arXiv subjects

M. Chiu

Publications and source records attributed to M. Chiu.

At least 73 records · Page 4Linked to original sources

arXiv2022

NEXO: Neutrinoless double beta decay search beyond $10^{28}$ year half-life sensitivity

The nEXO neutrinoless double beta decay experiment is designed to use a time projection chamber and 5000 kg of isotopically enriched liquid xenon to search for the decay in $^{136}$Xe. Progress in the detector design, paired with higher fidelity in its simulation and an advanced data analysis, based on the one used for the final results of EXO-200, produce a sensitivity prediction that exceeds the half-life of $10^{28}$ years. Specifically, improvements have been made in the understanding of production of scintillation photons and charge as well as of their transport and reconstruction in the detector. The more detailed knowledge of the detector construction has been paired with more assays for trace radioactivity in different materials. In particular, the use of custom electroformed copper is now incorporated in the design, leading to a substantial reduction in backgrounds from the intrinsic radioactivity of detector materials. Furthermore, a number of assumptions from previous sensitivity projections have gained further support from interim work validating the nEXO experiment concept. Together these improvements and updates suggest that the nEXO experiment will reach a half-life sensitivity of $1.35\times 10^{28}$ yr at 90% confidence level in 10 years of data taking, covering the parameter space associated with the inverted neutrino mass ordering, along with a significant portion of the parameter space for the normal ordering scenario, for almost all nuclear matrix elements. The effects of backgrounds deviating from the nominal values used for the projections are also illustrated, concluding that the nEXO design is robust against a number of imperfections of the model.

nucl-ex↗

arXiv2022

Transverse-single-spin asymmetries of charged pions at midrapidity in transversely polarized $p{+}p$ collisions at $\sqrt{s}=200$ GeV

In 2015, the PHENIX collaboration has measured single-spin asymmetries for charged pions in transversely polarized proton-proton collisions at the center of mass energy of $\sqrt{s}=200$ GeV. The pions were detected at central rapidities of $|η|<0.35$. The single-spin asymmetries are consistent with zero for each charge individually, as well as consistent with the previously published neutral-pion asymmetries in the same rapidity range. However, they show a slight indication of charge-dependent differences which may suggest a flavor dependence in the underlying mechanisms that create these asymmetries.

hep-ex↗

arXiv2022

Transverse single spin asymmetries of forward neutrons in $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV as a function of transverse and longitudinal momenta

In 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider recorded $p$$+$$p$, $p$$+$Al, and $p$$+$Au collision data at center of mass energies of $\sqrt{s_{_{NN}}}=200$ GeV with the proton beam(s) transversely polarized. At very forward rapidities $η>6.8$ relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction $x_F$. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultra-peripheral collisions, UPC. Events that are correlated with a hard collision indeed display a mostly negative asymmetry that increases in magnitude as a function of transverse momentum with only little dependence on $x_F$. In contrast, events that are not likely to have emerged from a hard collision display positive asymmetries for the nuclear collisions with a kinematic dependence that resembles that of a UPC based model. Because the UPC interaction depends strongly on the charge of the nucleus, those effects are very small for $p$$+$$p$ collisions, moderate for $p$$+$Al collisions, and large for $p$$+$Au collisions.

hep-ex↗

arXiv2022

Kinematic dependence of azimuthal anisotropies in $p$$+$Au, $d$$+$Au, $^3$He+Au at $\sqrt{s_{_{NN}}}$ = 200 GeV

There is strong evidence for the formation of small droplets of quark-gluon plasma in $p/d/^{3}$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and in $p$+$p$/Pb collisions at the Large Hadron Collider. In particular, the analysis of data at RHIC for different geometries obtained by varying the projectile size and shape has proven insightful. In the present analysis, we find excellent agreement with the previously published PHENIX at RHIC results on elliptical and triangular flow with an independent analysis via the two-particle correlation method, which has quite different systematic uncertainties and an independent code base. In addition, the results are extended to other detector combinations with different kinematic (pseudorapidity) coverage. These results provide additional constraints on contributions from nonflow and longitudinal decorrelations.

hep-ex↗

arXiv2022

Development of a $^{127}$Xe calibration source for nEXO

We study a possible calibration technique for the nEXO experiment using a $^{127}$Xe electron capture source. nEXO is a next-generation search for neutrinoless double beta decay ($0νββ$) that will use a 5-tonne, monolithic liquid xenon time projection chamber (TPC). The xenon, used both as source and detection medium, will be enriched to 90% in $^{136}$Xe. To optimize the event reconstruction and energy resolution, calibrations are needed to map the position- and time-dependent detector response. The 36.3 day half-life of $^{127}$Xe and its small $Q$-value compared to that of $^{136}$Xe $0νββ$ would allow a small activity to be maintained continuously in the detector during normal operations without introducing additional backgrounds, thereby enabling in-situ calibration and monitoring of the detector response. In this work we describe a process for producing the source and preliminary experimental tests. We then use simulations to project the precision with which such a source could calibrate spatial corrections to the light and charge response of the nEXO TPC.

physics.ins-det↗

arXiv2021

Probing gluon spin-momentum correlations in transversely polarized protons through midrapidity isolated direct photons in $p^\uparrow+p$ collisions at $\sqrt{s}=200$ GeV

Studying spin-momentum correlations in hadronic collisions offers a glimpse into a three-dimensional picture of proton structure. The transverse single-spin asymmetry for midrapidity isolated direct photons in $p^\uparrow+p$ collisions at $\sqrt{s}=200$ GeV is measured with the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). Because direct photons in particular are produced from the hard scattering and do not interact via the strong force, this measurement is a clean probe of initial-state spin-momentum correlations inside the proton and is in particular sensitive to gluon interference effects within the proton. This is the first time direct photons have been used as a probe of spin-momentum correlations at RHIC. The uncertainties on the results are a fifty-fold improvement with respect to those of the one prior measurement for the same observable, from the Fermilab E704 experiment. These results constrain gluon spin-momentum correlations in transversely polarized protons.

hep-ex↗

arXiv2021

Reflectivity of VUV-sensitive Silicon Photomultipliers in Liquid Xenon

Silicon photomultipliers are regarded as a very promising technology for next-generation, cutting-edge detectors for low-background experiments in particle physics. This work presents systematic reflectivity studies of Silicon Photomultipliers (SiPM) and other samples in liquid xenon at vacuum ultraviolet (VUV) wavelengths. A dedicated setup at the University of Münster has been used that allows to acquire angle-resolved reflection measurements of various samples immersed in liquid xenon with 0.45° angular resolution. Four samples are investigated in this work: one Hamamatsu VUV4 SiPM, one FBK VUV-HD SiPM, one FBK wafer sample and one Large-Area Avalanche Photodiode (LA-APD) from EXO-200. The reflectivity is determined to be 25-36% at an angle of incidence of 20° for the four samples and increases to up to 65% at 70° for the LA-APD and the FBK samples. The Hamamatsu VUV4 SiPM shows a decline with increasing angle of incidence. The reflectivity results will be incorporated in upcoming light response simulations of the nEXO detector.

physics.ins-det↗

arXiv2021

Event Reconstruction in a Liquid Xenon Time Projection Chamber with an Optically-Open Field Cage

nEXO is a proposed tonne-scale neutrinoless double beta decay ($0νββ$) experiment using liquid ${}^{136}Xe$ (LXe) in a Time Projection Chamber (TPC) to read out ionization and scintillation signals. Between the field cage and the LXe vessel, a layer of LXe ("skin" LXe) is present, where no ionization signal is collected. Only scintillation photons are detected, owing to the lack of optical barrier around the field cage. In this work, we show that the light originating in the skin LXe region can be used to improve background discrimination by 5% over previous published estimates. This improvement comes from two elements. First, a fraction of the $γ$-ray background is removed by identifying light from interactions with an energy deposition in the skin LXe. Second, background from ${}^{222}Rn$ dissolved in the skin LXe can be efficiently rejected by tagging the $α$ decay in the ${}^{214}Bi-{}^{214}Po$ chain in the skin LXe.

physics.ins-det↗

arXiv2021

Transverse single-spin asymmetries of midrapidity $π^0$ and $η$ mesons in polarized $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV

We present a measurement of the transverse single-spin asymmetry for $π^0$ and $η$ mesons in $p^\uparrow$$+$$p$ collisions in the pseudorapidity range $|η|<0.35$ and at a center-of-mass energy of 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider. In comparison with previous measurements in this kinematic region, these results have a factor of 3 smaller uncertainties. As hadrons, $π^0$ and $η$ mesons are sensitive to both initial- and final-state nonperturbative effects for a mix of parton flavors. Comparisons of the differences in their transverse single-spin asymmetries have the potential to disentangle the possible effects of strangeness, isospin, or mass. These results can constrain the twist-3 trigluon collinear correlation function as well as the gluon Sivers function.

hep-ex↗

arXiv2021

Transverse momentum dependent forward neutron single spin asymmetries in transversely polarized $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV

In 2015, the PHENIX collaboration has measured very forward ($η>6.8$) single-spin asymmetries of inclusive neutrons in transversely polarized proton-proton and proton-nucleus collisions at a center of mass energy of 200 GeV. A previous publication from this data set concentrated on the nuclear dependence of such asymmetries. In this measurement the explicit transverse-momentum dependence of inclusive neutron single spin asymmetries for proton-proton collisions is extracted using a bootstrapping-unfolding technique on the transverse momenta. This explicit transverse-momentum dependence will help improve the understanding of the mechanisms that create these asymmetries.

hep-ex↗

arXiv2020

Measurement of jet-medium interactions via direct photon-hadron correlations in Au$+$Au and $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We present direct photon-hadron correlations in 200 GeV/A Au$+$Au, $d$$+$Au and $p$$+$$p$ collisions, for direct photon $p_T$ from 5--12 GeV/$c$, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in $d$$+$Au collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in Au$+$Au compared to $p$$+$$p$ and $d$$+$Au. As the momentum fraction decreases, the yield of hadrons in Au$+$Au increases to an excess over the yield in $p$$+$$p$ collisions. The excess is at large angles and at low hadron $p_T$ and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.

hep-ex↗

arXiv2020

Production of $π^0$ and $η$ mesons in U$+$U collisions at $\sqrt{s_{_{NN}}}=192$ GeV

The PHENIX experiment at the Relativistic Heavy Ion Collider measured $π^0$ and $η$ mesons at midrapidity in U$+$U collisions at $\sqrt{s_{_{NN}}}=192$ GeV in a wide transverse momentum range. Measurements were performed in the $π^0(η)\rightarrowγγ$ decay modes. A strong suppression of $π^0$ and $η$ meson production at high transverse momentum was observed in central U$+$U collisions relative to binary scaled $p$$+$$p$ results. Yields of $π^0$ and $η$ mesons measured in U$+$U collisions show similar suppression pattern to the ones measured in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV for similar numbers of participant nucleons. The $η$/$π^0$ ratios do not show dependence on centrality or transverse momentum, and are consistent with previously measured values in hadron-hadron, hadron-nucleus, nucleus-nucleus, and $e^+e^-$ collisions.

hep-ex↗

arXiv2020

Measurements of Higher-Order Flow Harmonics in Au+Au Collisions at sqrt(s_NN) = 200 GeV

Flow coefficients v_n for n = 2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at sqrt(s_NN) = 200 GeV, are measured relative to event planes Ψ_n determined at large rapidity. We report v_n as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The v_n are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint improves precision of the extracted viscosity to entropy density ratio eta/s.

nucl-ex↗

arXiv2020

Polarization and cross section of midrapidity J/$ψ$ production in proton-proton collisions at $\sqrt{s}=510$ GeV

The PHENIX experiment has measured the spin alignment for inclusive $J/ψ\rightarrow e^{+}e^{-}$ decays in $p$+$p$ collisions at $\sqrt{s}=510$ GeV at midrapidity. The angular distributions have been measured in three different polarization frames, and the three decay angular coefficients have been extracted in a full two-dimensional analysis. Previously, PHENIX saw large longitudinal net polarization at forward rapidity at the same collision energy. This analysis at midrapidity, complementary to the previous PHENIX results, sees no sizable polarization in the measured transverse momentum range of $0.0<p_T<10.0$ GeV/$c$. The results are consistent with a previous one-dimensional analysis at midrapidity at $\sqrt{s}=200$ GeV. The transverse-momentum-dependent cross section for midrapidity $J/ψ$ production has additionally been measured, and after comparison to world data we find a simple logarithmic dependence of the cross section on $\sqrt{s}$.

hep-ex↗

arXiv2020

Production of $b\bar{b}$ at forward rapidity in $p$+$p$ collisions at $\sqrt{s}=510$ GeV

The cross section of bottom quark-antiquark ($b\bar{b}$) production in $p$+$p$ collisions at $\sqrt{s}=510$ GeV is measured with the PHENIX detector at the Relativistic Heavy Ion Collider. The results are based on the yield of high mass, like-sign muon pairs measured within the PHENIX muon arm acceptance ($1.2<|y|<2.2$). The $b\bar{b}$ signal is extracted from like-sign dimuons by utilizing the unique properties of neutral $B$ meson oscillation. We report a differential cross section of $dσ_{b\bar{b}\rightarrow μ^\pmμ^\pm}/dy = 0.16 \pm 0.01~(\mbox{stat}) \pm 0.02~(\mbox{syst}) \pm 0.02~(\mbox{global})$ nb for like-sign muons in the rapidity and $p_T$ ranges $1.2<|y|<2.2$ and $p_T>1$ GeV/$c$, and dimuon mass of 5--10 GeV/$c^2$. The extrapolated total cross section at this energy for $b\bar{b}$ production is $13.1 \pm 0.6~(\mbox{stat}) \pm 1.5~(\mbox{syst}) \pm 2.7~(\mbox{global})~μ$b. The total cross section is compared to a perturbative quantum chromodynamics calculation and is consistent within uncertainties. The azimuthal opening angle between muon pairs from $b\bar{b}$ decays and their $p_T$ distributions are compared to distributions generated using {\sc ps pythia 6}, which includes next-to-leading order processes. The azimuthal correlations and pair $p_T$ distribution are not very well described by {\sc pythia} calculations, but are still consistent within uncertainties. Flavor creation and flavor excitation subprocesses are favored over gluon splitting.

hep-ex↗

arXiv2020

Evolution of pi^0 suppression in Au+Au collisions from sqrt(s_NN) = 39 to 200 GeV

Neutral-pion, pi^0, spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at sqrt(s_NN) = 39 and 62.4 GeV and compared to earlier measurements at 200 GeV in the 1<p_T<10 GeV/c transverse-momentum (p_T) range. The high-p_T tail is well described by a power law in all cases and the powers decrease significantly with decreasing center-of-mass energy. The change of powers is very similar to that observed in the corresponding p+p-collision spectra. The nuclear-modification factors (R_AA) show significant suppression and a distinct energy dependence at moderate p_T in central collisions. At high p_T, R_AA is similar for 62.4 and 200 GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe R_AA well at 200 GeV, fail to describe the 39 GeV data, raising the possibility that the relative importance of initial-state effects and soft processes increases at lower energies. A conclusion that the region where hard processes are dominant is reached only at higher p_T, is also supported by the x_T dependence of the x_T-scaling power-law exponent.

nucl-ex↗

arXiv2020

Measurement of charged pion double spin asymmetries at midrapidity in longitudinally polarized $p$$+$$p$ collisions at $\sqrt{s}=510$ GeV

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the longitudinal double spin asymmetries, $A_{LL}$, for charged pions at midrapidity ($|η|<0.35$) in longitudinally polarized $p$$+$$p$ collisions at $\sqrt{s}=510$ GeV. These measurements are sensitive to the gluon spin contribution to the total spin of the proton in the parton momentum fraction $x$ range between 0.04 and 0.09. One can infer the sign of the gluon polarization from the ordering of pion asymmetries with charge alone. The asymmetries are found to be consistent with global quantum-chromodynamics fits of deep-inelastic scattering and data at $\sqrt{s}=200$ GeV, which show a nonzero positive contribution of gluon spin to the proton spin.

hep-ex↗

arXiv2020

Measurement of $J/ψ$ at forward and backward rapidity in $p$+$p$, $p$$+A$l, $p$$+A$u, and $^3$He+Au collisions at $\sqrt{s_{_{NN}}}=200~{\rm GeV}$

Charmonium is a valuable probe in heavy-ion collisions to study the properties of the quark gluon plasma, and is also an interesting probe in small collision systems to study cold nuclear matter effects, which are also present in large collision systems. With the recent observations of collective behavior of produced particles in small system collisions, measurements of the modification of charmonium in small systems have become increasingly relevant. We present the results of $J/ψ$ measurements at forward and backward rapidity in various small collision systems, $p$$+$$p$, $p$$+$Al, $p$$+$Au and $^3$He$+$Au, at $\sqrt{s_{_{NN}}}$=200 GeV. The results are presented in the form of the observable $R_{AB}$, the nuclear modification factor, a measure of the ratio of the $J/ψ$ invariant yield compared to the scaled yield in $p$$+$$p$ collisions. We examine the rapidity, transverse momentum, and collision centrality dependence of nuclear effects on $J/ψ$ production with different projectile sizes $p$ and $^3$He, and different target sizes Al and Au. The modification is found to be strongly dependent on the target size, but to be very similar for $p$$+$Au and $^{3}$He$+$Au. However, for 0%--20% central collisions at backward rapidity, the modification for $^{3}$He$+$Au is found to be smaller than that for $p$$+$Au, with a mean fit to the ratio of $0.89\pm0.03$(stat)${\pm}0.08$(syst), possibly indicating final state effects due to the larger projectile size.

hep-ex↗