Discussion Session 10/13/11

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UT Austin (c... Administator 216 posts

Search for supersymmetry in pp collisions at √s = 7 TeV in final states with missing transverse momentum, b-jets and one lepton with the ATLAS detector

Discussion led by Jay Wacker

UT Austin (c... Administator 216 posts

The link to the ATLAS note is here

SLAC (contac... Moderator 2 posts

edited 10 years ago

Are there are any comments related to last week’s forum on ATLAS-CONF-2011-130?

I had the following questions/comments:

  1. How are the background normalizations taken? Did they really just use ALPGEN straight from the can to get the W/Z+jets backgrounds?
  2. Markus pointed out the topology ppZHh(bb¯)(WW)p p \to Z' \to H h \to (b \overline{b}) (W W) and various other topologies. Markus, did you have a paper or note to reference?
  3. Any other questions??
SLAC (contac... Moderator 2 posts

edited 10 years ago

Dear organizers of the LHC Results Forum,

Thanks a lot for showing interest in this analysis and for your very useful comments which will help us to improve the presentation of our results in the future. Please find our answers below.

We’ve recent started the LHC Results Forum where theorists present individual analyses from the LHC so as to better understand the details of high profile analyses. The first meeting covered the ATLAS bjet + lepton + MET analysis.

The forum was impressed with the analysis and we had a few small comments.

  1. We had a bit of trouble following the discussion on the normalization of the background. In Sec. 2, the analysis says that QCD is data driven, and later say you use fix-order theoretical inclusive cross sections for top and W&Z+jets. Then in Section 4 the analysis normalizes the number of expected events in the signal region by N sigdata=N sigMC×(N CRdata/N CRMC)N_{sig data} = N_{sig MC} \times (N_{CR data}/ N_{CR MC}).
    Is the purpose of using NnLO cross sections is to get the relative contribution of the different backgrounds? When the plots are made with the different background contributions labeled, is this rescaling already performed? I’ve talked to Pelle Hansson and think I understand the answers to these questions, but on the first pass it was difficult to follow. A sentence in Sec. 2 sign-posting how the backgrounds will be normalized in the signal region might clear it up.

    The MC prediction for top and W&Z+jets processes is normalized using NNLO cross sections. This normalization is used to show the background contributions in all the plots and to get the MC based estimate of the number of background events in the signal region and in the control region, respectively N sigMCN_{sig MC} and N CRMCN_{CR MC}. However, we don’t use directly this estimate from MC (N sigMCN_{sig MC}) to derive the exclusion limits but we use a semi-data driven approach where the MC estimate is rescaled using the number of observed events in the control region (N CRdataN_{CR data}) according to the formula N sigdata=N sigMC×(N CRdata/N CRMC)N_{sig data} = N_{sig MC} \times (N_{CR data}/ N_{CR MC}). This rescaling is not applied on the plots. We will clarify this procedure in the next update of this analysis.

  2. Having the predictions from the other control regions in a table would have been nice to evaluate the statement that “CR2 and CR3 agree well with the background”.

    Yes you’re right, we will add these tables and the corresponding plots to the approved material for the next publication.

  3. A distribution of the invariant mass of the bjet and lepton would be useful. This can show end points in spectra such as the top but also in potential signals. Any structure (or lack there of) in this variable is important to know.

    This plot can certainly be added to the approved material. However, we are looking at very busy final states and the huge combinatorics background could make difficult the interpretation of any structure (or lack of structure) in this variable.

  4. Showing a distribution of the number of jets and the number of leptons would be helpful since many signals have high jet multiplicities and higher lepton multiplicities.

    We agree with you and we will also add these distributions to the approved material for the next publication.

  5. There are several non-susy (simplified) models that would be potentially constrained by this analysis. For instance, Markus Luty has proposed:
    ggA 0W ±H W ±(W h 0)W ±(W (bb¯))g g \to A^0 \to W^\pm H^\mp \to W^\pm (W^\mp h^0) \to W^\pm (W^\mp (b \overline{b})) Comparing against wildly different models is interesting to see how sensitive the searches are to kinematics. Even one mass point showing the acceptance would be useful.

    Thanks for pointing us this interesting model. Generally, our policy consists in providing limits on simplified models plus model independent upper limits on the effective cross-section for new physics (σ×efficiency×acceptance\sigma\times \text{efficiency}\times \text{acceptance}) which can then be used by theorists to derive limits on their specific models. Thanks to many discussions ongoing between experimentalists and theorists, we will certainly improve the presentation of our results in the future to ease their exploitation by the theorists.

UT Austin (c... Administator 216 posts

Jay’s slides can be found here.