Investigation of background subtraction technique for new physics

Transcription

1 Investigation of acground sutraction techniue for new physics M.S. Kim 1, D.H. Kim 1, T. Kamon 1,2 Kyungpoo Nat l University 1 Texas A&M University 2 23 April 2015 KPS Spring Meeting 2015

2 Outline Introduction Bacground estimation and sutraction Event-Mixing Method Concept and history Pros and cons Correlated distriution Modeling of cominatorial acground Application to new physics 2

3 A standard way to oserve resonance production mae an invariant mass distriution of particles which may e decay products of resonances 1ar produc/on in l + jets channel Introduction simulated data and predic/on Prog.Part.Nucl.Phys. 63 (2009) 239 pea correct permutation wrong permutation An estimation of the acground is essential if the acground has a simple falling or rising form, it will not e difficult to approximate the acground shape with an analytical function 3

4 Bacground estimation A potential prolem with an analytical function inematic restrictions induced y the experimental setup and the event selection can lead to wrong estimation it reuires further study to understand the effects of detector acceptance and event selection cuts in the analysis Event mixing to overcome these prolems artificial mass distriution otained from taing one particle from one event and the other particle from another event cross-event particle cominations are called mixed events 4

5 Bacground sutraction In hadron colliders, lots of particles produced in a single event reconstruct particles y misidentifying their decay product Reconstruction of Z 0 in di-lepton mass: Z 0 e + e - / μ + μ - collect a sample of opposite-sign same-flavor (OSSF) pairs cominatorial: opposite-sign opposite-flavor (OSOF) pairs h(ossf) - C h(osof) in m ll distriution However, such a sutraction techniue not availale for jets introduced & developed Bi-Event Sutraction Techniue (BEST) Phys. Lett. B 703 (2011) 475, arxiv: , and CMS PAS TOP

6 Event mixing To estimate the acground mass distriution with cominations of particles from different events (e.g., three jets in top uar reconstruction) event 1 2 event1 6

7 Mixed events To estimate the acground mass distriution with cominations of particles from different events (e.g., three jets in top uar reconstruction) event 1 2 event1 7

8 History of event mixing Idea came up in the 1970s in the meson physics interference analyses with di-pion sources: distriution calculated from two pions from different events is used for the modeling of the non-interfering contriution Modeling of the cominatorial acground in invariant two-particle mass spectra used to extract ρ and ω resonances from the di-pion spectrum at the ISR storage ring at CERN provided acground predictions in other di-particle mass spectra (aons, protons, and pions) searching for further resonances and setting limits ut, another experiment found that it is not a suitale method 8

9 Pros and cons Pros no application of inematical fitting to reconstruction of decay products of resonances Cons a correlation if same inematical restrictions are applied to event mixing comining two particles from different events whose mass distriution is narrower than that of the cominatorial acground 9

10 Selection of i-event Most important reuirements mixing events with the same numer of jets mixing jets with the angular separation of R > 0.5 R(,) > 0.5 R(,) > event njets=4 event 10

11 Correlated distriution Comining two particles from different events each jet contriutes a resonance (W ) in corresponding event 1 2 event1 event 11

12 Correction Negative in acground sutraction due to a correlation induced y event mixing D. Drijard, H. Fischer, and T. Naada Study of Event Mixing and its Applica/on to the Extrac/on of Resonance Signals Nucl. Instrum. Meth. A225 (1984) 367 doi: / (84) negative resulted y sutraction We should mae a correction for this 12

13 However Comining three particles from different events no such a correlation, thus no correction 1 2 event1 event 13

14 Second mixing Comining three particles from different events reuires that we perform mixing twice taing into account wrong permutation issue 1 2 event1 event 14

15 MC simulation Input: cominatorial acground of tt + jets events modeled y event mixing Di- jets invariant mass Three- jets invariant mass Cominations / 5 GeV f (8 TeV) CMS Wor in progress Simulation Com. acground Mixed events Cominations / 10 GeV f (8 TeV) CMS Wor in progress Simulation Com. acground Mixed events Bg/Mixed Ratio m jj (GeV) Bg/Mixed Ratio m jj (GeV) 15

16 New physics Bump search Dar matter (MET + 3 jets) SUSY tth (lepton + 6 jets) s- channel resonant case 16

17 Summary A very common approach for hints of a new physics search in the invariant mass distriution of oserved particles Modeling of the cominatorial acground presented a very rief review of an estalished acground estimation method with the event-mixing techniue developing event-mixing techniue to estimate tt acground, which needs to e tested and validated with simulated events Application to new physics In association with top uar (pairs) 17

18 Bacup

19 BEST Phys. Lett. B 703 (2011) 475, arxiv: , and CMS PAS TOP Bi-Event Sutraction Techniue is to model and sutract dominant cominatorial acground during reconstruction of particle decay chains at hadron colliders Event mixing is used to estimate the acground (the mass distriution with cominations of particles from different collisions) Basic idea is for the reconstruction of the W oson decaying into two jets at LHC A general application to the reconstruction of the top uar (t W) Event mixing (Event mixing presented y E. Kwon at LHCC poster) MC study W Top BEST is for oth the top uar mass and other measurements Interesting for new physics! Largest systematic sources: JES and QCD modeling 19