CERN Document Server: Theoretical Physics (TH)
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CERN Document Server latest documents in Theoretical Physics (TH)enWed, 28 Jan 2015 12:09:11 GMTInvenio 1.1.3.1106-62468cds.support@cern.ch36013959125http://cds.cern.ch/img/site_logo_rss.pngCERN Document Server
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Naturalness in the Dark at the LHC
http://cds.cern.ch/record/1982747
We revisit the Twin Higgs scenario as a "dark" solution to the little hierarchy problem, identify the structure of a minimal model and its viable parameter space, and analyze its collider implications. In this model, dark naturalness generally leads to Hidden Valley phenomenology. The twin particles, including the top partner, are all Standard-Model-neutral, but naturalness favors the existence of twin strong interactions -- an asymptotically-free force that confines not far above the Standard Model QCD scale -- and a Higgs portal interaction. We show that, taken together, these typically give rise to exotic decays of the Higgs to twin hadrons. Across a substantial portion of the parameter space, certain twin hadrons have visible and often displaced decays, providing a potentially striking LHC signature. We briefly discuss appropriate experimental search strategies.Craig, NathanielFri, 23 Jan 2015 07:09:59 GMThttp://cds.cern.ch/record/1982747Discrete Glimpses of the Physics Landscape after the Higgs Discovery
http://cds.cern.ch/record/1982737
What is the Higgs boson telling us? What else is there? How do we find it? This talk discusses these current topics in particle physics in the wake of the Higgs discovery, with particular emphasis on the discrete symmetries CP and R-parity, not forgetting flavour physics and dark matter, and finishing with some remarks about possible future colliders.Ellis, JohnFri, 23 Jan 2015 07:08:53 GMThttp://cds.cern.ch/record/1982737Neutrino versus antineutrino cross sections and CP violation
http://cds.cern.ch/record/1981097
We discuss the nuclear interactions of neutrinos versus those of antineutrinos, a relevant comparison for CP violation experiments in the neutrino sector. We consider the MiniBooNE quasielastic-like double differential neutrinos and antineutrinos cross sections which are flux dependent and hence specific to the MiniBooNE set-up. We combine them introducing their sum and their difference. We show that the last combination can bring a general information, which can be exploited in other experiments, on the nuclear matrix elements of the axial vector interference term. Our theoretical model reproduces well the two cross sections combinations. This confirms the need for a sizeable multinucleon component in particular in the interference term.Ericson, MWed, 14 Jan 2015 06:26:23 GMThttp://cds.cern.ch/record/1981097Wall-crossing made smooth
http://cds.cern.ch/record/1980359
In $D=4,N=2$ theories on $R^{3,1}$, the index receives contributions not only from single-particle BPS states, counted by the BPS indices, but also from multi-particle states made of BPS constituents. In a recent work [arXiv:1406.2360], a general formula expressing the index in terms of the BPS indices was proposed, which is smooth across walls of marginal stability and reproduces the expected single-particle contributions. In this note, I analyze the two-particle contributions predicted by this formula, and show agreement with the spectral asymmetry of the continuum of scattering states in the supersymmetric quantum mechanics of two non-relativistic, mutually non-local dyons. This provides a physical justification for the error function profile used in the mathematics literature on indefinite theta series, and in the physics literature on black hole partition functions.Pioline, BorisFri, 09 Jan 2015 07:08:58 GMThttp://cds.cern.ch/record/1980359Explaining $h\to\mu^\pm\tau^\mp$, $B\to K^* \mu^+\mu^-$ and $B\to K \mu^+\mu^-/B\to K e^+e^-$ in a two-Higgs-doublet model with gauged $L_\mu-L_\tau$
http://cds.cern.ch/record/1980012
The LHC observed so far three deviations from the Standard Model (SM) predictions in flavour observables: LHCb reported anomalies in $B\to K^* \mu^+\mu^-$ and $R(K)=B\to K \mu^+\mu^-/B\to K e^+e^-$ while CMS found an excess in $h\to\mu\tau$. We show, for the first time, how these deviations from the SM can be explained within a single well-motivated model: a two-Higgs-doublet model with gauged $L_\mu-L_\tau$ symmetry. We find that, despite the constraints from $\tau\to\mu\mu\mu$ and $B_s$--$\overline{B}_s$ mixing, one can explain $h \to\mu\tau$, $B\to K^* \mu^+\mu^-$ and $R(K)$ simultaneously, obtaining interesting correlations among the observables.Crivellin, AndreasWed, 07 Jan 2015 08:01:17 GMThttp://cds.cern.ch/record/1980012LHC constraints on gauge boson couplings to dark matter
http://cds.cern.ch/record/1979702
Collider searches for energetic particles recoiling against missing transverse energy allow to place strong bounds on the interactions between dark matter (DM) and standard model 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 mono-photon, mono-W, mono-Z, mono-W/Z, invisible Higgs-boson decays in the vector boson fusion mode and the mono-jet channel is presented. Depending on the parameter choices, either the mono-photon or the mono-jet 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.Crivellin, AndreasTue, 06 Jan 2015 06:25:40 GMThttp://cds.cern.ch/record/1979702Correlation functions of the chiral stress-tensor multiplet in N=4 SYM
http://cds.cern.ch/record/1979332
We give a new method for computing the correlation functions of the chiral part of the stress-tensor supermultiplet that relies on the reformulation of N=4 SYM in twistor space. It yields the correlation functions in the Born approximation as a sum of Feynman diagrams on twistor space that involve only propagators and no integration vertices. We use this unusual feature of the twistor Feynman rules to compute the correlation functions in terms of simple building blocks which we identify as a new class of N=4 off-shell superconformal invariants. Making use of the duality between correlation functions and planar scattering amplitudes, we demonstrate that these invariants represent an off-shell generalisation of the on-shell invariants defining tree-level scattering amplitudes in N=4 SYM.Chicherin, DmitryWed, 31 Dec 2014 07:02:10 GMThttp://cds.cern.ch/record/1979332Observable Effects of General New Scalar Particles
http://cds.cern.ch/record/1979299
We classify all possible new scalar particles that can have renormalizable linear couplings to Standard Model fields and therefore be singly produced at colliders. We show that this classification exhausts the list of heavy scalar particles that contribute at the tree level to the Standard Model effective Lagrangian to dimension six. We compute this effective Lagrangian for a general scenario with an arbitrary number of new scalar particles and obtain flavor-preserving constraints on their couplings and masses. This completes the tree-level matching of the coefficients of dimension five and six operators in the effective Lagrangian to arbitrary extensions of the Standard Model.de Blas, JorgeWed, 31 Dec 2014 07:01:44 GMThttp://cds.cern.ch/record/1979299Automated NNLL+NLO Resummation for Jet-Veto Cross Sections
http://cds.cern.ch/record/1979152
In electroweak-boson production processes with a jet veto, higher-order corrections are enhanced by logarithms of the veto scale over the invariant mass of the boson system. In this paper, we resum these Sudakov logarithms at next-to-next-to-leading logarithmic (NNLL) accuracy and match our predictions to next-to-leading order (NLO) fixed-order results. We perform the calculation in an automated way, for arbitrary electroweak final states and in the presence of kinematic cuts on the leptons produced in the decays of the electroweak bosons. The resummation is based on a factorization theorem for the cross sections into hard functions, which encode the virtual corrections to the boson production process, and beam functions, which describe the low-p_T emissions collinear to the beams. The one-loop hard functions for arbitrary processes are calculated using the MadGraph5_aMC@NLO framework, while the beam functions are process independent. We perform the resummation for a variety of processes, in particular for W+W- pair production followed by leptonic decays of the W bosons.Becher, ThomasTue, 30 Dec 2014 06:25:03 GMThttp://cds.cern.ch/record/1979152The ${\mathcal N}=2$ superconformal bootstrap
http://cds.cern.ch/record/1978848
In this work we initiate the conformal bootstrap program for ${\mathcal N}=2$ superconformal field theories in four dimensions. We promote an abstract operator-algebraic viewpoint in order to unify the description of Lagrangian and non-Lagrangian theories, and formulate various conjectures concerning the landscape of theories. We analyze in detail the four-point functions of flavor symmetry current multiplets and of ${\mathcal N}=2$ chiral operators. For both correlation functions we review the solution of the superconformal Ward identities and describe their superconformal block decompositions. This provides the foundation for an extensive numerical analysis discussed in the second half of the paper. We find a large number of constraints for operator dimensions, OPE coefficients, and central charges that must hold for any ${\mathcal N}=2$ superconformal field theory.Beem, ChristopherThu, 25 Dec 2014 07:16:17 GMThttp://cds.cern.ch/record/1978848Pinning down top dipole moments with ultra-boosted tops
http://cds.cern.ch/record/1978522
We investigate existing and future hadron-collider constraints on the top dipole chromomagnetic and chromoelectric moments, two quantities that are expected to be modified in the presence of new physics. We focus first on recent measurements of the inclusive top pair production cross section at the Tevatron and at the Large Hadron Collider. We then analyse the role of top-antitop events produced at very large invariant masses, in the context of the forthcoming 13-14 TeV runs of the LHC, and at a future 100 TeV proton-proton collider. In this latter case, the selection of semileptonic decays to hard muons allows to tag top quarks boosted to the multi-TeV regime, strongly reducing the QCD backgrounds, and leading to a significant improvement in the sensitivity to anomalous top couplings.Aguilar-Saavedra, J ATue, 23 Dec 2014 08:18:56 GMThttp://cds.cern.ch/record/1978522Constraints on dark radiation from cosmological probes
http://cds.cern.ch/record/1978463
We present joint constraints on the number of effective neutrino species N_eff and the sum of neutrino masses M_nu, based on a technique which exploits the full information contained in the one-dimensional Lyman-Alpha forest flux power spectrum, complemented by additional cosmological probes. In particular, we obtain N_eff=2.91(+0.21)(-0.22) (95% CL) and M_nu<0.15 eV (95% CL) when we combine BOSS Lyman-Alpha forest data with CMB (Planck+ACT+SPT+WMAP polarization) measurements, and N_eff=2.88(+0.20)(-0.20) (95% CL) and M_nu<0.14 eV (95% CL) when we further add baryon acoustic oscillations. Our results tend to favor the normal hierarchy scenario for the masses of the active neutrino species, provide evidence for the Cosmic Neutrino Background from N_eff~3 (N_eff=0 is rejected at more than 14 sigma), and rule out the possibility of a sterile neutrino thermalized with active neutrinos (i.e., N_eff=4) - or more generally any decoupled relativistic relic with Delta N_eff ~ 1 - at a significance of over 5 sigma, the strongest bound to date, implying that there is no need for exotic neutrino physics in the concordance LCDM model.Rossi, GrazianoTue, 23 Dec 2014 08:17:37 GMThttp://cds.cern.ch/record/1978463Scaling study of an improved fermion action on quenched lattices
http://cds.cern.ch/record/1978282
We present scaling studies for heavy-quark observables calculated with an $O(a^2)$-improved fermion action on tree-level Symanzik improved gauge configurations. Lattices of $1/a = $ 2.0-3.8 GeV with an equal physical volume 1.6 fm are used. The results are compared with the standard domain-wall and naive Wilson fermions.Cho, Yong-GwiMon, 22 Dec 2014 06:59:48 GMThttp://cds.cern.ch/record/1978282Automated next-to-leading order predictions for colored scalar production at the LHC
http://cds.cern.ch/record/1978043
We explore scenarios beyond the Standard Model where new colored scalar particles can be pair produced in hadron collisions. Using simplified models to describe the new field interactions with the Standard Model, we present precision predictions for the LHC by automatically matching next-to-leading order matrix elements with parton showers within the MadGraph5 aMC@NLO framework. We illustrate in this way, for the first time in our field, the full automation of Monte Carlo event generation at the next-to-leading order accuracy in QCD for new physics theories with an extended colored sector, the sole inputs being the model Lagrangian and the process of interest.Degrande, CélineSat, 20 Dec 2014 08:55:34 GMThttp://cds.cern.ch/record/1978043Robustness of cosmic neutrino background detection in the cosmic microwave background
http://cds.cern.ch/record/1978003
The existence of a cosmic neutrino background can be probed indirectly by CMB experiments, not only by measuring the background density of radiation in the universe, but also by searching for the typical signatures of the fluctuations of free-streaming species in the temperature and polarisation power spectrum. Previous studies have already proposed a rather generic parametrisation of these fluctuations, that could help to discriminate between the signature of ordinary free-streaming neutrinos, or of more exotic dark radiation models. Current data are compatible with standard values of these parameters, which seems to bring further evidence for the existence of a cosmic neutrino background. In this work, we investigate the robustness of this conclusion under various assumptions. We generalise the definition of an effective sound speed and viscosity speed to the case of massive neutrinos or other dark radiation components experiencing a non-relativistic transition. We show that current bounds on these effective parameters do not vary significantly when considering an arbitrary value of the particle mass, or extended cosmological models with a free effective neutrino number, dynamical dark energy or a running of the primordial spectrum tilt. We conclude that it is possible to make a robust statement about the detection of the cosmic neutrino background by CMB experiments.Audren, BenjaminSat, 20 Dec 2014 07:06:17 GMThttp://cds.cern.ch/record/1978003Gauged R-symmetry and its anomalies in 4D N=1 supergravity and phenomenological implications
http://cds.cern.ch/record/1977668
We consider a class of models with gauged U(1)_R symmetry in 4D N=1 supergravity that have, at the classical level, a metastable ground state, an infinitesimally small (tunable) positive cosmological constant and a TeV gravitino mass. We analyse if these properties are maintained under the addition of visible sector (MSSM-like) and hidden sector state(s), where the latter may be needed for quantum consistency. We then discuss the anomaly cancellation conditions in supergravity as derived by Freedman, Elvang and K\"ors and apply their results to the special case of a U(1)_R symmetry, in the presence of the Fayet-Iliopoulos term ($\xi$) and Green-Schwarz mechanism(s). We investigate the relation of these anomaly cancellation conditions to the "naive" field theory approach in global SUSY, in which case U(1)_R cannot even be gauged. We show the two approaches give similar conditions. Their induced constraints at the phenomenological level, on the above models, remain strong even if one lifted the GUT-like conditions for the MSSM gauge couplings. In an anomaly-free model, a tunable, TeV-scale gravitino mass may remain possible provided that the U(1)_R charges of additional hidden sector fermions (constrained by the cubic anomaly alone) do not conflict with the related values of U(1)_R charges of their scalar superpartners, constrained by existence of a stable ground state. This issue may be bypassed by tuning instead the coefficients of the Kahler connection anomalies (b_K, b_{CK}).Antoniadis, IThu, 18 Dec 2014 07:36:02 GMThttp://cds.cern.ch/record/1977668Instantaneous stochastic perturbation theory
http://cds.cern.ch/record/1977661
A form of stochastic perturbation theory is described, where the representative stochastic fields are generated instantaneously rather than through a Markov process. The correctness of the procedure is established to all orders of the expansion and for a wide class of field theories that includes all common formulations of lattice QCD.Lüscher, MartinThu, 18 Dec 2014 07:35:45 GMThttp://cds.cern.ch/record/1977661Phenomenology of theories of gravity without Lorentz invariance: the preferred frame case
http://cds.cern.ch/record/1977484
Theories of gravitation without Lorentz invariance are candidates of low-energy descriptions of quantum gravity. In this review we will describe the phenomenological consequences of the candidates associated to the existence of a preferred time directionBlas, DiegoThu, 18 Dec 2014 07:28:38 GMThttp://cds.cern.ch/record/1977484A Symbol of Uniqueness: The Cluster Bootstrap for the 3-Loop MHV Heptagon
http://cds.cern.ch/record/1976326
Seven-particle scattering amplitudes in planar super-Yang-Mills theory are believed to belong to a special class of generalised polylogarithm functions called heptagon functions. These are functions with physical branch cuts whose symbols may be written in terms of the 42 cluster A-coordinates on Gr(4,7). Motivated by the success of the hexagon bootstrap programme for constructing six-particle amplitudes we initiate the systematic study of the symbols of heptagon functions. We find that there is exactly one such symbol of weight six which satisfies the MHV last-entry condition and is finite in the $7 \parallel 6$ collinear limit. This unique symbol is both dihedral and parity-symmetric, and remarkably its collinear limit is exactly the symbol of the three-loop six-particle MHV amplitude, although none of these properties were assumed a priori. It must therefore be the symbol of the three-loop seven-particle MHV amplitude. The simplicity of its construction suggests that the n-gon bootstrap may be surprisingly powerful for n>6.Drummond, James MFri, 12 Dec 2014 07:09:25 GMThttp://cds.cern.ch/record/1976326Flavoured Dark Matter Beyond MFV
http://cds.cern.ch/record/1976306
We review a model of quark flavoured dark matter with new flavour violating interactions. This simplified model describes Dirac fermionic dark matter that is charged under a new U(3) flavour symmetry and couples to right-handed down quarks via a scalar mediator. The corresponding coupling matrix is assumed to be the only new source of flavour violation, which we refer to as the Dark Minimal Flavour Violation (DMFV) hypothesis. This ansatz ensures the stability of dark matter. We discuss the phenomenology of the simplest DMFV model in flavour violating observables, LHC searches, and direct dark matter detection experiments. Especially interesting is the non-trivial interplay between the constraints from the different sectors.Blanke, MonikaFri, 12 Dec 2014 07:09:07 GMThttp://cds.cern.ch/record/1976306Hamiltonian Truncation Study of the Phi^4 Theory in Two Dimensions
http://cds.cern.ch/record/1976298
We defend the Fock-space Hamiltonian truncation method, which allows to calculate numerically the spectrum of strongly coupled quantum field theories, by putting them in a finite volume and imposing a UV cutoff. The accuracy of the method is improved via an analytic renormalization procedure inspired by the usual effective field theory. As an application, we study the two-dimensional Phi^4 theory for a wide range of couplings. The theory exhibits a quantum phase transition between the symmetry-preserving and symmetry-breaking phases. We extract quantitative predictions for the spectrum and the critical coupling and make contact with previous results from the literature. Future directions to further improve the accuracy of the method and enlarge its scope of applications are outlined.Rychkov, SlavaFri, 12 Dec 2014 07:08:55 GMThttp://cds.cern.ch/record/1976298The Quark Condensate in Multi-Flavour QCD - Planar Equivalence Confronting Lattice Simulations
http://cds.cern.ch/record/1976072
Planar equivalence between the large $N$ limits of ${\cal N}=1$ Super Yang-Mills (SYM) theory and a variant of QCD with fermions in the antisymmetric representation is a powerful tool to obtain analytic non-perturbative results in QCD itself. In particular, it allows the quark condensate for $N=3$ QCD with quarks in the fundamental representation to be inferred from exact calculations of the gluino condensate in ${\cal N}=1$ SYM. In this paper, we review and refine our earlier predictions for the quark condensate in QCD with a general number $n_f$ of flavours and confront these with lattice results.Armoni, AdiThu, 11 Dec 2014 06:31:50 GMThttp://cds.cern.ch/record/1976072Generalized Born--Infeld Actions and Projective Cubic Curves
http://cds.cern.ch/record/1976061
We investigate $U(1)^{\,n}$ supersymmetric Born-Infeld Lagrangians with a second non-linearly realized supersymmetry. The resulting non-linear structure is more complex than the square root present in the standard Born-Infeld action, and nonetheless the quadratic constraints determining these models can be solved exactly in all cases containing three vector multiplets. The corresponding models are classified by cubic holomorphic prepotentials. Their symmetry structures are associated to projective cubic varieties.Ferrara, SThu, 11 Dec 2014 06:31:04 GMThttp://cds.cern.ch/record/1976061Softened Gravity and the Extension of the Standard Model up to Infinite Energy
http://cds.cern.ch/record/1975745
Attempts to solve naturalness by having the weak scale as the only breaking of classical scale invariance have to deal with two severe difficulties: gravity and the absence of Landau poles. We show that solutions to the first problem require premature modifications of gravity at scales no larger than $10^{11}$ GeV, while the second problem calls for many new particles at the weak scale. To build models that fulfil these properties, we classify 4-dimensional Quantum Field Theories that satisfy Total Asymptotic Freedom (TAF): the theory holds up to infinite energy, where all coupling constants flow to zero. We develop a technique to identify such theories and determine their low-energy predictions. Since the Standard Model turns out to be asymptotically free only under the unphysical conditions $g_1 = 0$, $M_t = 186$ GeV, $M_\tau = 0$, $M_h = 163$ GeV, we explore some of its weak-scale extensions that satisfy the requirements for TAF.Giudice, Gian FWed, 10 Dec 2014 06:46:07 GMThttp://cds.cern.ch/record/1975745The Beautiful Physics of LHC Run 2
http://cds.cern.ch/record/1975705
Run 2 of the LHC offers some beautiful prospects for new physics, including flavour physics as well as more detailed studies of the Higgs boson and searches for new physics beyond the Standard Model (BSM). One of the possibilities for BSM physics is supersymmetry, and flavour physics plays various important r\^oles in constraining supersymmetric models.Ellis, JohnWed, 10 Dec 2014 06:44:31 GMThttp://cds.cern.ch/record/1975705