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Elastic photon-initiated production at the LHC: the role of hadron-hadron interactions
by L. A. Harland-Lang, V. A. Khoze, M. G. Ryskin
This is not the latest submitted version.
This Submission thread is now published as SciPost Phys. 11, 064 (2021)
|As Contributors:||Lucian Harland-Lang|
|Arxiv Link:||https://arxiv.org/abs/2104.13392v1 (pdf)|
|Date submitted:||2021-05-07 10:39|
|Submitted by:||Harland-Lang, Lucian|
|Submitted to:||SciPost Physics|
We analyse in detail the role of additional hadron-hadron interactions in elastic photon-initiated (PI) production at the LHC, both in $pp$ and heavy ion collisions. We first demonstrate that the source of difference between our predictions and other results in the literature for PI muon pair production is dominantly due to an unphysical cut that is imposed in these latter results on the dimuon-hadron impact parameter. We in addition show that this is experimentally disfavoured by the shape of the muon kinematic distributions measured by ATLAS in ultraperipheral PbPb collisions. We then consider the theoretical uncertainty due to the survival probability for no additional hadron-hadron interactions, and in particular the role this may play in the tendency for the predicted cross sections to lie somewhat above ATLAS data on PI muon pair production, in both $pp$ and PbPb collisions. This difference is relatively mild, at the $\sim 10\%$ level, and hence a very good control over the theory is clearly required. We show that this uncertainty is very small, and it is only by taking very extreme and rather unphysical variations in the modelling of the survival factor that this tension can be removed. This underlines the basic, rather model independent, point that a significant fraction of elastic PI scattering occurs for hadron-hadron impact parameters that are simply outside the range of QCD interactions, and hence this sets a lower bound on the survival factor in any physically reasonable approach. Finally, other possible origins for this discrepancy are discussed.
Submission & Refereeing History
Published as SciPost Phys. 11, 064 (2021)
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- Report 3 submitted on 2021-07-03 17:14 by Anonymous
- Report 2 submitted on 2021-06-06 20:51 by Prof. Cudell
- Report 1 submitted on 2021-05-21 13:42 by Anonymous
Reports on this Submission
Anonymous Report 3 on 2021-7-3 (Invited Report)
- Cite as: Anonymous, Report on arXiv:2104.13392v1, delivered 2021-07-03, doi: 10.21468/SciPost.Report.3176
This paper describes the role of hadron-hadron interactions in inelastic photon-initiated processes ay the LHC.
In general, the paper is well written, detailed and describes new results. It definitely deserves publication once the few questions that I have are answered.
The authors discuss the impact of an impact parameter cut imposed on photon-induced processes in the Monte Calo when compared with the ATLAS measurement. In Fig. 1, I did not fully understand why the blue curve goes above the red one at high Q^2 when one imposes the impact parameter cut. In Table 1, I am wondering if the authors could comment about potential cuts at 2R_A, 3R_A, 4 R_A since the difference between 29.9 and 38.9 is quite large. Do Starlight and Superchic without survival probability also lead to the same results?
On page 9, it might be good to add a plot showing the dependence on the impact parameter cut or redo Fig.1 for different impact parameter cuts. I think this would illustrate better the discussion. Which parameter cut would then describe the ATLAS data best?
I am also wondering if the survival effect is indeed in “competition” with the impact parameter cut. It means that the survival probability will depend on the chosen impact parameter cut. In the same line, is it possible to use an impact parameter cut that is not a sharp cut, but some cuts dependent on kinematics (like the survival probability that depends on kinematics)?
There was also a recent observation by CMS that there is no further suppression of jet get jet events due to survival probabilities between 7 and 13 TeV, contrary to expectations. I am wondering of the situation is not even more complicated, and related to additional soft color exchanges that might modify the concept (and calculations) of survival probabilities. “Survival probabilities” can also be dependent on impact parameters.
On page 12, the authors might want to discuss briefly the validity of the EPA approximation especially at high mass?
On page 2, the authors might want to quote a recent paper published about ALPs in heavy ions: https://arxiv.org/abs/1903.04151
Report 2 by Jean-Rene Cudell on 2021-6-6 (Invited Report)
This paper is topical, and I believe it is correct. It shows that the usual "solution" to the overshoot of muon data by theoretical predictions is not correct. After precisely quantifying the discrepancy, it suggests possible leads towards a solution. I think it should be published in its present form.
Anonymous Report 1 on 2021-5-21 (Contributed Report)
- Cite as: Anonymous, Report on arXiv:2104.13392v1, delivered 2021-05-21, doi: 10.21468/SciPost.Report.2949
-The paper is well written.
-We can follow the reasoning of the authors linearly and the attempt of the author to make a proof of one effect is essentially fulfilled. There are three main aspects discussed in the text:
-1. The role and significance of the cut b>R in STARlight MC and some other models used for example in ATLAS to predict the cross section of exclusive pairs of muons production. The authors explain why this cut is unphysical while necessary to obtain a correct numerical prediction... The arguments are clearly exposed, with of course the subjectivity inherent to the authors. The authors discuss in particular why the predictions of SuperCHIC may differ from the standard version of STARlight, while by using a similar treatment of the cuts in impact parameter, the predictions tend to converge.
-2. How to account for uncertainties for the process here mentioned in a complete ways... For example in the survival factor determination etc. Again, this discussion is clear and can be easily followed.
- 3. points 1. and 2. are exposed both in the context of pp and PbPb collisions... Further comments are made in the text to discuss the predictions are very large impact parameter (>3R).
- I think that all these items are nice arguments in the field and this discussion is useful in the practical life of experimentalist as well as giving some ideas for mors fundamental views of the topic.
This is not really a weakness but more an outlook for such studies. We are all aware that these treatments are too simplistic to be correct in fundamental physics. Obviously, wave functions of the incoming particles and the behavior of such functions in the process of the hard event should ideally be computed, as well as their fluctuations in the quantum sense. We know that this will not be for tomorrow and in a sense all arguments posed in a way or another are by definition unphysical, even the ones of the authors.
In this sense, this may be useful to avoid to be too categoric in the conclusions that this or that does not follow a good rationale, because this is also true for the conclusions of the paper.
Based on my above arguments, I think the the criteria for a publication are met with no ambiguity.