Direct Photon Research Articles

GR@PPA 2.9: Radiation matching for simulating photon production processes in hadron collisions

We release an event generator package GR@PPA 2.9, for simulating the direct (single) photon and diphoton (double photon) production in hadron collisions. The included programs were used in our previous studies, in which we have explicitly shown large contributions from parton-associated processes. The programs consistently combine simulations based on matrix elements with parton-shower simulations that reproduce the multiple parton radiation and quark fragmentation to photons. The matrix elements include associated parton production processes up to two partons. We provide instructions for the installation and execution of the programs in this article. The practical performance is also presented. Program summaryProgram Title: GR@PPA 2.9Program Files doi:http://dx.doi.org/10.17632/7838c8djsr.1Licensing provisions: GPLv3Programming language: Fortran; with some included libraries coded in C and C++External routines/libraries: bash and Perl for the setup, and CERNLIB, ROOT, LHAPDF 5, PYTHIA 6.4 according to the choice of usersJournal reference of previous version: Comput. Phys. Comm. 183 (2012) 1014–1028Does the new version supersede the previous version?: No, this version supports processes not included in previous versions.Nature of problem: It is necessary to include jet-associated processes up to two jets for realistic simulations of photon production processes in high-energy hadron collisions. Photons also need to be considered as radiation. An appropriate matching method has to be introduced for combining those processes having different jet and photon multiplicities and parton shower simulations.Solution method: The limited leading-log (LLL) subtraction method has been extended to multi-jet processes and those including photon radiation. The final-state parton shower simulation has been improved to support QED photon radiation.Reasons for the new version: New processes have been supported.Summary of revisions: This program package provides event generators for realistic simulations of single and double photon production processes in high-energy hadron collisions. Users can consistently combine different jet- and photon-multiplicity processes up to two jets and two photons.

Direct photon production and PDF fits reloaded

Direct photon production in hadronic collisions provides a handle on the gluon PDF by means of the QCD Compton scattering process. In this work we revisit the impact of direct photon production on a global PDF analysis, motivated by the recent availability of the next-to-next-to-leading (NNLO) calculation for this process. We demonstrate that the inclusion of NNLO QCD and leading-logarithmic electroweak corrections leads to a good quantitative agreement with the ATLAS measurements at 8 and 13 TeV, except for the most forward rapidity region in the former case. By including the ATLAS 8 TeV direct photon production data in the NNPDF3.1 NNLO global analysis, we assess its impact on the medium-x gluon. We also study the constraining power of the direct photon production measurements on PDF fits based on different datasets, in particular on the NNPDF3.1 no-LHC and collider-only fits. We also present updated NNLO theoretical predictions for direct photon production at 13 TeV that include the constraints from the 8 TeV measurements.

Optical Helicity-Manipulated Photocurrents and Photovoltages in Organic Solar Cells

The performance of an organic functional device can be effectively improved through external field manipulation. Here, we experimentally demonstrate the optical polarization manipulation of the photocurrent or photovoltage in organic solar cells. Through switching the incident light from a linearly polarized light to a circularly polarized one, we find a pronounced change in the photocurrent, which is not observable in normal inorganic cells. There are two competing hypotheses for the primary process underlying the circular polarization-dependent phenomena in organic materials, one involving the inverse Faraday effect (IFE) and the other a direct photon spin–electron spin interaction. By way of ingenious device design and external magnetic field-induced stimuli, it is expected that the organic IFE can be a powerful experimental tool in revealing and elucidating excited-state processes occurring in organic spintronic and optoelectronic devices. Therefore, we believe that our results will potentially lead to the development of new multifunctional organic devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies.

Subleading-power corrections to the radiative leptonic B \u2192 \u03b3\u2113\u03bd decay in QCD

Applying the method of light-cone sum rules with photon distribution amplitudes, we compute the subleading-power correction to the radiative leptonic B → γℓν decay from the twist-two hadronic photon contribution at next-to-leading order in QCD; and further evaluate the higher-twist “resolved photon” corrections at leading order in αs, up to twist-four accuracy. QCD factorization for the vacuum-to-photon correlation function with an interpolating current for the B-meson is established explicitly at leading power in Λ/mb employing the evanescent operator approach. Resummation of the parametrically large logarithms of mb2/Λ2 entering the hard function of the leading-twist factorization formula is achieved by solving the QCD evolution equation for the light-ray tensor operator at two loops. The leading-twist hadronic photon effect turns out to preserve the symmetry relation between the two B → γ form factors due to the helicity conservation, however, the higher-twist hadronic photon corrections can yield symmetry-breaking effect already at tree level in QCD. Using the conformal expansion of photon distribution amplitudes with the non-perturbative parameters estimated from QCD sum rules, the twist-two hadronic photon contribution can give rise to approximately 30% correction to the leading-power “direct photon” effect computed from the perturbative QCD factorization approach. In contrast, the subleading-power corrections from the higher-twist two-particle and three-particle photon distribution amplitudes are estimated to be of mathcal{O}left(3sim 5%right) with the light-cone sum rule approach. We further predict the partial branching fractions of B → γℓν with a photon-energy cut Eγ ≥ Ecut, which are of interest for determining the inverse moment of the leading-twist B-meson distribution amplitude thanks to the forthcoming high-luminosity Belle II experiment at KEK.

Measurement of Neutral Mesons and Direct Photons with ALICE at the LHC

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Photon mapping with visible kernel domains

Despite the strong efforts made in the last three decades, lighting simulation systems still remain prone to various types of imprecisions. This paper specifically tackles the problem of biases due to density estimation used in photon mapping approaches. We study the fundamental aspects of density estimation and exhibit the need for handling visibility in the early stage of the kernel domain definition. We show that properly managing visibility in the density estimation process allows to reduce or to remove biases all at once. In practice, we have implemented a 3D product kernel based on a polyhedral domain, with both point-to-point and point-to-surface visibility computation. Our experimental results illustrate the enhancements produced at every stage of density estimation, for direct photon maps visualization and progressive photon mapping.

Tunneling Modulation of Transistor Lasers: Theory and Experiment

The coherent photons generated at the base quantum wells in the transistor laser (TL) interact with the collector field and “assist” electron tunneling from the valence band of the base to the conduction band states of the collector. The cavity coherent photon intensity effect on photon-assisted tunneling in the TL has resulted in the realization of a novel photon-field enhanced optical absorption. This intra-cavity photon-assisted tunneling (ICPAT) in the TL or the light-emitting transistor is the unique property of voltage (field) modulation and the basis for ultrahigh speed direct tunneling photon modulation and switching. In addition, a new tunneling current modulation gain in a “new form” of transistor is discovered based on the collector tunneling holes feedback to the base and dielectric relaxation base transport. The TL, owing to its unique three-terminal configuration and the complementary nature of its optical and electrical collector output signals, enables fast base recombination, collector tunneling, and OEO feedback, which has resulted in the realization of compact electro-optical applications, such as non-linear signal-mixing, frequency multiplication, OE tunneling transistor, and electrical and optical bistability.

Coherent Bichromatic Force Deflection of Molecules.

We demonstrate the effect of the coherent optical bichromatic force on a molecule, the polar free radical strontium monohydroxide (SrOH). A dual-frequency retroreflected laser beam addressing the X[over ˜]^{2}Σ^{+}↔A[over ˜]^{2}Π_{1/2} electronic transition coherently imparts momentum onto a cryogenic beam of SrOH. This directional photon exchange creates a bichromatic force that transversely deflects the molecules. By adjusting the relative phase between the forward and counterpropagating laser beams we reverse the direction of the applied force. A momentum transfer of 70ℏk is achieved with minimal loss of molecules to dark states. Modeling of the bichromatic force is performed via direct numerical solution of the time-dependent density matrix and is compared with experimental observations. Our results open the door to further coherent manipulation of molecular motion, including the efficient optical deceleration of diatomic and polyatomic molecules with complex level structures.

Time-dependent quantum description of molecular double-core-hole-state formation: Impact of the nuclear dynamics on sequential two-photon processes

We present a theoretical model to investigate double-core-hole-state formation in molecules through sequential absorption of two x-ray photons from a femtosecond laser pulse. A complete time-dependent quantum description taking into account x-ray absorption and nuclear dynamics explicitly and Auger decay phenomenologically is established within the local approximation. Using this model, we assess the impact of the nuclear dynamics on double-core photoionization processes in the case of the carbon monoxide molecule. We show that sequential absorption of two x-ray photons modifies significantly the vibrational distribution of the photoelectron spectra of the double-core-hole states compared to direct single x-ray photon absorption. Depending on the shape of the potential energy curves involved in the sequential absorption processes, lower or higher vibrational levels may be favored. Furthermore, in the case where the final state is dissociative, the electron spectrum is further broaden and blue-shifted in the two-photon process.

Low Momentum Direct Photon Measurement

Direct photons have long been considered as golden probes to study the properties of the Quark Gluon Plasma (QGP). They do not interact strongly with the medium and are produced at all stages of the collision, hence carrying information of the entire evolution of the system to the detectors. The PHENIX experiment discovered a large excess of low pT photons in Au+Au collisions at √sNN = 200 GeV compared to reference p+p collisions, which has been interpreted as thermal radiation from the QGP and hadron gas medium. At the same time, the excess photons show a large elliptic and triangular flow. These results are challenging for the current theoretical models to describe simultaneously, because on one hand the large yield suggests early stage emissions when the temperature is high, on the other hand the large anisotropy is expected to be formed only at later stages of the collision when the system has cooled off and the thermal photon production rate is expected to be smaller. Using a variety of high statistics datasets across different collision systems and energies in PHENIX, simultaneous analyses of yields and azimuthal asymmetries of direct photons with higher precision are performed to provide more constraints to the theoretical calculations. In this talk, we will present recent results on low pT direct photons measured via their external conversions to electron-positron pairs, including new results from Au+Au at lower beam energies of 39 and 62.4 GeV, as well as Cu+Cu at 200GeV.

Direct photon production in pp, p–Pb and Pb–Pb collisions measured with the ALICE experiment

We review recent ALICE results on direct photon production in pp, p–Pb and Pb–Pb collisions at LHC energies. In light systems, pp and p–Pb, no signal of direct photons at low pT < 3-5 GeV/c is observed within uncertainties, while at high pT our measurements are consistent with NLO pQCD calculations. In central and mid-central Pb–Pb collisions a thermal photon contribution is observed at low pT < 3 - 4 GeV/c with slopes Teff = (304 ± 11stat ± 40syst) MeV and (407 ± 61stat ± 96syst) MeV respectively. A collective elliptic flow of direct photons is measured and appeared to be close to the decay photon flow.

The STAR Detector Upgrades and Electromagnetic Probes in Beam Energy Scan Phase II

The Beam Energy Scan Phase II at RHIC, BES-II, is scheduled from year 2019 to 2020 and will explore the high baryon density region of the QCD phase diagram with high precision. The program will focus on the interesting energy region determined from the results of BES-I. Some of the key measurements anticipated are the chiral symmetry restoration and QGP thermal radiation in the dilepton and direct photon channels. The measurements will be possible with an order of magnitude better statistics provided by the electron cooling upgrade of RHIC and with the detector upgrades planned to extend STAR experimental reach. The upgrades are: the inner Time Projection Chamber sectors (iTPC), the Event Plane Detector (EPD), and the end-cap Time of Flight (eTOF). We present the BES-II program details and the physics opportunities in the dilepton and direct photon channels enabled by the upgrades.

Applicability of transverse mass scaling in hadronic collisions at energies available at the CERN Large Hadron Collider

We present a study on the applicability of transverse mass scaling for identified particle spectra in proton-proton collisions at $\sqrt{s}=7$ TeV based on data taken by the ALICE experiment at the LHC. The measured yields are parametrized and compared to estimates obtained from a generalized transverse mass scaling approach applied to different reference particle spectra. It is found that generalized transverse mass scaling is not able to describe the measured spectra over the full range in transverse momentum. At low $p_{\rm T}$, deviations of $20$% or more are obtained, in particular if pions are used as reference particles. A better scaling performance is obtained, when kaons are used as reference particles. At high $p_{\rm T}$ all tested spectra with the possible exception of the charged kaons exhibit a scaling behavior. Investigating the feed-down contributions from resonance decays to the charged pion yields reveals, that using them as reference a general scaling may not be achievable. Our findings imply that for precision measurements of direct photon and di-electron spectra at low transverse momentum one should measure the relevant hadronic background, instead of relying on $m_{\rm T}$ scaling for its estimate.

Applications of a pnCCD detector coupled to columnar structure CsI(Tl) scintillator system in ultra high energy X-ray Laue diffraction

Most charge coupled devices (CCDs) are made of silicon (Si) with typical active layer thicknesses of several microns. In case of a pnCCD detector the sensitive Si thickness is 450 μm. However, for silicon based detectors the quantum efficiency for hard X-rays drops significantly for photon energies above 10 keV . This drawback can be overcome by combining a pixelated silicon-based detector system with a columnar scintillator. Here we report on the characterization of a low noise, fully depleted 128×128 pixels pnCCD detector with 75×75 μm2 pixel size coupled to a 700 μm thick columnar CsI(Tl) scintillator in the photon range between 1 keV to 130 keV . The excellent performance of the detection system in the hard X-ray range is demonstrated in a Laue type X-ray diffraction experiment performed at EDDI beamline of the BESSY II synchrotron taken at a set of several GaAs single crystals irradiated by white synchrotron radiation. With the columnar structure of the scintillator, the position resolution of the whole system reaches a value of less than one pixel. Using the presented detector system and considering the functional relation between indirect and direct photon events Laue diffraction peaks with X-ray energies up to 120 keV were efficiently detected. As one of possible applications of the combined CsI-pnCCD system we demonstrate that the accuracy of X-ray structure factors extracted from Laue diffraction peaks can be significantly improved in hard X-ray range using the combined CsI(Tl)-pnCCD system compared to a bare pnCCD.

Color entanglement like effect in collinear twist-3 factorization

We study color entanglement effect for T-odd cases in collinear twist-3 factorization. As an example, we compute the transverse single spin asymmetry for direct photon production in pp collisions in pure collinear twist-3 approach. By analyzing the gauge link structure of the collinear gluon distribution on unpolarized target side, we demonstrate how color entanglement effect arises in the presence of the additional gluon attachment from polarized projectile. The result is consistent with that obtained from a hybrid approach calculation.

KK graviton resonance and cascade decays in warped gravity

We consider the Kaluza–Klein (KK) graviton and its decays in the presence of the light radion, the modulus of extra dimension, appearing naturally light due to classical scale invariance in warped gravity. Due to the presence of a sizable wave function overlap in the extra dimension between the KK graviton and the radion, the KK graviton can decay largely into a pair of radions, each of which decays into a pair of collimated photons or photon-jets. Focusing on the setup where the KK graviton has suppressed couplings to the Higgs boson and fermions, we discuss the bounds on the light radion in the parameter space where the photon-jets are relevant. Moreover, we pursue the possibility of distinguishing the scenario from the case with direct photon contributions by the angular distributions of photons in the LHC Run 2. Roles of KK graviton and radion as mediators of dark matter interactions, extensions with brane kinetic terms and the model with a holographic composite Higgs are also addressed.

Charge-state Resolved Infrared Multiple Photon Dissociation (IRMPD) Spectroscopy of Ubiquitin Ions in the Gas Phase

In this study, we obtained for the first time the direct infrared multiple photon dissociation (IRMPD) spectra of ubiquitin ions in the range 2700–3750 cm−1. Ubiquitin ions with different charge states showed absorption in the two regions of 2940–3000 cm−1 and 3280–3400 cm−1. The increase of the charge state of ubiquitin ions broadened the absorption peak on the high-frequency side in the second region, indicating some hydrogen bonds were weakened due to Coulomb interaction. It is also found that the relative intensity of the absorption peak in the first region compared to the absorption peak in the second region increased with increasing charge state, making the IRMPD spectra charge-state resolved. Although it is usually reasonable to suggest the origin of the absorption in the range 2940–3000 cm−1 as the C-H bond stretching modes, the results show significantly reduced absorption after the deuteration of all labile hydrogen atoms. A possible explanation for this is that the coupling coefficients between the C-H vibrational mode and other selective modes decreased greatly after the deuteration, reducing the rate of energy redistribution and probability of consecutive IR absorption.

Real-time robust direct and indirect photon separation with polarization imaging

National Science Foundation [NSF 1607358]; DARPA Revolutionary Enhancement of Visibility by Exploiting Active Light-fields [REVEAL HR0011-16-C-0026]

Photon production in Pb + Pb collisions at [formula omitted] TeV

We calculate the high energy photon production from the Pb + Pb collisions for different centrality classes at sNN=2.76 TeV Large Hadron Collider (LHC) energy. The jet energy loss in the jet fragmentation, jet-photon conversion and jet bremsstrahlung is considered by using the Wang–Huang–Sarcevic (WHS) and Baier–Dokshitzer–Mueller–Peigne–Schiff (BDMPS) models. We use the (1+1)-dimensional ideal relativistic hydrodynamics to study the collective transverse flow and space–time evolution of the quark gluon plasma (QGP). The numerical results agree well with the ALICE data of the direct photons from the Pb + Pb collisions (sNN=2.76 TeV) for 0–20%, 20–40% and 40–80% centrality classes.

Measuring the frequency-time two-photon wavefunction of narrowband entangled photons from cold atoms via stimulated emission

Cold atom based narrowband entangled photon sources are important for efficient atom–photon interaction, which is at the heart of long-distance quantum communication and quantum memory protocols. Complete characterization of the narrowband entangled photons requires acquiring the frequency-time two-photon wavefunction, involving both joint temporal intensity (JTI) and joint temporal phase (JTP) measurements. Here, we demonstrate stimulated emission tomography of the frequency-time two-photon wavefunction of narrowband entangled photons from cold atoms. We show more than six orders of magnitude (×106) improvement in the measurement time for obtaining JTI and JTP compared to the conventional direct photon counting method, thus paving the way toward ultrafast high-resolution quantum tomography of photonic quantum states.