# Generalized hydrodynamics regime from the thermodynamic bootstrap program

### Submission summary

 As Contributors: Milosz Panfil Arxiv Link: https://arxiv.org/abs/1909.08393v2 Date submitted: 2019-10-03 Submitted by: Panfil, Milosz Submitted to: SciPost Physics Discipline: Physics Subject area: Condensed Matter Physics - Theory Approach: Theoretical

### Abstract

Within the generalized hydrodynamics (GHD) formalism for quantum integrable models, it is possible to compute simple expressions for a number of correlation functions at the Eulerian scale. Specializing to integrable relativistic field theories, we show the same correlators can be computed as a sum over form factors, the GHD regime corresponding to the leading contribution with one particle-hole pair on a finite energy-density background. The thermodynamic bootstrap program (TBP) formalism was recently introduced as an axiomatic approach to computing such finite-energy-density form factors for integrable field theories. We derive a new axiom within the TBP formalism from which we easily recover the predicted GHD Eulerian correlators. We also compute higher form factor contributions, with more particle-hole pairs, within the TBP, allowing for the computation of correlation functions in the diffusive, and beyond, GHD regimes. The two particle-hole form factors agree with expressions recently conjectured within the~GHD.

###### Current status:
Editor-in-charge assigned

### Submission & Refereeing History

Submission 1909.08393v2 on 3 October 2019

## Reports on this Submission

### Strengths

1- the paper extends the axioms of thermodynamic bootstrap program formulated in the original reference [1] providing a systematic derivation of the one particle-hole form factor
2- the extended thermodynamic bootstrap program is validated by comparison with existing generalized hydrodynamics (GHD), both for correlation functions (1 particle-hole form factor) and for the diffusion matrix (2 particle-hole form factors)
3- a general method which leads to arbitrary particle hole form factors is given, which could be useful to write down a systematic gradient expansion beyond GHD

### Weaknesses

1- there is no check (even numerical) of all predictions beyond the state-of-the-art

### Report

This paper provides further results and confirmations about the thermodynamic bootstrap program, introduced in a recent publication by the same authors. This program aims at giving the minimal set of axioms needed to reconstruct the explicit formulas of form factors of a few excitations on top of a thermodynamic GGE state. This is both a relevant and ambitious direction and I found remarkable that so far, all the checks have been passed.
In particular, in this work, it is showed that the single particle-hole form factor at small momentum does indeed reproduce the known result from generalised hydrodynamcs. Additionally, the 2 particle-hole form factor is consistent with the already known diffusion matrix.

Overall, the paper is well-written, although being this an "axiomatic program", it is important to clarify which hypothesis are done for each derivations. I would recommend its publication once my comments have been addressed.

### Requested changes

1- the use of the subscript $\vartheta$ in Eq. (5,7,8) is not clear at that stage of the paper. It is only clear later on, but this should be stated explicitly in the text.
2- After Eq. (27), the expression "dressed form" factors is used. I found it confusing because it is not immediately clear if it is the same quantity introduced in Eq. (24) (which is however defined simply as "form factor") or it involves some further dressing operation.
3- After Eq. (57) is derived, the authors say that it can be added as an additional "axiom" to the bootstrap program. I think it would be useful if the logic and terminology were explained more clearly. Indeed, after the axioms (28-33) have been introduced, one would expect that the further derivations are a consequence of these axioms. But then, how can a new axiom be derived? Is it truly independent on the other ones (as they have been used in its derivation)? Could the authors clarify better the problem with the prefactor?
4- Eq. (59) involves a limit where $\kappa_1, \kappa_2 \to 0$ but the right-hand side depends clearly on the way the limit in two variables is approached. As far as I can see, here the idea is that rather than taking really a limit, the authors are analysing the structure of singularities which allows them to derive Eq. (62). Although these calculations and the notation can be standard within this literature, I think it would be useful to be more explicit. The same questions can be raised about Eq. (64).
5- It is stated that at large $t$, only one particle hole matters n Eq. (66). This should be motivated more clearly, perhaps estimating what is the correction due to a larger number of particle-holes.

• validity: high
• significance: top
• originality: high
• clarity: good
• formatting: excellent
• grammar: excellent

### Strengths

1- interesting and timely results and good calculations

### Report

In this paper, the authors further develop the thermodynamic bootstrap program which they have introduced recently. They evaluate exactly the single particle-hole pair form factor at low momentum in terms of a Leclair-Mussardo series. This, in turn, using bootstrap, allow them to evaluate other form factors where momenta of particles and holes coincide or are near to each other. They show that for two particle-hole pairs, this reproduces the conjecture of [30] that led to the calculation of hydrodynamic diffusion in integrable models. Further, they evaluate the form factor expansion to two-point function in the large-space-time limit and show that their framework reproduces the formula proposed in [13] from hydrodynamic projections / linear response theory.

I think this paper is very interesting, both for showing that the framework proposed in [1] is consistent and powerful, and for establishing results that were conjectured, and results that were obtained from hydrodynamic principles instead of microscopic calculations. The paper is well written, and should be published.

Besides minor comments (see below), I have only two comments which I'd like the authors to address; both should be simple to address, the first involves a bit more calculations but I think it is important to address it in order to clarify the situation.

1- Section 6: the authors mention that the higher-particle form factors do not contribute to the correlator. It would be nice if they could provide a more complete argument for this. In particular, is it indeed {\em not necessary} to perform Euler-scale fluid-cell averages in order to have the correct answer? Might higher-particle terms contribute non-vanishing oscillating terms which would go away just under averaging? If not, it would be good that the authors give a full argument, as the result, as stated, eq 65 and 73, is stronger than that predicted in ref [13], and this would be very helpful as it would clarify some Euler-scale subtleties.

2- Comment end of section 4: this is an interesting comment, which, if I understood well, was something discussed already in [B. Doyon, Finite-temperature form factors: a review, SIGMA 3, 011 (2007)], see section 4.4. There, a connection was discussed in the Ising (free fermion) case, where it is understood that analytic continuation changes the quantisation scheme between "on the circle" and "at finite temperature".

p5: also mention [B. Doyon and H. Spohn, Drude weight for the Lieb-Liniger Bose gas, SciPost Phys. 3, 039 (2017)]

p6: the logic was more that the form factor structure was assumed, then diffusion matrix found and checked in various ways.

P 14 : epsilon is not defined I think

Ref 43 names

eqs 65, 73: maybe define not as a limit (which is zero because of the $1/t$) but as an asymptotic, or alternatively define as a limit after multiplying by $t$

### Requested changes

• validity: high
• significance: high
• originality: high
• clarity: high
• formatting: perfect
• grammar: excellent

### Strengths

1- Original computation, based on the previous work by the same authors, which might open the way for many developments in the field of out-of-equilibrium integrable QFT.

2- The paper is clear and well-written.

3- Recent results from Generalized Hydrodynamics are reproduced with success.

### Weaknesses

1- A "modified LeClair-Mussardo" formula (25) is used, with a certain regularization scheme for the form factors singularities, however the results presented here at Euler scale do not allow to test the effect of this regularization scheme.

2-The computation still relies on some usual GHD assumptions, namely the vanishing of contribution from higher-order form factors at Euler scale or for diffusive contributions respectively (as opposed to a finite contribution resulting from the re-summation of all terms). While such assumptions are reasonable, they prevent the present results from being a completely independent check of GHD.

3- Lacks any form of numerical check.

### Report

The authors expand on their previous construction, the thermodynamic bootstrap program (TBP, ref. [1]), to compute the correlation functions of local operators at the Euler scale (which holds where time and space are commensurately sent to infinity) in the case where such theories have inhomogeneous density/energy/charge profiles, a regime nowadays commonly described by the generalized hydrodynamics (GHD).
In contrast with the traditional form factor bootstrap, the TBP formulates axioms for the form factors of local operators over finite energy backgrounds, typically finite-density eigenstates representing finite temperature or generalized Gibbs ensembles (GGE). In the inhomogeneous setting, such finite energy backgrounds vary continuously with space-time.
The Euler scale correlators are expressed from the TBP form factors through an adaptation of the Leclair-Mussardo series (eq. (25) in the paper), and the leading contribution (single particle-hole pairs, two particle-hole pairs and more) are therefore computed. Only the former contribute at Euler scale, and allow comparison with independent GHD results. Further contribution, in turn, are argued to describe diffusive corrections, and more generally a full gradient expansion of the hydrodynamic equations.

This is an overall very good and original paper, which I recommend for publication in SciPost once the following minor points have been addressed.

### Requested changes

1- As pointed in the "Weaknesses" section, the notion of representative state is introduced in p. 10, but used already much earlier in the paper. In fact, in eq. (5) the authors use the notation $\langle \ldots \rangle_{\vartheta}$ (instead of the previously used $\langle \ldots \rangle_{\rm GGE}$ with no further explanation. This should be clarified a bit.

2- Perhaps a naive question : in the form factor axioms, in particular eq. (28), one may have expected that the scattering factor $S()$ would need to be replaced by the dressed scattering phase. Could the authors clarify this point ? (not necessarily in the paper if there is a clear justification for that)

3- Misprint on p. 13, 3rd line : "rapidties" instead of "rapidities"

4- Reference [43] lacks an author

• validity: high
• significance: high
• originality: high
• clarity: top
• formatting: excellent
• grammar: excellent