Quadrupole anisotropy in dihadron azimuthal correlations in central d+Au collisions at √(s(NN))=200 GeV

The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in d+Au collisions at √(s(NN))=200 GeV. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central p+Pb collisions at √(s(NN))=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar, but larger, anisotropies in d+Au collisions at RHIC compared to those seen in p+Pb collisions at the LHC. The larger extracted v2 values in d+Au are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from p+Pb collisions. When both are divided by an estimate of the initial-state eccentricity the scaled anisotropies follow a common trend with multiplicity that may extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to arise from hydrodynamic flow.

Nuclear modification of ψ', χc, and J/ψ production in d+Au collisions at sqrt[s(NN)]=200 GeV

We present results for three charmonia states (ψ', χc, and J/ψ) in d+Au collisions at |y|<0.35 and sqrt[s(NN)]=200  GeV. We find that the modification of the ψ' yield relative to that of the J/ψ scales approximately with charged particle multiplicity at midrapidity across p+A, d+Au, and A+A results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large-impact-parameter collisions we observe a similar suppression for the ψ' and J/ψ, while in small-impact-parameter collisions the more weakly bound ψ' is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger ψ' suppression in central events is not explained by an increase of the nuclear absorption owing to meson formation time effects.

Medium modification of jet fragmentation in Au+Au collisions at √[s(NN)]=200 GeV measured in direct photon-hadron correlations

The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at √[s(NN)]=200 GeV. The p(T) of the photon is an excellent approximation to the initial p(T) of the jet and the ratio z(T)=p(T)(h)/p(T)(γ) is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I(AA), the ratio of hadron yield opposite the photon in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z(T). The associated hadron yield at low z(T) is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.

Cold-nuclear-matter effects on heavy-quark production in d+Au collisions at sqrt[S(NN)]=200 GeV

The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt[S(NN)]=200 GeV in the transverse-momentum range 0.85 ≤ p(T)(e) ≤ 8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R(dA) at 1.5<p(T)<5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at the Relativistic Heavy Ion Collider extends to the heavy D meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the π(0) and heavy-flavor-electron nuclear modification factors R(AA).

Evolution of π(0) suppression in Au+Au collisions from √(s(NN))=39 to 200 GeV

Neutral-pion π(0) spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at √(s(NN))=39 and 62.4 GeV and compared with earlier measurements at 200 GeV in a transverse-momentum range of 1<p(T)<10 GeV/c. 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 spectra for p+p collisions. The nuclear modification factors (R(AA)) show significant suppression, with a distinct energy, centrality, and p(T) dependence. Above p(T)=7 GeV/c, R(AA) is similar for √(s(NN))=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, for the same p(T) region, the relative importance of initial-state effects and soft processes increases at lower energies. The p(T) range where π(0) spectra in central Au+Au collisions have the same power as in p+p collisions is ≈5 and 7 GeV/c for √(s(NN))=200 and 62.4 GeV, respectively. For the √(s(NN))=39 GeV data, it is not clear whether such a region is reached, and the x(T) dependence of the x(T)-scaling power-law exponent is very different from that observed in the √(s(NN))=62 and 200 GeV data, providing further evidence that initial-state effects and soft processes mask the in-medium suppression of hard-scattered partons to higher p(T) as the collision energy decreases.

Observation of direct-photon collective flow in Au + Au collisions at √s(NN)] = 200 GeV

The second Fourier component v(2) of the azimuthal anisotropy with respect to the reaction plane is measured for direct photons at midrapidity and transverse momentum (p(T)) of 1-12 GeV/c in Au + Au collisions at √s(NN)] = 200 GeV. Previous measurements of this quantity for hadrons with p(T) < 6 GeV/c indicate that the medium behaves like a nearly perfect fluid, while for p(T) > 6 GeV/c a reduced anisotropy is interpreted in terms of a path-length dependence for parton energy loss. In this measurement with the PHENIX detector at the Relativistic Heavy Ion Collider we find that for p(T) > 4 GeV/c the anisotropy for direct photons is consistent with zero, which is as expected if the dominant source of direct photons is initial hard scattering. However, in the p(T) < 4 GeV/c region dominated by thermal photons, we find a substantial direct-photon v(2) comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region underpredict the observed v(2).

Measurements of higher order flow harmonics in Au+Au collisions at √s(NN)=200 GeV

Flow coefficients ν(n) for n=2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at √s(NN)=200 GeV, are measured relative to event planes Ψ(n), determined at large rapidity. We report ν(n) as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The ν(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 can serve to improve the precision of the extracted shear viscosity to entropy density ratio η/s.

Suppression of back-to-back hadron pairs at forward rapidity in d+Au collisions at √s(NN)=200 GeV

Back-to-back hadron pair yields in d+Au and p+p collisions at √s(NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |η|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<η<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case, the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with a low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p(T), and η points to cold nuclear matter effects arising at high parton densities.

Cold nuclear matter effects on J/ψ yields as a function of rapidity and nuclear geometry in d+A collisions at sqrt[s(NN)]=200 GeV

We present measurements of J/ψ yields in d+Au collisions at sqrt[s(NN)]=200  GeV recorded by the PHENIX experiment and compare them with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/ψ rapidity (-2.2<y<2.4) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. In order to remove model dependent systematic uncertainties we also compare the data to a simple geometric model. The forward rapidity data are inconsistent with nuclear modifications that are linear or exponential in the density weighted longitudinal thickness, such as those from the final state breakup of the bound state.

Cross section and parity-violating spin asymmetries of W± boson production in polarized p + p collisions at sqrt[s] = 500 GeV

Large parity-violating longitudinal single-spin asymmetries A(L)(e+) = -0.86(-0.14) (+0.30) and A(L)(e-) = 0.88(-0.71) (+0.12) are observed for inclusive high transverse momentum electrons and positrons in polarized p+p collisions at a center-of-mass energy of sqrt[s] = 500 GeV with the PHENIX detector at RHIC. These e± come mainly from the decay of W± and Z0 bosons, and their asymmetries directly demonstrate parity violation in the couplings of the W± to the light quarks. The observed electron and positron yields were used to estimate W± boson production cross sections for the e± channels of σ(pp → W+ X) × BR(W+ → e+ ν(e)) = 144.1 ± 21.2(stat)(-10.3) (+3.4) (syst) ± 21.6(norm)  pb, and σ(pp → W- X) × BR(W- → e- ν[over ¯](e)) = 31.7 ± 12.1(stat)(-8.2) (+10.1) (syst) ± 4.8(norm)  pb.

Azimuthal anisotropy of π⁰ production in Au+Au collisions at sqrt((s)NN)=200 GeV: path-length dependence of jet quenching and the role of initial geometry

We have measured the azimuthal anisotropy of π⁰ production for 1<p(T)<18  GeV/c for Au+Au collisions at sqrt((s)NN)=200  GeV. The observed anisotropy shows a gradual decrease for 3≲p(T)≲7-10  GeV/c, but remains positive beyond 10  GeV/c. The magnitude of this anisotropy is underpredicted, up to at least ∼10  GeV/c, by current perturbative QCD (PQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and fluctuations is insufficient to account for this discrepancy. Calculations that implement a path-length dependence steeper than what is implied by current PQCD energy-loss models show reasonable agreement with the data.

Elliptic and hexadecapole flow of charged hadrons in Au+Au collisions at sq.rt(s(NN))=200 GeV

Differential measurements of the elliptic (v(2)) and hexadecapole (v(4)) Fourier flow coefficients are reported for charged hadrons as a function of transverse momentum (p(T)) and collision centrality or number of participant nucleons (N(part)) for Au+Au collisions at sq.rt(s(NN))=200  GeV. The v(2,4) measurements at pseudorapidity |η|≤0.35, obtained with four separate reaction-plane detectors positioned in the range 1.0<|η|<3.9, show good agreement, indicating the absence of significant Δη-dependent nonflow correlations. Sizable values for v(4)(p(T)) are observed with a ratio v(4)(p(T),N(part))/v(2)(2)(p(T),N(part))≈0.8 for 50≲N(part)≲200, which is compatible with the combined effects of a finite viscosity and initial eccentricity fluctuations. For N(part)≳200 this ratio increases up to 1.7 in the most central collisions.

Transition in yield and azimuthal shape modification in dihadron correlations in relativistic heavy ion collisions

Hard-scattered parton probes produced in collisions of large nuclei indicate large partonic energy loss, possibly with collective produced-medium response to the lost energy. We present measurements of π^{0} trigger particles at transverse momenta p{T}{t}=4-12  GeV/c and associated charged hadrons (p{T}{a}=0.5-7  GeV/c) vs relative azimuthal angle Δϕ in Au+Au and p+p collisions at sqrt[s{NN}]=200  GeV. The Au+Au distribution at low p{T}{a}, whose shape has been interpreted as a medium effect, is modified for p{T}{t}<7  GeV/c. At higher p{T}{t}, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p{T}{a}, which quantitatively challenges some medium response models. The associated yield of hadrons opposing the trigger particle in Au+Au relative to p+p (I{AA}) is suppressed at high p{T} (I{AA}≈0.35-0.5), but less than for inclusive suppression (R{AA}≈0.2).

Measurement of bottom versus charm as a function of transverse momentum with electron-hadron correlations in p + p collisions at square root of s = 200 GeV

The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|<0.35 in p+p collisions at square root of s=200 GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2<pT<7 GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D-->e(+/-)K(-/+)X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4 GeV/c in pT. A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is sigma(bb)=3.2(-1.1)(+1.2)(stat)(-1.3)(+1.4)(syst)mub.

Gluon-spin contribution to the proton spin from the double-helicity asymmetry in inclusive pi0 production in polarized p+p collisions at [sqrt]s=200 GeV

The double helicity asymmetry in neutral pion production for pT=1 to 12 GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, DeltaG, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of micro2=4 GeV2 a next to leading order QCD analysis gives DeltaG[0.02,0.3]=0.2, with a constraint of -0.7<DeltaG[0.02,0.3]<0.5 at Deltachi2=9 (approximately 3sigma) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the DeltaG constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.

Onset of pi(0) suppression studied in Cu+Cu collisions at sqrt S NN=22.4, 62.4, and 200 GeV

Neutral pion transverse momentum (p(T)) spectra at midrapidity (|y| less than or approximately 0.35) were measured in Cu+Cu collisions at sqrt[s(NN)]=22.4, 62.4, and 200 GeV. Relative to pi_(0) yields in p+p collisions scaled by the number of inelastic nucleon-nucleon collisions (N(coll) the pi_(0) yields for p(T) more than or approximately 2 GeV/c in central Cu+Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu+Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.

Origins of heart rate variability: relationship of heart rate burst morphology to work duration and load

We are developing a lexicon of specific heart period changes, or lexons, that recur frequently and whose physiological meaning can be read into ambulatory electrocardiogram (ECG). The transient, reversible "burst" of tachycardia induced by exercise initiation can also be seen on ambulatory ECG. We hypothesized that burst morphology depended on the work that preceded it and on baroreceptor activation. Ten subjects with mean age 38 yr (range 17-69 yr) underwent two protocols of semisupine cycling in which load and duration were varied. Burst duration increased with longer cycling times (median values of 18.0, 25.5, and 23.7 s with 1, 3, and 5 s of cycling, respectively; P = 0.033). Burst shape as assessed by heart period exponential decay constant and burst magnitude did not change. To assess the impact of workload, subjects cycled for 5 s at loads of 0, 25, 50, and 75 W. No significant differences were seen in burst duration, burst magnitude, or burst shape. Tachycardia preceded hypotension by 4.6 +/- 2.2 s, which is inconsistent with baroreceptor involvement in the onset of burst tachycardia. Because burst morphology is a nearly quantal response to the initiation of exercise, the presence of a burst on an ambulatory ECG implies the onset of exercise.

Large-magnitude, transient, bradycardic events in rabbits

We propose that heart period sequences are organized similarly to sentences, with a lexicon of recurrent, similarly shaped words. These words should fulfill four criteria: universality, nonrandomness, central statistical tendencies, and specific associated physiology. Here we describe a large-magnitude, transient bradycardia (LMTB) and assess whether it constitutes a word. LMTBs were seen in 11 of 12 adult female rabbits. All shape parameters were different than those of the beat-randomized and phase-randomized surrogate sequences (P < 0.05-0.001). LMTBs were 8. 4 +/- 2.9 beats and 2.64 +/- 0.87 s long and were characterized by bradycardia of 77 +/- 49 ms over 1.09 +/- 0.49 s with a recovery to baseline over 1.56 +/- 0.61 s. The LMTBs had a slower recovery than onset in 9 of 11 rabbits and were highly peaked in 10 of 11 rabbits (P < 0.05). Scalar, magnitude, and shape parameters had values with central statistical tendencies. About 76% of LMTBs were accompanied by hypotension (mean -6.1 +/- 3.9 mmHg) that lagged 2 beats behind the onset of the bradycardia and that correlated with the bradycardia (-10.5 +/- 4.1 ms/mmHg). Thus transient bradycardic events are a distinct "word" in the lexicon of heart rate variability.

Origins of heart rate variability. Inducibility and prevalence of a discrete, tachycardic event

BACKGROUND

We propose that heart period sequences are linearly organized, like sentences, and that there is a lexicon of recurrent, similarly shaped transient structures like words. Each word (or lexon) has a characteristic physiological basis. One potential lexon is the transient, reversible tachycardia that is induced by exercise initiation under laboratory conditions. We hypothesized that this lexon was inducible and observable on ambulatory ECGs of most or all subjects, was morphologically similar in both induced and detected bursts, and shared a plausible origin in both circumstances.

METHODS AND RESULTS

Ten healthy subjects (mean age, 36 years) underwent a protocol in which subjects rolled themselves from supine to lateral decubitus positions and back. Transient tachycardias ("bursts") were seen in 36 of 40 rollovers. Bursts were characterized by an initial monoexponential heart period decay (K=0.39+/-0.23 s-1), a maximum heart period decrease of 277+/-109 ms after 10.8+/-4.5 seconds, and a subsequent return to baseline 23.3+/-10.8 seconds after roll initiation. The roll-induced bursts were detected with 97% sensitivity and 99% specificity with a search algorithm that incorporated morphological parameters. In 24-hour ambulatory ECGs of 10 healthy subjects (mean age, 38 years; range, 17 to 69 years), 117+/-59 bursts were detected. Induced and detected bursts were similar in most morphological parameters. Finally, many bursts occurred at night, when rolling over also occurs.

CONCLUSIONS

Bursts are inducible, transient tachycardias that occur clinically and constitute a lexon with an understandable physiology.

Temporally localized contributions to measures of large-scale heart rate variability

The purpose of this work was to determine the temporal origins of the standard deviation of successive 5-min mean heart period sequences (SDANN) and the power of the ultralow-frequency (ULF) spectral band (< 0.0033 Hz). We hypothesized that SDANN and ULF might have their origins in changes in human activity rather than slow oscillatory rhythms. Heart period sequences were obtained from 24-h Holter electrocardiograms of 10 healthy ambulatory subjects. There was no evidence of any persistent oscillation within the ULF band. Using moving 4-h windows in short-time Fourier transforms, we showed that the amplitude of ULF fluctuated markedly, particularly during times bordering sleep. The local ULF amplitude correlated (r = 0.59 +/- 0.09) with large-scale changes in heart period quantified with 2- and 4-h wavelet transforms. Local SDANN also fluctuated, mainly around times of sleep. Although the 24-h SDANN and ULF values correlated highly, there was little correlation between their temporal distributions (r = 0.10 +/- 0.25). The temporal distributions of measures of long-range heart period variability suggest that they reflect changes in human activity levels.