1 / 13

CMS/Totem document Some suggestion for treating PU background Some news from the CMS diff group

Signals/backgrounds etc. Monika Grothe Wisconsin/Turin. CMS/Totem document Some suggestion for treating PU background Some news from the CMS diff group. “Prospects for diffractive and forward physics at the LHC”, The CMS and TOTEM diffractive and forward physics group,

thea
Download Presentation

CMS/Totem document Some suggestion for treating PU background Some news from the CMS diff group

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Signals/backgrounds etc Monika Grothe Wisconsin/Turin CMS/Totem document Some suggestion for treating PU background Some news from the CMS diff group Monika Grothe, FP420 meeting, Arlington March 2007

  2. “Prospects for diffractive and forward physics at the LHC”, The CMS and TOTEM diffractive and forward physics group, CERN/LHCC 2006-039/G-124, CMS Note 2007/002, TOTEM Note 06-5, December 2006 Presented to the LHCC in the LHCC open session in January 2007 Monika Grothe, FP420 meeting, Arlington March 2007

  3. From Valentina Avati’s talk in the open LHCC session Monika Grothe, FP420 meeting, Arlington March 2007

  4. Conclusions The CMS and TOTEM collaborations intend to carry out a joint diffractive and forward physics program with an unprecedented rapidity coverage. The document outlines some aspects of this physics program. The document addresses, for the first time at the LHC, the central experimental issues in measuring fwd and diffractive physics by way of a number of exemplary processes A worthwhile program and we are optimistic that it can be done from my talk in the open LHCC session Monika Grothe, FP420 meeting, Arlington March 2007

  5. Summary CMS/Totem document • CMS/Totem document is not a TDR • Was given to the LHCC just for their information • No LHCC response was required, nor provided • Document presented to the LHCC in January • Document was used as argument in the subsequent request from Totem to the LHCC for 90m optics running • CMS position concerning this request not yet decided Monika Grothe, FP420 meeting, Arlington March 2007

  6. Pile-up background TOTEM d(epeXp)/dxL [nb] det@420 xL=P’/Pbeam= 1-x Number of PU events with protons within acceptance of near-beam detectors on either side: ~2 % with p @ 420m ~6 % with p @ 220m Translates into a probability of obtaining a fake DPE signature caused by protons from PU: Eg at 2x 1033 cm-2s-110% of the non-diff background that looks like the signal in the central detector will appear to have DPE signature. This is independent of the type of signal. S/B_PU hence depends on the relative cross sections of diff signal and non-diff background that looks like the signal in the central detector. Depends critically on the leading proton spectrum at the LHC which in turn depends on size of soft rescattering effects (rapidity gap survival factor) ! Monika Grothe, FP420 meeting, Arlington March 2007

  7. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive partPile-up background (II) Can be reduced by: Requiring correlation between ξ, M measured in the central detector and ξ, Mmeasured by the near-beam detectors Fast timing detectors that can determine whether the protons seen in the near-beam detector came from the same vertex as the hard scatter (currently R&D project) ; 12 s = M2 incl QCD di-jets + PU CEP H(120) bb (jets) (220m) CEP of H(120 GeV) → b bbar: Possible to retain O(10%) of signal up to 2 x 1033 cm-2 s-1 in a special forward detectors trigger stream S/B in excess of unity for a SM Higgs and up to 1000 for a MSSM Higgs appears achievable M(2-jets)/M(p’s) Monika Grothe, FP420 meeting, Arlington March 2007

  8. Part I: “Diffractive” part - High lumiImpact of pile-up background Effect of cuts on the S/B for the background caused by PU for 120GeV Higgs MC overestimates leading proton spectrum in acc region of 420m detectors 0.6/10 0.9/30 0.6/20 0.9/100 0.6/180 0.9/320 0.6/900 0.9/1900 Beware: Uncertainty of the leading proton spectrum normalization because of uncertainty in rap gap suppression factor and its energy dependence Beware: Further handles not yet employed against PU are track multiplicity cuts and more stringent vertex cuts which may lead to an additional reduction of PU by a factor 10-100 Monika Grothe, FP420 meeting, Arlington March 2007

  9. Goal of pile-up background studies Regular phone meetings between Andrew and Arnab in Arlington, Antonio in Turin, Marek in Prague and Andy Pilkington in Manchester Antonio will present his results on H-> WW tomorrow Results for H-> WW should be brought to some conclusion soon such that these results can be included in any write-up we want to provide to ATLAS/CMS In particular need to substantiate that PU background is indeed less of a problem in this channel compared to H->bb One suggestion for a new approach to treating PU background: next slide Monika Grothe, FP420 meeting, Arlington March 2007

  10. Statistical subtraction of PU bckgrd !? Suggestion: All our attempts so far to reduce PU background based on more and harder cuts Problem: Starve signal PU background is combinatorial Hence in invariant mass should be signal bump sitting on a smooth combinatorial pedestal from PU Select signal in the same way one selects a D0 signal: Use 2 distributions in fit: mass distribution Delta(xi) distribution (instead of Delta(M) as for D0) - Signal bump should be dominated by resolution, i.e. is essentially a delta function around Higgs mass convolved with a resolution function which can probably be described with one or two Gaussians - Pedestal from PU combinatorial background should be describable with a polynomial Delta(xi) distribution can be used to compute a weight for each event that describes its probability to be a signal event based on where it is located in the delta(xi) distribution Once one has a good fit to the PU background, can subtract it statistically, which is a lot easier on the signal statistics Monika Grothe, FP420 meeting, Arlington March 2007

  11. What is currently going on in CMS on diffraction and forward physics • For 2008 data-taking, define rap gap based program • Prepare trigger accordingly • In parallel, keep preparing, on a lower level, possible joint data-taking with Totem • Prepare our tools: Integrate all fwd detectors simulation/reconstruction into standard CMS software such that all fwd detectors can be included in the default CMS MC generation chain • We are in the process of interfacing HECTOR to the CMS software • Sasha’s FP420 detector simulation/reconstruction code is already part of CMS software (see Sasha’s talk earlier today) • Work on-going to interface with HECTOR and validate whole setup • Have asked the Protvino group to do simulation of beam-beam, beam-halo, beam-gas background for ZDC location, 220m (and 420m, see Marta’s talk) Monika Grothe, FP420 meeting, Arlington March 2007

  12. What is currently going on in CMS in diffraction and forward physics (II) Topics foreseen for first data in 2008: Measurement of the ratio of events with LRG and a hard scale at 14TeV, where hard scale: W Z dijets heavy quarks Measurement of the ratio of di-jet events with a LRG between the jets Measurement of rapidity gap survival probability from SD and DPE dijet events at 14TeV Study of the underlying event in diffractive topologies Measurements of forward jets and forward Drell-Yan electrons Study of gamma-gamma and gamma-proton interactions Definition of rapidity gap triggers Minimum bias trigger using forward detectors (CASTOR, possibly T1, T2) see: https://twiki.cern.ch/twiki/bin/view/CMS/DiffractionAndFwdPhysics Monika Grothe, FP420 meeting, Arlington March 2007

  13. What is currently going on in CMS in diff & fwd physics: Rapidity gap trigger Assume that our first analyses in 2008 will be based on rap gap selection As a figure of merit, assume we can get at most a few hundred pb-1 of single-interaction data Our request to the trigger group for diff trigger bits: 1. Exclusive two-lepton trigger bit -- calib channel for near-beam detectors at 220/420m Two isolated leptons (di-electrons or di-muons) AND (no jets with ET>10GeV) 2. Rap gap trigger bit. Since neither Castor nor ZDC software is available for rate studies in standard CMSSW, this has to be based, for the time being, on the HF. Two conditions are possible: a. 2-jets with ET>40GeV AND (no jet in HF with ET>10GeV on one side) b. 2-jets with ET>40GeV AND (sum ET in HF on one side < 50GeV) 3. Gap-between-jets trigger bit (Two jets, one in HF-right, one in HF-left, with ET>40GeV) AND (no central jets with ET>10GEV Desirable: Min bias trigger with CASTOR Currently foreseen min bias trigger requires activity in HF on both sides, i.e. is veto against rap gaps Require activity in CASTOR just on one side + prescale Monika Grothe, FP420 meeting, Arlington March 2007

More Related