NBI-Theoretical Particle Physics and Cosmology

Seminars and Events: Fall 2014

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• August 2014
• MSc defence

Aud. A, Wednesday 20 Aug at 13:00

Rasmus W. Rasmussen

Determination of the neutrino mixing angle theta_23 octant and differentiation among flavor symmetries

MSc defence

Determination of the neutrino mixing angle theta_23 octant and differentiation among flavor symmetries

Abstract:

This thesis studies the potential/reach for the Proposed IceCube Next Generation Upgrade (PINGU) with respect to the determination of the octant of the atmospheric mixing angle theta_23. In the neutrino sector, this is one of the still unknown parameters which have to be determined by future experiments. Present measurements of theta_23 are not precise enough to establish whether this mixing is maximal or not. If it is non-maximal, there are two solutions to the neutrino oscillation data in that theta_23 can either be below or above 45 degrees, i.e. it lies in the first or second octant.
From Monte Carlo simulations the event rates of neutrinos in PINGU were calculated and a chi square fit was done, assuming the theoretical predicted event rates were fitted in the wrong octant of the atmospheric angle. This provides a way to constrain this mixing parameter. In turn this can potentially differentiate among various flavour symmetries which can underly the neutrino mixing matrix and thereby the oscillation parameters.
By implementing multiple flavour symmetries in the simulations, a comparison of the sensitivities to the various models could be made. This in priciple provides a way to experimentally determine the flavour symmetry for neutrinos.

Details

Date: Wednesday 20 Aug

Time: 13:00

Place: Aud. A

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• NBIA Colloquium

Aud. A, Blegdamsvej, Friday 29 Aug at 14:15

Michael B. Green (Cambridge)

The Scope of String Theory

NBIA Colloquium

The Scope of String Theory

Abstract:

Interesting physical questions arise on vastly different distance scales, ranging from the nature of the fundamental sub-nuclear particles, to properties of macroscopic condensed matter systems, to features of the whole Universe. The talk will present a survey of approaches describing such disparate phenomena based on ideas emerging from String Theory, which has the potential for providing a theoretical link between areas of physics that otherwise appear to be only remotely connected.

Details

Date: Friday 29 Aug

Time: 14:15

Place: Aud. A, Blegdamsvej

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• September 2014
• PhD defence

Aud. A, Thursday 4 Sep at 14:15

Lisa Glaser

The price of Causality

PhD defence

The price of Causality

Abstract:

There are different ideas about how to quantize gravity that are currently pursued. In each approach trade-offs are made between different concepts that are clearly defined in quantum theory or general relativity but that can not be combined. One such fundamental concept is the Lorentzian structure of space-time. In this defense I will present my research on Causal Dynamical Triangulations and Causal Set Theory, and the consequences that their focus on Lorentzian structure engenders.

Details

Date: Thursday 4 Sep

Time: 14:15

Place: Aud. A

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• HET Seminar

Aud D(!), Blegdamsvej, Monday 8 Sep at 14:15

Rafael Sorkin (Syracuse & Perimeter Institute)

Projective Statistics (for Topological Geons) in Quantum Gravity

HET Seminar

Projective Statistics (for Topological Geons) in Quantum Gravity

Abstract:

To the extent that spacetime remains a manifold M on small scales, excitations of the spatial topology can function as particles called topological geons. In a first quantized theory of topological geons (aka continuum quantum gravity without topology change), different irreducible unitary representations of the mapping-class group $G$ of $M$, yield different superselection sectors of the theory. In some of these sectors the geons behave as fermions, even though gravitons themselves are of course bosons. A still more exotic possibility is "projective statistics", where the operators that permute identical geons only preserve group multiplication up to a phase-factor that cannot be eliminated. I will describe how it might be possible to realize this phenomenon with four $\RP3$ geons.

Details

Date: Monday 8 Sep

Time: 14:15

Place: Aud D(!), Blegdamsvej

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• Discovery Colloquium

Auditorium A, Tuesday 9 Sep at 14:15

Seon-Hee Seo (Seoul National University)

Dancing of the Neutrinos

Discovery Colloquium

Dancing of the Neutrinos

Abstract:

Neutrinos are the most mysterious among the fundamental particles in our universe even though they are the second most abundant. Neutrinos were postulated by Pauli in 1930 and the discovery of neutrinos was finally achieved using reactors by Reines and Cowan in 1956. We now know that there are three active flavors (or families) of neutrinos (nu_e, nu_mu, nu_tau) and they like to mix while traveling. As a result they can change their flavors like a magic show. This is possible because the neutrinos have mass and even though the masses are very tiny the mass eigenstates are very different than the flavor eigenstates. In this talk I will introduce the neutrinos and related oscillation experiments, and some interesting questions/challenges remained in neutrino physics.

Details

Date: Tuesday 9 Sep

Time: 14:15

Place: Auditorium A

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• HET journal club

Fb-6, Wednesday 10 Sep at 13:30
Start-up meeting

HET journal club

Start-up meeting

Details

Date: Wednesday 10 Sep

Time: 13:30

Place: Fb-6

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• Niels Bohr Lecture

Aud 3 at HCØ, Wednesday 10 Sep at 15:15

Bert Schellekens (NIKHEF)

Particle Physics in the Multiverse

Niels Bohr Lecture

Particle Physics in the Multiverse

Abstract:

Is our universe a single entity or part of a multiverse? Can the Standard Model of Particle Physics be derived uniquely from a fundamental theory, or is it just one of many possibilities? Insights from inflation and string theory in the past three decades suggests that it is not unique, and that the number of alternatives may be enormous.
This would have important implications for our attempts at understanding the Standard Model and the expectations for “new physics”.
It also implies that we cannot ignore an important bias in observations: the existence of observers. This leads inevitably to a controversial issue: the "anthropic principle”.

Details

Date: Wednesday 10 Sep

Time: 15:15

Place: Aud 3 at HCØ

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• HET Seminar

Aud A, Blegdamsvej, Thursday 11 Sep at 14:15

Bert Schellekens (NIKHEF)

GUTs without guts

HET Seminar

GUTs without guts

Abstract:

The discrete structure of the Standard Model (gauge group, representations
and in particular charge quantization) is derived in a class of intersecting brane models
from a mild anthropic constraint, namely the existence of electrodynamics.
Although Grand Unification is realized in a subclass, this offers no advantages whatsoever.
In all other cases the Standard Model is realized as a GUT with its intestines removed,
to keep only the good parts: a GUT without guts.

Details

Date: Thursday 11 Sep

Time: 14:15

Place: Aud A, Blegdamsvej

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• PhD defence

Aud A, Monday 22 Sep at 14:15

Hjalte Frellesvig

Generalized Unitarity Cuts and Integrand Reduction at Higher Loop Orders

PhD defence

Generalized Unitarity Cuts and Integrand Reduction at Higher Loop Orders

Abstract:

The huge amounts of data on particle scattering which have been collected by the LHC in the recent years, has enabled the investigation of interactions and other effects which so far have been too minuscule to be observed. A perfect example of this is the recent discovery of the Higgs particle.

The large amount of background to such processes has caused an ever increasing need for precise predictions by the fundamental theory. For most collision processes the leading contribution comes from QCD, the theory of the strong interactions, which is the main focus of this thesis. In practice the required precision corresponds to the next to next to leading order in the perturbative expansion of the cross section, which requires the calculation of Feynman diagrams with two or more loops.

For one-loop diagrams the corresponding problem has been solved to the extend that almost all one-loop amplitudes of physical interest are known, and their calculation automated. This is mainly due to the technique of generalized unitarity cuts combined with integrand reduction into what is known as the OPP method. This method is sufficiently easy and fast that the one-loop contributions can be incorporated into event-generation software on equal footing with the tree contributions.

More specifically, the OPP method calculates individually each topology in the expansion given by the integrand reduction. First it calculates the topologies with the most propagators, and when they are known one may calculate the lower topologies using the higher ones as subtraction terms. Combined with specialized methods to find the rational term, the OPP method provides a complete procedure for finding the one-loop corrections to any amplitude.

In this thesis we will develop a method to extend the OPP method to two loops and beyond. It is based on a categorization of the integrand using algebraic geometry, in which the set of propagators corresponding to each topology is identified with an algebraic ideal I. This allows for the identification of the set of terms which are allowed in the numerator corresponding to each topology, with those of the members of the quotient ring R/I which lives up to a set of renormalization constraints. The method works in both four and d space-time dimensions.

We start the thesis by an introduction to QCD, amplitudes, and unitarity cuts. Then we do a number of four-dimensional examples of our form of the OPP method in the context of the process gg -> gg We calculate the one-loop contribution in significant detail, the three two-loop, seven-propagator topologies called the double-box, the crossed box, and the pentagon-triangle, and finally we calculate the triple-box three-loop topology. We also take a brief look at another three-loop topology, the tennis court.

By the additional calculation of a number of six-propagator two-loop topologies, we illustrate two problems with our method in its four-dimensional version. The first is "the minor problem" in which a unitarity cut solutions for a topology coincide with a cut solution for one of its parent topologies causing infinities to appear in the calculation. The other is denoted "the major problem", and is characterized by the existence of terms which vanish on the cut without being a sum of terms proportional to products of the propagators.

It turns out that both of these problems can be solved by going to $d$ dimensions. We illustrate the d-dimensional method by repeating part of the one-loop gg -> gg calculation, showing how the higher dimensional contributions give rise to the rational term of the amplitude.

For doing d-dimensional two-loop calculations, we describe how to embed the higher-dimensional parts of the loop-momenta, denoted rho-parameters, into two extra dimensions, and we describe how to do calculations in these dimensions using the six-dimensional spinor-helicity formalism. As an example we calculate the planar two-loop contribution to gg -> gg for the case where all external gluons have the same helicity, and we get agreement with the known result.

The main calculation of these thesis is that of the planar part of gg -> ggg for the mentioned helicity configuration. There are eight topologies contributing to the amplitude and we find that the result agrees with numerical checks. We find a curious relation between the result and the MHV result for N=4 SYM, similar to a known relation at one-loop. We also calculate a few non-planar contributions to the amplitude.

We end the thesis by comparing the method to a number of alternative methods proposed by other groups, and by a number of appendices. Primary among the appendices are a detailed introduction to the six-dimensional spinor-helicity formalism, and a derivation of the method used to perform integrals over the higher dimensional rho-parameters.

Details

Date: Monday 22 Sep

Time: 14:15

Place: Aud A

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• HET journal club

Fb6, Wednesday 24 Sep at 12:15

HET journal club

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Date: Wednesday 24 Sep

Time: 12:15

Place: Fb6

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• Discovery Seminar

Aud A, Blegdamsvej, Thursday 25 Sep at 14:15

Patrick Fox

Dark Matter without Astrophysics: Halo-independent methods at Direct Detection Experiments

Discovery Seminar

Dark Matter without Astrophysics: Halo-independent methods at Direct Detection Experiments

Abstract:

Underground searches for dark matter involve a complicated interplay of particle physics, nuclear physics, atomic physics and astrophysics. I will describe attempts to remove the uncertainties associated with astrophysics by developing the means to map the observed signal in one experiment directly into a predicted rate at another. It is possible to make experimental comparisons that are completely free of astrophysical uncertainties. I will describe one recent approach that does not bin the data, and uses all available information, it is uniquely suited to the analysis of emerging DM direct detection hints. Applications of these approaches will be presented.

Details

Date: Thursday 25 Sep

Time: 14:15

Place: Aud A, Blegdamsvej

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• MSc defence

Aud. A, Blegdamsvej, Tuesday 30 Sep at 11:15

Dipole Moments of Black Branes

MSc defence

Dipole Moments of Black Branes

Abstract:

In recent years it has been shown that higher-order corrections, namely pole-dipole corrections, to the blackfold approach reveal that higher dimensional black branes possess some attributes analogous to conventional elastic materials. Pole-dipole order corrections to the effective stress-energy tensor of charged fluid branes living on a submanifold of a background metric result in bending moments, the effect of which is captured by a relativistic generalization of the Young modulus and piezoelectric moduli. In the defense, the dynamics of pole-dipole order black branes and a method of generating charged bent brane solutions will be presented, as well as how the response coefficients of the Young modulus and piezoelectric moduli are read from the solutions.

Details

Date: Tuesday 30 Sep

Time: 11:15

Place: Aud. A, Blegdamsvej

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• HET Seminar

Aud A, Blegdamsvej, Tuesday 30 Sep at 14:15

Larus Thorlacius (University of Iceland and Stockholm University)

Black hole complementarity - the inside view

HET Seminar

Black hole complementarity - the inside view

Abstract:

Hawking's black hole information paradox arises when the formation and
subsequent evaporation of a black hole is examined in the context of quantum
theory. In a gravity theory with a gauge theory dual, the paradox must be resolved
in favor of unitary evolution but the challenge is then to implement unitarity on the
gravity side. A key issue, which is at the center of the recent debate on firewalls, is
whether this requires giving up the equivalence principle. For a black hole formed
in a generic pure quantum state, we argue that a typical infalling observer will see
no drama on their way to the stretched horizon and a proposal is made for an
approximate interior effective field theory description where this conclusion holds
until the observer approaches the black hole singularity.

Details

Date: Tuesday 30 Sep

Time: 14:15

Place: Aud A, Blegdamsvej

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• October 2014
• HET journal club

Fb6, Wednesday 1 Oct at 12:15

HET journal club

Details

Date: Wednesday 1 Oct

Time: 12:15

Place: Fb6

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• Niels Bohr Lecture

Aud 3 at HCØ, Wednesday 1 Oct at 15:15

Mark Raizen

Breaking Barriers with Maxwell's Demon

Niels Bohr Lecture

Breaking Barriers with Maxwell's Demon

Abstract:

We are developing new approaches to the control of atomic motion. The starting point is the supersonic beam, an ultra-bright source of atoms. We use pulsed magnetic fields to stop the beam, and this approach is now proven to be optimum using an adiabatic slower. We further cool the atoms using a one-way wall, a direct realization of the historic thought experiment of Maxwell's Demon, proposed by James Clerk Maxwell in 1871.
This toolbox of new methods is an alternative to Laser Cooling, with much better predicted performance in terms of generality, flux of ultra-cold atoms, and phase-space density. We will use this ultra-bright source of atoms to pattern the nanoscale, bridging between atomic physics and condensed matter/material science.

Details

Date: Wednesday 1 Oct

Time: 15:15

Place: Aud 3 at HCØ

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• Discovery-HET Seminar

Aud A, Blegdamsvej, Thursday 2 Oct at 14:15

Simon Caron-Huot

Hidden symmetries of scattering amplitudes, old and new

Discovery-HET Seminar

Hidden symmetries of scattering amplitudes, old and new

Abstract:

Physical systems with unexpected, or `hidden,’ symmetries have often played an important role in physics, beginning with the classical Kepler problem whose Laplace-Runge-Lenz vector ensures the closure of planetary orbits. I will argue that precisely the same symmetry underlies the integrability of a unique four-dimensional quantum field theory, a maximally supersymmetric cousin of the strong-interaction Yang-Mills theory that has been the focus of much recent work. When considering massive particles, the combination of the Laplace-Runge-Lenz vector with relativity yields a novel way to calculate its spectrum of Hydrogen-like bound states, with full account of relativistic corrections.

Details

Date: Thursday 2 Oct

Time: 14:15

Place: Aud A, Blegdamsvej

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• journal club

Fb6, Wednesday 8 Oct at 12:15

TBA

journal club

TBA

Details

Date: Wednesday 8 Oct

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Aud A, Blegdamsvej, Thursday 9 Oct at 14:15

Arthur Lipstein (DESY/University of Hamburg)

Ambitwistor Strings at Null Infinity and Asymptotic Symmetries

Discovery-HET Seminar

Ambitwistor Strings at Null Infinity and Asymptotic Symmetries

Abstract:

Recently, Strominger and collaborators proposed a new way of understanding soft theorems for gravity amplitudes in terms of BMS symmetry. In this talk, I will describe how these theorems can be proven from the perspective of conformal field theory and extended in various ways using a chiral infinite tension limit of the RNS string known as ambitwistor string theory.

Details

Date: Thursday 9 Oct

Time: 14:15

Place: Aud A, Blegdamsvej

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• Discovery-HET Seminar

Aud A, Blegdamsvej, Monday 13 Oct at 14:15

Andrew Strominger (Harvard)

Semiclassical Virasoro Symmetry of the Quantum Gravity S-Matrix

Discovery-HET Seminar

Semiclassical Virasoro Symmetry of the Quantum Gravity S-Matrix

Abstract:

A recently discovered exact equivalence between soft theorems in quantum field theory and asymptotic symmetries in four-dimensional QED, non-abelian gauge theories and quantum gravity is discussed.

Details

Date: Monday 13 Oct

Time: 14:15

Place: Aud A, Blegdamsvej

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• Journal club

Fb6, Wednesday 15 Oct at 12:15

Matthias Christandl

Entangling Pauli

Journal club

Entangling Pauli

Details

Date: Wednesday 15 Oct

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Aud A, Blegdamsvej, Thursday 16 Oct at 14:15

Oliver Schlotterer

SYM amplitudes from BRST symmetry

Discovery-HET Seminar

SYM amplitudes from BRST symmetry

Abstract:

This talk describes a method to compute supersymmetric tree amplitudes and loop integrands in ten-dimensional super Yang-Mills theory. It relies on the constructive interplay between their cubic graph organization and BRST invariance of the underlying pure spinor superspace description. After a general introduction to this kind of superspace, we discuss a canonical set of multiparticle building blocks which represent tree level subdiagrams and are guided by their BRST transformation. These building blocks are shown to yield a compact solution for tree level amplitudes, and the applicability of the BRST approach to loop integrands is exemplified through recent examples at one-loop.

Details

Date: Thursday 16 Oct

Time: 14:15

Place: Aud A, Blegdamsvej

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• Journal club

Fb6, Wednesday 22 Oct at 12:15

Daniel Mayerson

Exotic Branes and 3D Supergravity

Journal club

Exotic Branes and 3D Supergravity

Abstract:

String theory has been known for quite a while to contain non-perturbative objects called D-branes in addition to the fundamental strings the theory is named after. However, fairly recently, new types of membranes in string theory are being studied with increasing interest: the so-called "exotic branes" (also known by other names). These exotic branes have the interesting feature that their spacetime is multi-valued or non-geometric; this means that when you travel in a circle around the brane, the resulting spacetime looks different from that which you started out with! It is of considerable interest to study and classify the possible types of such exotic branes that can exist in string theory. To study them, we can work in a 3D theory (of maximal supergravity), where the exotic branes are just point particles. In this theory, we can at least classify the supersymmetric point particles, thereby gaining insight into what kind of supersymmetric exotic branes exist in string theory. This problem reduces to that of classifying nilpotent orbits associated with the U-duality group (E_8), for which various mathematical results are known. It can be shown that the only allowed supersymmetric configurations are 1/2, 1/4, 1/8, and 1/16 PS,
and their respective nilpotent orbits can be given explicitly. On the other hand, it turns out exceedingly difficult to translate this classification into a simple criterion for supersymmetry in terms of the non-Abelian (monodromy) charges of the objects.

Details

Date: Wednesday 22 Oct

Time: 12:15

Place: Fb6

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• MSc defence

Aud. A, Thursday 23 Oct at 11:15

Isak Buhl-Mortensen

Subtracted Black Holes and Matter

MSc defence

Subtracted Black Holes and Matter

Abstract:

The Bekenstein-Hawking area law lacks a generally fulfilling statistical-mechanical interpretation. For certain extremal BPS black holes, a statistical derivation of the area law can be performed in the low coupling regime in terms of wound Dp-branes. However, string theory has so far been unable to reproduce the counting for non-extremal black holes. Progress is nonetheless being made towards a microscopic description. The recently proposed Kerr/CFT correspondence conjectures the existence of a conformal field theory (CFT) dual to extremal Kerr. Furthermore, so-called hidden conformal symmetry for non-extremal Kerr suggests that the horizon microstates of general black holes may have a dual CFT description. Hidden conformal symmetry is made manifest in the so called subtracted geometry, which is presented for the four-charged four-dimensional black holes in the STU model, along with key observations and result from the thesis being defended. Notably, a subtracted geometry whose near-horizon extremal Kerr (NHEK) limit coincides with the NHEK limit on the original black hole geometry is presented.

Details

Date: Thursday 23 Oct

Time: 11:15

Place: Aud. A

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• HET Seminar

Bk2, Thursday 23 Oct at 14:15

Ulf Gran (Chalmers Gothenburg)

Dynamical symmetry enhancement near black hole horizons

HET Seminar

Dynamical symmetry enhancement near black hole horizons

Abstract:

In this talk I will sketch a general proof of the (super)symmetry enhancement occurring near black hole horizons, a phenomenon previously observed only on a case by case basis. I will also show that the symmetry algebra for all supersymmetric black hole horizons with non-trivial fluxes includes an sl(2,R) subalgebra.

Details

Date: Thursday 23 Oct

Time: 14:15

Place: Bk2

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• journal club

Fb6, Wednesday 29 Oct at 12:15

Tobias Hansen

From strings in target space to conformal correlators in d dimensions

journal club

From strings in target space to conformal correlators in d dimensions

Abstract:

In recent years on-shell recursion relations have played a key role in advancing the understanding of field theory scattering amplitudes. The same on-shell methods also apply to string theory to a large extent, with the added complication that summing over the infinite tower of string states is required. Based on joint work with Rutger Boels I will show a method to avoid these sums using a smallish set of assumptions to arrive at the usual string S-matrix. These assumptions provide an alternative working definition of the tree level string S-matrix in a flat target space that does without any reference to the world sheet.

In a second part I will present recent work with Miguel Costa on an index-free formalism for efficiently working with mixed-symmetry tensors. The formalism makes it easy to determine the tensor structures in any conformal correlator of mixed-symmetry tensor operators. This is a crucial ingredient for the computation of conformal blocks with mixed-symmetry tensor exchange which in turn are a prerequisite for using conformal bootstrap techniques for operators with spin.

Details

Date: Wednesday 29 Oct

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Auditorium A, Blegdamsvej 17, Thursday 30 Oct at 14:15

Jonathan Davis (Durham)

DAMA without Dark Matter

Discovery-HET Seminar

DAMA without Dark Matter

Abstract:

Direct Detection experiments aim to detect evidence of Dark Matter scattering with nuclei. One such collaboration: DAMA/LIBRA has claimed a positive detection of Dark Matter, which takes the form of an annually modulated event rate. However the regions of Dark Matter parameter space favoured by these experiments are in strong tension with the null results from e.g. LUX, XENON100 and SuperCDMS. Hence in this talk I discuss an alternative explanation of the DAMA events which does not invoke Dark Matter. I show that the DAMA modulation is equally well explained by neutrons liberated by a combination of boron-8 solar neutrinos and atmospheric muons in the rock or shielding around the detector.
In addition I estimate that rates of neutrons from muons and neutrinos and address recent criticisms of these estimates. With current data my model gives as good a fit as Dark Matter and I discuss prospects for future experiments to discriminate between the two.

Details

Date: Thursday 30 Oct

Time: 14:15

Place: Auditorium A, Blegdamsvej 17

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• November 2014
• Discovery Seminar

Aud A, Blegdamsvej 17, Tuesday 4 Nov at 14:15

Alain Blondel (Geneva U/CERN)

FCC-ee the first step in a long term vision for particle physics

Discovery Seminar

FCC-ee the first step in a long term vision for particle physics

Abstract:

CERN is undertaking an integral design study of Future Circular Colliders of a circumference of 100km around Geneva. A possible first step is a high luminosity electron-positron (lepton) collider covering the energy range from the Z pole to above the top threshold, for the study of several TeraZ, okuW, MegaHiggs and Megatops. The ultimate goal is a 100 TeV pp collider. The project will be described with special attention to the electron machine.
The combination of the two machines offers a remarkable potential for discoveries, from a blend of precision measurements, high statistics, high energies and sensitivity to very small couplings. In particular the search for sterile right-handed neutrinos (aka neutral heavy leptons) will be shown to reach all the way up to the Z mass, in a way complementary to the SHIP experiment.

Details

Date: Tuesday 4 Nov

Time: 14:15

Place: Aud A, Blegdamsvej 17

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• journal club

Fb6, Wednesday 5 Nov at 12:15

journal club

Silvia Nagy

Abstract:

This talk will be based on arxiv:1408.4434 and arxiv:1312.6523.

Details

Date: Wednesday 5 Nov

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Fa10, Tuesday 11 Nov at 14:15

Daniel Jafferis (Harvard)

Toda theory from six dimensions

Discovery-HET Seminar

Toda theory from six dimensions

Abstract:

I will give a constructive derivation of the Alday-Gaiotto-Tachikawa correspondence. The correspondence relates certain observables of strongly interacting 4d quantum field theories with 8 supercharges to correlation functions in 2d Toda conformal field theory. The 4d theories in question are obtained by compactification of the 6d (2,0) theory on a Riemann surface, and the Toda theory lives on that Riemann surface. The derivation I will give involves reduction of the (2,0) theory on the 4-sphere to directly obtain the Toda theory.

Details

Date: Tuesday 11 Nov

Time: 14:15

Place: Fa10

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• Journal club

Fb6, Wednesday 12 Nov at 12:15

Journal club

Niels Obers

Abstract:

Niels Obers will talk about arxiv 1409.1522 and 1409.1519.

Details

Date: Wednesday 12 Nov

Time: 12:15

Place: Fb6

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• HET Seminar

Fa10, Thursday 13 Nov at 14:15

George Savvidy (Demokritos National Research Centre)

Asymptotic freedom of non-Abelian tensor gauge fields. Proton structure and tensor gluons

HET Seminar

Asymptotic freedom of non-Abelian tensor gauge fields. Proton structure and tensor gluons

Abstract:

We consider the possibility that inside the proton and, more generally, inside the hadrons there are additional partons tensor-gluons that can carry part of the proton momentum. The tensor-gluons have zero electric charge, like gluons, but have a larger spin. Inside the proton, the nonzero density of the tensor-gluons can be generated by the emission of tensor-gluons by gluons. The process of gluon splitting suggests that part of the proton momentum that was carried by neutral partons is shared between vector and tensor gluons.
We derive DGLAP evolution equations for the parton distribution functions that take into account these new processes. The momentum sum rule allows us to find the tensor-gluon contribution to the Callan–Simanzik beta function and to calculate the corresponding anomalous dimensions. The contribution is negative and corresponds to the asymptotically free theory. This also influences the unification scale at which the coupling constants of the standard model merge, shifting its value to lower energies of the order of 40 TeV.

Details

Date: Thursday 13 Nov

Time: 14:15

Place: Fa10

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• HET Seminar

Auditorium A, Blegdamsvej 17, Tuesday 18 Nov at 14:15

Niko Jokela (Helskinki)

Flux, flows, and Hall states in ABJ(M)

HET Seminar

Flux, flows, and Hall states in ABJ(M)

Abstract:

I will consider probe D6-branes with quantized internal flux in the ABJM background with dynamical massless flavors. This flux breaks parity and allows quantum Hall states. I will present a family of analytic, $\cal N=1/2$ supersymmetric, solutions with nonzero charge density, electric, and magnetic fields.

Details

Date: Tuesday 18 Nov

Time: 14:15

Place: Auditorium A, Blegdamsvej 17

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• Journal club

Fb6, Wednesday 19 Nov at 12:15

Journal club

Louise Anderson

Details

Date: Wednesday 19 Nov

Time: 12:15

Place: Fb6

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• Two-minute meeting

Aud. A, Tuesday 25 Nov at 14:15

Two-minute meeting

Abstract:

Every group member will present his/her field of research in two minutes using one slide.

Details

Date: Tuesday 25 Nov

Time: 14:15

Place: Aud. A

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• Journal club

Fb6, Wednesday 26 Nov at 12:15

Andi Hektor (CERN)

A mechanism for dark energy (and inflation): the adaptive screening

Journal club

A mechanism for dark energy (and inflation): the adaptive screening

Abstract:

We describe how known matter effects within a well-motivated particlephysics framework can explain the dark energy of the Universe. Byconsidering a cold gas of particles which interact via a vectormediator, we show that there exists a regime where the gas reproducesthe dynamics of dark energy. In this regime the screening mass of themediator is proportional to the number density of the gas, hence werefer to this phenomenon as “the adaptive screening mechanism”. As anexample, such screening mass could result from Anderson localization ofthe vector mediators. The proposed dark energy mechanism explains therecent BAO observations and can be tested by the EUCLID experiment, aswell as by studying properties of dark photons and sterile neutrinos.The talk is based on arXiv:1407.0389.

Details

Date: Wednesday 26 Nov

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Auditorium A, Thursday 27 Nov at 14:15

Michael Trott (NBI)

The Standard Model EFT(s) in the LHC era

Discovery-HET Seminar

The Standard Model EFT(s) in the LHC era

Abstract:

With the first run of LHC, a Standard Model (SM) like scalar
was discovered, and no beyond the SM particles were reported.
This has led to a surge of interest in Effective Field Theory approaches to the
SM in the Higgs sector and the systematic development of the SM as a true EFT.
I will review these developments, what we have learned about the Higgs so far,
and how we plan to put the Higgs under the microscope to unravel the effects
of physics beyond the SM, that is difficult to directly discover at LHC.

Details

Date: Thursday 27 Nov

Time: 14:15

Place: Auditorium A

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• December 2014
• MSc defence

Fb6, Wednesday 3 Dec at 11:00

Jacopo Dall'Aglio

D5 and D7 probe branes and their relevance to graphene

MSc defence

D5 and D7 probe branes and their relevance to graphene

Details

Date: Wednesday 3 Dec

Time: 11:00

Place: Fb6

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• Journal club

Fb6, Wednesday 3 Dec at 12:15

Matthias Wihelm (Humboldt University)

Amplitudes, Form Factors and the Dilatation Operator of N=4 SYM Theory

Journal club

Amplitudes, Form Factors and the Dilatation Operator of N=4 SYM Theory

Abstract:

Form factors form a bridge between the purely on-shell amplitudes and the purely off-shell correlation functions. In this talk, we study the form factor of a generic gauge-invariant local composite operator in N=4 SYM theory. At tree-level and for a minimal number of external fields, the form factor exactly realises the spin-chain picture of N=4 SYM theory in the language of scattering amplitudes. Via generalised unitarity, we obtain the cut-constructible part of the one-loop correction to the minimal form factor of a generic operator. Its UV divergence yields the complete one-loop dilatation operator of the theory. Proceeding to the next loop order, we compute the complete two-loop correction to the two-point form factor of the Konishi operator via unitarity and obtain the two-loop Konishi anomalous dimensions from it. For the Konishi operator as well as other non-protected operators, important subtleties arise which require an extension of the method of unitarity. The talk is based on the recent works 1410.6309 and 1410.8485.

Details

Date: Wednesday 3 Dec

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Auditorium A, Blegdamsvej 17, Thursday 4 Dec at 14:15

Ante Bilandzic (NBI)

New exact results in multiparticle correlation techniques

Discovery-HET Seminar

New exact results in multiparticle correlation techniques

Abstract:

Multiparticle correlation techniques are today the primary experimental tool used to study the anisotropic flow phenomenon in heavy-ion collisions. The publications on anisotropic flow measurements at Large Hadron Collider are, together with publications on the discovery of the Higgs boson, the most cited recent publications in the field of high energy physics, making it one of the “hottest” research topics in the field. Although multiparticle correlations are widely used in anisotropic flow measurements, mathematical and statistical properties of them still have to be fully understood, and the sources of remaining systematic biases have to be identified and removed. In this talk I will present a new generic framework which enables exact and fast evaluation of all multiparticle azimuthal correlations [1], and discuss the progress which has been made recently in the understanding of their fundamental mathematical and statistical properties [2].

[1] A. Bilandzic, C. H. Christensen, K. Gulbrandsen, A. Hansen and Y. Zhou,
“Generic framework for anisotropic flow analyses with multiparticle azimuthal correlations,”
Phys. Rev. C 89 (2014) 064904 [arXiv:1312.3572 [nucl-ex]].
[2] A. Bilandzic
“Higher order moments of multiparticle azimuthal correlations,”
arXiv:1409.5636 [nucl-ex]

Details

Date: Thursday 4 Dec

Time: 14:15

Place: Auditorium A, Blegdamsvej 17

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• HET Seminar

Auditorium A, Blegdamsvej 17, Tuesday 9 Dec at 14:15

Agnese Bissi (Oxford)

What can we learn from crossing symmetry at large N?

HET Seminar

What can we learn from crossing symmetry at large N?

Abstract:

In this talk I will discuss how to construct all solutions consistent with crossing symmetry in the limit of large central charge c ~ N^2, starting from the four-point correlator of the stress tensor multiplet in N=4 SYM. Unitarity forces the introduction of a scale \Delta_{gap} and these solutions organize as a double expansion in 1/c and 1/\Delta_{gap}. These solutions are valid to leading order in 1/c and to all orders in 1/\Delta_{gap} and reproduce, in particular, instanton corrections previously found. Comparison with such instanton computations allows to fix \Delta_{gap}. Using this gap scale one can explain the upper bounds for the scaling dimension of unprotected operators observed in the numerical superconformal bootstrap at large central charge. Furthermore, I will present connections between such upper bounds and positivity constraints arising from causality in flat space and I will discuss how certain relations derived from causality constraints for scattering in AdS follow from crossing symmetry.

Details

Date: Tuesday 9 Dec

Time: 14:15

Place: Auditorium A, Blegdamsvej 17

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• Journal club

Fb6, Wednesday 10 Dec at 12:15

Journal club

Alessandra Cagnazzo & Johannes Thürigen

Abstract:

Alessandra Cagnazzo from DESY will talk about sigma models on
symmetric and semi-symmetric backgrounds, their dualities, and
applications to AdS string theory.

The second speaker is Johannes Thürigen from MPI Potsdam.
*Title* Dimensional flow in superposed discrete quantum geometries.
*Abstract* In various approaches to quantum gravity one observes a
dimensional flow for the spectral dimension from the topological
dimension d on large length scales to some smaller value on small,
Planckian, scales.
While the origin of such a flow is rather well understood in theories of
smooth geometry such as the Asymptotic Safety scenario of quantum
gravity or Horava-Lifshitz gravity, in theories built on discrete
geometries an understanding of the underlying mechanism is missing.
I will shed some light on the issue by identifying a particular class of
quantum geometries with a dimensional flow in the spectral dimension,
given by superpositions of states defined on regular complexes.

The Journal club will probably take longer than usual. People who might
have constraints preventing them from listening to either presentation
are still welcome to come for just one.

Details

Date: Wednesday 10 Dec

Time: 12:15

Place: Fb6

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• Discovery-HET Seminar

Auditorium A, Blegdamsvej 17, Thursday 11 Dec at 14:15

Steven Abel (Durham)

Towards a Non-Supersymmetric String Phenomenology

Discovery-HET Seminar

Towards a Non-Supersymmetric String Phenomenology

Abstract:

We explore the properties of string models with spontaneous supersymmetry breaking introduced via Scherk-Schwarz compactification. Such models have a non-supersymmetric spectrum and a cosmological constant that vanishes in the decompactification limit, but we provide additional insight into their interpolation from non-tachyonic non-supersymmetric models in the small radius limit. We show that there exist "quasi-stable" models that have exponentially suppressed cosmological constant which are phenomenologically viable. The general phenomenological properties and myriad possibilities of these models are discussed.

Details

Date: Thursday 11 Dec

Time: 14:15

Place: Auditorium A, Blegdamsvej 17

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