Measurement of Di-photon Cross-section in pp Collisions at √s=7 TeV with the ATLAS Detector

Open Discussion ATLAS-STDM-2011-05-001Measurement of isolated di-photon cross-section in ppCollisions at√ s=7 TeV with the ATLAS Detector Corresponding Editors Marcello Fanti, Sandrine Laplace, Marco Delmastro Editorial Board Erez Etzion, Thomas Koffas, Cristobal Padilla, Peter Watkins 24/05/2011 To be submitted to Phys. Rev. D

Goal The purpose is measurement of the production cross-section of photon pairs in pp collisions, with differential distributions in the kinematic variables M(γγ), pT(γγ), ∆φ(γγ). The data is 2010, corresponding to an integrated luminosity of 37 pb-1. • The photon identification is based on the shower shape variables and on the calorimetric isolation. • The background (Mainly hadronic jets and isolated electrons) is estimated and subtracted using data-driven techniques. • The ID efficiency is taken from the inclusive photon analysis. • The reconstruction efficiency and the unfolding matrix are derived from Pythia and Sherpa di-photon MC. Erez Etzion

Current Publication Draft EdB formed - January 26, 2011 Kickoff meeting of analysis team, editors with EdB - February 1, 2011. Supporting COM Note : ATL-COM-PHYS-2011-301 STDM group approval, March 24, 2011. First draft circulated to the Collaboration, May 12, 2011. Deadline for comments, May 19, 2011. !3 lists of comments from 6 collaborators, 4 groups (Iowa, Birmingham, Queen Mary, UPenn) and 3 EdB/Editors. Most of the comments are positive and support rapid publication. The comments are composed of some Physics questions, structural comments and some are text related suggesting clarifications and language corrections. Authors + edb went through the All the comments, (including those arriving after the deadline) answered Physics related ones, and tried to address the structure and text related issues. Details in CDS. Erez Etzion

Measurement of isolated di-photon cross-section in pp Collisions at √ s=7 TeV with the ATLAS DetectorABSTRACT Erez Etzion

Plan for today’s meeting Approve the structure of the paper and the response to the comments of the collaboration to the 1st circulation. Approve the figures and analysis (as preliminary) to allow presentation at conferences (starting with Blois – next week). Erez Etzion

Major or repeating comments Requireexplantation to the enchantement around5 GeV of figure 2 – next page.. Improvethe explantation of several paragraphes, mainly the isolation section - Done Reduce the content oftable 1 – modifed structure but left the content Fix bug on the 2D fit yield - Done Add Feynman diagrams – refuse: 1. not very special, 2. (too) many options (ISR/FSR, fragmentation...) Erez Etzion

Verified that this fluctuation resulted in a downward fluctuation in the substracted non-tight sample, has no significant effect on the measurement. Added clarification on the caption. Reevaluated the isolation distribution and indeed it has some larger errors, namely less significant discontinuity Figure 2 Erez Etzion

Some Physics questions Q: Not clear if the isolation transverse energy method is valid/applied to the leading photon only or to the sub-leading photon as well. A: validated on leading photons on then applied to diphotons. Q: Photons produced after hadronization by neutral hadron decays..” Why is this limited to neutral hadron decays? Are photons from radiativedecays of other particles not also background? A: Yes, photons from radiative decays are also background, however they are a minor component wrtpi0 decays. Nevertheless rephrased. Q: Line style on figure 4 is not consistent with the legend? A: A mistake, corrected. Q: ..”is a diagram mediated by a quark box diagram” -gg interaction Is this what people usually mean by gluon gluon fusion? A: Thinking e.g. at Higgs, gg fusion has a quark loop in between, with an outgoing leg for the Higgs. Here we have two outgoing legs (the photons), so we believe it's ok Erez Etzion

Some Physics questions (2) Q: Not clear if the isolation transverse energy method is valid/applied to the leading photon only or to the sub-leading photon as well. A: validated on leading photons on then applied to diphotons. Q: Photons produced after hadronization by neutral hadron decays..” Why is this limited to neutral hadron decays? Are photons from radiativedecays of other particles not also background? A: Yes, photons from radiative decays are also background, however they are a minor component wrtpi0 decays. Nevertheless rephrased. Q: Line style on figure 4 is not consistent with the legend? A: A mistake, corrected. Q: ..”is a diagram mediated by a quark box diagram” -gg interaction Is this what people usually mean by gluon gluon fusion? A: Thinking e.g. at Higgs, gg fusion has a quark loop in between, with an outgoing leg for the Higgs. Here we have two outgoing legs (the photons), so we believe it's ok Erez Etzion

Some Physics questions Q: The M_gamma,gamma plot (figure 3 on the draft or 9 on the supporting note) has a peak at around 50 GeV which is not explained in the paper itself. A: New sentence : “Due to the kinematic selection, the distribution of the di-photon invariant mass exhibits a turn-on at M_gg>2 pT, clearly visible in Figure 3, followed by the usual decrease typical of the continuum processes. The region below the turn-on is populated by di-photon events with low delta phi Q: it is said that the DeltaPhi distribution is sensitive to the double fragmentation from the _same_ parton. I would say that it is sensitive to fragmentation, single or double, even from different partons. A: Accepted Erez Etzion

Structural comments Q: Provide a summary of the content of the paper . A: OK. Q: It would be helpful to add more explanation to motivate the equation (5). What assumptions have been made about the form of the? A: we preferred to keep it as it is now, this is specific method well detailed and motivated on ref [10] Q.Equation (6) and the paragraph following it are quite opaque. A: Added (back) clarifying terms. Q: Is it acceptable to reference ATLAS CONF notes? A: If there is no other choice.. Q: Why do you discuss in such details the three jet-fake BG determinations? A: We want to give complete (thus, with under control systematics) description of the three methods to add strength to the work. Being the first publication of diphoton analysis, we believe it is important document it exhaustively, also for future references. Erez Etzion

Structural comments (2) Q: Table 1 is very detailed: Please consider getting rid of the detailed error breakdown in every bin. It is hard to pick out what the final measurements are! A: Added some spacing between the overall results and the breakdown, this reads better Q: Simplify the ATLAS coordinate system description A: Accepted Q: suggested to remove the systematic error computed for DIPHOX is applied to ResBosby saying “The error bands come from scale and PDF uncertainties from DIPHOX” and not to mention ResBos in this respect A: Accepted Q: Photon reconstruction - "This is addressed as described in [9]"---> please add a short description of how this is addressed, referencing to [9] for the details. A: The ambiguity resolution is quite complex and involves several tests. We chose to drop a tentative description, which would anyway be too imprecise, and rely completely on the ref [9] Erez Etzion

Structural comments (3) Q: I would put the first paragarph later in the introduction. Before telling us which processes are signal and which are background, you should tell us what you are doing and why this is interesting. A:We think it is difficult to say "what we are doing" (i.e. what we are measuring) before giving first a precise definition of the signal we are studying. Photons may come from many sources, so it is important to state clearly which photons we are measuring. We prefer to keep the paragraphs as they are now. Erez Etzion

And the rest of the comments Most other comments are good suggestions mainly to do with the structure of the paper and the way the analysis is described in a clear way to the general Physicist. All the comment were answered in CDS, most of them were accepted and implemented. Changes were introduced to the text, hopefully making it clearer to read. Erez Etzion

Conclusions Erez Etzion

Conclusions (2) Erez Etzion

Review summary The author list can be seen at https://atlas-authdb.web.cern.ch/atlas-authdb/authorlists/papers.html. A new draft is under construction. The new draft will be released to ATLAS for a 2nd circulation after incorporating the comments on this meeting Submit to Phys Rev. D … when ready .. Erez Etzion

Thank you all It has been an extensive common effort of many people who are working on this very detailed analysis for a very long time. They made it possible to reach the open discussion of this paper at the beginning of the summer conference era as originally scheduled. Many thanks to all those who contributed and those who commented and helped us improving this paper which will be published very soon. Erez Etzion

Measurement of Di-photon Cross-section in pp Collisions at √s=7 TeV with the ATLAS Detector