arXiv subjects

Thomas A. Trainor

Publications and source records attributed to Thomas A. Trainor.

At least 37 records · Page 2Linked to original sources

arXiv2021

Fluctuations and Selection Bias in 5 and 13 TeV p-p Collisions: Where are the jets?

The ALICE collaboration recently reported high-statistics $\bf p_t$ spectra from 5 TeV and 13 TeV p-p collisions with intent to determine the role of jets in high-multiplicity collisions. In the present study a two-component (soft + hard) model (TCM) of hadron production in p-p collisions is applied to ALICE $\bf p_t$ spectra. As in previous TCM studies of A-B collision systems jet and nonjet contributions to $\bf p_t$ spectra are accurately separated over the entire $\bf p_t$ acceptance. The statistical significance of data-model differences is established leading to insights concerning selection bias and spectrum model validity.

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arXiv2021

Statistical evaluation of fitted models applied to $\bf p_t$ spectrum data from 5 TeV and 13 TeV $\bf p$-$\bf p$ collisions at the large hadron collider

In a recent analysis of high-statistics $p_t$ spectra from 5 and 13 TeV $p$-$p$ collisions a two-component (soft+hard) model (TCM) of hadron production near midrapidity, based on fixed model functions independent of collision multiplicity $n_{ch}$ or event selection criteria, was employed as a reference to determine data biases induced by the selection criteria. As in previous studies the fixed TCM accurately isolated jet-related and nonjet contributions to spectra. It was observed that two selection criteria (V0M and SPD), based on different pseudorapidity $η$ acceptances, bias the spectrum hard component (jet fragment distribution) in different but complementary ways whereas the soft component is not significantly biased. In the present study TCM model functions are adjusted (i.e. fitted) to accommodate data. The effect of selection bias is then represented by smooth evolution of model parameters with $n_{ch}$. To evaluate the quality of model fits, Z-scores (data-model differences divided by statistical uncertainties) are evaluated and compared with data/model ratios that are often used for such comparisons. Based on Z-scores the variable TCM is compared with two other frequently-invoked models: the Tsallis model and the blast-wave model as they have been recently applied to the same spectrum data. The results are relevant to recent claims that collectivity (various flow manifestations) as a possible manifestation of QGP formation is observed in small systems. Data systematics suggest that these $p$-$p$ $p_t$ spectra are consistent with conventional QCD.

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arXiv2021

A two-component model of hadron production applied to $\bf p_t$ spectra from 5 TeV and 13 TeV $\bf p$-$\bf p$ collisions at the large hadron collider

The ALICE collaboration at the large hadron collider (LHC) recently reported high-statistics $p_t$ spectrum data from 5 TeV and 13 TeV $p$-$p$ collisions. Particle data for each energy were partitioned into event classes based on the total yields within two disjoint pseudorapidity $η$ intervals denoted by acronyms V0M and SPD. For each energy the spectra resulting from the two selection methods were then compared to a minimum-bias INEL $> 0$ average over the entire event population. The nominal goal was determination of the role of jets in high-multiplicity $p$-$p$ collisions and especially the jet contribution to the low-$p_t$ parts of spectra. A related motivation was response to recent claims of "collective" behavior and other nominal indicators of quark-gluon plasma (QGP) formation in small collision systems. In the present study a two-component (soft + hard) model (TCM) of hadron production in $p$-$p$ collisions is applied to the ALICE spectrum data. As in previous TCM studies of a variety of A-B collision systems the jet and nonjet contributions to $p$-$p$ spectra are accurately separated over the entire $p_t$ acceptance. Distinction is maintained among spectrum normalizations, jet contributions to spectra and systematic biases resulting from V0M and SPD event selection. The statistical significance of data-model differences is established. The effect of {\em spherocity} (azimuthal asymmetry measure nominally sensitive to jet production) on ensemble-mean $p_t$ vs event multiplicity $n_{ch}$ is investigated and found to have little relation to jet production. The general results of the TCM analysis are as expected from a conventional QCD description of jet production in $p$-$p$ collisions.

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arXiv2020

A two-component model for identified-hadron $\bf p_t$ spectra from 5 TeV p-Pb collisions

In preparation for the heavy ion program at the relativistic heavy ion collider (RHIC) $d$-Au collisions were designated as a control experiment for possible discovery of a quark-gluon plasma (QGP) in more-central Au-Au collisions, and contrasting results from the two systems seemed to support such a discovery. In contrast, recent results ($p_t$-spectrum and angular-correlation features) from $p$-Pb collisions at the large hadron collider (LHC) have been interpreted to support claims of hydrodynamic flows and QGP formation even in small collision systems. The present study addresses such claims via a two-component (soft + hard) model (TCM) of identified-hadron (PID) $p_t$ spectra from 5 TeV $p$-Pb collisions. $p$-Pb centrality is adopted from a previous study of ensemble-mean $\bar p_t$ data from the same system. $p$-Pb $p_t$ spectra for pions, kaons, protons and Lambdas are described by the TCM within their point-to-point uncertainties. Invariance of the TCM hard component vs $p$-Pb centrality indicates that jet formation remains unchanged in $p$-Pb collisions relative to $p$-$p$ collisions, and radial-flow contributions to spectra are negligible. These $p$-Pb TCM results have implications for interpretation of similar data features from A-A collisions in terms of QGP formation.

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arXiv2020

Differential comparison of identified-hadron $\bf p_t$ spectra from high-energy A-B nuclear collisions based on a two-component model of hadron production

Identified-hadron (PID) spectra from 2.76 TeV Pb-Pb and $p$-$p$ collisions are analyzed via a two-component (soft + hard) model (TCM) of hadron production in high-energy nuclear collisions. The PID TCM is adapted with minor changes from a recent analysis of PID hadron spectra from 5 TeV $p$-Pb collisions. Results from LHC data are compared with a PID TCM for 200 GeV Au-Au pion and proton spectra. 2.76 TeV proton spectra exhibit strong inefficiencies above 1 GeV/c estimated by comparing the $p$-$p$ spectrum with the corresponding TCM. After inefficiency correction Pb-Pb proton spectra are very similar to Au-Au proton spectra. PID A-A spectra are generally inconsistent with radial flow. Jet-related Pb-Pb and Au-Au spectrum hard components exhibit strong suppression at higher $p_t$ in more-central collisions corresponding to results from spectrum ratio $R_{AA}$ but also, for pions and kaons, exhibit dramatic enhancements below $p_t = 1$ GeV/c that are concealed by $R_{AA}$. In contrast, enhancements of proton hard components appear only above 1 GeV/c suggesting that the baryon/meson "puzzle" is a jet phenomenon. Modification of spectrum hard components in more-central A-A collisions is consistent with increased gluon splitting during jet formation but with approximate conservation of leading-parton energy within a jet via the lower-$p_t$ enhancements.

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arXiv2019

QGP droplet formation in small asymmetric collision systems

The journal Nature recently published a letter titled "Creating small circular, elliptical, and triangular droplets of quark-gluon plasma" [1]. The basis for that claim is a combination of measured Fourier amplitudes $v_2$ and $v_3$ from collision systems $p$-Au, $d$-Au and $h$-Au (helion $h$ is the nucleus of atom $^3$He), Glauber Monte Carlo estimates of initial-state transverse collision geometries for those systems and hydro Monte Carlo descriptions of the $v_n$ data. Correspondence between hydro $v_n$ trends and data trends is interpreted as confirmation of "collectivity" occurring in the small collision systems, further interpreted to indicate QGP formation. QGP formation in small systems runs counter to pre-RHIC theoretical assumptions that QGP formation should require large collision systems (e.g. central A-A collisions). There is currently available a broad context of experimental data from $p$-$p$, A-A and $p$-Pb collisions at the RHIC and LHC against which the validity of the Nature letter claims may be evaluated. This talk provides a summary of such results and their implications. [1] Nature Phys. 15, no. 3, 214 (2019).}

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arXiv2019

Questioning quark-gluon plasma formation in small collision systems

A recent letter published in the journal Nature reports observation at the relativistic heavy ion collider (RHIC) of quark-gluon plasma (QGP) formation in small asymmetric collision systems denoted as $p$-Au, $d$-Au and $^3$He-Au. The claimed phenomenon, described as `short-lived QGP droplets,' is inferred from a combination of Glauber Monte Carlo simulations, measurement of two azimuth Fourier amplitudes $v_2$ and $v_3$ and hydro theory calculations. While that claim follows a trend in recent years to report `signals' conventionally attributed to QGP as appearing also in smaller collision systems, the new result remains surprising in the context of expectations before first RHIC operation that small systems, e.g. $d$-Au collisions, would provide control experiments in which a QGP was unlikely to appear. An alternative interpretation of the recent RHIC result is that small-system control experiments do convey an important message: The `signals' conventionally attributed to QGP formation in larger A-A collisions do not actually represent that phenomenon in any system. The present study reviews a broad array of experimental evidence for or against QGP formation in several collision systems. It examines in particular hydro theory descriptions of spectra and correlations usually interpreted to support QGP formation. Available evidence suggests that data features conventionally attributed to QGP formation represent either minimum-bias jet production or a nonjet azimuth quadrupole with properties inconsistent with a hydro hypothesis.

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arXiv2018

PYTHIA and the preoccupied proton

The PYTHIA Monte Carlo (PMC) has been applied broadly to simulations of high-energy $p$-$p$ and $p$-$\bar p$ collisions. The PMC is based on several assumptions, such as that most hadrons result from jet production (multiple parton interactions or MPIs), that $p$-$p$ centrality is relevant and that color reconnection (CR) strongly influences fragmentation to jets. An alternative description is provided by the two-component (soft + hard) model (TCM) of hadron production. TCM analysis of $p$-Pb ensemble-mean-$p_t$ data reveals centrality trends quite different from those estimated via a geometric Glauber model based on the eikonal approximation. Glauber estimates of binary-collision number are three times TCM estimates. Detailed study of $p$-Pb data conflicts with a basic Glauber assumption -- that a projectile proton may interact simultaneously with multiple target nucleons. Instead, in both $p$-$p$ and $p$-A collisions, a $p$-N collision once initiated is exclusive of other possible interactions (during that collision), and within the collision any pair of participant partons may interact -- a $p$-N collision is thus "all or nothing." In this presentation the PMC is challenged by an assortment of contradictory data, and evidence for $p$-N exclusivity is reviewed to make a case for the "preoccupied proton" of the title.

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arXiv2018

Comparing the PYTHIA Monte Carlo to a two-component (soft + hard) model of hadron production in high-energy p-p collisions

The PYTHIA Monte Carlo (PMC), first introduced more than thirty years ago, remains a popular simulation tool both for analysis of $p$-$p$ collision dynamics and for detector design and calibration. The PMC assumes that almost all produced hadrons result from parton-parton scatterings (interactions) described by pQCD (a hard component), and that multiple parton interactions per collision event (MPIs) are a common occurrence. In contrast, a two-component (soft + hard) model (TCM) of high-energy collisions, inferred inductively from a variety of data formats, attributes a majority of final-state hadrons to a soft component (projectile-nucleon dissociation) and a minority to a hard component representing minimum-bias dijet production (corresponding to measured jet spectra and fragmentation functions). The hard-component hadron yield is precisely proportional to the square of the soft-component yield over an interval corresponding to 100-fold increase in dijet production. The two data descriptions appear to be in conflict. This study presents a detailed comparison of the two models and their relations to a broad array of collision data. The PMC appears to disagree with some data, whereas the TCM provides an accurate and comprehensive data description.

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arXiv2018

Exclusivity of p-N interactions within p-A collisions

A geometric Glauber model applied to p-p or p-A collisions assumes that participant partons within a projectile nucleon independently follow eikonal trajectories, just as for projectile nucleons within target nuclei, and interact independently with any target nucleons encountered along their trajectory. A noneikonal quadratic relation for dijet production observed for p-p collisions suggests that such assumptions for p-N collisions may be incorrect. Data from p-A collisions supports that conclusion. For both isolated p-p collisions and for p-N interactions within p-A collisions a proton projectile interacts with only a single target nucleon within a brief time interval. Multiple overlapping p-N collisions are inhibited, suggesting the exclusivity of p-N interactions within p-A collisions.

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arXiv2018

Glauber-model analysis of 5 TeV $\bf p$-Pb centrality compared to a two-component (soft + hard) model of hadron production in high-energy nuclear collisions

A recent study of 5 TeV $p$-Pb centrality combined a Glauber model of $p$-Pb collision geometry with an assumption of linear scaling between $n_{ch}$ (charge) integrated within some $η$ acceptance and the number of nucleon participants $N_{part}$. The study concluded that $N_{part}$ increases to nearly 16 in central collisions, and the high-$p_t$ region of $p$-Pb $p_t$ spectra rescaled by the Glauber-estimated number of $p$-N binary collisions remains consistent with a $p$-$p$ spectrum for the same energy, independent of $p$-Pb centrality. However, the relation between $N_{part}$ and $n_{ch}$ derived from a two-component (soft + hard) model (TCM) study of ensemble-mean $\bar p_t$ data for the same system is quite different. This article reports a detailed analysis of the Glauber study and the question of centrality in $p$-A collisions. The Glauber centrality model is compared with the $\bar p_t$ TCM to understand the sources of major differences. The assumption of linear proportionality between $n_{ch}$ and $N_{part}$ is found to be inconsistent with $\bar p_t$ data. Properties of the convolution integral relating a differential cross section and hadron production model to an event distribution on $n_{ch}$ are examined. An alternative differential-cross-section distribution is inferred from charge-multiplicity data, and the upper limit on $N_{part}$ is estimated to be near 8. The TCM centrality model is then applied to $p_t$ spectrum ratios to predict results for $p$-Pb spectra. The spectrum TCM is tested with identified-pion spectra from 5 TeV $p$-Pb collisions and the result is consistent with previous $p$-$p$ TCM results. A TCM prediction that the spectrum ratio at high $p_t$ should increase to 14 for central $p$-Pb collisions due to quadratic dependence of dijet production on $n_{ch}$ is consistent with $\bar p_t$ data from the same system.

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arXiv2017

Collectivity and manifestations of minimum-bias jets in high-energy nuclear collisions

Collectivity, as interpreted to mean flow of a dense medium in high-energy A-A collisions described by hydrodynamics, has been attributed to smaller collision systems -- p-A and even p-p collisions -- based on recent analysis of LHC data. However, alternative methods reveal that some data features attributed to flows are actually manifestations of minimum-bias (MB) jets. In this presentation I review the differential structure of single-particle $p_t$ spectra from SPS to LHC energies in the context of a two-component (soft + hard) model (TCM) of hadron production. I relate the spectrum hard component to measured properties of isolated jets. I use the spectrum TCM to predict accurately the systematics of ensemble-mean $\bar p_t$ in p-p, p-A and A-A collision systems over a large energy interval. Detailed comparisons of the TCM with spectrum and correlation data suggest that MB jets play a dominant role in hadron production near midrapidity. Claimed flow phenomena are better explained as jet manifestations agreeing quantitatively with measured jet properties.

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arXiv2017

Ensemble-mean $\bf p_t$ and hadron production in high-energy nuclear collisions

A two-component (soft + hard) model (TCM) of hadron production in high-energy nuclear collisions is applied to ensemble-mean $p_t$ (denoted by $\bar p_t$) data for $p$-$p$, $p$-Pb and Pb-Pb collisions from the relativistic heavy ion collider (RHIC) and large hadron collider (LHC). This $\bar p_t$ TCM is directly related to a recently-published TCM for the charge-multiplicity $n_{ch}$ and collision-energy dependence of $p_t$ spectrum data from $p$-$p$ collisions. Multiplicity dependence of the $p$-$p$ spectrum hard component observed in the previous study is consistent with similar behavior for $\bar p_t$ hard component $\bar p_{th}$. $p$-$p$ $\bar p_t$ $n_{ch}$ dependence is observed to follow a noneikonal trend for the TCM hard component (dijet production $\propto n_{ch}^2$), whereas the trend for Pb-Pb collisions is consistent with the eikonal approximation assumed for the Glauber A-A centrality model. The $p$-Pb trend is intermediate, transitioning from the noneikonal $p$-$p$ trend for more-peripheral collisions to an eikonal trend for more-central collisions. The multiplicity dependence of participant number $N_{part}$ and binary-collision number $N_{bin}$ inferred from $p$-Pb $\bar p_t$ data differs strongly from a Glauber Monte Carlo model of that system. The rapid increase with $n_{ch}$ and large magnitude of $\bar p_t$ for the $p$-$p$ and $p$-Pb systems suggests that minimum-bias jets (TCM hard component) dominate $\bar p_t$ variation. The trend for $\bar p_{th}$ in Pb-Pb collisions is consistent with quantitative modification of jet formation in more-central A-A collisions.

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arXiv2017

Charge-multiplicity and collision-energy dependence of $p_t$ spectra from $p$-$p$ collisions at the relativistic heavy-ion collider and large hadron collider

A two-component (soft + hard) model (TCM) of hadron production in yields and spectra derived from the charge-multiplicity dependence of 200 GeV p-p collisions at the relativistic heavy ion collider (RHIC) is extended to describe p-p spectrum data from the large hadron collider (LHC) up to 13 TeV. The LHC data include spectrum ratios that provide only partial information on the TCM. The LHC ratio method is applied to well-understood 200 GeV spectrum data to derive an algebraic link between spectrum ratios and the full TCM. Some aspects of the form of the hard component on transverse momentum are found to be $n_{ch}$ dependent. LHC spectrum ratios are then analyzed to obtain $n_{ch}$ and collision-energy (over three orders of magnitude) dependence of isolated soft and hard TCM spectrum components. The energy dependence of the spectrum soft component is a new result suggesting a relation to Gribov diffusion. The spectrum hard component varies simply and smoothly with $n_{ch}$ suggesting bias of the underlying jet spectrum and linearly with QCD parameter $\log(s/s_0)$, its properties consistent with minimum-bias reconstructed-jet spectrum measurements.

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arXiv2017

Manifestations of minimum-bias dijets in high-energy nuclear collisions

Dijets observed near midrapidity in high-energy nuclear collisions result from large-angle scattering of low-$x$ partons (gluons) within projectile hadrons as a signature manifestation of QCD. Within the same collisions it has been claimed that hydrodynamic flows (radial, elliptic and "higher harmonic" flows) carried by a dense QCD medium or quark-gluon plasma (QGP) dominate the observed hadronic final state. The flow-QGP narrative is imposed {\em a priori} on primary particle data, and of all possible analysis methods a subset A that seems to support that narrative is preferred. The present study explores an alternative minimum-bias (MB) jet narrative -- quantitative correspondence of MB dijet manifestations in the hadronic final state with measured {\em isolated jet} properties. The latter incorporates a different set of methods B that emerge from inductive study of primary particle data without {\em a priori} assumptions. The resulting system of methods and data manifestations is represented by a two-component (soft + hard) model (TCM) of hadron production. A survey of methods reveals that type A tends to discard substantial information carried by primary particle data whereas type B retains almost all information in both primary particle data from nuclear collisions and from isolated jets. The main goal of the present study is a review of MB dijet contributions to high-energy collisions in small and large systems relative to measured isolated-jet properties. Representative analysis methods from types A and B are compared in the context of MB jet manifestations. This study suggests that at least some data features commonly attributed to flows actually result from MB dijets and thereby challenges the flow-QGP narrative.

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arXiv2016

Distinguishing the nonjet azimuth quadrupole from QCD jets and hydrodynamic flows via 2D angular correlations and quadrupole spectrum analysis

According to the flow narrative commonly applied to high-energy nuclear collisions a 1D cylindrical-quadrupole component of 2D angular correlations conventionally denoted by quantity $v_2$ is interpreted to represent elliptic flow: azimuth modulation of transverse or radial flow in noncentral nucleus-nucleus (A-A) collisions. The nonjet (NJ) quadrupole component exhibits various properties inconsistent with a flow or hydro interpretation, including the observation that NJ-quadrupole centrality variation in $A$-$A$ collisions has no relation to strongly-varying jet modification ("jet quenching") in those collisions commonly attributed to jet interaction with a dense flowing medium. In the present study I report isolation of quadrupole spectra from $p_t$-differential $v_2(p_t)$ data obtained at the relativistic heavy ion collider (RHIC) and large hadron collider (LHCr). I demonstrate that NJ quadrupole spectra have characteristics very different from the single-particle spectra for most hadrons, that quadrupole spectra indicate a common boosted hadron source for a small minority of hadrons that "carry" the quadrupole structure, that the narrow source-boost distribution is characteristic of an expanding thin cylindrical shell (also strongly contradicting a hydro interpretation), and that in the boost frame a single universal quadrupole spectrum (Lévy distribution) on transverse mass $m_t$ accurately describes data for several hadron species scaled according to their statistical-model abundances. The quadrupole spectrum shape changes very little from RHIC to LHC energies. Taken in combination those characteristics strongly suggest a unique {\em nonflow} (and nonjet) QCD mechanism for the NJ quadrupole conventionally represented by $v_2$.

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arXiv2016

Rescuing the nonjet (NJ) azimuth quadrupole from the flow narrative

According to the flow narrative commonly applied to high-energy nuclear collisions a cylindrical-quadrupole component of 1D azimuth angular correlations is conventionally denoted by quantity $v_2$ and interpreted to represent elliptic flow. Jet angular correlations may also contribute to $v_2$ data as "nonflow" depending on the method used to calculate $v_2$, but 2D graphical methods are available to insure accurate separation. The nonjet (NJ) quadrupole has various properties inconsistent with a flow interpretation, including the observation that NJ quadrupole centrality variation in A-A collisions has no relation to strongly-varying jet modification ("jet quenching") in those collisions commonly attributed to jet interaction with a flowing dense medium. In this presentation I describe isolation of quadrupole spectra from pt-differential $v_2(p_t)$ data from the RHIC and LHC. I demonstrate that quadrupole spectra have characteristics very different from the single-particle spectra for most hadrons, that quadrupole spectra indicate a common boosted hadron source for a small minority of hadrons that "carry" the NJ quadrupole structure, that the narrow source-boost distribution is characteristic of an expanding thin cylindrical shell (strongly contradicting hydro descriptions), and that in the boost frame a single universal quadrupole spectrum (Lévy distribution) on transverse mass $m_t$ accurately describes data for several hadron species scaled according to their statistical-model abundances. The quadrupole spectrum shape changes very little from RHIC to LHC energies. Taken in combination those characteristics strongly suggest a unique {\em nonflow} (and nonjet) QCD mechanism for the NJ quadrupole conventionally represented by $v_2$.

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arXiv2015

Optimal modeling of 1D azimuth correlations in the context of Bayesian inference

Analysis and interpretation of spectrum and correlation data from high-energy nuclear collisions is currently controversial because two opposing physics narratives derive contradictory implications from the same data---one narrative claiming collision dynamics is dominated by dijet production and projectile-nucleon fragmentation, the other claiming collision dynamics is dominated by a dense, flowing QCD medium. Opposing interpretations seem to be supported by alternative data models, and current model-comparison schemes are unable to distinguish between them. There is clearly need for a convincing new methodology to break the deadlock. In this study we introduce Bayesian Inference (BI) methods applied to angular correlation data as a basis to evaluate competing data models. For simplicity the data considered are projections of 2D angular correlations onto 1D azimuth from three centrality classes of 200 GeV AuAu collisions. We consider several data models typical of current model choices, including Fourier series (FS) and a Gaussian plus various combinations of individual cosine components. We evaluate model performance with BI methods and with power-spectrum (PS) analysis. We find that the FS-only model is rejected in all cases by Bayesian analysis which always prefers a Gaussian. A cylindrical quadrupole cos(2ϕ) is required in some cases but rejected for 0-5%-central AuAu collisions. Given a Gaussian centered at the azimuth origin "higher harmonics" cos(mϕ) for m > 2 are rejected. A model consisting of Gaussian + dipole cos(ϕ) + quadrupole cos(2ϕ) provides good 1D data descriptions in all cases.

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