Properties of jet fragmentation using charged particles measured with the ATLAS detector in pp collisions at root s=13 TeV
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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Search for a multi-Higgs-boson cascade in W + W - b b ¯ events with the ATLAS detector in p p collisions at s = 8 TeV
A search is presented for new particles in an extension to the Standard Model that includes a heavy Higgs boson (H 0 ), an intermediate charged Higgs-boson pair ( H ± ), and a light Higgs boson (h 0 ). The analysis searches for events involving the production of a single heavy neutral Higgs boson which decays to the charged Higgs boson and a W boson, where the charged Higgs boson subsequently decays into a W boson and the lightest neutral Higgs boson decaying to a bottom-antibottom-quark pair. Such a cascade results in a W -boson pair and a bottom-antibottom-quark pair in the final state. Events with exactly one lepton, missing transverse momentum, and at least four jets are selected from a data sample corresponding to an integrated luminosity of 20.3 fb -1 , collected by the ATLAS detector in proton-proton collisions at $ \sqrt{s}$ = 8 TeV at the LHC. The data are found to be consistent with Standard Model predictions, and 95% confidence-level upper limits are set on the product of cross section and branching ratio. Lastly, these limits range from 0.065 to 43 pb as a function of H 0 and H ± masses, with m h 0 fixed at 125 GeV.
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Measurement of jet charge in dijet events from $\sqrt{s}$ = 8 TeV pp collisions with the ATLAS detector
The momentum-weighted sum of the charges of tracks associated to a jet is sensitive to the charge of the initiating quark or gluon. This paper presents a measurement of the distribution of momentum-weighted sums, called jet charge, in dijet events using 20.3 fb$^{-1}$ of data recorded with the ATLAS detector at $\sqrt{s}$ = 8 TeV in $pp$ collisions at the LHC. The jet charge distribution is unfolded to remove distortions from detector effects and the resulting particle-level distribution is compared with several models. The $p_T$ dependence of the jet charge distribution average and standard deviation are compared to predictions obtained with several leading-order and next-to-leading-order parton distribution functions. The data are also compared to different Monte Carlo simulations of QCD dijet production using various settings of the free parameters within these models. The chosen value of the strong coupling constant used to calculate gluon radiation is found to have a significant impact on the predicted jet charge. There is evidence for a $p_T$ dependence of the jet charge distribution for a given jet flavor. In agreement with perturbative QCD predictions, the data show that the average jet charge of quark-initiated jets decreases in magnitude as the energy of the jet increases.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurements of the charge asymmetry in top-quark pair production in the dilepton final state at s = 8 TeV with the ATLAS detector
We presented measurements of the top-antitop quark pair production charge asymmetry in the dilepton channel, characterized by two high- pT leptons (electrons or muons), using data corresponding to an integrated luminosity of 20.3 fb $-$1 from pp collisions at a center-of-mass energy $\sqrt{s}$ = 8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the $t\bar{t}$ system are performed both in the full phase space and in a fiducial phase space closely matching the detector acceptance. Two observables are studied: A $ℓℓ\atop{C}$ based on the selected leptons and A$t\bar{t}\atop{C}$ based on the reconstructed $t\bar{t}$ final state. The inclusive asymmetries are measured in the full phase space to be A $ℓℓ\atop{C}$ = 0.008 ± 0.006 and A$t\bar{t}\atop{C}$= 0.021 ± 0.016 , which are in agreement with the Standard Model predictions of A $ℓℓ\atop{C}$ = 0.0064 ± 0.0003 and A$t\bar{t}\atop{C}$ = 0.0111 ± 0.0004 .
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Observation of seasonal variation of atmospheric multiple-muon events in the NOvA Near Detector
Using two years of data from the NOvA Near Detector at Fermilab, we report a seasonal variation of cosmic ray induced multiple-muon (Nμ≥2) event rates which has an opposite phase to the seasonal variation in the atmospheric temperature. The strength of the seasonal multiple-muon variation is shown to increase as a function of the muon multiplicity. However, no significant dependence of the strength of the seasonal variation of the multiple-muon variation is seen as a function of the muon zenith angle, or the spatial or angular separation between the correlated muons.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Measurement of the ZZ Production Cross Section in $pp$ Collisions at $\sqrt{s}$ = 13 TeV with the ATLAS Detector
The ZZ production cross section in proton-proton collisions at 13 TeV center-of-mass energy is measured using 3.2 fb$^{-1}$ of data recorded with the ATLAS detector at the Large Hadron Collider. The considered Z boson candidates decay to an electron or muon pair of mass 66-116 GeV. The cross section is measured in a fiducial phase space reflecting the detector acceptance. It is also extrapolated to a total phase space for Z bosons in the same mass range and of all decay modes, giving $16.7^{+2.2}_{-2.0}$(stat)$ ^{+0.9}_{-0.7}$(syst)$ ^{+1.0}_{-0.7}$(lumi) pb. The results agree with standard model predictions.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of the Inelastic Proton-Proton Cross Section at s = 13 TeV with the ATLAS Detector at the LHC
This Letter presents a measurement of the inelastic proton-proton cross section using 60 μ b -1 of p p collisions at a center-of-mass energy √ s of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region ( 2.07 < | η | < 3.86 ) of the detector. A cross section of 68.1 ± 1.4 mb is measured in the fiducial region ξ = M $2\atop{X}$ / s > 10 - 6 , where M X is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this ξ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with M X > 13 GeV . The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross section of 78.1 ± 2.9 mb is measured, consistent with the inelastic cross section increasing with center-of-mass energy.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Photoproduction of the f 2 ( 1270 ) Meson Using the CLAS Detector
The quark structure of the f 2 ( 1270 ) meson has, for many years, been assumed to be a pure quark-antiquark (q$\bar{q}$) resonance with quantum numbers J PC = 2 ++ . Recently, it was proposed that the f 2 ( 1270 ) is a molecular state made from the attractive interaction of two ρ mesons. Such a state would be expected to decay strongly to final states with charged pions due to the dominant decay ρ → π + π - , whereas decay to two neutral pions would likely be suppressed. Here, we measure for the first time the reaction γp → π 0 π 0 p , using the CEBAF Large Acceptance Spectrometer detector at Jefferson Lab for incident beam energies between 3.6 and 5.4 GeV. Differential cross sections, dσ/dt , for f 2 ( 1270 ) photoproduction are extracted with good precision due to low backgrounds and are compared to theoretical calculations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Dual-energy cone-beam CT with a flat-panel detector: Effect of reconstruction algorithm on material classification
Purpose: Cone-beam CT (CBCT) with a flat-panel detector (FPD) is finding application in areas such as breast and musculoskeletal imaging, where dual-energy (DE) capabilities offer potential benefit. The authors investigate the accuracy of material classification in DE CBCT using filtered backprojection (FBP) and penalized likelihood (PL) reconstruction and optimize contrast-enhanced DE CBCT of the joints as a function of dose, material concentration, and detail size. Methods: Phantoms consisting of a 15 cm diameter water cylinder with solid calcium inserts (50–200 mg/ml, 3–28.4 mm diameter) and solid iodine inserts (2–10 mg/ml, 3–28.4 mm diameter), as well as a cadaveric knee with intra-articular injection of iodine were imaged on a CBCT bench with a Varian 4343 FPD. The low energy (LE) beam was 70 kVp (+0.2 mm Cu), and the high energy (HE) beam was 120 kVp (+0.2 mm Cu, +0.5 mm Ag). Total dose (LE+HE) was varied from 3.1 to 15.6 mGy with equal dose allocation. Image-based DE classification involved a nearest distance classifier in the space of LE versus HE attenuation values. Recognizing the differences in noise between LE and HE beams, the LE and HE data were differentially filtered (in FBP) or regularized (in PL). Both a quadratic (PLQ) and a total-variation penalty (PLTV) were investigated for PL. The performance of DE CBCT material discrimination was quantified in terms of voxelwise specificity, sensitivity, and accuracy. Results: Noise in the HE image was primarily responsible for classification errors within the contrast inserts, whereas noise in the LE image mainly influenced classification in the surrounding water. For inserts of diameter 28.4 mm, DE CBCT reconstructions were optimized to maximize the total combined accuracy across the range of calcium and iodine concentrations, yielding values of ∼88% for FBP and PLQ, and ∼95% for PLTV at 3.1 mGy total dose, increasing to ∼95% for FBP and PLQ, and ∼98% for PLTV at 15.6 mGy total dose. For a fixed iodine concentration of 5 mg/ml and reconstructions maximizing overall accuracy across the range of insert diameters, the minimum diameter classified with accuracy >80% was ∼15 mm for FBP and PLQ and ∼10 mm for PLTV, improving to ∼7 mm for FBP and PLQ and ∼3 mm for PLTV at 15.6 mGy. The results indicate similar performance for FBP and PLQ and showed improved classification accuracy with edge-preserving PLTV. A slight preference for increased smoothing of the HE data was found. DE CBCT discrimination of iodine and bone in the knee was demonstrated with FBP and PLTV at 6.2 mGy total dose. Conclusions: For iodine concentrations >5 mg/ml and detail size ∼20 mm, material classification accuracy of >90% was achieved in DE CBCT with both FBP and PL at total doses <10 mGy. Optimal performance was attained by selection of reconstruction parameters based on the differences in noise between HE and LE data, typically favoring stronger smoothing of the HE data, and by using penalties matched to the imaging task (e.g., edge-preserving PLTV in areas of uniform enhancement)
62 RADIOLOGY AND NUCLEAR MEDICINE↗
Measurement of k T splitting scales in W→ℓν events at $\sqrt{s} = 7\ \mathrm{TeV}$ with the ATLAS detector
A measurement of splitting scales, as defined by the k T clustering algorithm, is presented for final states containing a W boson produced in proton–proton collisions at a centre-of-mass energy of 7 TeV. The measurement is based on the full 2010 data sample corresponding to an integrated luminosity of 36 pb -1 which was collected using the ATLAS detector at the CERN Large Hadron Collider. Cluster splitting scales are measured in events containing W bosons decaying to electrons or muons. The measurement comprises the four hardest splitting scales in a k T cluster sequence of the hadronic activity accompanying the W boson, and ratios of these splitting scales. Backgrounds such as multi-jet and top-quark-pair production are subtracted and the results are corrected for detector effects. Predictions from various Monte Carlo event generators at particle level are compared to the data. Overall, reasonable agreement is found with all generators, but larger deviations between the predictions and the data are evident in the soft regions of the splitting scales.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of the centrality and pseudorapidity dependence of the integrated elliptic flow in lead–lead collisions at $\sqrt{s_{\mathrm {NN}}}=2.76$ TeV with the ATLAS detector
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at $\sqrt{s_{\mathrm {NN}}} TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v 2 , was measured in the pseudorapidity range |η| ≤ 2.5 with the event-plane method. In order to include tracks with very low transverse momentum p T , thus reducing the uncertainty in v 2 integrated over p T , a 1 μ b -1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v 2 is compared to other measurements obtained with higher p T thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v 2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for heavy long-lived multi-charged particles in pp collisions at √s = 8 TeV using the ATLAS detector
A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at √s = 8 TeV from pp collisions corresponding to an integrated luminosity of 20.3 fb -1 are examined. Particles producing anomalously high ionisation, consistent with long-lived massive particles with electric charges from |q| = 2e to |q| = 6e are searched for. No signal candidate events are observed, and 95 % confidence level cross-section upper limits are interpreted as lower mass limits for a Drell–Yan production model. The mass limits range between 660 and 785 GeV.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for invisible decays of the Higgs boson produced in association with a hadronically decaying vector boson in pp collisions at √s = 8 with the ATLAS detector
A search for Higgs boson decays to invisible particles is performed using 20.3 fb⁻¹ of pp collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. The process considered is Higgs boson production in association with a vector boson (V = W or Z) that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain V H production followed by H decaying to invisible particles for the Higgs boson mass range 115 < m H < 300 GeV. The 95 % confidence-level observed upper limit on σ VH × BR(H → inv.) varies from 1.6 pb at 115 GeV to 0.13 pb at 300 GeV. Assuming Standard Model production and including the gg → H contribution as signal, the results also lead to an observed upper limit of 78% at 95% confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.
43 PARTICLE ACCELERATORS↗
Searches for scalar leptoquarks in pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector
Searches for pair-produced scalar leptoquarks are performed using 20 fb -1 of proton–proton collision data provided by the LHC and recorded by the ATLAS detector at $\sqrt{s}$ = 8 TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to $bν_τ\bar{b}\bar{ν}_τ$ and $tν_τ\bar{t}\bar{ν}_τ$ final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to m LQ1 < 1050 GeV for first-generation leptoquarks, m LQ2 < 1000 GeV for second-generation leptoquarks, m LQ3 < 625 GeV for third-generation leptoquarks in the $bν_τ\bar{b}\bar{ν}_τ$ channel, and 200 < m LQ3 < 640 GeV in the $tν_τ\bar{t}\bar{ν}_τ$ channel.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurements of fiducial cross-sections for $t\bar{t}$ production with one or two additional b-jets in pp collisions at $\sqrt{s}$ =8 TeV using the ATLAS detector
Fiducial cross-sections for $t\bar{t}$ production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb -1 of proton-proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector.
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Measurement of the centrality dependence of the charged-particle pseudorapidity distribution in proton-lead collisions at root s(NN)=5.02 TeV with the ATLAS detector
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗