 54.4±4.0(stat)±3.9(syst)±2.0(lumi) pb, consistent with the Standard Model prediction of σ(pp→WW) = 44.4±2.8 pb. Limits on anomalous electroweak triple-gauge couplings are extracted from a fit to the transverse-momentum distribution of the leading charged lepton in the event.

A search for the Standard Model Higgs boson in proton–proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb−1 collected at √s=7 TeV in 2011 and 5.8 fb−1 at √s=8 TeV in 2012. Individual searches in the channels H→ZZ(*)→4ℓ, H→γγ and H→WW(*)→eνμν in the 8 TeV data are combined with previously published results of searches for H→ZZ(*), WW(*), bb and τ+τ− in the 7 TeV data and results from improved analyses of the H→ZZ(⁎)→4ℓ and H→γγ channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0±0.4(stat)±0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10−9, is compatible with the production and decay of the Standard Model Higgs boson.

Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga–electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself.

The LHC luminosity upgrade, known as the HL-LHC, will require high-speed optical links to read out data from the detectors. Such links must be capable of withstanding high doses whilst being kept at low temperatures. Two single-mode and two multi-mode fibres were exposed to 200 kGy(Si) at a dose rate of about 700 Gy(Si)/hr, whilst being kept at about -25°C. The radiation induced attenuation of these fibres was measured as the fibres accumulated dose. A conservative estimate has been made of the total attenuation expected for a realistic fibre route through the ATLAS detector after a lifetime dose at the HL-LHC. With safety factors, the maximum dose extrapolated to was 375 kGy(Si). All four fibres performed extremely well and were qualified for use at HL-LHC detectors.

DC Hall, P Hamilton, BT Huffman, PK Teng, and AR Weidberg. 2012. “The radiation tolerance of MTP and LC optical fibre connectors to 500 kGy(Si) of gamma radiation.” Journal of Instrumentation, 7: P04014. Publisher's Version Abstract

The LHC luminosity upgrade, known as the High Luminosity LHC (HL-LHC), will require high-speed optical links to read out data from the detectors. The optical fibre connectors contained within such a link must have a small form factor and be capable of operating in the harsh radiation environment at the HL-LHC. MTP ribbon fibre connectors and LC single fibre connectors were exposed to 500 kGy(Si) of gamma radiation and their radiation hardness was investigated. Neither type of connector exhibited evidence for any significant radiation damage and both connectors could be qualified for use at HL-LHC detectors.

F Vasey, D Hall, T Huffman, S Kwan, A Prosser, C Soos, J Troska, A Weidberg, A Xiang, and J. Ye. 2012. “The Versatile Link common project: feasibility report.” Journal of Instrumentation, 7: C01075. Publisher's Version Abstract

The Versatile Link is a bi-directional digital optical data link operating at rates up to 4.8 Gbit/s and featuring radiation-resistant low-power and low-mass front-end components. The system is being developed in multimode or singlemode versions operating at 850 nm or 1310 nm wavelength respectively. It has serial data interfaces and is protocol-agnostic, but is targeted to operate in tandem with the GigaBit Transceiver (GBT) serializer/deserializer chip being designed at CERN. This paper gives an overview of the project status three and a half years after its launch. It describes the challenges encountered and highlights the solutions proposed at the system as well as the component level. It concludes with a positive feasibility assesment and an outlook for future project development directions.