Doing LHC analyses without knowing everything

Transcription

1 Doing LHC analyses without knowing everything Alan Barr University of Oxford 27/03/2013 Alan Barr, University of Oxford 1

2 Not knowing everything Absolute ignorance Limits & bounds Relative ignorance Significance-like variables Working out what you do know Analysis design matters 27/03/2013 Alan Barr, University of Oxford 2

3 (S)particles SM SUSY Spin-1/2 Spin-1 Spin-0 quarks (L&R) leptons (L&R) neutrinos (L&?) γ Z 0 W ± gluon h 0 H 0 A 0 H ± B W 0 squarks (L&R) sleptons (L&R) sneutrinos (L&?) Bino Wino 0 Wino ± gluino ~ H 0 ~ H ± Spin-1/2 Spin-0 After Mixing 4 x neutralino gluino 2 x chargino Extended higgs sector (2 doublets) Alan Barr 14 October 2005

4 Motivational arguments We should look for WIMPs at the LHC 27/03/2013 Alan Barr, University of Oxford 4

5 The basic example Jet Don t know momenta of invisible(s) Jet P T miss 27/03/2013 Alan Barr, University of Oxford 5

6 Missing transverse momentum Know that momentum beam was zero in initial state Can measure momentum beam in final state for visible particles Construct Sum of P T of invisibles Construct from negative sum P T of visibles 27/03/2013 Alan Barr, University of Oxford 6

7 \begin{digression} 27/03/2013 Alan Barr, University of Oxford 7

8 Missing E T / MET ARRGH! These are not well defined quantities What is a transverse energy? Under transverse projection: P x and P y components should stay the same P z component should be set to zero What happens to energy component? Three different procedures (at least ) 27/03/2013 Alan Barr, University of Oxford 8

9 arxiv: /03/2013 Alan Barr, University of Oxford 9

10 \end{digression} 27/03/2013 Alan Barr, University of Oxford 10

11 Answer 1 DESIGNING AN ANALYSIS TO BE INSENSITIVE TO THE UNKNOWNS 27/03/2013 Alan Barr, University of Oxford 11

12 A useful guiding principle: In the absence of complete information we should place the best possible bound on any Lorentz invariants of interest 27/03/2013 Alan Barr, University of Oxford 12

13 A familiar example W Lepton Neutrino Transverse mass: 27/03/2013 Alan Barr, University of Oxford 13

14 Mathematical meaning? m T is the greatest lower boundon the mass of the parent (W) given the unknowns and the constraints For each signal event m W > m T 27/03/2013 Alan Barr, University of Oxford 14

15 The use of m T in discovery physics For B lυ : m T < m B For W lυ : m T < m W Range m B < m T < m W is signal dominated 27/03/2013 Alan Barr, University of Oxford 15

16 Similar search for new W at ATLAS 27/03/2013 Alan Barr, University of Oxford 16

17 Identical pair decays: W Lepton Invisible W Lepton 27/03/

18 Example: the stop squarksearch Stop should be light if SUPERSYMMETRY is NATURAL Major background is top quark pair production 27/03/2013 Alan Barr, University of Oxford 18

19 The top quark background t t Jet W W Jet Lepton Lepton Invisible 27/03/2013 Alan Barr, University of Oxford 19

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21 Higgs WW* (l υ) (l υ) H W W Lepton Lepton Invisible An example from the Higgs world 27/03/2013 Alan Barr, University of Oxford 21

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23 A rather general situation 27/03/2013 Alan Barr, University of Oxford 23

24 Constructing a bound Know the momenta P for visible particles Choose some momenta Q for the invisible particles Each M a can provide a lower bound for the selected Q Constructing the bound achieved by minimising over our ignorance 27/03/2013 Alan Barr, University of Oxford 24

25 A table of examples arxiv:

26 Intermezzo KNOW WHAT CAN GO WRONG 27/03/2013 Alan Barr, University of Oxford 26

27 JETS + MISSING MOMENTUM Measurements Jets: φ cut Reduce: Had. Calorimeter E.M. Calorimeter Tracks from vertex Measure remainder at small φ 27/03/2013 Alan Barr, University of Oxford 27

28 JETS + MISSING MOMENTUM From cosmics Reduce by: (a) requiring tracks with jets (b) look for muon hits Measure remainder: (a) no beam (b) timing 27/03/2013 Alan Barr, University of Oxford 28

29 JETS + MISSING MOMENTUM From beam halo Reduce by requiring tracks with jets Measure remainder with single beam / timing 27/03/2013 Alan Barr, University of Oxford 29

30 JETS + MISSING MOMENTUM Calorimeter noise Reduce by requiring tracks with jets Measure remainder (a) no beam (b) timing 27/03/2013 Alan Barr, University of Oxford 30

31 Early 2010 data SUSYx10

32 Real missing momentum: W υl Early 2010 data Simulation normalized to data 27/03/

33 Pile-up 27/03/2013 Alan Barr, University of Oxford 33

34 Dealing with pile-up Pile up suppression by e.g. associating jets with primary vertex 27/03/ See ATLAS-CONF

35 Answer 2 KNOW HOW WELL YOU GET THINGS WRONG (RESOLUTION) 27/03/2013 Alan Barr, University of Oxford 35

36 ATLAS-CONF An unexpected channel Each top quark can decay to 3 jets Want to be sensitive to e.g. 9 (!) jets + P T miss No way to make this prediction with current MC Absolutelynot something done in the pre-data preparation (there we went up to 4 jets ) 27/03/2013 Alan Barr, University of Oxford 36

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38 Multijets+ missing transverse momentum: QCD background Nature knows the multi-jet cross section We know the detector behaviour Can determine QCD BG! 27/03/2013 Alan Barr, University of Oxford 38

39 A quick reminder on jet resolution QCD backgrounds dominated by FAKE P T miss Resolution (largely) determined by Poisson sampling statistics* Expect σ E E Has almost universal distribution for QCD events * There are also noise and constant terms which the experts will be well aware of 27/03/2013 Alan Barr, University of Oxford 39

40 Proof of principal universal shape 6-jet distribution 27/03/2013 from 5-jet template Alan Barr, University of Oxford 40

41 Other backgrounds also depend largely on measurementsfrom control regions Jets Jets Z υυ + jets 27/03/2013 Alan Barr, University of Oxford 41

42 Jets Jets Z μμ+ jets 27/03/2013 Alan Barr, University of Oxford 42

43 A nine jet prediction 27/03/2013 Alan Barr, University of Oxford 43

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45 Excluded 27/03/2013 Alan Barr, University of Oxford 45

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48 ATLAS-CONF lepton + 4(1 b-)jets + P T miss 27/03/2013 Alan Barr, University of Oxford 48

49 ATLAS-CONF leptons + P T miss 27/03/2013 Alan Barr, University of Oxford 49

50 ATLAS-CONF leptons + P T miss 27/03/2013 Alan Barr, University of Oxford 50

51 Beyond Run 1 INTO THE FUTURE ULTIMATE GOALS? 27/03/2013 Alan Barr, University of Oxford 51

52 A luminosity upgrade: additional reach 27/03/2013 Alan Barr, University of Oxford Page 52

53 Reach of a 33 TeVmachine? 27/03/2013 Alan Barr, University of Oxford 53

54 THE END 27/03/2013 Alan Barr, University of Oxford 54

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