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We show how the BRST identities can be used to control the renormalization of the background gauge in QCD, in spite of the fact that certain one-particle reducible graphs have to be omitted in general. We obtain the all order renormalized effective action for the background field theory.
The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in $t\bar{t}$ events. The topology of the t → Wb decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 fb -1 , were collected in proton–proton collisions during the years 2015–2017 at a centre-of-mass energy $ \sqrt{s}$ = 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.
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We study a warped extra-dimension scenario where the standard model fields lie in the bulk, with the addition of a fourth family of fermions. We concentrate on the flavor structure of the Higgs couplings with fermions in the flavor anarchy ansatz. Even without a fourth family, these couplings will be generically misaligned with respect to the standard model fermion mass matrices. The presence of the fourth family typically enhances the misalignment effects and we show that one should expect them to be highly nonsymmetrical in the (34) intergenerational mixing. The radiative corrections from the new fermions and their flavor-violating couplings to the Higgs affect negligibly known experimental precision measurements such as the oblique parameters and Z{yields}bb or Z{yields}{mu}{sup +}{mu}{sup -}. On the other hand, {Delta}F=1, 2 processes, mediated by tree-level Higgs exchange, as well as radiative corrections to b{yields}s{gamma} and {mu}{yields}e{gamma} put some generic pressure on the allowed size of the flavor-violating couplings. But more importantly, these couplings will alter the Higgs decay patterns as well as those of the new fermions, and produce very interesting new signals associated to Higgs phenomenology in high energy colliders. These signals might become very important indirect signals for these type of models as they would be present even when the KK mass scale is high and no heavy KK particle is discovered.
Azimuthal correlations of charged particles in xenon-xenon collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=5.44$ TeV are studied. The data were collected by the CMS experiment at the LHC with a total integrated luminosity of 3.42μb -1 . The collective motion of the system formed in the collision is parametrized by a Fourier expansion of the azimuthal particle density distribution. The azimuthal anisotropy coefficients $v_2$, $v_3$, and $v_4$ are obtained by the scalar-product, two-particle correlation, and multiparticle correlation methods. Within a hydrodynamic picture, these methods have different sensitivities to noncollective and fluctuation effects. The dependence of the Fourier coefficients on the size of the colliding system is explored by comparing the xenon-xenon results with equivalent lead-lead data. Model calculations that include initial-state fluctuation effects are also compared to the experimental results. The observed angular correlations provide new constraints on the hydrodynamic description of heavy ion collisions.
Electroweak corrections can play a crucial role in dark matter annihilation. The emission of gauge bosons, in particular, leads to a secondary flux consisting of all Standard Model particles, and may be described by electroweak fragmentation functions. To assess the quality of the fragmentation function approximation to electroweak radiation in dark matter annihilation, we have calculated the flux of secondary particles from gauge-boson emission in models with Majorana fermion and vector dark matter, respectively. For both models, we have compared cross sections and energy spectra of positrons and antiprotons after propagation through the galactic halo in the fragmentation function approximation and in the full calculation. Fragmentation functions fail to describe the particle fluxes in the case of Majorana fermion annihilation into light fermions: the helicity suppression of the lowest-order cross section in such models cannot be lifted by the leading logarithmic contributions included in the fragmentation function approach. However, for other classes of models like vector dark matter, where the lowest-order cross section is not suppressed, electroweak fragmentation functions provide a simple, model-independent and accurate description of secondary particle fluxes.
A search for a Higgs boson has been performed in the H{yields}WW{yields}l{nu}jj channel in 1.04 fb{sup -1} of pp collision data at {radical}(s)=7 TeV recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range 240 GeV<m{sub H}<600 GeV. The best sensitivity is reached for m{sub H}=400 GeV, where the 95% confidence level upper bound on the cross section for H{yields}WW production is 3.1 pb, or 2.7 times the standard model prediction.
Spaces with a Weyl-type connection and torsion of a special type that are determined by the structure of the differentiability conditions in the algebra of complex quaternions are considered. These conditions are consistent only if the curvature of the connection is self-dual. The Maxwell and SL (2,ℂ) Yang-Mills fields associated with the irreducible components of the connection also turn out to be self-dual, so that the corresponding equations are fulfilled on the solutions of the generating system. Using the twistor structure of the latter, its general solution is obtained. The singular locus has a string-like (particle-like) structure generating the self-consistent algebraic dynamics of the string system.
We present measurements of J/{psi} yields in d+Au collisions at {radical}(s{sub NN})=200 GeV recorded by the PHENIX experiment and compare them with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/{psi} rapidity (-2.2<y<2.4) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. In order to remove model dependent systematic uncertainties we also compare the data to a simple geometric model. The forward rapidity data are inconsistent with nuclear modifications that are linear or exponential in the density weighted longitudinal thickness, such as those from the final state breakup of the bound state.
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So far the squarks have not been detected at the LHC indicating that they are heavier than a few hundred GeVs, if they exist. The lighter stop can be considerably lighter than the other squarks. We study the possibility that a supersymmetric partner of the top quark, stop, is the next-to-lightest supersymmetric particle in the constrained supersymmetric standard model. Various constraints, on top of the mass limits, are taken into an account, and the allowed parameter space for this scenario is determined. Observing stop which is the next-to-lightest supersymmetric particle at the LHC may be difficult.
Results of a search for H → ττ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb –1 and 20.3 fb –1 at centre-of-mass energies of √s=7 TeV and √s=8 TeV respectively. All combinations of leptonic (τ → ℓνν¯ with ℓ = e, μ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalized to the Standard Model expectation, of μ = 1.43 –0.37 +0.43 is consistent with the predicted Yukawa coupling strength in the Standard Model.
A search for pair production of vector-like quarks, both up-type (T) and down-type (B), as well as for four-top-quark production, is presented. The search is based on pp collisions at \( \sqrt{s}=8 \) TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb –1 . Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. Dedicated analyses are performed targeting three cases: a T quark with significant branching ratio to a W boson and a b-quark \( \left(T\overline{T}\to Wb+\mathrm{X}\right) \), and both a T quark and a B quark with significant branching ratio to a Higgs boson and a third-generation quark (\( T\overline{T}\to Ht+X\;\mathrm{and}\;B\overline{B}\to Hb+\mathrm{X} \) respectively). No significant excess of events above the Standard Model expectation is observed, and 95% CL lower limits are derived on the masses of the vector-like T and B quarks under several branching ratio hypotheses assuming contributions from T → Wb, Zt, Ht and B → Wt, Zb, Hb decays. The 95% CL observed lower limits on the T quark mass range between 715 GeV and 950 GeV for all possible values of the branching ratios into the three decay modes, and are the most stringent constraints to date. In addition, the most restrictive upper bounds on four-top-quark production are set in a number of new physics scenarios.
ATLAS results on the production of photons and Z bosons in lead-lead collisions at the LHC are presented. Their production rates are found to scale with the number of binary collisions, as expected from QCD factorization at large transverse momentum. This makes them an ideal tool for probing the modification of jets which balance their transverse momentum. Despite the large differences in available statistics, both channels show a strong modification of the energy of the recoil jets, which increases with centrality.
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