REVIEW
Event-by-Event Fluctuations in Heavy Ion Collisions and the QCD Critical Point
High Energy Physics - Phenomenology · Submitted 9 Mar 1999 · 34 references
Pith's verdict: The analytic core is solid and the NA49 comparison is honest; the 'easily observable' claim leans on a correlation length the authors' own finite-time estimate does not support.
T0 · AI review
Why Pith says this
Highlight any text to comment on it.
Referee report
The paper
The paper claims
The paper argues that event-by-event fluctuations of charged-pion multiplicity and mean transverse momentum should show a nonmonotonic bump when heavy-ion collisions freeze out near the QCD critical endpoint, and that the bump is large en
The reading
What carries the argument
The argument is carried by the microscopic correlator $langleDelta n_pDelta n_krangle$, the covariance between fluctuations of pion occupation numbers in momentum modes $p$ and $k$. In an ideal Bose gas this is diagonal, $v_p^2delta_{pk}$ with $v_p^2=langle n_prangle(1+langle n_prangle)$; resonance decays add small off-diagonal terms; thermal contact with a finite heat capacity $C_A$ adds a negative off-diagonal term from energy conservation; and $\sigma$ exchange adds the critical term $frac{1}{V m_sigma^2}G^2T,frac{v_p^2}{omega_p}frac{v_k^2}{omega_k}$. Once this correlator is known, any observable built from pion occupation numbers follows by weighting with the single-particle quantity and dividing by $langle Nrangle^2$. The finite-size and finite-time discussion then replaces the divergent $1/m_sigma^2$ by a capped correlation length $xi_{max}$, using scaling arguments $xi_{max}sim R$ and $xi_{max}simtau^{1/Delta_t}$.
Extended reading notes
Core claim
The central claim is that the QCD critical endpoint leaves a measurable imprint on the pions already observed in heavy-ion collisions. Near the endpoint the $\sigma$ field becomes light and its long-wavelength fluctuations grow; through the $Gsigma pi pi$ coupling these fluctuations add a term to the pion occupation-number correlator proportional to $G^2 T/m_sigma^2$, which diverges as $m_sigma to 0$. The paper turns this into estimates for the scaled event-by-event variance of mean transverse momentum, $F_sigma=1+0.35(G/300,mathrm{MeV})^2(xi/6,mathrm{fm})^2$ at $mu_pi=60,mathrm{MeV}$, and for the multiplicity variance, with both growing near the endpoint. It also argues that the noncritical baseline—Bose enhancement, resonance decays, flow fluctuations, and energy-conservation anticorrelations—is broadly consistent with NA49 data, so that a nonmonotonic departure from that baseline would be evidence for critical freeze-out.
Load-bearing premise
The load-bearing premise is that freeze-out occurs from an equilibrated hadronic system close enough to the critical endpoint that the $\sigma$ correlation length reaches several femtometers, because the predicted enhancement scales as $xi^2$ and a correlation length of only about $2,mathrm{fm}$ would shrink the $p_T$ signal to roughly the percent level.
Editorial extensions
If this is right
- As collision energy, centrality, or ion size is varied, the scaled $p_T$ variance and the multiplicity variance should first rise, peak at the same setting, and then fall.
- The critical increase should be strongest for soft pions, so a low-$p_T$ cut should amplify the nonmonotonic signal relative to the full-acceptance variance.
- The cross correlation $langleDelta NDelta p_Trangle$, normally small, should grow in magnitude near the endpoint because it receives a large sigma-exchange contribution.
- If freeze-out is near the endpoint, a nonthermal population of pions with mean $p_Tsim m_pi/2$ should appear from late sigma decays and then disappear as the endpoint is passed.
- Off-diagonal correlations between $pi^+$ and $pi^-$ fluctuations should grow near the endpoint, since the diagonal Bose term is absent in that channel.
Reading between the lines
- A null result in an energy scan would not by itself rule out the endpoint; it could mean the correlation length stayed below the finite-time cap, so the measurement would bound the combination $Gxi$ rather than the existence of the endpoint.
- Because the predicted signal depends quadratically on $xi$, an independent interferometric or fluctuation-based estimate of the homogeneous patch size at freeze-out would sharpen the test considerably.
- The same correlator logic suggests that a differential scan in pion momentum, rather than a single acceptance-averaged number, would separate the sigma-exchange term from the Bose and resonance backgrounds.
- If the soft-pion excess from late sigma decays is seen, its angular and charge correlations could distinguish a critical source from ordinary resonance decays.
Original abstract
The event-by-event fluctuations of suitably chosen observables in heavy ion collisions at SPS, RHIC and LHC can tell us about the thermodynamic properties of the hadronic system at freeze-out. By studying these fluctuations as a function of varying control parameters, it is possible to learn much about the phase diagram of QCD. As a timely example, we stress the methods by which present experiments at the CERN SPS can locate the second-order critical endpoint of the first-order transition between quark-gluon plasma and hadron matter. Those event-by-event signatures which are characteristic of freeze-out in the vicinity of the critical point will exhibit nonmonotonic dependence on control parameters. We focus on observables constructed from the multiplicity and transverse momenta of charged pions. We first consider how the event-by-event fluctuations of such observables are affected by Bose-Einstein correlations, by resonances which decay after freeze-out and by fluctuations in the transverse flow velocity. We compare our thermodynamic predictions for such noncritical event-by-event fluctuations with NA49 data, finding broad agreement. We then focus on effects due to thermal contact between the observed pions and a heat bath with a given (possibly singular) specific heat, and due to the direct coupling between the critical fluctuations of the sigma field and the observed pions. We also discuss the effect of the pions produced in the decay of sigma particles just above threshold after freeze-out on the inclusive pion spectrum and on multiplicity fluctuations. We estimate the size of these nonmonotonic effects which appear near the critical point, including restrictions imposed by finite size and finite time, and conclude that they should be easily observable.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The event-by-event fluctuations of suitably chosen observables in heavy ion collisions at SPS, RHIC and LHC can tell us about the thermodynamic properties of the hadronic system at freeze-out. By studying these fluctuations as a function of varying control parameters, it is possible to learn much about the phase diagram of QCD. As a timely example, we stress the methods by which present experiments at the CERN SPS can locate the second-order critical endpoint of the first-order transition between quark-gluon plasma and hadron matter. Those event-by-event signatures which are characteristic of freeze-out in the vicinity of the critical point will exhibit nonmonotonic dependence on control parameters. We focus on observables constructed from the multiplicity and transverse momenta of charged pions. We first consider how the event-by-event fluctuations of such observables are affected by Bose-Einstein correlations, by resonances which decay after freeze-out and by fluctuations in the transverse flow velocity. We compare our thermodynamic predictions for such noncritical event-by-event fluctuations with NA49 data, finding broad agreement. We then focus on effects due to thermal contact between the observed pions and a heat bath with a given (possibly singular) specific heat, and due to the direct coupling between the critical fluctuations of the sigma field and the observed pions. We also discuss the effect of the pions produced in the decay of sigma particles just above threshold after freeze-out on the inclusive pion spectrum and on multiplicity fluctuations. We estimate the size of these nonmonotonic effects which appear near the critical point, including restrictions imposed by finite size and finite time, and conclude that they should be easily observable.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The event-by-event fluctuations of suitably chosen observables in heavy ion collisions at SPS, RHIC and LHC can tell us about the thermodynamic properties of the hadronic system at freeze-out. | background 0.97 | 0.10 | 0.10 | hedged possibility 0.80 |
| s1 | By studying these fluctuations as a function of varying control parameters, it is possible to learn much about the phase diagram of QCD. | background 0.87 | 0.08 | 0.07 | hedged possibility 0.83 |
| s2 | As a timely example, we stress the methods by which present experiments at the CERN SPS can locate the second-order critical endpoint of the first-order transition between quark-gluon plasma and hadron matter. | contribution 0.68 | 0.25 | 0.14 | asserted 0.52 |
| s3 | Those event-by-event signatures which are characteristic of freeze-out in the vicinity of the critical point will exhibit nonmonotonic dependence on control parameters. | result 0.35 | 0.16 | 0.30 | asserted 0.94 |
| s4 | We focus on observables constructed from the multiplicity and transverse momenta of charged pions. | method 0.63 | 0.68 | 0.03 | asserted 0.87 |
| s5 | We first consider how the event-by-event fluctuations of such observables are affected by Bose-Einstein correlations, by resonances which decay after freeze-out and by fluctuations in the transverse flow velocity. | method 0.71 | 0.67 | 0.03 | asserted 0.67 |
| s6 | We compare our thermodynamic predictions for such noncritical event-by-event fluctuations with NA49 data, finding broad agreement. | result 0.98 | 0.93 | 0.07 | asserted 0.99 |
| s7 | We then focus on effects due to thermal contact between the observed pions and a heat bath with a given (possibly singular) specific heat, and due to the direct coupling between the critical fluctuations of the sigma field and the observed pions. | contribution 0.54 | 0.62 | 0.04 | asserted 0.52 |
| s8 | We also discuss the effect of the pions produced in the decay of sigma particles just above threshold after freeze-out on the inclusive pion spectrum and on multiplicity fluctuations. | contribution 0.88 | 0.61 | 0.04 | asserted 0.85 |
| s9 | We estimate the size of these nonmonotonic effects which appear near the critical point, including restrictions imposed by finite size and finite time, and conclude that they should be easily observable. | result 0.81 | 0.91 | 0.76 | asserted 0.69 |
What was examined, and what was not
- arXiv listing abstract v2
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:hep-ph/9903292 by the authors.
References
How later papers used it
220 later papers cite it
1 later paper by other authors is listed below: 1 used its data or values. 1 more paper by the same authors is listed: 1 used its data or values.
Uses its data or values 2 of 5 papers with this label · 1 by the same authors
-
Took a coupling G = 300 MeV from this paper, where its value near the critical point is extracted from considerations using a Ginzburg-Landau effective potential and the linear sigma model.
Critical versus spurious fluctuations in the search for the QCD critical point (2015) · 1.00 · paraphrased: the licence does not allow quoting
By the same authors
-
Used G = 300 MeV from this paper as a benchmark value throughout its work.
Ratios of Fluctuation Observables in the Search for the QCD Critical Point (2010) (same authors) · 0.99 · paraphrased: the licence does not allow quoting
How the literature uses it
186 arXiv papers cite this in 301 sentences
Papers citing it · 214 sentences without a confident label
Contests it 1 paper
-
The approach in Refs. [ref] leading to Eq. ([ref]) leaves not only the magnitude, but also the sign of $g_A$ undetermined.
Maximum entropy freezeout of hydrodynamic fluctuations · Comparison with existing methods · contest 0.81
Builds on it 1 paper
-
Given the importance of the baryon number fluctuations in the phenomenology of CEP searching in the experiments [ref] , in this work we would like to extend relevant studies in Refs. [ref] from two to 2+1 flavors, based on the 2+1 flavor Polyakov-loop improved quark-meson (PQM) effective model, see, e.g. [ref] for more details about the model, and Refs. [ref] for other relevant 2+1 flavor low energy effective models of the same class.
Baryon number fluctuations in the 2+1 flavor low energy effective model · Introduction · 0.97
Uses its method or tool 6 papers
-
This contribution can be found using an intuitive picture described in Ref. [ref] .
Non-Gaussian fluctuations near the QCD critical point · Introduction · 0.90
-
We use the same effective theory as Refs. [ref] .
Critical versus spurious fluctuations in the search for the QCD critical point · Effective theory · 0.99
-
The contribution of critical fluctuations to the 2-, 3- and 4- particle correlators due to $σ$-exchanges can be calculated using the diagrammatic method developed in Ref. [ref] (see also Refs. [ref] and [ref] ).
Using Higher Moments of Fluctuations and their Ratios in the Search for the QCD Critical Point · Critical point contribution to correlators · 1.00
Uses its data or values 5 papers
-
A coupling $G = 300$ MeV is taken from Ref. [ref] , where its value near the critical point is extracted from considerations using a Ginzburg-Landau effective potential and the linear sigma model.
Critical versus spurious fluctuations in the search for the QCD critical point · Results · 1.00
-
This constant is known only roughly – the estimate was made in Ref. [ref] .
Non-Gaussian fluctuations near the QCD critical point · Introduction · 0.95
-
Throughout this paper we will use $G = 300$ MeV (see Ref. [ref] ) and $g_p=7$ (see, e.g., [ref] ) as benchmark values.
Ratios of Fluctuation Observables in the Search for the QCD Critical Point · Cumulant calculations · 0.99
Compares with it 1 paper
-
This data is compared to the effect of the critical point as discussed in [ref] and at this workshop [ref] .
Event-by-Event Fluctuations and the Search for the Critical Point within the NA49 Experiment · Fluctuations at the Critical Point · 0.94
Cites it as support 2 papers
-
However, a more stringent constraint comes from the fact that the system does not spend enough time in the critical region for the correlation length to build up to its equilibrium value [ref] .
Mapping the Phases of Quantum Chromodynamics with Beam Energy Scan · Finite time dynamics and limitations on the correlation length in heavy-ion collisions · 0.98
-
The critical slowing down makes it increasingly important to consider non-equilibrium evolution of fluctuations near the critical point, as earlier estimates and model calculations demonstrate [ref] .
Non-Gaussian fluctuation dynamics in relativistic fluids · Introduction · 0.98
Cites it as background 60 papers
-
Whether the analytic crossover turns into a first-order phase transition at finite baryon density at a QCD critical point (CP) remains an open question studied both theoretically, and experimentally through heavy-ion collisions [ref] .
Chemical freeze-out curve in heavy-ion collisions and the QCD critical point · Introduction · 0.91
-
The standard picture [ref] assumes the crossover line for small $μ$ meets a line of first order transitions at a critical endpoint as $μ$ increases and $T$ decreases.
Fluctuations in cool quark matter and the phase diagram of Quantum Chromodynamics · Relation to QCD · 0.98
-
Experimentally, the QCD phase structure can be probed through event-by-event fluctuations of the net-proton number measured in relativistic heavy-ion collisions [ref] , most notably in the RHIC Beam Energy Scan (BES) program, which covers baryochemical potentials up to in collider mode [ref] .
Extraction of baryon number susceptibilities at finite density from heavy-ion collisions · Introduction · 0.81
Discussion
Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.