Showing papers on "Elementary particle published in 2015"

Observation of the rare B-s(0)->mu(+)mu(-) decay from the combined analysis of CMS and LHCb data

Abstract: The standard model of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The probabilities, or branching fractions, of the strange B meson (B-s(0)) and the B-0 meson decaying into two oppositely charged muons (mu(+) and mu(-)) are especially interesting because of their sensitivity to theories that extend the standard model. The standard model predicts that the B-s(0)->mu(+)mu(-) and B-0 ->mu(+)mu(-) decays are very rare, with about four of the former occurring for every billion B-s(0) mesons produced, and one of the latter occurring for every ten billion B-0 mesons(1). A difference in the observed branching fractions with respect to the predictions of the standard model would provide a direction in which the standard model should be extended. Before the Large Hadron Collider (LHC) at CERN2 started operating, no evidence for either decay mode had been found. Upper limits on the branching fractions were an order of magnitude above the standard model predictions. The CMS (Compact Muon Solenoid) and LHCb(Large Hadron Collider beauty) collaborations have performed a joint analysis of the data from proton-proton collisions that they collected in 2011 at a centre-of-mass energy of seven teraelectronvolts and in 2012 at eight teraelectronvolts. Here we report the first observation of the B-s(0)->mu(+)mu(-) decay, with a statistical significance exceeding six standard deviations, and the best measurement so far of its branching fraction. Furthermore, we obtained evidence for the B-0 ->mu(+)mu(-) decay with a statistical significance of three standard deviations. Both measurements are statistically compatible with standard model predictions and allow stringent constraints to be placed on theories beyond the standard model. The LHC experiments will resume taking data in 2015, recording proton-proton collisions at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double the production rates of B-s(0) and B-0 mesons and lead to further improvements in the precision of these crucial tests of the standard model.

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Abstract: Results of a search for H -> tau tau 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 root s = 7TeV and root s = 8 TeV respectively. All combinations of leptonic (tau -> l nu(nu) over bar with l = e, mu) and hadronic (tau -> hadrons nu) 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, normalised to the Standard Model expectation, of mu = 1.43(-0.37)(+0.43) is consistent with the predicted Yukawa coupling strength in the Standard Model.

A taste of dark matter: flavour constraints on pseudoscalar mediators

Abstract: Dark matter interacting via the exchange of a light pseudoscalar can induce observable signals in indirect detection experiments and experience large self-interactions while evading the strong bounds from direct dark matter searches. The pseudoscalar mediator will however induce flavour-changing interactions in the Standard Model, providing a promising alternative way to test these models. We investigate in detail the constraints arising from rare meson decays and fixed target experiments for different coupling structures between the pseudoscalar and Standard Model fermions. The resulting bounds are highly complementary to the information inferred from the dark matter relic density and the constraints from primordial nucleosynthesis. We discuss the implications of our findings for the dark matter self-interaction cross section and the prospects of probing dark matter coupled to a light pseudoscalar with direct or indirect detection experiments. In particular, we find that a pseudoscalar mediator can only explain the Galactic Centre excess if its mass is above that of the B mesons, and that it is impossible to obtain a sufficiently large direct detection cross section to account for the DAMA modulation.

Twin Higgs mechanism and a composite Higgs boson

Abstract: We combine the twin Higgs mechanism with the paradigm of composite Higgs models. In this class of models the Higgs is a pseudo-Nambu-Goldstone boson from a strongly coupled sector near the TeV scale, and it is additionally protected by a discrete symmetry due to the twin mechanism. We discuss the model-building issues associated with this setup and quantify the tuning needed to achieve the correct electroweak vacuum and the Higgs mass. In contrast to standard composite Higgs models, the lightest resonance associated with the top sector is the uncolored mirror top, while the colored top partners can be made parameterically heavier without extra tuning. In some cases, the vector resonances are predicted to lie in the multi-TeV range. We present models where the resonances---both fermions and vectors---being heavier alleviates the pressure on naturalness coming from direct searches demonstrating that theories with low tuning may survive constraints from the Large Hadron Collider.

The Hunt for the Rest of the Higgs Bosons

Abstract: We assess the current state of searches at the LHC for additional Higgs bosons in light of both direct limits and indirect bounds coming from coupling measurements of the Standard Model-like Higgs boson. Given current constraints, we identify and study three LHC searches that are critical components of a comprehensive program to investigate extended electroweak symmetry breaking sectors: production of a heavy scalar or pseudoscalar with decay to $$ t\overline{t} $$ ; $$ b\overline{b} $$ and $$ t\overline{t} $$ associated production of a heavy scalar or pseudoscalar with decay to invisible final states; and $$ t\overline{b} $$ associated production of a charged Higgs with decay to $$ t\overline{b} $$ . Systematic experimental searches in these channels would contribute to robust coverage of the possible single production modes of additional heavy Higgs bosons.

Gravitational spin Hamiltonians from the S matrix

Abstract: We utilize generalized unitarity and recursion relations combined with effective field theory techniques to compute spin-dependent interaction terms for an inspiralling binary system in the post-Newtonian (PN) approximation. Using these methods offers great computational advantage over traditional techniques involving Feynman diagrams, especially at higher orders in the PN expansion. As a specific example, we reproduce the spin-orbit (up to 2.5PN order) and the leading-order ${S}^{2}$ (2PN) Hamiltonian for a binary system with one of the massive objects having nonzero spin using the $S$-matrix elements of elementary particles. For the same system, we also obtain the ${S}^{3}$ (3.5PN) spin Hamiltonian for an arbitrary massive object, which was until now known only for a black hole. Furthermore, we derive the missing ${S}^{4}$ Hamiltonian at leading order (4PN), again for an arbitrary massive object and establish that the minimal coupling of an elementary particle to gravity automatically captures the physics of a spinning black hole. Finally, the Kerr metric is obtained as a series in ${G}_{N}$ by comparing the action of a test particle in the vicinity of a spinning black hole to the derived potential.

The Composite Nambu-Goldstone Higgs

Abstract: The composite Higgs scenario, in which the Higgs emerges as a composite pseudo-Nambu-Goldstone boson, is extensively reviewed in these Notes. The material is presented in a pedagogical fashion, with great emphasis on the conceptual and technical foundations of the construction. A comprehensive summary of the flavor, collider and electroweak precision phenomenology is also presented.

Higgs inflation from Standard Model criticality

Abstract: In this chapter, we investigate the possibility that the SM Higgs boson plays the role of an inflation in light of the discovery of the Higgs boson. In 2007, Bezurkov and Shaposhnikov first pointed out this possibility. The successful Higgs inflation is realized if the non-minimal coupling between the Higgs H and scalar curvature \(\mathcal {R}\), \(\xi |H|^2 \mathcal {R}\), is introduced. \(\xi \) is the coupling constant, and very large value, \(\xi \sim 10^5\), is required for successful inflation. However, the observation of the Higgs and determination of its mass changes the situation. As we have seen in Chap. 2, the Higgs self coupling and its beta function becomes zero at very high scale, which means that the Higgs potential is very flat around string/Planck scale. This opens up the new possibilities of the Higgs inflation, which we will present here. In Sect. 3.1, we present the general argument of the Higgs inflation above the SM cutoff \(\Lambda \). In Sect. 3.2, we show that non-minimal coupling \(\xi \) can be as small as \(\mathcal {O}(10)\).

Fully covering the MSSM Higgs sector at the LHC

Abstract: In the context of the Minimal Supersymmetric extension of the Standard Model (MSSM), we reanalyze the search for the heavier CP-even H and CP-odd A neutral Higgs bosons at the LHC in their production in the gluon-fusion mechanism and their decays into gauge and lighter h bosons and into top quark pairs. We show that only when considering these processes, that one can fully cover the entire parameter space of the Higgs sector of the model. Indeed, they are sensitive to the low tan β and high Higgs mass ranges, complementing the traditional searches for high mass resonances decaying into τ -lepton pairs which are instead sensitive to the large and moderate tan β regions. The complementarity of the various channels in the probing of the complete [tan β, M A ] MSSM parameter space at the previous and upcoming phases of the LHC is illustrated in a recently proposed simple and model independent approach for the Higgs sector, the hMSSM, that we also refine in this paper.

Higgs pair production at next-to-next-to-leading logarithmic accuracy at the LHC

Abstract: We perform the threshold resummation for Higgs pair production in the dominant gluon fusion channel to next-to-next-to-leading logarithmic (NNLL) accuracy. The calculation includes the matching to the next-to-next-to-leading order (NNLO) cross section obtained in the heavy top-quark limit, and results in an increase of the inclusive cross section up to 7% at the LHC with centre-of-mass energy E cm = 14 TeV, for the choice of factorization and renormalization scales μ F = μ R = Q, being Q the invariant mass of the Higgs pair system. After the resummation is implemented, we estimate the theoretical uncertainty from the perturbative expansion to be reduced to about ±5.5%, plus ∼ 10% from finite top-mass effects. The resummed cross section turns out to be rather independent of the value chosen for the central factorization and renormalization scales in the usual range (Q/2, Q).

Neutron Electric Dipole Moment and Tensor Charges from Lattice QCD

Abstract: We present lattice QCD results on the neutron tensor charges including, for the first time, a simultaneous extrapolation in the lattice spacing, volume, and light quark masses to the physical point in the continuum limit. We find that the "disconnected" contribution is smaller than the statistical error in the "connected" contribution. Our estimates in the modified minimal subtraction scheme at 2 GeV, including all systematics, are g_{T}^{d-u}=1.020(76), g_{T}^{d}=0.774(66), g_{T}^{u}=-0.233(28), and g_{T}^{s}=0.008(9). The flavor diagonal charges determine the size of the neutron electric dipole moment (EDM) induced by quark EDMs that are generated in many new scenarios of CP violation beyond the standard model. We use our results to derive model-independent bounds on the EDMs of light quarks and update the EDM phenomenology in split supersymmetry with gaugino mass unification, finding a stringent upper bound of d_{n}<4×10^{-28} e cm for the neutron EDM in this scenario.

Higgs bosons in heavy supersymmetry with an intermediate m A

Abstract: The minimal supersymmetric standard model leads to precise predictions of the properties of the light Higgs boson degrees of freedom that depend on only a few relevant supersymmetry breaking parameters. In particular, there is an upper bound on the mass of the lightest neutral Higgs boson, which for a supersymmetric spectrum of the order of a TeV is barely above the one of the Higgs resonance recently observed at the LHC. This bound can be raised by considering a heavier supersymmetric spectrum, relaxing the tension between theory and experiment. In a previous article, we studied the predictions for the lightest CP-even Higgs mass for large values of the scalar-top and heavy Higgs boson masses. In this article we perform a similar analysis, considering also the case of a CP-odd Higgs boson mass $m_A$ of the order of the weak scale. We perform the calculation using effective theory techniques, considering a two-Higgs doublet model and a Standard Model-like theory and resumming the large logarithmic corrections that appear at scales above and below $m_A$, respectively. We calculate the mass and couplings of the lightest CP-even Higgs boson and compare our results with the ones obtained by other methods.

WIMPs at the galactic center

Abstract: Simple models of weakly interacting massive particles (WIMPs) predict dark matter annihilations into pairs of electroweak gauge bosons, Higgses or tops, which through their subsequent cascade decays produce a spectrum of gamma rays. Intriguingly, an excess in gamma rays coming from near the Galactic center has been consistently observed in Fermi data. A recent analysis by the Fermi collaboration confirms these earlier results. Taking into account the systematic uncertainties in the modelling of the gamma ray backgrounds, we show for the first time that this excess can be well fit by these final states. In particular, for annihilations to (WW, ZZ, hh, tt{sup -bar}), dark matter with mass between threshold and approximately (165, 190, 280, 310) GeV gives an acceptable fit. The fit range for bb{sup -bar} is also enlarged to 35 GeV≲m{sub χ}≲165 GeV. These are to be compared to previous fits that concluded only much lighter dark matter annihilating into b, τ, and light quark final states could describe the excess. We demonstrate that simple, well-motivated models of WIMP dark matter including a thermal-relic neutralino of the MSSM, Higgs portal models, as well as other simplified models can explain the excess.

Towards an understanding of the correlations in jet substructure

Abstract: Over the past decade, a large number of jet substructure observables have been proposed in the literature, and explored at the LHC experiments. Such observables attempt to utilize the internal structure of jets in order to distinguish those initiated by quarks, gluons, or by boosted heavy objects, such as top quarks and W bosons. This report, originating from and motivated by the BOOST2013 workshop, presents original particle-level studies that aim to improve our understanding of the relationships between jet substructure observables, their complementarity, and their dependence on the underlying jet properties, particularly the jet radius and jet transverse momentum. This is explored in the context of quark/gluon discrimination, boosted W boson tagging and boosted top quark tagging.

Higgs production in association with bottom quarks

Abstract: We study the production of a Higgs boson in association with bottom quarks in hadronic collisions, and present phenomenological predictions relevant to the 13 TeV LHC. Our results are accurate to the next-to-leading order in QCD, and matched to parton showers through the MC@NLO method; thus, they are fully differential and based on unweighted events, which we shower by using both Herwig++ and Pythia8. We perform the computation in both the four-flavour and the five-flavour schemes, whose results we compare extensively at the level of exclusive observables. In the case of the Higgs transverse momentum, we also consider the analytically-resummed cross section up to the NNLO+NNLL accuracy. In addition, we analyse at $$ \mathcal{O}\left({\alpha}_S^3\right) $$ the effects of the interference between the $$ b\overline{b}H $$ and gluon-fusion production modes.

Vector interaction enhanced bag model for astrophysical applications

Abstract: For quark matter studies in astrophysics the thermodynamic bag model (tdBAG) has been widely used. Despite its success it fails to account for various phenomena expected from Quantum-ChromoDynamics (QCD). We suggest a straightforward extension of tdBAG in order to take the dynamical breaking of chiral symmetry and the influence of vector interactions explicitly into account. As for tdBAG the model mimics confinement in a phenomenological approach. It is based on an analysis of the Nambu–Jona-Lasinio (NJL) model at finite density. Furthermore, we demonstrate how NJL and bag models in this regime follow from the more general and QCD based framework of DysonSchwinger (DS) equations in medium by assuming a simple gluon contact interaction. Based on our simple and novel model, we construct quark hadron hybrid equations of state (EoS) and study systematically chiral and deconfinement phase transitions, the appearance of s-quarks and the role of vector interaction. We further study these aspects for matter in β-equilibrium at zero temperature, with particular focus on the current ∼ 2 M⊙ maximum mass constraint for neutron stars. Our approach indicates that the currently only theoretical evidence for the hypothesis of stable strange matter is an artifact of tdBAG and results from neglecting the dynamical breaking of chiral symmetry. Subject headings: dense matter — equation of state — elementary particles: quarks — stars: neutron

Lepton-flavor-violating Higgs decay h → μτ and muon anomalous magnetic moment in a general two Higgs doublet model

Abstract: A two Higgs doublet model (2HDM) is one of minimal extensions of the Standard Model (SM), and it is well-known that the general setup predicts the flavor-violating phenomena, mediated by neutral Higgs interactions. Recently the CMS collaboration has reported an excess of the lepton-flavor-violating Higgs decay in h → μτ channel with a significance of 2.4 σ. We investigate the CMS excess in a general 2HDM with tree-level Flavor Changing Neutral Currents (FCNCs), and discuss its impact on the other physical observations. Especially, we see that the FCNCs relevant to the excess can enhance the neutral Higgs contributions to the muon anomalous magnetic moment, and can resolve the discrepancy between the measured value and the SM prediction. We also find that the couplings to be consistent with the anomaly of the muon magnetic moment as well as the CMS excess in h → μτ predict the sizable rate of τ → μγ, which is within the reach of future B factory.

Mass effects in the Higgs-gluon coupling : boosted vs. off-shell production.

Abstract: In the upcoming LHC run we will be able to probe the structure of the loopinduced Higgs-gluon coupling through kinematics. First, we establish state-of-the-art simulations with up to two jets to next-to-leading order including top mass effects. They allow us to search for deviations from the low-energy limits in boosted Higgs production. In addition, the size of the top mass effects suggests that they should generally be included in Higgs studies at the LHC. Next, we show how off-shell Higgs production with a decay to four leptons is sensitive to the same top mass effects. We compare the potential of both methods based on the same top-Higgs Lagrangian. Finally, we comment on related model assumptions required for a Higgs width measurement.

Singlet-like Higgs bosons at present and future colliders

Abstract: The presence of extra scalar singlets is a feature of several motivated extensions of the Standard Model, and the mixing of such a singlet with the Higgs boson is allowed to be quite large by current experiments. In this paper we perform a thorough phenomeno-logical study of this possibility. We consider both direct and indirect searches, and we quantify the current constraints as well as the prospects for future hadron and lepton machines — from the forthcoming LHC run up to a futuristic 100 TeV proton-proton collider. The direct reaches are obtained extrapolating the current limits with a technique that we discuss and check with various tests. We find a strong complementarity between direct and indirect searches, with the former dominating for lower values of the singlet mass. We also find that the trilinear Higgs coupling can have sizeable deviations from its Standard Model value, a fact for which we provide an analytical understanding. The results are first presented in a general scalar singlet extension of the Standard Model, taking advantage of the very small number of parameters relevant for the phenomenology. Finally, we specify the same analysis to a few most natural models, i.e. the Next-to-Minimal Supersymmetric Standard Model, Twin Higgs and Composite Higgs.

Higher-order QCD predictions for dark matter production at the LHC in simplified models with s-channel mediators.

Abstract: Weakly interacting dark matter particles can be pair-produced at colliders and detected through signatures featuring missing energy in association with either QCD/EW radiation or heavy quarks. In order to constrain the mass and the couplings to standard model particles, accurate and precise predictions for production cross sections and distributions are of prime importance. In this work, we consider various simplified models with s-channel mediators. We implement such models in the FeynRules/MadGraph5_aMC@NLO framework, which allows to include higher-order QCD corrections in realistic simulations and to study their effect systematically. As a first phenomenological application, we present predictions for dark matter production in association with jets and with a top-quark pair at the LHC, at next-to-leading order accuracy in QCD, including matching/merging to parton showers. Our study shows that higher-order QCD corrections to dark matter production via s-channel mediators have a significant impact not only on total production rates, but also on shapes of distributions. We also show that the inclusion of next-to-leading order effects results in a sizeable reduction of the theoretical uncertainties.

Supersymmetric dark matter after LHC run 1

Abstract: Different mechanisms operate in various regions of the MSSM parameter space to bring the relic density of the lightest neutralino, [Formula: see text], assumed here to be the lightest SUSY particle (LSP) and thus the dark matter (DM) particle, into the range allowed by astrophysics and cosmology. These mechanisms include coannihilation with some nearly degenerate next-to-lightest supersymmetric particle such as the lighter stau [Formula: see text], stop [Formula: see text] or chargino [Formula: see text], resonant annihilation via direct-channel heavy Higgs bosons H / A, the light Higgs boson h or the Z boson, and enhanced annihilation via a larger Higgsino component of the LSP in the focus-point region. These mechanisms typically select lower-dimensional subspaces in MSSM scenarios such as the CMSSM, NUHM1, NUHM2, and pMSSM10. We analyze how future LHC and direct DM searches can complement each other in the exploration of the different DM mechanisms within these scenarios. We find that the [Formula: see text] coannihilation regions of the CMSSM, NUHM1, NUHM2 can largely be explored at the LHC via searches for [Formula: see text] events and long-lived charged particles, whereas their H / A funnel, focus-point and [Formula: see text] coannihilation regions can largely be explored by the LZ and Darwin DM direct detection experiments. We find that the dominant DM mechanism in our pMSSM10 analysis is [Formula: see text] coannihilation: parts of its parameter space can be explored by the LHC, and a larger portion by future direct DM searches.

What's in the loop? The anatomy of double Higgs production

Abstract: Determination of Higgs self-interactions through the double Higgs production from gluon fusion is a major goal of current and future collider experiments. We point out this channel could help disentangle and resolve the nature of ultraviolet contributions to Higgs couplings to two gluons. Analytic properties of the double Higgs amplitudes near kinematic threshold are used to study features resulting from scalar and fermionic loop particles mediating the interaction. Focusing on the hh invariant mass spectrum, we consider the effect from anomalous top and bottom Yukawa couplings, as well as from scalar and fermionic loop particles. In particular, the spectrum at high hh invariant mass is sensitive to the spin of the particles in the loop.

The Higgs legacy of the LHC Run I

Abstract: Based on Run I data we present a comprehensive analysis of Higgs couplings. For the first time this SFitter analysis includes independent tests of the Higgs-gluon and top Yukawa couplings, Higgs decays to invisible particles, and off-shell Higgs measurements. The observed Higgs boson is fully consistent with the Standard Model, both in terms of coupling modifications and effective field theory. Based only on Higgs total rates the results using both approaches are essentially equivalent, with the exception of strong correlations in the parameter space induced by effective operators. These correlations can be controlled through additional experimental input, namely kinematic distributions. Including kinematic distributions the typical Run I reach for weakly interacting new physics now reaches 300 to 500 GeV.

Fiducial cross sections for Higgs boson production in association with a jet at next-to-next-to-leading order in QCD

Abstract: We extend the recent computation of Higgs boson production in association with a jet through next-to-next-to-leading order in perturbative QCD by including decays of the Higgs boson to electroweak vector bosons. This allows us to compute ducial cross sections and kinematic distributions including realistic selection criteria for the Higgs boson decay products. As an illustration, we present results for pp! H +j! +j closely following the ATLAS 8 TeV analysis and for pp! H +j! W + W +j! e + +j in a CMS-like 13 TeV setup.

Light Higgs bosons in two-Higgs-doublet models

Abstract: We explore the possibilities in two-Higgs-doublet models (2HDMs) of Type I and Type II for Higgs states with mass below about 60 GeV, i.e. less than half of the ~125 GeV mass of the observed SM-like Higgs boson. We identify the latter as either the lighter or the heavier CP-even state, h or H, and employ scans of the 2HDM parameter space taking into account all relevant theoretical and experimental constraints, including the most up-to-date Higgs signal strength measurements. We find that, in both Type I and Type II models, such light Higgs states are phenomenologically viable and can lead to interesting signatures. Part of the relevant parameter space may be testable with the existing 8 TeV LHC data, e.g. by looking for direct production of the light state via gg-fusion or bb-associated-production using its \tau^+ \tau^- and \mu^+ \mu^- decays at low invariant mass.

The Orbifold Higgs

Abstract: We introduce and systematically study an expansive class of “orbifold Higgs” theories in which the weak scale is protected by accidental symmetries arising from the orbifold reduction of continuous symmetries The protection mechanism eliminates quadratic sensitivity of the Higgs mass to higher scales at one loop (or more) and does not involve any new states charged under the Standard Model The structures of the Higgs and top sectors are universal and determined exclusively by group theoretical considerations The twin Higgs model fits within our framework as the simplest example of an orbifold Higgs Our models admit UV completions as geometric orbifolds in higher dimensions, and fit naturally within frameworks of low scale gauge coupling unification

LHC constraints on gauge boson couplings to dark matter

Abstract: Collider searches for energetic particles recoiling against missing transverse energy $({\overline{)E}}_{T})$ allow us to place strong bounds on the interactions between dark matter (DM) and standard model (SM) particles. In this article we update and extend LHC constraints on effective dimension-7 operators involving DM and electroweak gauge bosons. A concise comparison of the sensitivity of the monophoton, mono-$W$, mono-$Z$, mono-$W/Z$, and invisible Higgs-boson decays in the vector boson fusion mode and the monojet channel is presented. Depending on the parameter choices, either the monophoton or the monojet data provide the most stringent bounds at the moment. We furthermore explore the potential of improving the current 8 TeV limits at 14 TeV. Future strategies capable of disentangling the effects of the different effective operators involving electroweak gauge bosons are discussed as well.