Supersymmetric Dark Matter : aspects of sfermion coannihilations

There is very strong evidence that ordinary matter in the Universe is outweighed by almost ten times as much so-called dark matter. Dark matter does neither emit nor absorb light and we do not know what it is. One of the theoretically favoured candidates is a so-called neutralino from the supersymmetric extension of the Standard Model of particle physics. A theoretical calculation of the expected cosmic neutralino density must include the so-called coannihilations. Coannihilations are particle processes in the early Universe with any two supersymmetric particles in the initial state and any two Standard Model particles in the final state. In this thesis we discuss the importance of these processes for the calculation of the relic density. We will go through some details in the calculation of coannihilations with one or two so-called sfermions in the initial state. This includes a discussion of Feynman diagrams with clashing arrows, a calculation of colour factors and a discussion of ghosts in non-Abelian field theory. Supersymmetric models contain a large number of free parameters on which the masses and couplings depend. The requirement, that the predicted density of cosmic neutralinos must agree with the density observed for the unknown dark matter, will constrain the parameters. Other constraints come from experiments which are not related to cosmology. For instance, the supersymmetric loop contribution to the rare b -> sγ decay should agree with the measured branching fraction. The principles of the calculation of the rare decay are discussed in this thesis. Also on-going and planned searches for cosmic neutralinos can constrain the parameters. In one of the accompanying papers in the thesis we compare the detection prospects for several current and future searches for neutralino dark matter.

A new conceptual model of nitrification: A key factor in resolving the metabolic state of the ocean

Máster en Oceanografía

The dark side of the board: advances in chess Kriegspiel

While imperfect information games are an excellent model of real-world problems and tasks, they are often difficult for computer programs to play at a high level of proficiency, especially if they involve major uncertainty and a very large state space. Kriegspiel, a variant of chess making it similar to a wargame, is a perfect example: while the game was studied for decades from a game-theoretical viewpoint, it was only very recently that the first practical algorithms for playing it began to appear. This thesis presents, documents and tests a multi-sided effort towards making a strong Kriegspiel player, using heuristic searching, retrograde analysis and Monte Carlo tree search algorithms to achieve increasingly higher levels of play. The resulting program is currently the strongest computer player in the world and plays at an above-average human level.

Heteronuclear recoupling methods in solid-state NMR

This thesis is focused on the development of heteronuclear correlation methods in solid-state NMR spectroscopy, where the spatial dependence of the dipolar coupling is exploited to obtain structural and dynamical information in solids. Quantitative results on dipolar coupling constants are extracted by means of spinning sideband analysis in the indirect dimension of the two-dimensional experiments. The principles of sideband analysis were established and are currently widely used in the group of Prof. Spiess for the special case of homonuclear 1H double-quantum spectroscopy. The generalization of these principles to the heteronuclear case is presented, with special emphasis on naturally abundant 13C-1H systems. For proton spectroscopy in the solid state, line-narrowing is of particular importance, and is here achieved by very-fast sample rotation at the magic angle (MAS), with frequencies up to 35 kHz. Therefore, the heteronuclear dipolar couplings are suppressed and have to be recoupled in order to achieve an efficient excitation of the observed multiple-quantum modes. Heteronuclear recoupling is most straightforwardly accomplished by performing the known REDOR experiment, where pi-pulses are applied every half rotor period. This experiment was modified by the insertion of an additional spectroscopic dimension, such that heteronuclear multiple-quantum experiments can be carried out, which, as shown experimentally and theoretically, closely resemble homonuclear double-quantum experiments. Variants are presented which are well-suited for the recording of high-resolution 13C-1H shift correlation and spinning-sideband spectra, by means of which spatial proximities and quantitative dipolar coupling constants, respectively, of heteronuclear spin pairs can be determined. Spectral editing of 13C spectra is shown to be feasible with these techniques. Moreover, order phenomena and dynamics in columnar mesophases with 13C in natural abundance were investigated. Two further modifications of the REDOR concept allow the correlation of 13C with quadrupolar nuclei, such as 2H. The spectroscopic handling of these nuclei is challenging in that they cover large frequency ranges, and with the new experiments it is shown how the excitation problem can be tackled or circumvented altogether, respectively. As an example, one of the techniques is used for the identification of a yet unknown motional process of the H-bonded protons in the crystalline parts of poly(vinyl alcohol).

Supramolecular order and dynamics of discotic materials studied by solid-state NMR recoupling methods

The topic of this thesis is the investigation of structure,order and dynamics in discotic mesogens by advancedsolid-state NMR spectroscopy. Most of the discotic mesogensunder investigation are hexa-peri-hexabenzocoronene (HBC)derivatives which are of particular interest for potentialdevice applications due to their high one-dimensional chargecarrier mobilities. The supramolecular stacking arrangement of the discoticcores was investigated by 2D 1H-1H double-quantum (DQ)methods, which were modified by incorporating the WATERGATEsuppression technique into the experiments in order toovercome severe phase problems arising from the strongsignal of the long alkyl sidechains. Molecular dynamics and sample orientation was probed throughthe generation of sideband patterns by reconversion rotorencoding in 2D recoupling experiments. These experimentswere extended by new recoupling schemes to enable thedistinction of motion and orientation effects. The solid-state NMR studies presented in this work aim tothe understanding of structure-property relationships in theinvestigated discotic materials, while the experimentsapplied to these materials include new recoupling schemeswhich make the desired information on molecular orientationand dynamics accessible without isotope labelling.

Characterisation of supramolecular structures by novel recoupling methods in solid-state NMR

In this work, solid-state NMR methods suitable for the investigation of supramolecular systems were developed and improved. In this context, special interest was focussed on non-covalent interactions responsible for the formation of supramolecular structures, such as pi-pi interacions and hydrogen-bonds. In the first part of this work, solid-state NMR methods were presented that provide information on molecular structure and motion via the investigation of anisotropic interactions, namely quadrupole and dipole-dipole couplings, under magic-angle spinning conditions. A two-dimensional 2H double quantum experiment was developed, which is performed under off magic-angle conditions and correlates 2H isotropic chemical shifts with quasistatic DQ-filtered line shapes. From the latter, the quadrupole coupling parameters of samples deuterated at multiple sites can be extracted in a site-selective fashion. Furthermore, 7Li quadrupole parameters of lithium intercalated into TiO2 were determined by NMR experiments performed under static and MAS conditions, and could provide information on the crystal geometry. For the determination of 7Li-7Li dipole-dipole couplings, multiple-quantum NMR experiments were performed. The 1H-13C REREDOR experiment was found to be capable of determining strong proton-carbon dipole-dipole couplings with an accuracy of 500~Hz, corresponding to a determination of proton-carbon chemical-bond lengths with picometer accuracy In the second part of this work, solid-state NMR experiments were combined with quantum-chemical calculations in order to aid and optimise the interpretation of experimental results. The investigations on Calix[4]hydroquinone nanotubes have shown that this combined approach can provide information on the presence of disordered and/or mobile species in supramolecular structures. As a second example, C3-symmetric discs arranging in helical columnar stacks were investigated. In these systems, 1H chemical shifts experience large pi-shifts due to packing effects, which were found to be long-ranged. Moreover, quantum-chemical calculations revealed that helicity in these systems is induced by the propeller-like conformation of the core of the molecules.

Regulation of wake-sleep states and state-dependent cardiovascular function in diet-induced obesity rats

Obesity often predisposes to coronary heart disease, heart failure, and sudden death. Also, several studies suggest a reciprocal enhancing interaction between obesity and sleep curtailment. Aim of the present study was to go deeper in the understanding of sleep and cardiovascular regulation in an animal model of diet-induced obesity (DIO). According to this, Wake-Sleep (W-S) regulation, and W-S dependent regulation of cardiovascular and metabolic/thermoregulatory function was studied in DIO rats, under normal laboratory conditions and during sleep deprivation and the following recovery period, enhancing either wake or sleep, respectively. After 8 weeks of the delivery of a hypercaloric (HC) diet, treated animals were heavier than those fed a normocaloric (NC) diet (NC: 441 ±17g; HC: 557±17g). HC rats slept more than NC ones during the activity period (Dark) of the normal 12h:12h light-dark (LD) cycle (Wake: 67.3±1.2% and 57.2 ±1.6%; NREM sleep (NREMS): 26.8±1.0% and 34.0±1.4%; REM sleep (REMS): 5.7±0. 6% and 8.6±0.7%; for NC and HC, respectively; p<0.05 for all). HC rats were hypertensive throughout the W-S states, as shown by the mean arterial blood pressure values across the 24-h period (Wake: 90.0±5.3 and 97.3±1.3; NREMS: 85.1±5.5 and 92.2±1.2; REMS: 87.2±4.5 and 96.5±1.1, mmHg for NC and HC, respectively; p<0.05 for all). Also, HC rats appeared to be slightly bradycardic compared to NC ones (Wake: 359.8±9.3 and 352.4±7.7; NREMS: 332.5±10.1 and 328.9±5.4; REMS: 338.5±9.3 and 334.4±5.8; bpm for NC and HC, respectively; p<0.05 for Wake). In HC animals, sleep regulation was not apparently altered during the sleep rebound observed in the recovery period following sleep deprivation, although REMS rebound appeared to be quicker in NC animals. In conclusion, these results indicate that in the rat obesity interfere with W-S and cardiovascular regulation and that DIO rats are suitable for further studies aimed at a better understanding of obesity comorbidities.

Effizientes Vierwellenmischen durch Ausnutzen von Resonanzen in Quecksilber

In dieser Arbeit wird eine kohärente, kontinuierliche Strahlungsquelle im vakuum-ultravioletten (VUV) Spektrum vorgestellt. Sie basiert auf einem Vierwellenmischprozess in Quecksilberdampf mit Fundamentalstrahlen bei 253,7 nm, 407,9 nm und 545,5 nm Wellenlänge. Diese fundamentalen Strahlen werden durch frequenzverdoppelte bzw. frequenzvervierfachte Festkörperlasersysteme bereit gestellt. Durch das Ausnutzen der 6^1S-7^1S Zweiphotonenresonanz und zusätzlich der 6^1S-6^3P Einphotonenresonanz kann der Vierwellenmischprozess deutlich effizienter betrieben werden als zuvor. Eine nahe Einphotonenresonanz verringert die optimale Phasenanpassungstemperatur des Vierwellenmischprozesses, wodurch Druck- und Stoßverbreiterungen des Quecksilbers verkleinert und damit der nichtlineare Prozess effizienter wird. So können Leistungen bis zu 0,3 nW bei 121,56 nm, dem 1S-2P Lyman-alpha Übergang von Wasserstoff, erzeugt werden. Diese Lyman-alpha Quelle soll für die zukünftige Laserkühlung von Antiwasserstoff genutzt werden. rnrnNeben der Generierung von Strahlung bei Lyman-alpha kann, durch Veränderung der dritten Fundamentalwellenlänge, auch Strahlung in der Nähe zu einer Einphotonresonanz im VUV bei dem 6^1S-12^1P Übergang in Quecksilber durch Vierwellenmischen erzeugt werden. Durch diese weitere Einphotonresonanz kann die nichtlineare Suszeptibilität, verantwortlich für das Vierwellenmischen, stark vergrößert werden, ohne Einfluss auf die Phasenanpassung zu haben. Damit lässt sich der Mischprozess um drei Größenordnungen effizienter gestalten und Leistungen von 6 µW im VUV konnten realisiert werden. Dies entspricht einer Leistungsteigerung um einen Faktor 30 im Vergleich zu früheren Quellen. rnrnDarüberhinaus konnte die Zweiphotonenresonanz in Quecksilber ausführlich untersucht werden. Man erhält bei kleinen Rabifrequenzen der Fundamentalstrahlen eine geschwindigkeitsselektive Doppelresonanz, die den gleichen Ursprung wie Dunkelresonanzen in Lambda-Systemen hat. Bei hohen Rabifrequenzen kann die Anregung in das Zweiphotonenniveau so stark werden, dass auf dem 7^1S-6^1P Übergang ein Laserprozess initiiert wird. Dieser Prozess wurde hier zum ersten Mal mit kontinuierlichen Fundamentallasern nachgewiesen. Es wird gezeigt, dass der zusätzliche Laserprozess das Vierwellenmischen nicht beeinträchtigt.

The scintillating fiber focal plane detector for the use of Kaos as a double arm spectrometer

The upgrade of the Mainz Mikrotron (MAMI) electron accelerator facility in 2007 which raised the beam energy up to 1.5,GeV, gives the opportunity to study strangeness production channels through electromagnetic process. The Kaon Spectrometer (KAOS) managed by the A1 Collaboration, enables the efficient detection of the kaons associated with strangeness electroproduction. Used as a single arm spectrometer, it can be combined with the existing high-resolution spectrometers for exclusive measurements in the kinematic domain accessible to them.rnrnFor studying hypernuclear production in the ^A Z(e,e'K^+) _Lambda ^A(Z-1) reaction, the detection of electrons at very forward angles is needed. Therefore, the use of KAOS as a double-arm spectrometer for detection of kaons and the electrons at the same time is mandatory. Thus, the electron arm should be provided with a new detector package, with high counting rate capability and high granularity for a good spatial resolution. To this end, a new state-of-the-art scintillating fiber hodoscope has been developed as an electron detector.rnrnThe hodoscope is made of two planes with a total of 18432 scintillating double-clad fibers of 0.83 mm diameter. Each plane is formed by 72 modules. Each module is formed from a 60deg slanted multi-layer bundle, where 4 fibers of a tilted column are connected to a common read out. The read-out is made with 32 channels of linear array multianode photomultipliers. Signal processing makes use of newly developed double-threshold discriminators. The discriminated signal is sent in parallel to dead-time free time-to-digital modules and to logic modules for triggering purposes.rnrnTwo fiber modules were tested with a carbon beam at GSI, showing a time resolution of 220 ps (FWHM) and a position residual of 270 microm m (FWHM) with a detection efficiency epsilon>99%.rnrnThe characterization of the spectrometer arm has been achieved through simulations calculating the transfer matrix of track parameters from the fiber detector focal plane to the primary vertex. This transfer matrix has been calculated to first order using beam transport optics and has been checked by quasielastic scattering off a carbon target, where the full kinematics is determined by measuring the recoil proton momentum. The reconstruction accuracy for the emission parameters at the quasielastic vertex was found to be on the order of 0.3 % in first test realized.rnrnThe design, construction process, commissioning, testing and characterization of the fiber hodoscope are presented in this work which has been developed at the Institut für Kernphysik of the Johannes Gutenberg - Universität Mainz.

Production of antihydrogen via double charge exchange

Spectroscopy of the 1S-2S transition of antihydrogen confined in a neutral atom trap and comparison with the equivalent spectral line in hydrogen will provide an accurate test of CPT symmetry and the first one in a mixed baryon-lepton system. Also, with neutral antihydrogen atoms, the gravitational interaction between matter and antimatter can be tested unperturbed by the much stronger Coulomb forces.rnAntihydrogen is regularly produced at CERN's Antiproton Decelerator by three-body-recombination (TBR) of one antiproton and two positrons. The method requires injecting antiprotons into a cloud of positrons, which raises the average temperature of the antihydrogen atoms produced way above the typical 0.5 K trap depths of neutral atom traps. Therefore only very few antihydrogen atoms can be confined at a time. Precision measurements, like laser spectroscopy, will greatly benefit from larger numbers of simultaneously trapped antihydrogen atoms.rnTherefore, the ATRAP collaboration developed a different production method that has the potential to create much larger numbers of cold, trappable antihydrogen atoms. Positrons and antiprotons are stored and cooled in a Penning trap in close proximity. Laser excited cesium atoms collide with the positrons, forming Rydberg positronium, a bound state of an electron and a positron. The positronium atoms are no longer confined by the electric potentials of the Penning trap and some drift into the neighboring cloud of antiprotons where, in a second charge exchange collision, they form antihydrogen. The antiprotons remain at rest during the entire process, so much larger numbers of trappable antihydrogen atoms can be produced. Laser excitation is necessary to increase the efficiency of the process since the cross sections for charge-exchange collisions scale with the fourth power of the principal quantum number n.rnThis method, named double charge-exchange, was demonstrated by ATRAP in 2004. Since then, ATRAP constructed a new combined Penning Ioffe trap and a new laser system. The goal of this thesis was to implement the double charge-exchange method in this new apparatus and increase the number of antihydrogen atoms produced.rnCompared to our previous experiment, we could raise the numbers of positronium and antihydrogen atoms produced by two orders of magnitude. Most of this gain is due to the larger positron and antiproton plasmas available by now, but we could also achieve significant improvements in the efficiencies of the individual steps. We therefore showed that the double charge-exchange can produce comparable numbers of antihydrogen as the TBR method, but the fraction of cold, trappable atoms is expected to be much higher. Therefore this work is an important step towards precision measurements with trapped antihydrogen atoms.

Optisch detektierte Magnetresonanz an NV-Zentren in Diamant

Die rasante Entwicklung der Computerindustrie durch die stetige Verkleinerung der Transistoren führt immer schneller zum Erreichen der Grenze der Si-Technologie, ab der die Tunnelprozesse in den Transistoren ihre weitere Verkleinerung und Erhöhung ihrer Dichte in den Prozessoren nicht mehr zulassen. Die Zukunft der Computertechnologie liegt in der Verarbeitung der Quanteninformation. Für die Entwicklung von Quantencomputern ist die Detektion und gezielte Manipulation einzelner Spins in Festkörpern von größter Bedeutung. Die Standardmethoden der Spindetektion, wie ESR, erlauben jedoch nur die Detektion von Spinensembles. Die Idee, die das Auslesen von einzelnen Spins ermöglich sollte, besteht darin, die Manipulation getrennt von der Detektion auszuführen.rn Bei dem NV−-Zentrum handelt es sich um eine spezielle Gitterfehlstelle im Diamant, die sich als einen atomaren, optisch auslesbaren Magnetfeldsensor benutzen lässt. Durch die Messung seiner Fluoreszenz sollte es möglich sein die Manipulation anderer, optisch nicht detektierbaren, “Dunkelspins“ in unmittelbarer Nähe des NV-Zentrums mittels der Spin-Spin-Kopplung zu detektieren. Das vorgeschlagene Modell des Quantencomputers basiert auf dem in SWCNT eingeschlossenen N@C60.Die Peapods, wie die Einheiten aus den in Kohlenstoffnanoröhre gepackten Fullerenen mit eingefangenem Stickstoff genannt werden, sollen die Grundlage für die Recheneinheiten eines wahren skalierbaren Quantencomputers bilden. Die in ihnen mit dem Stickstoff-Elektronenspin durchgeführten Rechnungen sollen mit den oberflächennahen NV-Zentren (von Diamantplatten), über denen sie positioniert sein sollen, optisch ausgelesen werden.rnrnDie vorliegende Arbeit hatte das primäre Ziel, die Kopplung der oberflächennahen NV-Einzelzentren an die optisch nicht detektierbaren Spins der Radikal-Moleküle auf der Diamantoberfläche mittels der ODMR-Kopplungsexperimente optisch zu detektieren und damit entscheidende Schritte auf dem Wege der Realisierung eines Quantenregisters zu tun.rn Es wurde ein sich im Entwicklungsstadium befindende ODMR-Setup wieder aufgebaut und seine bisherige Funktionsweise wurde an kommerziellen NV-Zentrum-reichen Nanodiamanten verifiziert. Im nächsten Schritt wurde die Effektivität und Weise der Messung an die Detektion und Manipulation der oberflächennah (< 7 nm Tiefe) implantieren NV-Einzelzenten in Diamantplatten angepasst.Ein sehr großer Teil der Arbeit, der hier nur bedingt beschrieben werden kann, bestand aus derrnAnpassung der existierenden Steuersoftware an die Problematik der praktischen Messung. Anschließend wurde die korrekte Funktion aller implementierten Pulssequenzen und anderer Software-Verbesserungen durch die Messung an oberflächennah implantierten NV-Einzelzentren verifiziert. Auch wurde der Messplatz um die zur Messung der Doppelresonanz notwendigen Komponenten wie einen steuerbaren Elektromagneten und RF-Signalquelle erweitert. Unter der Berücksichtigung der thermischen Stabilität von N@C60 wurde für zukünftige Experimente auch ein optischer Kryostat geplant, gebaut, in das Setup integriert und charakterisiert.rn Die Spin-Spin-Kopplungsexperimente wurden mit dem sauerstoffstabilen Galvinoxyl-Radikalals einem Modell-System für Kopplung durchgeführt. Dabei wurde über die Kopplung mit einem NVZentrum das RF-Spektrum des gekoppelten Radikal-Spins beobachtet. Auch konnte von dem gekoppelten Spin eine Rabi-Nutation aufgenommen werden.rn Es wurden auch weitere Aspekte der Peapod Messung und Oberflächenimplantation betrachtet.Es wurde untersucht, ob sich die NV-Detektion durch die SWCNTs, Peapods oder Fullerene stören lässt. Es zeigte sich, dass die Komponenten des geplanten Quantencomputers, bis auf die C60-Cluster, für eine ODMR-Messanordnung nicht detektierbar sind und die NV-Messung nicht stören werden. Es wurde auch betrachtet, welche Arten von kommerziellen Diamantplatten für die Oberflächenimplantation geeignet sind, für die Kopplungsmessungen geeignete Dichte der implantierten NV-Zentren abgeschätzt und eine Implantation mit abgeschätzter Dichte betrachtet.

A double role of sperm in scorpions: the mating plug of Euscorpius italicus (Scorpiones: Euscorpiidae) consists of sperm

Mating plugs occluding the female gonopore after mating are a widespread phenomenon. In scorpions, two main types of mating plugs are found: sclerotized mating plugs being parts of the spermatophore that break off during mating, and gel-like mating plugs being gelatinous fluids that harden in the female genital tract. In this study, the gel-like mating plug of Euscorpius italicus was investigated with respect to its composition, fine structure, and changes over time. Sperm forms the major component of the mating plug, a phenomenon previously unknown in arachnids. Three parts of the mating plug can be distinguished. The part facing the outside of the female (outer part) contains sperm packages containing inactive spermatozoa. In this state, sperm is transferred. In the median part, the sperm packages get uncoiled to single spermatozoa. In the inner part, free sperm is embedded in a large amount of secretions. Fresh mating plugs are soft gelatinous, later they harden from outside toward inside. This process is completed after 3-5 days. Sperm from artificially triggered spermatophores could be activated by immersion in insect Ringer's solution indicating that the fluid condition in the females' genital tract or females' secretions causes sperm activation. Because of the male origin of the mating plug, it has likely evolved under sperm competition or sexual conflict. As females refused to remate irrespective of the presence or absence of a mating plug, females may have changed their mating behavior in the course of evolution from polyandry to monandry.

Testing the TICT State Hypothesis: an Investigation into the Solvatochromic Behavior of 2-Naphthalenylmethylene Malononitrile

Towards the goal of investigating the possible Twisted Intramolecular Charge Transfer (TICT) state mechanism of fluorescence emission, two aromatic dicyanovinyl compounds, 2-(naphthalene-2-ylmethylene) malononitrile (DCN) and a rigidified analogue, 3,4-dihydrophenanthren-1(2H)-ylidene)malononitrile (RDCN) were synthesized and their absorption and steady-state fluorescence emission spectra characterized. The spectral characterization was divided into two studies: first, DCN and RDCN were characterized in liquid solvents of increasing polarity; second, DCN and RDCN were characterized in viscous solvents and rigid glass media. The absorption spectra for both DCN and RDCN in all solvents demonstrated little to no solvatochromism. Emission results for DCN and RDCN in liquid solvents of increasing polarity showed DCN possessing strong solvatochromism while RDCN showed much less solvatochromism. Using the Lippert-Mataga equation, the difference between the ground and excited state dipole moment for DCN was estimated to be 8.4 + 0.4 Debye and between ~3.0 to 5.0 Debye for RDCN. Quantum yield measurements for DCN and RDCN in hexane, diethyl ether and acetonitrile were less than 0.01 and independent of polarity for both both solvents, with DCN generally possessing a quantum yield 3-4 times greater than RDCN. Experiments in glass media for DCN and RDCN showed a lessening of their solvatochromic character in both polar and non-polar glasses. These data provide strong evidence for a link between molecular flexibility and solvatochromism. However, while these data are consistent with a TICT state hypothesis for the emission mechanism, an alternative mechanism proposed by Maroncelli et al.10 involving rotation about the dicyanovinyl double bond in the excited state remains a possibility as well.

Collimated Field Emission Beams from Metal Double-Gate Nanotip Arrays Optically Excited via Surface Plasmon Resonance

The generation of collimated electron beams from metal double-gate nanotip arrays excited by near infrared laser pulses is studied. Using electromagnetic and particle tracking simulations, we showed that electron pulses with small rms transverse velocities are efficiently produced from nanotip arrays by laser-induced field emission with the laser wavelength tuned to surface plasmon polariton resonance of the stacked double-gate structure. The result indicates the possibility of realizing a metal nanotip array cathode that outperforms state-of-the-art photocathodes.

The dark side of the moon

When talking about flow, most people probably think of a highly desirable state associated with a broad variety of positive outcomes in terms of positive motivation, well-being, and performance. In contrast, this chapter suggests that the characteristics of flow also have the potential to be evil. First, we will explain how flow can lead to addiction when exercising, playing games, and using the Internet. Then we will consider how flow is linked to impaired risk perception and risky behavior. As a third negative facet of flow, we will outline how it can also be experienced in antisocial contexts and during combat. This chapter ends with some broader comments on the dark and bright sides of flow, including flow as a universal experience, the implications for practical interventions, ethical questions related to flow, and future research questions.