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Probing the time structure of the quark-gluon plasma with top quarks
http://cds.cern.ch/record/2292566
The tiny droplets of Quark Gluon Plasma (QGP) created in high-energy nuclear collisions experience fast expansion and cooling with a lifetime of some $\text{fm}/c$. Despite the information provided by probes such as jet quenching and quarkonium suppression, and the excellent description by hydrodynamical models, direct access to the time evolution of the system remains elusive. We point out that the study of hadronically-decaying $W$ bosons in events with a top-antitop quark pair can provide unique insight into the time structure of the QGP. This is because of the finite lifetimes of the top and $W$ particles, and a time-delay in the interaction of the (colour-singlet) $W$-boson's decay products with the medium. All three times are correlated with the kinematics of the top quark, allowing the approximate determination of the time at which the interaction with the QGP begins. We carry out a simple Monte Carlo feasibility study and find that the LHC has the potential to bring first, limited information on the time structure of the QGP. Substantially increased LHC heavy-ion luminosities or future higher-energy colliders would open opportunities for more extensive studies.Apolinário, LilianaMilhano, José GuilhermeSalam, Gavin P.Salgado, Carlos A.Sat, 11 Nov 2017 07:10:21 GMT2017-11-08CERN-TH-2017-237http://cds.cern.ch/record/22925662017237Single top-quark production with SHERPA
http://cds.cern.ch/record/2292297
We present results at next-to-leading order accuracy in QCD for single top-quark production in the $t$, $s$ and $tW$ channels at the LHC at a centre-of-mass energy of 8 TeV, obtained with the SHERPA event generator. We find them in very good agreement with measured values and quantify their theory uncertainties. Uncertainties stemming from the choice between the four- and the five-flavour scheme are found to be typically of the order of 5-10 % over large ranges of phase space. We discuss the impact of parton distribution functions, and in particular of the bottom PDF. We also show how different cuts on QCD radiation patterns improve the signal-to-background ratio in realistic fiducial volumes.Bothmann, EnricoKrauss, FrankSchönherr, MarekThu, 09 Nov 2017 06:58:38 GMT2017-11-07CERN-TH-2017-234http://cds.cern.ch/record/22922972017234Local multiboson factorization of the quark determinant
http://cds.cern.ch/record/2292080
We discuss the recently proposed multiboson domain-decomposed factorization of the gauge-field dependence of the fermion determinant in lattice QCD. In particular, we focus on the case of a lattice divided in an arbitrary number of thick time slices. As a consequence, multiple space-time regions can be updated independently. This allows to address the exponential degradation of the signal-to-noise ration of correlation functions with multilevel Monte Carlo sampling. We show numerical evidence of the effectiveness of a two-level integration for pseudoscalar propagators with momentum and for vector propagators, in a two active regions setup. These results are relevant to lattice computation of the hadronic contributions to the anomalous magnetic moment of the muon and to heavy meson decay form factors.Cè, MarcoGiusti, LeonardoSchaefer, StefanWed, 08 Nov 2017 06:55:09 GMT2017-11-05HIM-2017-06http://cds.cern.ch/record/2292080201706Differential Higgs production at $N^3$LO beyond threshold
http://cds.cern.ch/record/2292076
We present several key steps towards the computation of differential Higgs boson cross sections at N$^3$LO in perturbative QCD. Specifically, we work in the framework of Higgs-differential cross sections that allows to compute precise predictions for realistic LHC observables. We demonstrate how to perform an expansion of the analytic N$^3$LO coefficient functions around the production threshold of the Higgs boson. Our framework allows us to compute to arbitrarily high order in the threshold expansion and we explicitly obtain the first two expansion coefficients in analytic form. Furthermore, we assess the phenomenological viability of threshold expansions for differential distributions. In addition, we report on an interesting obstacle for the computation of N$^3$LO corrections with LHAPDF parton distribution functions and our solution. We provide files containing the analytic expressions for the partonic cross sections together with the arXiv submission.Dulat, FalkoMistlberger, BernhardPelloni, AndreaWed, 08 Nov 2017 06:55:01 GMT2017-10-09arXiv:1710.03016http://cds.cern.ch/record/2292076['arXiv:1710.03016']arXiv:1710.03016Lattice QCD on new chips: a community summary
http://cds.cern.ch/record/2291711
I review the most recent evolutions of the QCD codes on new architectures, with a focus on the performances obtained by the different coding strategies as presented during the Lattice2017 conference.Rago, AntonioMon, 06 Nov 2017 11:47:46 GMT2017-11-03CERN-TH-2017-231http://cds.cern.ch/record/22917112017231Rigid limit for hypermultiplets and five-dimensional gauge theories
http://cds.cern.ch/record/2291128
We study the rigid limit of a class of hypermultiplet moduli spaces appearing in Calabi-Yau compactifications of type IIB string theory, which is induced by a local limit of the Calabi-Yau. We show that the resulting hyperkahler manifold is obtained by performing a hyperkahler quotient of the Swann bundle over the moduli space, along the isometries arising in the limit. Physically, this manifold appears as the target space of the non-linear sigma model obtained by compactification of a five-dimensional gauge theory on a torus. This allows to compute dyonic and stringy instantons of the gauge theory from the known results on D-instantons in string theory. Besides, we formulate a simple condition on the existence of a non-trivial local limit in terms of intersection numbers of the Calabi-Yau, and find an explicit form for the hypermultiplet metric including corrections from all mutually non-local D-instantons, which can be of independent interest.Alexandrov, SergeiBanerjee, SibasishLonghi, PietroWed, 01 Nov 2017 06:31:55 GMT2017-10-29L2C:17-128http://cds.cern.ch/record/2291128L2C:17128Texture Zero Neutrino Models and Their Connection with Resonant Leptogenesis
http://cds.cern.ch/record/2291127
Within the low scale resonant leptogenesis scenario, the cosmological CP asymmetry may arise by radiative corrections through the charged lepton Yukawa couplings. While in some cases, as one expects, decisive role is played by the $\lambda_{\tau }$ coupling, we show that in specific neutrino textures only by inclusion of the $\lambda_{\mu }$ the cosmological CP violation is generated at 1-loop level. With the purpose to relate the cosmological CP violation to the leptonic CP phase $\delta $, we consider an extension of MSSM with two right handed neutrinos (RHN), which are degenerate in mass at high scales. Together with this, we first consider two texture zero 3x2 Dirac Yukawa matrices of neutrinos. These via see-saw generated neutrino mass matrices augmented by single $\Delta L=2$ dimension five (d=5) operator give predictive neutrino sectors with calculable CP asymmetries. The latter is generated through $\lambda_{\mu , \tau }$ coupling(s) at 1-loop level. Detailed analysis of the leptogenesis is performed. We also revise some one texture zero Dirac Yukawa matrices, considered earlier, and show that addition of a single $\Delta L=2$, d=5 entry in the neutrino mass matrices, together with newly computed 1-loop corrections to the CP asymmetries, give nice accommodation of the neutrino sector and desirable amount of the baryon asymmetry via the resonant leptogenesis even for rather low RHN masses (~few TeV - $10^7$ GeV).Achelashvili, AvtandilTavartkiladze, ZurabWed, 01 Nov 2017 06:31:54 GMT2017-10-27CERN-TH-2017-202http://cds.cern.ch/record/22911272017202Multi-boson block factorization of fermions
http://cds.cern.ch/record/2290741
The numerical computations of many quantities of theoretical and phenomenological interest are plagued by statistical errors which increase exponentially with the distance of the sources in the relevant correlators. Notable examples are baryon masses and matrix elements, the hadronic vacuum polarization and the light-by-light scattering contributions to the muon g-2, and the form factors of semileptonic B decays. Reliable and precise determinations of these quantities are very difficult if not impractical with state-of-the-art standard Monte Carlo integration schemes. I will review a recent proposal for factorizing the fermion determinant in lattice QCD that leads to a local action in the gauge field and in the auxiliary boson fields. Once combined with the corresponding factorization of the quark propagator, it paves the way for multi-level Monte Carlo integration in the presence of fermions opening new perspectives in lattice QCD. Exploratory results on the impact on the above mentioned observables will be presented.Giusti, LeonardoCè, MarcoSchaefer, StefanFri, 27 Oct 2017 05:21:53 GMT2017-10-25CERN-TH-2017-195http://cds.cern.ch/record/22907412017195Predictions for exclusive Higgs cross sections
http://cds.cern.ch/record/2290474
I give an overview of recent theoretical results for exclusive Higgs-production cross sections, focusing in particular on processes where precise predictions will be relevant in the near future.Re, EmanueleWed, 25 Oct 2017 05:26:21 GMT2017-10-24CERN-TH-2017-223http://cds.cern.ch/record/22904742017223openQ*D simulation code for QCD+QED
http://cds.cern.ch/record/2290473
The openQ*D code for the simulation of QCD+QED with C$^\star$ boundary conditions is presented. This code is based on openQCD-1.6, from which it inherits the core features that ensure its efficiency: the locally-deflated SAP-preconditioned GCR solver, the twisted-mass frequency splitting of the fermion action, the multilevel integrator, the 4th order OMF integrator, the SSE/AVX intrinsics, etc. The photon field is treated as fully dynamical and C$^\star$ boundary conditions can be chosen in the spatial directions. We discuss the main features of openQ*D, and we show basic test results and performance analysis. An alpha version of this code is publicly available and can be downloaded from http://rcstar.web.cern.ch/ .Campos, IsabelFritzsch, PatrickHansen, MartinMarinković, Marina KrstićPatella, AgostinoRamos, AlbertoTantalo, NazarioWed, 25 Oct 2017 05:26:19 GMT2017-10-24CERN-TH-2017-217http://cds.cern.ch/record/22904732017217Simulations of QCD and QED with C* boundary conditions
http://cds.cern.ch/record/2290472
We present exploratory results from dynamical simulations of QCD in isolation, as well as QCD coupled to QED, with C* boundary conditions. In finite volume, the use of C* boundary conditions allows for a gauge invariant and local formulation of QED without zero modes. In particular we show that the simulations reproduce known results and that masses of charged mesons can be extracted in a completely gauge invariant way. For the simulations we use a modified version of the HiRep code. The primary features of the simulation code are presented and we discuss some details regarding the implementation of C* boundary conditions and the simulated lattice action.Hansen, MartinLucini, BiagioPatella, AgostinoTantalo, NazarioWed, 25 Oct 2017 05:26:18 GMT2017-10-24CP3-ORIGINS-2017-046-DNRF90http://cds.cern.ch/record/2290472046DNRF90On classical de Sitter and Minkowski solutions with intersecting branes
http://cds.cern.ch/record/2290471
Motivated by the connection of string theory to cosmology or particle physics, we study solutions of type II supergravities having a four-dimensional de Sitter or Minkowski space-time, with intersecting Dp-branes and orientifold Op-planes. Only few such solutions are known, and we aim at a better characterisation. Modulo a few restrictions, we prove that there exists no classical de Sitter solution for any combination of D3/O3 and D7/O7, while we derive interesting constraints for intersecting D5/O5 or D6/O6, or combinations of D4/O4 and D8/O8. Concerning classical Minkowski solutions, we understand some typical features, and propose a solution ansatz. Overall, a central information appears to be the way intersecting Dp/Op overlap each other, a point we focus on.Andriot, DavidWed, 25 Oct 2017 05:26:17 GMT2017-10-24CERN-TH-2017-222http://cds.cern.ch/record/22904712017222The Dawn of the Post-Naturalness Era
http://cds.cern.ch/record/2290470
In an imaginary conversation with Guido Altarelli, I express my views on the status of particle physics beyond the Standard Model and its future prospects.Giudice, Gian FrancescoWed, 25 Oct 2017 05:26:15 GMT2017-10-18CERN-TH-2017-205http://cds.cern.ch/record/22904702017205Alternatives to the stochastic "noise vector" approach
http://cds.cern.ch/record/2290179
Several important observables, like the quark condensate and the Taylor coefficients of the expansion of the QCD pressure with respect to the chemical potential, are based on the trace of the inverse Dirac operator and of its powers. Such traces are traditionally estimated with "noise vectors" sandwiching the operator. We explore alternative approaches based on polynomial approximations of the inverse Dirac operator.de Forcrand, PhilippeJaeger, BenjaminMon, 23 Oct 2017 05:16:56 GMT2017-10-19CERN-TH-2017-210http://cds.cern.ch/record/22901792017210A Lower Bound on Inelasticity in Pion-Pion Scattering
http://cds.cern.ch/record/2290009
Assuming that the pion-pion scattering amplitude and its absorptive part are analytic inside an ellipse in $t$- plane with foci $t=0$, $u=0$ and right extremity $t=4 m_{\pi}^2 +\epsilon $, ($\epsilon > 0$), except for cuts prescribed by Mandelstam representation for $t\geq 4 m_{\pi}^2$, $u\geq 4 m_{\pi}^2$ , and bounded by $s^N$ on the boundary of this domain, we prove that for $s\rightarrow \infty$, \sigma_{inel} (s) > \frac{Const}{s^{5/2} }\exp {[-\frac{\sqrt{s}}{4} (N+5/2) \ln {s} ]}.Martin, AndréRoy, S.M.Fri, 20 Oct 2017 05:41:41 GMT2017-10-19CERN-TH-2017-215http://cds.cern.ch/record/22900092017215Ergodicity of the LLR method for the Density of States
http://cds.cern.ch/record/2290007
The LLR method is a novel algorithm that enables us to evaluate the density of states in lattice gauge theory. We present our study of the ergodicity properties of the LLR algorithm for the model of Yang Mills SU(3). We show that the use of the replica exchange method alleviates significantly the topological freeze-out that severely affects other algorithms.Cossu, GuidoLucini, BiagioPellegrini, RobertoRago, AntonioFri, 20 Oct 2017 05:41:37 GMT2017-10-17CERN-TH-2017-213http://cds.cern.ch/record/22900072017213Search for Dark Matter Effects on Gravitational Signals from Neutron Star Mergers
http://cds.cern.ch/record/2289724
Motivated by the recent detection of the gravitational wave signal emitted by a binary neutron star merger, we analyse the possible impact of dark matter on such signals. We show that dark matter cores in merging neutron stars may yield an observable supplementary peak in the gravitational wave power spectral density following the merger, which could be distinguished from the features produced by the neutron components.Ellis, JohnHektor, AndiHütsi, GertKannike, KristjanMarzola, LucaRaidal, MarttiVaskonen, VilleWed, 18 Oct 2017 06:02:28 GMT2017-10-16CERN-TH-2017-208http://cds.cern.ch/record/22897242017208Constraints from microlensing experiments on clustered primordial black holes
http://cds.cern.ch/record/2289221
It has recently been proposed that massive primordial black holes (PBH) could constitute all of the dark matter, providing a novel scenario of structure formation, with early reionization and a rapid growth of the massive black holes at the center of galaxies and dark matter halos. The scenario arises from broad peaks in the primordial power spectrum that give both a spatially clustered and an extended mass distribution of PBH. The constraints from the observed microlensing events on the extended mass function have already been addressed. Here we study the impact of spatial clustering on the microlensing constraints. We find that the bounds can be relaxed significantly for relatively broad mass distributions if the number of primordial black holes within each cluster is typically above one hundred. On the other hand, even if they arise from individual black holes within the cluster, the bounds from CMB anisotropies are less stringent due to the enhanced black hole velocity in such dense clusters. This way, the window between a few and ten solar masses has opened up for PBH to comprise the totality of the dark matter.García-Bellido, JuanClesse, SebastienMon, 16 Oct 2017 05:57:12 GMT2017-10-12CERN-TH-2017-206http://cds.cern.ch/record/22892212017206Double parton scattering theory overview
http://cds.cern.ch/record/2288654
The dynamics of double hard scattering in proton-proton collisions is quite involved compared with the familiar case of single hard scattering. In this contribution, we review our theoretical understanding of double hard scattering and of its interplay with other reaction mechanisms.Diehl, MarkusGaunt, Jonathan R.Fri, 13 Oct 2017 05:37:44 GMT2017-10-12CERN-TH-2017-204http://cds.cern.ch/record/22886542017204Strong-Coupling Lattice QCD on Anisotropic Lattices
http://cds.cern.ch/record/2288317
Anisotropic lattice spacings are mandatory to reach the high temperatures where chiral symmetry is restored in the strong coupling limit of lattice QCD. Here, we propose a simple criterion for the nonperturbative renormalisation of the anisotropy coupling $\gamma$ in strongly-coupled SU($N$) or U($N$) lattice QCD with massless staggered fermions. We then compute the renormalised anisotropy $\xi(\gamma)$, and the strong-coupling analogue of Karsch's coefficients (the running anisotropy), for $N=3$. We achieve high precision by combining diagrammatic Monte Carlo and multi-histogram reweighting techniques. We observe that the mean field prediction in the continuous time limit captures the nonperturbative scaling, but receives a large, previously neglected correction on the unit prefactor. Using our nonperturbative prescription in place of the mean field result, we observe large corrections of the same magnitude to the continuous time limit of the static baryon mass, and of the location of the phase boundary associated with chiral symmetry restoration. In particular, the phase boundary, evaluated on different finite lattices, has a dramatically smaller dependence on the lattice time extent. We also estimate, as a byproduct, the pion decay constant and the chiral condensate of massless SU(3) QCD in the strong coupling limit at zero temperature.de Forcrand, PhilippeUnger, WolfgangVairinhos, HelvioThu, 12 Oct 2017 05:18:05 GMT2017-10-02CERN-TH-2017-198http://cds.cern.ch/record/22883172017198Dark matter and LHC: Complementarities and limitations
http://cds.cern.ch/record/2288104
It is well known that dark matter density measurements, indirect and direct detection experiments, importantly complement the LHC in setting strong constraints on new physics scenarios. Yet, dark matter searches are subject to limitations which need to be considered for realistic analyses. For illustration, we explore the parameter space of the phenomenological MSSM and discuss the interplay of the constraints from dark matter searches and the LHC, and analyse the impact of the astrophysical uncertainties in some detail.Robbins, G.Mahmoudi, F.Arbey, A.Boudaud, M.Wed, 11 Oct 2017 05:28:47 GMT2017-10-10CERN-TH-2017-203http://cds.cern.ch/record/22881042017203Integrable deformations of the $G_{k_1} \times G_{k_2}/G_{k_1+k_2}$ coset CFTs
http://cds.cern.ch/record/2287602
We study the effective action for the integrable $\lambda$-deformation of the $G_{k_1} \times G_{k_2}/G_{k_1+k_2}$ coset CFTs. For unequal levels theses models do not fall into the general discussion of $\lambda$-deformations of CFTs corresponding to symmetric spaces and have many attractive features. We show that the perturbation is driven by parafermion bilinears and we revisit the derivation of their algebra. We uncover a non-trivial symmetry of these models parametric space, which has not encountered before in the literature. Using field theoretical methods and the effective action we compute the exact in the deformation parameter $\beta$-function and explicitly demonstrate the existence of a fixed point in the IR corresponding to the $G_{k_1-k_2} \times G_{k_2}/G_{k_1}$ coset CFTs. The same result is verified using gravitational methods for $G=SU(2)$. We examine various limiting cases previously considered in the literature and found agreement.Sfetsos, KonstantinosSiampos, KonstantinosMon, 09 Oct 2017 05:17:51 GMT2017-10-06CERN-TH-2017-199http://cds.cern.ch/record/22876022017199Monodromy inflation at strong coupling: $4\pi$ in the sky
http://cds.cern.ch/record/2285565
We present a simple effective field theory formulation of a general family of single field flux monodromy models for which strong coupling effects at large field values can flatten the potential and activate operators with higher powers of derivatives. These models are radiatively and non-perturbatively stable and can easily sustain $\ga 60$ efolds of inflation. The dynamics combines features of both large field chaotic inflation and $k$-inflation, both of which can suppress the tensor amplitude. Reducing the tensor-scalar ratio below the observational bound $r \lesssim 0.1$ while keeping the scalar spectral index $n_s$ within experimental bounds either yields equilateral nongaussianity $f_{NL}^{eq} \simeq {\cal O}(1)$, close to the current observational bounds, or ultimately gives very small $r$.D'Amico, GuidoKaloper, NemanjaLawrence, AlbionSat, 23 Sep 2017 06:34:17 GMT2017-09-20BRX-TH-6323http://cds.cern.ch/record/2285565TH6323The S-matrix Bootstrap III: Higher Dimensional Amplitudes
http://cds.cern.ch/record/2285239
We consider constraints on the S-matrix of any gapped, Lorentz invariant quantum field theory in 3+1 dimensions due to crossing symmetry, analyticity and unitarity. We extremize cubic couplings, quartic couplings and scattering lengths relevant for the elastic scattering amplitude of two identical scalar particles. In the cases where our results can be compared with the older S-matrix literature they are in excellent agreement. We also extremize a cubic coupling in 2+1 dimensions which we can directly compare to a universal bound for a QFT in AdS. This paper generalizes our previous 1+1 dimensional results of arXiv:1607.06109 and arXiv:1607.06110.Paulos, Miguel F.Penedones, JoaoToledo, Jonathanvan Rees, Balt C.Vieira, PedroThu, 21 Sep 2017 12:21:25 GMT2017-08-22arXiv:1708.06765http://cds.cern.ch/record/2285239['arXiv:1708.06765']arXiv:1708.06765Primordial black holes from single field models of inflation
http://cds.cern.ch/record/2285007
Primordial black holes (PBH) have been shown to arise from high peaks in the matter power spectra of multi-field models of inflation. Here we show, with a simple toy model, that it is also possible to generate a peak in the curvature power spectrum of single-field inflation. We assume that the effective dynamics of the inflaton field presents a near-inflection point which slows down the field right before the end of inflation and gives rise to a prominent spike in the fluctuation power spectrum at scales much smaller than those probed by Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) observations. This peak will give rise, upon reentry during the radiation era, to PBH via gravitational collapse. The mass and abundance of these PBH is such that they could constitute the totality of the Dark Matter today. We satisfy all CMB and LSS constraints and predict a very broad range of PBH masses. Some of these PBH are light enough that they will evaporate before structure formation, leaving behind a large curvature fluctuation on small scales. This broad mass distribution of PBH as Dark Matter will be tested in the future by AdvLIGO and LISA interferometers.Garcia-Bellido, JuanRuiz Morales, EsterWed, 20 Sep 2017 07:14:53 GMT20 Sep 2017CERN-TH-2017-196http://cds.cern.ch/record/22850072017196