1. 6-10 September 2010 Edmund Widl, Robert Schöfbeck Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy 60. Jahrestagung der ÖPG

2. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy 2Edmund Widl (HEPHY Vienna) 6-10 September 2010 References  Martin, S., “A Supersymmetry Primer”, arXiv:hep-ph/9709356v5  CMS Collaboration, “The CMS Experiment at the CERN LHC”, JINST 3, S08004 (2008)  CMS Collaboration, “CMS Jet Performance in pp Collisions at √s = 7 TeV”, CMS Physics Analysis Summary, CMS PAS JME-10- 003  CMS Collaboration, “CMS MET Performance in Jet Events from pp Collisions at √s = 7 TeV”, CMS Physics Analysis Summary, CMS PAS JME-10-004  CMS Collaboration, “Performance of Methods for Data-Driven Background Estimation for SUSY Searches in the CMS Experiment”, CMS Physics Analysis Summary, CMS PAS SUS-10- 001

3. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Why jets and missing energy? The theoretical framework of supersym- metry allows for a very, very large amount of contrasting phenomenological features: –different mass spectra with distinct decay channels –different cross sections and branching ratios –even different conservation laws What is expected by the LHC experiments: –strong production of gluinos and squarks dominant SUSY processes in pp-collisions source of high-p T jets –decay chains ending with a stable, unchar- ged sparticle (neutralino) dark matter candidate for cosmology source of high excess of missing energy 3Edmund Widl (HEPHY Vienna) 6-10 September 2010 Example: mass spectra for three different SUSY scenarios

4. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy 4Edmund Widl (HEPHY Vienna) 6-10 September 2010

5. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy 5Edmund Widl (HEPHY Vienna) 6-10 September 2010 Commissioning of CMS QCD backgrounds are expected to be orders of magnitude larger than any SUSY signal Experimental requirements –a good understanding of complex physics observables –control over backgrounds from standard model processes Careful commissioning is the key to success (since end of May) –performance of the reconstruction of jets and missing transverse energy (MET) –validation of methods for background suppression and data-driven estimation study QCD backgrounds validate predictions of background distributions –comparison with predictions from simulation The first proton-proton collision data at a center-of-mass energy of 7 TeV recorded by CMS has been used to do exactly that –first collisions at 7 TeV on March 30 th –high efficieny data-taking (>90%) since the end of April

6. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of jet reconstruction (1) 6Edmund Widl (HEPHY Vienna) 6-10 September 2010 Signature for the production of colored particles –in SUSY: production of squarks and gluinos hadronizing to high-p T jets The instrumentation of CMS allows to reconstruct jets in various ways Two examples: –calorimeter jets uses only information about energy deposits from calorimeters –particle-flow jets (PF) aims to reconstruct, identify and calibrate each single particle by combining all sub-systems individual particles are the "bundled" to define jets

7. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of jet reconstruction (2) 7Edmund Widl (HEPHY Vienna) 6-10 September 2010 Jet energy correction (JEC) done in three steps: –offset correction due to electronics noise and additional pp-interactions in same bunch crossing (pile-up) from data: retrieve from zero-bias and minimum-bias data –relative correction due to non-linear and non-uniform response of calorimeter from data: use p T balance from back-to-back di-jet events –absolute correction calibrate absolute jet energy scale from data: use p T balance from back-to-back  +jet events, exploiting the extremely precise p T measurement of the  from the electromagnetic calorimeter

8. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of jet reconstruction (3) 8Edmund Widl (HEPHY Vienna) 6-10 September 2010 jet resolution for jets with 0.0 ≤ |η| ≤ 1.4 determined with "di-jet asymmetry method" –asymmetry variable A = (p T, jet1 - p T, jet2 ) / (p T, jet1 +p T, jet2 ) –for approximately equally values of p T one finds  (p T )/p T = √2  A

9. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of MET reconstruction (1) Signature for the production of uncharged, weakly interacting, long- lived particles –in SUSY: long-lived neutralinos (WIMPs) causing high excess of MET CMS has implemented various algorithms to reconstruct missing transverse energy Two examples: –calorimeter MET (caloMET) based on calorimeter energies, using the tower geometry of the hadron calorimeter –particle-flow MET (pfMET) calculated using a complete particle-flow technique 9Edmund Widl (HEPHY Vienna) 6-10 September 2010

10. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of MET reconstruction (2) 10Edmund Widl (HEPHY Vienna) 6-10 September 2010 Performance of MET reconstruction in di-jet events.

11. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of MET reconstruction (3) Removal of anomalous signals in the calorimeters –caused by particles hitting the transducers or rare random discharges –can be characterized by unphysical charge sharing between neighboring channels as well as timing/pulse shape information Removal of beam-induced backgrounds (beam halo) –veto on parallel-to-beam trajectories reconstructed in the endcaps of the muon system Corrections due to non-compensating nature of the calorimeter –response of hadronic and electromagnetic component of calorimeter showers differs (hence non-compensating) –energy of hadrons therefore tends to be undermeasured –correction of type I: apply jet energy correction to all jets with electromagnetic energy fraction 20 GeV –correction of type II: account for the remaining soft jets below threshold and energy deposits not clustered in any jet 11Edmund Widl (HEPHY Vienna) 6-10 September 2010

12. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of MET reconstruction (4) 12Edmund Widl (HEPHY Vienna) 6-10 September 2010 ECAL: electromagnetic calorimeterHE: hadronic calorimeter endcap region HF: hadronic calorimeter very forward regionHB: hadronic calorimeter barrel region

13. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Performance of MET reconstruction (5) 13Edmund Widl (HEPHY Vienna) 6-10 September 2010 Resolution for MET –caloMET: calorimetry only –pfMET: particle flow algorithm –tcMET: combined information from calorimetry and tracking Measured from photon+jet sample –use the very precise photon momentum measurement as reference The good agreement between data and simulation confirms that the MET reconstruction (including also cleaning proce- dures) works properly!

14. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy SUSY backgrounds at the LHC SUSY cross sections are expected to be orders of magnitude smaller than typical QCD cross sections: –backgrounds from QCD processes have to be understood and controlled –the corresponding tails in MET distributions have to be quantified accordingly Modeling and controlling the high-energy tails of QCD processes is very difficult: –QCD cross sections are not predicted with high enough precision –key topological and kinematic distributions such as the number of jets and their p T spectra are difficult to predict Strategy pursued by CMS: –determine backgrounds using data-driven methods whenever possible –use multiple methods of cross-checks 14Edmund Widl (HEPHY Vienna) 6-10 September 2010

15. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Example: Suppression of QCD contributions 15Edmund Widl (HEPHY Vienna) 6-10 September 2010 Kinematic variables H T and  T -characterize the overall transverse momentum balance of the event  in practice, QCD background is largely confined to the region  T <0.5 -suppression improves with increasing H T

16. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Background estimation: Templates The concept of the "template method": –assume that the signal of channel A looks like the background for channel B –retrieve the shape of signal A and normalize it to unity –use this shape as a template for estimating the background of B 16Edmund Widl (HEPHY Vienna) 6-10 September 2010 Example:  use multi-jet QCD events to model  +jets events with artificial MET  channel  +jets is also a relevant background for SUSY searches

17. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Conclusions and outlook The CMS Collaboration has used the first data from pp-collisions at a center-of-mass energy of 7 TeV to commission the detector. This commissioning included also the proper processing and handling of complex physics observables likes jets and MET. The performance of jet and MET reconstruction has come up to the expectations of the collaboration. This performance also allowed to successfully launch the physics programs, including the search for supersymmetry. Even though supersymmetry studies will still lack of statistics for some time, studies concerning backgrounds are progressing well. 17Edmund Widl (HEPHY Vienna) 6-10 September 2010

18. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Backup Slides 18Edmund Widl (HEPHY Vienna) 6-10 September 2010

19. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Backup: beam halo event 19Edmund Widl (HEPHY Vienna) 6-10 September 2010 Event display for beam halo event faking significant caloMET (224 GeV).

20. Preparation for Supersymmetry Searches at CMS: Jets and Missing Energy Backup: Another example of background suppression 20Edmund Widl (HEPHY Vienna) 6-10 September 2010 Kinematic variable  ∗ : –defined as the minimum angle bet- ween one jet and the MHT computed using the remaining jets –tests whether there is at least one jet which, if rescaled by a certain factor, would be able to balance the event –peaks around zero for QCD events –distributions of  *: red: multi-jet events black: multi-jet events with on jet removed mimicking SUSY events