CERN Document Server: Theoretical Physics (TH)
http://cds.cern.ch/collection/Theoretical%20Physics%20%28TH%29
CERN Document Server latest documents in Theoretical Physics (TH)enSun, 21 Sep 2014 00:03:25 GMTInvenio 1.1.3.1106-62468cds.support@cern.ch36013888125http://cds.cern.ch/img/site_logo_rss.pngCERN Document Server
http://cds.cern.ch
Search Search this site:p
http://cds.cern.ch/search
Higgs production in association with bottom quarks
http://cds.cern.ch/record/1756681
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 ${\cal O}(\alpha_S^3)$ the effects of the interference between the $b\bar{b}H$ and gluon-fusion production modes.Wiesemann, MFri, 19 Sep 2014 06:15:05 GMThttp://cds.cern.ch/record/1756681$tt^*$ equations, localization and exact chiral rings in 4d N=2 SCFTs
http://cds.cern.ch/record/1756083
We compute exact 2- and 3-point functions of chiral primaries in four-dimensional N=2 superconformal field theories, including all perturbative and instanton contributions. We demonstrate that these correlation functions are nontrivial and satisfy exact differential equations with respect to the coupling constants. These equations are the analogue of the $tt^*$ equations in two dimensions. In the SU(2) N=2 SYM theory coupled to 4 hypermultiplets they take the form of a semi-infinite Toda chain. We provide the complete solution of this chain using input from supersymmetric localization. To test our results we calculate the same correlation functions independently using Feynman diagrams up to 2-loops and we find perfect agreement up to the relevant order. As a spin-off, we perform a 2-loop check of the recent proposal of arXiv:1405.7271 that the logarithm of the sphere partition function in N=2 SCFTs determines the K\"ahler potential of the Zamolodchikov metric on the conformal manifold. We also present the $tt^*$ equations in general SU(N) N=2 superconformal QCD theories and comment on their structure and implications.Baggio, MarcoWed, 17 Sep 2014 05:55:03 GMThttp://cds.cern.ch/record/1756083Exact correlation functions in SU(2) N=2 superconformal QCD
http://cds.cern.ch/record/1756059
We report an exact solution of 2- and 3-point functions of chiral primary fields in SU(2) N=2 super-Yang-Mills theory coupled to four hypermultiplets. It is shown that these correlation functions are non-trivial functions of the gauge coupling, obeying differential equations which take the form of the semi-infinite Toda chain. We solve these equations recursively in terms of the Zamolodchikov metric that can be determined exactly from supersymmetric localization on the four-sphere. Our results are verified independently in perturbation theory with a Feynman diagram computation up to 2-loops. This is a short version of a companion paper that contains detailed technical remarks, additional material and aspects of an extension to SU(N) gauge group.Baggio, MarcoWed, 17 Sep 2014 05:54:39 GMThttp://cds.cern.ch/record/1756059The Discovery of the Gluon
http://cds.cern.ch/record/1756020
Soon after the postulation of quarks, it was suggested that they interact via gluons, but direct experimental evidence was lacking for over a decade. In 1976, Mary Gaillard, Graham Ross and the author suggested searching for the gluon via 3-jet events due to gluon bremsstrahlung in $e^+ e^-$ collisions. Following our suggestion, the gluon was discovered at DESY in 1979 by TASSO and the other experiments at the PETRA collider.Ellis, JohnWed, 17 Sep 2014 05:53:32 GMThttp://cds.cern.ch/record/1756020Interplay and Characterization of Dark Matter Searches at Colliders and in Direct Detection Experiments
http://cds.cern.ch/record/1756012
We present and discuss a concrete proposal for the consistent interpretation of Dark Matter searches at colliders and in direct detection experiments. Based on a specific implementation of simplified models of vector and axial-vector mediator exchanges, this proposal demonstrates how the two search strategies can be compared on an equal footing. Summary of the Brainstorming Workshop held at Imperial College London on May 29th, 2014.Malik, SarahWed, 17 Sep 2014 05:53:31 GMThttp://cds.cern.ch/record/1756012On holography for (pseudo-)conformal cosmology
http://cds.cern.ch/record/1755912
We propose a holographic dual for (pseudo-)conformal cosmological scenario, with a scalar field that forms a moving domain wall in adS_5. The domain wall separates two vacua with unequal energy densities. Unlike in the existing construction, the 5d solution is regular in the relevant space-time domain.Libanov, MWed, 17 Sep 2014 05:52:12 GMThttp://cds.cern.ch/record/1755912N=4 superconformal Ward identities for correlation functions
http://cds.cern.ch/record/1754745
In this paper we study the four-point correlation function of the energy-momentum supermultiplet in theories with N=4 superconformal symmetry in four dimensions. We present a compact form of all component correlators as an invariant of a particular abelian subalgebra of the N=4 superconformal algebra. This invariant is unique up to a single function of the conformal cross-ratios which is fixed by comparison with the correlation function of the lowest half-BPS scalar operators. Our analysis is independent of the dynamics of a specific theory, in particular it is valid in N=4 super Yang-Mills theory for any value of the coupling constant. We discuss in great detail a subclass of component correlators, which is a crucial ingredient for the recent study of charge-flow correlations in conformal field theories. We compute the latter explicitly and elucidate the origin of the interesting relations among different types of flow correlations previously observed in arXiv:1309.1424.Belitsky, A VWed, 10 Sep 2014 05:36:59 GMThttp://cds.cern.ch/record/1754745Ambient cosmology and spacetime singularities
http://cds.cern.ch/record/1754677
We present a new approach to the issues of spacetime singularities and cosmic censorship in general relativity. This is based on asymptotically splitting 5-dimensional bulk space that has standard 4-dimensional spacetime as its boundary geometry, and constructing a unique, complete, smooth 'ambient' metric which has the latter as its conformal infinity. We then find that the existence of spacetime singularities is intimately linked to breaking the boundary conformal structure, while the 5-dimensional metric is only sensitive to conformal infinity without singular points. It follows that uniqueness of the ambient cosmological metric crucially depends on cosmic censorship holding on the boundary.Antoniadis, IgnatiosWed, 10 Sep 2014 05:35:18 GMThttp://cds.cern.ch/record/1754677A Cheap Alternative to the Lattice?
http://cds.cern.ch/record/1754001
We show how to perform accurate, nonperturbative and controlled calculations in quantum field theory in d dimensions. We use the Truncated Conformal Space Approach (TCSA), a Hamiltonian method which exploits the conformal structure of the UV fixed point. The theory is regulated in the IR by putting it on a sphere of a large finite radius. The QFT Hamiltonian is expressed as a matrix in the Hilbert space of CFT states. After restricting ourselves to energies below a certain UV cutoff, an approximation to the spectrum is obtained by numerical diagonalization of the resulting finite-dimensional matrix. The cutoff dependence of the results can be computed and efficiently reduced via a renormalization procedure. We work out the details of the method for the phi^4 theory in d dimensions with d not necessarily integer. A numerical analysis is then performed for the specific case d = 2.5, a value chosen in the range where UV divergences are absent. By going from weak to intermediate to strong coupling, we are able to observe the symmetry-preserving, symmetry-breaking, and conformal phases of the theory, and perform rough measurements of masses and critical exponents. As a byproduct of our investigations we find that both the free and the interacting theories in non integral d are not unitary, which however does not seem to cause much effect at low energies.Hogervorst, MatthijsMon, 08 Sep 2014 05:25:58 GMThttp://cds.cern.ch/record/1754001Alpha- and Omega-Deformations from fluxes in M-Theory
http://cds.cern.ch/record/1753681
We discuss an SL(2,R) family of deformed N=2 four-dimensional gauge theories which we derive from a flux background in M-theory. In addition to the Omega-deformation this family includes a new deformation, which we call the Alpha-deformation, which can be viewed as an S-dual to the Omega-deformation. We study these gauge theories in two ways: by constructing a non-Abelian (but UV-complete) Lagrangian, and by their strong coupling lift to M-theory where their low-energy dynamics can be determined by examining the equation of motion of a single M5-brane wrapped on a Riemann surface.Lambert, NeilFri, 05 Sep 2014 05:35:29 GMThttp://cds.cern.ch/record/1753681Challenges for New Physics in the Flavour Sector
http://cds.cern.ch/record/1753632
In these proceedings I present a personal perspective of the challenges for new physics (NP) searches in the flavour sector. Since the CKM mechanism of flavour violation has been established to a very high precision, we know that physics beyond the Standard Model can only contribute sub-dominantly. Therefore, any realistic model of physics beyond the Standard Model (SM) must respect the stringent constrains from flavour observables like $b\to s \gamma$, $B_s\to\mu^+\mu^-$, $\Delta F=2$ processes etc., in a first step. In a second step, it is interesting to ask the question if some deviations from the SM predictions (like the anomalous magnetic moment of the muon or recently observed discrepancies in tauonic $B$ decays or $B\to K^*\mu^+\mu^-$) can be explained by a model of NP without violating bounds from other observables.Crivellin, AndreasFri, 05 Sep 2014 05:34:21 GMThttp://cds.cern.ch/record/1753632Transverse momentum resummation for Higgs production via gluon fusion in the MSSM
http://cds.cern.ch/record/1753256
The resummed transverse momentum distribution of supersymmetric Higgs bosons produced through gluon fusion at NLO+NLL is presented, including the exact quark and squark mass dependences. Considering various MSSM scenarios, we compare our results to previous ones within the POWHEG approach. We analyze the impact of the bottom loop which becomes the dominant contribution to the gluon fusion cross section for a wide range of the parameter space for the pseudo-scalar and heavy Higgs.Harlander, Robert VWed, 03 Sep 2014 05:46:04 GMThttp://cds.cern.ch/record/1753256Cosmic backgrounds of relic gravitons and their absolute normalization
http://cds.cern.ch/record/1752916
Provided the consistency relations are not violated, the recent Bicep2 observations pin down the absolute normalization, the spectral slope and the maximal frequency of the cosmic graviton background produced during inflation. The properly normalized spectra are hereby computed from the lowest frequencies (of the order of the present Hubble rate) up to the highest frequency range in the GHz region. Deviations from the conventional paradigm cannot be excluded and are examined by allowing for different physical possibilities including, in particular, a running of the tensor spectral index, an explicit breaking of the consistency relations and a spike in the high-frequency tail of the spectrum coming either from a post-inflationary phase dominated by a stiff fluid of from the contribution of waterfall fields in a hybrid inflationary context. The direct determinations of the tensor to scalar ratio at low frequencies, if confirmed by the forthcoming observations, will also affect and constrain the high-frequencies uncertainties. The limits on the cosmic graviton backgrounds coming from wide-band interferometers (such as Ligo/Virgo, Lisa and Bbo/Decigo) together with a more accurate scrutiny of the tensor B mode polarization at low frequencies will set direct bounds on the post-inflationary evolution and on other unconventional completions of the standard lore.Giovannini, MassimoTue, 02 Sep 2014 05:23:37 GMThttp://cds.cern.ch/record/1752916Coulomb Branch and The Moduli Space of Instantons
http://cds.cern.ch/record/1752721
The moduli space of instantons on C^2 for any simple gauge group is studied using the Coulomb branch of N=4 gauge theories in three dimensions. For a given simple group G, the Hilbert series of such an instanton moduli space is computed from the Coulomb branch of the quiver given by the over-extended Dynkin diagram of G. The computation includes the cases of non-simply-laced gauge groups G, complementing the ADHM constructions which are not available for exceptional gauge groups. Even though the Lagrangian description for non-simply laced Dynkin diagrams is not currently known, the prescription for computing the Coulomb branch Hilbert series of such diagrams is very simple. For instanton numbers one and two, the results are in agreement with previous works. New results and general features for the moduli spaces of three and higher instanton numbers are reported and discussed in detail.Cremonesi, StefanoMon, 01 Sep 2014 05:56:26 GMThttp://cds.cern.ch/record/1752721Chiral algebras of class S
http://cds.cern.ch/record/1752414
Four-dimensional N=2 superconformal field theories have families of protected correlation functions that possess the structure of two-dimensional chiral algebras. In this paper, we explore the chiral algebras that arise in this manner in the context of theories of class S. The class S duality web implies nontrivial associativity properties for the corresponding chiral algebras, the structure of which is best summarized in the language of generalized topological quantum field theory. We make a number of conjectures regarding the chiral algebras associated to various strongly coupled fixed points.Beem, ChristopherFri, 29 Aug 2014 05:39:27 GMThttp://cds.cern.ch/record/1752414Catching sparks from well-forged neutralinos
http://cds.cern.ch/record/1752392
In this paper we present a new search technique for electroweakinos, the superpartners of electroweak gauge and Higgs bosons, based on final states with missing transverse energy, a photon, and a dilepton pair, $\ell^+\,\ell^- + \gamma + \displaystyle{\not} E_T$. Unlike traditional electroweakino searches, which perform best when $m_{\widetilde{\chi}^0_{2,3}} - m_{\widetilde{\chi}^0_1}, m_{\widetilde{\chi}^{\pm}} - m_{\widetilde{\chi}^0_1} > m_Z$, our search favors nearly degenerate spectra; degenerate electroweakinos typically have a larger branching ratio to photons, and the cut $m_{\ell\ell} \ll m_Z$ effectively removes on-shell Z boson backgrounds while retaining the signal. This feature makes our technique optimal for `well-tempered' scenarios, where the dark matter relic abundance is achieved with inter-electroweakino splittings of $\sim 20 - 70\,\text{GeV}$. Additionally, our strategy applies to a wider range of scenarios where the lightest neutralinos are almost degenerate, but only make up a subdominant component of the dark matter -- a spectrum we dub `well-forged'. Focusing on bino-Higgsino admixtures, we present optimal cuts and expected efficiencies for several benchmark scenarios. We find bino-Higgsino mixtures with $m_{\widetilde{\chi}^0_{2,3}} \lesssim 190\,\text{GeV}$ and $m_{\widetilde{\chi}^0_{2,3}} - m_{\widetilde{\chi}^0_1} \cong 30\,\text{GeV}$ can be uncovered after roughly $600\,\text{fb}^{-1}$ of luminosity at the 14 TeV LHC. Scenarios with lighter states require less data for discovery, while scenarios with heavier states or larger mass splittings are harder to discriminate from the background and require more data. Unlike many searches for supersymmetry, electroweakino searches are one area where the high luminosity of the next LHC run, rather than the increased energy, is crucial for discovery.Bramante, JosephFri, 29 Aug 2014 05:38:53 GMThttp://cds.cern.ch/record/1752392A perturbative approach to the hydrodynamics of heavy ion collisions
http://cds.cern.ch/record/1752386
Initial fluctuations in hydrodynamic fields such as energy density or flow velocity give access to understanding initial state and equilibration physics as well as thermodynamic and transport properties. We provide evidence that the fluid dynamic propagation of fluctuations of realistic size can be based on a background-fluctuation splitting and a systematic perturbative expansion in the fluctuating fields. Initial conditions are characterized by a Bessel-Fourier expansion for single events, event-by-event correlations and probability distributions. The evolution equations can be solved order-by-order in the expansion which allows to study the fluid dynamical propagation of single modes, the study of interaction effects between modes, the determination of the associated particle spectra and the generalization of the whole program to event-by-event correlations and distributions.Floerchinger, StefanFri, 29 Aug 2014 05:38:50 GMThttp://cds.cern.ch/record/1752386Gauss-Bonnet assisted Braneworld Inflation in light of BICEP2 and Planck
http://cds.cern.ch/record/1752311
Motivated by the idea that quantum gravity corrections usually suppress the power of the scalar primordial spectrum (E-mode) more than the power of the tensor primordial spectrum (B-mode), in this Letter we construct a concrete gravitational theory in five-dimensions for which $V(\phi)\propto \phi^n$-type inflation ($n\ge 1$) generates an appropriate tensor-to-scalar ratio that may be compatible with the BICEP2 and Planck data together. The true nature of gravity is five-dimensional and described by the action $S = \int d^5{x} \sqrt{|g|} M^3 \left(- 6\lambda M^2 + R + \alpha M^{-2} {\cal R}^2\right)$ where $M$ is the five-dimensional Planck mass and ${\cal R}^2=R^2-4 R_{ab} R^{ab} + R_{abcd} R^{abcd}$ is the Gauss-Bonnet (GB) term. The five-dimensional `bulk' spacetime is anti-de Sitter ($\lambda<0$) for which inflation ends naturally. The effects of ${\cal R}^2$ term on the magnitudes of scalar and tensor fluctuations and spectral indices are shown to be important at the energy scale of inflation. For GB-assisted $m^2\phi^2$-inflation, inflationary constraints from BICEP2 and Planck, such as, $n_s\simeq 0.9603~(\pm 0.0073)$, $r=0.16~(+0.06-0.05)$ and $V_*^{1/4} \sim 1.5\times 10^{16}~GeV$ are all satisfied for $ (-\lambda \alpha) \simeq (3-300)\times 10^{-5}$.Neupane, Ishwaree PFri, 29 Aug 2014 05:36:20 GMThttp://cds.cern.ch/record/1752311SIMPle Dark Matter: Self-Interactions and keV Lines
http://cds.cern.ch/record/1752287
We consider a simple supersymmetric hidden sector: pure SU(N) gauge theory. Dark matter is made up of hidden glueballinos with mass $m_X$ and hidden glueballs with mass near the confinement scale $\Lambda$. For $m_X \sim 1~\text{TeV}$ and $\Lambda \sim 100~\text{MeV}$, the glueballinos freeze out with the correct relic density and self-interact through glueball exchange to resolve small-scale structure puzzles. An immediate consequence is that the glueballino spectrum has a hyperfine splitting of order $\Lambda^2 / m_X \sim 10~\text{keV}$. We show that the radiative decays of the excited state can explain the observed 3.5 keV X-ray line signal from clusters of galaxies, Andromeda, and the Milky Way.Boddy, Kimberly KFri, 29 Aug 2014 05:36:18 GMThttp://cds.cern.ch/record/1752287Theory Summary and Prospects
http://cds.cern.ch/record/1751766
This talk reviews some of the theoretical progress and outstanding issues in QCD, flavour physics, Higgs and electroweak physics and the search for physics beyond the Standard Model at the Tevatron and the LHC, and previews some physics possibilities for future runs of the LHC and proposed future hadron colliders.Ellis, JohnTue, 26 Aug 2014 05:53:00 GMThttp://cds.cern.ch/record/1751766W gamma production in hadronic collisions using the POWHEG+MiNLO method
http://cds.cern.ch/record/1751762
We detail a calculation of W gamma production in hadronic collision, at Next-to-Leading Order (NLO) QCD interfaced to a shower generator according to the POWHEG prescription supplemented with the MiNLO procedure. The fixed order result is matched to an interleaved QCD+QED parton shower, in such a way that the contribution arising from hadron fragmentation into photons is fully modeled. In general, our calculation illustrates a new approach to the fully exclusive simulation of prompt photon production processes accurate at the NLO level in QCD. We compare our predictions to those of the NLO program MCFM, which treats the fragmentation contribution in terms of photon fragmentation functions. We also perform comparisons to available LHC data at 7 TeV, for which we observe good agreement, and provide phenomenological results for physics studies of the W gamma production process at the Run II of the LHC. The new tool, which includes W leptonic decays and the contribution of anomalous gauge couplings, allows a fully exclusive, hadron-level description of the W gamma process, and is publicly available at the repository of the POWHEG BOX. Our approach can be easily adapted to deal with other relevant isolated photon production processes in hadronic collisions.Barze, LucaTue, 26 Aug 2014 05:52:59 GMThttp://cds.cern.ch/record/1751762On the Flavor Structure of Natural Composite Higgs Models & Top Flavor Violation
http://cds.cern.ch/record/1751232
We explore the up flavor structure of composite pseudo Nambu-Goldstone-boson Higgs models, where we focus on the flavor anarchic minimal $SO(5)$ case. We identify the different sources of flavor violation in this framework and emphasise the differences from the anarchic Randall-Sundrum scenario. In particular, the fact that the flavor symmetry does not commute with the symmetries that stabilize the Higgs potential may constrain the flavor structure of the theory. In addition, we consider the interplay between the fine tuning of the model and flavor violation. We find that generically the tuning of this class of models is worsen in the anarchic case due to the contributions from the additional fermion resonances. We show that, even in the presence of custodial symmetry, large top flavor violating rate are naturally expected. In particular, $t\to cZ$ branching ratio of order of $10^{-5}$ is generic for this class of models. Thus, this framework can be tested in the next run of the LHC as well as in other future colliders. We also find that the top flavor violation is weakly correlated with the increase amount of fine tuning. Finally, other related flavor violation effects, such as $t \to ch$ and in the $D$ system, are found to be too small to be observed by the current and near future colliders.Azatov, AleksandrFri, 22 Aug 2014 08:14:03 GMThttp://cds.cern.ch/record/1751232The chiral phase transition in two-flavor QCD from imaginary chemical potential
http://cds.cern.ch/record/1751225
We investigate the order of the finite temperature chiral symmetry restoration transition for QCD with two massless fermions, by using a novel method, based on simulating imaginary values of the quark chemical potential $\mu=i\mu_i,\mu_i\in\mathbb{R}$. Our method exploits the fact that, for low enough quark mass $m$ and large enough chemical potential $\mu_i$, the chiral transition is decidedly first order, then turning into crossover at a critical mass $m_c(\mu)$. It is thus possible to determine the critical line in the $m - \mu^2$ plane, which can be safely extrapolated to the chiral limit by taking advantage of the known tricritical indices governing its shape. We test this method with standard staggered fermions and the result of our simulations is that $m_c(\mu=0)$ is positive, so that the phase transition at zero density is definitely first order in the chiral limit, on our coarse $N_t=4$ lattices with $a\simeq 0.3\,\mathrm{fm}$.Bonati, ClaudioFri, 22 Aug 2014 08:13:11 GMThttp://cds.cern.ch/record/1751225Dark matter direct detection constraints from gauge bosons loops
http://cds.cern.ch/record/1751207
While many interactions of dark matter (DM) with the standard model (SM) affect direct detection and LHC searches, there are only a few operators generating annihilation of DM into photons. All of these operators, except four of them, give rise to unsuppressed rates, rendering indirect detection superior to other search strategies. For two of the four effective interactions with velocity-suppressed annihilation cross sections, we identify a new type of loop effect which significantly enhances the associated direct detection rates. As a result, relevant constraints from next-generation direct detection experiments on DM-SM interactions, so far only bounded by the LHC, are obtained.Crivellin, AndreasFri, 22 Aug 2014 06:26:30 GMThttp://cds.cern.ch/record/1751207EFT Beyond the Horizon: Stochastic Inflation and How Primordial Quantum Fluctuations Go Classical
http://cds.cern.ch/record/1751142
We identify the effective theory describing inflationary super-Hubble scales and show it to be a special case of effective field theories appropriate to open systems. Open systems allow information to be exchanged between the degrees of freedom of interest and those that are integrated out, such as for particles moving through a fluid. Strictly speaking they cannot in general be described by an effective lagrangian; rather the appropriate `low-energy' limit is instead a Lindblad equation describing the evolution of the density matrix of the slow degrees of freedom. We derive the equation relevant to super-Hubble modes of quantum fields in near-de Sitter spacetimes and derive two implications. We show the evolution of the diagonal density-matrix elements quickly approaches the Fokker-Planck equation of Starobinsky's stochastic inflationary picture. This provides an alternative first-principles derivation of this picture's stochastic noise and drift, as well as its leading corrections. (An application computes the noise for systems with a sub-luminal sound speed.) We argue that the presence of interactions drives the off-diagonal density-matrix elements to zero in the field basis. This shows why the field basis is the `pointer basis' for the decoherence of primordial quantum fluctuations while they are outside the horizon, thus allowing them to re-enter as classical fluctuations, as assumed when analyzing CMB data. The decoherence process is efficient, occurring after several Hubble times even for interactions as weak as gravitational-strength. Crucially, the details of the interactions largely control only the decoherence time and not the nature of the final late-time stochastic state, much as interactions can control the equilibration time for thermal systems but are largely irrelevant to the properties of the resulting equilibrium state.Burgess, C PFri, 22 Aug 2014 06:25:03 GMThttp://cds.cern.ch/record/1751142