arXiv subjects

Sebastian Wild

Publications and source records attributed to Sebastian Wild.

66 records · Page 4Linked to original sources

arXiv2015

Dirac dark matter with a charged mediator: a comprehensive one-loop analysis of the direct detection phenomenology

We analyze the direct detection signals of a toy model consisting of a Dirac dark matter particle which couples to one Standard Model fermion via a scalar mediator. For all scenarios, the dark matter particle scatters off nucleons via one loop-induced electromagnetic and electroweak moments, as well as via the one-loop exchange of a Higgs boson. Besides, and depending on the details of the model, the scattering can also be mediated at tree level via the exchange of the scalar mediator or at one loop via gluon-gluon interactions. We show that, for thermally produced dark matter particles, the current limits from the LUX experiment on these scenarios are remarkably strong, even for dark matter coupling only to leptons. We also discuss future prospects for XENON1T and DARWIN and we argue that multi-ton xenon detectors will be able to probe practically the whole parameter space of the model consistent with thermal production and perturbativity. We also discuss briefly the implications of our results for the dark matter interpretation of the Galactic GeV excess.

hep-ph↗

arXiv2015

Average Case and Distributional Analysis of Dual-Pivot Quicksort

In 2009, Oracle replaced the long-serving sorting algorithm in its Java 7 runtime library by a new dual-pivot Quicksort variant due to Vladimir Yaroslavskiy. The decision was based on the strikingly good performance of Yaroslavskiy's implementation in running time experiments. At that time, no precise investigations of the algorithm were available to explain its superior performance - on the contrary: Previous theoretical studies of other dual-pivot Quicksort variants even discouraged the use of two pivots. Only in 2012, two of the authors gave an average case analysis of a simplified version of Yaroslavskiy's algorithm, proving that savings in the number of comparisons are possible. However, Yaroslavskiy's algorithm needs more swaps, which renders the analysis inconclusive. To force the issue, we herein extend our analysis to the fully detailed style of Knuth: We determine the exact number of executed Java Bytecode instructions. Surprisingly, Yaroslavskiy's algorithm needs sightly more Bytecode instructions than a simple implementation of classic Quicksort - contradicting observed running times. Like in Oracle's library implementation we incorporate the use of Insertionsort on small subproblems and show that it indeed speeds up Yaroslavskiy's Quicksort in terms of Bytecodes; but even with optimal Insertionsort thresholds the new Quicksort variant needs slightly more Bytecode instructions on average. Finally, we show that the (suitably normalized) costs of Yaroslavskiy's algorithm converge to a random variable whose distribution is characterized by a fixed-point equation. From that, we compute variances of costs and show that for large n, costs are concentrated around their mean.

cs.DS↗

arXiv2014

Analysis of Quickselect under Yaroslavskiy's Dual-Pivoting Algorithm

There is excitement within the algorithms community about a new partitioning method introduced by Yaroslavskiy. This algorithm renders Quicksort slightly faster than the case when it runs under classic partitioning methods. We show that this improved performance in Quicksort is not sustained in Quickselect; a variant of Quicksort for finding order statistics. We investigate the number of comparisons made by Quickselect to find a key with a randomly selected rank under Yaroslavskiy's algorithm. This grand averaging is a smoothing operator over all individual distributions for specific fixed order statistics. We give the exact grand average. The grand distribution of the number of comparison (when suitably scaled) is given as the fixed-point solution of a distributional equation of a contraction in the Zolotarev metric space. Our investigation shows that Quickselect under older partitioning methods slightly outperforms Quickselect under Yaroslavskiy's algorithm, for an order statistic of a random rank. Similar results are obtained for extremal order statistics, where again we find the exact average, and the distribution for the number of comparisons (when suitably scaled). Both limiting distributions are of perpetuities (a sum of products of independent mixed continuous random variables).

cs.DS↗

arXiv2014

Analysis of Branch Misses in Quicksort

The analysis of algorithms mostly relies on counting classic elementary operations like additions, multiplications, comparisons, swaps etc. This approach is often sufficient to quantify an algorithm's efficiency. In some cases, however, features of modern processor architectures like pipelined execution and memory hierarchies have significant impact on running time and need to be taken into account to get a reliable picture. One such example is Quicksort: It has been demonstrated experimentally that under certain conditions on the hardware the classically optimal balanced choice of the pivot as median of a sample gets harmful. The reason lies in mispredicted branches whose rollback costs become dominating. In this paper, we give the first precise analytical investigation of the influence of pipelining and the resulting branch mispredictions on the efficiency of (classic) Quicksort and Yaroslavskiy's dual-pivot Quicksort as implemented in Oracle's Java 7 library. For the latter it is still not fully understood why experiments prove it 10% faster than a highly engineered implementation of a classic single-pivot version. For different branch prediction strategies, we give precise asymptotics for the expected number of branch misses caused by the aforementioned Quicksort variants when their pivots are chosen from a sample of the input. We conclude that the difference in branch misses is too small to explain the superiority of the dual-pivot algorithm.

cs.DS↗

arXiv2014

Pivot Sampling in Dual-Pivot Quicksort

The new dual-pivot Quicksort by Vladimir Yaroslavskiy - used in Oracle's Java runtime library since version 7 - features intriguing asymmetries in its behavior. They were shown to cause a basic variant of this algorithm to use less comparisons than classic single-pivot Quicksort implementations. In this paper, we extend the analysis to the case where the two pivots are chosen as fixed order statistics of a random sample and give the precise leading term of the average number of comparisons, swaps and executed Java Bytecode instructions. It turns out that - unlike for classic Quicksort, where it is optimal to choose the pivot as median of the sample - the asymmetries in Yaroslavskiy's algorithm render pivots with a systematic skew more efficient than the symmetric choice. Moreover, the optimal skew heavily depends on the employed cost measure; most strikingly, abstract costs like the number of swaps and comparisons yield a very different result than counting Java Bytecode instructions, which can be assumed most closely related to actual running time.

cs.DS↗

arXiv2014

Sharp Gamma-ray Spectral Features from Scalar Dark Matter Annihilations

The search for sharp features in the gamma-ray spectrum is a promising approach to identify a signal from dark matter annihilations over the astrophysical backgrounds. In this paper we investigate the generation of gamma-ray lines and internal bremsstrahlung signals in a toy model where the dark matter particle is a real scalar that couples to a lepton and an exotic fermion via a Yukawa coupling. We show that the Fermi-LAT and H.E.S.S. searches for line-like spectral features severely constrain regions of the parameter space where the scalar dark matter is thermally produced. Finally, we also discuss the complementarity of the searches for sharp spectral features with other indirect dark matter searches, as well as with direct and collider searches.

hep-ph↗

arXiv2014

Antihelium from Dark Matter

Cosmic-ray anti-nuclei provide a promising discovery channel for the indirect detection of particle dark matter. Hadron showers produced by the pair-annihilation or decay of Galactic dark matter generate anti-nucleons which can in turn form light anti-nuclei. Previous studies have only focused on the spectrum and flux of low energy antideuterons which, although very rarely, are occasionally also produced by cosmic-ray spallation. Heavier elements ($A\geq3$) have instead entirely negligible astrophysical background and a primary yield from dark matter which could be detectable by future experiments. Using a Monte Carlo event generator and an event-by-event phase space analysis, we compute, for the first time, the production spectrum of \antihe and \antiT for dark matter annihilating or decaying to $b\bar{b}$ and ${W^+}{W^-}$ final states. We then employ a semi-analytic model of interstellar and heliospheric propagation to calculate the \antihe flux as well as to provide tools to relate the anti-helium spectrum corresponding to an arbitrary antideuteron spectrum. Finally, we discuss prospects for current and future experiments, including GAPS and AMS-02.

hep-ph↗

arXiv2014

Higher order dark matter annihilations in the Sun and implications for IceCube

Dark matter particles captured in the Sun would annihilate producing a neutrino flux that could be detected at the Earth. In some channels, however, the neutrino flux lies in the MeV range and is thus undetectable at IceCube, namely when the dark matter particles annihilate into $e^+e^-$, $μ^+μ^-$ or light quarks. On the other hand, the same interaction that mediates the annihilations into light fermions also leads, via higher order effects, to the production of weak gauge bosons (and in the case of quarks also gluons) that generate a high energy neutrino flux potentially observable at IceCube. We consider in this paper tree level annihilations into a fermion-antifermion pair with the associated emission of one gauge boson and one loop annihilations into two gauge bosons, and we calculate the limits on the scattering cross section of dark matter particles with protons in scenarios where the dark matter particle couples to electrons, muons or light quarks from the non-observation of an excess of neutrino events in the direction of the Sun. We find that the limits on the spin-dependent scattering cross section are, for some scenarios, stronger than the limits from direct detection experiments.

hep-ph↗

arXiv2013

High-energy neutrino signals from the Sun in dark matter scenarios with internal bremsstrahlung

We investigate the prospects to observe a high energy neutrino signal from dark matter annihilations in the Sun in scenarios where the dark matter is a Majorana fermion that couples to a quark and a colored scalar via a Yukawa coupling. In this minimal scenario, the dark matter capture and annihilation in the Sun can be studied in a single framework. We find that, for small and moderate mass splitting between the dark matter and the colored scalar, the two-to-three annihilation $q \bar q g$ plays a central role in the calculation of the number of captured dark matter particles. On the other hand, the two-to-three annihilation into $q \bar q Z$ gives, despite its small branching fraction, the largest contribution to the neutrino flux at the Earth at the highest energies. We calculate the limits on the model parameters using IceCube observations of the Sun and we discuss their interplay with the requirement of equilibrium of captures and annihilations in the Sun and with the requirement of thermal dark matter production. We also compare the limits from IceCube to the limits from direct detection, antiproton measurements and collider searches.

hep-ph↗

arXiv2013

Average Case Analysis of Java 7's Dual Pivot Quicksort

Recently, a new Quicksort variant due to Yaroslavskiy was chosen as standard sorting method for Oracle's Java 7 runtime library. The decision for the change was based on empirical studies showing that on average, the new algorithm is faster than the formerly used classic Quicksort. Surprisingly, the improvement was achieved by using a dual pivot approach, an idea that was considered not promising by several theoretical studies in the past. In this paper, we identify the reason for this unexpected success. Moreover, we present the first precise average case analysis of the new algorithm showing e.g. that a random permutation of length $n$ is sorted using $1.9n\ln n-2.46n+\mathcal{O}(\ln n)$ key comparisons and $0.6n\ln n+0.08n+\mathcal{O}(\ln n)$ swaps.

cs.DS↗

arXiv2013

Prospects of antideuteron detection from dark matter annihilations or decays at AMS-02 and GAPS

The search for cosmic antideuterons has been proposed as a promising method to indirectly detect dark matter, due to the very small background flux from spallations expected at the energies relevant to experiments. The antideuteron flux from dark matter annihilations or decays is, however, severely constrained by the non-observation of an excess in the antiproton-to-proton fraction measured by PAMELA. In this paper we calculate, for representative dark matter annihilation and decay channels, upper limits on the number of antideuteron events at AMS-02 and GAPS from requiring that the associated antiproton flux is in agreement with the PAMELA data. To this end, we first analyze in detail the formation of antideuterons in the coalescence model using an event-by-event Monte Carlo simulation and using data from various high energy experiments. We find that the resulting coalescence momentum shows a dependence on the underlying process and on the center of mass energy involved. Then, we calculate, using a diffusion model, the flux of antideuterons at the Earth from dark matter annihilations or decays. Our results indicate that, despite the various sources of uncertainty, the observation of an antideuteron flux at AMS-02 or GAPS from dark matter annihilations or decays will be challenging.

hep-ph↗

arXiv2013

Determination of the Cosmic Antideuteron Flux in a Monte Carlo approach

We investigate in this paper the antideuteron flux produced in high energy collisions of cosmic rays with the interstellar matter. We employ the Monte Carlo generator DPMJET-III together with the coalescence model to simulate, in an event-by-event basis, the antideuteron production in cosmic ray collisions in our Galaxy. Then, we use a diffusion model to calculate the expected flux. We find a secondary antideuteron flux at the Earth which is approximately a factor of two smaller than in previous calculations.

astro-ph.HE↗