Showing papers in "Physics Letters B in 2012"

Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC

Abstract: 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 View the MathML source in 2011 and 5.8 fb−1 at View the MathML source 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(⁎), View the MathML source 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 View the MathML source 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.

Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC

Abstract: Results are presented from searches for the standard model Higgs boson in proton-proton collisions at sqrt(s)=7 and 8 TeV in the CMS experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at 7 TeV and 5.3 inverse femtobarns at 8 TeV. The search is performed in five decay modes: gamma gamma, ZZ, WW, tau tau, and b b-bar. An excess of events is observed above the expected background, a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, gamma gamma and ZZ; a fit to these signals gives a mass of 125.3 +/- 0.4 (stat.) +/- 0.5 (syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.

Top-pair production at hadron colliders with next-to-next-to-leading logarithmic soft-gluon resummation

Abstract: Incorporating all recent theoretical advances, we resum soft-gluon corrections to the total t t ¯ cross-section at hadron colliders at the next-to-next-to-leading logarithmic (NNLL) order. We perform the resummation in the well established framework of Mellin N-space resummation. We exhaustively study the sources of systematic uncertainty like renormalization and factorization scale variation, power suppressed effects and missing two- and higher-loop corrections. The inclusion of soft-gluon resummation at NNLL brings only a minor decrease in the perturbative uncertainty with respect to the NLL approximation, and a small shift in the central value, consistent with the quoted uncertainties. These numerical predictions agree with the currently available measurements from the Tevatron and LHC and have uncertainty of similar size. We conclude that significant improvements in the t t ¯ cross-sections can potentially be expected only upon inclusion of the complete NNLO corrections.

Higgs mass implications on the stability of the electroweak vacuum

Abstract: We update instability and metastability bounds of the Standard Model electroweak vacuum in view of the recent ATLAS and CMS Higgs results. For a Higgs mass in the range 124–126 GeV, and for the current central values of the top mass and strong coupling constant, the Higgs potential develops an instability around 1011 GeV, with a lifetime much longer than the age of the Universe. However, taking into account theoretical and experimental errors, stability up to the Planck scale cannot be excluded. Stability at finite temperature implies an upper bound on the reheat temperature after inflation, which depends critically on the precise values of the Higgs and top masses. A Higgs mass in the range 124–126 GeV is compatible with very high values of the reheating temperature, without conflict with mechanisms of baryogenesis such as leptogenesis. We derive an upper bound on the mass of heavy right-handed neutrinos by requiring that their Yukawa couplings do not destabilize the Higgs potential.

Combined search for the Standard Model Higgs boson using up to 4.9 fb-1 of pp collision data at s=7TeV with the ATLAS detector at the LHC

Abstract: A combined search for the Standard Model Higgs boson with the ATLAS experiment at the LHC using datasets corresponding to integrated luminosities from 1.04 fb(-1) to 4.9 fb(-1) of pp collisions col ...

Implications of LHC searches for Higgs-portal dark matter

Abstract: The search for the a Standard Model Higgs boson at the LHC is reaching a critical stage as the possible mass range for the particle has become extremely narrow and some signal at a mass of about 125 GeV is starting to emerge. We study the implications of these LHC Higgs searches for Higgs-portal models of dark matter in a rather model independent way. Their impact on the cosmological relic density and on the direct detection rates are studied in the context of generic scalar, vector and fermionic thermal dark matter particles. Assuming a sufficiently small invisible Higgs decay branching ratio, we find that current data, in particular from the XENON experiment, essentially exclude fermionic dark matter as well as light, i.e. with masses below ≈60 GeV, scalar and vector dark matter particles. Possible observation of these particles at the planned upgrade of the XENON experiment as well in collider searches is discussed.

Implications of a 125 GeV Higgs for supersymmetric models

Abstract: Preliminary results of the search for a Standard Model like Higgs boson at the LHC with 5 fb−1 data have just been presented by the ATLAS and CMS Collaborations and an excess of events at a mass of ≈ 125 GeV has been reported. If this excess of events is confirmed by further searches with more data, it will have extremely important consequences in the context of supersymmetric extensions of the Standard Model and, in particular the minimal one, the MSSM. We show that for a standard-like Higgs boson with a mass 123 M h 127 GeV , several unconstrained or constrained (i.e. with soft supersymmetry-breaking parameters unified at the high scale) MSSM scenarios would be excluded, while the parameters of some other scenarios would be severely restricted. Examples of constrained MSSM scenarios which would be disfavoured as they predict a too light Higgs particle are the minimal anomaly and gauge mediated supersymmetry-breaking models. The gravity mediated constrained MSSM would still be viable, provided the scalar top quarks are heavy and their trilinear coupling large. Significant areas of the parameter space of models with heavy supersymmetric particles, such as split or high-scale supersymmetry, could also be excluded as, in turn, they generally predict a too heavy Higgs particle.

The top quark and Higgs boson masses and the stability of the electroweak vacuum

Abstract: The ATLAS and CMS experiments observed a particle at the LHC with a mass ≈ 126 GeV, which is compatible with the Higgs boson of the Standard Model. A crucial question is, if for such a Higgs mass value, one could extrapolate the model up to high scales while keeping the minimum of the scalar potential that breaks the electroweak symmetry stable. Vacuum stability requires indeed the Higgs boson mass to be MH > 129 ±1 GeV, but the precise value depends critically on the input top quark pole mass which is usually taken to be the one measured at the Tevatron, m exp = 173.2±0.9 GeV. However, for an unambiguous and theoretically well-defined determination of the top quark mass one should rather use the total cross section for top quark pair production at hadron colliders. Confronting the latest predictions of the inclusive p¯ p → t¯ + X cross section up to next-to-next-to-leading order in QCD to the experimental measurement at the Tevatron, we determine the running mass in the MS-scheme to be m MS (mt) = 163.3 ± 2.7 GeV which gives a top quark pole mass of m pole = 173.3 ± 2.8 GeV. This leads to the vacuum stability constraint MH ≥ 129.8±5.6 GeV to which a ≈ 126 GeV Higgs boson complies as the uncertainty is large. A very precise assessment of the stability of the electroweak vacuum can only be made at a future high-energy electron-positron collider, where the top quark pole mass could be determined with a few hundred MeV accuracy.

Interpreting the LHC Higgs Search Results in the MSSM

Abstract: Recent results reported by the ATLAS and CMS experiments on the search for a SM-like Higgs boson both show an excess for a Higgs mass near 125 GeV, which is mainly driven by the γγ and Z Z ⁎ decay channels, but also receives some support from channels with a lower mass resolution. We discuss the implications of this possible signal within the context of the minimal supersymmetric Standard Model (MSSM), taking into account previous limits from Higgs searches at LEP, the Tevatron and the LHC. The consequences for the remaining MSSM parameter space are investigated. Under the assumption of a Higgs signal we derive new lower bounds on the tree-level parameters of the MSSM Higgs sector. We also discuss briefly an alternative interpretation of the excess in terms of the heavy CP-even Higgs boson, a scenario which is found to be still viable.

Event-by-event fluctuations of magnetic and electric fields in heavy ion collisions

Abstract: We show that fluctuating proton positions in the colliding nuclei generate, on the event-by-event basis, very strong magnetic and electric fields in the direction both parallel and perpendicular to the reaction plane. The magnitude of E and B fields in each event is of the order of m π 2 ≈ 10 18 Gauss. Implications on the observation of electric dipole in heavy ion collisions is discussed, and the possibility of measuring the electric conductivity of the hot medium is pointed out.

Ghost free massive gravity in the Stückelberg language

Abstract: Massive gravity in 4 dimensions has been shown to be free of the Boulware–Deser (BD) ghost in the ADM language for a specific choice of mass terms. We show here how this is consistent with the Stuckelberg language beyond the decoupling limit, and how the constraint required to remove the BD ghost arises in this framework non-perturbatively, without the use of field redefinitions. We emphasize a subtlety in obtaining this constraint, that has been overlooked in previous literature. In both the ADM and Stuckelberg formalisms the vanishing of the determinant of a Hessian guarantees the absence of the BD ghost.

The lightest Higgs boson mass in pure gravity mediation model

Abstract: We discuss the lightest Higgs boson mass in the minimal supersymmetric Standard Model with “pure gravity mediation”. By requiring that the model provides the observed dark matter density, we find that the lightest Higgs boson is predicted to be below 132 GeV. We also find that the upper limit on the lightest Higgs boson mass becomes 128 GeV, if we further assume thermal leptogenesis mechanism as the origin of baryon asymmetry of universe. The interrelations between the Higgs boson mass and the gaugino masses are also discussed.

Constraints on Low-Mass WIMP Interactions on $^{19}F$ from PICASSO

Abstract: Recent results from the PICASSO dark matter search experiment at SNOLAB are reported. These results were obtained using a subset of 10 detectors with a total target mass of 0.72 kg of 19F and an exposure of 114 kgd. The low backgrounds in PICASSO allow recoil energy thresholds as low as 1.7 keV to be obtained which results in an increased sensitivity to interactions from Weakly Interacting Massive Particles (WIMPs) with masses below 10 GeV/c 2 . No dark matter signal was found. Best exclusion limits in the spin dependent sector were obtained for WIMP masses of 20 GeV/c 2 with a cross section on protons of σ p S D = 0.032 pb (90% C.L.). In the spin independent sector close to the low mass region of 7 GeV/c 2 favoured by CoGeNT and DAMA/LIBRA, cross sections larger than σ p S I = 1.41 × 10 − 4 pb (90% C.L.) are excluded.

Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in lead-lead collisions at sNN=2.76 TeV with the ATLAS detector

Abstract: This Letter describes the measurement of elliptic flow of charged particles in lead-lead collisions at root s(NN) = 2.76 TeV using the ATLAS detector at the Large Hadron Collider (LHC). The results ...

Higgs portal, fermionic dark matter, and a Standard Model like Higgs at 125 GeV

Abstract: We show that fermionic dark matter (DM) which communicates with the Standard Model (SM) via the Higgs portal is a viable scenario, even if a SM-like Higgs is found at around 125 GeV. Using effective field theory we show that for DM with a mass in the range from about 60 GeV to 2 TeV the Higgs portal needs to be parity violating in order to be in agreement with direct detection searches. For parity conserving interactions we identify two distinct options that remain viable: a resonant Higgs portal, and an indirect Higgs portal. We illustrate both possibilities using a simple renormalizable toy model.

Vector Higgs portal dark matter and the invisible Higgs

Abstract: The Higgs sector of the Standard Model offers a unique probe of the hidden sector. In this work, we explore the possibility of renormalizable Higgs couplings to the hidden sector vector fields which can constitute dark matter (DM). Abelian gauge sectors with minimal field content, necessary to render the gauge fields massive, have a natural Z2 parity. This symmetry ensures stability of the vector fields making them viable dark matter candidates, while evading the usual electroweak

Flavour-Changing Decays of a 125 GeV Higgs-like Particle

Abstract: The ATLAS and CMS experiments at the LHC have reported the observation of a possible excess of events corresponding to a new particle h with mass ∼125 GeV that might be the long-sought Higgs boson, or something else. Decyphering the nature of this possible signal will require constraining the couplings of the h and measuring them as accurately as possible. Here we analyze the indirect constraints on flavour-changing h decays that are provided by limits on low-energy flavour-changing interactions. We find that indirect limits in the quark sector impose such strong constraints that flavour-changing h decays to quark–antiquark pairs are unlikely to be observable at the LHC. On the other hand, the upper limits on lepton-flavour-changing decays are weaker, and the experimental signatures less challenging. In particular, we find that either B ( h → τ μ ¯ + μ ¯ τ ) or B ( h → τ e ¯ + e ¯ τ ) could be O ( 10 ) % , i.e., comparable to B ( h → τ + τ − ) and potentially observable at the LHC.

Harmonic decomposition of two particle angular correlations in Pb-Pb collisions at root s(NN)=2.76 TeV

Abstract: Angular correlations between unidentified charged trigger (t) and associated (a) particles are measured by the ALICE experiment in Pb-Pb collisions at root s(NN) = 2.76 TeV for transverse momenta 0.25 p(T)(a). The shapes of the pair correlation distributions are studied in a variety of collision centrality classes between 0 and 50% of the total hadronic cross section for particles in the pseudorapidity interval |eta| 0.8, and are referred to as "long-range correlations". Fourier components V-n Delta equivalent to are extracted from the long-range azimuthal correlation functions. If particle pairs are correlated to one another through their individual correlation to a common symmetry plane, then the pair anisotropy V-n Delta (p(T)(t), p(T)(a)) is fully described in terms of single-particle anisotropies v(n)(p(T)) as V-n Delta(p(T)(t), p(T)(a)) = v(n)(p(T)(t))v(n)(p(T)(a)). This expectation is tested for 1 <= n <= 5 by applying a global fit of all V-n Delta(p(T)(t), p(T)(a)) to obtain the best values v(n){GF}(p(T)). It is found that for 2 <= n <= 5, the fit agrees well with data up to p(T)(a) similar to 3-4 GeV/c, with a trend of increasing deviation as p(T)(t) and p(T)(a) are increased or as collisions become more peripheral. This suggests that no pair correlation harmonic can be described over the full 0.25 < p(T) < 15 GeV/c range using a single v(n)(p(T)) curve: such a description is however approximately possible for 2 <= n <= 5 when p(T)(a) < 4 GeV/c. For the n = 1 harmonic, however, a single v(1)(p(T)) curve is not obtained even within the reduced range p(T)(a) < 4 GeV/c. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.

Proof of consistency of nonlinear massive gravity in the Stückelberg formulation

Abstract: We address some recent concerns about the absence of the Boulware–Deser ghost in the Stuckelberg formulation of nonlinear massive gravity. First we provide general arguments for why any ghost analysis in the Stuckelberg formulation has to agree with existing consistency proofs that have been carried out without using Stuckelberg fields. We then demonstrate the absence of the ghost at the completely nonlinear level in the Stuckelberg formulation of the minimal massive gravity action. The constraint that removes the ghost field and the associated secondary constraint that eliminates its conjugate momentum are computed explicitly, confirming the consistency of the theory in the Stuckelberg formulation.

Is the resonance at 125 GeV the Higgs boson

Abstract: The recently discovered resonance at 125 GeV has properties remarkably close to those of the Standard Model Higgs boson. We perform model-independent ts of all presently available data. The non

Hydrodynamic elliptic and triangular flow in Pb–Pb collisions at sNN=2.76A TeV

Abstract: It is shown that a simultaneous comparison of both elliptic and triangular flow from ( 2 + 1 ) -dimensional viscous fluid dynamics with recent measurements in Pb + Pb collisions at the Large Hadron Collider (LHC) favors a small specific shear viscosity ( η / s ) QGP ≈ 1 / ( 4 π ) for the quark–gluon plasma. Using this viscosity value, the relative magnitude of the elliptic and triangular flow is well described with Monte Carlo Glauber (MC-Glauber) initial conditions while Monte Carlo Kharzeev–Levin–Nardi (MC-KLN) initial conditions require twice as large viscosity to reproduce the elliptic flow and then underpredict triangular flow by about 30%.

The nucleon axial mass and the MiniBooNE Quasielastic Neutrino-Nucleus Scattering problem

Abstract: The charged-current double differential neutrino cross section, measured by the MiniBooNE Collaboration, has been analyzed using a microscopical model that accounts for, among other nuclear effects, long range nuclear (RPA) correlations and multinucleon scattering. We find that MiniBooNE data are fully compatible with the world average of the nucleon axial mass in contrast with several previous analyses which have suggested an anomalously large value. We also discuss the reliability of the algorithm used to estimate the neutrino energy.

Current experimental constraints on the lightest Higgs boson mass in the constrained MSSM

Abstract: We examine the parameter space of the constrained MSSM by considering various experimental constraints. For the dark matter sector, we require the neutralino dark matter to account for the relic density measured by the WMAP and satisfy the XENON limits on its scattering rate with the nucleon. For the collider constraints, we consider all relevant direct and indirect limits from LEP, Tevatron and LHC as well as the muon anomalous magnetic moment. Especially, for the limits from B-s -> mu(+)mu(-), we either directly consider its branching ratio with the latest LHC data or alternatively consider the double ratio of the purely leptonic decays defined by Br(B-s -> mu(+)mu(-))/Br(B-mu -> tau nu(tau))/Br(D-s -> tau nu(tau))/Br(D -> tau mu(tau)). We find that under these constraints, the mass of the lightest Higgs boson (h) in both the CMSSM and the NUHM2 is upper bounded by about 124 GeV (126 GeV) before (after) considering its theoretical uncertainty. We also find that for these models the di-photon Higgs signal at the LHC is suppressed relative to the SM prediction, and that the lower bound of the top-squark mass goes up with nth. reaching 600 GeV for m(h) = 124 GeV. (C) 2012 Elsevier B.V. All rights reserved.

Hadron–hadron interactions from imaginary-time Nambu–Bethe–Salpeter wave function on the lattice

Abstract: Imaginary-time Nambu–Bethe–Salpeter (NBS) wave function is introduced to extend our previous approach for hadron–hadron interactions on the lattice. Scattering states of hadrons with different energies encoded in the NBS wave function are utilized to extract non-local hadron–hadron potential. “The ground state saturation”, which is commonly used in lattice QCD but is hard to be achieved for multi-baryons, is not required. We demonstrate that the present method works efficiently for the nucleon–nucleon interaction (the potential and the phase shift) in the S 0 1 channel.

A higher order GUP with minimal length uncertainty and maximal momentum

Abstract: We present a higher order generalized (gravitational) uncertainty principle (GUP) in the form [ X , P ] = i ℏ / ( 1 − β P 2 ) . This form of GUP is consistent with various proposals of quantum gravity such as string theory, loop quantum gravity, doubly special relativity, and predicts both a minimal length uncertainty and a maximal observable momentum. We show that the presence of the maximal momentum results in an upper bound on the energy spectrum of the momentum eigenstates and the harmonic oscillator.

Models for little rip dark energy

Abstract: We examine in more detail specific models which yield a little rip cosmology, i.e., a universe in which the dark energy density increases without bound but the universe never reaches a finite time singularity. We derive the conditions for the little rip in terms of the inertial force in the expanding universe and present two representative models to illustrate in more detail the difference between little rip models and those which are asymptotically de Sitter. We derive conditions on the equation of state parameter of the dark energy to distinguish between the two types of models. We show that coupling between dark matter and dark energy with a little rip equation of state can alter the evolution, changing the little rip into an asymptotic de Sitter expansion. We give conditions on minimally coupled phantom scalar field models and on scalar-tensor models that indicate whether or not they correspond to a little rip expansion. We show that, counterintuitively, despite local instability, a little rip cosmology has an infinite lifetime.

WIMP-nucleon cross-section results from the second science run of ZEPLIN-III

Abstract: We report experimental upper limits on WIMP-nucleon elastic scattering cross sections from the second science run of ZEPLIN-III at the Boulby Underground Laboratory. A raw fiducial exposure of 1344 kg⋅days was accrued over 319 days of continuous operation between June 2010 and May 2011. A total of eight events was observed in the signal acceptance region in the nuclear recoil energy range 7–29 keV, which is compatible with background expectations. This allows the exclusion of the scalar cross-section above 4.8 × 10 − 8 pb near 50 GeV / c 2 WIMP mass with 90% confidence. Combined with data from the first run, this result improves to 3.9 × 10 − 8 pb . The corresponding WIMP-neutron spin-dependent cross-section limit is 8.0 × 10 − 3 pb . The ZEPLIN programme reaches thus its conclusion at Boulby, having deployed and exploited successfully three liquid xenon experiments of increasing reach.

The constrained NMSSM and Higgs near 125 GeV

Abstract: We assess the extent to which various constrained versions of the NMSSM are able to describe the recent hints of a Higgs signal at the LHC corresponding to a Higgs mass in the range 123–128 GeV.

Neutral pion and eta meson production in proton-proton collisions at root s=0.9 TeV and root s=7 TeV

Abstract: The first measurements of the invariant differential cross sections of inclusive pi(0) and eta meson production at mid-rapidity in proton-proton collisions root s = 0.9 TeV and root s = 7 TeV are reported. The pi(0) measurement covers the ranges 0.4 < p(T) < 7 GeV/c and 0.3 < p(T) < 25 GeV/c for these two energies, respectively. The production of eta mesons was measured at root s = 7 TeV in the range 0.4 < p(T) < 15 GeV/c. Next-to-Leading Order perturbative QCD calculations, which are consistent with the pi(0) spectrum at root s = 0.9 TeV, overestimate those of pi(0) and eta mesons at root s = 7 TeV, but agree with the measured eta/pi(0) ratio at root s = 7 TeV. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.

Analytic regularization in Soft-Collinear Effective Theory

Abstract: In high-energy processes which are sensitive to small transverse momenta, individual contributions from collinear and soft momentum regions are not separately well-defined in dimensional regularization. A simple possibility to solve this problem is to introduce additional analytic regulators. We point out that in massless theories the unregularized singularities only appear in real-emission diagrams and that the additional regulators can be introduced in such a way that gauge invariance and the factorized eikonal structure of soft and collinear emissions is maintained. This simplifies factorization proofs and implies, at least in the massless case, that the structure of Soft-Collinear Effective Theory remains completely unchanged by the presence of the additional regulators. Our formalism also provides a simple operator definition of transverse parton distribution functions.