| 1. |
- Abelev, Betty, et al.
(författare)
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Long-range angular correlations on the near and away side in p-Pb collisions at root S-NN=5.02 TeV
- 2013
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 719:1-3, s. 29-41
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Tidskriftsartikel (refereegranskat)abstract
- Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 < P-T,P-assoc < P-T,P-trig < 4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS Collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and p(T) bins, and the widths show no significant evolution with event multiplicity or p(T). These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge. (c) 2013 CERN. Published by Elsevier B.V. All rights reserved.
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| 2. |
- Abelev, Betty, et al.
(författare)
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Measurement of prompt J/psi and beauty hadron production cross sections at mid-rapidity in pp collisions at root s=7 TeV
- 2012
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Ingår i: Journal of High Energy Physics. - 1029-8479. ; :11
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Tidskriftsartikel (refereegranskat)abstract
- The ALICE experiment at the LHC has studied J/psi production at mid-rapidity in pp collisions at root s = 7 TeV through its electron pair decay on a data sample corresponding to an integrated luminosity L-int = 5.6 nb(-1). The fraction of J/psi from the decay of long-lived beauty hadrons was determined for J/psi candidates with transverse momentum p(t) > 1,3 GeV/c and rapidity vertical bar y vertical bar < 0.9. The cross section for prompt J/psi mesons, i.e. directly produced J/psi and prompt decays of heavier charmonium states such as the psi(2S) and chi(c) resonances, is sigma(prompt J/psi) (p(t) > 1.3 GeV/c, vertical bar y vertical bar < 0.9) = 8.3 +/- 0.8(stat.) +/- 1.1 (syst.)(-1.4)(+1.5) (syst. pol.) mu b. The cross section for the production of b-hadrons decaying to J/psi with p(t) > 1.3 GeV/c and vertical bar y vertical bar < 0.9 is a sigma(J/psi <- hB) (p(t) > 1.3 GeV/c, vertical bar y vertical bar < 0.9) = 1.46 +/- 0.38 (stat.)(-0.32)(+0.26) (syst.) mu b. The results are compared to QCD model predictions. The shape of the p(t) and y distributions of b-quarks predicted by perturbative QCD model calculations are used to extrapolate the measured cross section to derive the b (b) over bar pair total cross section and d sigma/dy at mid-rapidity.
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| 3. |
- Abelev, Betty, et al.
(författare)
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Underlying Event measurements in pp collisions at root s=0.9 and 7 TeV with the ALICE experiment at the LHC
- 2012
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Ingår i: Journal of High Energy Physics. - 1029-8479. ; :7
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Tidskriftsartikel (refereegranskat)abstract
- We present measurements of Underlying Event observables in pp collisions at root s = 0 : 9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum p(T),L-T in the event. Different regions are defined with respect to the azimuthal direction of the leading (highest transverse momentum) track: Toward, Transverse and Away. The Toward and Away regions collect the fragmentation products of the hardest partonic interaction. The Transverse region is expected to be most sensitive to the Underlying Event activity. The study is performed with charged particles above three different p(T) thresholds: 0.15, 0.5 and 1.0 GeV/c. In the Transverse region we observe an increase in the multiplicity of a factor 2-3 between the lower and higher collision energies, depending on the track p(T) threshold considered. Data are compared to PYTHIA 6.4, PYTHIA 8.1 and PHOJET. On average, all models considered underestimate the multiplicity and summed p(T) in the Transverse region by about 10-30%.
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| 4. |
- Clark, Andrew G., et al.
(författare)
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Evolution of genes and genomes on the Drosophila phylogeny
- 2007
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 450:7167, s. 203-218
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Tidskriftsartikel (refereegranskat)abstract
- Comparative analysis of multiple genomes in a phylogenetic framework dramatically improves the precision and sensitivity of evolutionary inference, producing more robust results than single-genome analyses can provide. The genomes of 12 Drosophila species, ten of which are presented here for the first time (sechellia, simulans, yakuba, erecta, ananassae, persimilis, willistoni, mojavensis, virilis and grimshawi), illustrate how rates and patterns of sequence divergence across taxa can illuminate evolutionary processes on a genomic scale. These genome sequences augment the formidable genetic tools that have made Drosophila melanogaster a pre-eminent model for animal genetics, and will further catalyse fundamental research on mechanisms of development, cell biology, genetics, disease, neurobiology, behaviour, physiology and evolution. Despite remarkable similarities among these Drosophila species, we identified many putatively non-neutral changes in protein-coding genes, non-coding RNA genes, and cis-regulatory regions. These may prove to underlie differences in the ecology and behaviour of these diverse species.
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| 5. |
- Albacete, Javier L., et al.
(författare)
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Predictions for p + Pb Collisions at sN N = √5 TeV : Comparison with Data
- 2016
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Ingår i: International Journal of Modern Physics E. - 0218-3013. ; 25:9
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Forskningsöversikt (refereegranskat)abstract
- Predictions made in Albacete et al. [Int. J. Mod. Phys. E 22 (2013) 1330007] prior to the LHC p+Pb run at sNN = 5 TeV are compared to currently available data. Some predictions shown here have been updated by including the same experimental cuts as the data. Some additional predictions are also presented, especially for quarkonia, that were provided to the experiments before the data were made public but were too late for the original publication.
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| 6. |
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| 7. |
- El-Sayed, Najib M., et al.
(författare)
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The genome sequence of Trypanosoma cruzi, etiologic agent of Chagas disease.
- 2005
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 309:5733, s. 409-15
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Tidskriftsartikel (refereegranskat)abstract
- Whole-genome sequencing of the protozoan pathogen Trypanosoma cruzi revealed that the diploid genome contains a predicted 22,570 proteins encoded by genes, of which 12,570 represent allelic pairs. Over 50% of the genome consists of repeated sequences, such as retrotransposons and genes for large families of surface molecules, which include trans-sialidases, mucins, gp63s, and a large novel family (>1300 copies) of mucin-associated surface protein (MASP) genes. Analyses of the T. cruzi, T. brucei, and Leishmania major (Tritryp) genomes imply differences from other eukaryotes in DNA repair and initiation of replication and reflect their unusual mitochondrial DNA. Although the Tritryp lack several classes of signaling molecules, their kinomes contain a large and diverse set of protein kinases and phosphatases; their size and diversity imply previously unknown interactions and regulatory processes, which may be targets for intervention.
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| 8. |
- Kharitonov, Konstantin, et al.
(författare)
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Flexible ptychography platform to expand the potential of imaging at free electron lasers
- 2021
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Ingår i: Optics Express. - 1094-4087. ; 29:14, s. 22345-22365
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Tidskriftsartikel (refereegranskat)abstract
- Ptychography, a scanning coherent diffraction imaging method, can produce a high-resolution reconstruction of a sample and, at the same time, of the illuminating beam. The emergence of vacuum ultraviolet and X-ray free electron lasers (FELs) has brought sources with unprecedented characteristics that enable X-ray ptychography with highly intense and ultra-fast short-wavelength pulses. However, the shot-to-shot pulse fluctuations typical for FEL pulses and particularly the partial spatial coherence of self-amplified spontaneous emission (SASE) FELs lead to numerical complexities in the ptychographic algorithms and ultimately restrict the application of ptychography at FELs. We present a general adaptive forward model for ptychography based on automatic differentiation, which is able to perform reconstructions even under these conditions. We applied this model to the first ptychography experiment at FLASH, the Free electron LASer in Hamburg, and obtained a high-resolution reconstruction of the sample as well as the complex wavefronts of individual FLASH pulses together with their coherence properties. This is not possible with more common ptychography algorithms.
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| 9. |
- Pop, Mihai, et al.
(författare)
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Characterization of FEL spectra using specific figures of merit
- 2019
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Ingår i: Proceedings of the 39th International Free-Electron Laser Conference, FEL 2019. - 9783954502103 ; , s. 388-391
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Konferensbidrag (refereegranskat)abstract
- There is an increasing demand from the user community for high quality FEL radiation. The spectrum of this radiation can prove to be a useful tool in characterizing the FEL process. Starting from a tool initially developed at FERMI we extend its capabilities to be able to analyze the modal components of the FEL spectrum. In this paper we will describe and compare two different figures of merit and offer initial bench-marking with respect to classic figure of merit for spectra such as FWHM and RMS.
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| 10. |
- Pop, Mihai, et al.
(författare)
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Considerations on implementing EEHG with a strong linear chirp
- 2019
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Ingår i: Proceedings of the 39th International Free-Electron Laser Conference, FEL 2019. - 9783954502103 ; , s. 262-265
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Konferensbidrag (refereegranskat)abstract
- Due to the stochastic nature of SASE radiation its longitudinal coherence is limited to a small fraction of the electron bunch. By pre-bunching the electrons before entering a radiator, the FEL radiation is ensured to have the same initial phase all through the bunch. Echo enabled harmonic generation (EEHG) is a technique that, by cleverly using two modulators and two dispersive sections, creates microbunches of electrons at a high harmonic of the seed laser used in the modulators. In this paper we will present some of the challenges of using this technique in combination with a strongly chirped beam and indicate a few ways to overcome said challenges.
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| 11. |
- Pop, Mihai, et al.
(författare)
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Mitigation of CSR induced spectral broadening in EEHG FEL
- 2023
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 1048
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Tidskriftsartikel (refereegranskat)abstract
- Externally seeded schemes have offered a major leap in improving the longitudinal coherence of FELs in the EUV and soft X-ray range. The Echo Enabled Harmonic Generation (EEHG) technique creates bunching at wavelengths of a few nanometers in two stages of longitudinal phase space manipulation, each consisting of an energy modulator and a chicane. Compared to other seeding techniques, EEHG is less sensitive to electron beam phase space distortions occurring during acceleration and compression. However the strong chicane needed in the first stage makes the system vulnerable to Coherent Synchrotron Radiation (CSR), which imprints a long-wavelength energy modulation on the electron bunch after the EEHG process has been initiated. Coupled with the second dispersive section, this modulation creates a position dependent change in the final bunching wavelength. In this work we show that tailoring the second seed laser wavelength can successfully compensate for the varying energy profile induced by CSR.
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| 12. |
- Pop, Mihai, et al.
(författare)
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On the coherence properties of FEL
- 2017
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Ingår i: IPAC 2017 - Proceedings of the 8th International Particle Accelerator Conference. - 9783954501823 ; , s. 2753-2755
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- FELs are well known for generating highly coherent light with excellent brilliance. In order to improve FEL coherence we have to follow how coherence evolves along the undulator and determine the factors that influence it the most. This paper presents qualitative coherence features that shine new light on the importance of coherence studies in the design of an FEL. By simulating and then analyzing the the FEL light we arrive at results linking coherence to parameters such as undulator polarization or seed laser power density.
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| 13. |
- Pop, Mihai
(författare)
-
Probing and improving coherence in Free-electron lasers
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Free electron lasers or FELs, have transformed the way science has been done in the last decades by providing coherent, highly brilliant radiation at wavelengths not achievable with conventional lasers. Their most important characteristic, along side brightness, is their coherence, which allows users to perform experiments that rely on precise phase characteristics of the probing radiation.The FEL generates intense radiation by transferring energy from a relativistic electron beam to the EM field. Both the electron beam properties and the FEL-process itself determine the final characteristics of the FEL radiation. Through specific numerical and experimental tools it is possible to probe the coherence properties of FELs and gain insight into the fundamental processes influencing them. By understanding these processes, we can improve the coherence of the resulting FEL radiation using optical lasers or magnetic elements to act on specific features of the electron beam.Investigations of transverse coherence by a Young-like experimental setup revealed differences between SASE and seeded FELs in the way that coherence is built up during the amplification process. Using the existing formalism, we used a double slit experiment to determine the transverse coherence of an FEL at various stages in the amplification process in both seeded and SASE operation. The results revealed significant differences in the way coherence is built up in the two FEL operation modes.Longitudinal coherence improvement by EEHG seeding requires a strong magnetic chicane, which can makes it vulnerable to collective instabilities in the electron beam. This work shows how one can mitigate the effect of these instabilities by a creating a wavelength dependence with time of the seed laser. The EEHG technique is also explored here as a seeding option for SXL, the soft X-ray FEL at MAX IV.
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| 14. |
- Pop, Mihai, et al.
(författare)
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Single-shot transverse coherence in seeded and unseeded free-electron lasers: A comparison
- 2022
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 25:040701
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Tidskriftsartikel (refereegranskat)abstract
- The advent of x-ray free-electron lasers (FELs) drastically enhanced the capabilities of several analytical techniques, for which the degree of transverse (spatial) coherence of the source is essential. FELs can be operated in self-amplified spontaneous emission (SASE) or seeded configurations, which rely on a qualitatively different initialization of the amplification process leading to light emission. The degree of transverse coherence of SASE and seeded FELs has been characterized in the past, both experimentally and theoretically. However, a direct experimental comparison between the two regimes in similar operating conditions is missing, as well as an accurate study of the sensitivity of transverse coherence to key working parameters. In this paper, we carry out such a comparison, focusing in particular on the evolution of coherence during the light amplification process.
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| 15. |
- Qin, Weilun, et al.
(författare)
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The FEL in the SXL project at MAX IV
- 2021
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Ingår i: Journal of Synchrotron Radiation. - : International Union Of Crystallography. - 1600-5775 .- 0909-0495. ; 28, s. 707-717
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Tidskriftsartikel (refereegranskat)abstract
- In this paper the design of the free-electron laser (FEL) in the SXL (Soft X-ray Laser) project at the MAX IV Laboratory is presented. The target performance parameters originate in a science case put forward by Swedish users and the SXL FEL is foreseen to be driven by the existing MAX IV 3 GeV linac. The SXL project is planned to be realized in different stages and in this paper the focus is on Phase 1, where the basic operation mode for the FEL will be SASE (self-amplified spontaneous emission), with an emphasis on short pulses. Simulation results for two linac bunches (high and low charge) with different pulse duration are illustrated, as well as the performance for two-color/two-pulses mode and power enhancement through tapering. Besides standard SASE and optical klystron configurations, the FEL setup is also tailored to allow for advanced seeding schemes operations. Finally possible upgrades that will be implemented in a second phase of the project are discussed.
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| 16. |
- Rebernik Ribič, P., et al.
(författare)
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Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser
- 2019
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Ingår i: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 13, s. 555-561
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
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