| 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. |
- Apaydin, Dogukan, 1991, et al.
(författare)
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UVC photonic crystal surface-emitting lasers with low-divergent far-fields
- 2023
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Photonic crystal surface-emitting lasers (PCSEL) emitting in the ultraviolet (UV) C spectral range are exciting devices due to their low divergence and single-mode emission capable of high output powers as already demonstrated in the infrared [1] and blue spectral range [2]. This is due to their unique design, which incorporates a photonic crystal leading to a large optical gain area. PCSELs are based upon in-plane feedback from the photonic crystal and out-of-plane emission by the diffraction of the modes with zero group velocity at the photonic band edges. We recently demonstrated, to the best of our knowledge, the first UVC PCSEL with an emission at 279 nm. The device structure consists of 3 x 2 nm AlGaN quantum wells (QW) in a 60 nm Al0.70Ga0.30N waveguide and AlN cladding layers. The 140x140 μm large photonic crystal is dry etched into the top AlN cladding layer with a hexagonal lattice consisting of circular holes with a lattice constant of 140 nm and an etch depth of 65 nm, leaving 65 nm between the bottom of the photonic crystals and the first quantum well. Lasing in these PCSELs was achieved by resonant pumping of the QWs by a 266 nm pulsed laser with a spot size of 82 μm at room temperature. The devices exhibit threshold pump power densities from 25 down to 13 MW/cm2 showing a spectral narrowing down to 25 pm. Far-field patterns and band structures were investigated for a range of filling factors (fraction of the surface that is etched) between 10% to 26%, and the far-fields contain emission bands that were not yet reported in PCSELs at longer wavelengths. Changing the filling factor affects the photonic crystal band structure and thereby the optical mode at the Γ-point that will reach threshold first. This feature enables us to intentionally select the lasing mode with the desired far-field pattern. By a proper choice of filling factor, the intensity in the angular emission bands is diminished, resulting in a far-field with a narrow beam divergence of <1°.
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| 5. |
- Yapparov, Rinat, 1992-
(författare)
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Carrier dynamics in blue and green InGaN LED structures
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on effects that are critical to achieving high internal quantum efficiency (IQE) in GaN-based light-emitting diodes (LEDs) that emit in a broad spectral range, from violet to green-yellow. These effects include interwell carrier transport in multiple quantum well (QW) structures, lateral transport in the QW plane, and radiative and nonradiative recombination. The investigation is conducted with the time-integrated and time-resolved near- and far-field photoluminescence (PL) spectroscopy. Measurements are performed on polar single and multiple InxGa1-xN QW structures of different alloy compositions, which are supplemented with a self-consistent solution of one-dimensional Schrödinger and Poisson equations and an evaluation of the carrier density dynamics. Interwell carrier transport is studied to determine the conditions required for a uniform interwell carrier distribution in an LED active region. Such a distribution would decrease the detrimental impact of the nonradiative Auger recombination and increase the IQE. Since the hole transport is the bottleneck for this process, ambipolar interwell transport, determined by the slower holes, is studied. Standard time-resolved PL measurements are performed on multiple QW structures with a different number of In0.12Ga0.88N QWs and different barrier parameters in terms of thickness and material. Photoexcited carrier transport over the multiple QW structure is monitored by measuring PL rise times from a deeper detector QW. Such measurements make it possible to distinguish the interwell transport mechanism at high temperatures (e.g., thermionic emission - ns range) and low temperatures (e.g., ballistic - sub-ps range). In standard InGaN/GaN structures, the interwell hole transport is found to be inefficient. Studies of transport and IQE in structures with InGaN barriers of different compositions, as well as thin GaN or AlGaN interlayers between the QWs and barriers, allowed the design of structures with fast, efficient interwell transport and high IQE. These measurements are performed for blue LED structures; however, the conclusions could be extended to QWs emitting at longer wavelengths, for which the issue of the nonuniform interwell carrier distribution is even more severe. Studies of the carrier recombination and IQE are performed on single QWs with a focus on long wavelength (green, green-yellow) emitting structures, in which the IQE is much smaller than for the violet and blue-emitting wells. Radiative and nonradiative carrier recombination times are determined at different temperatures, revealing a record-high IQE of ∼60% in the green-yellow QWs. Since nonradiative recombination is often assigned to extended defects, near-field spectroscopy is applied to study the impact of V-defects related to dislocations in polar GaN-based structures. The parameters of PL spectra, as well as radiative and nonradiative recombination times, show large spatial variations. The increased nonradiative recombination related to the dislocations is revealed only in their immediate vicinity, suggesting that their impact on the IQE and device performance, contrary to common belief, should be small.
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