Annihilation Products Research Articles

Status of PHOKHARA

A review of the status of the Monte Carlo event generator PHOKHARA, developed for experiments using the radiative return method. The four-pion production in electron-positron annihilation and in τ -lepton decays and the narrow resonances studies are described.

Nonresonant muon pair production in electron-positron annihilation in the field of light wave

In the frame of the Born approximation it was theoretically investigated nonresonant muon pair production in e + e − collision in the field of elliptically polarized electromagnetic wave in the general relativistic case. It was analyzed in detail the reaction kinematics. It was derived the expressions for the amplitude and the cross section of the nonresonant muon pair production in e + e − annihilation in the moderately strong field. It was demonstrated that the partial cross-section is factorized as a product of the cross section of the muon pair production from e + e − pair in the absence of the external field and the probability of emission and absorption of photons from the wave.

Dark stars: Implications and constraints from cosmic reionization and extragalactic background radiation

Dark stars powered by dark matter annihilation have been proposed as the first luminous sources in the Universe. These stars are believed to form in the central dark matter cusp of low-mass minihalos. Recent calculations indicate stellar masses up to $\ensuremath{\sim}1000{M}_{\ensuremath{\bigodot}}$ and/or have very long lifetimes. The UV photons from these objects could therefore contribute significantly to cosmic reionization. Here we show that such dark star models would require a somewhat artificial reionization history, based on a double-reionization phase and a late star burst near redshift $z\ensuremath{\sim}6$, in order to fulfill the WMAP (Wilkinson Microwave Anisotropy Probe) constraint on the optical depth as well as the Gunn-Peterson constraint at $z\ensuremath{\sim}6$. This suggests that, if dark stars were common in the early universe, then models are preferred which predict a number of UV photons similar to conventional Pop. III stars. This excludes $800{M}_{\ensuremath{\bigodot}}$ dark stars that enter a main-sequence phase and other models that lead to a strong increase in the number of UV photons. We also derive constraints for massive as well as light dark matter candidates from the observed x-ray, gamma-ray, and neutrino background, considering dark matter profiles which have been steepened during the formation of dark stars. This increases the clumping factor at high redshift and gives rise to a higher dark matter annihilation rate in the early universe. We furthermore estimate the potential contribution from the annihilation products in the remnants of dark stars, which may provide a promising path to constrain such models further, but which is currently still uncertain.

High strangeness production in antiproton annihilation at rest on few nucleon systems as a possible signature of quark deconfinement or QGP occurrence

The dependence of the K + and K − production on the number of nucleons involved in the annihilation process is investigated experimentally in the p ¯ annihilation at rest on hydrogen, deuterium, 3He and 4He gas targets. Annihilations with any number of prongs (charged pions and kaons, protons and deuterons) are analyzed. Events with and without production of neutral mesons and with and without emission of fast neutrons (that is neutrons involved in the annihilation process) are recognized. The results are consistent with our previous ones on a more restricted sample of annihilation reactions and put in evidence that the strangeness production is lower or higher depending on the reaction channel. As a general trend, the strangeness production is higher in events without neutral mesons and still higher in events with the involvement of a higher number of nucleons. Both K + and K − productions increase with the number of involved nucleons, but K + much more. The maximum K + production is observed in the reaction K + 2 π + 2 π − 3 n on 4He (with the involvement of 3–4 nucleons); compared with the production on hydrogen in the reaction K + π + 2 π − , the production on 4He is higher by a factor of 31.7 ± 5.5 . In the light of some theoretical speculations, this enhancement factor is too high to be explainable in terms of hadronic interactions and could be interpreted as a signature of quark deconfinement and of formation of a quark–gluon plasma.

Electroweak bremsstrahlung in dark matter annihilation

A conservative upper bound on the total dark matter annihilation rate can be obtained by constraining the appearance rate of the annihilation products which are hardest to detect. The production of neutrinos, via the process $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}\ensuremath{\nu}$, has thus been used to set a strong general bound on the dark matter annihilation rate. However, standard model radiative corrections to this process will inevitably produce photons which may be easier to detect. We present an explicit calculation of the branching ratios for the electroweak bremsstrahlung processes $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}\ensuremath{\nu}Z$ and $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}eW$. These modes inevitably lead to electromagnetic showers and further constraints on the dark matter annihilation cross section. In addition to annihilation, our calculations are also applicable to the case of dark matter decay.

Internal motion of massive charmed quarks in double charmonium production in electron-positron annihilation

The cross section for double charmonium production in electron-positron annihilation at the interaction energy of √s = 10.6 GeV is estimated. Both the form of the charmonium wave function and a nonzero charmed-quark mass are taken into account.

Initial-state showering based on colour dipoles connected to incoming parton lines

A parton-shower model for hadronic collisions based on the emission properties of QCD dipoles is proposed. This proposal therefore extends the well-known radiation pattern of pure final-state colour dipoles to QCD initial-state radiation, both of which are treated perturbatively. Corresponding dipole splitting functions are derived and the kinematics of all dipole splittings is discussed. The application of the resulting dipole-shower model to hadron production in electron-positron annihilation, to Drell-Yan lepton-pair and QCD jet production yields encouraging results.

Dark matter annihilation in the Milky Way halo

To constrain the particle properties of dark matter using their annihilation products, robust predictions of the annihilation rates are critical. Since the annihilation rate scales as the dark matter density squared, a precise knowledge of the distribution of dark matter is required. Given the popular galactic center contains large uncertainties in the dark matter density, we focus on the signal arising from the whole Milky Way halo. This is less sensitive to uncertainties in the dark matter distribution, and in terms of flux it is larger than the cosmic signal for halos flatter than NFW. We apply this to a dark matter model in which the principal annihilation products are neutrinos. Being the least detectable Standard Model particle, the neutrino defines a strong and general upper bound on the total annihilation cross section. We show that using the Milky Way halo signal improves previous limits by 1-2 orders of magnitude.(This article is based on H. Yuksel, S. Horiuchi, J. F. Beacom, S. Ando, ‘Neutrino Constraints on the Dark Matter Total Annihilation Cross Section,’ (2007 Phys. Rev. D 76 123506, arXiv:0707.0196).)

Four-pion production inτdecays ande+e−annihilation: An update

An improved description of four-pion production in electron-positron annihilation and in $\ensuremath{\tau}$-lepton decays is presented. The model amplitude is fitted to recent data from BABAR which cover a wide energy range and which were obtained exploiting the radiative return. Predicting $\ensuremath{\tau}$-decay distributions from ${e}^{+}{e}^{\ensuremath{-}}$ data and comparing these predictions with ALEPH and CLEO results, the validity of isospin symmetry is confirmed within the present experimental errors. A good description of two- and three-pion subdistributions is obtained. Special emphasis is put on the predictions for $\ensuremath{\omega}\ensuremath{\pi}(\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0})$ in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation and in $\ensuremath{\tau}$ decay. The model amplitude is implemented in the Monte Carlo generator PHOKHARA.

Fragmentation functions for pions, kaons, protons and charged hadrons

We present new sets of pion, kaon, proton and inclusive charged hadron fragmentation functions obtained in NLO combined analyses of single-inclusive hadron production in electron-positron annihilation, proton-proton collisions, and deep-inelastic lepton-proton scattering with either particles identified in the final state. At variance with all previous fits, the present analyses take into account data where hadrons of different electrical charge are identified, which allow to discriminate quark from anti-quark fragmentation functions.

Extracting the gamma ray signal from dark matter annihilation in the galactic center region

The GLAST satellite mission will study the gamma ray sky with considerably greater exposure than its predecessor EGRET. In addition, it will be capable of measuring the arrival directions of gamma rays with much greater precision. These features each significantly enhance GLAST's potential for identifying gamma rays produced in the annihilations of dark matter particles. The combined use of spectral and angular information, however, is essential if the full sensitivity of GLAST to dark matter is to be exploited. In this paper, we discuss techniques for separating dark matter annihilation products from astrophysical backgrounds, focusing on the Galactic Center region, and perform a forecast for such an analysis. We consider both point-like and diffuse astrophysical backgrounds and model them using a realistic point-spread-function for GLAST. While the results of our study depend on the specific characteristics of the dark matter signal and astrophysical backgrounds, we find that in many scenarios it is possible to successfully identify dark matter annihilation radiation, even in the presence of significant astrophysical backgrounds.

Prospects for detecting dark matter with GLAST in light of the WMAP haze

Observations by the Wilkinson Microwave Anisotropy Probe (WMAP) experiment have identified an excess of microwave emission from the center of the Milky Way. It has previously been shown that this ``WMAP haze'' could be synchrotron emission from relativistic electrons and positrons produced in the annihilations of dark matter particles. In particular, the intensity, spectrum, and angular distribution of the WMAP haze is consistent with an electroweak scale dark matter particle (such as a supersymmetric neutralino or Kaluza-Klein dark matter in models with universal extra dimensions) annihilating with a cross section on the order of $\ensuremath{\sigma}v\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}26}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ and distributed with a cusped halo profile. No further exotic astrophysical or annihilation boost factors are required. If dark matter annihilations are in fact responsible for the observed haze, then other annihilation products will also be produced, including gamma rays. In this article, we study the prospects for the GLAST satellite to detect gamma rays from dark matter annihilations in the Galactic Center region in this scenario. We find that by studying only the inner 0.1\ifmmode^\circ\else\textdegree\fi{} around the Galactic Center, GLAST will be able to detect dark matter annihilating to heavy quarks, gauge bosons, or tau leptons over astrophysical backgrounds with $5\ensuremath{\sigma}$ ($3\ensuremath{\sigma}$) significance if they are lighter than approximately 320--500 GeV (500--750 GeV). If the angular window is broadened to study the dark matter halo profile's angular extension (while simultaneously reducing the astrophysical backgrounds), weakly interacting, massive particles (WIMPs) as heavy as several TeV can be identified by GLAST with high significance. Only if the dark matter particles annihilate mostly to electrons or muons will GLAST be unable to identify the gamma ray spectrum associated with the WMAP haze.

Testing MeV dark matter with neutrino detectors

MeV particles have been advocated as dark matter (DM) candidates in different contexts. This hypothesis can be tested indirectly by searching for the standard model (SM) products of DM self-annihilations. As the signal from DM self-annihilations depends on the square of the DM density, we might expect a sizable flux of annihilation products from our galaxy. Neutrinos are the least detectable particles in the SM and a null signal in this channel would allow to set the most conservative bound on the total annihilation cross section. Here, we show that neutrino detectors with good energy resolution and low energy thresholds can not only set bounds on the annihilation cross section but actually test the hypothesis of the possible existence of MeV DM, i.e. test the values of the cross section required to explain the observed DM density. At present, the data in the (positron) energy interval [18--82] MeV of the Super-Kamiokande experiment is already able to put a very stringent bound on the annihilation cross section for masses between $\ensuremath{\sim}15--130\text{ }\text{ }\mathrm{MeV}$. Future large experiments, like megaton water-\ifmmode \check{C}\else \v{C}\fi{}erenkov or large scintillator detectors, will improve the present limits and, if MeV DM exists, would be able to detect it.

Neutrino constraints on the dark matter total annihilation cross section

In the indirect detection of dark matter through its annihilation products, the signals depend on the square of the dark matter density, making precise knowledge of the distribution of dark matter in the Universe critical for robust predictions. Many studies have focused on regions where the dark matter density is greatest, e.g., the galactic center, as well as on the cosmic signal arising from all halos in the Universe. We focus on the signal arising from the whole Milky Way halo; this is less sensitive to uncertainties in the dark matter distribution, and especially for flatter profiles, this halo signal is larger than the cosmic signal. We illustrate this by considering a dark matter model in which the principal annihilation products are neutrinos. Since neutrinos are the least detectable standard model particles, a limit on their flux conservatively bounds the dark matter total self-annihilation cross section from above. By using the Milky Way halo signal, we show that previous constraints using the cosmic signal can be improved on by 1--2 orders of magnitude; dedicated experimental analyses should be able to improve both by an additional 1--2 orders of magnitude.

Upper Bound on the Dark Matter Total Annihilation Cross Section

We consider dark matter annihilation into standard model particles and show that the least detectable final states, namely, neutrinos, define an upper bound on the total cross section. Calculating the cosmic diffuse neutrino signal, and comparing it to the measured terrestrial atmospheric neutrino background, we derive a strong and general bound. This can be evaded if the annihilation products are dominantly new and truly invisible particles. Our bound is much stronger than the unitarity bound at the most interesting masses, shows that dark matter halos cannot be significantly modified by annihilations, and can be improved by a factor of 10-100 with existing neutrino experiments.

Herwig++Monte Carlo at next-to-leading order fore+e−annihilation and lepton pair production

This paper describes the MC@NLO method for matching next-to-leading order (NLO) perturbative QCD with the parton shower and hadronization model of the Monte Carlo (MC) event generator tt Herwig++, for e+e- annihilation and Drell-Yan lepton pair production. Details of the event generation method as well as spin, flavour, momentum and colour assignments are presented. We obtain predictions for various distributions which arecompared with experimental data.

Global analysis of fragmentation functions for protons and charged hadrons

We present new sets of fragmentation functions for protons and inclusive charged hadrons obtained in combined next-to-leading order QCD analyses of single-inclusive hadron production in electron-positron annihilation, proton-proton collisions, and deep-inelastic lepton-proton scattering. These analyses complement previous results for pion and kaon fragmentation functions with charge and flavor discrimination. The Lagrange multiplier technique is used to assess the uncertainties in the extraction of the new sets of fragmentation functions.

Collider phenomenology of unparticle physics

Low-energy phenomenology of the unparticle physics associated with an exact scale invariant sector possessing a nontrivial infrared fixed point at a higher energy scale is explored for both electron-positron and hadronic colliders. Feynman rules for a spin 0, 1, or 2 unparticle coupled to a variety of standard model gauge invariant operators that are relevant to many low-energy processes involving either real emissions of unparticles or their virtual propagator effects are presented. Missing energy and/or recoil mass distributions of the unparticle in the associated production of the unparticle together with a photon or Z boson at LEP2 and ILC as well as in Z decay into an unparticle plus a fermion-antifermion pair are studied. In addition, monojet production with missing energy from the unparticle at hadronic collisions are explored. The complex phase in the unparticle propagator that can give rise to interesting interference effects between an unparticle-exchange diagram and the standard model amplitudes are studied in details for the Drell-Yan process as well as muon pair and diphoton production in electron-positron annihilation. These energy and/or recoil mass distributions (with the exception in hadron colliders) and interference effects are found sensitively depending not only on the scaling dimension but also onmore » the spin of the unparticle. For the spin-2 unparticle, its physical effects are found to resemble that of a tower of Kaluza-Klein gravitons, which strongly indicates that the underlying unparticle physics may have root in a higher dimensional theory. A connection between unparticle physics and theories of large extra dimensions is speculated. Experimental constraints on the unparticle scale are deduced from the LEP2 data on monophoton production and from the 4-fermion contact interactions.« less

Atomic collision experiment using ultra-slow antiproton beams

The development of techniques to decelerate, cool and confine antiprotons in vacuo with an electromagnetic trap has opened a new research field of atomic physics of 'cold' antiprotons, including synthesis of antihydrogen atoms. At the Antiproton Decelerator (AD) facility at CERN, we the MUSASHI group of ASACUSA collaboration have so far achieved efficient confinement of millions of antiprotons in a Multi-Ring electrode Trap (MRT) installed in a superconducting magnet of 2.5 T, by a sequential combination of the AD (down to 5.3 MeV), an RFQD (Radio-Frequency Quadrupole Decelerator; down to 50–120 keV) and the MRT. Antiprotons, cooled to energies less than an electronvolt by preloaded electrons in the trap, was then extracted out of the magnetic field and transported along a 3-m beamline as a monoenergetic beam of 10–500 eV. With this unique ultra-low-energy antiproton beam, we are now planning the first atomic collision experiments under single collision conditions, to measure ionization and atomic capture cross sections of antiprotons against helium atoms. A supersonic atomic gas-jet target is prepared and crossed with the antiproton beam. Antiprotons as well as electrons emitted during the reaction will be detected by a microchannel plate (MCP) while the antiproton annihilation will be recognized by detection of annihilation products—mostly pions—by surrounding scintillation counters. When the antiproton is captured, it forms a neutral antiprotonic helium atom, some in a metastable state whose level structures have been well studied with spectroscopic methods. Severe identification of particles and atoms plays an essential role in the design of the experiment, to distinguish the small number of reaction events out of a huge pile of background events. Our strategies for our near-future experiments are discussed.

Global analysis of fragmentation functions for pions and kaons and their uncertainties

We present new sets of pion and kaon fragmentation functions obtained in next-to-leading order combined analyses of single-inclusive hadron production in electron-positron annihilation, proton-proton collisions, and deep-inelastic lepton-proton scattering with either pions or kaons identified in the final state. At variance with all previous fits, the present analyses take into account data where hadrons of different electrical charge are identified, which allow one to discriminate quark from antiquark fragmentation functions without the need of nontrivial flavor symmetry assumptions. The resulting sets are in good agreement with all data analyzed, which cover a much wider kinematical range than in previous fits. An extensive use of the Lagrange multiplier technique is made in order to assess the uncertainties in the extraction of the fragmentation functions and the synergy from the complementary data sets in our global analysis.