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A massive variable flavour number scheme for the DrellYan process
by R. Gauld
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Submission summary
Authors (as registered SciPost users):  Rhorry Gauld 
Submission information  

Preprint Link:  scipost_202107_00031v1 (pdf) 
Date submitted:  20210720 10:29 
Submitted by:  Gauld, Rhorry 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Phenomenological 
Abstract
The prediction of differential crosssections in hadronhadron scattering processes is typically performed in a scheme where the heavyflavour quarks ($c, b, t$) are treated either as massless or massive partons. In this work, a method to describe the production of coloursinglet processes which combines these two approaches is presented. The core idea is that the contribution from power corrections involving the heavyquark mass can be numerically isolated from the rest of the massive computation. These power corrections can then be combined with a massless computation (where they are absent), enabling the construction of differential crosssection predictions in a massive variable flavour number scheme. As an example, the procedure is applied to the lowmass DrellYan process within the LHCb fiducial region, where predictions for the rapidity and transversemomentum distributions of the lepton pair are provided. To validate the procedure, it is shown how the $n_f$dependent coefficient of a massless computation can be recovered from the massless limit of the massive one. This feature is also used to differentially extract the massless ${\rm N3LO}$ coefficient of the DrellYan process in the gluonfusion channel.
Current status:
Reports on this Submission
Anonymous Report 1 on 2021818 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202107_00031v1, delivered 20210818, doi: 10.21468/SciPost.Report.3406
Report
The paper contains the description of a new method to combine
fixedorder (FO) computations performed considering the heavyflavour
quarks as massless or massive. In particular, the contributions from
power suppressed terms of the form (m_q/Q)^k (where m_q is the mass of
the heavy flavour and Q the typical hard scale of the process) are
consistently combined with a massless computation, thereby obtaining
differential distributions in a "massive variable flavour number"
scheme. Such scheme allows to obtain predictions where collinear logs
(log(m_q)) are resummed to all orders and exact (m_q)^k dependence is
kept to the FO accuracy.
Apart from the proposed "massive variable flavour number" scheme, the
paper also discusses how, using the formalism, one can numerically
extract unknown results for the gginduced DY differential cross
section at order as^3.
Although I cannot claim to be a real expert on the specific topic
addressed by the author and the associated literature (i.e. I have
never specifically worked on the details and subtleties related to the
treatment of heavy flavours in the initial state and the combination
of massless with massive flavour schemes), I believe the paper
contains original results.
Such results are timely too, in the sense that, as also remarked by
the author, the issues discussed in the paper are likely to become
more and more relevant in the years to come, in the context of
precision phenomenology at the LHC.
As far as the presentation is concerned, I've found the paper
wellwritten and clear. Similarities with previous works are properly
commented, and technical points are discussed when needed, giving
details at a level that I find appropriate.
For the above reasons, I recommend the paper for publication in
SciPost Physics, after the following comments are addressed by the
author.
1. "Massive computation, dsigma^M" (sec. 2, page 3): isn't there also
a massdependent contribution at order as^2 coming from "realvirtual"
terms for the process pp>Z+1jet (gq >Zq), where mass terms might
arise from closed massive fermionic loops ? These terms are finite (no
UV or IR divergent) and cancel for the vectorial coupling of the Z
boson to the massive fermionic line, but for the axialvector coupling
the cancellation is exact only for doublets degenerate in mass
(e.g. (u,d) for m_u=m_d=0), whereas a leftover scaling as (m_i)^2 
(m_j)^2 is expected to remain, m_i and m_j being the masses of the
quarks belonging to the same SU(2)_L doublet (this is discussed in
several places; see, for instance,
Radiative Corrections to the Ratio of Z and W Boson Production
Dicus, Willenbrock
Phys.Rev.D 34 (1986) 148
).
Can the author add a comment on how these terms are included (I
suppose that, for consistency, they are included) ?
2. "Logarithmic computation" (sec. 2, end of page 4): "...required to
reconstruct the zeromass limit of the massive computation". Can the
author be more clear about this point? I understand it as follows:
 there are terms without logs, without mass dependence, and not
arising from nftype contributions (i.e. terms that don't belong to
a_{0,0} in eq. 7)
 these terms are there also in the full massive computations
 eq. 3 allows one to build them and, by consequence, in the paper,
such terms live in dsigma^{ln(m)}.
Is this correct, or not? Perhaps adding a further sentence to clarify
this point could be useful.
3. page 7: the sentence where the author describes how PDF uncertainty
have been computed is not very clear, i.e. I don't understand,
operationally, what has been done. Does the author mean that, for a
given replica, dsigma_replica = dsigma_0 * K, where dsigma_0 is the
prediction at order as^2 for the central set, and K would be instead
disgmatilde_replica/dsigmatilde_0, where both the disgmatilde's have
been computed at order as?
4. eq. 7. I guess there's a typo: m^(2i) > (m^2)^i ?
On a related note, can the author comment on why one starts with
(m^2)^i instead of m^i ?
Always on the same equation: up to which value i and j are allowed to
run in the fit ?
I understand that, physically, what matters is the log(m)
dependence. However, I assume that, also in practice, a given
dimensional scale mu must have been used to fit the coefficients,
i.e. that, in practice, the author used log(m/mu). Is this the case ?
If so, which value of mu has been used ?
Requested changes
See report for points to be addressed.
Author: Rhorry Gauld on 20210826 [id 1709]
(in reply to Report 1 on 20210818)1) How are the finite corrections from virtual heavyquark contributions proportional to the axialvector treated?
These contributions have been included in my calculation (Refs. [9294] of the original manuscript). This includes both doublevirtual and realvirtual contributions, for all subprocesses. Practically, I define the massive power corrections per eachquark flavour as the difference between the contribution when the calculated with either a massive or massless quark (when fixing the mass of the other quark in the doublet). Numerically these contributions were extremely small for the results shown in this paper (which are far below mll ̄ << mZ, and are therefore strongly power suppressed). For the referees interest, in the region of mll ̄ ∼ mZ these contributions are at the permille level for the bquark with mb = 4.75 GeV.
I suggest to extend the citations to include reference to the work of Dicus and Willenbrock noted by theReferee. Practically, I recalculated the realvirtual contributions myself and used the doublevirtual results from the literature I had previously cited. I have now explicitly noted the inclusion of these (axialvector) contributions in the manuscript.
2) Clarification on the ‘constant’ terms which are built by eq. (3).
The understanding of the Referee is correct, and this is certainly a tricky point. I have defined a0,0 in such a way that it is constructed from inputs that are defined in the renormalisation scheme (for PDFs and αs) where the heavyflavour quark is massive (then built with the massless partonic crosssection). As such, these extra constant terms (i.e. the mindependent terms at the crosssection level) are absent from a0,0. However, they are part of the massive crosssection and must be constructed somehow (i.e. through knowledge of these decoupling relations for the PDFs, αs [and m its self at higher orders] computed by others in the past).
3) Definition of the PDF uncertainties.
This (the one quoted by the referee) is the definition that has been used throughout. Including a formula in the text would be appropriate to clarify this
4) The numerical fit and power corrections.
In the fit, j may run up to n where n is the order of the coefficient (i.e. 2 for αs2 and 3 for αs3). This is guided by the fact that there can be maximally these powers of collinear logarithms present (which I believe should dictates the possible powers of j present). The integer i was allowed to run up to 2, as it was found increasing this value made little difference on the fit (i.e. these higherpower terms were largely suppressed).
The ln[μ] dependence (which is certainly there, and is required from dimensional grounds as noted by the Referee) is absorbed into the constant coefficients of the fit. e.g. b_{0,1} ln[m/μ] → a_{0,1} ln[m] + a_{0,0}. I have used a dynamical scale choice μ0 = ET,ll ̄ in the numerical computation, and so practically I see this as the most obvious way to perform the fit.
Finally, concerning the choice of linear vs quadratic powercorrections in the fit. I have used (m2)i (and have corrected the typo, thank you) for the form of the power corrections, which is based on the behaviour of the squared matrixelement and phase space corrections which both contain corrections of the m2/Q2.