Measurement of the Z/γ * boson transverse momentum distribution in pp collisions at √ s = 7 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.chAbstract: This paper describes a measurement of the Z/γ * boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of √ s = 7 TeV at the LHC. The measurement is performed in the Z/γ * → e + e − and Z/γ * → µ + µ − channels, using data corresponding to an integrated luminosity of 4.7 fb −1 . Normalized differential cross sections as a function of the Z/γ * boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/γ * rapidities up to 2.4, as well as in three rapidity bins. The channel results are combined, compared to perturbative and resummed QCD calculations and used to constrain the parton shower parameters of Monte Carlo generators. The ATLAS collaboration 31 Keywords: Hadron-Hadron Scattering IntroductionThe transverse momentum distribution of W and Z bosons produced in hadronic collisions is a traditional probe of strong interaction dynamics. The low transverse momentum (p T ) range is governed by initial-state parton radiation (ISR) and the intrinsic transverse momentum of the initial-state partons inside the proton, and modeled using soft-gluon resummation [1] or parton shower models [2,3]. Quark-gluon scattering dominates at high p T and is described by perturbative QCD [4][5][6]. The correct modelling of the vector boson p T distribution is important in many physics analyses at the LHC for which the production of W or Z bosons constitutes a significant background. Moreover, it is crucial for a precise measurement of the W boson mass. The transverse momentum distribution also probes the gluon density of the proton [7]. Vector boson p T distribution measurements were published by ATLAS [8, 9] and CMS [10] based on 35-40 pb −1 of proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV. The typical precision of these measurements is 4% to 10%.-1 - JHEP09(2014)145This paper presents a measurement of the normalized Z boson transverse momentum distribution (p Z T ) with the ATLAS detector, in the Z/γ * → e + e − and Z/γ * → µ + µ − channels, using LHC proton-proton collision data taken in 2011 at a centre-of-mass energy of √ s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb −1 [11]. The large integrated luminosity allows the measurement to be performed in three different Z boson rapidity (y Z ) bins, probing the transverse momentum dynamics over a wide range of the initial-state parton momentum fraction. With respect to previous results, the present analysis aims at reduced uncertainties, finer binning and extended measurement range.Reconstructed from the final-state lepton kinematics, p Z T is affected by lepton energy and momentum measurement uncertainties. To minimize the impact of these uncertainties, the φ η observable 1 was introduced as an alternative probe of p Z T [12], pioneered at the Tev...
We investigate pair creation by an electric field in four-dimensional de Sitter space. The expectation value of the induced current is computed, using the method of adiabatic regularization. Under strong electric fields the behavior of the current is similar to that in flat space, while under weak electric fields the current becomes inversely proportional to the mass squared of the charged field. Thus we find that the de Sitter space obtains a large conductivity under weak electric fields in the presence of a charged field with a tiny mass. We then apply the results to constrain electromagnetic fields in the early universe. In particular, we study cosmological scenarios for generating large-scale magnetic fields during the inflationary era. Electric fields generated along with the magnetic fields can induce sufficiently large conductivity to terminate the phase of magnetogenesis. For inflationary magnetogenesis models with a modified Maxwell kinetic term, the generated magnetic fields cannot exceed 10 −30 G on Mpc scales in the present epoch, when a charged field carrying an elementary charge with mass of order the Hubble scale or smaller exists in the Lagrangian. Similar constraints from the Schwinger effect apply for other magnetogenesis mechanisms.
A search is performed for narrow resonances decaying into W W , W Z, or ZZ boson pairs using 20.3 fb −1 of proton-proton collision data at a centre-of-mass energy of √ s = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution of dijets where both jets are tagged as a boson jet, compatible with a highly boosted W or Z boson decaying to quarks, using jet mass and substructure properties. The largest deviation from a smoothly falling background in the observed dijet invariant mass distribution occurs around 2 TeV in the W Z channel, with a global significance of 2.5 standard deviations. Exclusion limits at the 95% confidence level are set on the production cross section times branching ratio for the W Z final state of a new heavy gauge boson, W , and for the W W and ZZ final states of Kaluza-Klein excitations of the graviton in a bulk Randall-Sundrum model, as a function of the resonance mass. W bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level. The ATLAS collaboration 22 IntroductionThe substantial dataset of Large Hadron Collider (LHC) proton-proton (pp) collisions at √ s = 8 TeV collected by the ATLAS experiment provides a distinct opportunity to search for new heavy resonances at the TeV mass scale. This paper presents a search for narrow diboson resonances (W W , W Z and ZZ) decaying to fully hadronic final states. The fully hadronic mode has a higher branching fraction than leptonic and semileptonic decay modes, and is therefore used to extend the reach of the search to the highest possible resonance masses.W and Z bosons resulting from the decay of very massive resonances are highly boosted, so that each boson's hadronic decay products are reconstructed as a single jet. The signature of the heavy resonance decay is thus a resonance structure in the dijet invariant mass spectrum. The dominant background for this search is due to dijet events from QCD processes, which produce a smoothly falling spectrum without resonance structures. To cope with this large background, jets are selected using a boson tagging procedure based on -1 - JHEP12(2015)055a reclustering-mass-drop filter (BDRS-A, similar to the method introduced in ref.[1]), jet mass, and further substructure properties. The tagging procedure strongly suppresses the dijet background, although these QCD processes still overwhelm the expected backgrounds from single boson production with one or more jets, Standard Model (SM) diboson production, single-top and top-pair production. As all of these background sources produce dijet invariant mass distributions without resonance peaks, the expected background in the search is modelled by a fit to a smoothly falling distribution.Diboson resonances are predicted in several extensions to the SM, such as technicolour [2][3][4], warped extra dimensions [5][6][7], and Grand Unified Theories [...
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