arXiv subjects
Publications and source records attributed to Thomas A. Trainor.
Recent developments in the area of soft interactions and multiparticle correlations are reviewed, including higher harmonic flows conjectured to result from the initial-state geometry which might coincide with some jet-like correlation features in more-central A-A collisions.
An $η$-elongated same-side 2D peak ("soft ridge") in minimum-bias angular correlations from heavy ion collisions has been attributed both to jet formation and to initial-state geometry structure coupled to radial flow. We consider evidence for both interpretations.
Angular correlations measured in \pp and heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) include a same-side (SS) 2D peak. In peripheral åand p-p collisions the SS peak properties are consistent with predicted minimum-bias jet correlations. However, in more-central \auau collisions the SS peak becomes elongated on pseudorapidity $η$. Arguments have been proposed to explain the SS peak $η$ elongation in terms of possibly-fluctuating initial-state geometry multipoles coupled with radial flow to produce final-state momentum-space multipoles. Such arguments are based on Fourier decomposition of 2D angular correlations projected onto 1D azimuth. In this analysis we show that measured correlation structure on $η$ (large curvatures) establishes a clear distinction between the SS 2D (jet) peak and 1D multipoles. Measured 2D peak systematics can predict inferred 1D Fourier amplitudes interpreted as "higher harmonic flows." But 1D Fourier amplitudes alone cannot describe 2D angular correlations. The SS 2D peak remains a unique structure which can be interpreted in terms of parton scattering and fragmentation in all cases.
A Glasma flux-tube model has been proposed to explain strong elongation on pseudorapidity $η$ of the same-side 2D peak in minimum-bias angular correlations from $\sqrt{s_{NN}} = 200$ GeV \auau collisions. The same-side peak or "soft ridge" is said to arise from coupling of flux tubes to radial flow. Gluons radiated transversely from flux tubes are boosted by radial flow to form a narrow structure or ridge on azimuth. In this study we test the conjecture by comparing predictions for particle production, spectra and correlations from the Glasma model and conventional fragmentation processes with measurements. We conclude that the Glasma model is contradicted by measured hadron yields, spectra and correlations, whereas a two-component model of hadron production, including minimum-bias parton fragmentation, provides a quantitative description of most data, although $η$ elongation remains unexplained.
The CMS collaboration has recently reported the appearance of a same-side "ridge" structure in two-particle angular correlations from 7 TeV p-p collisions. The ridge in p-p collisions at 7 TeV has been compared to a ridge structure in more-central Au-Au collisions at 0.2 TeV interpreted by some as evidence for a dense, flowing QCD medium. In this study we make a detailed comparison between 0.2 TeV p-p correlations and the CMS results. We find that 7 TeV minimum-bias jet correlations are remarkably similar to those at 0.2 TeV, even to the details of the same-side peak geometry. Extrapolation of azimuth quadrupole systematics from 0.2 TeV suggests that the same-side ridge at 7 TeV is a manifestation of the azimuth quadrupole with amplitude enhanced by applied cuts.
Angular correlations measured in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) include a same-side (SS) 2D peak which is strongly elongated on pseudorapidity $η$ in more-central \auau collisions. The elongated peak has been referred to as a ``soft ridge.'' The SS peak is consistent with expected jet correlations in peripheral åand p-p collisions. A saturation-scale argument has been proposed to explain the origin of the elongated SS peak in terms of correlations from Glasma flux tubes interacting with radial flow. In this analysis we review the details of the proposed argument in comparison to perturbative QCD predictions of jet yields and correlations. We find that the proposal is inconsistent with several features of measured spectra and correlations.
The hydrodynamic (hydro) model applied to data from the relativistic heavy ion collider (RHIC) suggests that a dense QCD medium opaque to partons is formed in more-central Au-Au collisions. However, two-component spectrum analysis reveals a hard component, consistent with parton fragmentation described by pQCD, which can masquerade as "radial flow." Minimum-bias angular correlations reveal that most scattered partons survive as "minijets" even in central Au-Au collisions. Such alternative methods quantitatively describe spectrum and correlation structure via pQCD calculations. RHIC collisions appear to be dominated by parton scattering and fragmentation even in central Au-Au collisions.
The hydrodynamic (hydro) model has been extensively applied to heavy ion data from the relativistic heavy ion collider (RHIC). Results are interpreted to conclude that a dense QCD medium nearly opaque to most partons, a strongly-coupled quark-gluon plasma (sQGP), is formed in more-central Au-Au collisions. The sQGP may have a very small viscosity (``perfect liquid''). However, other analysis methods provide contradictory evidence. Two-component analysis of single-particle hadron spectra reveals a spectrum hard component consistent with a parton fragment distribution described by pQCD which can masquerade as ``radial flow'' in some hydro-motivated analysis. Minimum-bias angular correlations reveal that a large number of back-to-back jets from initial-state scattered partons with energies as low as 3 GeV survive as ``minijet'' hadron correlations even in central Au-Au collisions, suggesting near transparency to partons. In this talk I present methods by which almost all spectrum and correlation structure, even in the most-central Au-Au collisions at 200 GeV, can be described quantitatively by pQCD calculations. The evolution of nuclear collisions is apparently dominated by parton scattering and fragmentation even in the most-central A-A collisions, albeit the fragmentation process is strongly modified.
Spectrum hard components from 200 GeV \auau collisions are accurately described by pQCD parton fragment distributions, indicating that a substantial population of parton fragments is present in hadron spectra at low $p_t$. Minimum-bias angular correlations contain jet-like correlation structure with most-probable hadron momentum 1 GeV/c. In this study we convert minimum-bias jet-like angular correlations to single-particle yields and compare them with spectrum hard components. We find that jet-like correlations in central 200 GeV \auau collisions correspond quantitatively to pQCD predictions, and the jet-correlated hadron yield comprises one third of the final state.
Parametrizations of fragmentation functions (FFs) from $e^+$-$e^-$ and p-p collisions are combined with a parton spectrum model in a pQCD folding integral to produce minimum-bias {\em fragment distributions}. A model of in-medium FF modification is included. Calculated fragment distributions are compared with {\em hard components} from p-p and Au-Au $p_t$ spectra. Data are well described by pQCD over a large kinematic region for a range of Au-Au centralities.
comment on Phys. Rev. Lett. 103, 042302 (2009)
The hydrodynamic (hydro) model applied to heavy ion data from the relativistic heavy ion collider (RHIC) in the form of single-particle spectra and correlations seems to indicate that a dense QCD medium nearly opaque to partons, a strongly-coupled quark-gluon plasma (sQGP), is formed in more-central Au-Au collisions, and that the sQGP may have a very small viscosity ("perfect liquid"). Measurements of radial and elliptic flows, with possible coalescence of "constituent quarks" to form hadrons, seem to support the conclusion. However, other measurements provide contradictory evidence. Unbiased angular correlations indicate that a large number of back-to-back jets from initial-state scattered partons with energies as low as 3 GeV survive as "minijet" hadron correlations even in central Au-Au collisions, suggesting near transparency. Two-component analysis of single-particle hadron spectra reveals a corresponding spectrum hard component (parton fragment distribution described by pQCD) which can masquerade as "radial flow" in some spectrum analysis. Reinterpretation of "elliptic flow" as a QCD scattering process resulting in fragmentation is also possible. In this paper I review analysis methods and results in the context of two paradigms: the conventional hydrodynamics/hard-probes paradigm and an alternative quadrupole/minijets paradigm. Based on re-interpretation of fiducial data I argue that hydrodynamics may not be relevant to RHIC collisions. Collision evolution may be dominated by parton scattering and fragmentation, albeit the fragmentation process is strongly modified in more-central A-A collisions.
Dihadron azimuth correlations can provide combinatoric access to jet structure in nuclear collisions. To isolate true jet yields a background must be subtracted, including a constant offset and a contribution from "elliptic flow" (azimuth quadrupole measured by $v^2_2$). The principle of "zero yield at minimum" (ZYAM) has been introduced to determine the constant offset. Independent measurements determine $v_2^2$. This analysis demonstrates that the ZYAM concept is invalid (offset typically overestimated) and $v_2^2$ is also overestimated by conventional measurements. Jet yields are thus substantially underestimated in more-central A-A collisions, and the "away-side" azimuth peak (back-to-back jet correlations) is strongly distorted, leading to incorrect inference of "Mach shocks."
Hard components of $p_t$ spectra can be identified with minimum-bias parton fragmentation in nuclear collisions. Minimum-bias fragment distributions (FDs) can be calculated by folding a power-law parton energy spectrum with parametrized fragmentation functions (FFs) derived from $e^+$-$e^-$ and p-p collisions. Alterations to FFs due to parton "energy loss" or "medium modification" in Au-Au collisions are modeled by adjusting FF parametrizations consistent with rescaling QCD splitting functions. The parton spectrum is constrained by comparison with a p-p $p_t$ spectrum hard component. The reference for all nuclear collisions is the FD derived from in-vacuum $e^+$-$e^-$ FFs. Relative to that reference the hard component for p-p and peripheral Au-Au collisions is found to be {\em strongly suppressed} for smaller fragment momenta. At a specific point on centrality the Au-Au hard component transitions to enhancement at smaller momenta and suppression at larger momenta, consistent with FDs derived from medium-modified $e^+$-$e^-$ FFs.
Minimum-bias fragment distributions (FDs) are calculated by folding a power-law parton energy spectrum with parametrized fragmentation functions (FFs) derived from $e^+$-$e^-$ and p-p collisions. Changes in FFs due to parton "energy loss" or "medium modification" are modeled by altering FF parametrizations consistent with rescaling QCD splitting functions. The common parton spectrum is constrained by comparison with a p-p $p_t$ spectrum hard component. In-vacuum and in-medium FDs are compared with spectrum hard components from 200 GeV Au-Au collisions for several centralities. The reference for all nuclear collisions is the FD derived from in-vacuum $e^+$-$e^-$ FFs. The hard component for p-p and peripheral Au-Au collisions is found to be {\em strongly suppressed} for smaller fragment momenta, consistent with the FD derived from in-vacuum p-p FFs. At a particular centrality the Au-Au hard component transitions to enhancement at smaller momenta and suppression at larger momenta, consistent with FDs derived from in-medium $e^+$-$e^-$ FFs. Fragmentation systematics suggest that QCD color connections change dramatically in more-central A-A collisions. Spectrum systematics are inconsistent with saturation-scale arguments and parton thermalization.
I present the first isolation of azimuth quadrupole components from published $v_2(p_t)$ data (called elliptic flow) as spectra on transverse rapidity $y_t$ for identified pions, kaons and lambdas/protons from minimum-bias Au-Au collisions at 200 GeV. The form of the spectra on $y_t$ indicates that the three hadron species are emitted from a common boosted source with boost $Δy_{t0} \sim 0.6$. The quadrupole spectra have a Lévy form similar to the soft component of the single-particle spectrum, but with significantly reduced ($\sim 0.7\times$) slope parameters $T$. Comparison of quadrupole spectra with single-particle spectra suggests that the quadrupole component comprises a small fraction ($< 5$%) of the total hadron yield, contradicting the hydrodynamic picture of a thermalized, flowing bulk medium. The form of $v_2(p_t)$ is, within a constant factor, the product of $p'_t$ ($p_t$ in the boost frame) times the ratio of quadrupole spectrum to single-particle spectrum. That ratio in turn implies that above 0.5 GeV/c the form of $v_2(p_t)$ is dominated by the hard component of the single-particle spectrum (interpreted as due to minijets). It is therefore unlikely that so-called {\em constituent-quark scaling} attributed to $v_2$ is relevant to soft hadron production mechanisms (e.g., chemical freezeout).
A two-component analysis of spectra to $p_t = 12$ GeV/c for identified pions and protons from 200 GeV Au-Au collisions is presented. The method is similar to an analysis of the $n_{ch}$ dependence of $p_t$ spectra from p-p collisions at 200 GeV, but applied to Au-Au centrality dependence. The soft-component reference is a Lévy distribution on transverse mass $m_t$. The hard-component reference is a Gaussian on $y_t$ with exponential ($p_t$ power-law) tail. Deviations of data from the reference are described by hard-component ratio $r_{AA}$ which generalizes nuclear modification factor $R_{AA}$. The analysis suggests that centrality evolution of pion and proton spectra is dominated by changes in parton fragmentation. The structure of $r_{AA}$ suggests that parton energy loss produces a negative boost $Δy_t$ of a large fraction (but not all) of the minimum-bias fragment distribution, and that lower-energy partons suffer relatively less energy loss, possibly due to color screening. The analysis also suggests that the anomalous $p/π$ ratio may be due to differences in the parton energy-loss process experienced by the two hadron species. This analysis provides no evidence for radial flow.
Large elliptic flow at RHIC seems to indicate that ideal hydrodynamics provides a good description of Au-Au collisions, at least at the maximum RHIC energy. The medium formed has been interpreted as a nearly perfect (low-viscosity) liquid, and connections have been made to gravitation through string theory. Recently, claimed observations of large flow fluctuations comparable to participant eccentricity fluctuations seem to confirm the ideal hydro scenario. However, determination of the azimuth quadrupole with 2D angular autocorrelations, which accurately distinguish ``flow'' (quadrupole) from ``nonflow'' (minijets), contradicts conventional interpretations. Centrality trends may depend only on the initial parton geometry, and methods used to isolate flow fluctuations are sensitive instead mainly to minijet correlations. The results presented in this paper suggest that the azimuth quadrupole may be a manifestation of gluonic multipole radiation.