Seminars 2011



  • Thursday 13 January 2011 at 10.15 in A315: Particle theory 2-minute meeting.
    Abstract: Everybody working in theoretical particle physics has up to two minutes to present his/her activities.
    Blackboard, transparencies, the local computer or none of those can be used (remember the strictly enforced time limit).
  • Tuesday 1 February 2011 at 10.15 in E204: Samu Kurki (Helsinki)
    Measuring transverse shape with virtual photons
    Abstract: The photon is an excellent probe of hadrons at any virtuality q^2. For example, the charge radius of the nucleon is determined from the slope of the nucleon form factor measured in eN -> eN as q^2 -> 0. At higher q^2 one expects the photon to be able to probe the charge density at higher resolution. However, the standard three-dimensional Fourier transform is not applicable for obtaining the density, since the quarks are highly relativistic, and the time difference between spatially seperated photon interactions cannot be neglected.
    The correct relation between hadron form factors F(q^2) and the spatial density of quarks rho(x) was long obscure. It was only recently understood that the density of quarks in hadrons as a function of their transverse position (impact parameter b) is given by the two-dimensional Fourier transform of the exclusive form factors. The densities are defined in the infinite momentum frame, where the motion of quarks in the transverse direction slows down, and the light-front Fock expansion of the hadron state is applicable.
    The transverse densities of the nucleon and deuteron have been determined from form factor data, and studied for the electron using QED []. We show how to apply this method to any virtual photon induced process, such as gamma* + N -> pi N. The Fourier transforms of the amplitude and the cross section allow new insight into the transverse dynamics of strong interactions. All values of q^2 < 0 contribute to the Fourier transform, whereas in the case of parton distributions one is dependent on the Bjorken limit of -q^2 -> infty. We illustrate the method with QED amplitudes.
  • Tuesday 8 February 2011 at 10.15 in E204: Valery Khoze (Durham)
    Luminosity measurements at the LHC: theoretical assessment
    Abstract: The LHC collider at CERN is doing extremely well, and has a potential of becoming a precision machine. This, in turn, requires an accurate luminosity calibration. Recently the luminosity measurements at the LHC have reached a 10% level accuracy, and the precision of around 5% is on agenda. In this talk we address the importance of knowing the cross section scale with high precision and discuss the main ‘indirect’ methods of luminosity calibration from a theoretical perspective. The main aim is to identify the issues which require further theoretical efforts and to evaluate the size of expected theoretical uncertainties in different approaches. Thursday 10 February 2011 at 10.15 in A315: Volker Ziemann (Uppsala)
    A milestone on the road to CLIC: Testing the the two-beam acceleration scheme in the Two-beam test stand at CERN
    Abstract: The Compact Linear Collider (CLIC) will be the lepton-accelerator of choice to perform precision measurements of new physics if experimental results from LHC indicate new phenomena at energies on or beyond the TeV scale. In order to reach center-of-mass energies in collisions on that scale within a reasonable length of the accelerator the two-beam acceleration scheme was invented. It uses a low-energy, but high-intensity electron drive beam to generate microwaves that are subsequently used to accelerate a low-intensity beam to extremely high energies. The scheme is presently experimentally tested in the Two-beam test stand of the CLIC test facility at CERN. After a general overview of the CLIC accelerator complex I will focus on the two-beam test stand and experimental results where the acceleration gradients needed for CLIC were recently achieved, followed by an outlook on further developments.
  • Tuesday 15 February 2011 at 10.15 in E204: Antonello Polosa (Roma Sapienza University)
    From Scalar Mesons to XYZ Spectroscopy
    Abstract: Since 2005 a number of new charmonium-like resonances have been discovered at Belle and Babar. Some of them have been observed also at Tevatron and, recently, at LHC experiments. Most of these particles are considered to be ‘exotic’ hadrons because, despite of their resemblance to charmonium particles, as apparent from decay modes, they do not match a standard charmonium assignment. This has opened a broad discussion on the subject. I will review the main experimental facts and the theoretical interpretations and controversies raised in the last few years. In particular I will discuss the diquark-antidiquark model of some of the XYZ particles which has his roots in some studies on the problem of light scalar mesons.
  • Kollokvio: Tiistaina 22 helmikuuta 2011 klo 10.15 salissa E204: Seppo Manninen (Fysiikan laitos)
    Fysiikan laitoksella pro gradu-tutkielman tai väitöskirjan vv. 2000-2009 tehneiden sijoittuminen työmarkkinoille
    Yhteenveto: Fysikaalisten tieteiden oppiaineista fysiikan laitokseen kuuluivat v. 2000 fysiikka, teoreettinen fysiikka ja lisäksi fysiikan opettaja. Vuonna 2001 laitokseen liitettiin myös geofysiikka ja meteorologia. Vuosina 2000-2009 tehtyjen pro gradu-tutkielmien määrä on lähes 600 ja väitöskirjoja hyväksyttiin n. 200. Näiden opinnäytetöiden tehneiden työura on muutamaa poikkeusta lukuunottamatta selvitetty tutkinnon suorittamishetkestä nykyhetkeen (v. 2010 loppu) saakka. Työpaikat on jaettu kuuteen luokkaan (i) oma laitos, HIP mukaanluettuna, (ii) HY:n muu laitos tai muu Suomen yliopisto, (iii) sektoritutkimuslaitokset, (iv) koululaitos (kunnallinen ja amk), (v) ulkomainen yliopisto tai tutkimuslaitos ja (vi) yksityinen sektori. Tulokset esitetään oppiaineittain pro gradu-tutkielman ja väitöskirjan tehneiden osalta.
    Fysiikan ja teoreettisen fysiikan osalta on myös selvitetty, miten alkuperäinen työpaikkajakauma poikkeaa nykyisestä vv. 2000-2004 ja vv. 2005-2009 aineistoon perustuen. Lopuksi tarkastellaan luetteloa yksittäisistä työpaikoista merkittävyysjärjestyksessä ja laitokselta valmistuneiden tohtorien ulkomaisia post.doc.-paikkoja.
    Tähtitieteen laitos liitettiin fysiikan laitokseen v. 2010 alussa, joten sieltä valmistuneet opiskelijat eivät ole mukana yllä kuvatussa tarkastelussa. Heidän osaltaan esitetään oma tilastonsa vuosilta 2000-2009.
  • Tuesday 1 March 2011 at 10.15 in E204: Rose Lerner (HIP)
    Higgs inflation: what happens next?
    Abstract: In this talk, we discuss the future of Higgs inflation in the context of its detectability in the near future, and in the context of the recent discussions on unitarity violation. First, we introduce the model of Higgs inflation (where the inflaton is the standard model Higgs boson with a large non-minimal coupling to gravity) and its variants (which include non-minimally coupled scalar dark matter). We then present new results showing how the Higgs mass (m_H) and CMB spectral index (n) are sufficient to distinguish between Higgs inflation and a model where a gauge singlet scalar is the inflaton. We find that radiative corrections cause the common classical value (n = 0.966) to decrease with m_H for Higgs inflation and increase with m_H where the inflaton is scalar dark matter. The models are clearly observationally distinct and well within the reach of Planck and LHC. We then review recent results relating to unitarity violation in this class of models and discuss the implications for Higgs inflation. We present a new, unitarity-conserving model of Higgs inflation which predicts a larger classical value for the spectral index (n = 0.974) than the original model. Thus we conclude that Higgs inflation and its variants are important models which should not be dismissed before confrontation with data from LHC, Planck and dark matter detection experiments.
  • Tuesday 22 March 2011 at 10.15 in E204: Claudio Pica (CP3-Origins, Odense)
    Topics in strong gauge dynamics with infrared fixed points
    Abstract: I will present a brief overview of some recent investigations of strongly interecting gauge theories featuring a conformal dynamics at low energies, and their relevance to high energy physics. I will discuss:
    * the fixed-point structure of gauge theories at 4-loop in perturbation theory and beyond
    * recent lattice results for the Minimal Walking Technicolor model, featuring an IR fixed point.
  • Thursday 24 March 2011 at 10.15 in A315: Jorge Casalderrey Solana (CERN)
    Jet Quenching via Jet Collimation
    Abstract: The ATLAS Collaboration recently reported strong modifications of dijet properties in heavy ion collisions. In this work, we discuss to what extent these first data constrain already the microscopic mechanism underlying jet quenching. Simple kinematic arguments lead us to identify a frequency collimation mechanism via which the medium efficiently trims away the soft components of the jet parton shower. Through this mechanism, the observed dijet asymmetry can be accomodated with values of the kinetic coefficient qhat that lie in the expected order of magnitude.
  • Tuesday 5 April 2011 at 10.15 in E204: David Weir (Imperial College)
    Dynamics of classical and quantum monopoles
    Abstract: Not only are monopoles the source of many interesting problems in themselves, but they also provide us with a narrative thread that links together many areas of physics. In this talk I shall discuss recent work on simulations of monopole formation in the early universe. Possible connections with monopole formation in the `spin ice’ materials of solid state physics shall be discussed, potentially leading to the idea of `solid state cosmology’.
    For quantum field theory, the quantum properties of monopoles pose a particular challenge for perturbative calculations. Therefore, I will also discuss non-perturbative work with ‘t Hooft-Polyakov monopoles, including recent successes with lattice simulations. I will comment quantitatively on the efficacy of various Monte Carlo algorithms when studying topological solitons in lattice quantum field theory.
  • Tuesday 12 April 2011 at 10.15 in E204: Emiliano Sefusatti (CEA/Saclay, Paris)
    Testing the initial conditions with the large-scale structure of the Universe
    Abstract: The possible detection of a non-Gaussian component in the Cosmic Microwave Background anisotropies by the Planck mission would constitute a major result for contemporary cosmology. Such a discovery would rule-out, per se, single- field, slow-roll, canonical model for inflation and significantly help in discriminating between high-energy models of the early Universe. On the other hand, the detection of primordial non-Gaussianity in the large-scale structure would provide a fundamental confirmation. I will review the recent but intense activity on the effects of non-Gaussian initial conditions on the galaxy and cluster distributions. I will discuss the large-scale correction to the bias of halos and the galaxy power spectrum and I will focus, in particular, on the possibilities offered by the measurement of higher-order correlation functions such as the galaxy bispectrum.
  • Thursday 14 April 2011 at 10.15 in A315: Jarno Rantaharju (Helsinki)
    Walking dynamics on the lattice
    Abstract: Applications to technicolor models have created an interest in strongly interacting theories with a slowly running coupling and QCD-like behaviour in the large coupling limit. Possible candidates for this kind of walking behaviour lie near the lower end of the conformal window. Non-perturbatice methods are required to establish walking behaviour, since it typically takes place at strong coupling. We present lattice studies of the running coupling of SU(2) gauge theories with fermions in the fundametal and adjoint representations. The studies show that there are large discretisation errors in the measurements. We have minimized these errors using the Sheikholeslami-Wohlert improved action. Repeating one of the studies using the improved actions has produced significantly different results.
  • Monday 18 April 2011 at 12.15 in A311 (note time and place!): Ari Hietanen (Swansea)
    Eguchi-Kawai model with adjoint fermions
    Abstract: Eguchi-Kawai model is a single site SU(N) matrix model, which was argued to agree with infinite volume SU(N) pure gauge theory at the large-N limit. However, the model has been proven to be incorrect at the continuum limit. I will first review the standard Eguchi-Kawai model and problems associated with it. Then I’ll discuss EK-model with adjoint fermions, which cure the problems of the original theory.
  • Tuesday 19 April 2011 at 10.15 in E204: Miklos Långvik (Helsinki)
    Dirac Quantization of Monopoles in Noncommutative Space-Time
    Abstract: String theory and a high-energy Gedanken experiment motivate the possibility that space-time should be noncommutative at very short distances. In this context the notion of a point is an ill-defined object and one might expect results related to the existence of single points in a commutative space-time, such as the quantization of monopoles due to Dirac, to change in the noncommutative setting. In this talk, the topological formulation of magnetic monopoles due to Wu and Yang is generalized to noncommutative space. The solution of the noncommutative Maxwell equations is calculated perturbatively and it is concluded that, at least perturbatively, the quantization condition of Dirac for point-like monopoles does not hold in a noncommutative space-time.
  • Tuesday 26 April 2011 at 10.15 in E204: Keijo Kajantie (Helsinki)
    Response functions of hot QCD matter from 5-dimensional gravity
    Abstract: Response functions – like the dielectric constant – express how a medium reacts to an external perturbation. The medium we are interested in is finite temperature matter of quarks and gluons and the perturbation is shear in flow. For a very special conformally invariant (completely scalefree) matter the problem has a remarkable solution: the response functions can be computed from 5-dimensional gravity. For infinite wave length and zero frequency this leads to the celebrated prediction of viscosity/entropy=hbar/(4pi). I outline the method and describe how it can be implemented for all wave lengths and frequencies and also how extensions to the physically relevant case of QCD matter with scales, hadron masses and critical temperatures, could be modeled.
  • Tuesday 3 May 2011 at 10.15 in E204: Jussi Lehtola (Helsinki)
    Inelastic x-ray scattering as a tool in materials science
    Abstract: Inelastic x-ray scattering (IXS) experiments can be used to study the structure and dynamics of materials. IXS can be used to extract unique information that is not available with conventional methods, such as x-ray or neutron diffraction. However, due to the nature of the interaction, the interpretation of the experimental results relies heavily on computational modeling.
    In this talk I will give a brief overview of the computational methods used in the modeling of the scattering cross sections (focusing mostly on electronic structure calculations), and discuss some of our newest computational and experimental results.
  • Tuesday 10 May 2011 at 10.15 in E204: Diego Blas (Lausanne)
    Quantum gravity without relativity
    Abstract: I will review the recent proposal of P. Horava for a consistent theory of quantum gravity. After introducing the idea of anisotropic scaling (and its relation to renormalization) with a toy model, I will show how this idea is implemented in gravitational theories. Once the proposal is understood, I will discuss the phenomenological implications. My plan is to make a pedagogical review aimed at anyone interested in quantum gravity.
  • Tuesday 17 May 2011 at 10.15 in E204: Sebastian Szybka (Jagellonian University, Cracow)
    Angular diameter distance in Swiss-Cheese models
    Abstract: I will present numerical results concerning the effect of inhomogeneities on light propagation in Swiss-Cheese models. In the models I have studied, inhomogeneities may partially mimic the accelerated expansion of the Universe provided that light propagates through regions with lower than the average density. The effect of inhomogeneities is small and full randomization of the photons’ trajectories reduces it to an insignificant level.
  • Tuesday 31 May 2011 at 10.15 in E204: Lauri Wendland (Helsinki)
    Status of Higgs boson searches in the CMS experiment at the CERN LHC
    Abstract: The last unconfirmed part of the standard model of particle physics is the spontaneous breaking of the electroweak symmetry, which is assumed to account for the generation of masses of different particles through the Higgs mechanism. The Higgs mechanism requires, however, the existence of one or more Higgs bosons, which so far have not been experimentally discovered. Since the mass of the Higgs boson is limited by theoretical constraints below the TeV energy scale, the LHC and its general-purpose detectors, including the CMS experiment, are expected to be in a position to either confirm or exclude the existence of the Higgs boson.
    The current status of efforts for finding the Higgs boson at the CMS experiment is discussed for both the standard model (SM) and the minimal supersymmetric standard model (MSSM). Results based on the 36 pb-1 of data collected in 2010 are presented [1].
    [1] arXiv:1102.5429, arXiv:1104.1619, CMS-HIG-11-001, CMS-HIG-11-002,
  • Monday 6 June 2011 at 13.00 in A315 (Note time!!): Claus Montonen (Helsinki)
    On Arnold’s principle and further digressions
    Abstract: Arnold’s principle states that a discovery named after someone did not originate with that person. I shall try to convince you that there are exceptions to this principle, and also follow some side-tracks stretching back almost forty years.
  • Tuesday 7 June 2011 at 10.15 in A315: Juan Garcia-Bellido (Madrid)
    The growth factor as a key observable for exploring the nature of dark energy
    Abstract: While background observables like the rate of expansion and the matter content give us crucial information on the nature of dark energy, it is the growth of matter perturbations that will be able to break degeneracies among the different alternatives: quintessence, modified gravity, extra dimensions or even local curvature. Future surveys like the Dark Energy Survey and Euclid will be able to chart the Universe out to redshift z ~ 2, and allow us to disentangle the various alternatives.
  • Tuesday 14 June 2011 at 10.15 in A315: Priyotosh Bandyopadhyay (KIAS, South Korea)
    Aspects of Higgs searches in CP-violating MSSM at the Large Hadron Collider
    Abstract: In CP violating MSSM benchmark scenario (known in the literature as `CPX-scenario’), there exists a light neutral Higgs (h_1, with m_{h_1}< 60 GeV) which is not ruled out by LEP, leaving a not probed ‘hole’ in the m_{h_1}-tan(beta) plane. We have studied this scenario in the context of the Large Hadron Collider (LHC). Many studies have been performed to probe the ‘LEP hole’ at the LHC. In particular I will discuss the associated Higgs productions, cascade production and pair productions at the LHC. These multi-channel analyses are worth studying to probe the ‘hole’ and to reconstruct the Higgs mass(es). Also charged Higgs lighter than top quark mass leads to non-trivial signal topologies different from CP concerving MSSM. The size of the hole depends also on the soft SUSY breaking terms, M_SUSY. We also try to find out the reach of M_SUSY to probe the ‘hole’ in the context of the LHC. It has been shown that with low to moderate accumulated luminosity, we will be able to probe the existing `hole’ in the m_{h_1}-tan(beta) plane.
  • Tuesday 21 June 2011 at 10.15 in A315: Otto Kong (NCU, Taiwan)
    A New Paradigm for Dynamical Symmetry Breaking with Supersymmetry
    Abstract: From the theoretical point of view, the scalar, Higgs, sector of the SM is unnatural and somewhat problematic, while the rest is close to perfection. It also hides the mystery of the EWSB. The situation is improved with supersymmetry, but not to our complete satisfaction. On the other hand, the Nambu–Jona-Lasinio model provides a mechanism to have the Higgs arisen as a bi-fermion composite with the vacuum condensation giving the symmetry breaking. We will tell the story of a new supersymmetric version of the model that can give rise to the Higgs sector of the MSSM dynamically and outline its key features and phenomenological implications. Note that the old version, connected to a volume of famous publications on the related RGE and IQFP analyses around the early 1990s, is no long phenomenologically viable. The new model embodies a completely different way for dynamical symmetry breaking with (softly broken) supersymmetric models; even a new perspective for dynamical symmetry breaking in general.
  • Wednesday 22 June 2011 at 10.30 (Note time!!) in A315: Jorma Louko (Nottingham)
    Localised quantum information for accelerated observers
    Abstract: The entanglement in a quantum state shared by an inertial observer and a relativistic accelerated observer tends to degrade with increasing acceleration. We analyse how this degradation depends on the spatial localisation of the quantum states and (if time permits) on the temporal localisation of the measurements.
  • Tuesday 5 July 2011 at 10.15 in A315: Mark Strikman (PennState University)
    Transverse structure of fast nucleons and pp collisions at the LHC
    Abstract: Studies of exclusive processes at HERA allowed to determine the single parton small x transverse structure of the nucleon, giving important constrains on the pp collision dynamics at the LHC. In particular this allows to explain the observed saturation of the underlying multiplicities in events with a jet trigger. Theoretical developments in the studies of multiparton correlations are summarized which suggest the presence of significant parton – parton transverse correlations already at a low virtuality scale. It is suggested that large fluctuations in the shape of the nucleon wave function as well as in the gluon densities occur and a method to probe these fluctuations is suggested. A possible relation of these effects to the structure of the high multiplicity events reported by CMS is also outlined.
  • Tuesday 16 August 2011 at 10.15 in A315: Kazuo Fujikawa (Institute of Quantum Science, Nihon University, Tokyo)
    Remark on the subtractive renormalization of quadratically divergent scalar mass
    Abstract: The quadratically divergent scalar mass is subtractively renormalized unlike other divergences which are multiplicatively renormalized. We re-examine some technical aspects of the subtractive renormalization, in particular, the mass independent renormalization of massive lambda phi^4 theory with higher derivative regularization. We then discuss an unconventional scheme to introduce the notion of renormalization point mu to the subtractive renormalization in a theory defined by a large fixed cut-off M. The resulting renormalization group equation generally becomes inhomogeneous but it is transformed to be homogeneous. The renormalized scalar mass consists of two components in this scheme, one with the ordinary anomalous dimension and the other which is proportional to the renormalization scale mu. This scheme interpolates between the theory defined by dimensional regularization and the theory with un-subtracted quadratic divergences. [Phys. Rev. D83 (2011) 105012, arXiv:1104.3396]
  • Tuesday 23 August 2011 at 10.15 in A315: Johanna Erdmenger (Munich)
    String Theory Methods for the Real World
    Abstract: We give an overview over the subject of gauge/gravity duality and its applications. Based on string theory, gauge/gravity duality maps strongly coupled quantum field theories to weakly coupled gravity theories and thus provides a new framework for studying physical phenomena in strongly coupled systems. We describe the main features of this approach and discuss its range of applicability. Moreover, we give examples of applications in both elementary particle/nuclear physics and condensed matter physics. The examples presented include the quark-gluon plasma, mesons, quantum phase transitions and superconductors.
  • Tuesday 30 August 2011 at 10.15 in A315: Shahin Sheikh-Jabbari (IPM, Tehran)
    Gauge-flation: Inflation from Non-Abelian Gauge Fields
    Abstract: We introduce an inflationary scenario, Non-Abelian Gauge Field Inflation or gauge-flation for short, in which slow-roll inflation is driven by non-Abelian gauge field minimally coupled to gravity. I present a detailed analysis, both numerical and analytical, of the gauge-flation. By studying the phase diagrams of the theory, we show that getting enough number of e-folds during a slow-roll inflation is fairly robust to the choice of initial gauge field values. In addition, I show the results of the gauge-flation cosmic perturbation theory which has its own specific features and novelties. The specific gauge-flation model I analyze in this talk has two parameters, a cutoff scale Lambda and the gauge coupling g. Fitting our results with the current cosmological data fixes Lambda ~ 10 H ~ 10^{15} GeV (H is the Hubble parameter) and g ~ 10^{-4}, which are in the natural range of parameters in generic particle physics beyond standard models. Our model also predicts a tensor-to-scalar ratio r>0.05, in the range detectable by the Planck satellite.
  • Tuesday 6 September 2011 at 10.15 in E204: Ritva Kinnunen (Helsinki)
    Higgs results from CMS
    Abstract: Search for Higgs bosons with integrated luminosity of at least 1 inverse femtobarn of LHC data, collected with the CMS detector during 2010 and 2011, is discussed.
  • Tuesday 13 September 2011 at 10.15 in E204: K. Österberg (Helsinki)
    First physics results from TOTEM at LHC
    Abstract: The TOTEM experiment at the LHC focuses on measuring of the total pp cross-section as well as on studying elastic scattering, diffraction and forward event topologies. The first physics measurement of TOTEM: the measurement of the elastic scattering cross-section using data taken in 2010 will be presented in detail. For the first time after the ISR measurements almost 40 years ago, the t-distribution (-t > 0.4 GeV2) of elastic pp scattering (at ~ 100 times larger energy) has been measured, exhibiting an exponential behavior at smallest |t|-values, followed by a diffractive minimum and subsequently a broad peak, just as at the ISR. The TOTEM measurement confirms the energy dependence of the cross-section behavior indicating an increase of the effective proton radius with increasing energy. In addition, following the successful commissioning of a dedicated LHC optics over the summer, the physics prospects of special runs with this optics in autumn 2011 for a first total cross-section measurement will be discussed.
  • Friday 16 September 2011 at 14.15 in A315: Shaaban Khalil (BUE, Cairo). Note day!
    Phenomenological aspects of SUSY B-L extension of the SM
    Abstract: Supersymmetric (SUSY) B-L (Baryon minus Lepton number) extension of the Standard Model (SM) is one of the best candidate for physics beyond the SM that accounts for TeV scale seesaw mechanisms and provides an attractive solution for the Higgs naturalness problem. In the context of SUSY B-L with an inverse seesaw mechanism, we calculate the one-loop radiative corrections due to right-handed (s)neutrinos to the mass of the lightest Higgs boson when the latter is SM-like. We show that such effects can be as large as O(100) GeV, thereby giving an absolute upper limit on such a mass around 200 GeV. We also show that the lightest right-handed sneutrino in this model is a viable candidate for cold dark matter. We find that it accounts for the observed dark matter relic abundance in a wide range of parameter space.
  • Tuesday 20 September 2011 at 10.15 in E204: Kim Splittorff (NBI, Copenhagen)
    The Wilson Spectrum Close to the Continuum
    Abstract: The unquenched spectrum of the Wilson Dirac operator in the microscopic (epsilon) regime including order a^2-corrections is discussed. We explain how to obtain exact analytical expressions for the microscopic spectral density from the supersymmetric method and Wilson chiral perturbation theory. The results are derived for individual topological sectors, where topology at nonzero lattice spacing is defined by the index of the Wilson Dirac operator. The microscopic spectral density shows in detail the transition to the Aoki phase. Finally, we discuss constraints on the coefficients of Wilson chiral perturbation theory.
  • Tuesday 27 September 2011 at 10.15 in E204: David Mulryne (Queen Mary, University of London)
    Evolution of non-Gaussianity in multi-field models
    Abstract: The study of the statistics of perturbations produced by inflation has the power to tell us much about the inflationary potential, and hence the physics of the very early universe. In complex multi-field models, however, extracting information about the potential from observation is extremely difficult. I will discuss some recent work which is a step towards this goal. The work is concerned with how the non-linearity parameter fnl, which parametrises the three-point function, can evolve during multi-field inflation. Particular features in the inflationary potential lead to a particular sign of fnl as it evolves, and tuning the initial conditions on these features leads to a large but transitory magnitude. Ultimately we should consider the value of fnl at an adiabatic limit, when it becomes constant, and I further discuss the conditions needed for fnl to be large at that time if the limit is reached “naturally”. I show that for concrete models of inflation numerical techniques often become essential to reliably calculate the value of fnl at an adiabatic limit, and give a number of explicit examples, including axion potentials, inflection potentials and the hybrid potential.
  • Tuesday 4 October 2011 at 10.15 in A315: Aleksi Kurkela (McGill, Montreal) (note place: this is a blackboard talk)
    Thermalisation in collisions of extremely large nuclei at extremely large energies
    Abstract: Hydrodynamical analysis of experimental data of ultrarelativistic heavy ion collisions seems to indicate that the hot QCD matter created in the collisions thermalizes very quickly. Theoretically, we have no idea why this should be true. In my talk, I will describe how the thermalization takes place in the most theoretically clean limit — that of large nuclei at asymptotically high energy per nucleon, where the system is described by weak-coupling QCD. In this limit, plasma instabilities dominate the dynamics from immediately after the collision until well after the plasma becomes nearly in equilibrium at time Qt ~ alpha^(-5/2).
  • Tuesday 18 October 2011 at 10.15 in E204: Paul Hoyer (Helsinki)
    Bound states in field theory
    Abstract: Bound states form an important part of observable phenomena, and are a prominent feature of quantum field theories. Atoms provide precision tests of QED and serve as a conceptual model for hadrons, the relativistic bound states of QCD. The field theory description of bound states is nevertheless often omitted in standard textbooks and courses. Perhaps for this reason there are basic principles concerning relativistic bound states that receive little attention. I illustrate with three instructive questions:
    1. What is the wave function of a QED atom which is in relativistic CM motion?
    2. How can the single particle Dirac wave function describe bound states with an infinite number of pairs?
    3. Is there a Born term (of lowest order in hbar) for bound states?
    The answers provide guidance in addressing the more difficult question: Is there an expansion of QCD which at lowest order is similar to the Quark Model picture of hadrons?
  • Tuesday 25 October 2011 at 10.15 in E204: Oleg Antipin (Odense)
    Light dilaton
    Abstract: I will present the infrared dynamics of a nonsupersymmetric SU(X) gauge theory featuring an adjoint fermion, Nf Dirac flavors and an Higgs-like complex Nf x Nf scalar which is a gauge singlet. First, I establish the existence of an infrared stable perturbative fixed point and then investigate the spectrum near this point. I demonstrate that this theory naturally features a light scalar degree of freedom to be identified with the dilaton and elucidate its physical properties. The spectrum of the theory can be computed and I will demonstrate that at low energy the nonperturbative part of the spectrum of the theory is the one of pure supersymmetric Yang-Mills. Therefore the exact nonperturbative fermion condensate and relevant properties of the nonperturbative spectrum of the theory can be deduced. I will also show that the intrinsic scale of super Yang-Mills is exponentially smaller than the scale associated to the breaking of conformal and chiral symmetry of the theory. [1107.2932]
  • Tuesday 1 November 2011 at 10.15 in E204: Matts Roos (Helsinki)
    Glashow’s cake problem
    and other memories from the fifties, sixties and seventies.
  • Thursday 3 November 2011 at 10.15 in A315: Jari Laamanen (Nijmegen)
    Stop NLSP in cMSSM
    So far the squarks have not been detected at the LHC indicating that they are heavier than a few hundred GeVs, if they exist. The lighter stop can be considerably lighter than the other squarks. In this talk, the possibility that a supersymmetric partner of the top quark, stop, is the next-to-lightest supersymmetric particle in the constrained supersymmetric standard model is studied. We’ll find that it cannot be ruled out. Observing this kind of stop at the LHC may be difficult.
  • Tuesday 8 November 2011 at 10.15 in E204: Matti Antola (Helsinki)
    Supersymmetric Technicolor
    Abstract: In supersymmetric models scalars are naturally as light as the supersymmetry breaking scale. This explanation of the Higgs mass is minimal since one can use the same degrees of freedom to give mass to the electroweak gauge sector and to the matter fields. However, the large hadron collider has seen no trace of light superpartners and already beforehand one knew that the SUSY breaking sector must conserve flavor to a suprisingly high accuracy. In our model these facts are explained by increasing the SUSY breaking scale, and correspondingly the Higgs mass, to 10-100 TeV. The electroweak scale is instead explained as in technicolor – it is analogous to the QCD scale. The fundamental Higgses do not participate in electroweak symmetry breaking but simply act as messengers between the symmetry breaking sector and the quarks and leptons. I will present a specific realization of this idea, with the added interest that the supersymmetric technicolor sector is N=4 super-Yang-Mills.
  • Tuesday 15 November 2011 at 10.15 in E204: Kirtiman Ghosh (Helsinki)
    Diphoton Events with Large Missing Transverse Energy at the LHC
    Abstract: The search for diphoton events with large missing transverse energy was recently communicated by the ATLAS collaboration. The result is based on the data collected with the ATLAS detector in proton-proton collisions at 7 TeV center of mass energy and 3.1 inverse femtobarn integrated luminosity. Above the Standard Model background prediction, no excess of such events was reported. In the context of a specific model with one Universal Extra Dimension with compactification radius R and gravity-induced decays, we reanalyze the ATLAS search result and put constraints on the parameters of this model.
  • Thursday 8 December 2011 at 10.15 in A315: Lars Fritz (Köln)
    Interaction effects in transport in Dirac systems
    Abstract: In this talk I will discuss the effect of interactions as they manifest themselves in transport properties in Dirac systems, most notably in graphene.
    In a first part I discuss the role of weak long-range interactions in understanding transport properties in the vicinity of the Dirac point. Most importantly, based on a microscopic approach employing kinetic equations, I will show that the transport properties are fully accounted for by a hydrodynamic approach. In a second part I address the role of strong correlations and transport characteristics in the vicinity of an interaction driven transition towards an excitonic insulator. In a final part I discuss the possible tunability of the chiral symmetry breaking transition in Dirac systems in topological insulators.
  • Tuesday 13 December 2011 at 10.15 in E204: Heribert Weigert (Cape Town, South Africa)
    The Color Glass Condensate: QCD at modern collider facilities
    Abstract: Nonlinear effects become more and more important in our understanding of QCD as experimental efforts progress to higher energies and/or large nuclear targets. I will present an infrared safe, nonlinear renormalization group approach, justified for central collisions at small Bjorken x (large energies) and large nuclei. The emergence of a new density driven scale leads to a selfconsistent treatment with consequences that apply to a wide variety of experiments such as ep, eA and AA collisions as performed or planned at HERA, EIC, RHIC and LHC.